Pressure drop and flow behaviour of refrigerant two phase flow in a smooth hairpin

Abstract

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.

Two-phase flow of refrigerant R134a in a hairpin was studied. The hairpin consists of a smooth tube with an inner diameter of 8mm with a U-bend radius of 11mm. Because of the forces exerted on the flow in the U-bend the flow needs to develop again downstream of the U-bend. The effects of this phenomenon on the pressure drop are studied and linked to a visual observation of the flow. Pressure drop and videos of the flow are recorded in the straight sections upstream and downstream of the U-bend. Afterwards they are compared to the flow and pressure drop for developed flow. The pressure drop downstream of the U-bend was found to be consistently higher than that for developed flow. It exceeded the pressure drop in case of developed flow by an average of 30% for all data points. Each video of the flow behaviour was reduced to a single image by calculating the standard deviation of the time signal per pixel in time. The standard deviation profiles were compared in order to quantitatively evaluate the change in the flow behaviour. For the flow downstream of the U-bend a disturbance was observed that stretches out over more than 30 diameters downstream of the U-bend.