Influence factors of two-phase flow and heat transfer in vertical ground heat exchangers

Aiming at optimizing U-pipe heat exchangers in direct expansion ground-source heat pump systems, utilizes user defined functions adding the quality and heat transfer source between phase interfaces, establishes a modified two-phase flow mixture model, simulates three dimensional steady flow field and heat transfer of underground exchangers in heating conditions, and analyses the influences of inlet dryness and flow of working fluid on flow and heat transfer in ground source heat exchangers. The results show that the model can meet the requirement of engineering in accuracy; temperature rise in the U-pipe mainly occurs in 0 to 15 m section from the inlet; in a certain quality flow condition, there exists an optimum dryness making heat transfer rate optimal, or in a certain dryness condition, there is an optimum quality flow.