fundamentals of engineering supplied-reference handbook - Ventech!

There are three modes of heat transfer: conduction,
convection, and radiation. Boiling and condensation are
classified as convection.
CONDUCTION
Fourier's Law of Conduction
Q� = −kA(
dT dx).
, where
Q = rate of heat transfer.
�
Conduction Through a Plane Wall:
( T T ) L
Q kA 2 1 − − = � , where
k = the thermal conductivity of the wall,
A = the wall surface area,
L = the wall thickness, and
T1, T2 = the temperature on the near side and far side of
the wall respectively.
Thermal resistance of the wall is given by
R = L/(kA)
Resistances in series are added.
Composite Walls:
R1 =
Rtotal = R1 + R2, where
L1/(k1A), and
R2 = L2/(k2A).
HEAT TRANSFER
67
To Evaluate Surface or Intermediate Temperatures:
T = T − Q�
R ; T = T − Q�
R
2
1
1
3
Conduction through a cylindrical wall is given by
π
Q�
2
=
kL(
T1
− T2
)
ln(
r2
r1
)
( r r )
2
ln 2 1 R =
2πkL
CONVECTION
Convection is determined using a convection coefficient
(heat transfer coefficient) h.
� = hA T − T , where
Q w
( )
∞
A = the heat transfer area,
Tw = wall temperature, and
T∞ = bulk fluid temperature.
Resistance due to convection is given by
R = 1/(hA)
FINS: For a straight fin,
Q� = hpkA T − T tanh mL , where
c
( b ∞ ) c
h = heat transfer coefficient,
p = exposed perimeter,
k = thermal conductivity,
Ac = cross-sectional area,
Tb = temperature at base of fin,
T∞ = fluid temperature,
m = hp ( kA ) , and
c
Lc = L + Ac /p, corrected length.
2