Study of High Efficiency Micro Heat Exchanger and Refrigerant Distribution using Throttle Mechanism

对于使用小直径管的多路热交换器，分配器将制冷剂供应到热交换器管的每个通道。制冷剂不均匀会降低热交换器的效率。本文研究了制冷剂在并联小管中的分配规律和制冷剂R-134 a的沸腾换热特性，为紧凑型小管换热器的设计提供了理论依据。分配器连接到并联的蒸发器管，向每个蒸发器供应制冷剂。在蒸发管前端连接的分配器上安装有两块孔板，分别为0.1mm、0.15mm和0.2mm三种喷嘴直径。在蒸发器入口干度小于0. 05、质量流量为100 ~ 250 kg/m^2·s、热通量为4.8 ~ 11.8kW/m^2的工况下进行了实验研究。结果表明：1）喷嘴直径对制冷剂的分配没有影响; 2)随着喷嘴直径的减小，制冷剂流量趋于稳定，这意味着节流减小了制冷剂流量的波动。3)蒸发器中发生的两相振荡流被节流阀抑制。对于节流对沸腾换热的影响，当质量流量为100和150 kg/m^2s时，直径为0.1mm的喷嘴的沸腾换热系数优于直径为0.2mm的喷嘴。而在质量流量为250 kg/m^2s时，直径为0.1mm和0.2mm的沸腾传热系数几乎相同。在低质量流量下，沸腾传热系数得到提高。在以往的研究中，蒸发器内的振荡流动会降低传热系数。在蒸发器入口处设置喉道可以抑制流动不稳定性，提高沸腾换热系数。

For the multi-path heat exchanger using small diameter tubes, a distributor supplies refrigerant to each passage of heat exchanger tubes. No evenness of refrigerant decreases the efficiency of the heat exchanger. In the present study, the distribution of refrigerant to parallel small tubes and the boiling heat transfer for refrigerant R-134a were investigated to provide useful data for the design of compact heat exchangers using small tubes. The distributor, which is connected to the parallel evaporator tubes, supplies refrigerant to each evaporator. Two orifice plates, which are three kinds of nozzle diameters(0.1mm, 0.15mm and 0.2mm), are installed in the distributor connected the front ends of the evaporator tubes. The experimental investigation was performed under the mass flux 100 to 250kg/m^2s, heat flux from 4.8 to 11.8kW/m^2 and inlet refrigerant vapor quality of the evaporator < 0. Results showed that 1) the distribution of refrigerant was not affected by nozzle diameter. 2) the refrigerant flow was steady with decreasing the nozzle diameter, which meant that the fluctuation of refrigerant flow decreased by throttle. 3) two-phase oscillating flow occurred in the evaporator was suppressed by throttle. For the effect of throttle for the boiling heat transfer, when mass fluxes were 100 and 150kg/m^2s, the boiling heat transfer coefficient for the nozzle diameter 0.1mm was better than for the diameter 0.2mm. But, in the case of the mass flux 250kg/m^2s, both the boiling heat transfer coefficients for the diameter 0.1mm and for 0.2mm were almost same values. The boiling heat transfer coefficient was improved at low mass flux. In previous study, it was known that the oscillating flow in the evaporator decreased the heat transfer coefficient. Throat on the entrance of evaporator suppressed flow instability and as a result, the boiling heat transfer coefficient was improved.