Chapter 1: General Introduction

Energy is one of the major building blocks of society and it is needed to create goods from natural resources. Global economics development and improved standards of energy are complex processes that share a common denominator i.e. the availability of an adequate and reliable supply of clean energy. With an oil embargo in 1973, continuing with the Iranian revolution of 1979 and the Persian Gulf War of 1991, political events had made many people aware of how crucial clean energy is for everyday functioning of our society. The energy crises of the 1970s were almost forgotten by the 1980s, that period brought an increased awareness of other environmental issues. The global warming, acid rain and radioactive waste are still very much with us today, and each of these topics is related to our energy security.

In the present scenario all sectors of society, e.g. labour, environment, economics and international relations, etc. in addition to our own personal livings, i.e. housing, food, transportation, recreation and communication, etc., strongly depends on energy. The use of energy resources has relieved us from much drudgery and made our efforts more productive. Human beings once had to depend on their own muscle energy to provide the energy necessary to do the daily work. Today, muscle energy supplies less than 1% of the work done in the industrialised world.

Energy is a globally conserved quantity, i.e. the total amount energy in the universe is constant. Energy can neither be created nor destroyed. It can only be transformed from one state to another. Two billiard balls colliding, for example, may come to rest, with the resulting energy becoming sound and perhaps a bit of heat at the point of collision.

Energy, environment and economic development are closely related. The proper use of energy requires consideration of social impact as well as technological ones. Indeed, sustained economic growth of a country in this century along with improvements in the quality of everyone's lives may be possible only by the well planned and efficient use of fossil fuel and other resources and the development of new renewable energy technologies.

In physics, energy is defined as “the capacity of a physical system to perform work”. The word is used by each of us with many different connotations, but in physics, it has a very definite meaning:

A force of one Newton (N) acting through a distance of one meter in the same direction performs an amount of work equivalent to one joule (J). It is also important to note that work, the capacity for doing work and energy has the same units. A system may possess energy even when no work is being done. Since energy is measured by the total amount of work that the body can do, hence energy is expressed in the same unit of work as mentioned above.

There is an important law known as the “Law of conservation of energy” that states that the total amount of energy in a closed system remains constant. Energy may change from one form to another, but the total amount in any closed system does not change. This law is extremely important in order to understand a variety of phenomenon. We will begin by identifying various forms of energy and transformations of energy from one form to another as mentioned below.

The kinetic energy of an object is the extra energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its current velocity. Mathematically, kinetic energy can be expressed an

This is the energy of an object due to its elevation in a gravitational field. Mathematically, potential energy can be expressed an

Chemical energy is the energy stored in the chemical bonds of molecules. It arises out of the capacity of atoms to evolve heat as they combine or separate. It is the chemical energy in coal, natural gas, wood, oil that heats our homes, powers cars and is used to generate electricity.

This is defined as the capacity of moving electrons to evolve heat, electromagnetic radiation and magnetic fields.

This is the energy of a material due to the random motion of its particles. It is also called thermal energy. The word “heat” is used when energy is transferred from one substance to another.

Radiant energy is the energy emitted by electrons as they change orbit and by atomic nuclei during fission and fusion; on striking matter, such energy appears ultimately as heat.

This is the energy stored in the nucleus of an atom. According to Einstein when the mass of some system is reduced by an amount Δm, as in nuclear reaction, then the amount of energy released is

The various forms, sources and end users of energy are summarised in Table 1.1. Further, Table 1.2 shows a number of devices to illustrate conversions of energy from one form to another. For example, a solar cell illustrates the conversion of light energy to electrical energy; a battery converts chemical energy into electrical energy. The kinetic part of mechanical energy of a car converts into heat when the brakes are applied.

Energy that is used by the human beings on planet Earth can be classified as: (i) renewable energy sources and (ii) nonrenewable energy sources.

1. Renewable energy sources: These are the energy sources that are derived from natural sources that replenish themselves over short periods of time. These resources include the Sun, wind, moving water, organic plant and waste material (biomass), and the Earth's heat (geothermal). These resources are also called nonconventional sources of energy. This renewable energy sources can be used to generate electricity as well as for other applications. For example, biomass may be used as a boiler fuel to generate steam heat; solar energy may be used to heat water or for passive space heating; and landfill methane gas can be used for heating or cooking.

2. Nonrenewable energy sources: These are the energy sources that are derived from finite and static stocks of energy. It cannot be produced, grown, generated or used on a scale that can sustain its consumption rate. These resources often exist in a fixed amount and are consumed much faster than nature can create them. Examples of these types of resources are fossil fuels such as coal, petroleum, and natural gas and nuclear power (uranium). Due to its exhaustibility in nature, these types of energy resources are sometimes also called conventional sources of energy.

The basic different between renewable and nonrenewable energy are presented in Figure 1.1. Table 1.3 provides the features of comparison between renewable and nonrenewable sources of energy.

There are six sources of useful energy utilised by human beings on planet Earth. These sources are given below:

1. the Sun (thermal and electric);

2. geothermal energy from cooling, chemical reactions and radioactive decay in the Earth (thermal and electric);

3. the gravitational potential and planetary motion among Sun, Moon and Earth;

4. chemical energy from reactions among mineral sources;

5. fossil fuels such as coal, petroleum products and natural gases (thermal and electric); and

6. nuclear energy from nuclear reactions on the Earth.

Renewable energy is obtained from sources (i), (ii) and (iii), whereas nonrenewable energy is derived from sources (iv), (v) and (vi).

The continuous flow of natural energy as renewable energy on Earth is shown in Figure 1.2. The total solar flux incident on Earth at sea level is about 1.2×10¹⁷ W and the solar flux per person (for around 6×10⁹ persons) is approximately 20 MW. This is equivalent to the power generating capacity of seven very large sized diesel power plants. The maximum solar flux density perpendicular to the solar beam on Earth is around 1 kW/m².

Figure 1.2 provides a rough idea of the availability of natural energy on Earth. They have little value for practical purposes as renewable energy, which is site specific. Each region has a different environment that determines the amount of energy to be harnessed from the renewable energy sources.

The conversion of energy from one form to another generally affects the environment. Hence, without considering the impact of energy on the environment, the study of energy is not complete. Fossil fuels have been powering the industrial growth and the amenities of modern life that we are enjoying now since the 1700s. But this has not been without any undesirable side effects. From the soil we farm, the water we drink and the air we breathe, the environment has been paying a heavy price for it.

During the combustion of fossil fuels the emitted pollutants are strongly responsible for smog, acid rain, global warming and climate change. The environmental pollution has reached such a high level that it becomes a serious threat for vegetables growth, wild life and human health. Air pollution can cause health problems and it can also damage the environment and property. It has caused thinning of the protective ozone layer of the atmosphere, which is leading to climate change.

Hundreds of elements and compounds such as benzene and formaldehyde are known to be emitted during the combustion of coal, oil, natural gas, engine of vehicles, furnaces and even fireplaces. Air pollution results from a variety of causes, not all of which are within human control. Dust storms in desert areas and smoke from forest fires and grass fires contribute to chemical and particulate pollution of the air. The source of pollution may be in one country but the impact of pollution may be felt elsewhere. Major air pollutants and their sources are listed below:

Carbon monoxide (CO): This is a colourless, odourless gas that is produced by the incomplete burning of carbon-based fuels including petrol, diesel and wood. It is also produced from the combustion of natural and synthetic products such as cigarettes. It lowers the amount of oxygen that enters our blood. It can slow our reflexes and make us confused and sleepy.

Carbon dioxide (CO₂): This is the principle greenhouse gas emitted as a result of human activities such as the burning of coal, oil, and natural gases.

Chlorofluorocarbons (CFC): These are gases that are released mainly from air conditioning systems and refrigeration. When released into the air, CFCs rise to the stratosphere, where they come in contact with other gases, which lead to a reduction of the ozone layer that protects the Earth from the harmful ultraviolet rays of the Sun.

Lead: This is present in petrol, diesel, lead batteries, paints, hair dye products, etc. Lead affects children in particular. It can cause nervous system damage and digestive problems and, in some cases, cause cancer.

Ozone (O₃): This occurs naturally in the upper layers of the atmosphere. This important gas shields the Earth from the harmful ultraviolet rays of the Sun. However, at the ground level, it is a pollutant with highly toxic effects. Vehicles and industries are the major source of ground level ozone emissions. Ozone makes our eyes itch, burn, and water. It lowers our resistance to colds and pneumonia.

Nitrogen oxide (NO_x): This causes smog and acid rain. It is produced from burning fuels including petrol, diesel, and coal. Nitrogen oxides can make children susceptible to respiratory diseases in winters.

Suspended particulate matter (SPM): This consists of solids in the air in the form of smoke, dust, and vapour that can remain suspended for extended periods and is also the main source of haze, which reduces visibility. The finer of these particles, when breathed in can lodge in our lungs and cause lung damage and respiratory problems.

Sulfur dioxide (SO₂): This is a gas produced from burning coal, mainly in thermal power plants. Some industrial processes, such as production of paper and smelting of metals, produce sulfur dioxide. It is a major contributor to smog and acid rain. Sulfur dioxide can lead to lung diseases.

The major areas of environmental problems may be classified as follows:

• water pollution;

• ambient air quality;

• hazardous air pollutants;

• maritime pollution;

• solid waste disposal;

• land use and siting impact;

• acid rain;

• stratospheric ozone depletion;

• global climate change (greenhouse effect).

Among these environmental issues, the internationally most vital problems are the acid precipitation, the stratospheric ozone depletion and the global climate change.

Acid rain is a widespread term used to describe all forms of acid precipitation (rain, snow, hail, fog, etc.) Atmospheric pollutants, particularly oxides of sulfur and nitrogen, can cause precipitation to become more acidic when converted to sulfuric and nitric acids, hence the term acid rain. Motor vehicles also contribute to SO₂ emissions since petrol and diesel fuel also contains small amounts of sulfur.

The sulfur oxides (SO₂) and nitric oxides (NO) react with water vapour (H₂O) and other chemicals in the atmosphere in the presence of sunlight to form sulfuric acid (H₂SO₄) and nitric acid (HNO₃) as follows:

1. Sulfur dioxide reacts with water to form sulfurous acid (H₂SO₃):

   

   Sulfur dioxide (SO₂) can be oxidised gradually to sulfur trioxide (SO₃):

   

   Sulfur trioxide (SO₃) reacts with water to form sulfuric acid (H₂SO₄):

   

2. Carbon dioxide reacts with water to form carbonic acid:

   

   Since carbonic acid is a weak acid it partially dissociates:

   

   Nitrogen dioxide reacts with water to form a mixture of nitrous acid and nitric acid:

   

These are shown in Figure 1.3. The acids formed usually dissolve in the suspended water droplets in clouds or fogs. These acid-laden droplets are washed from the air to the soil by rain or snow onto the Earth. This is known as acid rain, which is as acidic as lemon juice. The soil is capable of neutralising a certain amount of acid. However, the power plant, which uses high-sulfur coal, pollutes many lakes and rivers in industrial areas that have become too acidic for fish to grow. Forests in different regions of the Earth also experience a slow death due to absorption of acids from acid rain through the leaves, needles and roots of the trees.

It is well known that the natural build up of oxygen in the atmosphere gradually led to the formation of the ozone layer. This layer is found between 19 and 30 kilometres (km) above the ground. The ozone layer filters out incoming radiation from the Sun that is harmful to life on Earth. The development of the ozone layer allowed more advanced lifeforms to evolve. Most ozone is produced naturally in the stratosphere, a layer of atmosphere between 10 and 50 km above the Earth's surface, but it can be found throughout the whole of the atmosphere. The ozone layer plays a natural and equilibrium maintaining role for the Earth through the absorption of ultraviolet (UV) radiation (240–320 nm) and absorption of infrared radiation. A global environmental problem is the distortion and regional depletion of the stratospheric ozone layer. This effect is shown in Figure 1.4 due to the emissions of NO_x and CFCs, etc. Ozone depletion in the stratosphere can lead to increased levels of damaging ultraviolet radiation reaching the ground. This increases rates of skin cancer, eye damage and other harm to many biological species. Chlorofluorocarbons (CFCs) and NO_x emissions are produced by fossil fuel and biomass combustion processes and play the most significant role in ozone depletion. Hence, the major pollutant, NO_x emissions, needs to be minimised to prevent stratospheric ozone depletion.

The greenhouse effect is a process by which radiative energy leaving a planetary surface is absorbed by some atmospheric gases, called greenhouse gases. They transfer this energy to other components of the atmosphere, and it is reradiated in all directions, including back down towards the surface. This transfers energy to the surface and lower atmosphere, so the temperature there is higher than it would be if direct heating by solar radiation were the only warming mechanism.

The greenhouse effect is also experienced on a larger scale on Earth. This warms up as a result of the absorption of solar energy (short wavelength) during the day, cools down at night by radiating part of its energy into deep space as infrared radiation (long wavelength). Carbon dioxide (CO₂), water vapour and trace amounts of some other gases such as methane (CH₄) and nitrogen oxides act like a blanket and keep the Earth warm at night by blocking the heat radiation from the Earth, as shown in the Figure 1.5. Therefore, they are called “greenhouse effect” gases. In this case, the CO₂ is the primary component. Water vapour is usually taken out of this list. Since it comes down as rain or snow as part of the water cycle, the man's activities in producing water do not make much difference on its concentration in the atmosphere. It is mostly due to evaporation from rivers, lakes, oceans etc. The CO₂ is different due to man's activities that do make a difference in CO₂ concentration in the atmosphere.

The greenhouse effect makes human life on the planet Earth feasible by keeping the Earth warm at about 30 °C. However, excessive amounts of greenhouse gases emitted by human being disturb the delicate balance by trapping too much energy. This causes the average temperature of the Earth to rise and the climate generally changes at some localities. These undesirable features of the greenhouse effect are generally referred to as global warming or climate change.

The excessive use of fossil fuels such as coal, petroleum products and natural gas in electric power generation, transportation and manufacturing processes is responsible for global climate change. The present concentration of CO₂ in the atmosphere is about 360 ppm (0.36 percent). This is 20 percent higher than the level a century ago. Further, it is projected to increase over 700 ppm by the year 2100. Under normal conditions, vegetables consume CO₂ and release CO₂ during the photosynthesis process, thus keeping the CO₂ concentration in the atmosphere in check. A mature growing tree consumes about 12 kg of CO₂ a year and exhales enough oxygen to support a family of four. However, deforestation and the huge increase in CO₂ production due to the fast growing industrialisation in recent decades has disturbed this balance. Also, a major source of greenhouse gas emissions is transportation. Each litre of petrol burned by a vehicle produces about 2.5 kg of CO₂. Also, a car emits about 6000 kg of CO₂ to the atmosphere in a year, which is nearly 4 times the weight of a car.

There should be an effort to find ways to replace fossil fuels with more environmental friendly alternatives, particularly renewable energy resources, as potential solutions to the current environmental problems associated with the harmful pollutant emissions from fossil fuels. The use of renewable energy should be encouraged worldwide, with incentives. It is necessary to make the Earth a better place to live in. In recent times the advancements in thermodynamics have greatly contributed to improving conversion efficiencies of systems and thus to reduce pollution for clean environment. As individuals, we can also help in sustainable environment by using efficient energy conservation devices and by making energy efficiency a high priority in our purchases. Some of the potential solutions to environmental problems are as follows:

• clean renewable energy technologies;

• efficient energy conservation devices;

• clean alternative energy for transportation;

• energy source switching from fossil fuel to environmentally benign energy forms;

• energy storage technologies for better use;

• clean coal-based technologies;

• recycling method;

• encouraging forestation;

• use of locally available renewable energy resources;

• changing life style;

• increasing public awareness among users;

• educating and training for clean energy-based technologies.

The continuing depletion of fossil fuels, and the environmental hazard problems posed by fast growing industrial development, are gradually shifting the path of devolvement towards (i) environmental sustainability, (ii) better sociability and (iii) climate change. This in turn emphasises the need for use of renewable energy sources by human beings. The area of renewable energy sources is expanding rapidly. Numerous innovations as well as its applications based on renewable energy sources are taking place. The decentralised renewable energy systems has been recognised the world over as an answer to meeting the energy demands both in the household and in the agroindustrial sector. The exhaustion of natural sources and the accelerated demand of conventional energy have forced planners and policy makers to look for alternate sources, i.e. clean renewable energy sources.

A secure supply of energy resources is generally agreed to be a necessary but not sufficient requirement for development within a society. Furthermore, sustainable development demands a sustainable supply of energy sources that, in the long term, is readily and sustainably available at reasonable cost and can be utilised for all required tasks without causing negative social impacts. Supplies of energy resources like fossil fuels (coal, oil and natural gas) and uranium are generally acknowledged to be finite and other energy sources such as sunlight, wind and falling water (hydro) are generally considered renewable, and therefore sustainable over the relative long term. Waste and biomass fuels are also usually viewed as sustainable energy sources. In general, the implications of these statements are numerous and depend on how the word “sustainable” is defined.

Environmental concerns are an important factor in sustainable development. For a variety of reasons, activities that continually degrade the environment are not sustainable overtime, i.e. cumulative impact of such activities on the environment often leads over time to a variety of health, ecological and other problems. A large portion of the environmental impact in a society is associated with its utilisation. Ideally, a society seeking sustainable development utilises only energy resources that cause no environmental impact (e.g. that release no emissions to the environment). However, since all energy resources lead to some environmental impact, it is reasonable to suggest that some (not all) of the concerns regarding the limitations imposed on sustainable development by environmental emissions and their negative impacts, can be in part overcome through increased energy efficiency. Clearly, a strong relation exists between energy efficiency and environmental impact, since for the same services or products, less resource utilisation and pollution is normally associated with increased energy efficiency.

Presently, even though commercial energy sources like coal, oil, and natural gas are being utilised to a large extent, renewable sources of energy are slowly gaining importance. Renewable energy plays a basic role in sustainable development. Such sources can supply the energy we need for indefinite periods of time polluting far less than fossil fuels. The advantages of renewables are well known, as far as they enhance diversity in energy supply markets; secure long-term sustainable energy supplies; reduce local and global atmospheric emissions; and create new employment opportunities, offering possibilities for local manufacturing.¹ 

According to the International Energy Agency (IEA), renewable energy includes hydropower, biomass, wind, solar, geothermal and marine energy. Figure 1.6 shows the world share of total primary energy consumption (TPES) from 1997.² 

Figure 1.6 shows that renewable energy represented 13.4% of total world combustion in 1973 which increased up to 18.7% in 2008 as shown by Figure 1.7.³ 

Support is also focused upon the so-called “new” renewables such as wind energy, modern biomass and solar photovoltaic. Modern biomass in the sustainable biomass resources used for the production of modern energy vectors such as electricity and a variety of liquid as well as gaseous fuels using advanced technologies. Similarly, traditional biomass is the biomass resource used for domestic heating and cooking, mainly in developing countries based on the collection and combustion of wood and dung in crude stoves/fireplaces. Hence, biomass produced in a sustainable way is called modern biomass, whereas biomass produced in an unsustainable way is called traditional biomass.

The economic development of the country is strongly depends on its energy utilisation. Presently, India ranks as the world's seventh largest producer and accounts for about 2.5% of the world's total annual energy production. This country is also the world's fifth largest energy consumer and accounts for about 3.5% of the world's total annual energy consumption.⁴ 

The 62^nd report of the National Sample Survey Organisation (NSSO) states that 74% of households in rural area in India still depend on firewood as their cooking fuel, about 9% use dung cake and only 9% use LPG as cooking fuel. About 56% of households in rural India use electricity for lighting purpose, while 42% use kerosene. This reveals that a large section of rural India still depends on traditional biomass.⁵ 

In terms of the primary energy demand the country ranks fourth in the world and fifth when biomass is excluded. If it continues sustained economic growth, achieving 8–10% of GDP growth per annum till 2003, its primary energy supply will need to grow by three to four times and its electricity supply five to seven times.⁶ 

The energy transfer occurs by heat transfer through mechanical and electrical processes. Heat transfer is a well established topic. It does not require detailed discussions for small and moderate renewable energy applications due to small temperature difference (ΔT), less complicated geometrical configurations and lower energy fluxes.

An understanding of the basic principles of heat transfer is of vital importance in dealing with the renewable energy technologies. The object of this section is to review the fundamentals of basic heat transfer in connection with the renewable energy analysis. Heat is the form of energy that can be transferred from one system to another as a result of a temperature difference (ΔT). The science that deals with the determination of the rates of such energy transfer is heat transfer.

No net heart transfer between two media at the same temperature can take place as a temperature difference between two mediums is the first requirement. The rate of heat transfer in a certain direction depends on the magnitude of the temperature gradient in that direction. The higher the temperature gradient, the higher the rates of heat transfer.

Heat can be transferred as (i) conduction, (ii) convection and (iii) radiation. All modes of heat transfer are from the high temperature surface to a lower temperature one. Practical heat transfer problems are generally limited to a single method of heat transfer. Usually all three methods are involved in an overall heat transfer problems.⁷ 

Conduction is the transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones. Conduction can take place in solids, liquids or gases. Conduction is due to the collisions and diffusion of molecules during their random motion in gases and liquids. In solids, it is due to the combination of vibrations of the molecules in a lattice and the energy transport by free electrons. For example, a cold canned drink in a warm room eventually warms to the room air temperature as a result of heat transfer from the room to the drink through the aluminium by conduction.

The rate of heat conduction through a medium depends on (i) the geometry of the medium, (ii) its thickness (iii) the material of the medium and (iv) the temperature difference across the medium. For example, an insulated hot water tank reduces the rate of heat loss from the tank. The thicker the insulation, the smaller is the heat loss. A hot water tank will lose heat at a higher rate when the temperature of the room housing the tank is lowered. Further, tanks with larger surface area have a higher rate of heat loss.

Let us consider steady-state heat condition through a large plane wall of thickness Δx=L and surface area A. For ΔT=T₂ – T₁. On the basis of experiments it can be shown that the rate of heat transfer through the wall is doubled when the temperature difference ΔT across the wall or the area A normal to the direction of heat transfer is doubled. But it is halved when thickness L is doubled. Thus, one can conclude that the rate of heat conduction through a plane layer is proportional to ΔT across the layer and the heat transfer area (A), but is inversely proportional to the thickness of the layer (L). That is,

or,

where the constant K is the conductivity of the materials (Appendix V), which is a measure of the ability of a material to conduct heat. In the limiting case of Δx→0, the above measure equation reduces to the different form

which is known as Fourier's law of heat conduction. Here, is the temperature gradient, which is the slope of the temperature curve on a T–x diagram at location x. The above relation indicates that “the rate of heat conduction in a direction is proportional to the temperature gradient in that direction”. Heat is conducted in the direction of decreasing temperature and the temperature gradient becomes negative when temperature decreases with increasing x. Therefore, a negative sign is added to Eq. (1.2) to make heat transfer in the positive direction. The heat transfer area A is always normal to the direction of heat transfer.

The rate of conduction heat transfer under steady conditions (Eq. (1.1)) can also be viewed as the defining equation for thermal conductivity (K). Thus, the thermal conductivity of a material (K) can be defined as the rate of heat transfer through a unit thickness of the material (Δx=1) per unit area per unit temperature difference (ΔT=1 °C). The thermal conductivity of a material is a measure of how fast heat flows through material. A large value of thermal conductivity indicates that the material is a good heat conductor. A low value indicates that the material is a poor heat conductor or insulator. The value of thermal conductivity of a few commonly used materials is given in Appendix V.

Thermal conductivity of gases, liquids and solids depends on temperature. Experimental studies have shown that for many materials, the dependence of thermal conductivity on temperature can be assumed to be linear.

where K₀ is the thermal conductivity at temperature T₀ and β is a constant for the material. The values of K will increase for T>T₀ and decrease for T<T₀, however, the value of K is unaffected in the medium temperature range of renewable energy technologies in the present book.

The thermal diffusivity is another material property that appears in heat conduction analysis. It represents how fast heat diffuses through a material and is defined as

Here, the heat capacity (ρC_p) represents how much energy a material stores per unit volume. C_p and ρ are the specific heat and the density of the material, respectively. Therefore, the thermal diffusivity of a material can be viewed as the ratio of the heat conducted through the material to the heat stored per unit volume (Eq. (1.4)). A material that has a high thermal conductivity or a low heat capacity will obviously have a large thermal diffusivity (α). The larger the thermal diffusivity, the faster is the propagation of heat into the medium. A small value of thermal diffusivity means that heat is mostly absorbed by the material and a small amount of heat will be conducted further. Thermal diffusivities of some common materials are given in Appendix V.

Similarly, one dimensionless parameter is also used in heat conduction problems known as the Biot number (Bi) and is given by

or,

Here, K is thermal conductivity of solid.

Therefore, the Biot number is defined as the ratio of the convective heat transfer coefficient at the surface to the conductive heat transfer coefficient within the body. When a solid body is being heated by the hotter fluid surrounding it, heat is first convected to the body and subsequently conducted within the body. The Biot number can also be defined as the ratio of the internal thermal resistance of a body to heat conducted to its external thermal resistance to heat convection. Hence, a small Biot number represents small resistance to heat conduction and thus small temperature gradients within the body.

The practical problems in heat transfer involve a medium consisting of several different parallel layers each having different thermal conductivity or involve two or more of the heat transfer modes, namely, conduction, convection and radiation. In such a situation, the concept of an overall heat transfer coefficient (U) is applied to predict the one-dimensional steady-state heat transfer rate.

Consider a composite wall as shown in Figure 1.8a through which heat is transferred from the hot fluid at temperature T_A to the cold fluid at temperature T_B. Assuming steady state, i.e. the heat-transfer rate, through the structure is the same through each layer and we can write,

where terms like h ΔT represent heat transfer by convection and terms like K (ΔT/L) represent heat transfer by conduction through various layers.

Also, the rate of heat transfer per unit area is

where the Rs are thermal resistances at various surfaces and layers and are defined by,

Equation (1.6b) can also be written the following form:

where,

An overall heat transfer coefficient, U, is related to the total thermal resistance “R” of the composite wall by,

Consider a concrete greenhouse wall/roof with an air cavity of air conductance “C”, as shown in Figure 1.8b.

The variation of thermal air conductance (C) with thickness of air gap (L) is given in Figure 1.8c.

The heat is transferred from the hot surface at temperature T_A to the cold surface temperature T_B (Figure 1.8b). For steady-state conditions, the rate of heat transfer per unit area of walls/roof will be the same at each layer boundary. Then, we can write,

The previous equation can be derived as done earlier and its expression is given below:

where,

The expression for an overall heat transfer coefficient (U) for other configurations of parallel slabs with air cavities can be written as follows:

The rate of heat transfer by convection between the fluid and the boundary surface may be evaluated by using the following expression,

where, h is the local convective heat transfer coefficient and the rate of heat flow at the fluid and body interface is related to the temperature difference between the surface of the body concerned and its surroundings.

The convection equations contain the following dimensionless number depending on physical parameters of fluid:

where

The above dimensionless numbers given by Eqs. (1.12) can be obtained by using the physical properties of dry and moist air and water given in Appendix V.

The Reynolds (Re), Prandtl (Pr) and Grashof (Gr) numbers are calculated by using the physical properties of fluid at the average temperatures (T_f) of the hot surface (T₁) and surrounding air (T₂) i.e.

The thermal expansion coefficient (β^′) should be calculated at

(a) temperature of surrounding air (T₂) for exposed surface, i.e.

and (b) average temperature (T_f) for parallel plate, i.e.

The characteristic dimension (X) of other shape is given by

where A and P are the area and perimeter of the surface, which is generally used for irregular shapes.

Sometimes, for a rectangular horizontal surface (L₀×B₀), the characteristic dimension can also be calculated by

Nusselt number (Nu): This is the ratio of convective heat transfer to heat transfer by conduction in the fluid and is usually unknown in problems of convection, since it involves the heat transfer coefficient, h, which is an unknown parameter. Although the Nusselt number resembles the Biot number, the two are essentially different. The Biot number includes the thermal conductivity of a solid, the Nusselt number that of a fluid.

Reynolds number (Re): This is the ratio of the fluid dynamic force (ρv₀²) to the viscous drag force (μv₀/X) where, ρ is the density and μ the dynamic viscosity. It indicates the flow behaviour in forced convection and serves as a criterion for the stability of laminar flow.

Prandtl number (Pr): This is the ratio of momentum diffusivity (μ/ρ) to the thermal diffusivity (K/ρC_p); C_p is the specific heat at constant pressure. It gives the relation of heat transfer to fluid motion.

Grashof number (Gr): This is the ratio of the buoyancy force to the viscous force; β is the coefficient of volumetric thermal expansion, g is the gravitational acceleration and T is the temperature.

Rayleigh number (Ra): This is the ratio of the thermal buoyancy to viscous inertia.

For free convection, the terrestrial gravitational field, acting on the fluid with a nonuniform density distribution owing to the temperature difference between the fluid and the contacting surface, causes the fluid motion.

The coefficient of heat transfer, h, usually incorporated with Nusselt number, depends on whether the flow is laminar or turbulent, free or forced.

For free convection,

where the relationship is obtained by the method of dimensional analysis. The constants C^′ and n are determined by the correlation of experimental data of geometrically similar bodies. The correlation factor, K′, is introduced to represent the entire physical behaviour of the problem.⁸ 

Some empirical relations used for free convention are given in Table 1.4.

In forced convection, the fluid motion is artificially induced, say with a pump or a fan that forces the fluid flow over the surface. The external energy is supplied to maintain the process in which there are two types of forces (a) the fluid pressure related to flow velocity (1/2)ρv² and (b) the frictional force produced by viscosity (μ.dv/dy). Their relative importance in heat transfer is signified by the nondimensional Reynolds number. It also controls the flow, laminar or turbulent, in the boundary layer with which the rate of heat transfer is closely connected. The heat transfer by forced convection is represented by the following Nusselt equation

where, C and n are constants for a given type of flow and geometry. K is a correction factor (shape factor) added, to obtain a greater accuracy.

The empirical relation for forced convective heat transfer through cylindrical tubes may be represented as,

where D=4A/P, is the hydraulic diameter (m); P is the perimeter of the section (m) and K_th is the thermal conductivity (W/m K).

The values of C, m, n and K for various conditions are given in Table 1.5.

For fully developed laminar flow in tubes at constant wall temperature the following relation applies,

The heat transfer coefficient calculated from this relation is the average value over the entire length of the tube. When the tube is sufficiently long the Nusselt number approaches a constant value of 3.66. For the plate heated over its entire length, the Nusselt number can be obtained by integrating the equation given below over the length of the plate,

Now,

The heat transfer from a flat plate exposed to outside winds has been analysed by several workers. The following equation for convective heat transfer coefficient is generally used

where V is the wind speed, m/s.

The above equation for zero wind speed gives heat loss by natural convection. It may be mentioned here that the process taking place is not as simple as it appears, as the wind may not always be blowing parallel to the surface.

It is probable that in this equation the effects of free convection and radiation are included. For this reason, this equation should be,

The sensibility of these parameters is also demonstrated through a comparison, another relation for convective heat transfer coefficient is given by,

Several other correlations are also available in the literature and generally, h_c is determined from an expression in the formed expressed as

where, a=2.8, b=3 and n=1 for V_a<5 m/s and a=0, b=6.15 and n=0.8 for V_a>5 m/s.

(The source and reference of Eqs. (1.20) can be obtained from Tiwari (2002).)

Thermal radiation involves the transfer of heat from a body at a higher temperature to another at a lower temperature by electromagnetic waves (0.1 to 100 μm). Temperature is transmitted in the space in the form of electromagnetic waves. Thermal radiation is in the infrared range and obeys all the rules as that of light, namely, travels in straight lines through a homogenous medium, is converted into heat when it strikes any body that can absorb it and is reflected and refracted according to the same rules as those of light.

When radiant energy falls on a body, a part of it is reflected, another part is absorbed and the rest is transmitted through it. The conservation of energy states that the total sum must be equal to the incident radiation, thus,

or,

where, ρ^′, α^′ and τ are the reflectivity, absorptivity and transmissivity of the intercepting body, respectively. The ratio of the energy reflected to that which is incident is called the reflectivity. The ratio of the energy absorbed and the energy transmitted to that which is incident are the absorptivity and transmissivity, respectively.

For an opaque surface, τ=0, therefore ρ^′+α^′=1. However, when ρ^′=τ=0; α=1, that is, the substance absorbs the whole of the energy incident on it. Such a substance is called a blackbody. Similarly, for a white body that reflects the whole of the radiation falling on it, α^′=τ=0, ρ^′=1.

The energy that is absorbed is converted into heat and this heated body, by virtue of its temperature, emits radiation. The radiant energy emitted per unit area of a surface in unit time is referred to as the emissive power (E_λ). However, if defined as the amount of energy emitted per second per unit area perpendicular to the radiating surface in a cone formed by a unit solid angle between the wavelengths lying in the range dλ, it is called spectral emissive power (e_λ). Further, emissivity, defined as the ratio of the emissive power of a surface to the emissive power of a blackbody of the same temperature, is the fundamental property of a surface.

This states that for a body in thermal equilibrium, the ratio of its emissive power to that of a blackbody at the same temperature is equal to its absorptivity, i.e.

Thus, a body can absorb as much incident radiation as it can emit at a given temperature. However, it may not be valid if the incident radiation comes from a source at different temperature. Further, it applies to surfaces bearing the grey surface characteristics, namely radiation intensity is taken to be a constant proportional to that of a blackbody. The radiative properties α_λ, ε_λ and ρ_λ are assumed to be uniform over the entire wavelength spectrum.

Laws of thermal radiations have been obtained for black bodies and conditions of thermodynamic equilibrium.

The emission of energy with respect to wavelength is not uniform and depends on temperature. Planck's law establishes the relation of the spectral emissive power, wavelength and temperature and is written as,

where, C₁=3.742×10⁸ W μm⁴/m² (=3.7405×10⁻⁶ W m²) and C₂=1.4387×10⁴ μm K (= 0.01439 mK) are called Planck's first and second radiation constants, respectively. Planck's law has two limiting cases depending on the relative value of C₂ and λT:

a) when λT ≫C₂

b) when λT≪ C₂

The wavelength corresponding to the maximum intensity of blackbody radiation for a given temperature T is given by this law:

where, C₃=2897.6 μm K. Hence, an increase in temperature shifts the maximum blackbody radiation intensity towards the shorter wavelength.

This law relates the hemispherical total emissive power, namely total energy and temperature. By integrating Planck's law over all wavelengths, the total energy emitted by a blackbody is found to be,

where σ=5.6697 ×10⁻⁸ W/m² K⁴ is the Stefan–Boltzmann constant.

In order to evaluate radiation exchange between a body and the sky, certain equivalent blackbody sky temperature is defined. This accounts for the fact that the atmosphere is not at a uniform temperature and that it radiates only in certain wavelength regions. Thus, the net radiation to a surface with emittance ε and temperature T is,

In order to express the equivalent sky temperature T_sky, in terms of ambient air temperature, various expressions have been given. These relations, although simple to use, are only approximations. The sky temperature to the local air temperature can be given by the relation,

where, T_sky and T_a are both in Kelvin.

Another commonly used relation is given as

or,

The radiant heat exchange between two infinite parallel surfaces per m² at temperatures T₁ and T₂ may be given as,

where,

The above equation may be rewritten as,

where ΔR=σ[(T_a+273)⁴–(T_sky+273)⁴] is the difference between the long-wavelength radiation incident on the surface from sky and surroundings and the radiation emitted by a blackbody at ambient temperature. The reduction of Q_r in the form of Eq. (1.25c) will enable one to find the exact closed form solution for T₁. It may be noted here that this solution is based on the assumption that T_a and T_sky are constant.

In a moving single-component medium, heat is transferred by conduction and convection; the process is known as convective heat transfer.

where T_w and T_a are the fluid (water) and the surrounding air temperatures, respectively. By analogy, the process of molecular and molar transport of matter; in a moving heterogeneous medium, is called convective mass transfer. The surface of the liquid phase plays a role similar to that of a solid wall in heat transfer process without accompanying diffusion. The process of heat and mass transfer are of practical interest in evaporation, condensation, etc. The heat transfer is based on Newton's law.

Mass transfer rate is based on a similar equation:

where m is the rate of mass flow per unit area, (kg/m²/s), h_D the mass transfer coefficient [(kg/s)(sqm/kg/m³)], ρ_w⁰ the partial mass density of water vapour, kg/m³, ρ_a⁰ is the partial mass density of air (kg/m³).

According to the Lewis relation for an air and water vapour mixture

From the perfect gas equation for 1 mole of air,

By assuming T_w=T_a=T at the water/air interface, Eq. (1.26b) becomes

The rate of heat transfer on account of mass transfer of water vapour is

where L is the latent heat of vapourisation and P_w and P_a are the partial pressures of water vapour and air respectively.

Let

Using the perfect gas equation for air, (for 1 mole of air) and by substituting in the expression for H₀

For small values of P_w, P_T=P_a the above equation becomes

where P_T is the total pressure of the air–vapour mixture. The values of different parameters used in Eq. (1.26h) are as follows:

• L=Latent heat of vapourisation=2200 kJ/kg

• C_pa=Specific heat of air=1.005 kJ/kg °C

• M_w=18 kg/mol (molar mass of water)

• P_T=Total pressure of air–vapour mixture=1 atm

• 1 atm=101 325 N/m²

Substituting all these values in Equation (1.26h) and solving

The best representation of heat and mass transfer phenomenon is obtained if the values of is taken to be 16.27×10⁻³ instead of 0.013. Thus, the rate of heat transfer on account of mass transfer is written as

If the surface is exposed to atmosphere, then the above equation reduces to

where γ is the relative humidity of air.

Also,

The values of P_w and P_a for the ranges of temperature (10–90 °C) can be obtained from the following expression (Fernanez and Chargoy, 1990).

A physical system containing a large number of atoms or molecules is called the thermodynamic system if macroscopic properties, such as the temperature, pressure, mass density, heat capacity, etc., are the properties of main interest. The number of atoms or molecules contained, and hence the volume of the system, must be sufficiently large so that the conditions on the surfaces of the system do not affect the macroscopic properties significantly. From the theoretical point of view, the size of the system must be infinitely large, and the mathematical limit in which the volume, and proportionately the number of atoms or molecules, of the system are taken to infinity is often called the thermodynamic limit.

The thermodynamic process is a process in which some of the macroscopic properties of the system change in the course of time, such as the flow of matter or heat and/or the change in the volume of the system. It is stated that the system is in thermal equilibrium if there is no thermodynamic process going on in the system, even though there would always be microscopic molecular motions taking place. The system in thermal equilibrium must be uniform in density, temperature, and other macroscopic properties.

If two thermodynamic systems, A and B, each of which is in thermal equilibrium independently, are brought into thermal contact, one of two things will take place: either (i) a flow of heat from one system to the other or (ii) no thermodynamic process will result. In the latter case the two systems are said to be in thermal equilibrium with respect to each other.

According to the zeroth law of thermodynamics, “If two systems are in thermal equilibrium with each other and there is a physical property that is common to the two systems, this common property is called the temperature.”

Let the condition of thermodynamic equilibrium between two physical systems A and B be symbolically represented by

if and , then .

Based on preceding observations, some of the physical properties of the system C can be used as a measure of the temperature, such as the volume of a fixed amount of the chemical element mercury under some standard atmospheric pressure. The zeroth law of thermodynamics is the assurance of the existence of a property called the temperature.

Let us consider a situation in which a macroscopic system has changed state from one equilibrium state P₁ to another equilibrium state P₂, after undergoing a succession of reversible processes. Here, the processes mean that a quantity of heat energy Q has cumulatively been absorbed by the system and an amount of mechanical work W has cumulatively been performed upon the system during these changes.

According to the first law of thermodynamics, there are many different ways or routes to bring the system from state P₁ to the state P₂; however, it turns out that the sum is independent of the ways or the routes as long as the two states P₁ and P₂ are fixed, even though the quantities W and Q may vary individually depending upon the different routes. 

There are several different ways of expressing the second law of thermodynamics, and the following are three examples.

Clausius’ principle: No cyclic process exists that has as its sole effect the transference of heat from a colder body to a hotter body.

Kelvin's principle: No cyclic process exists that produces no other effect than the extraction of heat from a body and its conversion into an equivalent amount of work.

Caratheodory's principle: There are states of a system, differing infinitesimally from a given state, which is unattainable from that state by any quasistatic adiabatic process.

The second law (especially in the Caratheodory form) allows one to order the states according to the direction that the system is allowed to evolve. A parameter, called empirical entropy, is ascribed to each state, such that this parameter never decreases spontaneously. In the absence of external agents doing work on the system, any change in the system is either reversible, which involves no change in entropy, or irreversible, which involves an increase in entropy.

A bucket of warm water with a warm cannon ball has higher entropy than the bucket of cold water and the red hot cannon ball, so that the evolution is irreversible and can go in only one direction. To reverse the change we need to do work on the system: mechanical work to lift the cannon ball, and thermodynamic work, using some form of refrigerator to decrease the temperature of the bucket of water, and some other process to heat the cannon ball.

The entropy of a system approaches a constant value as the temperature approaches absolute zero.

The third law is relatively easy to understand from a statistical point of view in which entropy is associated with disorder. As absolute zero is approached, all thermal motions cease, and any system must approach an ordered state in which the particles do not move. Hence, the entropy of a system is defined only to within an arbitrary constant and only changes in entropy have physical significance. The changes in entropy become negligibly small as absolute zero is approached.

The moving fluid follows the fundamental laws of mechanics related to the conversation of mass, energy and momentum for its transfer of energy. The energy balance for solar, wind, hydro and wave energy devices are based on the energy transfer of moving fluids (both liquids and gases). Compressibility provides the important point for distinguishing a liquid from gas. A gas is more compressible than a liquid. The flow pattern in fluids are mostly steady, i.e. flow does not change with time. Here, we shall consider broadly the incompressible flow.

Bernoulli's equation: The Bernoulli equation is a relation between pressure, velocity and elevation in steady, incompressible, frictionless flow. Despite its simplicity, it has proven to be a very powerful tool in liquid mechanics. A key assumption in the derivation of the Bernoulli equation is to consider the viscous effects to be negligible and thus fluid to be inviscid. Such flows are usually designated as frictionless flow. There is no fluid with zero viscosity and thus this assumption is valid only when the viscous effect is small compared with other effects such as gravity and pressure. Therefore, care should be exercised when making this assumption. In the absence of frictional effect and less common effects such as surface tension, the fluid motion is governed by the combined effects of pressure and gravity forces.

The motion of a particle and the path it follows are described by the vector velocity as a function of time and space coordinates and the initial position of the particle. When the flow is steady (no change with time at a specific location), all particles that pass through the same point will follow the same path (which is the streamline) and the velocity vectors remain tangent to the path at every point. The flow stays within well defined (though imaginary) stream tubes, i.e. tubes bounded by streamlines. Here, we assume no work is done.

Figure 1.9 shows a streamline that rises from a height of h₁ to a height of h₂. The tube is narrow enough that h will probably be consistent over each section of the tube. We consider a control volume bounded by the streamlines and two perpendicular slices across the stream tube at 1 and 2.

A mass m=ρA₁v₁Δt enters the control volume at 1 and an equal m=ρA₁v₁Δt leaves at 2 due to conservation of mass. For Figure 1.9, the energy balance on the fluid can be written as

where the pressure forces p₁A₁ and p₂A₂ act through a distance v₁Δt and v₂Δt, respectively. E_t is the thermal heat energy generated by the friction in the flow channel.

After neglecting the effect of fluid friction, Eq. (1.28) can be written as

or,

The above equation is termed Bernoulli's equation. In Bernoulli's equation each term has the dimension of length and represents some kind of head. is the pressure head, which represents the height of a fluid column that produces the static pressure p. h is the elevation head, which represents the potential energy of the fluid. is the velocity head, which represents the elevation needed for a fluid to reach the velocity v during frictionless free fall.

Equation (1.28b) can also be written as,

where H denotes the total head for the flow. Therefore, Bernoulli's equation can also be expressed in terms of head. The sum of the pressure head, elevation head and velocity heads along with a streamline is constant during steady flow when the compressibility and frictional effects are negligible. Equation (1.28c) can be rewritten as

Here, is pressure energy, hg is potential energy and is kinetic energy per unit mass. Therefore, Bernoulli's equation can also be expressed in terms of energy as: the sum of the pressure, potential and kinetic energies of a fluid particle is constant along a streamline during steady flow when compressibility and frictional effects are negligible.

Newton's second law of motion for particles can be generalised for fluids as “At any instant in steady flow, the resultant force acting on the moving fluid within a fixed volume of space, equals the next rate of outflow of momentum from the closed surface bounding that volume”. This is known as the momentum theorem.

Let us consider the fluid passing across a turbine in a pipe as shown in Figure 1.10. The dotted line in the Figure 1.10 shows the control surface over which the momentum theorem is applied.

In Figure 1.10 fluid flowing at speed v₁ into the left of the control surface carries momentum per unit volume, where is the unit vector in the direction of flow. In time Δt, the volume entering the surface is . Therefore, the rate at which the momentum is entering the control surface is

Similarly, the rate at which momentum is leaving the control volume is . The next rate of outflow momentum can be given as,

where is the mass flow. The momentum theorem tells us that F₁ is the force on the fluid and so, by Newton's third law, −F₁ is the force exerted on the turbine and pipe by the fluid. Normally, v₂<v₁, so that F₁ points in the negative x direction and –F (i.e. the force on the turbine) is in the direction of flow, as expected.

There are two points to note in applying the momentum theorem: (i) momentum is a vector and (ii) the expression for flow of momentum (e.g., ) typically involves products of velocity.

1. Energy is defined as

   1. The rate of doing work

   2. The rate of applying force

   3. The rate of displacement

   4. None of them

2. The unit of energy is

   1. J

   2. Wh

   3. KWh

   4. all

3. Renewable energy has

   1. Infinite source

   2. Zero source

   3. Finite source

   4. None

4. Nonrenewable energy has

   1. Infinite source

   2. Zero source

   3. Finite source

   4. None

5. Fossil fuel has

   1. Infinite source

   2. Zero source

   3. Finite source

   4. None

6. Fossil fuel sources are

   1. Increasing

   2. Depleting

   3. Constant

   4. None

7. Chemical energy is the energy

   1. Stored in the chemical bonds of molecules

   2. Due to random motion of particles in solids

   3. Due to elevation of objects in gravitational field

   4. All of them

8. Potential energy is the energy

   1. Stored in the chemical bonds of molecules

   2. Due to random motion of particles in solids

   3. Due to elevation of objects in gravitational field

   4. All of them

9. Heat energy is the energy

   1. Stored in the chemical bonds of molecules

   2. Due to random motion of particles in solids

   3. Due to elevation of objects in gravitational field

   4. All of them

10. Kinetic energy is the energy

    1. Stored in the chemical bonds of molecules

    2. Due to random motion of particles in solids

    3. Due to elevation of objects in gravitational field

    4. Due to motion of an object

11. Nuclear energy is the energy

    1. Stored in the chemical bonds of molecules

    2. Due to random motion of particles in solids

    3. Due to elevation of objects in gravitational field

    4. Stored in the nucleus of an atom

12. The Sun is the source of

    1. All renewable energy

    2. All nonrenewable energy

    3. Both renewable and nonrenewable energy

    4. All of them

13. The cause of increase of CO₂ in environment is burning of

    1. Coal

    2. Oil

    3. Natural gas

    4. All

14. The acid rain is due to interaction between

    1. SO₂ and H₂O

    2. NO₂ and H₂O

    3. SO₂ and NO₂

    4. O₃ and H₂O

15. The increase of SO₂ in the atmosphere is due to

    1. Burning coal in thermal power plant

    2. Industrial processes

    3. Burning of biomass for domestic uses

    4. All of them

16. The main greenhouse gases are

    1. CO₂

    2. CH₄

    3. NO_x

    4. SO_x

    5. None

17. Most of infrared radiation is

    1. Blocked by greenhouse molecules

    2. Allowed to escaped by greenhouse molecules

    3. Absorbed by greenhouse molecules

    4. None of them

18. The thermal conductivity of material depends on

    1. Temperature

    2. Length

    3. Thickness

    4. None

19. The thermal conductivity of insulating material is

    1. Low

    2. High

    3. Infinity

    4. Zero

20. The thermal conductivity of conducting material is

    1. Infinite

    2. Zero

    3. Very high

    4. Low

21. The heat transfer coefficient is inversely proportional to

    1. Thermal resistance

    2. Thermal conductivity

    3. Thickness

    4. None

22. The rate of heat transfer from higher to lower temperature is due to

    1. Conduction

    2. Convection

    3. Radiation

    4. All

23. The conductive heat transfer is governed by

    1. Fourier's law

    2. Stefan–Boltzmann law

    3. Wien's displacement law

    4. None

24. The radiation heat transfer is governed by

    1. Stefan–Boltzmann's law

    2. Fourier's law

    3. Wien's displacement law

    4. None

25. The wavelength of radiation depends inversely to

    1. Temperature

    2. Temperature difference

    3. Area of surface and

    4. None

26. Expression for an overall heat transfer coefficient (U) is derived under

    1. Transient conditions

    2. Periodic conductions

    3. Quasisteady state

    4. Steady-state conditions

27. The unit of thermal conductance of air is

    1. Same as heat transfer coefficient

    2. Different from heat transfer coefficient

    3. Same as thermal conductivity of air

    4. None of them

28. The thermal conduction of air is unaffected for

    1. Larger air cavity

    2. Smaller air cavity

    3. Infinity air cavity

    4. Zero air cavity

29. The thermal conductance of air is very large for

    1. Smallest air cavity

    2. Largest air cavity

    3. Zero air cavity

    4. None of them

30. The conduction, convection and radiation losses are

    1. Dependent on each other

    2. Independent of each other

    3. Independent of temperature

    4. None of them

31. Thermal expansion coefficients depend on

    1. Temperature

    2. Thermal air conductance

    3. Thermal conductivity

    4. None of these

32. The convective heat transfer depends on

    1. Physical properties of fluid

    2. Physical properties of solid

    3. Characteristics dimension

    4. All

33. The forced convective heat transfer at higher temperature is higher then

    1. Free convective

    2. Conductive

    3. Radiative

    4. Evaporative

34. The radiation heat transfer between two surfaces is mainly due to

    1. Short wavelength radiation

    2. Infrared

    3. UV

    4. Long wavelength radiation

35. The evaporative heat transfer coefficient (h_ew) is

    1. Proportional to h_cw (convective heat transfer coefficient)

    2. Inversely proportional to h_cw

    3. Independent of convective heat transfer coefficient

    4. None

36. The evaporative heat transfer coefficient (h_ew) depends on convective heat transfer coefficient due to

    1. Lewis relation

    2. Newton's law

    3. Fourier's law

    4. None

37. The shape (geometrical) factor for parallel surfaces is

    1. One

    2. Ten

    3. Less than one

    4. Infinity

38. The shape (geometrical) factor for nonparallel surfaces is

    1. One

    2. Less than one

    3. Ten

    4. Infinity

39. The radiative heat transfer coefficient for parallel surfaces having a temperature difference by 1°C is

    1. 6 W/m² K

    2. 60 W/m² K

    3. 0.6 W/m² K

    4. None

40. The expression for radiative heat transfer coefficient (h) for a surfaces having temperatures almost the same but different is

    1. 4εσT⁴

    2. 4εσT³

    3. 

    4. 

41. Expression for an overall heat transfer coefficient for single slab is

    1. 

    2. 

    3. 

    4. None

42. For an inclined surface, an expression for free convective heat transfer coefficient can be obtained from

    1. Nu=C(GrPr)ⁿ

    2. Nu=C(GrPr sin θ)ⁿ

    3. 

    4. Here, C=0.54 and

43. For horizontal surface facing upward, an expression for free convection is

    1. 

    2. Nu=C(GrPr)ⁿ

    3. Nu=C(GrPr cos θ)ⁿ

    4. None Here, C=0.54 and

44. For forced convection, an expression for convective heat transfer coefficient is

    1. Nu=CRe^mPrⁿ·k

    2. Nu=C(Re cos θ)^mPrⁿ·k

    3. Nu=C(Re^m·(Pr cos θ)ⁿ)·k

    4. None

45. An expression for wind dependent convective heat transfer coefficient is

    1. h=3+2.8V

    2. h=2.8+3V

    3. h=3+3V

    4. None

46. An expression for wind dependent convective and radiative heat transfer coefficient is

    1. h=3.8+5.7V

    2. h=3.8+3V

    3. h=5.7+3.8V

    4. None

47. The properties of a selective surface are

    1. High value of absorptivity and emissivity of surface

    2. Low value of absorptivity and emissivity of surface

    3. High value of absorptivity and low value of emissitivity

    4. None

48. The sky temperature with respect to ambient temperature is

    1. Less

    2. More

    3. Equal

    4. None

49. The partial vapour pressure depends on temperature

    1. Linearly

    2. Proportionally

    3. Exponentially

    4. None

50. The value of heat transfer is increased by

    1. Increasing the volume

    2. Increasing the mass

    3. Decreasing the surface area

    4. Increasing the surface area

51. The value of heat transfer of a given surface area is increased by using

    1. Black surface

    2. Reflected surface

    3. Fins

    4. None of these

52. The value of heat transfer for a hot surface facing upward is maximum for

    1. Horizontal surface

    2. Inclined surface

    3. Vertical surface

    4. None

53. The value of heat transfer is significantly effected by

    1. Streamline flow

    2. Turbulent flow

    3. Steady-state flow

    4. Constant flow rate

54. The convective heat transfer coefficient in the case of water in comparison with air as fluid for a hot surface facing upward is

    1. Significantly higher

    2. Equal value

    3. Less value

    4. None of these

55. The partial vapour pressure depends on temperature at low operating temperature range up to 40°C

    1. Nonlinear

    2. Constant

    3. Linear

    4. None

56. According to Bernoulli's theorem

    1. Sum of pressure, potential and kinetic energy is constant

    2. Sum of pressure, potential and kinetic energy varies

    3. Sum of pressure, potential and kinetic energy is zero

    4. None of them

57. Conservation of momentum for fluids flowing through pipes depends on

    1. Cross-sectional area of pipe and flow velocity

    2. Radius of pipe and flow velocity

    3. Diameter of pipe and flow velocity

    4. All of them

58. The second law of thermodynamics states that heat transfer takes place

    1. From a colder body to a hotter body

    2. From a hotter body to a colder body

    3. From same-temperature bodies

    4. None of them

59. The second law of thermodynamics is

    1. Reversible

    2. Irreversible

    3. Both

    4. None

60. The first law of thermodynamics is

    1. Reversible

    2. Irreversible

    3. Both

    4. None

61. Energy conservation process depends on

    1. The first law of thermodynamics

    2. The second law of thermodynamics

    3. Zeroth law of thermodynamics

    4. Third law of thermodynamics

62. The existence of temperature depends on

    1. The first law of thermodynamics

    2. The second law of thermodynamics

    3. Zeroth law of thermodynamics

    4. Third law of thermodynamics

1.1 (a); 1.2 (d); 1.3 (a); 1.4 (c); 1.5 (c); 1.6 (b); 1.7 (a); 1.8 (c); 1.9 (b); 1.10 (d); 1.11 (d); 1.12 (a); 1.13 (d); 1.14 (a); 1.15 (d); 1.16 (a); 1.17 (a); 1.18 (a); 1.19 (a); 1.20 (c); 1.21 (a); 1.22 (d); 1.23 (a); 1.24 (a); 1.25 (a); 1.26 (d); 1.27 (a); 1.28 (a) & (c); 1.29 (a); 1.30 (b); 1.31 (a); 1.32 (a) & (c); 1.33 (a) & (b); 1.34 (d); 1.35 (a); 1.36 (a); 1.37 (a); 1.38 (b); 1.39 (a); 1.40 (b); 1.41 (b); 1.42 (d); 1.43 (b); 1.44 (a); 1.45 (b); 1.46 (c); 1.47 (c); 1.48 (a); 1.49 (c); 1.50 (d); 1.51 (c); 1.52 (a); 1.53 (b); 1.54 (a); 1.55 (c); 1.56 (a); 1.57 (d); 1.58 (b); 1.59 (b); 1.60 (a); 1.61 (a); 1.62 (c).