Experimentation and correlation of R744 two-phase flow through short tubes

Abstract

Refrigerant-744 flow through a short-tube orifice was investigated. The flow of R744 through a short-tube orifice is greatly different from that of a conventional refrigerant such as R134a. R744 flow depending upon upstream pressure, upstream superheating, downstream pressure, and short-tube geometries were examined. A correlation for mass flow rate prediction was developed based on a large amount of experimental data. Ten sharp-edged short tubes and three chamfered tubes with lengths between 8.02 and 25.42 mm and diameters between 0.83 and 1.53 mm were tested to investigate the flowing characteristics of R744. The dependence upon upstream temperature was examined for condensing temperatures ranging from 35.3 to 45.5 °C. The effects of upstream pressure and downstream pressure were tested respectively. The high pressure was varied from 7.55 to 10.24 MPa, and the low pressure ranged from 3.03 to 5.02 MPa. A specially designed short tube was used in the tests to study the flow inside the tube. Experimental results indicated that choked flow existed in all cases. The R744 flow was strongly affected by the diameter of the tube; it was proportional to the upstream pressure but was insensitive to the inlet and exit chamfering.