Heat Transfer
The thermal efficiency of a reversible heat engine operating between two given thermal reservoirs is 0.4. The device is used either as a refrigerator or as a heat pump between the same reservoirs. Then the coefficient of performance as a refrigerator (COP)R and the co-efficient of performance as a heat pump (COP)HP are

(COP)R = 2.5; (COP)HP = 1.5
(COP)R = (COP)HP = 0.6
(COP)R = 1.5; (COP)HP = 2.5
(COP)R = (COP)HP = 2.5

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Heat Transfer
Nusselt number is the ratio of the

Temperature difference to the temperature gradient at the wall
Heat flux at the wall to that across the entire pipe
None of these
Temperature gradient of the wall to that across the entire pipe

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