In a furnace made of bricks, Inside heat transfer coefficient = 35 W/m$^2$K, Outside heat transfer coefficient = 25 W/m$^2$K, Thermal conductivity of bricks (15 cm thick) = 0.15 W/mK. The overall heat transfer coefficient (in W/m$^2$K) will be closer to the
- A. Inside heat transfer coefficient
- B. Outside heat transfer coefficient
- C. Thermal conductivity of bricks
- D. Heat transfer coefficient based on the thermal conductivity of the bricks alone
Answer: D. Heat transfer coefficient based on the thermal conductivity of the bricks alone
Explanation: Resistance $R = 1/h_i + L/k + 1/h_o$. $L/k = 0.15/0.15 = 1$. Convection resistances are small ($<0.04$). The dominant resistance is conduction, so U depends mostly on $k/L$.