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Heat Transfer

Q1: Pick out the wrong statement.

A In process heat exchangers, saturated steam is preferred over the superheated steam.

B The maximum is the emissive power of a surface at a temperature T1 occurs at a wavelength of λ1. If the surface temperature is halved, the maximum in the emissive power would occur at a wavelength of 0.5 λ1.

C When a vertical plate is heated in infinite air environmental under natural convection conditions, the velocity profile in air, normal to the plate, exhibits a maximum.

D A body at 925 K emits an energy of 1.42 x 1011σW/m2 (σ is the Stefan-Boltzman constant) in the wavelength band between 3 μm to 4 μm. The fraction of this energy in the total energy emitted over the entire wavelength range is equal to emissivity.

ANS:B - The maximum is the emissive power of a surface at a temperature T1 occurs at a wavelength of λ1. If the surface temperature is halved, the maximum in the emissive power would occur at a wavelength of 0.5 λ1.

The wrong statement is: "The maximum is the emissive power of a surface at a temperature T1​ occurs at a wavelength of λ1​. If the surface temperature is halved, the maximum in the emissive power would occur at a wavelength of 0.5 1λ1​." This statement is incorrect because the wavelength of maximum emissive power does not depend on the temperature of the emitting surface. According to Wien's displacement law, the wavelength of maximum emissive power is inversely proportional to the absolute temperature of the surface. Therefore, halving the surface temperature would not result in a change in the wavelength of maximum emissive power to 0.5 times the original wavelength.



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