## 1 Introduction  
Due to its high efficiency and performance, a heat pump is commonly used as an energy-saving residential heating and cooling unit, and is being consistently developed for the improvement of system components, the optimization of configuration, and the control of the systems. Generally, a heat pump has a dry compression cycle, where the irreversibility exists by a superheated refrigerant entering the compressor inlet [1]. However, some heat pump systems, such as a fixed orifice expansion valve, can occasionally run vapor-liquid compression due to charge balancing factors. The vapor-liquid compression in a heat pump system is called wet compression, which brings it closer to the Ideal Carnot Cycle for a pure refrigerant and closer to the Lorenz Cycle for non-azeotropic refrigerants, because the irreversibilities caused by vapor superheating during compression are prevented. It was shown that the advantage of a wet compression cycle offers the highest possible efficiency for thermodynamic cycles operating between two temperatures with a heat sink and heat source [2; 3]. While some earlier works [4; 5] showed improvements in the isentropic and volumetric efficiency of the compressor, they did not recommend using wet compression due to the following possible dangers. Firstly, wet compression may cause an abrupt increase of pressure at the cylinder head, which may damage the valve or cylinder. Another risk is the acceleration of the wear, because the droplets of liquid wash the lubricating oil from the walls of the cylinder [6]. In fact, the destructive increase of cylinder pressure during wet compression can only occur when the vapor quality is under 0.4 at the compressor inlet, but this condition is rare [7]. In addition, the accelerated wear of a cylinder could be reduced by fully evaporating the droplets of liquid during compression [8]. Thus, the number of liquid droplets and a finite time required for heat transfer during compression, are considered to be important for preventing an abrupt increase in the pressure and a greater wear of the cylinder. [PERSON] et al. [9] reported that a 5% liquid injection at the compressor inlet enhances the performance of a heat pump. [PERSON] et al. [10] reported that the liquid refrigerant injection, under a low speed, increases the leakage between scroll wraps, but is very effective under a high speed. In general, for improving the performance and extending the operating range of the heat pumps, the wet compression injected refrigerant at the compressor inlet has been frequently used with a complicated injection circuit, in order to decrease the discharge temperature of the compressor inlet in severe climatic conditions [11; 12]. R32, which is a recent potential substitute for R410A, shows a higher discharge temperature of the compressor when compared to R410A, which is a main factor that can reduce the operating range of the heat pumps [13]. If the injection cycle is used for R32 in extending the operating range, it may experience difficulties during the operation, due to the low mass flow rate and the small molecular weight, which increase the internal leakage in the compressor, causing a reduction in the performance of the compressor. These problems could be solved by wet compression, which could decrease both the internal leakage and the discharge temperature during compression. Instead of adopting a complicated injection circuit for this purpose, the Electronic Expansion Valve (EEV) could be used to obtain the wet refrigerant state of the compressor suction, by controlling the EEV opening; this alternative motivated the present study. Due to its wider working range, higher control accuracy, and fast response, the EEV could be more popular in heat pump applications [13]. It could also be implemented in a much simpler configuration in the heat pump system, with an accurate control of the wet refrigerant state at the compressor inlet.