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This quiz contains multiple-choice problems on conduction through various mechanisms, including plane wall, composite wall, cylindrical wall and a sphere, shape factor, variable conductivity effect, and critical insulation thickness.
The value of critical radius in case of a cylindrical hollow object is
Chose the correct statement with respect to the critical radius of insulation.
There is more heat loss, i.e., conductive
There occurs a decrease in heat flux
Heat loss increases with addition of insulation
Heat loss decreases with addition of insulation
A heat exchanger shell of outer radius 15 cm is insulated with glass wool of thermal conductivity 0.0825 W/m degree. The surface temperature is 280 °C and it is assumed to remain constant after the insulation layer is applied to the shell. The convective film coefficient between the glass wool’s outside surface and the surrounding air is estimated to be 8 W/m^2 degree. What is the value of the critical radius?
9.31 mm
10.31 mm
11.31 mm
12.31 mm
For spherical objects, the value of critical radius would be
2k/3h
3k/h
2k/h
k/h
The maximum value of critical radius is
0.01 m
0.04 m
0.06 m
0.0001 m
An electric aluminium cable (k = 240 W/m degree) is insulated with rubber (k = 6 W/m^2 degree). If the cable is in air (h = 6 W/square meter degree), determine its critical radius.
80 mm
160 mm
40 mm
25 mm
A wire of radius 3 mm and 1.25 m length is maintained at 60 °C by insulating it with a material having a thermal conductivity of 0.175 W/m K. The surrounding temperature is 20 °C with heat transfer coefficient of 8.5 W/ m2 K. What is the percentage increase of heat loss due to insulation?
134.46 %
124.23 %
100.00 %
12.55 %
A pipe of outer diameter 20 mm is insulated with asbestos which has a mean thermal conductivity of 0.1 W/m degree. The local coefficient of convective heat to the surroundings is 5 W/m^2 degree. What is the critical radius of insulation for optimum heat transfer from a pipe?
10 mm
20 mm
30 mm
40 mm
For insulation to be adequately effective in restricting heat transmission, the pipe radius r{0} should be
Greater than critical radius
Less than critical radius
Equal to critical radius
Greater than or equal to critical radius
The heat conduction equation in terms of Cartesian coordinates is expressed as
d^(2)t/dx^2 + d^(2)t/dy^2 + d^(2)t/dz^2 + qg = (1/α)(dt/dT)
2*d^(2)t/dx^2 + d^(2)t/dy^2 + d^(2)t/dz^2 + 34qg = (dt/dT)
d^(2)t/dx^2 + 3*d^(2)t/dy^2 + d^(2)t/dz^2 = (1/α)(dt/dT)
4*d^(2)t/dx^2 + d^(2)t/dy^2 + d^(2)t/dz^2 + (1/2)qg = (1/α)(dt/dT)
The temperature distribution in a large thin plate with uniform surface temperature will be (assuming the plate is undergoing steady-state conduction)
Logarithmic
Hyperbolic
Parabolic
Linear
Consider two walls of equal thickness and cross-sectional area whose thermal conductivities are in the ratio 1/2 and the temperature difference is the same across the wall faces. The ratio of heat flow will be
1
1/2
2
4
An oven’s interior is maintained at a temperature of 850 °C using suitable control apparatus. The oven walls are 500 mm thick and fabricated from a material of thermal conductivity 0.3 W/m degree. For an outside wall temperature of 250 °C, determine the resistance to heat flow.
0.667 degree/W
1.667 degree/W
2.667 degree/W
3.667 degree/W
In the case of a homogeneous plane wall, the linear temperature distribution is expressed as
t = t{1} + (t{2} - t{1}) δ/x
t = t{2} - (t{2}-t{1}) x/δ
t = t{1} + (t{2} - t{1}) x
t = t{1} + (t{2}-t{1}) x/δ
A cable of 10 mm is laid outside at an atmosphere of 25 °C (h = 12.5 W/m^2 degree), and its surface temperature is likely to be 75 °C due to the heat generated. How would the heat flow from the cable be affected if it is insulated with rubber having thermal conductivity k = 0.15 W/m degree?
43.80 W/m
53.80 W/m
63.80 W/m
73.80 W/m
