Heat and Mass Transfer
When heat is transferred from hot body to cold body, in a straight line, without affecting the intervening medium, it is referred as heat transfer by

Radiation
Conduction and convection
Conduction
Convection

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Heat and Mass Transfer
The heat transfer by conduction through a thick cylinder (Q) is given by (where T₁ = Higher temperature, T₂ = Lower temperature, r₁ = Inside radius, r₂ = Outside radius, l = Length of cylinder, and k = Thermal conductivity)

Q = [2π (T₁ - T₂)]/2.3 lk log (r₂/r₁)
Q = [2πlk (T₁ - T₂)]/2.3 log (r₂/r₁)
Q = 2.3 log (r₂/r₁)/[2πlk (T₁ - T₂)]
Q = = 2πlk/2.3 (T₁ - T₂) log (r₂/r₁)

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

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Heat and Mass Transfer
Fourier's law of heat conduction is (where Q = Amount of heat flow through the body in unit time, A = Surface area of heat flow, taken at right angles to the direction of heat flow, dT = Temperature difference on the two faces of the body, dx = Thickness of the body, through which the heat flows, taken along the direction of heat flow, and k = Thermal conductivity of the body)

(dT/dx)
k. (dx/dT)
k.
k.
k. (dT/dx)
(dx/dT)

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