Analysis and optimization of a novel two-phase spray injection heat pump suitable for extremely cold conditions

Low-temperature air-source heat pumps equipped with a two-phase refrigerant injection compressor can effectively address issues such as excessively high discharge temperature and performance degradation faced by traditional single-stage compression heat pump systems during cold seasons. However, excessively low injection quality may increase the risk of liquid slugging in the compressor, while excessively high injection quality fails to adequately reduce the discharge temperature. To further explore the performance limits of two-phase injection and eliminate potential liquid slugging risks, a novel cycle with two-phase spray injection is proposed. Subsequently, a numerical model incorporating the processes of two-phase refrigerant atomization, droplet motion, and mass transfer is subsequently developed and validated. Based on this model, a comparison of the evaporation rates and mass flow rates was conducted between the compressor with non-spray and spray two-phase injection. Additionally, the effects of different injection pressures and qualities on the flow characteristics and performance of the novel cycle with two-phase spray injection were investigated. Ultimately, an optimized operating strategy based on maximizing the heating capacity and COP while controlling the discharge temperature was proposed. The optimization results indicate that, within the evaporation temperature range of −30 °C to −10 °C, the proposed strategy achieved an increase in heating capacity by 18–32 %, an improvement in COP by 5–9 %, and a reduction in discharge temperature by 7–34 °C compared to the system without injection.