Explanation of the differences between radiation, convection
heat by collisions to other water molecules, which is known as conduction However, in that example, convection was the most dominant mode of heat transfer If you wish to know the exact difference between these modes of transfer, you would perhaps benefit from enrolling in an intermediate-level engineering course
Convection, Conduction & Radiation
Convection, Conduction & Radiation 1 Convection, Conduction & Radiation There are three basic ways in which heat is transferred: convection, conduction and radiation In gases and liquids, heat is usually transferred by convection, in which the motion of the gas or liquid itself carries heat from one place to another
Heat Transfer: Conduction, Convection, and Radiation
Conduction is how heat travels between objects that are touching Conduction travels fastest through solids, but liquids and gases can also conduct heat Some materials, like metal, can conduct heat very quickly, while other materials (like plastic or wood) conduct heat very slowly Convection is how heat travels through fluids – liquids and
Heat Transfer conduction and convection
• Temperature difference analogous to potential difference • Both follow Ohm’s law with appropriate resistance term 10 Thermal Resistance II • Conduction ( ) kA L R R T T Q L kA T T Q cond cond ⇒ = − ⇒ = − &= 1 2 & 1 2 • Convection hA R R T T Q hAT T Q conv conv s f s f 1 ⇒ = − = − ⇒& = • Radiation rad rad A T T T T
Conduction, Convection, & Radiation
Conduction, convection, or radiation • Tell students we are going to “see” conduction, convection and radiation by using the liquid crystal sensors Learning Goals: •Students explain that energy is transferred through a solid during conduction, a liquid/gas during convection, and any medium during radiation
Conduction, Convection and Sensible and Latent Heat
Conduction, Convection and Sensible and Latent Heat So far we have only considered transfer of heat through the atmosphere by radiative processes Energy may also be transferred through conduction and convection Conduction: The process of conduction occurs by the transfer of kinetic energy from one molecule to an adjacent one
•Diffusion •Convection •Adsorption
•Convection •Adsorption Physiology: • Diffusion: Process of movement of molecules across an area due to differences in concentration gradients – Moves from high-to-low concentration until the concentrations for both areas are equal – Adequate for clearing small molecules (
2-D Conduction: Finite-Difference Methods
outer surface is exposed to convection with a fluid at 300 K A symmetrical element with a 2-dimensional grid is shown and temperatures for nodes 1,3,6, 8 and 9 are given (a) Derive finite-difference equations for nodes 2, 4 and 7 and determine the temperatures T2, T4 and T7 (b) Calculate heat loss per unit length 430 K 394 K 492 K 600 600
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Steady Heat Transfer February 14, 2007
ME 375 - Heat Transfer1
Steady Heat Transfer with Steady Heat Transfer withConduction and Convection
Conduction and Convection
Larry Caretto
Mechanical Engineering 375
Heat TransferHeat Transfer
February 14, 2007
2Outline
• Review last lecture • Equivalent circuit analyses - Review basic concept - Application to series circuits with conduction and convection - Application to composite materials - Application to other geometries • Two-dimensional shape factors 3Review Steady, 1-D,
• Rectangular• Cylindrical shell 1212ln2 r rTTk
LQ• Spherical shell
2112/1/14 rrTTkQ L TTk A Qq L0 is an average thermal conductivity (or a constant value) if k is constantk T 0 , T L = temperatures at x = 0,L; T 1 , T 2 temperatures at inner (r 1 ) and outer(r 2 ) radii 0e gen
4Review Heat Generation
• Various phenomena in solids can generate heat • Define as the heat generated per unit volume per unit time gen eFigure 2-21 from Çengel,
Heat and Mass Transfer
2222 A I LAALI V RIe gen 5
Review Heat Generation II
• Temperature and heat flux equations LTTkLxeq
Lgen 0 22L xTT k xLe k xeTT
Lgengen
02 0 22Lxgenx
QEQ 06Plot of (T - T0)/(TL - T0) for Heat Generation in a Slab
00.20.40.60.811.21.41.61.82
0 0.10.20.30.40.50.60.70.80.9 1
x/LTemperatureDifference Ratio
H = 0H = .01
H = .1
H = 1 H = 2 H = 5H = 10
02TTkeLH
LgenSteady Heat Transfer February 14, 2007
ME 375 - Heat Transfer2
7Steady Heat Transfer Definition
• In steady heat transfer the temperature and heat flux at any coordinate point do not change with time • Both temperature and heat transfer can change with spatial locations, but not with time • Steady energy balance (first law of thermodynamics) means that heat in plus heat generated equals heat out 8Rectangular Steady Conduction
Figure 2-63 from Çengel,
Heat and Mass TransferFigure 3-2 from
Çengel, Heat
and MassTransfer
The heat
transfer is constant in this 1D rectangle for both constant & variable k dxdTkqA Q 9Thermal Resistance
• Heat flow analogous to current • Temperature difference analogous to potential difference • Both follow Ohm's law with appropriate resistance term 10Thermal Resistance II
• Conduction kALRRTTQLTTAkQ cond cond 2121• Convection hARRTTQTThAQ conv convfs fs 1 • Radiation radrad hATTTTTTAR
12211223231121
11 F 11Where Does the Heat Go?
Energy conservation
requires that conduction heat through wall equals the heat leaving the wall by convection and radiation 321QQQ
Figure 1-18 from
Çengel, Heat and
Mass Transfer
12Where Does the Heat Go? II
Figure 1-18
from Çengel,Heat and
MassTransfer
Figure 3-5 from Çengel,
Heat and Mass Transfer
Steady Heat Transfer February 14, 2007
ME 375 - Heat Transfer3
13Parallel Resistances (T
= T surr radconvtotalRRR111
radsconvs total hAhAR1 surr TTDefine total heat transfer
coefficient, h total radconv totalstotal hhRAh1Figure 3-5
from Çengel,Heat and
Mass Transfer
14Combined Modes
Convection or
convection plus radiationConvection or convection plus radiationConduction
11 TThq L TTkq 21L
All values are the same
q 22TThq
Figure 3-6 from
Çengel, Heat and
Mass Transfer
15Combined Modes II
1,1,2121
convwallconvtotal RRRTT RTTQ 21212121
1111
hkL hTT AQq AhkAL AhTTQ L
A is area normal
to heat flowSeries
Resistance
Formula
Figure 3-6 from
Çengel, Heat and
Mass Transfer
16Combined Modes III
212111 hkL hTT AQq L
A is area normal
to heat flowIf you know h
1 , h 2 , L, k, T 1 and T 2 , but you do not know T 1 and T 2 , can you find the heat flux?Once you found the heat flux from the
information give, can you find T 1 and T 2 222111
hqTThqTT
Figure 3-6 from
Çengel, Heat and
Mass Transfer
17Problem
A house has a 4 in thick brick wall with k = 0.6
Btu/hr·ft·
oF. The interior temperature is 70
o F and the exterior temperature is 0 oF. The inside and
outside convection plus radiation coefficients are3 Btu/hr·ft
2 oF and 4 Btu/hr·ft
2 oF, respectively.
Find the heat flux through the wall.
Given:Wall with L = 4 in = 4/12 ft and k =0.6
Btu/hr·ft·
oF has convection on two sides. T
1 70o F, T 2 = 0 o F, h 1 = 3 Btu/hr·ft 2 o
F and h
2 = 4Btu/hr·ft
2 o F. Find: AQq 18Solution
LA is area normal
to heat flowBtuFfthr
BtuFfthrftBtuFfthrFF
hkL hTT AQq ooooo46.0124
307011 22
2121