Modeling of Refrigerant Flow Through Flexible Short Tube Orifices

Abstract

Single-phase flow of refrigerants R-22, R-13a, and R-41a through flexible short tube orifices with three different geometries and a range in upstream operating conditions was modeling using the finite element method, FEM. A commercial package, ANSYS with its CFD counterpart, FLOTRAN, was used. Three tube moduli of elasticity, 5513 kPa (800 psi). 7084 kPa (1028 psi), and 9889 kPa (1435 psi) with three different L/D ratios as 5.5, 6.4, and 11.2 were investigated. The predicted mass flow rate values were compared to available published experimental results. The study showed that upon deformation, the short tube resembled the shape of a converging-diverging nozzle. Both tube inlet and outlet had a chamfered-like shape after deformation which reduced the sharp pressure drop at the tube inlet. The less modulus the tube, the larger chamfered-like angle at the inlet. Further, the more flexible tube, the higher pressure drop along the tube due to higher tube contraction. The mass flow rates estimated with the numerical model were 14% over those from the experimental results. The model also indicated a pressure dip due to the contraction in the tube area just downstream of the vena-contracta region. The results illustrate that as the upstream pressure increased by 45%, the tube area deformed by 35-60% related to the tube geometry. The study showed that the more flexible tube restricted the mass flow rate by 2-6% less than the less flexible tube depending on the upstream pressure. The non-linear correlation to estimate the flow rate of a single-phase flow through flexible short tubes was predicted based on the model results. The percentage difference between the predicted mass flow rate and the experimental results was 13%. The tube deformed more in the case of R-410a and less in the case of R-134a compared to R-22 at the same condensing temperature, 46.2 C. R-410a showed a higher mass flow rate than R-22; while R-134a showed less variation in flow rate compared to R-22. This can be attributed to the higher pressure differential in case of R-410a compared to R-134a, and R-22 at the same condensing and evaporating temperatures.