| Summary: | 碩士 === 國立雲林科技大學 === 機械工程系碩士班 === 99 === There are several heat exchangers frequently applied in thermal management field by using inlet header to distribute working fluid into parallel subchannels for increasing heat exchange area, and then collected all the fluid form parallel channels by an outlet header to exit the heat exchanger. These parallel flow heat exchangers provide a high thermal performance, thus, they are widely utilized in very high demands of heat transfer applications, such as boilers, cooling system of nuclear reactor, solar energy flat-plate collector and automobile cooling system. The flow distribution in these heat exchangers is often not uniform. For a two-phase flow, the flow distribution appears even more complicated. The two-phase flow maldistribution in parallel subchannels of heat exchanger greatly reduces the thermal and hydraulic performances, thus, the object of this study is to analyze the two phase flow distribution in parallel flow heat exchangers. The influences of flow orientation (upstream flow, downstream flow, and horizontal flow), inlet pipe direction to the header (U-type and Z-type), two-phase flow structure in header, total mass flow rate and gas quality were taken as parameters in this study. Air-water mixture flow was used as the working fluid to simulate the two-phase flow into the parallel flow heat exchanger consisted of 9 glass round tubes with a diameter of 3mm as parallel subchannels and inlet/outlet headers were made of transparent acrylic plates with cross section area of 12 x 12 mm for flow visualization.
The results indicate that phase separation of air-water two-phase flow occurs in the header due to the density difference between air and water, meanwhile, the jet flow happens in the header as well because of abrupt flow area change from the inlet tube connecting to the header. The jet flow effect raises the phase separation phenomenon of air-water two-phase flow that makes the flow distribution in parallel channels more unequal, even some parallel channels appear full of water and some of them with very few water flow rate. Also, the influence of gravity affects the flow structure in the header. All of the air and water flow distributions give the better distribution in low gas quality except for the air flow distribution in downstream orientation.
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