Experimental study on flow boiling characteristics of R-245fa in circular tube under non-uniform heat flux

Abstract

The direct vapor generation with parabolic trough solar collector utilizing organic working fluids has a significant potential of large-scale application. In such a system, the non-uniform heat flux commonly exists outside the collector tube which would result in the unique characteristics of two-phase flow and moreover boiling heat transfer. However, few studies pay attention to the effects of the non-uniform heat flux on flow pattern transitions and flow boiling, which have important influence on the long-term stable operation of the system. Therefore, in this study, an experimental system was established to study the two-phase flow boiling of R-245fa in a horizontal tube under the condition of the non-uniform heat flux. The experiment was performed with the mass flow rate from 192.9 to 385.8 kg/(m2 s), heat fluxes from 9.95 to 35.9 kW/m2 and saturation temperatures from 40 to 60 °C. Using a high-speed camera and the image processing, the differences between the flow pattern with non-uniform and uniform heating were analyzed. A comparison study on four available flow pattern I/A transition lines in the literature has been demonstrated as well. The effects of vapor quality, mass velocity, saturation temperature and heat flux on the two-phase heat transfer coefficient were analyzed. Furthermore, based on the experimental data, the correlation of boiling heat transfer of R245fa under non-uniform heat flux was developed. The results indicate that under the same heat addition, the vapor quality where the non-uniform I/A phase transition is lower than the uniform one. Furthermore, the non-uniform heat flux makes R245fa in the tube easier to form nucleation sites, therefore the nucleation boiling can be significantly enhanced. Taking account of the strengthening factor “S” of nucleate boiling, the new correlation of boiling heat transfer can reproduce 94% of data sets under non-uniform heat flux within the uncertainty of ±20%.