Fourier’s Law
and the
Heat Equation
Chapter Two
Fourier’s Law
Fourier’s Law
• A rate equation that allows determination of the conduction heat flux
from knowledge of the temperature distribution in a medium
• Its most general (vector) form for multidimensional conduction is:


q    k  T
Implications:
– Heat transfer is in the direction of decreasing temperature
(basis for minus sign).
– Fourier’s Law serves
to define the thermal conductivity of the




medium  k   q /  T 


– Direction of heat transfer is perpendicular to lines of constant
temperature (isotherms).
– Heat flux vector may be resolved into orthogonal components.
Heat Flux Components
• Cartesian Coordinates:
T  x, y , z 

T  T  T 
q  k
i k
jk
k
x
y
z
qx
qz
qy
• Cylindrical Coordinates:
(2.3)
T  r, , z 

T 
T  T 
q    k
i k
jk
k
r
r 
z
qr
q
(2.18)
qz
T  r ,  , 


T 
T 
T
q  k
i k
jk
k
r
r 
r sin  
q
q
qr
• Spherical Coordinates:
(2.21)
Heat Flux Components (cont.)
• In angular coordinates or  ,  , the temperature gradient is still
based on temperature change over a length scale and hence has
units of C/m and not C/deg.
• Heat rate for one-dimensional, radial conduction in a cylinder or sphere:
– Cylinder
qr  Ar qr  2 rLqr
or,
qr  Ar qr  2 rqr
– Sphere
qr  Ar qr  4 r 2 qr
Heat Equation
The Heat Equation
• A differential equation whose solution provides the temperature distribution in a
stationary medium.
• Based on applying conservation of energy to a differential control volume
through which energy transfer is exclusively by conduction.
• Cartesian Coordinates:
  T
k
x  x
   T
  k
 y  y
   T
  k
 z  z
Net transfer of thermal energy into the
control volume (inflow-outflow)
T
 •
  q  c p
t

Thermal energy
generation
(2.13)
Change in thermal
energy storage
Heat Equation (Radial Systems)
• Cylindrical Coordinates:
1   T
 kr
r r  r
T
 1   T    T  •

k

k

q


c
p


 2


t
 r     z  z 
(2.20)
• Spherical Coordinates:
1   2 T 
1
  T 
1
 
T  •
T
kr

k

k
sin


q


c
p






r  r 2 sin 2      r 2 sin   
 
t
r 2 r 
(2.33)
Heat Equation (Special Case)
• One-Dimensional Conduction in a Planar Medium with Constant Properties
and No Generation
 2T
x 2


1 T
 t
k
 thermal diffusivity of the medium
c p
Boundary Conditions
Boundary and Initial Conditions
• For transient conduction, heat equation is first order in time, requiring
specification of an initial temperature distribution: T  x, t t 0  T  x,0
• Since heat equation is second order in space, two boundary conditions
must be specified. Some common cases:
Constant Surface Temperature:
T  0, t   Ts
Constant Heat Flux:
Applied Flux
Insulated Surface
k
T
|x  0  qs
x
k
T
|x  0  h T  T  0, t  
x
Convection
T
|x  0  0
x
Properties
Thermophysical Properties
Thermal Conductivity: A measure of a material’s ability to transfer thermal
energy by conduction.
Thermal Diffusivity: A measure of a material’s ability to respond to changes
in its thermal environment.
Property Tables:
Solids: Tables A.1 – A.3
Gases: Table A.4
Liquids: Tables A.5 – A.7
Conduction Analysis
Methodology of a Conduction Analysis
• Solve appropriate form of heat equation to obtain the temperature
distribution.
• Knowing the temperature distribution, apply Fourier’s Law to obtain the
heat flux at any time, location and direction of interest.
• Applications:
Chapter 3:
Chapter 4:
Chapter 5:
One-Dimensional, Steady-State Conduction
Two-Dimensional, Steady-State Conduction
Transient Conduction
Problem : Thermal Response of Plane Wall
Problem 2.46 Thermal response of a plane wall to convection heat transfer.
KNOWN: Plane wall, initially at a uniform temperature, is suddenly exposed to convective
heating.
FIND: (a) Differential equation and initial and boundary conditions which may be used to find
the temperature distribution, T(x,t); (b) Sketch T(x,t) for the following conditions: initial (t 
0), steady-state (t  ), and two intermediate times; (c) Sketch heat fluxes as a function of
time at the two surfaces; (d) Expression for total energy transferred to wall per unit volume
(J/m3).
SCHEMATIC:
Problem: Thermal Response (Cont.)
ASSUMPTIONS: (1) One-dimensional conduction, (2) Constant properties, (3) No internal
heat generation.
ANALYSIS: (a) For one-dimensional conduction with constant properties, the heat equation
has the form,
 2T 1  T

2
 t
x
and the
conditions are:
Initial, t  0 : T  x,0   Ti


Boundaries: x=0  T/ x)0  0

x=L  k T/ x) L = h T  L,t   T 


uniform temperature
adiabatic surface
surface convection
(b) The temperature distributions are shown on the sketch.
Note that the gradient at x = 0 is always zero, since this boundary is adiabatic. Note also that
the gradient at x = L decreases with time.
Problem: Thermal Response (Cont.)
bg
c) The heat flux, q x x, t , as a function of time, is shown on the sketch for the surfaces x = 0 and x =
L.
d) The total energy transferred to the wall may be expressed as

Ein   qconv As dt
0
Ein  hAs 

0
 T  T  L,t  dt
Dividing both sides by AsL, the energy transferred per unit volume is
Ein h 

T  T  L,t  dt
V L 0 
 J/m3 


Problem: Non-Uniform Generation due
to Radiation Absorption
Problem 2.28 Surface heat fluxes, heat generation and total rate of radiation
absorption in an irradiated semi-transparent material with a
prescribed temperature distribution.
KNOWN: Temperature distribution in a semi-transparent medium subjected to radiative flux.
FIND: (a) Expressions for the heat flux at the front and rear surfaces, (b) The heat generation rate
q x , and (c) Expression for absorbed radiation per unit surface area.
bg
SCHEMATIC:
Problem: Non-Uniform Generation (Cont.)
ASSUMPTIONS: (1) Steady-state conditions, (2) One-dimensional conduction in medium, (3)
Constant properties, (4) All laser irradiation is absorbed and can be characterized by an internal
volumetric heat generation term q x .
bg
ANALYSIS: (a) Knowing the temperature distribution, the surface heat fluxes are found using
Fourier’s law,
q x   k
dT O L A
O
L
-ax


k

a
e

B
b
g
M
P
M
Ndx P
Q Nka2
Q
Front Surface, x=0:
Rear Surface, x=L:
A
A
O
L
O

1

B


 kBP
bg L
M
P
M
Nka Q Na Q
A -aL
L+ A e-aL  BO
L
O
q x bg
L  kM


e

kB
.
P
P
Nka
QM
Na
Q
q x 0   k +
(b) The heat diffusion equation for the medium is
F
IJ
G
HK
bg L
M
N
F
IJ
G
HK
d dT
q
d dT
  0 or q = -k
dx dx
k
dx dx
d
A
q x   k
 e-ax  B  Ae -ax .
dx ka
O
P
Q
(c ) Performing an energy balance on the medium,
 E


E
in
out  E g  0
<
<
Problem: Non-Uniform Generation (Cont.)
On a unit area basis
bg bg e
A
E g   E in  E out  q x 0  q x L   1  e-aL .
a
j
Alternatively, evaluate E g by integration over the volume of the medium,
z bg z
L
 g  L q x dx = L Ae-axdx = - A e-ax  A 1  e-aL .
E
0
0
0
a
a
e
j