Gas-liquid two-phase flow distributions in multi-pass channels 多分岐構造をもつ流路内の気液二相分配

博士論文

application/pdf

In order to improve the performance of a heat pump system, this research has focused on the improvement of the thermal performance of an evaporator. In particular, the gas-liquid flow distributions in multi-pass channels that simulate a compact evaporator which is currently used for an automobile air-conditioning system and soon will be introduced to other heat pump appliances has been examined experimentally. In this research, the study was divided into two parts. 1. Study on two-phase flow distribution behavior of air-water flow in a multi-pass channel In this study, the influences of several parameters on the two-phase flow distributions in the multi-pass channel have been investigated in an isothermal air-water flow system. The test channel has a horizontal header with a square cross section of 20mm × 20mm and a length of 255mm, and ten upward branches with a length of 200mm were connected to it. The superficial air velocity jG and superficial water velocity jW at the entrance of the header are determine based on the quality and mass flow rate of the refrigerant encountered in a real evaporator. In this study, jG and jW are set equal to the superficial velocities of vapor and liquid of the refrigerant flow (R-134a) in a real evaporator of a automobile air-conditioner, and they are 1.0 - 5.0 m/s and 0.015 - 0.045 m/s, respectively. These velocity conditions correspond to the range of quality x = 0.2 - 0.7 and mass flow rate M = 40 - 160 kg/h in R-134a refrigerant flow in a real evaporator. Special attention was directed to influences of (i) flow-inlet condition at the header entrance (stratified-flow inlet and mist-flow inlet), (ii) pressure condition at the branch outlets (uniform backpressure and non-uniform backpressure), (iii) pressure-loss characteristics of branches (flat tubes and multi-port tubes) on the gas-liquid distribution characteristics. Based on the results of the experiments, he most influenced parameter to the flow distribution uniformity is determined by using the design of experiment method. In addition to the gas-liquid distributions to branches, the pressure distributions in the headers are measured to make clear the pressure condition in a real evaporator. It has been found that the outlet pressure condition of branches exerts great influence on the gas-liquid distributions to branches in the channel with flat tube branches, but it has only minor influence in the channel with multi-port tube branches. The flow-inlet condition at the header entrance has significant influence on the gas-liquid distribution, and the uniformity of the liquid distribution to branches is improved under the mist-flow inlet condition. The pressure in the headers shows uniform distributions in the stream-wise direction, suggesting that the uniform backpressure condition at the branch outlets is appropriate for reproducing the flow in a real compact evaporator with multi-pass channels. The flow distribution uniformity of gas phase is influenced mostly by superficial air velocity, and the flow distribution uniformity of liquid phase is mostly influence by 2-way interaction of parameters which are the flow-inlet condition at the header entrance and the superficial air velocity. 2. Study on similarity of two-phase flow distribution behaviors of real refrigerant (R-134a) flow and air-water flow in a multi-pass channel As a next stage of the study, an experimental study was conducted on the gas-liquid two-phase flow distributions in the multi-pass channel that simulated a compact evaporator in a heat pump system. Attention was directed to the similarity between the refrigerant (R-134a) flow and the air-water flow in upward and continue with downward channel. This to confirm the result on similarity of real refrigerant with air-water flow distribution behavior in upward multi-pass channel is significance in other orientation of multi-pass channel, i.e., in this study shall be the downward multi-pass channel. The body of the test channel was made of transparent PVC to allow the optical access, and the multiport aluminum tubes were used as branches. The horizontal dividing and combining headers with cross sections of 20mm x 20mm were connected by 22 branches with cross sections of 20mm x 2mm, lengths of 120 mm and pitches of 12mm. v At first, concentrate on multi-pass upward channel, the refrigerant two-phase flow was visualized to clarify the flow characteristics in the headers. Then, using exactly the same test channel, the air-water flow in the headers was observed under following four air and water velocity conditions at the dividing header entrance to seek 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. From a comparison of the flow patterns in the dividing header, it was found that the air-water flow under the inlet conditions of the equal kinetic energies and equal Baker's flow pattern map parameters could simulate the refrigerant flow quite 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 refrigerant flow in the combining header revealed that the inlet condition of equal Baker's flow pattern map parameters gave better results than the equal kinetic energy condition. Secondly, using the same research flow and same testing apparatus and method, the multi-pass downward channel was examined. From a comparison of the flow patterns in the dividing header, it was found that the air-water flow under the inlet conditions of the equal kinetic energies and equal superficial velocities of gas and liquid phase of real refrigerant could simulate the refrigerant flow quite closely. Then, based on this result, the air and water distribution ratios in the branches were measured under all four conditions to examine the influence of the flow inlet conditions on the flow distribution characteristics. A close comparison of the air-water distributions and refrigerant flow in the combining header revealed that the inlet condition of superficial velocities of gas and liquid phase of real refrigerant gave better results than the equal kinetic energy condition. Thirdly, to confirm the result in downward test channel, the inlet pipe has been swaged at the end of it to be a smaller inner diameter, i.e., 1.5mm, as a new parameter for further study on this flow distribution behavior similarities between real refrigerant and air-water flow. Also as a counter measure to improve the flow distribution uniformity. From the result of comparing the qualitative result of real refrigerant visual observation and air-water flow distribution ratio measurement, it was found that the inlet condition of air-water flow that equal to superficial velocities of those in real refrigerant are the one that could simulated the real refrigerant flow distribution Summary vibehavior in multi-pass downward channel. However, to fit both upward and downward oriented multi-pass channel flow distribution of liquid phase flow simultaneously, it is better and safer to use the inlet condition that equal to kinetic energies of real refrigerant. Thus, these studies and new conclusions are able to make the development of new compact evaporators with higher thermal performance, more efficient and faster.

三重大学大学院工学研究科博士後期課程システム工学専攻

23, 148p