01-05-2012, 10:23 AM
HEAT CONDUCTION EQUATION
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Question 1
Starting with an energy balance on a volume element, Derive the two dimensional transient heat
conduction equation in rectangular coordinates for T(x,y,z) for the case of constant thermal
conductivity and no heat generation.
STEADY HEAT CONDUCTION
Question 1
A wall is constructed of two layers of 1.5cm thick sheetrock (k=0.17W/m.K) which is a
plasterboard made of two layers of heavy papers separated by a layer of gypsum, placed 18 cm
apart. The space between the sheetrock is filled with fiberglass insulation (k=0.035W/m.K).
Determine:
a) The thermal resistance of the wall
Figure 1
Question 2
The wall of refrigerator is constructed of fiberglass insulation (k=0.035W/m.K) sandwiched
between two layers of 1mm thick sheet metal (k=15.1W/m.K). The refrigerated space is
maintained at 2oC and the average heat transfer coefficient at the inner and the outer surfaces of
the wall are 4W/m2.K and 9W/m2.K respectively. The kitchen temperatures averages 24oC. It is
observed that condensation occurs on the outer surfaces of the refrigerator when the temperature
of the outer surfaces drops to 20oC. Determine the minimum thickness of fiberglass insulation
that needs to be used in the wall in order to avoid condensation on the outer surfaces.
FUNDAMENTALS OF CONVECTION
Question 2
Consider fluid flow over a surface with a velocity profile given as:
u(y)=100(y + 2y2 - 0.5y3 )m/ s
Determine the shear stress at the wall surface, if the fluid is
a) Air at 1 atm at 20oC
b) Liquid water at 20oC
EXTERNAL FORCED CONVECTION
Question 1
The local atmospheric pressure in Genting Highland is 83.4kPa. Air at this pressure and at 30oC
flows with a velocity of 6m/s over a 2.5m x 8m flat plate whose temperature is 120oC. Determine
the rate of heat transfer from the plate if the air flows parallel to the:
a) 8m long side
b) 2.5m m side
Question 2
A stainless steel ball (ρ=8055kg/m3, cp=480J/kg.K) of diameter D=15cm is removed from the
oven at a uniform temperature of 350oC. The ball is then subjected to the flow of air at 1 atm
pressure and 30oC with a velocity of 6m/s. The surface temperature of the ball eventually drops
to 250oC.
a) Determine the average convection heat transfer coefficient during this cooling process.
b) Estimate how long this process has taken