R600a水平管内两相流型转换及摩擦压降特性

Experiment investigation of two-phase flow pattern transition and frictional pressure drop of R600a in a horizontal tube

R600a作为天然工质,因其环境友好性和出色的制冷能力在欧美国家得到大范围推广,被用作可替换制冷剂之一.R600a两相流型转换和摩擦压降的精确预测对制冷系统的设计和优化具有重要参考意义.针对这个问题,开展了在内径为6 mm的水平管中R600a的两相流型以及摩擦压降的实验研究,实验工况为饱和温度282.4~304 K,质量流率范围67~194 kg/(m^2 s).实验中利用高速摄像仪,观察到塞状流(plug flow)、层流波动流(stratified-wavy flow)、弹状流(slug flow)、环状流(annular flow)4种流型,并绘制流型图,将流型图与文献中经典的I/A流型转换曲线进行比较.在综合考虑了气体惯性力、液体的黏性力和液体的表面张力对流型转换的影响,引入修正后的韦伯数We~*,提出了新的流型转换曲线关联式.同时基于环状流和非环状流的流型划分,对摩擦压降分区变化进行针对性的讨论,发现摩擦压降在不同的流型区呈现出不同的变化规律.将摩擦压降实验值与9种摩擦压降关联式进行对比,结果显示Gr?nnerud关联式和Muller-Steinhagen & Heck关联式分别能够较好预测非环状流和环状流的两相摩擦压降.

With the growing concern of environment problems, there are urgent demands for environment-friendly refrigerants in the refrigeration industry. As a natural refrigerant, R600a not only has zero ozone depletion potential and quite low global warming potential, but also has better cooling performance than other refrigerants. Due to its good cooling performance and environment-friendly characteristics, R600a has been used as an alternative refrigerant in heat transfer applications such as refrigerators, freezers, and heat pumps. It is known that accurate prediction of two phase flow pattern transition and frictional pressure drop of R600a in a horizontal tube play an important role in design and optimization of refrigerators, freezers, and heat pumps. According to this issue, a detailed experimental study was carried out to investigate the two-phase flow pattern transition and frictional pressure drop characteristics of R600a in a smooth horizontal tube with an inner diameter of 6 mm. The experiments were performed at conditions covering saturation temperature from 282.4 to 304 K, mass fluxes from 67 to 194 kg/（m2 s）. Based on a high speed camera, four main flow regimes can be observed： plug flow, stratified-wavy flow, slug flow and annular flow. The flow map of R600a has been drawn and comparisons between the experimental data of flow patterns and transition lines in the literature have been made. The results showed that with the increase of mass flux, the inception of the annular flow regime occurs earlier. The intermittent flow regime takes place over a narrow range of vapor quality and annular flow regime occupies a wide range of vapor quality. In addition, the trend of Barbieri and Ong and Thome I/A transitions lines are consistent with the experimental data. However, both of them overestimate the inception of the annular flow regime. After taking the influence of vapor inertia, liquid viscous and surface tension forces into account, a modified Weber number We＊ has been introduced to the new flow pattern transition model. Based on the relationship of Xn and We＊, a new correlation has been proposed and it can well fit with the experimental data of flow pattern. Furthermore, a detailed discuss of frictional pressure drop of annular flow and non-annular flow has been made. The results indicated that frictional pressure drop increases with the increases of vapor quality. The tendency of frictional pressure drop presents different the changing laws in different flow regime. The frictional pressure drop increases slowly in the non-annular flow regime while it increases rapidly when the flow pattern turns into the annular flow regime. The reason is that the vapor-phase pressure drop often dominates the total two-phase frictional pressure drop. The annular flow regime owns higher void fraction than the non-annular flow regime. Nine classic prediction correlations of pressure drop were evaluated and the results showed that the correlation of Grrnnerud and Muller-Steinhagen and Heck give the best fit to the experimental frictional pressure drop of annular flow and non-annular flow with a mean absolute relative deviation of 38.5% and 19.7% respectively.