| Summary: | 碩士 === 雲林科技大學 === 機械工程系碩士班 === 98 === Flow distribution from a header to parallel channels in heat exchangers is widely used in the conventional heat exchangers, recently also utilized on compact and micro/electronic systems for cooling. Normally, the cooling fluid flows into the header, and distribute to the parallel flow channels, then the flows from each small channel merge into the outlet header for going out of the heat exchanger. The flow rates of single-phase distribution through the channels are often not even. The complex issue of flow distribution in parallel flow heat exchangers had been examined by several authors. The difficulties of obtaining uniform flow distribution were confirmed. Many parameters, such as flow conditions, geometric structure, orientation of header and channels, affect the flow distribution, and definitely more data are needed on this subject. Particularly, the effect of the flow disturbance generated by the baffle plate or tube with tiny holes for even flow distribution has not been well investigated.
The object of this study is to study the single-phase flow into parallel flow heat exchangers with inlet and outlet square headers and 9 circular tubes by experimentally and numerically simulating the flow through parallel flow heat exchangers. The flow rates for the 9 channels are calculated by the measured pressure drop across each tube. The effects of the flow conditions, the diameter of the parallel channels and header sizes, the Z and U flow directions through the heat exchanger, as well as the baffle plate or tube with tiny holes are investigated.
The results find the jet flow induced at the header inlet with eddy flows circulated at the sides of the expanded jet flow. The jet flow is pushed by its momentum through the header. Thus, the flow rates for the channels near the end of the header increased, while the flow rates to the channels closed to the inlet decreased. The flow distribution for the U flow direction is more even than the Z flow direction since the differences of the pressure drop within each channel for the 9 parallel channels are smaller for U flow direction than Z flow direction. For improving the flow distribution, different sizes of baffle plate and tube have been installed in the inlet header for testing. The results indicate that the baffle tube has better flow distribution than the baffle plate since the expansion effect through the baffle tube is less than the baffle plate as observed from tests and numerical simulation. For adjusting the hole size at the baffle tube, the flow distribution could be more even from analysis for obtaining a better flow distribution on the design of parallel flow heat exchanger.
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