Question

# In an experimaental set-up, air flows between two stations P and Q adiabatically. The direction of flow depdns on the pressure and temperature conditions maintained at P and Q. The conditions at P are 150kPa and 35K. The temperature at station Q is 300K. The following are the properties and relations pertaining to air: Specific heat at constant pressure, cp=1.005kG/kgK; Specific heat at constnat volume, cv=0.718kJ/kgK; Characteristic gas constnat, R=0.287kJ/kgK Enthalpy, h=cpT. Internal energy, u=cvT. If the air has to flow from station P to station Q, te maximum possible value of pressure in kPa at station Q is close to

A
50
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B
128
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C
150
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D
87
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Solution

## The correct option is D 87Method I: Since air is flowing rom P to Q, then to get maximum value of pressure at station Q, process must be reversible. cp=1.005kJ/kgK cv=0.718kJ/kgK R=0.287kJ/kgK γ=cpcv=1.0050.718=1.4 For reversible adiabatic process P−Q TQTP=(pQpP)γ−1γ pQ150=(300350)3.5=0.586 or pQ=87.45kPa Method II: Since air is flowing P to Q, then to get maximum value of pressure at station Q. Chnage in entropy, SQ−SP≥0

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