Variable refrigerant flow (VRF) refrigeration systems use variable flow regulation technology to better match the system load. But the frequency of the compressor and the opening of the electronic expansion valve (EV) directly affect the system's stability and performance. To study cooperative control and stability of performance parameters (superheat, refrigeration capacity, and coefficient of performance (COP)), a small variable frequency rolling rotor refrigeration system of R32 was set up. Experimental research on the stability of the system performances was studied with the effect of superheat, then collaborative control of the fitting model was established. The test campaign was carried out on the R32 variable refrigerant flow refrigeration system with a cooling capacity of 5 kW, compressor frequencies from 40 Hz to 60 Hz, pressure ratio from 2.93 to 3.2, and expansion valve opening from 20% to 80%. Results showed that (1) with the increase of refrigerant mass flow, the system came into the unstable oscillation range of the minimal stable signal (MSS) line, and then the system performances (e.g., superheat) started to oscillate. So, the superheated vapor flow and mist flow alternated, which led to the change of the heat transfer mechanism. (2) There was one inflection point (minimum stable superheat point) for the refrigeration capacity at the certain superheat for the different compressor frequencies. When superheat was near the minimum stable point, it was the best choice to balance refrigeration capacity (or coefficient of performance) and the control difficulty. (3) When the compressor frequency was from 40 Hz to 60 Hz under constant pressure ratio, the increase of the compressor frequency had a more direct impact on the compressor power consumption compared with refrigeration capacity. Therefore, there was a negative correlation between the COP and compressor frequency. (4) When the expansion valve opening was between 28% and 32% and the compressor frequency was from 44.5 Hz to 46.5 Hz, it was more difficult to control superheat. The relationship between the compressor frequency and the EV opening could reduce the phenomenon of overshoot or hysteresis. And, the system could reach a stable state quickly with the method of the synchronous control.

To improve the efficiency of the heat exchanger tube, one smooth tube and four doubly enhanced tubes (EX1, EX2, EX3 and EX4) have been investigated for the pool boiling heat transfer experiments in this paper. The indicate that the pool boiling heat transfer coefficient of the doubly enhanced tubes increased visibly with the augmentation of heat flux through the bubble behavior. Heat transfer reinforcement effect of the doubly enhanced tubes are significantly better than that of the smooth tube. Additionally, pool boiling heat transfer coefficient can be further enhanced by raising the saturate temperature. Entropy generation minimization analysis demonstrates that the heat transfer characteristics of like T-shaped tubes (EX1 and EX2) are superior to that of low fin tubes (EX3 and EX4). Particularly, tube EX1 exhibits higher pool boiling heat transfer efficiency. It is observed that a reasonable fin pitch is more advantageous for improving heat transfer characteristics. The utilization of entropy generation minimization analysis provides theoretical support for the design and optimization of doubly enhanced tubes.

A performance testing setup for an R290 compressor refrigeration system in electric vehicles was constructed to study the effects of variable compressor speed, evaporation temperature, condensation temperature, and oil circulation rate (OCR) on compressor performance, as well as the impact of system parameters on the OCR. The results show that the coefficient of performance (COP) and isentropic efficiency increase with rising compressor speed and evaporation temperature but are negatively correlated with condensation temperature. Under the same conditions, the R290 compressor has high power consumption, but its cooling capacity and COP are better than R134a, and it has a smaller charge amount and lower discharge temperature.

This paper establishes a simulation model for the performance of an R290 variable frequency compressor in automotive air conditioning and sets up a compressor performance testing system. It investigates the effects of different evaporation temperatures, condensation temperatures, compressor speeds, and pressure ratios on the oil circulation rate (OCR), as well as the impact of various oil circulation rates on the performance of the R290 compressor. As the comparison between simulation and experimental data shows, compressor performance predictions from the simulation model align with experimental results when the OCR is not taken into consideration. Experimental results indicate that the OCR increases with a rising evaporation temperature, decreases with a lowering condensation temperature, and increases with higher compressor speeds. The simulation model shows a minor deviation when predicting volumetric efficiency, while errors are larger when predicting isentropic efficiency and the discharge temperature. Isentropic efficiency and the discharge temperature show a notable impact from the OCR. Additionally, for system cooling capacity, power, and COP predictions, when the OCR is within the range of 2~10%, the accuracy of the simulation model proves satisfactory, with deviations within 5%.