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In thermal management applications using two-phase flow boiling, rectangular microchannels hold significant promise due to their ease of manufacturing and effective heat transfer characteristics. In this work, we combined experimental and theoretical analyses to propose a theoretical model based on thin liquid film evaporation for predicting heat transfer performance in rectangular cross-sectional microchannels. The heat transfer model is segmented into five zones based on two-phase flow patterns and transient liquid film thickness. These zones represent different flow boiling heat transfer mechanisms over time in microchannels: the liquid slug zone, elongated bubble zone, long-side wall dryout zone, corner liquid evaporation zone, and full dryout zone. The new model comprehensively explains experimental phenomena observed, including long-side wall dryout and thinning of the liquid film on the short-side wall. To validate our model, numerical solutions were computed to study the spatial and temporal variations in heat transfer coefficients. The results exhibited a consistent trend with experimental data regarding average heat transfer coefficients. We also analyzed factors influencing flow boiling characteristics, such as microchannel aspect ratio, hydraulic diameter, measurement location, fluid mass flux, and wall heat flux. [ABSTRACT FROM AUTHOR]

This paper presents the results of an experimental study on the heat transfer and pressure drop characteristics of a novel spiral plate mini-channel gas cooler designed for use with supercritical CO2. The CO2 channel of the mini-channel spiral plate gas cooler has a circular spiral cross-section with a radius of 1 mm, while the water channel has an elliptical cross-section spiral channel with a long axis of 2.5 mm and a short axis of 1.3 mm. The results show that increasing the mass flux of CO2 can effectively enhance the overall heat transfer coefficient when the water side mass flow rate is 0.175 kg·s−1 and the CO2 side pressure is 7.9 MPa. Increasing the inlet water temperature can also improve the overall heat transfer coefficient. The overall heat transfer coefficient is higher when the gas cooler is vertically oriented compared to horizontally oriented. A Matlab program was developed to verify that the correlation based on Zhang's method has the highest accuracy. The study found a suitable heat transfer correlation for the new spiral plate mini-channel gas cooler through experimental research, which can provide a reference for future designs. [ABSTRACT FROM AUTHOR]

A gas cooler is one of the important parts of a carbon dioxide (CO2) heat pump water heater, and it must meet the needs of not only pressurization but also heat transfer. It is important to study gas coolers. In this paper, a heat exchanger with a spiral channel is studied. ANSYS CFX software was used to analyze the flow and heat transfer characteristics of the heat exchanger (single-plate model). The influences of the cooling pressure of CO2, the mass flux of CO2, the mass flux of water and the channel radius of CO2 are discussed. In this paper, the results show that the cooling pressure of CO2, the mass flux of CO2 and the channel radius of CO2 all have a large influence on the local heat transfer coefficient: with an increase in the cooling pressure of CO2, the peak value of the heat transfer coefficient of CO2 decreases and the average heat transfer coefficient decreases; with an increase in the mass flux of CO2, the peak value of the heat transfer coefficient of CO2 increases and the average heat transfer coefficient increases; and with a decrease in the channel radius of CO2, the peak value of the heat transfer coefficient of CO2 increases. The water mass flux has only a slight effect on heat transfer, and the lower cooling pressure of CO2 corresponds to a higher peak heat transfer coefficient, which can reach 27.5 kW∙m−2∙K−1 at 9 MPa. [ABSTRACT FROM AUTHOR]