Question

Refrigerant-134a enters a diffuser steadily as saturated vapor at 800 kPa with a velocity of 120 m/s, and it leaves at 900 kPa and 40°C. The refrigerant is gaining heat at a rate of 2 kJ/s as it passes through the diffuser. If the exit area is 80 percent greater than the inlet area, determine (a) the exit velocity and (b) the mass flow rate of the refrigerant.

Answer 1

R-134a is decelerated in a diffuser from a velocity of 120 m/s. The exit velocity of R-134a and the mass flow rate of the R-134a 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

Properties:

From the R-134a tables (Tables A-11 through A-13)

Analysis:

(a) There is only one inlet and one exit, and thus \(\overset • m_1 = \overset • m_2 = \overset • m\). Then the exit velocity of R-134a is determined from the steady-flow mass balance to be

(b) We take diffuser 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