Question

Refrigerant-134a at 700 kPa and 120°C enters an adiabatic nozzle steadily with a velocity of 20 m/s and leaves at 400 kPa and 30°C. Determine (a) the exit velocity and (b) the ratio of the inlet to exit area A₁/A₂.

Answer 1

R-134a is accelerated in a nozzle from a velocity of 20 m/s. The exit velocity of the refrigerant and the ratio of the inlet to exit area of the nozzle are to be determined.

Assumptions:

1. This is a steady-flow process since there is no change with time. 
2. Potential energy changes are negligible.
3. There are no work interactions.
4. The device is adiabatic and thus heat transfer is negligible.

Properties: From the refrigerant tables (Table A-13)

 Analysis:

(a) There is only one inlet and one exit, and thus \(\overset •m_1 = \overset •m_2 = \overset •m\). We take nozzle as the system, which is a control volume since mass crosses the boundary. The energy balance for this steady-flow system can be expressed in the rate form as

(b) The ratio of the inlet to exit area is determined from the conservation of mass relation,