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ME 259 Heat Transfer Lecture Slides I

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Heat Transfer Lecture Slides I Dept. of Mechanical Engineering, 1/22/05 ME 259 * ... – PowerPoint PPT presentation

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Title: ME 259 Heat Transfer Lecture Slides I


1
ME 259Heat TransferLecture Slides I
  • Dept. of Mechanical Engineering,

2
Introduction
  • Reading Incropera DeWitt
  • Chapter 1

3
Heat Transfer as a Course
  • Has a reputation for being one of the most
    challenging courses in ME
  • Why??
  • Physically diverse thermodynamics, material
    science, diffusion theory, fluid mechanics,
    radiation theory
  • Higher-level math vector calculus, ODEs, PDEs,
    numerical methods
  • Physically elusive heat is invisible developing
    intuition takes time
  • Appropriate assumptions required to simplify and
    solve most problems
  • However, Heat Transfer is interesting, fun, and
    readily applicable to the real world

4
Relevance of Heat Transfer
  • Electric Power Generation
  • Alternate Energy Systems
  • Combustion/Propulsion Systems
  • Building Design
  • Heating Cooling Systems
  • Domestic Appliances
  • Materials/Food Processing
  • Electronics Cooling Packaging
  • Cryogenics
  • Environmental Processes
  • Space Vehicle Systems

5
Definition of Heat Transfer
  • Flow of energy due solely to a temperature
    difference
  • all other forms of energy transfer are
    categorized as work
  • from 2nd Law of Thermodynamics, heat flows in
    direction of decreasing temperature
  • heat energy can be transported through a solid,
    liquid, gas, or vacuum

6
Heat Quantities
7
Relationship Between the Study of Heat Transfer
Thermodynamics
  • 1st Law of Thermodynamics for Closed System
  • Thermodynamics - allows calculation of total heat
    transferred (Q) during a process in which system
    goes from one equilibrium state to another (i.e.,
    the big picture)
  • Heat Transfer - provides important physical laws
    that allow calculation of instantaneous heat
    rate, length of time required for process to
    occur, and temperature distribution within
    material at any time (i.e., the details
    required for design)

8
Heat Transfer Modes
  • Conduction
  • transfer of heat due to random molecular or
    atomic motion within a material (aka diffusion)
  • most important in solids
  • Convection
  • transfer of heat between a solid surface and
    fluid due to combined mechanisms of a) diffusion
    at surface b) bulk fluid flow within boundary
    layer
  • Radiation
  • transfer of heat due to emission of
    electromagnetic waves, usually between surfaces
    separated by a gas or vacuum

9
Heat Transfer Modes - Conduction
  • Rate equation (Fourier Biot, ?1820) is known as
    Fouriers law for 1-D conduction,
  • where qx heat rate in x-direction (W)
  • qx heat flux in x-direction (W/m2)
  • T temperature (C or K)
  • A area normal to heat flow (m2)
  • k thermal conductivity of material
  • (W/m-K) see Tables A.1-A.7

10
Heat Transfer Modes - Conduction
  • Steady-state heat conduction through a plane
    wall

T1
T2
k
L
q? (T1gtT2)
x
11
Heat Transfer Modes - Conduction
  • Example What thickness of plate glass would
    yield the same heat flux as 3.5? of glass-fiber
    insulation with the same S-S temperature
    difference (T1-T2) ?

12
Heat Transfer Modes - Conduction
  • Insulation R-value
  • where 1 W/m-K 0.578 Btu/hr-ft-F

13
Heat Transfer Modes - Convection
  • Rate equation (Newton, ?1700) is known as
    Newtons law of cooling
  • where q heat flux normal to surface
  • q heat rate from or to
    surface As
  • Ts surface temperature
  • T? freestream fluid
    temperature
  • As surface area exposed to fluid
  • h convection heat transfer coefficient
  • (W/m2-K)

q?
Fluid flow, T?
Ts (gtT?)
As
14
Heat Transfer Modes - Convection
  • The convection heat transfer coefficient (h)
  • is not a material property
  • is a complicated function of the many parameters
    that influence convection such as fluid velocity,
    fluid properties, and surface geometry
  • is often determined by experiment rather than
    theory
  • will be given in most HW problems until we reach
    Chapter 6

15
Heat Transfer Modes - Convection
  • Types of Convection
  • Forced convection flow caused by an external
    source such as a fan, pump, or atmospheric wind
  • Free (or natural) convection flow induced by
    buoyancy forces such as that from a heated plate
  • Phase change convection flow and latent heat
    exchange associated with boiling or condensation

16
Heat Transfer Modes - Radiation
  • Rate equation is the Stefan-Boltzmann law which
    gives the energy flux due to thermal radiation
    that is emitted from a surface for a black body
  • For non-black bodies,
  • where E emissive power (W/m2)
  • ? Stefan-Boltzmann constant
  • 5.67x10-8 W/m2-K4
  • ? emissivity (0lt ?lt1) of surface
  • Ts surface temperature in absolute
  • units (K)

17
Heat Transfer Modes - Radiation
  • Radiation incident upon an object may be
    reflected, transmitted, or absorbed
  • where
  • G irradiation (incident radiation)
  • ? reflectivity (fraction of G that is
    reflected)
  • ? transmissivity (fraction of G that is
    transmitted
  • ? absorptivity (fraction of G that is
    absorbed)
  • ? emissivity (fraction of black body
    emission)
  • E and the interaction of G with each
    participating object determines the net heat
    transfer between objects

G
?G
?G
?G
18
Heat Transfer Modes - Radiation
  • Heat transfer between a small object and larger
    surroundings (AsltltAsur)
  • where ? emissivity of small object
  • As surface area of small object
  • Ts surface temperature of small
  • object (K)
  • Tsur temperature of surroundings (K)

Tsur
q
Ts
? , As
19
Conservation of Energy Control Volume
  • Control volume energy balance
  • from thermodynamics
  • Incropera DeWitt text notation

Q
mass out
W
mass in
20
Conservation of Energy Control Volume
  • Energy rates
  • where

21
Conservation of Energy Control Surface
  • Surface energy balance
  • since a control surface is a special control
    volume that contains no volume, energy generation
    and storage terms are zero this leaves

Eout
Ein
22
Summary The Laws Governing Heat Transfer
  • Fundamental Laws
  • Conservation of mass
  • Conservation of momentum
  • Conservation of energy
  • Heat Rate Laws
  • Fouriers law of heat conduction
  • Newtons law of convection
  • Stefan-Boltzmann law for radiation
  • Supplementary Laws
  • Second law of thermodynamics
  • Equations of state
  • ideal gas law
  • tabulated thermodynamic properties
  • caloric equation (definition of specific heat)

23
Objectives of a Heat Transfer Calculation
  • ANALYSIS
  • Calculate T(x,y,z,t) or q for a system undergoing
    a specified process
  • e.g., calculate daily heat loss from a house
  • e.g., calculate operating temperature of a
    semiconductor chip with heat sink/fan
  • DESIGN
  • Determine a configuration and operating
    conditions that yield a specified T(x,y,z,t) or
    q
  • e.g., determine insulation needed to meet a
    specified daily heat loss from a house
  • e.g., determine heat sink and/or fan needed to
    keep operating temperature of a semiconductor
    chip below a specified value

24
Classes of Heat Transfer Problems
  • Thermal Barriers
  • insulation
  • radiation shields
  • Heat Transfer Enhancement (heat exchangers)
  • boilers, evaporators, condensers, etc.
  • solar collectors
  • finned surfaces
  • Temperature Control
  • cooling of electronic components
  • heat treating quenching of metals
  • minimizing thermal stress
  • heating appliances (toaster, oven, etc.)
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