Korean Journal of Air-Conditioning and Refrigeration Engineering, 2011
Recently, many researchers have analyzed the performance of the transcritical CO2 refrigeration c... more Recently, many researchers have analyzed the performance of the transcritical CO2 refrigeration cycle in order to identify opportunities to improve the system energy efficiency. The reduction of the expansion process losses is one of the key issues to improve the efficiency of the transcritical CO2 refrigeration cycle. In this study, the analytical study on the performance characteristics of CO2 cycle with an ejector carried out with a variation of outdoor temperature, gascooler inlet air velocity, evaporator inlet air velocity, and evaporator inlet air temperature. As a result, the system performance could be improved over 85% by using an ejector for various operating condition because of the reduction of compressor work. Moreover, the cooling capacity increased about 18% for variable outdoor condition. Therefore, the high performance of an ejector system could be maintained for wide operating conditions and system reliability could be improved compared to that of a basic system.
Abstract The electroencephalogram (EEG), photoplethysmography (PPG), subjective questionnaires, s... more Abstract The electroencephalogram (EEG), photoplethysmography (PPG), subjective questionnaires, surface temperature, and concentration of subjects for various resting conditions in the building under cooling and heating conditions were investigated to improve their quality of rest and examine the effect of environmental changes on various biosignals comprehensively. As a result, the use of a cold water seat during a rest under cooling condition activated θ wave and α wave in the subjects and decreased low frequency/high frequency (LF/HF) ratio and stress. In addition, the hot water seat used during a rest under heating conditions provided subjects with a stable rest environment because it activated the PPG and θ wave of the subjects and decreased the LF/HF ratio. However, the stress index of the subjects was slightly higher when electric heating and hot water seats were used. The subjective questionnaire showed that the use of an electric heating seat provided subjects with a comfortable environment and high concentration compared to those of other seat types.
In this study, computational fluid dynamics (CFD) analysis was performed to investigate the cause... more In this study, computational fluid dynamics (CFD) analysis was performed to investigate the cause of the thermal stratification in the channel and the temperature non-uniformity of the plate heat exchanger. The flow velocity maldistribution of the channel and the merging parts caused temperature non-uniformity in the channel width direction. The non-uniformity of flow velocity and temperature in the channel is shown in Section 1 > Section 3 > Section 2 from the heat exchanger. The non-uniform temperature distribution in the channel caused channel stratification and non-uniform outlet temperature. Stratification occurred at the channel near the merging due to the flow rate non-uniformity in the channel. In particular, as the mass flow rate increased from 0.03 to 0.12 kg/s and the effectiveness increased from 0.436 to 0.615, the cold-side stratified volume decreased from 4.06 to 3.7 cm3, and the temperature difference between the stratified area and the outlet decreased from 1.2...
Journal of Mechanical Science and Technology, 2016
The thermal performance of an individual pipe in an evacuated tube solar collector with a heat pi... more The thermal performance of an individual pipe in an evacuated tube solar collector with a heat pipe is investigated by an analytical method based on the energy balance of the collector. A simulation method was developed and compared to experimental results under different operating conditions, such as different ambient temperature and changes in weather. After comparing the simulation and experimental results, a discrepancy of about 0–6% is observed due to the simple assumptions in the simulation process and unavoidable human factors. The results show that under conditions of poor radiation and low ambient temperature, the efficiency changes rapidly, while it is relatively steady in high-radiation situations, and is influenced by differences in the ambient temperature and the collector inlet temperature. In addition to the change in the tilt angle of the solar collectors, the solar radiation collected by the absorber unit varies, which leads to differences in the outlet temperature and the efficiency.
Korean Journal of Air-Conditioning and Refrigeration Engineering, 2015
This paper presents a numerical study on the performance of a vapor compression cycle equipped wi... more This paper presents a numerical study on the performance of a vapor compression cycle equipped with an ejector as an expansion device to improve the COP by reducing the expansion loss and compressor work. The simulation is carried out using a model based on the conservation of mass, energy and momentum in the ejector. From the results of the simulation, the vapor compression cycle equipped with an ejector showed a maximum COP improvement of 14.0% when using R134a refrigerant and 16.8% when using R1234yf. In addition, the performance of the system with an ejector represents the increased performance as the temperature difference between condensing and evaporating increased.
International Journal of Automotive Technology, 2015
The formation of frost causes a decrease in the heat exchanger performance because of increased t... more The formation of frost causes a decrease in the heat exchanger performance because of increased thermal resistance. In this study, the performance of a refrigeration system in a refrigerator truck with frost growth was determined and the performance characteristics were analyzed considering the indoor and outdoor temperature, outdoor air velocity, and compressor rotating speed. As a result, under frost growth conditions, refrigeration capacity was decreased by 30.4% when the operating time passed by 3 hours which had a blocking ratio of 40.1%. In addition, the mass flow rate was decreased, but the pressure ratio was increased with an increase in frost thickness, leading to a decrease in the refrigeration capacity and COP. The maximum system COP was found at a compressor rotating speed of 1500 rpm: 2.30 and 1.86 for a blocking ratio of 0% and 40.1%, respectively. It gradually decreased over 1500 rpm.
Numerical method was developed to calculate the thermal performance of the molten salt cavity rec... more Numerical method was developed to calculate the thermal performance of the molten salt cavity receiver. Complicated photo-thermal conversion process in the molten salt receiver was studied in detail. Optimum cavity geometry was obtained with a lesser depth for improving thermal efficiency.
This paper discusses the forced convective heat transfer characteristics of water–ethylene glycol... more This paper discusses the forced convective heat transfer characteristics of water–ethylene glycol (EG)-based Fe3O4 nanofluid and Fe3O4–MWCNT hybrid nanofluid under the effect of a magnetic field. The results indicated that the convective heat transfer coefficient of magnetic nanofluids increased with an increase in the strength of the magnetic field. When the magnetic field strength was varied from 0 to 750 G, the maximum convective heat transfer coefficients were observed for the 0.2 wt% Fe3O4 and 0.1 wt% Fe3O4–MWNCT nanofluids, and the improvements were approximately 2.78% and 3.23%, respectively. The average pressure drops for 0.2 wt% Fe3O4 and 0.2 wt% Fe3O4–MWNCT nanofluids increased by about 4.73% and 5.23%, respectively. Owing to the extensive aggregation of nanoparticles by the external magnetic field, the heat transfer coefficient of the 0.1 wt% Fe3O4–MWNCT hybrid nanofluid was 5% higher than that of the 0.2 wt% Fe3O4 nanofluid. Therefore, the convective heat transfer can be...
In this study, electroencephalogram (EEG) and cardiac activity status of the human body while usi... more In this study, electroencephalogram (EEG) and cardiac activity status of the human body while using various types of seats during rest were analyzed in indoor summer conditions. Thermal comfort was also evaluated through a subjective survey. The EEG, cardiac activity status, and subjective survey during rest indicated that the use of ventilation and cold water-cooling seats was effective. This effectiveness was because of the θ-wave and α-wave activation, sensorimotor rhythm, β-wave reduction, and left hemisphere activation, demonstrating that the conditions applied were suitable for rest. According to the analysis of the subjective questionnaire survey, the use of ventilation and cold water-cooling seats provided a more pleasant state than the basic seat, improving the subject’s warmth and comfort, and also the concentration. In addition, the use of a cold water-cooling seat provided the highest satisfaction level, being the most favorable condition for rest.
This paper presents an analytical evaluation of the performance of low-GWP refrigerants in a finn... more This paper presents an analytical evaluation of the performance of low-GWP refrigerants in a finned-tube evaporator used for residential cooling applications. The study employed an evolutionary-computation optimization module to examine the effect of a refrigerant circuitry design on performance of twelve alternative refrigerants for R22 and R410A. The study showed that high-pressure fluids benefited most from refrigerant circuitry optimization, with R744 achieving a capacity increase of over 13 %. Among lower-pressure fluids, the capacity of an R717 evaporator increased by over 10 %. The effect of optimized refrigerant circuitries on the system performance was estimated. The results show that zeotropic blends with a large temperature glide are particularly sensitive to the refrigerant circuitry and may suffer significant performance degradation in heat exchangers with improper design.
This study reports the in-depth investigation of the thermophysical properties and thermal reliab... more This study reports the in-depth investigation of the thermophysical properties and thermal reliability of caprylic acid-stearyl alcohol (CA-SA) eutectic phase change material (PCM) for cooling applications. The phase diagram of CA-SA showed a eutectic point at a 90:10 molar ratio. The onset melting/freezing temperature and latent heat of fusion of caprylic acid-stearyl alcohol from the differential scanning calorimetry (DSC) were 11.4 °C/11.8 °C and 154.4/150.5 J/g, respectively. The thermal conductivity for the prepared eutectic PCM in the solid phase was 0.267 W/m.K (0 °C), whereas, in the liquid phase, it was 0.165 W/m.K (20 °C). In addition, the maximum relative percentage difference (RPD) marked at the end of 200 thermal cycles was 5.2% for onset melting temperature and 18.9% for phase change enthalpy. The Fourier transform infrared spectroscopy (FT-IR) result shows that the eutectic PCM holds good chemical stability. Corrosion tests showed that caprylic acid-stearyl alcohol co...
The Solar and geothermal energy have many advantage like low cost, non-toxic, and unlimited. But ... more The Solar and geothermal energy have many advantage like low cost, non-toxic, and unlimited. But those the have very low energy efficiency. In this study, the theoretical study of performance in a sola-geothermal hybrid heat pump with operating conditions has carried out. As a result, as the solar radiation increases from 1 to 20 , the heat pump operating time decreases by 19.5% from 18 times to 14.5 times and the heat pump heat decreases by 23%. Besides, the heating COP increases by 21.4% when the evaporator inlet temperature increases from to . By adapting the geothermal system into a solar hybrid R22 heat pump, the system performance and reliability increases significantly for variable operating conditions during winter season.
In this study, electroencephalogram (EEG), photo-plethysmography (PPG), and surface temperature m... more In this study, electroencephalogram (EEG), photo-plethysmography (PPG), and surface temperature measurements of subjects were taken while performing a driving simulation when the cabin and vent discharge air temperature in summer were changed from discomfort to comfort conditions. Additionally, subjective questionnaires were used to analyze the subject’s thermal comfort under the various driving environments. As a result, the surface temperatures of the forehead, left hand, right hand, and abdomen of the subject during driving were reduced by 2, 0.97, 2.18, and 5.86 °C, respectively, by operating a 12.5 °C vent cooling function at a cabin temperature of 35 °C. As a comprehensive analysis of the subjective survey, PPG, and EEG results, total power (TP), the standard deviation of N-N interval (SDNN), and the root mean square of successive differences (RMSSD) of subjects increased and stress index decreased at cabin and vent discharge air temperatures of 30–27.5 °C and 16.5–18.5 °C, re...
The performance of a finned tube heat exchanger is strongly affected by the velocity distribution... more The performance of a finned tube heat exchanger is strongly affected by the velocity distribution of the air that passes through it. The air velocity distribution for finned-tube heat exchangers has gone largely undocumented because it is difficult to measure accurately. In this study, the air velocity distribution upstream of three heat exchangers installed in a horizontal duct is examined; a vertically oriented single-slab coil, a single-slab coil placed at an angle of 65o to the duct wall, and a two-slab A-Shaped coil with a 34o apex angle. The data was taken using a non-intrusive, laser based technique called Particle Image Velocimetry (PIV). The measurement results show that the air inlet velocity profile is strongly influenced by the heat exchanger’s design and application.
Korean Journal of Air-Conditioning and Refrigeration Engineering, 2011
Recently, many researchers have analyzed the performance of the transcritical CO2 refrigeration c... more Recently, many researchers have analyzed the performance of the transcritical CO2 refrigeration cycle in order to identify opportunities to improve the system energy efficiency. The reduction of the expansion process losses is one of the key issues to improve the efficiency of the transcritical CO2 refrigeration cycle. In this study, the analytical study on the performance characteristics of CO2 cycle with an ejector carried out with a variation of outdoor temperature, gascooler inlet air velocity, evaporator inlet air velocity, and evaporator inlet air temperature. As a result, the system performance could be improved over 85% by using an ejector for various operating condition because of the reduction of compressor work. Moreover, the cooling capacity increased about 18% for variable outdoor condition. Therefore, the high performance of an ejector system could be maintained for wide operating conditions and system reliability could be improved compared to that of a basic system.
Abstract The electroencephalogram (EEG), photoplethysmography (PPG), subjective questionnaires, s... more Abstract The electroencephalogram (EEG), photoplethysmography (PPG), subjective questionnaires, surface temperature, and concentration of subjects for various resting conditions in the building under cooling and heating conditions were investigated to improve their quality of rest and examine the effect of environmental changes on various biosignals comprehensively. As a result, the use of a cold water seat during a rest under cooling condition activated θ wave and α wave in the subjects and decreased low frequency/high frequency (LF/HF) ratio and stress. In addition, the hot water seat used during a rest under heating conditions provided subjects with a stable rest environment because it activated the PPG and θ wave of the subjects and decreased the LF/HF ratio. However, the stress index of the subjects was slightly higher when electric heating and hot water seats were used. The subjective questionnaire showed that the use of an electric heating seat provided subjects with a comfortable environment and high concentration compared to those of other seat types.
In this study, computational fluid dynamics (CFD) analysis was performed to investigate the cause... more In this study, computational fluid dynamics (CFD) analysis was performed to investigate the cause of the thermal stratification in the channel and the temperature non-uniformity of the plate heat exchanger. The flow velocity maldistribution of the channel and the merging parts caused temperature non-uniformity in the channel width direction. The non-uniformity of flow velocity and temperature in the channel is shown in Section 1 > Section 3 > Section 2 from the heat exchanger. The non-uniform temperature distribution in the channel caused channel stratification and non-uniform outlet temperature. Stratification occurred at the channel near the merging due to the flow rate non-uniformity in the channel. In particular, as the mass flow rate increased from 0.03 to 0.12 kg/s and the effectiveness increased from 0.436 to 0.615, the cold-side stratified volume decreased from 4.06 to 3.7 cm3, and the temperature difference between the stratified area and the outlet decreased from 1.2...
Journal of Mechanical Science and Technology, 2016
The thermal performance of an individual pipe in an evacuated tube solar collector with a heat pi... more The thermal performance of an individual pipe in an evacuated tube solar collector with a heat pipe is investigated by an analytical method based on the energy balance of the collector. A simulation method was developed and compared to experimental results under different operating conditions, such as different ambient temperature and changes in weather. After comparing the simulation and experimental results, a discrepancy of about 0–6% is observed due to the simple assumptions in the simulation process and unavoidable human factors. The results show that under conditions of poor radiation and low ambient temperature, the efficiency changes rapidly, while it is relatively steady in high-radiation situations, and is influenced by differences in the ambient temperature and the collector inlet temperature. In addition to the change in the tilt angle of the solar collectors, the solar radiation collected by the absorber unit varies, which leads to differences in the outlet temperature and the efficiency.
Korean Journal of Air-Conditioning and Refrigeration Engineering, 2015
This paper presents a numerical study on the performance of a vapor compression cycle equipped wi... more This paper presents a numerical study on the performance of a vapor compression cycle equipped with an ejector as an expansion device to improve the COP by reducing the expansion loss and compressor work. The simulation is carried out using a model based on the conservation of mass, energy and momentum in the ejector. From the results of the simulation, the vapor compression cycle equipped with an ejector showed a maximum COP improvement of 14.0% when using R134a refrigerant and 16.8% when using R1234yf. In addition, the performance of the system with an ejector represents the increased performance as the temperature difference between condensing and evaporating increased.
International Journal of Automotive Technology, 2015
The formation of frost causes a decrease in the heat exchanger performance because of increased t... more The formation of frost causes a decrease in the heat exchanger performance because of increased thermal resistance. In this study, the performance of a refrigeration system in a refrigerator truck with frost growth was determined and the performance characteristics were analyzed considering the indoor and outdoor temperature, outdoor air velocity, and compressor rotating speed. As a result, under frost growth conditions, refrigeration capacity was decreased by 30.4% when the operating time passed by 3 hours which had a blocking ratio of 40.1%. In addition, the mass flow rate was decreased, but the pressure ratio was increased with an increase in frost thickness, leading to a decrease in the refrigeration capacity and COP. The maximum system COP was found at a compressor rotating speed of 1500 rpm: 2.30 and 1.86 for a blocking ratio of 0% and 40.1%, respectively. It gradually decreased over 1500 rpm.
Numerical method was developed to calculate the thermal performance of the molten salt cavity rec... more Numerical method was developed to calculate the thermal performance of the molten salt cavity receiver. Complicated photo-thermal conversion process in the molten salt receiver was studied in detail. Optimum cavity geometry was obtained with a lesser depth for improving thermal efficiency.
This paper discusses the forced convective heat transfer characteristics of water–ethylene glycol... more This paper discusses the forced convective heat transfer characteristics of water–ethylene glycol (EG)-based Fe3O4 nanofluid and Fe3O4–MWCNT hybrid nanofluid under the effect of a magnetic field. The results indicated that the convective heat transfer coefficient of magnetic nanofluids increased with an increase in the strength of the magnetic field. When the magnetic field strength was varied from 0 to 750 G, the maximum convective heat transfer coefficients were observed for the 0.2 wt% Fe3O4 and 0.1 wt% Fe3O4–MWNCT nanofluids, and the improvements were approximately 2.78% and 3.23%, respectively. The average pressure drops for 0.2 wt% Fe3O4 and 0.2 wt% Fe3O4–MWNCT nanofluids increased by about 4.73% and 5.23%, respectively. Owing to the extensive aggregation of nanoparticles by the external magnetic field, the heat transfer coefficient of the 0.1 wt% Fe3O4–MWNCT hybrid nanofluid was 5% higher than that of the 0.2 wt% Fe3O4 nanofluid. Therefore, the convective heat transfer can be...
In this study, electroencephalogram (EEG) and cardiac activity status of the human body while usi... more In this study, electroencephalogram (EEG) and cardiac activity status of the human body while using various types of seats during rest were analyzed in indoor summer conditions. Thermal comfort was also evaluated through a subjective survey. The EEG, cardiac activity status, and subjective survey during rest indicated that the use of ventilation and cold water-cooling seats was effective. This effectiveness was because of the θ-wave and α-wave activation, sensorimotor rhythm, β-wave reduction, and left hemisphere activation, demonstrating that the conditions applied were suitable for rest. According to the analysis of the subjective questionnaire survey, the use of ventilation and cold water-cooling seats provided a more pleasant state than the basic seat, improving the subject’s warmth and comfort, and also the concentration. In addition, the use of a cold water-cooling seat provided the highest satisfaction level, being the most favorable condition for rest.
This paper presents an analytical evaluation of the performance of low-GWP refrigerants in a finn... more This paper presents an analytical evaluation of the performance of low-GWP refrigerants in a finned-tube evaporator used for residential cooling applications. The study employed an evolutionary-computation optimization module to examine the effect of a refrigerant circuitry design on performance of twelve alternative refrigerants for R22 and R410A. The study showed that high-pressure fluids benefited most from refrigerant circuitry optimization, with R744 achieving a capacity increase of over 13 %. Among lower-pressure fluids, the capacity of an R717 evaporator increased by over 10 %. The effect of optimized refrigerant circuitries on the system performance was estimated. The results show that zeotropic blends with a large temperature glide are particularly sensitive to the refrigerant circuitry and may suffer significant performance degradation in heat exchangers with improper design.
This study reports the in-depth investigation of the thermophysical properties and thermal reliab... more This study reports the in-depth investigation of the thermophysical properties and thermal reliability of caprylic acid-stearyl alcohol (CA-SA) eutectic phase change material (PCM) for cooling applications. The phase diagram of CA-SA showed a eutectic point at a 90:10 molar ratio. The onset melting/freezing temperature and latent heat of fusion of caprylic acid-stearyl alcohol from the differential scanning calorimetry (DSC) were 11.4 °C/11.8 °C and 154.4/150.5 J/g, respectively. The thermal conductivity for the prepared eutectic PCM in the solid phase was 0.267 W/m.K (0 °C), whereas, in the liquid phase, it was 0.165 W/m.K (20 °C). In addition, the maximum relative percentage difference (RPD) marked at the end of 200 thermal cycles was 5.2% for onset melting temperature and 18.9% for phase change enthalpy. The Fourier transform infrared spectroscopy (FT-IR) result shows that the eutectic PCM holds good chemical stability. Corrosion tests showed that caprylic acid-stearyl alcohol co...
The Solar and geothermal energy have many advantage like low cost, non-toxic, and unlimited. But ... more The Solar and geothermal energy have many advantage like low cost, non-toxic, and unlimited. But those the have very low energy efficiency. In this study, the theoretical study of performance in a sola-geothermal hybrid heat pump with operating conditions has carried out. As a result, as the solar radiation increases from 1 to 20 , the heat pump operating time decreases by 19.5% from 18 times to 14.5 times and the heat pump heat decreases by 23%. Besides, the heating COP increases by 21.4% when the evaporator inlet temperature increases from to . By adapting the geothermal system into a solar hybrid R22 heat pump, the system performance and reliability increases significantly for variable operating conditions during winter season.
In this study, electroencephalogram (EEG), photo-plethysmography (PPG), and surface temperature m... more In this study, electroencephalogram (EEG), photo-plethysmography (PPG), and surface temperature measurements of subjects were taken while performing a driving simulation when the cabin and vent discharge air temperature in summer were changed from discomfort to comfort conditions. Additionally, subjective questionnaires were used to analyze the subject’s thermal comfort under the various driving environments. As a result, the surface temperatures of the forehead, left hand, right hand, and abdomen of the subject during driving were reduced by 2, 0.97, 2.18, and 5.86 °C, respectively, by operating a 12.5 °C vent cooling function at a cabin temperature of 35 °C. As a comprehensive analysis of the subjective survey, PPG, and EEG results, total power (TP), the standard deviation of N-N interval (SDNN), and the root mean square of successive differences (RMSSD) of subjects increased and stress index decreased at cabin and vent discharge air temperatures of 30–27.5 °C and 16.5–18.5 °C, re...
The performance of a finned tube heat exchanger is strongly affected by the velocity distribution... more The performance of a finned tube heat exchanger is strongly affected by the velocity distribution of the air that passes through it. The air velocity distribution for finned-tube heat exchangers has gone largely undocumented because it is difficult to measure accurately. In this study, the air velocity distribution upstream of three heat exchangers installed in a horizontal duct is examined; a vertically oriented single-slab coil, a single-slab coil placed at an angle of 65o to the duct wall, and a two-slab A-Shaped coil with a 34o apex angle. The data was taken using a non-intrusive, laser based technique called Particle Image Velocimetry (PIV). The measurement results show that the air inlet velocity profile is strongly influenced by the heat exchanger’s design and application.
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Papers by Honghyun Cho