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The objective of this study is to investigate the effects of vapor injection techniques on the heating performance of a CO2 heat pump. The performances of the flash tank vapor injection (FTVI), sub-cooler vapor injection (SCVI) and FTVI with a suction line heat exchanger (FTSX) cycles were measured and analyzed with variations of the outdoor temperature, compressor frequency, and injection mass flow rate. At the outdoor temperature of −15 °C and compressor frequency of 55 Hz, the heating capacity and COP of the optimized SCVI cycle were 12.1% and 12.7% higher than those of the optimized FTVI cycle, respectively, because the total mass flow rate in the SCVI cycle was higher than that in the FTVI cycle by the large temperature and pressure differences in the sub-cooler of the SCVI cycle. In addition, the optimum injection flow rate ratios in the vapor injection CO2 cycles yielding the maximum COP were determined at various compressor frequencies.

This paper explores the thermodynamic performance limits of the vapor compression cycle. We have applied evolutionary algorithms to explore the performance of hypothetical refrigerants defined by the thermodynamic parameters used by the extended corresponding states model for fluid properties. We identified optimal values of these parameters required to reach the performance limits. The study confirmed the fundamental trade-off between the coefficient of performance (COP) and volumetric capacity, and indicated refrigerant critical temperature as the dominant parameter influencing the tradeoff. Thermodynamic performance limits depend on the operating conditions and the cycle design. These limits are represented by Pareto fronts developed for the objective functions COP and volumetric capacity. As expected, the performance of current refrigerants falls below the Pareto front limits. We demonstrate that for practical cycles the developed methodology and resulting Pareto fronts are more realistic benchmarks for the performance potentials of refrigerants than is COP alone, which is an efficiency only metric.

The condensation heat transfer coefficient and pressure drop of CO 2 in a multiport microchannel with a hydraulic diameter of 1.5 mm was investigated with variation of the mass flux from 400 to 1000 kgm −2 s −1 and of the condensation temperature from −5 to 5 °C. The heat transfer coefficient and pressure drop increased with the decrease of condensation temperature and the increase of mass flux. However, the rate of increase of the heat transfer coefficient was retarded by these changes. The gradient of the pressure drop with respect to vapor quality is significant with the increase of mass flux. The existing models for heat transfer coefficient overpredicted the experimental data, and the deviation increased at high vapor quality and at high heat transfer coefficient. The smallest mean deviation of ±51.8% was found by the Thome et al. model. For the pressure drop, the Mishima and Hibiki model showed mean deviation of 29.1%.

The objective of this study is to investigate the control methods of the gas-cooler pressure in a CO2 heat pump. The cooling performance of the CO2 heat pump was measured by varying the refrigerant charge amount, EEV opening, compressor frequency, and outdoor fan speed at various outdoor temperatures. The effects of the EEV opening and the outdoor fan speed on the gas-cooler pressure and the COP were analyzed by using the experimental data. In the standard cooling condition at the compressor frequency of 45 Hz, the optimum gas-cooler pressure and the maximum COP were 9200 kPa and 3.04, respectively, at the optimum EEV opening of 41% and outdoor fan speed of 500 rpm. As the compressor frequency increased from 45 Hz to 55 Hz at the standard cooling condition, the optimum outdoor fan speed increased from 500 rpm to 700 rpm.

In residential air-source heat pump systems, the refrigerant injection technique has rapidly developed in recent years due to its outstanding performance at low ambient temperatures. A numerical simulation model of a flash tank vapor-injection heat pump with a twin rotary compressor is newly presented in this paper. The performance of the twin rotary compressor with refrigerant injection was simulated using a loss and efficiency model. The model was validated by comparing the predictions with measured data. In addition, the model was used to analyze the performance characteristics of the vapor-injection heat pump according to the cylinder volume ratio of the twin rotary compressor. Based on the simulation results, optimum cylinder volume ratios were suggested for various design conditions, including for the fixed first cylinder volume, the fixed total cylinder volume, and the fixed heating capacity.

The condensation heat transfer characteristics of CO 2 flowing in a horizontal smooth tube were investigated. The tube diameter was 5.15 mm. The condensation temperature ranged from −10 to 5 °C, and the mass flux was from 600 to 1000 kg m −2 s −1 . When the temperature changed from 0 to −10 °C, the increase rate of the heat transfer coefficient was from 9.4 to 14.6%, and the pressure drop increased from 6.2 to 52.9%. When the mass flux increased from 600 to 1000 kg m −2 s −1 , the heat transfer coefficient increased from 6 to 35%, and the pressure drop increased from 60 to 165%, which were dependent on the condensation temperature. The increases come from the change of vapor velocity and thermophysical properties with condensation temperature. Considering large variation of mass flux from 200 to 1200 kg m −2 s −1 by including the existing studies, the effect of mass flux on condensation heat transfer coefficient was minor.

A cooling system is adopted to control the thermal load from avionic equipment in a reconnaissance aircraft such as an unmanned aerial vehicle that undergoes large and rapid environmental changes according to the flight altitude and velocity. In this study, an avionic cooling system using R236fa was manufactured by adopting a vapor compression cycle with a closed-loop air-circulation system. Before the normal tests, the cooling system was optimized by varying the refrigerant charge amount and the expansion valve opening. The performance characteristics of the cooling system were measured by varying the compressor rotating speed, thermal load, evaporator and condenser air flow rates, and condenser inlet air temperature. The performance of the cooling system was analyzed according to these operating parameters, and possible control methods for stable operation of the cooling system were suggested.

The refrigerant injection technique has rapidly developed in recent years due to its outstanding performance at low ambient temperatures, and various control methods for a heat pump with injection have been presented. However, most studies on cycle control have been theoretical, and practical control methods for cycle optimization based on experimental results hardly have been presented. In this study, an optimum cycle control method was proposed for a refrigerant injection heat pump with a double expansion sub-cooler based on the intermediate pressure and the injection ratio. The optimum sub-cooler pressure ratio was proposed from 0.4 to 0.7 in view of the heating capacity, and from 0.7 to 0.8 in view of the COP. The optimum injection ratio increased from 0.1 to 0.3 with an increase in the compression ratio.

The performance degradation of air-source heat pumps cannot be avoided when they operate at both very low and high ambient temperatures. The refrigerant injection technique has rapidly developed in recent years due to its outstanding performance at low ambient temperatures. This study measured the heating performance of air-source heat pumps in which novel vapor injection techniques of a combined flash tank and sub-cooler (FTSC) cycle and a double expansion sub-cooler (DESC) cycle were applied. The performance of these cycles was compared with that of a flash tank (FT) and a sub-cooler (SC) cycle. The average heating capacities of the FT, FTSC, and DESC cycles were higher by 14.4%, 6.0%, and 3.8%, respectively, relative to that of the SC cycle, but the average COPs for the respective cycle options were very similar.

A heat pump has received much attention as substitute for the conventional boiler or heating coil because of its high efficiency. For the wide application of the heat pump, the most important design factor is the performance degradation upon its installation in tropical and cold regions. In this study, the effects of flash tank vapor injection on the heating performance of a two-stage heat pump with an inverter-driven twin rotary compressor were measured and analyzed for compressor frequency ranging from 50 to 100 Hz at ambient temperatures of −15, −5, and 5 °C. The COP and heating capacity of the injection cycle were enhanced by 10% and 25%, respectively, at the ambient temperature of −15 °C. The total mass flow rate of the injection cycle was 30–38% higher than that of the non-injection cycle.

Condensation heat transfer characteristics of R134a on the integral-fin tubes are experimentally investigated. The test tubes are made of stainless steel, and the root diameter of the tubes is 13.27 mm. The height of fin is 1.19 mm, and the densities of the integral fin are 19 fpi and 26 fpi. The present tests were conducted at the saturation temperatures of 20 °C and 30 °C. The condensation heat transfer coefficients of the tubes having 19 fpi and 26 fpi at the saturation temperature of 20 °C are higher than that of the plain tube by 4.4 and 3.1 times, respectively. When the temperature difference across the condensate film is less than 0.7 °C, the enhancement of the tube of 19 fpi is much larger than that of the tube of 26 fpi. The Honda and Nozu model shows the smallest mean deviation between the estimated values and experimental results among the existing models.