Question

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?

Answer 1

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

Step-by-step 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