HEAT TRANFER LECTURE NOTES

download HEAT TRANFER LECTURE NOTES

of 30

  • date post

    04-Jun-2018
  • Category

    Documents

  • view

    221
  • download

    0

Embed Size (px)

Transcript of HEAT TRANFER LECTURE NOTES

  • 8/13/2019 HEAT TRANFER LECTURE NOTES

    1/30

    HEAT AND MASS TRANSFER

    Prabal Talukdar Associate Professor

    De artment of Mechanical En ineerin

    IIT Delhi

    prabal@mech.iitd.ac.in

    MECH/IITD

  • 8/13/2019 HEAT TRANFER LECTURE NOTES

    2/30

    Course Coordinator: Dr. Prabal Talukdar Room No: III, 368

    E-mail: prabal@mech.iitd.ac.in

    Lectures: Tue, Wed, Fri: 9-9.50 a.m.(Room No: IV LT1)

    Tut: 1-1.50 p.m.Course webpage:http://web.iitd.ac.in/~prabal/courses.htmlPre-requisite: Fluid Mechanics (AML 160)

    (Tentative Room no: III352

    MEL 242: Heat and Mass Transfer (3-1-0)Syllabus (for total 42 lectures)Introduction and basics of to heat transfer : Modes of heat transfer, Fouriers law, conductivity, diffusivity.Heat conduction e uation : 1D Heat conduction General heat conduction e uation Boundar and initialconditions, Heat generation.Steady heat conduction : Heat conduction in plane wall, cylinder, sphere, network analysis, critical radius of insulation, heat transfer from fins.Transient heat conduction: Lumped system analysis, transient heat conduction in large plane walls, longcy n ers an sp eres w spa a e ec , e s er an ro er c ar sNumerical methods of heat conduction : Finite difference formulation, numerical methods for 1D and 2D steadystate heat conduction.( 10 lectures)Introduction to convection : Fundamentals, Velocity and thermal boundary layer, laminar, turbulent flows,

    conservation equations for mass, momentum and energy, solution of boundary layer equations, Analogy betweenheat and momentum transfer, Non-dimensional numbersExternal heat transfer : Drag and heat transfer, parallel flow over flat plates, flow across cylinders and spheresInternal heat transfer : Mean velocity and mean temperature, entrance region, constant heat flux and temperature

    , ,Natural/free convection : Equation of motion of Grashof number, natural convection over surfaces and inside

    enclosures( 13 lectures)

  • 8/13/2019 HEAT TRANFER LECTURE NOTES

    3/30

    Boiling and condensation : Boiling heat transfer, pool boiling, flow boiling, condensation heat transfer, filmcondensation, heat transfer correlations.

    Heat Exchangers : Types of heat exchangers, overall heat transfer coefficient, analysis of heat exchangers, thelog mean temperature method, -NTU method.( 4 lectures)Introduction to radiation : Fundamentals, radiative properties of opaque surfaces, Intensity, emissive power,ra os ty, anc s aw, en s sp acement aw, ac an ray sur aces, m ss v ty, a sorpt v ty, pectra an

    directional variations, Stephan Boltzmann law, Kirchhoffs lawView factors : Definitions and relations, radiation heat transfer between two black surfaces, between diffuse graysurfaces, network method above two surfaces, re-radiating surface, radiation shield, radiation effects ontem erature measurements.( 7 lectures)Mass Transfer : Introduction, analogy between heat and mass transfer, mass diffusion, Ficks Law, boundaryconditions, steady mass diffusion through a wall, cylinder and sphere, water vapour migration in buildings,transient mass diffusion, mass transfer in a moving medium, diffusion of vapor through a stationary gas: Stefan

    ( 4 lectures)

    Quiz Quiz 1 Quiz 2Tentative Date Au ust 27 ovember 5

    Evaluation:Tuts and Quiz (2 nos): 20% (Closed note, book)

    ,Minor Test II: 25% (Open note, closed book)Major Test: 35% (Open note, closed book)Total: 100%Textbook: Fundamental of Heat and Mass Transfer: F. P.

    P.TALUKDAR/IITD

    Incropera and D. P. DewittHeat Transfer: Yunus A. Cengel

    Heat Transfer: J.P. Holmann

  • 8/13/2019 HEAT TRANFER LECTURE NOTES

    4/30

    Heat Transfer as a Course Has a reputation for being one of the most challenging,fundamental, conceptual courses in ME. It is the heart oft erma eng neer ng

    Why?? , ,

    theory, fluid mechanics, radiation theory

    Higher-level math: vector calculus, ODEs, PDEs, numerical

    Physically elusive: heat is invisible; developing intuition takestime

    Appropriate assumptions: required to simplify and solve most problems

    However, Heat Transfer is interesting, fun, and readily

    P.TALUKDAR/IITD

    applicable to the real world

  • 8/13/2019 HEAT TRANFER LECTURE NOTES

    5/30

    Heat Transfer Applications

    Heat transfer is commonly encountered in engineering systems andother aspects of life, and one does not need to go very far to see some

    .

    P.TALUKDAR/IITD

  • 8/13/2019 HEAT TRANFER LECTURE NOTES

    6/30

    Human bod

    P.TALUKDAR/IITD

  • 8/13/2019 HEAT TRANFER LECTURE NOTES

    7/30

    Heat Transfer - Thermod namics Thermodynamics is concerned with the amount of heat transfer as a

    system undergoes a process from one equilibrium state to another,

    and it gives no indication about how long the process will take. A thermodynamic analysis simply tells us how much heat must be

    conservation of energy principle.

    We are normally interested in how long it takes for thehot coffee in a thermos to cool to a certaintemperature, which cannot be determined from a

    . Determining the rates of heat transfer to or from a

    P.TALUKDAR/IITD

    ,variation of the temperature, is the subject of heat transfer

  • 8/13/2019 HEAT TRANFER LECTURE NOTES

    8/30

    Definition Heat transfer is energy transfer due to a temperature difference in a

    medium or between two or more media

    Different types of heat transfer processes are called different modesof heat transfer

    Conduction heat transfer is due to a temperature gradient in astationary medium or media

    Convection heat transfer occurs between a surface and a movin fluid at different temperatures

    Radiation heat transfer occurs due to emission of energy in the

    orm o e ec romagne c waves y a o es a ove a so u e zerotemperature Net radiation heat transfer occurs when there exists a temperature

    P.TALUKDAR/IITD

    difference between two or more surfaces emitting radiation energy

  • 8/13/2019 HEAT TRANFER LECTURE NOTES

    9/30

    Conduction Conduction heat transfer is due to random molecular and atomic

    vibrational, rotational and translational motions High temperature and more energetic molecules vibrate more and

    transfer energy to less energetic particles as a result of molecularcollisions or interactions

    & is characterized by a transport property know as the Thermal Conductivity, k (W / m K)

    x

    W = watts m = Meters K = temperature in Kelvin

    P.TALUKDAR/IITD

  • 8/13/2019 HEAT TRANFER LECTURE NOTES

    10/30

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

    Conduction can take place in solids, liquids, or gases. In gases andliquids, conduction is due to the collisions and diffusion of themolecules during their random motion. In solids, it is due to thecombination of vibrations of the molecules in a lattice and theenergy transport by free electrons

    geometry of the medium, its thickness, and the material of themedium, as well as the temperature difference across the medium

    P.TALUKDAR/IITD

  • 8/13/2019 HEAT TRANFER LECTURE NOTES

    11/30

    Fouriers Law(W)

    TkA

    TTkAQ 12cond

    =

    =&

    In the limiting case of x 0, the equation above reduces to thedifferential form Fouriers law of heat

    conduction after J. Fourier,who expressed it first in his

    heat transfer text in 1822

    (W)

    dx

    dTkAQ cond =

    &

    transfer in the positive x direction is a

    positive quantity

    T1=T2 =

    P.TALUKDAR/IITD

  • 8/13/2019 HEAT TRANFER LECTURE NOTES

    12/30

    Thermal Conductivit Specific heat C p is a measure of a materials ability to store thermal

    ener . For exam le C = 4.18 kJ/k C for water and C = 0.45

    kJ/kgC for iron at room temperature, which indicates that watercan store almost 10 times the energy that iron can per unit mass.

    ,ability to conduct heat . For example, k = 0.608 W/mC for water

    and k = 80.2 W/mC for iron at room temperature, which indicatesa ron con uc s ea more an mes as er an wa er can.

    Thus water is a poor heat conductor relative to iron, althoughwater is an excellent medium to store thermal energy

    P.TALUKDAR/IITD

  • 8/13/2019 HEAT TRANFER LECTURE NOTES

    13/30

    Ran e of Thermal Conductivit The thermal conductivities of gases

    such as air vary by a factor of 10 4

    from those of pure metals such ascopper.

    Note that pure crystals and metals

    have the highest thermal,insulating materials the lowest.

    P.TALUKDAR/IITD

  • 8/13/2019 HEAT TRANFER LECTURE NOTES

    14/30

    A simple experimental setup to

    determine the thermal conductivity .

    P.TALUKDAR/IITD

  • 8/13/2019 HEAT TRANFER LECTURE NOTES

    15/30

    The range of

    of various materialsat room temperature

    P.TALUKDAR/IITD

  • 8/13/2019 HEAT TRANFER LECTURE NOTES

    16/30

    The thermal conductivity of a substance is

    in the gas phase.

    Unlike gases, the thermal conductivities ofmost qu s ecrease w t ncreas ngtemperature, with water being a notableexception.

    In solids, heat conduction is due to twoeffects: the lattice vibrational waves induced

    b the vibrational motions of the molecules positioned at relatively fixed positions in a periodic manner called a lattice , and the

    electrons in the solid .The thermal conductivity of a solid is obtained by adding the lattice

    P.TALUKDAR/IITD

    .of pure metals are primarily due to the electronic component .