11. Hot coffee in a cup is allowed to cool. Its cooling rate is measured and found to be greater than the value calculated by conduction, convection and radiation measurement. The difference is due to?
A. properties of coffee changing with temperature
B. currents of airflow in the room
C. underestimation of the emissivity of coffee
12. Heisler charts are used to determine transient heat flow rate and temperature distribution when-
A. Solids possess an infinitely large thermal conductivity
B. Internal conduction resistance is large and the convective resistance is small
C. Internal conduction resistance is small and the convective resistance is large
D. Both conduction and convection resistance are almost of equal significance
13. After expansion from a gas turbine, the hot exhaust gases are used to heat the compressed air from a compressor with the help of a cross-flow compact heat exchanger of 0.8 effectiveness. What is the number of transfer units of the heat exchanger?
14. What is the shape factor of a hemispherical body placed on a flat surface with respect to itself?
15. Heat is conducted through a 10 cm thick wall at the rate of 30 W/m² when the temperature difference across the wall is 10 °C. Find the thermal conductivity of the wall?
A. 0.03 W/mK
B. 0.3 W/mK
C. 0.003 W/mK
D. 3 W/mK