Consider a wall that consists of two layers, A and B, with the following values: a. kA = 0.7 W/m.ºC, b. LA = 6 cm, c. kB = 0.25 W/m.ºC, d. LB = 3 cm. If the temperature drop across the wall is 21ºC, what is the rate of heat transfer through the wall per unit area of the wall?

rate of heat transfer through wall per unit area of the wall is 102 W/m²

Explanation:

We know that the thermal resistance of a composite wall withe two layers, is equal to the sum of the resistance of each wall:

R = R1 + R2

where,

R = total resistance of composite wall

R1 = resistance of layer A = LA/KA (Area) = (0.06 m) / (0.7 W/m. °C) (Area)

R1 = (0.08571 W/°C) / (Area)

R2 = resistance of layer B = LB/KB (Area) = (0.03 m) / (0.25 W/m. °C) (Area)

R2 = (0.12 W/°C) / (Area)

therefore,

R = (0.08571 W/°C) / (Area) + (0.12 W/°C) / (Area m²)

R = (0.20571 W/°C) / (Area)

Now, we know that:

R = ΔT/Q

where,

R = total thermal resistance = (0.20571 W/°C) / (Area m²)

ΔT = Temperature drop = 21° C

Q = Heat transfer

Therefore,

(0.20571 W/°C) / (Area m²) = 21°C/Q

Q/Area m² = q = 102 W/m²

q = 102 W/m²