Heat Transfer
The Fourier number (defined as a.t/L²) is used in the analysis of problem involving heat transfer by

Forced convection
Transient conduction
Natural convection
Steady state conduction

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

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Heat Transfer
Air is to be heated by condensing steam. Two heat exchangers are available (i) a shell and tube heat exchanger and (ii) a finned tube heat exchanger. Tube side heat transfer area are equal in both the cases. The recommended arrangement is

Finned tube heat exchanger with air inside and steam outside
Shell and tube heat exchanger with air inside tubes and steam on shell side
Finned tube heat exchanger with air outside and steam inside
Shell and tube heat exchanger with air on shell side and steam inside tubes

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