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Heat Transfer - Engineering

Q1:

Pick out the wrong statement.

A In drying a solid containing moisture above the critical moisture content the number of degrees of freedom is 2.

B Sherwood number in mass transfer corresponds to Nusselt number in heat transfer and Schmidt number to Prandtl number.

C Forced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger.

D Hot gases at moderate pressure are usually in the shell side of shell and tube heat exchangers. At higher pressure, however, it is customary to put gas in the tube side.

ANS:C - Forced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger.

The wrong statement is: "Forced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger." Explanation:

  • The Schmidt number (Sc) is a dimensionless quantity that represents the ratio of momentum diffusivity (kinematic viscosity) to mass diffusivity. It is defined as Sc = ν/D, where ν is the kinematic viscosity and D is the mass diffusivity.
  • In mass transfer, a larger Schmidt number indicates that momentum diffusivity dominates over mass diffusivity, meaning that the fluid has a higher ability to transport momentum compared to the transport of mass.
  • Forced convection is more effective in increasing the rate of heat transfer, not mass transfer. In heat transfer, a larger Prandtl number (Pr) is associated with more efficient heat transfer in forced convection due to the higher ratio of momentum diffusivity to thermal diffusivity. However, this does not directly apply to mass transfer.
  • Therefore, the statement linking forced convection to the effectiveness of mass transfer based on the Schmidt number is incorrect.