Chapter 1: Introduction and Basic Concepts

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Objectives• Understand how thermodynamics and heat transfer are related to

each other,• Distinguish thermal energy from other forms of energy, and heat

transfer from other forms of energy transfer,• Perform general energy balances as well as surface energy

balances,• Understand the basic mechanisms of heat transfer, which are

conduction, convection, and radiation, and Fourier's law of heat conduction, Newton's law of cooling, and the Stefan–Boltzmann law of radiation,

• Identify the mechanisms of heat transfer that occur simultaneously in practice,

• Understand Global Warming and its implications.

Thermodynamics and Heat Transfer• The science of thermodynamics deals with the

amount of heat transfer as a system undergoes a process from one equilibrium state to another, and makes no reference to how long the process will take.

• The science of heat transfer dealswith the determination of the rates of energy that can be transferred from one system to another as a result of temperature difference.

• Thermodynamics deals with equilibrium states and changes from one equilibrium state to another. Heat transfer, on the other hand, deals with systems that lack of thermal equilibrium, and thus it is a non-equilibrium phenomenon.

• Therefore, the study of heat transfer cannot be based on the principles of thermodynamics alone.

• However, the laws of thermodynamics lay the framework for the science of heat transfer.

Thermodynamics and Heat Transfer

Heat Transfer• The basic requirement for heat transfer is the presence

of a temperature difference.• The second law requires that heat

be transferred in the direction of decreasing temperature.

• The temperature difference is the driving force for heat transfer.

• The rate of heat transfer in a certain direction depends on the magnitude of the temperature gradient in that direction.

• The larger the temperature gradient, the higher the rate of heat transfer.

Application Areas of Heat Transfer

Heat and Other Forms of Energy• Energy can exist in numerous forms such as:

– thermal, – mechanical, – kinetic, – potential, – electrical, – magnetic, – chemical, and – nuclear.

• Their sum constitutes the total energy E (or e on a unit mass basis) of a system.

• The sum of all microscopic forms of energy is called the internal energy of a system.

• Internal energy may be viewed as the sum of the kinetic and potential energies of the molecules.

• The kinetic energy of the molecules is called sensible heat.

• The internal energy associated with the phase of a system is called latent heat.

• The internal energy associated with the atomic bonds in a molecule is called chemical (or bond) energy.

• The internal energy associated with the bonds within the nucleus of the atom itself is called nuclear energy.

Internal Energy and Enthalpy• In the analysis of systems

that involve fluid flow, we frequently encounter the combination of properties u and Pv.

• The combination is defined as enthalpy (h=u+Pv).

• The term Pv represents the flow energy of the fluid (also called the flow work).

Specific Heats of Gases, Liquids, and Solids

• Specific heat is defined as the energy required to raise the temperature of a unit mass of a substance by one degree.

• Two kinds of specific heats: – specific heat at constant volume cv, and – specific heat at constant pressure cp.

• The specific heats of a substance, in general, depend on two independent properties such as temperature and pressure.

• For an ideal gas, however, they depend on temperature only.

Specific Heats• At low pressures all real gases approach ideal gas

behavior, and therefore their specific heats depend on temperature only.

• A substance whose specific volume (or density) does not change with temperature or pressure is called an incompressible substance.

• The constant-volume and constant-pressure specific heats are identical for incompressible substances.

• The specific heats of incompressible substances depend on temperature

only.

Energy Transfer• Energy can be transferred to or from a given mass by two

mechanisms: – heat transfer, and – work.

• The amount of heat transferred during a process is denoted by Q. • The amount of heat transferred per unit time is called heat

transfer rate, and is denoted by Q.• The total amount of heat transfer Q during a time interval t can

be determined from

• The rate of heat transfer per unit area normal to the direction of heat transfer is called heat flux, and the average heat flux is expressed as

0

(J)t

Q Qdt

2 (W/m )QqA

(1-6)

(1-8)

The First Law of Thermodynamics• The first law of thermodynamics states that energy

can neither be created nor destroyed during a process; it can only change forms.

• The energy balance for any system undergoing any process can be expressed as (in the rate form)

Total energyentering the

system

Total energyleaving the

system

Change in thetotal energy of

the system- =

(W)in out systemE E dE dt

Rate of net energy transferby heat, work, and mass

Rate of change in internal kinetic, potential, etc., energies

(1-9)

(1-11)

• In heat transfer problems it is convenient to write a heat balance and to treat the conversion of nuclear, chemical, mechanical, and electrical energies into thermal energy as heat generation.

• The energy balance in that case can be expressed as

, (J)in out gen thermal systemQ Q E E

Net heat transfer

Change in thermal energy of the system

Heat generation

(1-13)

Energy BalanceClosed systems

• Stationary closed system, no work:

Steady-Flow Systems• For system with one inlet and

one exit:

• When kinetic and potential energies are negligible, and there is no work interaction

(J)vQ mc T (kg/s)in outm m m

(kJ/s)pQ m h mc T

(1-15)

(1-18)

Heat Transfer Mechanisms• Heat can be transferred in three basic modes:

– conduction,– convection,– radiation.

• All modes of heat transfer require the existence of a temperature difference.

• All modes are from the high-temperature medium to a lower-temperature one.

Conduction• Conduction is the transfer of energy from the more

energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles.

• Conduction can take place in solids, liquids, or gases– In gases and liquids conduction is due to

the collisions and diffusion of the molecules during their random motion.

– In solids conduction is due to the combination of vibrations of the molecules in a lattice and the energy transport by free electrons.

Conduction

1 2 (W)condT T TQ kA kA

x x

Area Temperature differenceRate of heat conduction

Thickness

where the constant of proportionality k is the thermal conductivity of the material.

In differential form

which is called Fourier’s law of heat conduction.

(1-21)

(1-22) (W)conddTQ kAdx

Thermal Conductivity,k

• The thermal conductivity of a material is a measure of the ability of the material to conduct heat.

• High value for thermal conductivity good heat conductor

• Low value poor heat conductor or

insulator.

Thermal Conductivities of Materials

• The thermal conductivities of gases such as air vary by a factor of 104 from those of pure metals such as copper.

• Pure crystals and metals have the highest thermal conductivities, and gases and insulating materials the lowest.

Thermal diffusivity

• The thermal diffusivity represents how fast heat diffuses through a material.

• Appears in the transient heat conduction analysis.• A material that has a high thermal conductivity or a

low heat capacity will have a large thermal diffusivity.• The larger the thermal diffusivity, the faster the

propagation of heat into the medium.

2Heat conducted ( m s )Heat stored p

kc

(1-23)

Convection

• Convection is the mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion.

• Convection is commonly classified into three sub-modes:– Forced convection,– Natural (or free) convection,– Change of phase (liquid/vapor,

solid/liquid, etc.)

Convection = Conduction + Advection (fluid motion)

Convection

• The rate of convection heat transfer is expressed by Newton’s law of cooling as

• h is the convection heat transfer coefficient in W/m2°C.

• h depends on variables such as the surface geometry, the nature of fluidmotion, the properties of the fluid, and the bulk fluid velocity.

( ) (W)conv s sQ hA T T (1-24)

Radiation• Radiation is the energy emitted by matter in the form of

electromagnetic waves (or photons) as a result of the changes in the electronic configurations of the atoms or molecules.

• Heat transfer by radiation does not require the presence of an intervening medium.

• In heat transfer studies we are interested in thermal radiation (radiation emitted by bodies because of their temperature).

• Radiation is a volumetric phenomenon. However, radiation is usually considered to be a surface phenomenon for solids that are opaque to thermal radiation.

• The maximum rate of radiation that can be emitted from a surface at a thermodynamic temperature Ts (in K or R) is given by the Stefan–Boltzmann law as

• =5.670X108 W/m2·K4 is the Stefan–Boltzmann constant.• The idealized surface that emits radiation at this maximum

rate is called a blackbody. • The radiation emitted by all real surfaces is less than the

radiation emitted by a blackbody at the same temperature, and is expressed as

• is the emissivity of the surface.

4,max (W)emit s sQ A T

4,max (W)

0 1emit s sQ A T

Radiation - Emission

(1-25)

(1-26)

Radiation - Absorption• The fraction of the

radiation energy incident on a surface that is absorbed by the surface is termed the absorptivity .

• Both and of a surface depend on the temperature and the wavelength of the radiation.

0 1

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ENERGY AND ENVIRONMENT• The conversion of energy from one form to another often affects the environment

and the air we breathe in many ways, and thus the study of energy is not complete without considering its impact on the environment.

• Pollutants emitted during the combustion of fossil fuels are responsible for smog, acid rain, and global warming.

• The environmental pollution has reached such high levels that it became a serious threat to vegetation, wild life, and human health.

Energy conversion processes are often accompanied by environmental pollution.

Motor vehicles are the largest source of air pollution.

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The Greenhouse Effect: Global

Warming

• Greenhouse effect: Glass allows the solar radiation to enter freely but blocks the infrared radiation emitted by the interior surfaces. This causes a rise in the interior temperature as a result of the thermal energy buildup in a space (i.e., car).

• The surface of the earth, which warms up during the day as a result of the absorption of solar energy, cools down at night by radiating part of its energy into deep space as infrared radiation.

• Carbon dioxide (CO2), water vapor, and trace amounts of some other gases such as methane and nitrogen oxides act like a blanket and keep the earth warm at night by blocking the heat radiated from the earth. The result is global warming.

• These gases are called “greenhouse gases,” with CO2 being the primary component.

• CO2 is produced by the burning of fossil fuels such as coal, oil, and natural gas.

The greenhouse effect on earth.

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• A 1995 report: The earth has already warmed about 0.5°C during the last century, and they estimate that the earth’s temperature will rise another 2°C by the year 2100.

• A rise of this magnitude can cause severe changes in weather patterns with storms and heavy rains and flooding at some parts and drought in others, major floods due to the melting of ice at the poles, loss of wetlands and coastal areas due to rising sea levels, and other negative results.

• Improved energy efficiency, energy conservation, and using renewable energy sources help minimize global warming.

The average car produces several times its weight in CO2 every year (it is driven 20,000 km a year, consumes 2300 liters of gasoline, and produces 2.5 kg of CO2 per liter).

Renewable energies such as wind are called “green energy” since they emit no pollutants or greenhouse gases.

End