Evaporation heat transfer and pressure drop for geothermal heat pumps working with refrigerants R134a and R407C

This paper presents experimental results obtained during tests on the evaporation heat transfer inside tube of a coaxial heat exchanger for residential and industrial geothermal heat pumps. The experimental device and instrumentation of the test section using evaporation inside a water heated double tube, have been described. The results obtained for evaporation local heat transfer of refrigerants R134a and R407C have been compared to the existing correlations. The aim of this comparison is to evaluate the validation of the well-known correlations to predict evaporation heat transfer in the geothermal operating conditions. A complete experimental analysis of the local evaporation of the R134a and R407C is carried out as a function of vapor quality inside a smooth horizontal tube. Various refrigerant mass fluxes are tested. Moreover, new correlations for local heat transfer and pressure drop predictions have been developed and validated basing on the measurements.