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
A. ACmin /U
B. U/ACmin
C. AUCmin
D. AU/Cmin
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Reynolds number is the ratio of
A. Energy transferred by convection to that by conduction
B. Kinematic viscosity to thermal diffusivity
C. Inertia force to viscous force
D. None of the above
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If the temperature of a solid surface changes form 27°C to 627°C, then its emissive power changes in the ratio of
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Kirchhoff's law states that
A. The total radiation from a black body per second per unit area is directly proportional to the fourth power of the absolute temperature
B. The wave length corresponding to the maximum energy is proportional to the absolute temperature
C. The ratio of the emissive power and absorptive power of all bodies is the same and is equal to the emissive power of a perfectly black body
D. None of the above
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Metals are good conductors of heat because
A. Their atoms collide frequently
B. Their atoms are relatively far apart
C. They contain free electrons
D. They have high density
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Thermal conductivity of water at 20°C is of the order of
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Thermal conductivity of air at room temperature in kcal/m hr °C is of the order of
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According to Stefan's law, the total radiation from a black body per second per unit area is proportional to
A. Absolute temperature
B. T2
C. T5
D. T4
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The critical temperature is the temperature
A. Below which a gas does not obey gas laws
B. Above which a gas may explode
C. Below which a gas is always liquefied
D. Above which a gas will never liquefied
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Thermal conductivity of non-metallic amorphous solids with decrease in temperature
A. Increases
B. Decreases
C. Remain constant
D. May increase or decrease depending on temperature
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Thermal conductivity of air with rise in temperature
A. Increases
B. Decreases
C. Remain constant
D. May increase or decrease depending on temperature
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Thermal conductivity of water in general with rise in temperature
A. Increases
B. Decreases
C. Remain constant
D. May increase or decrease depending on temperature
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Heat is closely related with
A. Liquids
B. Energy
C. Temperature
D. Entropy
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The thermal diffusivities for gases are generally
A. More than those for liquids
B. Less than those for liquids
C. More than those for solids
D. Dependent on the viscosity
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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
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Reynolds number (RN ) is given by (where h = Film coefficient, l ρ p = Specific heat at constant pressure and μ
A. R N = k h l
B. R N = k μ c p
C. R N = μ ρ V l
D. R N = t c p V 2
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An ordinary passenger aircraft requires a cooling system of capacity.
A. 2 TR
B. 4 TR
C. 8 TR
D. 10 TR
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A cube at high temperature is immersed in a constant temperature bath. It loses heat from its top, bottom and side surfaces with heat transfer coefficients of h1 , h2 and h3 respectively. The average heat transfer coefficient for the cube is
A. h 1 + h 2 + h 3
B. ( h 1 h 2 h 3 ) 3 1
C. h 1 1 + h 2 1 + h 3 1
D. None of these
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Moisture would find its way into insulation by vapour pressure unless it is prevented by
A. High thickness of insulation
B. High vapour pressure
C. Less thermal conductivity insulator
D. A vapour seal
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The ratio of surface convection resistance to the internal conduction resistance is known as
A. Grashoff number
B. Biot number
C. Stanton number
D. Prandtl number
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