Heat transfer characteristics in circulating solar heat tubes based on gas-liquid two-phase flow

Abstract: The flood irrigation method adopted in the saline-alkali land in Southern Xinjiang produced a large amount of brackish water, so realizing the resource utilization of brackish water had an important value for improving the local water shortage. Vacuum glass tube had a long history of development and a proven production process, and it was one of the indispensable components of the brackish water desalination device using solar energy. In order to improve the heating level of solar collector, study the influence of different entrance mass flow on the heating efficiency of solar collector, and solve the outstanding problems such as small receiving surface area and high heat loss rate, this paper studied and designed a kind of circulating solar heat tube with high efficiency of absorbing light and heat preservation. The circulating solar heat tube was composed of quartz vacuum straight tube, quartz elbow and connection segment. A vacuum was formed between the inner and outer tubes of the quartz straight tube to reduce the convective and conduction heat loss, and such tubes were connected in series to expand the surface area and increased the received solar energy. In this paper, the energy balance equations of the liquid phase and the gas phase of the two-fluid in the circulating solar heat tube were deduced by the micro-element analysis method, and the mathematical model of the two-phase flow in the circulating solar heat tube was established. The solution region of the two-phase flow model was discretized and iteratively solved, so that the position and mass fraction of water vapor generated in the tube under different entrance mass flow were obtained. An experiment was designed to verify the two-phase flow model. The thermal performance of the circulating solar heat tube under single-phase flow and two-phase flow conditions were analyzed, and the entrance mass flow was optimized synthetically. The results showed that the entrance mass flow of 4.42 kg/h was the critical value for the division of single-phase flow and gas-liquid flow, and when the entrance mass flow below the value of 4.42 kg/h, two-phase flow occured. The fluid temperature in the tube was inversely correlated with the entrance mass flow, and the maximum temperature can reach 120 ℃. The heat collecting efficiency of the tube was positively correlated with the inlet mass flow rate, and the maximum efficiency can reach 0.87. The optimal entrance mass flow was 4.15 kg/h though comprehensive consideration of heat collection efficiency and fluid temperature. The test results showed that the theoretical results were coincident with the measured results of two-phase flow model (pressure is 0.1MB), and the error between them was less than 2%. The results of this paper can be a useful supplement to the two-phase flow analysis in vacuum collector tubes, and also can provide reference for the local application of brackish water desalination in Southern Xinjiang.