Two-phase flow pattern and heat transfer coefficients of R245fa/R134a under non-uniform heat flux

Abstract

The non-uniform distribution of the heat flux commonly occurs in systems where vapor-liquid phase changes, such as nuclear reactors, and aerospace thermal control systems. This poses a great challenge on the study of pressure drop, heat transfer coefficient, and flow pattern prediction. In addition, flow patterns and heat transfer characteristics are usually affected by differences in fluid properties (viscosity, density, and surface tension), thermodynamic conditions (pressure, temperature), and geometric dimensions. At present, most of the researches on non-uniform heat flux are concentrated in systems with pure working fluids. However, compared to the pure working fluid, the mixed working fluid, particularly the zeotropic working fluid can improve the cycle efficiency and has great application potential, chiefly due to the temperature glide. There are few studies on the two-phase flow pattern and heat transfer characteristics of the zeotropic working fluid under non-uniform heat flux. Aiming at the problem of vapor-liquid phase change of zeotropic working fluid under non-uniform heat flux boundary conditions, a two-phase flow boiling experimental system in a horizontal tube with a 10 mm inner diameter under non-uniform heat flow was established. The studied zeotropic working fluid is R245fa/R134a with a mass fraction of 0.7:0.3. Experiments were performed at conditions where the mass flux ranges from 175 to 373 kg/(m2 s), while the heat flux ranges from 9.95 to 47.57 kW/m2. Using a high-speed camera and image processing technology, the gray values of the observed bubble flow, plug flow, stratified flow, and annular flow were analyzed, and the flow patterns under both non-uniform and uniform heat flux were demonstrated. Effects of the vapor quality, mass flux, heat flux and saturation temperature on the two-phase heat transfer coefficient were analyzed. And on the basis of experimental data, the prediction accuracy of four commonly used correlations was evaluated. The results show that the gray value of bubble flow and plug flow fluctuate widely, while the gray value fluctuation range of stratified flow and annular flow is relatively small. The typical flow pattern under non-uniform heat flux is the same as flow pattern under uniform heat flux. With the same amount of the additional heat, under the condition of non-uniform heat flux, the initial vapor quality of annular flow is advanced, and the proportion of the annular flow is larger than the non-annular flow. The main reason for this phenomenon is that the flow instability caused by non-uniform heat flux accelerates the transition of intermittent flow to annular flow. In addition, the heat transfer coefficient changes little at the lower vapor quality, and the heat transfer coefficient of the intermittent flow region is independent of vapor quality. In the annular flow region with higher vapor quality, the heat transfer coefficient increases with the increase of vapor quality. The heat transfer coefficient increases with the increase of mass flux and heat flux. The saturation temperature has little effect on the heat transfer coefficient in the convection boiling region and has a certain influence in the nucleate boiling region. Furthermore, the prediction ability of the four correlations is poor in the non-annular flow region. Relatively, the prediction accuracy of the Sun & Mishima correlation is the highest in the annular flow region.