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For the first time, an epitaxial beryllium oxide (BeO) film was grown on 4H silicon carbide (4H-SiC) by atomic layer deposition (ALD) at a low temperature of 250 °C. The BeO film had a large lattice mismatch with the substrate (>7–8%), but it was successfully grown to a single crystal by domain-matching epitaxy (DME). The bandgap energy, dielectric constant, and thermal conductivity properties of crystalline BeO are suitable for power transistors that require low leakage currents and fast heat dissipation in high electric fields. Physical characterization confirmed the single-crystalline BeO (0 0 2). Raman analysis showed that the E1 and A1 phonon modes of ALD BeO were intermixed with the E2 and A1 phonon modes of SiC, resulting in a significant increase in phonon intensity. After heat treatment at a high temperature, a small amount of SiO2 interfacial oxide was formed but the stoichiometry of BeO was maintained. From the capacitance-voltage (C-V) curves, we obtained a dielectric constant of 6.9 and calculated a low interface trap density of 6 × 1010 cm−2·eV−1 using the Terman method at Ec-Et = 0.6 eV. The high bandgap, thermal conductivity, and excellent crystallinity reduced the dangling bonds at the interface of BeO-on-SiC.

The objective of this study is to investigate the performance characteristics of an R600a domestic refrigerator-freezer (RF) adopting a condenser outlet split (COS) ejector cycle. Experiments are conducted to measure the performances of conventional and COS ejector-based domestic RFs using R600a. A test bench is used to analyze the pressure lifting effect, mass flow rate variation, and coefficient of performance (COP) improvement with respect to the entrainment ratio (ER). For entire cycle operation at similar cooling capacity condition, the overall COP improvement of the test bench adopting COS ejector cycle over the baseline cycle is 11.4% at the ER of 0.18. Moreover, the COS ejector-based domestic RF is tested to examine its feasibility in actual applications. The COS ejector-based domestic RF with a compressor speed of 1450 rpm exhibits a temperature profile in the freezer compartment that is similar to that of the baseline domestic RF. At similar cooling capacity condition, the energy consumption of the COS ejector-based domestic RF with the compressor speed of 1450 rpm is 10.9% lower than that of the baseline domestic RF, owing to the pressure lifting effect.

In this paper, a conceptual design for a reusable unmanned space vehicle was studied with multiple objective functions. To achieve this goal, a multi-objective genetic algorithm was used, and the performance of the space vehicle was evaluated based on weight, propulsion, aerothermodynamics, and trajectory analysis. Minimization of weight and landing speed and maximization of the highest CL in the supersonic flight regime were selected as the objective functions. The maximum limits in the dynamic pressure and the heat flux were applied as constraints. All objective functions are in trade-off relationships with each other. The geometry that produced the smallest weight had a very small wing size. Also, the vehicle that had the maximum CL in the supersonic flight regime had a closer angle between the flow and the lower surface of the wing, which showed the highest CL on the flat surface. The vehicle design with the lowest landing speed had the largest wing, which generated sufficient lift.

Heat pumps, which enable the cooling and heating of vehicular cabins, consume a significant portion of the total energy consumption in electric vehicles (EVs). The efficiency of the heat pump is typically degraded owing to cold-weather conditions, so the refrigerant-injection technique has been proposed for improving the system performance and compressor reliability. In this study, a simulation model for an R134a heat pump with vapor injection is developed and validated by performing thermodynamic analyses with geometrical information. The effects of the injection-port design are investigated using the developed numerical model. Single-injection and dual-injection ports are considered to optimize the coefficient of performance (COP) and isentropic efficiency by controlling the injection mass flow rate. The optimal angles of the single- and dual-injection ports are determined to be 440° and 535°/355° (for pocket A/B), respectively, while the corresponding COPs are improved by 7.5% and 9.8%, respectively, compared to the non-injection heat pump at an outdoor temperature of −10 °C.

The objective of this study was to investigate the effects of ejector geometries on the performance of an ejector-expansion air conditioner (EEAC) considering the cooling seasonal performance factor (CSPF). The performance of the EEAC using R410A was measured and analyzed by varying the compressor speed, outdoor-bin temperature, operating pressures, nozzle-throat diameter, and mixing-section diameter. The EEAC in the medium-capacity mode exhibited maximum coefficient of performance (COP) improvement, i.e., 7.5%, over the baseline (conventional) cycle. The optimum mixing-section diameter was determined to be 9 mm based on the CSPF. In addition, the optimum mixing-section diameter increased with an increase in the annual average outdoor temperature. The CSPF of the EEAC with the optimized mixing-section diameter improved in the range of 16.0–20.3% over the baseline cycle depending on the climatic conditions.

In a liquid injection heat pump, it is very essential to control the compressor discharge temperature without wet-compression problems at extreme outdoor conditions. The objective of this study was to optimize the injection-hole geometries of a liquid injection heat pump in order to prevent the risk of wet-compression while reducing compressor discharge temperature at overload cooling conditions. In this study, a simulation model for predicting the performance of a liquid injection heat pump was developed and validated. The optimum injection-hole geometries were determined to obtain the maximum multiplication ratio, which led to a lower instant injection mass flow rate in terms of R- and θ-directional positions. In addition, the injection-hole diameter was minimized to prevent wet-compression while obtaining the target injection mass flow rate. The discharge temperature of the optimized compressor was decreased by 9.2 °C over the baseline compressor while maintaining the same risk for wet-compression at the overload cooling test condition.

The objective of this study is to investigate the performance characteristics of an injection CO 2 (carbon dioxide) HPWH (heat pump water heater) at various operating conditions. In the standard and standby heat loss test conditions, the performance of a two-stage CO 2 HPWH adopting a SCVI (sub-cooler vapor injection HPWH) cycle was measured and analyzed by varying the compressor frequency, the water flow rate, and the injection ratio at various outdoor temperatures. In the standard tests at the outdoor temperature of −15 °C, the COP (coefficient of performance) of the optimized SCVI HPWH was 7.6% higher than that of the non-injection HPWH. During the standby loss tests at the water flow rate of 200 kg h −1 , the COP of the optimized SCVI HPWH was 7.1% higher than that of the non-injection HPWH.

A heat pump has been considered as a thermal management unit for electric vehicles, including the heating and cooling of the cabin. However, the heat pump shows performance degradation at low outdoor temperatures or high compressor speeds. In this study, a R-134a heat pump for an electric vehicle was designed to improve system efficiency, by applying vapor injection with an internal heat exchanger. The heating performance characteristics of the vapor injection heat pump were analyzed at various compressor speeds and outdoor temperatures. The vapor injection heat pump showed 13.3% COP improvement over the non-injection heat pump, when the heating capacity was fixed at 5.2 kW. In addition, the heating capacity of the vapor injection system increased by 9.6%, as compared to the non-injection system.

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.

The injection mass flow rate in a vapor injection CO2 cycle is an important design factor to improve the heating performance of a heat pump at low ambient temperatures. The intermediate pressure in a twin rotary compressor strongly affects the injection mass flow rate, and can be adjusted by the cylinder volume ratio of the second-stage to the first-stage. In this study, the heating performances of the vapor injection CO2 cycles including the flash tank vapor injection (FTVI) and sub-cooler vapor injection (SCVI) cycles were measured by varying the cylinder volume ratio of the twin rotary compressor at the ambient temperature of −15 °C. From the results, the optimum injection ratio yielding the maximum COP was determined for each cylinder volume ratio. In addition, in the FTVI and SCVI cycles, the optimum cylinder volume ratio was determined as 0.7 in order to achieve the maximum COP with the designed heating capacity.

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.

A cooling system is adopted to control the thermal load from the avionic equipments in an aircraft for stable operation. In this study, an avionic cooling system was designed and manufactured by adopting a vapor compression cycle with a closed-loop air-circulation system to investigate the operating characteristics of an alternative refrigerant. The performance characteristics of a cooling system adopting R236fa as an alternative refrigerant were experimentally determined by varying the refrigerant charging amount, expansion valve opening, and compressor rotation speed. The experimental results were analyzed and compared with those of a cooling system adopting R124 as a refrigerant. The possibility of the adoption of R236fa as an alternative refrigerant was verified, and design solutions were suggested to improve the system efficiency.

In this study, a simulation model for a CO 2 heat pump using vapor injection was developed and validated. It was used to predict the improvement of the heating performance of the CO 2 heat pump at various operating conditions. The simulation results showed consistent results with the measured data. The heating performances of the vapor injection and non-injection heat pumps were compared by varying the outdoor temperature and compressor frequency. The heating capacity of the vapor injection heat pump was 40% higher than that of the non-injection heat pump at the outdoor temperature of -8℃. The performance of the vapor injection heat pump was consistently higher than that of the non-injection heat pump even when the compressor frequency was reduced to 35 Hz at the outdoor temperature of -3℃.Keywords: CO 2 heat pump(이산화탄소 열펌프), Vapor injection(가스인젝션), Simulation(시뮬레이션) †Corresponding author Tel.: +82-2-3290-3366; fax: +82-2-921-5439 E-mail address: yongckim@korea.ac.kr 기 호 설 명