Jaehyeok Heo

TWO-PHASE FLOW BOILING AND CONDENSATION HEAT TRANSFER CHARACTERISTICS OF NATURAL REFRIGERANTS: REVIEW

The existing artificial and chemical refrigerants have been phased out due to environmental concerns, and they have been replaced with environmentally friendly refrigerants. Among them, carbon dioxide, ammonia, and hydrocarbons are paid attention as next generation refrigerants, and their application has been widely expanded. Therefore, in this paper, the latest studies of flow boiling and condensation heat transfer characteristics of carbon dioxide, ammonia, and hydrocarbon are reviewed. The heat transfer characteristics of ammonia and hydrocarbon show the relatively similar trends with the conventional refrigerants compared to those of carbon dioxide. The general trends and recommendable models of flow boiling and condensation heat transfer with carbon dioxide, ammonia, and hydrocarbons are summarized.

Numerical Study on the Distribution Characteristics of Aluminum Plate-Fin Heat Exchangers According to the Distributor Aspect Ratio

Aluminum plate-fin heat exchangers (ALPHEs) are widely used in petroleum and gas-treated processes. The performance of the ALPHE is strongly dependent on the distribution characteristics of the header and the distributors of each fluid. In this paper, we define the aspect ratio in the side-entry type distributor and analyze the distribution characteristics using CFD-based numerical methods. The phenomena of velocity deviation and distribution with the aspect ratio and the inlet Reynolds number were analyzed by applying relative and absolute maldistribution parameters, and an optimum aspect ratio with the inlet Reynolds number was presented.

Operational Energy Saving Potential of Thermal Effluent Source Heat Pump System for Greenhouse Heating in Jeju

Massive thermal effluent energy from power plant is mostly released to the sea, and only a little is used for fishing culture and agriculture in South Korea. The thermal effluent from the power plant can be an efficient heat source of the heat pump system to provide heating energy for the greenhouse, but energy loss and pump power by long distance pipeline installation from a power plant to the greenhouse should be considered. In this paper, an operational energy saving potential of a thermal effluent source heat pump system for the greenhouse heating was investigated. For the estimation of thermal load, three cases of greenhouse were categorized, and the thermal performance and operating energy consumption during the heating season were compared with those of a conventional ground source heat pump (GSHP) system. The model for heat pump system was newly derived to estimate the energy performance of the proposed system, and then detailed simulations for each system under three cases of greenhouse were conducted. The results showed that the operational energy of the proposed system can be saved by 17–20% than that of the conventional GSHP system.

Analytical Study on the Heating Performance Improvement of a CO 2 Heat Pump Using Vapor Injection

ABSTRACT: 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

Experimental Study on the Performance of a CO 2 Heat Pump Water Heater under Various Operating Conditions

In this study, the steady state performance of a heat pump water heater was measured with a variation of operating conditions such as refrigerant charge amount, compressor frequency, EEV opening, and water mass flow rate. Transient state performance tests were also conducted to investigate major system effects associated with the interaction between the heat pump water heater and the water tank. Optimum refrigerant charge amount for the system was 1600 g. At compressor frequencies of 50 Hz and 60 Hz, water mass flow rates of 95 kg/h and 105 kg/h, and EEV opening of 8% and 16%, the water heating temperatures were and and COPs were 3.0 and 2.8, respectively. In the transient condition, the instantaneous COP decreased with an increase in the inlet water temperature.

Condensation Heat Transfer Characteristics of R-134a with Wall Thickness and Surface Roughness on Stainless Steel Horizontal Plain Tubes

The filmwise condensation heat transfer coefficients of R-134a on the horizontal copper and stainless steel tubes were measured and analyzed. The outside diameter of the tubes was 15.88 mm, and the tube thickness ranged from 0.89 to 1.65 mm. The polished stainless steel tube had an RMS surface roughness() of 0.37 m, and commercial stainless steel tubes had an surface roughness() of 1.855 m. The tests were conducted at the saturation temperatures of 20 and , and the liquid wall subcoolings from 0.4 to . The measured condensation heat transfer coefficients were significantly lower than the predicted data by the Nusselt analysis. This trend in the stainless steel tube was explained by the effects of thermal resistance of tube material and surface roughness. Based on the experimental data with respect to wall thickness and surface roughness, it was suggested that the existing correlation on external condensation should be modified by considering material and surface roughness factors. The revised correlation was developed by introducing the effects of wall thickness and surface roughness into the Nusselt equation. The average deviation of the revised correlation was 13.0 %.

Pool Boiling Heat Transfer Characteristics of R-l34a in Titanium Horizontal Plain and Low Finned Tubes

Pool boiling heat transfer characteristics of R-134a were investigated in titanium plain and low finned tubes. The diameter of test tube was 15.88 mm and the fin density was 33 fpi. Tests were conducted at saturation temperatures of and . Heat fluxes varied from 5000 W/ to 50,000 W/ based on surface area of the plain tube. The pool boiling heat transfer coefficients of the titanium horizontal plain tube are lower than those of the copper plain tube by . The boiling heat transfer coefficients of the low finned tube are averagely higher than those of the plain tubes by . The average deviation of the Slipcevic correlation from the present data for the low finned tube is .