Chapter 1 Heat Transfer
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Lecture L: lntroduction Learning Outcomes: 1. Understanding the basic mechanism of heat transfer 2. Able to derive different rate of heat transfer based on Fourier's law, Newton's law of cooling and Stefan-Boltzmann law KC21402 Definitions Heat transfer is (thermal) energy transfer that is induced by a temperature differenc e (or g rad ie nt) Thermodynamics - amount of energy transfer as the system undergoes a process from one equilibrium to another Heat transfer - science that deals with the rate of such energy transfer KC21402
Transcript of Chapter 1 Heat Transfer
Lecture L: lntroduction
Learning Outcomes:
1. Understanding the basic mechanism of heat transfer
2. Able to derive different rate of heat transfer based onFourier's law, Newton's law of cooling and Stefan-Boltzmannlaw
KC21402
Definitions
Heat transfer is (thermal) energy transfer that is induced by atemperature differenc e (or g rad ie nt)
Thermodynamics - amount of energy transfer as the systemundergoes a process from one equilibrium to another
Heat transfer - science that deals with the rate of such energytransfer
KC21402
Defin itio ns
Modes of heat transfer. Conduction heat transfer: Occurs when a temperature
gradient exists through a solid or a stationary fluid (liquid orgas).
. Cbnvection heat transfer: Occurs within a moving fluid, orbetween a solid surface and a moving fluid, when they are atdifferent tem peratu res
. Thermal radiation: Heat transfer between two surfaces (thatare not in contact), often in the absence of an interveningmedium.
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1. ConductionTransfer of energy from the more energetic to less energeticparticles of a substance as a results of interactions betweenatoms and/or molecules.
) Solid - Lattice vibration, free electron transport
) Liquids & gases - random molecular motion (collision &diffusion)
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Consider a brick wall, of thickness x which in a cold winter dayis exposed to a constant inside temperature, T, and a constantoutside temperatu re, I r.
. The heat transfer rate is(Fourier's law of heatconduction):
- ,- ,cr|'Area,A Y' *-^'tT
. The proportionality constant is atransport property, known as
thermal conductivity k (unitsw/m.K)
. Thermal conductivity is
temperature dependent. lt is ameasure of the ability of thematerialto conduct heat (Table
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General: Heat flux (unit: W/m2 ) is heat transfer rate in x direction perunit area perpendicular to the direction of transfer
.. .AT9* = -k-
X
Example: A furnace has a 0.15m thick brick wall (thermalconductivity of L.7 W/m.K). Steady state operation showsthat inner wall temperature is 1400K and j_150K for outerwall. What is the rate of heat loss if wall is 0.5m height and1.2m wide ?
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2. Convection
Energy transfer by random molecular motion (as inconduction) plus bulk (macroscopic) motion of the fluid.
- Convection: transport by random motion of moleculesand by bulk motion of fluid.
- Advection: transport due solely to bulk fluid motion.
F forced convection: Caused by external means such as fan,pump, wind
Natural (free) convection: flow induced by buoyancy forces,arising from density differences due to temperaturevariations in the fluid
Heat transfer involving phase change (latent heat exchange )
- boiling and condensation.
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) Example: air at 20"C blows over a hot plate, which ismaintained at a temperature Tr=300"C and has an area of A.
r? = 30rf c
} The convective heat flux is proportional to q", x T, _ Tn
F The proportionality constant is the convection heot tronsfercoeffici e nt, h (W/m2. K).
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) The convection heat transfer rate is:
Q" = h.A.(\ - L) Newton's law of cooting
F ftre heat transfer coefficient depends on surface geometry,nature of the fluid motion, as well as fluid properties. Fortypical ranges of values, see Table 1-3 textbook.
F Example: A 2m long, O.3cm diameter electrical wire in aroom with T of 15C. Heat is generated in the wire andsurface T is 152c at steady state. Voltage drop and current ismeasured at 60V and L.5A, determine h between wire andair.
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3. Radiation
F Thermal radiation is energy emitted by matter because oftheir temperature
) Energy is transported by electromagnetic radiation.
F Emission is due to changes in electron configuration.
) Can occur from solid surfaces, liquids and gases.
F Does not require presence of a medium
F Emissive power E is the rate at which energy is released perunit area (W/m2) (radiation emitted/rom the surface)
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. For an ideal radiator, or blackbody (max rate of radiation):
Qemitecr - oATr' Stefan-Boltzmann law
where \ is the absolute temperature of the surface (K) ando is the Stefan-Boltzmann constant, (o = 5.67x10 8 W/m2.1+1
. For a real (non-ideal) surface:
Qemitted -SOAI4 0<s<l
e is the emissivity measuring how closely the surface approx.blackbody
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' lf a heat transfer su.rface at T, is surrounded (encrosed) bya larger surface with Tsu' the net radiation
"*.f,rng"i,.
eraa _ eoA(T,u * qX,). lf radiation occurs parallel to conduction or convection, acombined heat transfer coefficient h.orb is used.
. The total heat transfer rate is:
, r gtour =h**oA{T, _T;)
. Example: Two infinite black plates at 8OOC and 300Cexchange heat by radiation. Calculate the heat transferper unit area.
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Summarv
Modes of Heat Transfer:
Conduction Convection Radiation
" ,dT n rq.=-k* Q* =h(Tr-T*) Qrad=co(T! -T,0,,)
g*" (W /rnzl is the heot fluxq, (W=J/s) is the heat rote
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How is heat transfer minimized in a thermos bottle?
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Conservation of EnergySurroundings, S
(contro:orum)
\ (cv) /Boundary B
z;;\->r
\orurirge) L.st trlAddition \ ^ .. i /Lossthrough inr"t\Ie*tnll)/ throush outet
Lrn Eo,,,
Energy conservation on a rate basis:
Units W=J/s
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) lnflow and outflow are surface phenomena
F Generation and accumulation are volumetric phenomena) Eu = KE + pE + U; U is Ur"n dhd U t","nt
8,,,+Eu*tou,=*=t*
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