A vapor-liquid adjustment evaporator exhibits superior heat transfer performance by adjusting vapor quality and mass flux. In this study, two vapor-liquid adjustment evaporators with circuitries of 4-2-4-6-4(evaporator 1) and 5-4-3-4-4-4(evaporator 2) are applied to heat pump water heaters(respectively referred to as systems A and B), and an experimental comparative study is conducted. The results show that compared with that in evaporator 1, the refrigerant is more uniformly distributed in evaporator 2, and its pressure drop is reduced by 10.91%. Under nominal working conditions, the heating capacity and power consumption of system B are reduced by 1.69% and 1.45%, respectively, compared to system A; the coefficients of performance(COPs) of the two systems are generally the same. The pressure at the compressor outlet of system B is 26.35-84.78 kPa lower than that of system A; this range is conducive to the safe operation of the compressor. At different ambient temperatures, the power consumption of the compressor of system B is lower than that of system A. When the ambient temperature is 30 ℃, the COP of system B is 2.1% higher than that of system A.

The use of high-temperature heat pumps to recover industrial waste heat has a high potential for energy conservation. High-temperature heat pumps require high critical temperatures, and the majority of the available working fluids are dry. However, when the dry working fluid is compressed from the saturated vapor phase, the compression process enters the two-phase region, resulting in a risk of liquid slugging that is detrimental to the operation of the compressor and high-temperature heat pump. Two improved vapor injection heat pump cycles (Cycle A and Cycle B) using isohexane, R1336mzz (Z), and R1233zd (E) as working fluids are proposed. The effects of compressor isentropic efficiency, evaporating temperature, and condensing temperature on the minimum superheating degree and heat pump performance are analyzed. The results indicate that, for cycle B, the evaporation temperature increases from 50 ℃ to 80 ℃. For R1336mzz(Z), the maximum COP (coefficient of performance) can be increased by 2.56%, and the maximum volumetric heating capacity (VHC) can be increased by 3.18%. For R1233zd(E), the maximum COP can be increased by 0.44%, and the maximum VHC can be increased by 0.54%. Cycle A has good adaptability to the isentropic efficiency. For cycle B, when the isentropic efficiency is higher than 0.6, isohexane is not suitable as a working fluid. When the isentropic efficiency is higher than 0.95, R1336mzz (Z) is also not suitable.

Vapor-liquid separation technology can enhance heat transfer while reducing pressure drop. The vapor-liquid separation unit is key to achieving efficient vapor-liquid separation. A visualization experiment of the header-orifice separator is conducted in this study using the zeotropic mixture R1234ze (E)/R32 (mass fraction ratio, 80/20) to investigate the vapor-liquid separation characteristics under different conditions and obtain the effective separation range. The results show that increasing the inlet vapor quality, reducing the inlet mass-flow rate, increasing the flow cross-sectional area of the lower outlet branch, and expanding the separation aperture can improve the separation efficiency, among which the separation efficiency is most sensitive to the inlet mass-flow rate. In the effective separation area, when the flow rate increases from 18 g/s to 12 g/s, the separation efficiency increases by 14.0%. The inlet mass-flow rate, valve opening, and separation aperture minimally affects the size of the effective separation dryness range; however, for the deviation of the effective separation area dryness range, the inlet mass-flow rate exerts the greatest impact, followed by the separation aperture, whereas the valve opening exerts the least impact.