Refrigerant distribution effects on the performance of microchannel evaporators.
The performance of air to refrigerant heat exchangers can be affected greatly by both refrigerant and air flow distribution. Maldistribution of either fluid stream can lead to reduced heat transfer effectiveness of the heat exchanger and thus lower system efficiency and capacity. Microchannel evaporators are especially susceptible to refrigerant maldistribution as they typically have many parallel channels fed by a common header or manifold. This paper outlines general trends in refrigerant flow distribution in such evaporators taking into account various heat exchanger orientations. In addition, links between heat exchanger performance and refrigerant flow distribution characteristics in microchannel evaporators are made using both analytical and experimental methods. These results show that unification of the liquid refrigerant flow distribution in the evaporator can lead to quantifiable improvements in evaporator performance.
Two-phase flow distribution of air–water annular flow in a parallel flow heat exchanger
Abstract The air and water flow distribution are experimentally studied for a round header – flat tube geometry simulating a parallel flow heat exchanger. The number of branch flat tube is 30. The effects of tube outlet direction, tube protrusion depth as well as mass flux, and quality are investigated. The flow at the header inlet is identified as annular. For the downward flow configuration, the water flow distribution is significantly affected by the tube protrusion depth. For flush-mounted configuration, most of the water flows through frontal part of the header. As the protrusion depth increases, more water is forced to the rear part of the header. The effect of mass flux or quality is qualitatively the same as that of the protrusion depth. Increase of the mass flux or quality forces the water to rear part of the header. For the upward flow configuration, however, most of the water flows through rear part of the header. The protrusion depth, mass flux, or quality does not significantly alter the flow pattern. Possible explanations are provided based on the flow visualization results. Negligible difference on the water flow distribution was observed between the parallel and the reverse flow configuration.
Distribution of two-phase annular flow at header–channel junctions
Abstract The main objective of this work is to examine the distribution of two-phase annular flow at header–channel junctions simulating the corresponding parts of compact heat exchangers. The parameters considered were mass quality at the inlet of the vertical header and intrusion depth of the horizontal channels into the header wall. The cross-section of the header and the channels were fixed to 24 mm × 24 mm and 22 mm × 1.8 mm, respectively. Three different intrusion depths (0, 6 and 12 mm beyond the inner wall of the header) were tested for the mass flux and the mass quality ranges of 54–134 kg/m 2  s and 0.2–0.5, respectively. Air and water were used as the test fluids. With zero intrusion-depth, less amount of liquid was separated out through the channels at the rear part. However, with the deeper intrusion (6 and 12 mm), the trend was reversed. The deeper intrusion hindered the liquid to flow into the channels installed at the fore part of the header. Finally, a uniform liquid-flow distribution through the parallel channels could be achieved simply by adjusting the intrusion depth.