KNOWN: Size and surface temperatures of a cubical freezer. Materials, thicknesses and interface
resistances of freezer wall.
FIND: Cooling load.
Lins = 100 mm
Lst = 6.35 mm
Lal = 6.35 mm
Ts,o = 22oC
Ts,i = -6oC
R = 2.5
530 Chapter 8 l Internal F low
Friction factor correlations for turbulent ow are based on limited data. Furthermore,
heat transfer augmentation due to the secondary ow is minor when the ow is turbulent
and is less than 10% for C/D Z 20. As such, augmentat
Chapter 5 l Transient Conduction
5.75 A sphere 30 mm in diameter initially at 800 K is
quenched in a large bath having a constant temperature
of 320 K with a convection heat transfer coefcient of
75 W/mZ-K. The therrnophysical properties of the
of 60C be recorded if the convection coefcient is
300 W/m2 - K?
Assess the effect of thermal diffusivity on the ther-
mal response of the material by computing center
and surface temperature histories for a = 106,
105, and 104 m2/s. Plot your re
378 Chapter 6 I Introduction to Convection
i hus far we have focused on heat transfer by conduction and have considered convection
only to the extent that it provides a possible boundary condition for conduction problems. In
Section 1.2.2 we used the term
Chapter 7 l External Flow
7.4.2 Convection Heat Transfer
Experimental results for the variation of the local Nusselt number with 0 are shown in
Figure 7.10 for the cylinder in a cross ow of air. Not unexpectedly, the results are strongly
(d) What advantage, if any, is there in not making
Ax = Ay for this situation?
With Ax = Ay = 2 mm, calculate the temperature
eld within the plate and the rate of heat transfer
from the plate. Under no circumstances may the
temperature at any l
Chapter 7 l External Flow
The rate of heat transfer from the fth heater is then
gm, = (74 WIm2 - K x 0.25 m 67 W/m2 - K x 0.20 m)
X 1 m(230 25)C
qcouv,5 : 1050 W
Similarly, the power requirement for the sixth heater may be obtained by subtracting the
The therrnophysical properties of the bar are p =
2600 kg/m3, c = 1030 J/kg-K, and k = 3.50 W/m-K.
(a) How long should the bar remain in the bath in
order that, when it is removed and allowed to equi-
librate while isolated from any surroundings
Heat Transfer, 6502481
Name: Prof. F. J. Diez
Quiz No. 1
An electric heater with the total surface area of 0.25 m2 and emissivity 0.75 is in a room where the air has a
temperature of 20 C and the walls are at 10 C. When the heater consumes 500 W
(e) Determine F12 using the results of Figure 13.4 if
the dimensions are increased to w = L = 2 m.
13.13 Consider the parallel planes of innite extent normal
to the page having opposite edges aligned as shown in
392 Chapter 6 I Introduction to Convection
always be treated as incompressible, density variations in owing gases should be considered
when the velocity approaches or exceeds the speed of sound. Specically, a gradual transition
from incompressible to comp
KNOWN: Surface conditions and thickness of a solar collector absorber plate. Temperature of
FIND: (a) Differential equation which governs plate temperature distribution, (b) Form of the
KNOWN: Window surface area and thickness, inside and outside heat transfer coefficients, outside
and passenger compartment temperatures.
FIND: Heat loss through the windows for high and low inside heat transfer coefficients.
KNOWN: A layer of fatty tissue with fixed inside temperature can experience different
outside convection conditions.
FIND: (a) Ratio of heat loss for different convection conditions, (b) Outer surface
temperature for different convection cond
KNOWN: Inner and outer radii of a tube wall which is heated electrically at its outer surface
and is exposed to a fluid of prescribed h and T. Thermal contact resistance between heater
and tube wall and wall inner surface temperature.
KNOWN: Dimensions of spherical, stainless steel liquid oxygen (LOX) storage container. Boiling
point and latent heat of fusion of LOX. Environmental temperature.
FIND: Thermal isolation system which maintains boil-off below 1 kg/day.
KNOWN: Thermal conductivity and diameter of a pin fin. Value of the heat transfer coefficient and
FIND: (a) Length of fin, (b) Fin effectiveness.
D = 4 mm
f = 0.65
k = 160 W/mK
h = 220 W/m2K
ASSUMPTIONS: (1) St
KNOWN: Diameter, thermal conductivity and microbial energy generation rate in cylindrical hay
bales. Ambient conditions.
FIND: The maximum hay temperature for q = 1, 10, and 100 W/m3.
T = 0C, h = 25 W/m 2K
q = 1, 10 or 100
KNOWN: Thermal conductivities and thicknesses of original wall, insulation layer, and glass layer.
Interior and exterior air temperatures and convection heat transfer coefficients.
FIND: Heat flux through original and retrofitted walls.
KNOWN: Long rod experiencing uniform volumetric generation of thermal energy, q, concentric
with a hollow ceramic cylinder creating an enclosure filled with air. Thermal resistance per unit
length due to radiation exchange between enclosure s
KNOWN: Cylindrical shell with uniform volumetric generation is insulated at inner surface
and exposed to convection on the outer surface.
FIND: (a) Temperature distribution in the shell in terms of ri , ro , q, h, T and k, (b)