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5. Find the heat flow rate
through the composite wall as shown in
Fig. 15.12. Assume one dimensional flow.
kA = 150 W/m°C,
kB = 30 W/m°C,
kC = 65 W/m°C and
kD = 50 W/m°C.
mild steel tank of wall thickness 12 mm contains water at 95°C. The
thermal conductivity of mild steel is 50 W/m°C, and the heat transfer coefficients dfor the inside
and outside the tank are 2850 and 10 W/m2°C, respectively. If the atmospheric temperature is
15°C, calculate :
(i) The rate of heat loss per m2 of the tank surface area ;
(ii) The temperature of the outside surface of the tank.
20 g of gas at 20oC and 1 bar pressure is compressed to 9 bar by the law pV1.4 = C.
Taking the gas constant R = 287 J/kg K calculate the work done. (Note that for a compression
process the work will turn out to be positive if you correctly identify the initial and final
conditions)
A gas is compressed from 120 kPa and 15oC to 800 kPa. Calculate the final temperature when
the process is
i. Isothermal (n=1)
ii. Polytropic (n=1.3)
iii Adiabatic (γ=1.4)
iv. Polytropic (n= 1.5)
a car of mass M is moving with speed v. the brake of mass m and specific heat capacity c, is used to stop the car. if half of the K.E is absorbed by the brake thenwhat is the increase in the temperature of the brake
The interior of a refrigerator having inside dimensions of 0.5 m × 0.5 m
base area and 1 m height, is to be maintained at 6°C. The walls of the refrigerator are constructed
of two mild steel sheets 3 mm thick (k = 46.5 W/m°C) with 50 mm of glass wool insulation (k =
0.046 W/m°C) between them. If the average heat transfer coefficients at the inner and outer
surfaces are 11.6 W/m2°C and 14.5 W/m2°C respectively, calculate :
(i) The rate at which heat must be removed from the interior to maintain the specified
temperature in the kitchen at 25°C, and
(ii) The temperature on the outer surface of the metal sheet.
A furnace wall is made up of three layers of thicknesses 250 mm, 100 mm
and 150 mm with thermal conductivities of 1.65, k and 9.2 W/m°C respectively. The inside is
exposed to gases at 1250°C with a convection coefficient of 25 W/m2°C and the inside surface is
at 1100°C, the outside surface is exposed air at 25°C with convection coefficient of 12 W/m2°C.
Determine :
(i) The unknown thermal conductivity ‘k’ ;
(ii) The overall heat transfer coefficient ;
(iii) All surface temperatures.
An exterior wall of a house may be approximated by a 0.1 m layer of
common brick (k = 0.7 W/m°C) followed by a 0.04 m layer of gypsum plaster (k = 0.48 W/m°C).
What thickness of loosely packed rock wool insulation (k = 0.065 W/m°C) should be added to
reduce the heat loss or (gain) through the wall by 80 per cent?
