Flow Characteristics In Microchannels With Abrupt Cross Section

In recent years,with the development of science and technology and the development trend of high-power miniaturization of electronic equipment,the technical requirements for cooling and heat dissipation of electronic equipment are becoming higher and higher.Microchannel heat exchanger,with its compact structure and high efficiency of heat transfer,has become the first choice for cooling electronic devices.The flow distribution characteristics of microchannel heat exchanger have an important impact on the normal operation of the heat exchanger.In this paper,theoretical analysis,experimental verification and numerical simulation methods are used to analyze the single-phase flow and two-phase flow in microchannel heat exchanger with different structures The conclusion of this paper provides important theoretical and technical support for the practical application of microchannel heat exchanger in the field of electronic equipment cooling.In this paper,the flow characteristics of single-phase flow in microchannels with different header structures and channel structures are studied.The effects of inlet and outlet position,header size,channel equivalent diameter,channel width to height ratio and channel number on flow distribution unevenness and pressure drop are simulated with deionized water as working fluid.The main conclusions are as follows: 1)the influence characteristics of header position: the Z-type heat exchanger with inlet and outlet located on the left and right sides of header has the strongest heat transfer ability.At the inlet speed of 0.6m/s,the total flow distribution nonuniformity of Z-type heat exchanger is 28.8% lower than that of I-type,and 38.4% lower than that of C-type.The change of inlet and outlet structure position and header size has a great influence on the overall pressure drop of the test piece 2)the influence characteristics of header size: by comparing different header sizes,it is found that increasing the header size is conducive to the uniform flow distribution of the microchannel.Combined with this experiment,the header size with the channel width of 8.2 mm and 20 mm × 20 mm has the best flow uniformity;3)the influence characteristics of the channel structure size: when the channel area of the heat exchanger is kept unchanged and the equivalent diameter is the largest,The results show that the uniformity of flow distribution is the best;keeping the channel width unchanged and changing the ratio of channel height to width,the peak value of the nonuniformity of flow distribution appears in advance with the increase of flow velocity;comparing the influence of channel number on flow distribution and pressure drop,it is found that the heat exchanger with 6 channels has the most uniform flow distribution and the least pressure drop.Next,the flow characteristics of two-phase flow in the microchannel are simulated by using nitrogen and deionized water as working medium.The main conclusions are as follows: 1)in the case of two-phase flow,the Z-type arrangement with the inlet and outlet located on the left and right sides of the header is the strongest.At the real speed of 2m/s,the flow distribution heterogeneity of Z-type microchannel heat exchanger is 6% lower than that of I and L type,and 20% lower than that of C-type microchannel heat exchanger;2)under the same two-phase flow rate,comparing with different header sizes,it is found that increasing the size of header is advantageous to the uniform distribution of flow of microchannel,but the size of header is too large or too small is not conducive to flow distribution The results show that the flow distribution is the best when the channel area is the same,the uniformity of flow distribution is the best and the pressure drop is the lowest when the equivalent diameter is the largest;3)the channel width is kept unchanged and the channel height width ratio is changed.It is concluded that when the channel height to width ratio is 1:1,the flow distribution is the most uniform and the pressure drop is the smallest;compared with the influence of channel number on the flow characteristics of the micro channel heat exchanger,it is found that the heat exchanger with 6 channels is the best channel structure.Finally,in order to further verify the reliability of the theoretical simulation,based on the optimized channel structure,the flow characteristics of two-phase flow in the microchannel heat exchanger were experimentally studied.Combined with the processing technology,I-type channel was used,nitrogen and water were used as working fluids,the volume fraction of nitrogen was 50%,and the flow situation in the channel was recorded by high-speed camera under different flow rates.The experimental results show that the gas distribution in the middle channel is the most,and the gas distribution in the two channels is the least.With the increase of gas-liquid flow rate,the gas elasticity will become smaller and smaller,which leads to the increase of uneven flow distribution in the microchannel heat exchanger.The experimental results are in good agreement with the theoretical calculation.In this paper,the flow distribution and heat transfer characteristics of single-phase flow and two-phase flow in microchannel heat exchanger with different header structure and microchannel structure are studied by theoretical simulation and experimental research.The best matching structure and matching size of header structure and microchannel structure under different working conditions are found,which can provide the basis for the application of electronic equipment cooling in microchannel heat exchanger Domain design,processing and production laid a theoretical foundation and data support.