A study on the coefficient of discharge through the suction or discharge ports of RV expander

• Main Author: Seow, Ee Hung.
• Other Authors: Ooi Kim Tiow
• Format: Final Year Project
• Language: English
• Published: 2010
• Subjects: DRNTU::Engineering::Mechanical engineering::Fluid mechanics
• Online Access: http://hdl.handle.net/10356/40052
• Institution: Nanyang Technological University

The carbon dioxide (CO2) refrigeration system has seen increasing interest in recent years. Since the ban on conventional ozone-depleting refrigerants, many have proposed CO2 as an alternative refrigerant. But high throttling losses involved in CO2 refrigeration cycle leads to low efficiency of the system. It is suggested that by replacing the conventional expansion valve with an expander, the efficiency of such a system can be greatly improved. Based on the successful RV-i mechanism, the Revolving Vane expander has been introduced and studied. The mathematical model for RV expander has proved that it is highly efficient with exceptionally low frictional losses. In order to further improve its efficiency, the mass flow rate of fluid through the suction and discharge ports of a RV expander must be investigated. The fluid mass flow rate can affect the working pressures inside the expander, which in turn affects the power produced. The actual mass flow rate is different from the ideal mass flow rate which can be easily calculated. The coefficient that relates the theoretical mass flow rate to the actual one is known as the coefficient of discharge (Cd). In this report, the relationships between Cd and various parameters are investigated using CFD simulations. The Cd values are calculated using 4 equations and the results are presented in 6 cases. In case 1, the type and density of meshing done for all the simulations is justified by balancing the accuracy with running time. Case 2 shows Cd increases by 11.5% with increasing pressure difference. Case 3 reveals that Cd is independent of size of opening. Case 4 discloses that increasing depth of port by more than 10 mm has no effect on Cd. Case 5 and 6 prove that a maximum Cd exist when the converging or diverging angles are increased from o to 35 degree. At peak, Cd increases by 18% for converging ports and 16% for diverging ports. The assumptions and limitations of all equations used will be discussed. A comparison of Cd values obtained from different equations will also be made. Based on the results, recommendations are made to optimize the port design.