The performance improvement of heat pump depends on the appropriate injection style such as liquid injection, vapour injection and two-phase injection. In this work, experimental and theoretical investigation on the heat pump with R410A refrigerant reveals three kinds of injection styles can be selectively achieved in the same injection device by suitable control of the sub-cooling electronic expansion valve. Liquid injection and vapour injection improve the heating capacity (Qh), mainly due to the increase of heat exchange in the sub-cooler and compressor compression work, respectively. Two-phase injection lifts the Qh owing to these two effects. Subsequently, the effect of two-phase injection on improving the heat pump performance is superior to the other two injection styles. For instance, the maximum Qh were 14.4 kW at 60 rps with the liquid injection, 12.5 kW and 13.4 kW at 85 rps with vapour injection and two-phase injection, and 8.1%, 7.0% and 18.3% higher than those without injection, respectively. The corresponding coefficient of performance was improved by 10.4%, 4.5% and 10.6%, respectively. Furthermore, the model based on the pressure-enthalpy diagram has been developed to analyze the experimental results and expound the main factors affecting the heat pump performance under different ambient temperatures.
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