Heat and Mass Transfer
Two balls of same material and finish have their diameters in the ratio of 2: 1 and both are heated to same temperature and allowed to cool by radiation. Rate of cooling by big ball as compared to smaller one will be in the ratio of

0.16736111111111
0.043055555555556
0.042361111111111
0.084027777777778

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Heat and Mass Transfer
Film coefficient is defined as the ratio of

Temperature drop through the films of fluids to the thickness of film of fluids
Thermal conductivity to the equivalent thickness of the film of fluid
Thickness of film of fluid to the temperature drop through the films of fluids
Thickness of film of fluid to the 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 = (Δt1 - Δt2)/ loge (Δt1/Δt2)
tm = loge (Δt1 - Δt2)/ Δt1/Δt2
tm = tm = (Δt1 - Δt2) loge (Δt1/Δt2)
tm = loge (Δt1/Δt2)/ (Δt1 - Δt2)

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