Heat and Mass Transfer
Thermal diffusivity is a

All of these
Dimensionless parameter
Function of temperature
Physical property of a substance

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Heat and Mass Transfer
According to Prevost theory of heat exchange

All bodies above absolute zero emit radiation
Heat transfer by radiation requires no medium
Heat transfer in most of the cases takes place by combination of conduction, convection and radiation
It is impossible to transfer heat from low temperature source to t high temperature source

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Heat and Mass Transfer
Which of the following would lead to a reduction in thermal resistance?

In convection, stirring of the fluid and cleaning the heating surface.
All of these
In radiation, increasing the temperature and reducing the emissivity.
In conduction, reduction in the thickness of the material and an increase in thermal conductivity.

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Heat and Mass Transfer
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)

tm = tm = (Δt1 - Δt2) loge (Δt1/Δt2)
tm = (Δt1 - Δt2)/ loge (Δt1/Δt2)
tm = loge (Δt1/Δt2)/ (Δt1 - Δt2)
tm = loge (Δt1 - Δt2)/ Δt1/Δt2

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