Review of two-phase flow and flow boiling of mixtures in small and mini channels

The objective of this study is to present test results and to develop a dimensionless correlation on the basis of the experimental data of adiabatic capillary tubes for R22 and its alternatives, R407C (R32/125/134a, 23/25/52 wt.%) and R410A (R32/125, 50/50 wt.%). Several capillary tubes with different length and inner diameter were selected as test sections. Mass flow rate through the capillary tube was measured for several condensing temperatures and various degrees of subcooling at the inlet of each capillary tube. Experimental conditions for the condensing temperatures were selected as 40, 45 and 50°C, and the degrees of subcooling were adjusted to 1.5, 5 and 10°C. Mass flow rates of R407C and R410A were compared with those of R22 for the same test conditions. The results for straight capillary tubes were also compared with those of coiled capillary tubes. A new correlation based on Buckingham π theorem to predict the mass flow rate through the capillary tubes was presented based on extensive experimental data for R22, R407C and R410A. Dimensionless parameters were chosen considering the effects of tube geometry, capillary tube inlet conditions, and refrigerant properties. Dimensionless correlation predicted experimental data within relative deviations ranging from −12% to +12% for every test condition for R22, R407C and R410A. The predictions by the developed correlation were in good agreement with the results in the open literature.

Experimental investigation of the performance of R22, R407C and R410A in several capillary tubes for air-conditioners

Modeling and application of direct-expansion solar-assisted heat pump for water heating in subtropical Hong Kong

The use of high temperature heat pumps (HTHPs) operating with natural fluids has been shown to be a potential environmentally friendly solution to increase energy efficiency in industrial processes. Industrial processes release a significant amount of energy as low quality waste heat to the environment. This paper reviews the research and development of efficient and cost effective HTHP technology that can utilize this waste heat. Natural fluids are of focus with consideration given to the comparable technologies using synthetic fluids. This review reveals the different challenges from fluid selection, component development to system optimization. The various innovative solutions to these challenges and promising technologies for further studies are discussed. The purpose of this paper is to serve as a start point for research by bringing together ideas, simulations and experimental results as a resource or reference tool for future development in HTHP using natural working fluids.

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Review of vapour compression heat pumps for high temperature heating using natural working fluids

Towards net zero energy design for low-rise residential buildings in subtropical Hong Kong

In this paper, pressure drop through a capillary tube is modeled in an attempt to predict the size of capillary tubes used in residential air conditioners and also to provide simple correlating equations for practicing engineers. Stoecker's basic model was modified with the consideration of various effects due to subcooling, area contraction, different equations for viscosity and friction factor, and finally mixture effect. McAdams' equation for the two-phase viscosity and Stoecker's equation for the friction factor yielded the best results among various equations. With these equations, the modified model yielded the performance data that are comparable to those in the ASHRAE handbook. After the model was validated with experimental data for CFC12, HFC134a, HCFC22, and R407C, performance data were generated for HCFC22 and its alternatives, HFC134a, R407C, and R410A under the following conditions: condensing temperature; 40, 45, 50, 55°C, subcooling; 0, 2.5, 5°C, capillary tube diameter; 1.2–2.4 mm, mass flow rate; 5–50 g/s. These data showed that the capillary tube length varies uniformly with the changes in condensing temperature and subcooling. Finally, a regression analysis was performed to determine the dependence of mass flow rate on the length and diameter of a capillary tube, condensing temperature, and subcooling. Thus determined simple practical equations yielded a mean deviation of 2.4% for 1488 data obtained for two pure and two mixed refrigerants examined in this study.

Capillary tube selection for HCFC22 alternatives

In this study, computer simulation programs were developed for multi-stage condensation heat pumps and their performance was examined for CFC11, HCFC123, HCFC141b under the same condition. The results showed that the coefficient of performance (COP) of an optimized ‘non-split type’ three-stage condensation heat pump was 25–42% higher than that of a conventional single-stage heat pump. The increase in COP differed among the fluids examined. The improvement in COP was due largely to the decrease in average temperature difference between the refrigerant and water in the condensers, which resulted in a decrease in thermodynamic irreversibility. For the three-stage heat pump, the highest COP was achieved when the total condenser area was evenly distributed to the three condensers. For the two-stage heat pump, however, the optimum distribution of total condenser area varied with working fluids. For the three-stage system, splitting the condenser cooling water for the use of intermediate and high pressure subcoolers helped increase the COP further. When the individual cooling water for the intermediate and high pressure subcoolers was roughly 10% of the total condenser cooling water, the optimum COP was achieved showing an additional 11% increase in COP as compared to that of the ‘non-split type’ for the three-stage heat pump system.

Byungjin Park

Papers

Capillary tube selection for HCFC22 alternatives

A study on the performance of multi-stage condensation heat pumps