Transcript Heat transfer
Heat Transfer: Overview of Heat Transfer Analysis
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Objectives
Section 6 – Thermal Analysis Module 1: Overview of Heat Transfer Page 2 Understand the basics of heat transfer analysis.
Study Conduction, Convection and Radiation modes of heat transfer.
Identify the considerations required for solving heat transfer problems.
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Overview of Heat Transfer Analysis
Section 6 – Thermal Analysis Module 1: Overview of Heat Transfer Page 3
Heat – energy produced as a result of combustion, chemical reaction,
electrical resistance, friction, fission, fusion, incident solar radiation, microwaves, etc .
Heat transfer – the exchange of heat from one body to another, the
study of which is applicable across a broad range of applications.
Common heat transfer problems include the calculation of heat loss/gain : Through windows In electronic chips Through pipes carrying steam Through fins on a radiator And many more… The Sun is by far the largest natural source of heat for our planet.
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Modes of Heat Transfer
Section 6 – Thermal Analysis Module 1: Overview of Heat Transfer Page 4 Transfer of heat takes place from a body at a higher temperature to a body at a lower temperature via one or more of the following mechanisms: Conduction Convection Radiation Heat transfer can occur through a body via all three modes simultaneously.
Cases with occurrence of more than one mode are termed as conjugate heat transfer problems.
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Conduction
Section 6 – Thermal Analysis Module 1: Overview of Heat Transfer Page 5 Fourier Equation:
Q
kA
T
x
Q = heat transferred K = thermal conductivity A = area
T
x
= Gradient of temperature Q The negative sign in the Fourier equation serves to counter the negative gradient of temperature.
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Convection
Section 6 – Thermal Analysis Module 1: Overview of Heat Transfer Page 6
Q
UhA
(
T b
T a
) Where:
T a
U = Velocity of moving fluid h = Convection coefficient A = area of the plate
T b
T b = Temperature of the solid body T a = Ambient fluid temperature Heat loss to air moving across fins on a radiator is a common engineering problem that involves convection.
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Conduction and Convection
Diffusion Advection Diffusion Advection Section 6 – Thermal Analysis Module 1: Overview of Heat Transfer Page 7 Conduction Convection © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk.
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Conduction and Convection
Ra
g
TL
3 Where: Volumetric Thermal Expansivity Thermal Diffusivity Kinematic Viscosity Section 6 – Thermal Analysis Module 1: Overview of Heat Transfer Page 8 If the objective is to stop heat transfer, then convection can be curbed by placing obstacles in the direction of flow.
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Conduction and Convection
Section 6 – Thermal Analysis Module 1: Overview of Heat Transfer Page 9 Both conduction and convection can be solved by the energy equation.
The energy equation in simplified form:
q x
x
q y
y
q
z z
divq
Energy in Change in Energy Within the system Energy Out The energy equation in full form:
D
1 2 (
u
2
v
2
w
2 )
Dt
u
.
gradp
u
(
x xx
yx
y
z zx
) (
x xy
yy
y
zy
z
)
w
(
x xz
y yz
z zz
)
u
.
S M
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Radiation
Radiative exchange between two bodies is expressed by the equation:
Q
A
(
T b
4
T a
4 ) Where:
A
Surface Emissivity Stephan Boltzman Constant Area of surface
T b
Temperature in K of radiant body
T a
Temperature in K of ambient sink Radiative loss to the ambient at absolute zero:
Q
A
(
T
4 ) Section 6 – Thermal Analysis Module 1: Overview of Heat Transfer Page 10
T a T b
Unlike convection and conduction, no medium is required. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk.
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Solving Heat Transfer Problems
Section 6 – Thermal Analysis Module 1: Overview of Heat Transfer Page 11 Heat transfer problems, just like fluid flow problems, are now solved predominantly using computational techniques.
Previously, heat transfer calculations involved calculating heat gain and loss via different modes using the formulae mentioned in the above slides.
For convection, the majority of calculations are done using regressions evaluated by experimental techniques.
Finite difference methods (FDM) are often taught at the graduate level for solving 2D conductive heat transfer.
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Summary
Section 6 – Thermal Analysis Module 1: Overview of Heat Transfer Page 12 Heat transfer affects us in profound ways. Our very existence depends upon the transfer of heat. The subject of heat transfer in engineering sciences is fundamental and its application is almost inescapable. Conduction, convection and radiation are the three modes of heat transfer.
Conduction requires a stationary medium, while convection requires a moving medium. Heat transfer problems can be solved though numerical analysis. © 2011 Autodesk Freely licensed for use by educational institutions. Reuse and changes require a note indicating that content has been modified from the original, and must attribute source content to Autodesk.
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Summary
Section 6 – Thermal Analysis Module 1: Overview of Heat Transfer Page 13 The fundamental equation for conduction is the Fourier equation.
For convection cases, dimensionless parameters measured through experiments are used. In the case of radiation, the Stephen Boltzmann law applies and surface properties influence this mode of heat transfer. Today, heat transfer problems are solved with the help of 3D computer software.
This has greatly reduced the need for experiments, thus cutting costs and product turnaround times.
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