Performance analysis of multi-split variable refrigerant flow (VRF) system with vapor-injection in cold season

•Numerical model was developed to predict the performance of multi-split variable refrigerant flow (VRF) systems using the vapor-injection with the geometry-based model of a scroll compressor.•The performance of multi-split VRF system was affected by the variation of the intermediate pressure due to the variation in the sub-cooling degree and the isentropic efficiency of a scroll compressor.•The temperature difference in each indoor room affected the intermediate pressure to maintain the preset degree of superheating at the exit of injection line. The performance of the variable refrigerant flow (VRF) system becomes poor at a very low ambient temperature because of the decrease in the suction mass flow rate. The vapor injection is applied to the heat pump system to enhance the performance of the systems in a cold season. In this study, the influence of the vapor injection under different intermediate pressures and outdoor ambient temperatures was investigated mathematically, and the results were validated with experimental data. In addition, the performance variation was observed with different indoor ambient temperatures in nine indoor units. The intermediate pressure affected the heating capacity, input power, coefficient of performance (COP), and isentropic efficiency of the compressor. The improvement of the COP with vapor injection was dominant at a low ambient temperature. The heating capacity at different indoor ambient temperatures was observed with different refrigerant mass flow rates in each indoor unit.