Q.1
The emissivity for a black body is
Q.2
Heat is closely related with
Q.3
The thermal diffusivities for gases are generally
Q.4
Two long parallel surfaces each of emissivity 0.7 are maintained at different temperatures and accordingly have radiation heat exchange between them. It is desired to reduce 75% of the radiant heat transfer by inserting thin parallel shields of emissivity 1 on both sides. The number of shields should be
Q.5
Thermal conductivity of air at room temperature in kcal/m hr °C is of the order of
Q.6
A heat exchanger with heat transfer surface area A and overall heat transfer coefficient U handles two fluids of heat capacities Cmax and Cmin. The number of transfer units (NTU) used in the analysis of heat exchanger is specified as
Q.7
Reynolds number is the ratio of
Q.8
According to Stefan's law, the total radiation from a black body per second per unit area is proportional to
Q.9
The critical temperature is the temperature
Q.10
Thermal conductivity of non-metallic amorphous solids with decrease in temperature
Q.11
If the temperature of a solid surface changes form 27°C to 627°C, then its emissive power changes in the ratio of
Q.12
Kirchhoff's law states that
Q.13
Metals are good conductors of heat because
Q.14
The transfer of heat by molecular collision is smallest in
Q.15
According to Stefan Boltzmann law, the total radiation from a black body per second per unit area is directly proportional to the
Q.16
Cork is a good insulator because it has
Q.17
Pick up the wrong case. Heat flowing from one side to other depends directly on
Q.18
Thermal conductivity of solid metals with rise in temperature normally
Q.19
The logarithmic mean temperature difference (tm) is given by (where Δt1 and Δt2 are temperature differences between the hot and cold fluids at entrance and exit)
Q.20
Emissivity of a white polished body in comparison to a black body is
Q.21
The heat of sun reaches to us according to
Q.22
Thermal diffusivity of a substance is
Q.23
The insulation ability of an insulator with the presence of moisture would
Q.24
In heat exchangers, degree of approach is defined as the difference between temperatures of
Q.25
Fouling factor is used
Q.26
The natural convection air cooled condensers are used in
Q.27
The concept of overall coefficient of heat transfer is used in heat transfer problems of
Q.28
An electric cable of aluminium conductor (k = 240 W/mK) is to be insulated with rubber (k = 0.15 W/mK). The cable is to be located in air (h = 6 W/m²). The critical thickness of insulation will be
Q.29
Which of the following is expected to have highest thermal conductivity?
Q.30
Which of the following has maximum value of thermal conductivity?
0 h : 0 m : 1 s