APK ELGI Heat Transfer

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    FUNDAMENTALS OF HEAT TRANSFER

    By

    Dr. P. KARTHIKEYAN

    Faculty from Department of Automobile Engineering

    DEPARTMENT OF MECHANICAL ENGINEERING

    PSG COLLEGE OF TECHNOLOGY

    COIMBATORE –  641 0004, INDIA.

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    THERMODYNAMICS AND HEAT TRANSFER ?

    Both are deals with the transfer of energy at thermal equilibrium

    Thermodynamics deals with transfer of energy in terms of heat

    and work transfer - Conservation of Energy

     Heat Transfer: It is the form of energy that can be transferred

    from one system to another as a result of temperature difference

    Driving Force - Temperature difference

    Modes of heat transfer (Mechanisms) and Rate of heat transfer

    (Time taken)

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    TRANSFER OF ENERGY ?

    How does the energy move from a hotter to a colderobject?

    Three mechanisms:

    Conduction

    Convection

    Radiation

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    METHODS OF HEAT TRANSFER

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    CONDUCTION

    Particles in a solid are always vibrating.

    If the solid gets hotter, the particlesvibrate more.

    Note: the particles don't swap places, ormove around they just vibrate more on the

    spot.

    Solids are better at conducting heat than ………………. and

    …………. because their particles are closer together. If the particles

    are too spaced out it makes it ……………… for the energy to pass

    along.Having said that, some solids conduct heat more than others. Metals

    are ……….. conductors. Non-metals are …………… conductors 

    (insulators). 

    Heat energy 

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    Conduction Con … 

     Conduction is the process whereby heat is transferred directly

    through a material, any bulk motion of the material playing no rolein the transfer.

     Those materials that conduct heat well are called thermal

    conductors, while those that conduct heat poorly are known as

    thermal insulators . 

    Most metals are excellent thermal conductors, while wood, glass,

    and most plastics are common thermal insulators.

     The free electrons in metals are responsible for the excellentthermal conductivity of metals.

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    Conduction of Heat Through a Material

    Rate of heat transfer by conduction, Q/t  through the length,  L across

    the cross-sectional area,  A is given by the following equation, where

    k  is the thermal conductivity and  ΔT  is the temperature difference

     between the two ends. Based on Fourier Law of Heat Conduction,

     L

    T kAQ  

    SI Unit of Thermal Conductivity:  W/( m · K)

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    Q T kAt d 

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    Isotropic and Anisotropic Materials

    In some material, the thermal conductivity varies with direction ofheat flow. Isotropic materials have the same thermal conductivityin all directions. Materials which show different thermalconductivities in different directions are known as Anisotropic

    materials. Examples for anisotropic materials are fiber-reinforced polymers and woods.

    Thermal conductivity for many materials is approximately as alinear function of temperature for limited temperature ranges

      k(T) = ko (1 + k T)

    Q =1 2( )

    avk A

    T T  L

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    Substance Thermal Conductivity, k  [J/(s · m · C°)]

    Metals  Aluminum 240

    Brass 110

    Copper 390

    Iron 79

    Lead 35

    Silver 420

    Steel (stainless) 14

    Gases  

    Air 0.0256

    Hydrogen (H2) 0.180

     Nitrogen (N2) 0.0258

    High conductivity

    High conductivity

    High conductivity

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    CONVECTION

    When a liquid is heated it becomes less

    dense (lighter) so it rises, cold liquid 

    takes its place.

    The same thing happens when air is

    heated. The hot air rises and cold air takes its place.

    This heat transfer can happen through liquidsand gases.

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    CONVECTION

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    Explains why breezes come from the ocean

    in the day and from the land at night

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    Convective Heat Transfer Evaluating processes where there is convective heat transferred

    to/from a solid surface

     External or Internal

     Gas or liquid

    Two Major Types

      Forced Convection: Flow is driven by external force –  Pump,

    Blower and Fan etc.. (Viscous Effect)

     Natural Convection: Flow is driven by density difference due to

    temperature gradient in the fluid (Buoyancy Effect)

    Convective Heat Transfer

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    Types of Flow –  Convection

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    General Approach to Calculating “h” 

     Determine Natural or Forced Convection

     Collect the appropriate physical data of the fluid:

    Thermal conductivity, Viscosity, Prandtl Number

     Calculate appropriate dimensionless numbers

     Use proper correlations to determine Nu

     Calculate “h” from Nu 

    Evaluating Physical Properties

     Film Temperature - Average between Tsur  and Tamb - Used for forcedconvection, and natural convection

     Average Temperature - Average between the two surface temperature

    of an enclosure

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     Nusselt Number = Convective Heat Transfer / Conductive Heat Transfer

    Reynolds Number = Inertia Force / Viscous Force

    Prandtl Number = Viscous Effect / Diffusion Effect

    Forced Convection

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    Internal Flows

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    Combined convection and conduction and overall

    heat transfer coefficient 

    In many engineering systems, heat transfer takes place between two fluidsseparated by a wall and the combined heat transfer coefficient is known as overall

    heat transfer coefficient.

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    Combined heat transfer equation is written as

    Q = UA (Th –  Tc)

    The overall heat transfer coefficient

    U =1

    1 1

    h c

     L

    h k h

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    The radiation heat transfer between two black bodies at

    temperatures T1 and T2 is given as   Q1-2 =  A1 F1-2 (T14 – T24)

    Where F1-2 is the shape or view or configuration factorwhich accounts for the fraction of the total radiationleaving gray surface 1 and reaching the gray surface 2.

    For two grey bodies this factor is given by

      F12 =

    Where 1 and 2 are the emissivities of the two bodies ofsurface area A1 and A2 and F12b is the view factor oftwo similar black bodies.

     As per the reciprocating theorem, A1F12 = A2F21

    1

    1 12 2 2

    1

    1 1 1

    1 1b

     A

     F A  

    Shape Factor

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    Properties of Radiation Thermal radiation is an important concept in

    thermodynamics as it is partially responsible for heatexchange between objects, as warmer bodies radiate moreheat than colder ones.

    (Other factors are convection and conduction.) Theinterplay of energy exchange is characterized by the

    following equation:   α + ρ + Ψ =1  Here, represents spectral absorption factor, spectral

    reflection factor and spectral transmission factor. All theseelements depend also on the wavelength. The spectralabsorption factor is equal to the emissivity ; this relation isknown as Kirchhoff's law of thermal radiation.

    http://en.wikipedia.org/wiki/Thermodynamicshttp://en.wikipedia.org/wiki/Heat_transferhttp://en.wikipedia.org/wiki/Heat_transferhttp://en.wikipedia.org/wiki/Physical_bodyhttp://en.wikipedia.org/wiki/Convectionhttp://en.wikipedia.org/wiki/Heat_conductionhttp://en.wikipedia.org/wiki/Emissivityhttp://en.wikipedia.org/wiki/Kirchhoff's_law_of_thermal_radiationhttp://en.wikipedia.org/wiki/Kirchhoff's_law_of_thermal_radiationhttp://en.wikipedia.org/wiki/Emissivityhttp://en.wikipedia.org/wiki/Heat_conductionhttp://en.wikipedia.org/wiki/Convectionhttp://en.wikipedia.org/wiki/Physical_bodyhttp://en.wikipedia.org/wiki/Heat_transferhttp://en.wikipedia.org/wiki/Heat_transferhttp://en.wikipedia.org/wiki/Thermodynamics

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     A warm or hot object emit (gives off) infrared radiation as

    heat waves, which can be absorbed (taken in) by anotherobject, heating it up

    The weird thing is that the surface colour of an object

    makes a difference

    Radiation

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    Radiation

    Energy carried by electromagnetic waves

     Light, microwaves, radio waves, x-rays

     Wavelength is related to vibration frequency

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    Radiation

    average frequency   absolute temperature

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    Black Body

    A mater ial that is a good absorber, l ikelampblack, is also a good emitter, and a

    mater ial that is a poor absorber, l ike

    polished silver, is also a poor emitter.

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    Absorption of Radiation

    Matt dark surfaces are betterabsorbers of infrared radiation thanshiny light surfaces.

    Which metal block will heat up quicker?

    If you lived in a hot country, would you paint your

    house a light colour or a dark colour?

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    Emission of Radiation

    Matt dark surfaces are betteremitters of infrared radiation thanshiny light surfaces.

    Hot soup62ºC

    Hot soup62ºC

    Which hot soup cup will lose heat quicker?If you had a food home delivery business, would youdeliver the hot food in dark coloured containers orAluminium foil?

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    Summer Clothing

    Q: People are uncomfortable wearing dark clothes during the

    summer. Why?

    A: Dark clothes absorb a large fraction of the sun's radiation andthen reemit it in all directions. About one-half of the emitted

    radiation is directed inward toward the body and creates the

    sensation of warmth. Light-colored clothes, in contrast, are cooler

    to wear, since they absorb and reemit relatively little of theincident radiation.

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    A White Sifaka Lemur

    To warm up in the morning, they turn their dark bellies toward

    the sun.

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    Why is the mother

    shielding her cub?

    Haven’t we seen

    this before?

    Ratio of the surface area

    of a cub to its volume ismuch larger than for its

    mother.

    To cool food, we cut it into smaller pieces, why?

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    Thermos Bottle

    A thermos bottle minimizes energy transfer

    due to convection, conduction, and radiation.

    Stopper- minimize conduction.

    Double-walled glass vessel with the space

     between the walls is evacuated to minimize

    energy losses due to conduction and

    convection.

    The silvered surfaces reflect most of the

    radiant energy that would otherwise enter or

    leave the liquid in the thermos.

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    Halogen Cooktop

    In a halogen cooktop, quartz-iodine lamps emit a large amount of

    electromagnetic energy that is absorbed directly by a pot or pan.

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    Highly reflective metal foil covering this satellite minimizes heat

    transfer by radiation.

    Metal foil

    HEAT EXCHANGERS

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    HEAT EXCHANGERS 

    Classification of heat exchangers

    Heat exchanger is a device, whose primary purpose is the transfer of heatenergy between two fluids. Heat exchangers are classified into regenerators,open-type exchangers and closed-type exchangers or recuperators. Theregenerators are heat exchangers in which the hot and cold fluids flowalternately through the same space with as little physical mixing between thetwo streams as possible.

    The Open-type heat exchangers are devices wherein physical mixing of the twofluid streams actually occurs. Hot and cold fluids enter open-type heatexchangers and leave as a single stream. The recuperator is a type in which thehot and cold fluid streams do not come into direct contact with each other butare separated by a tube wall or a surface.

    Heat exchangers are also classified according to the relative directions of thetwo streams, parallel flow if the fluids flow in the same direction, counter flowif the fluids flow in opposite directions and cross flow if the two fluids flow atright angles to one another.

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    Indirect contact parallel flow 

    Indirect contact counter flow 

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    Indirect contact cross flow 

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    Direct contact heat exchanger 

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    Overall heat transfer coefficients for heat exchangers 

    Types of heat exchanger  U[W/(m2K) 

    Gas-to-gas

    Water-to-air in finned tubes (water in tubes)

    Water-to-oil

    Water-to-gasoline or kerosene

    Water-to-water

    Feedwater heaters

    Steam-to-air in finned tubes (steam in tubes)

    Steam-to-light fuel oil

    Steam-to-heavy fuel oil

    Steam condenser

    Freon condenser (water cooled)

    Ammonia condenser (water cooled)

    Alcohol condensers (water cooled)

    10 –  40

    30 –  60

    100 –  350

    300 –  1000

    850 –  1700

    1000 –  8500

    30 –  300

    200 –  400

    50 –  200

    1000 –  6000

    300 –  1000

    800 –  1400

    250 –  700

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    Temperature profiles in parallel flow heat exchanger  

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    Temperature profiles in counter flow heat exchanger  

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    Log mean temperature difference (LMTD) 

    The temperatures of hot and cold fluids in a heat exchanger aregenerally not constant and vary from entry to exit due totransfer of heat from the hotter to the colder fluid. Due to thevariation of temperature difference at various sections, the rateof heat flow vary along the exchanger even for a constantthermal resistance. Neglecting the heat loss to the surroundings

    and potential and kinetic energy change, the general heattransfer equation for a heat exchanger is

    Heat lost by the hot fluid (Qh) = Heat transfer rate in heat

    exchanger = Heat gained by the cold fluid (Qc)

    mh Cph (Thi- Tho) = UA LMTD = mc Cpc (Tco –  Tci)

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