This case study models a capillary tube used as an expansion device in vapor-compression refrigeration systems with refrigerant R22. Using Flownex, the mass flow through a 400 mm long, 2 mm diameter tube was simulated with given inlet conditions (1990 kPa, 43 ºC) and outlet pressure (680 kPa). Flownex accurately calculated two-phase flow, including choking at the tube exit, with a mass flow of 0.0467428 kg/s.
CHALLENGE
The objective of the simulation is to determine the mass flow through the capillary tube.
BENEFITS
This case study demonstrates the modeling of a capillary tube. Capillary tubes are used as expansion devices on some vapor-compression refrigeration systems. The refrigerant used in this case study is R22.
SOLUTION
Flownex was used to determine the mass flow of R22 through a capillary tube given the inlet pressure and temperature and the outlet pressure. This case study demonstrated Flownex’s ability to calculate two-phase flow in a tube, including choking at the exit.
INTRODUCTION
This case study demonstrates the modeling of a capillary tube. Capillary tubes are used as expansion devices on some vapor-compression refrigeration systems. The refrigerant used in this case study is R22.
SYSTEM DESCRIPTION
A tube with a length of 400 mm and an inside diameter of 2 mm is considered. The fluid is R22 with an inlet pressure of 1990 kPa and an inlet temperature of 43 ºC. The outlet pressure is 680 kPa.
OBJECTIVE OF SIMULATION
The objective of the simulation is to determine the mass flow through the capillary tube.
FLOWNEX MODEL
The Flownex model of the tube is shown in Figure 1.
Element 2 is a Restrictor with Discharge coefficient (Flownex’s RD element). This type of element is used to model the exit losses, which can be very large. RD elements can model exit losses accurately, even in the case of choked two-phase flow.
DESCRIPTION OF SIMULATION
A pressure and temperature boundary condition is specified at node 1 while only a pressure boundary condition is specified at node 2.
RESULTS
Flownex calculates the mass flow as 0.0467428 kg/s. Figure 2 shows the variation of pressure and Figure 3 shows the variation of quality over the length of the tube. The large drop in pressure at the exit indicates that the flow is choked at the exit.
CONCLUSION
Flownex was used to determine the mass flow of R22 through a capillary tube given the inlet pressure and temperature and the outlet pressure. This case study demonstrated Flownex’s ability to calculate two-phase flow in a tube including choking at the exit.