Heat and Mass Transfer
A grey body is one whose absorptivity

Does not vary with temperature and. wavelength of the incident ray
Varies with temperature
Varies with wavelength of the incident ray
Is equal to its emissivity

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Heat and Mass Transfer
Thermal conductivity of a material may be defined as the

Quantity of heat flowing in one second through one cm cube of material when opposite faces are maintained at a temperature difference of 1°C
All of these
Heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces
Quantity of heat flowing in one second through a slab of the material of area one cm square, thickness 1 cm when its faces differ in temperature by 1°C

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

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