Flow Pattern Based Heat Transfer And Pressure Drop Model For R32 Flow Boiling Inside Horizontal Smooth Micro/Mini-tubes

As one of the most promising substitute refrigerants for R22, R32 shows good performance in heat transfer and fluidity as well as being environmental friendly, safe and easily accessible for industry application. Micro-/mini-channels Flow boiling for R32 in micro-/mini-channels differs evidently from that in conventional channels. Therefore, investigations on two-phase flow patterns for R32 during flow boiling in micro-channels shed light on these differences, which is of great significance both theoretically and practically.Visualization experiments of flow boiling heat transfer and pressure drop were conducted for R32 in a horizontal circular channel with an internal diameter of 1 mm and 2 mm, under the conditions of vapor quality ranging from 0.03- 0.97, mass flux ranging from 50-600 kg/m2 s, heat flux ranging from 10-30 kW/m2, and saturation temperature of 10 and 20℃. Following conclusions could be summarized from the results of the experiments.Six different flow patterns are identified during flow boiling for R32, which are slug flow, churn flow, annular flow, dryout, mist flow and stratified flow. Slug flow zone narrows down with an increasing mass flux and expands with an increasing heat flux and saturation temperature. Annular flow zone narrows down with an increasing mass flux, heat flux and saturation temperature. Mist flow zone expands with an increasing mass flux, heat flux and saturation temperature. A new flow pattern map is developed based on models proposed by Costa-Patry et al., Ong et al., Cheng et al. and Zurcher et al., giving a better comparison result that 97% of all the experimental data points match the new map.Heat transfer coefficients are measured and a new flow pattern based correlation for horizontal smooth micro-/mini-tube is proposed. While under a relatively large mass flux, heat transfer coefficient increases as vapor quality increases until reaching a peak at dryout incipience quality, after which heat transfer coefficient drops sharply and stays stable at a low level in mist flow zone. While under a relatively small mass flux, heat transfer coefficient decreases all along with an increasing vapor quality and remains stable in mist flow zone. Dryout incipience quality decreases with an increasing mass flux, heat flux and saturation temperature and a decreasing channel diameter. An increasing mass flux, heat flux and saturation temperature and a decreasing channel diameter increase heat transfer coefficient for R32 during flow boiling. A new flow pattern based heat transfer coefficient correlation is developed using general heat transfer model proposed by Kattan-Thome-Favrat, with 96% of points in dataset falling within ±25% error band.Friction pressure drops are measured and a new flow pattern based correlation for horizontal smooth micro-/mini-tube is proposed. While under a relatively large mass flux, friction pressure drop increases as vapor quality increases and decreases in mist flow zone. While under a relatively small mass flux, friction pressure drop change follows the same pattern but with a slow rate. An increasing mass flux, heat flux and a decreasing saturation temperature and channel diameter increase friction pressure drop for R32 during flow boiling. A new flow pattern based friction pressure drop correlation is developed using friction pressure drop model proposed by Moreno Quibén, with 89% of points in dataset falling within ±25% error band.