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.

Energy shortage has become one of the most concerning issues in the world today, and improving energy utilization efficiency is a key area of research for experts and scholars worldwide. Small-diameter heat exchangers offer advantages such as reduced material usage, lower refrigerant charge, and compact structure. However, they also face challenges, including increased refrigerant pressure drop and smaller heat transfer area inside the tubes. This paper combines the advantages and disadvantages of both small and large-diameter tubes and proposes a combined-diameter heat exchanger, consisting of large and small diameters, for use in the indoor units of split-type air conditioners. There are relatively few studies in this area. In this paper, A theoretical and numerical computation method is employed to establish a theoretical-numerical calculation model, and its reliability is verified through experiments. Using this model, the optimal combined diameters and flow path design for a combined-diameter heat exchanger using R32 as the working fluid are derived. The results show that the heat transfer performance of all combined diameter configurations improves by 2.79% to 8.26% compared to the baseline design, with the coefficient of performance (COP) increasing from 4.15 to 4.27~4.5. These designs can save copper material, but at the cost of an increase in pressure drop by 66.86% to 131.84%. The scheme IIIH, using R32, is the optimal combined-diameter and flow path configuration that balances both heat transfer performance and economic cost.

The frequency and the suction refrigerant state are complex in the actual operation of variable compressors, which leads to the variability of compressor performance under different operating conditions. The characterization and the modeling of the compressor have become hot research topics. For modeling, the electrical efficiency model of the compressor, which could be applied to the suction refrigerant state at both the superheated and the two-phase stages, the compressor frequency, the pressure ratio, and the evaporation temperature, is analyzed through experiments in this paper. The results show that the linear electrical efficiency decreases with the declining superheated temperature and the increasing suction vapor quality when the system is at the fixed and evaporating temperature. Moreover, the slope of the two-phase suction is above that of the superheated suction. In addition, the system’s electrical efficiency at the fixed pressure ratio is inversely proportional to the evaporation temperature, and the electrical efficiency of the system at the fixed evaporating temperature is also inversely proportional to the pressure ratio. The higher the variation of the pressure ratio, the smaller the evaporation temperature influences. To demonstrate the precision of the model, the theoretical value is compared with a confirmatory experiment in this paper. The maximum relative error is 1.81%, and the minimum is 0.035%.

Balancing heat transfer performance with material cost and refrigerant charge remains a key challenge in split air conditioning systems. To address this issue, the present study proposes a finned-tube heat exchanger with a variable-diameter configuration, combining 5.2 mm and 7.3 mm tubes for use with R290 refrigerant. Three hybrid arrangements are examined against a conventional baseline with uniform 7.3 mm tubes, differing in the number and spacing of the 5.2 mm tubes integrated within the heat exchanger layout, thereby enabling targeted structural and thermal optimization of the indoor unit. An integrated methodology, based on a theoretical iterative algorithm and supported by numerical simulations and experimental validation, is employed to characterize heat transfer and fluid flow under rated cooling conditions. The results show that the proposed configurations achieve substantial reductions in refrigerant charge, by up to 11.5%, and copper usage, by up to 7.78%, while simultaneously enhancing the system coefficient of performance by as much as 3.75% compared to the reference design. Configurations with a higher proportion and tighter spacing of 5.2 mm tubes yield the greatest improvement in energy efficiency, whereas those maximizing the substitution of 7.3 mm tubes with 5.2 mm tubes achieve the most pronounced reductions in material usage and refrigerant charge. Overall, the findings demonstrate that variable-diameter tube heat exchangers provide an effective strategy for optimizing the trade-off between performance, cost, and environmental impact in R290-based split air conditioning systems.