Question

find the expression for rate of flow of heat per unit area through a compostie slab mado of different material ?​

Answer 1

Explanation:

Heat flows through a composite slab, as shown. The depth of the slab is 1 m. The k values are in W/mK,

To find the overall thermal resistance in K/W

Solution:

There is no variation in the horizontal direction. Therefore, we consider a 1 m

deep and 1 m high portion of the slab, since it representative of the entire wall. Assuming any cross-section of the slab normal to the x - direction to be isothermal, the thermal resistance network for the slab is shown in the figure.

The corresponding thermal resistance network is as shown in above fig(ii).

The individual thermal resistance is, (substitute values from above fig)

R1=K1A1L1⟹R1=0.02(1×1)0.5⟹R1=25

Similarly we will calculate the other two thermal resistance value

R2=K2A2L2⟹R2=0.10(0.5×1)0.25⟹R2=5

R3=K3A3L3⟹R3=0.04(0.5×1)0.25⟹R3=12.5

Now R2 and R3 are in parallel.

So the equivalent resistance becomes.