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.

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.