6. A heat pipe is widely used because –
A. acts as an insulator
B. act as conductor and insulator
C. acts as a superconductor
D. acts as a fin
7. A counterflow heat exchanger is used to heat water from 20 °C to 80 °C by using hot exhaust gas entering at 140 °C and leaving at 80 °C. Find the log mean temperature difference for the exchanger is?
A. 60 °C
B. 80 °C
C. 110 °C
D. 120 °C
8. Solar radiation of 1200 W/m² falls perpendicularly on a gray opaque surface of emissivity 0.5. If the surface temperature is 50 °C and surface emissive power is 600 W/m², the radiosity of that surface will be?
A. 600 W/m²
B. 800 W/m²
C. 1200 W/m²
D. 1500 W/m²
9. Which one of the following heat exchangers gives a parallel straight-line pattern of temperature distribution for both cold and hot fluid?
A. Parallel flow with unequal heat capacities
B. Parallel flow with equal heat capacities
C. Counterflow with unequal heat capacities
D. Counterflow with equal heat capacities
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10. The average Nusselt number in laminar natural convection from a vertical wall at 180 °C with still air at 20 °C is found to be 48. If the wall temperature becomes 30 °C, all other parameters remaining the same, the average Nusselt number will be?