Experimental and dimensional study of the draining process in drainback solar thermal systems

This study aims at analyzing the phenomenon of siphon draining occurring in drainback solar thermal systems. Although several experimental studies have been previously carried out, there is a lack of theoretical work in the literature. Since describing the draining process from a theoretical point of view should include complex phenomena as two-phase flow and stochastic events, a simpler representation is sought through dimensional analysis. The main physical parameters impacting the draining are at first identified with the equations of fluid mechanics before being individually assessed with experiments. The piping diameter, the type of fluid, the piping length, the drained volume of the circuit as well as the singular pressure losses are for this purpose varied throughout the experiments. A methodology for obtaining an empirical equation using dimensionless variables is finally developed, showing that the impact of all the studied parameters on the draining time could be accurately described for piping with an inner diameter of 10 and 16 mm.