Experimental study on gas–liquid flow distributions in upward multi-pass channels—Comparison of R-134a flow and air–water flow

An experimental study was conducted on the gas–liquid two-phase flow distributions in a multi-pass channel that simulated a compact evaporator in a heat-pump system, with a focus on the similarity between the refrigerant (R-134a) flow and the air–water flow. The body of the test channel was made of transparent polyvinyl chloride to allow optical access, and multiport aluminum tubes were used as branches. Horizontal dividing and combining headers with cross sections of 20 mm × 20 mm were connected by 22 upward branches with cross sections of 20 mm × 2 mm, lengths of 120 mm, and pitches of 12 mm. Initially, the refrigerant two-phase flow was visualized to clarify the characteristics of the flow in the headers. Then, using the same test channel, the air–water flow in the headers was observed under four air and water velocity conditions at the dividing header entrance to investigate the similarity with the flow pattern of the refrigerant flow: (i) superficial gas and liquid velocities equal to those of the refrigerant flow, (ii) equal kinetic energies, (iii) equal quality and mass flow rate, (iv) equal Baker's flow pattern map parameters. A comparison of the flow patterns in the dividing header revealed that the air–water flow under the inlet conditions of the equal kinetic energies and equal Baker's flow pattern map parameters simulated the refrigerant flow closely. Then, based on this result, the air and water distribution ratios in the branches were measured under these two conditions to examine the influence of the flow inlet conditions on the flow-distribution characteristics. A close comparison of the air–water distributions and the refrigerant flow in the combining header revealed that the inlet condition of equal Baker's flow pattern map parameters yielded better results than the equal-kinetic energy condition.