Abstract
It has been well known that the heat pump system of air heat source can not be efficiently applied in cold regions, because the evaporator in heating mode is very easy to be frosted and COP will be low. Instead of air heat source, the ground can be used as a heat source in cold regions. The experiments of evaporating heat transfer were made in a vertical long-length U-tube for heat extraction in the direct-expansion ground coupled heat pump system. The experiments were carried out using R22 for pressures of 0.42 to 0.64MPa and mass flow rates of 90 to 300kg/m^2・s. The test tube is a smooth copper tube with 8.0mm I.D. and 9.5mm O.D. and the whole length of 12m. The temperatures of the flow fluid and test tube surface, local heat transfer coefficients and pressure drop were measured. The effects of the various parameters on the heat transfer coefficient, such as the evaporating pressure and heat flux, were discussed. The results of local heat transfer coefficients were compared with those given by other investigators, and a dimensionless correlation of the heat transfer coefficients was suggested, which was in good agreement with the present experimental data.
