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7 May, 17:55

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?

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  1. 7 May, 18:29
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    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
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