**21. If the temperature of a solid surface change from 27 °C to 627 °C, then find the ratio of its emissive power change?**

A. 6:1

B. 9:1

C. 27:1

D. 81:1

**22. When a liquid flows through a tube with sub-cooled or saturated boiling, what is the process called?**

A. Pool Boiling

B. Bulk Boiling

C. Convection Boiling

D. Forced Convection Boiling

**23. A wall having to comprise two layers, i.e. layer A and layer B. Find the ratio of the thickness of two layers?**

**(Given :- Ta = 1325 °C, Tb = 1200 °C, Length (L) = 300 mm, K _{a}/K_{b} = 2, Ambient Temp. = 25 °C)**

A. 0.213

B. 0.845

C. 0.559

D. 0.15

**24. A steel plate of thickness 5 cm and thermal conductivity 20 W/mK is subjected to a uniform heat flux of 800 W/m² on one surface A and transfers heat by convection with a heat transfer coefficient of 80 W/m²K from the other surface B into the ambient air at 25 °C. The temperature of the surface B transferring heat by convection is –**

A. 25 °C

B. 35 °C

C. 45 °C

D. 55 °C

**25. Consider two infinitely long black body concentric cylinders with a diameter ratio D _{2}/D_{1} = 3. Find the shape factor for the outer cylinder with itself?**

A. 0

B. 1

C. 2/3

D. 1/3