Rin Yun

Heat Transfer and Pressure Drop Simulation of CO2-Hydrate Mixture in Tube

The formation of CO2 hydrate during CO2 transportation presents a complex two-phase flow within tube. A two-dimensional CFD model for CO2 hydrate mixture flow in tube is derived based on the Eulerian multiphase flow modeling approach in which the two phases consist of CO2 gas and CO2 hydrate particles. A coupled Eulerian multiphase and nonisothermal flow model without phase-change is developed based on COMSOL Multiphysics built-in application modes. The model couples the mass, momentum, and energy equations for the two phases to solve the temperature and flow characteristics of the CO2 hydrate mixture flow in tube. CO2 hydrate particles are found to settle down during flow even under high velocity operation. The pressure drop increased linearly with inlet volume fraction from 1.29kPa for 0.1–5.2kPa for 0.5, and the related overall heat transfer coefficients of the CO2 hydrate mixture computed from the model ranged from 980 to 4000W/m2K with variation of CO2 hydrate volume fraction.

Operation Strategy and Parametric Analysis of a CHP and a Tri-Generation System for Integrated Community

Thermal efficiencies of Combined Heat and Power (CHP) and Tri-generation system, which are installed in a residential district and a general hospital, respectively, are investigated by using RETScreen. Economic feasibility and amount of CO2 reduction are also estimated by comparing them with separate energy system, which separately supplies electricity, heating and cooling energy to consumer. When power generation capacity of the CHP system is changed from 26 to 40 MW, the efficiency is the highest at 26 MW. It is found that equity payback period is 5.7 years. Power generation capacity of the Tri-generation system for a general hospital was estimated to be 3.2 MW. Depending on the insulation amount of the building, profits and operating efficiency of the Tri-generation system are significantly changed. As an operating strategy, the type of following electricity load shows the highest thermal efficiency of 86%. When cost of electricity is raised by 10% and 20% of the present price, simple payback period is reduced from 5.2 years to 3.0 and 2.1 years, respectively.

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.

INFLOW CONDENSATION HEAT TRANSFER CHARACTERISTICS OF CO2 IN MICROCHANNEL

In this study, we investigated two-phase flow patterns and effects of oil concentration on the heat transfer coefficient and pressure drop of CO2 undergoing condensation process in a microchannel, and from the data collected, we developed a prediction model of CO2 condensation heat transfer coefficient in smooth tube and microchannels. The narrow single rectangular channel was utilized to observe flow patterns of CO2 under the condensation process. Experimental results show that the transition of vapor quality from intermittent flow to annular flow advances with increase of mass flux and with decrease of condensation temperature. The heat transfer coefficient decreased by 50% as compared to that of the pure CO2 when the oil concentration was increased from 0.7 to 1.2 wt.% for the mass flux of 600 kg ⋅ m-2 ⋅ s-1. The pressure drop slightly decreased with oil concentration as compared to that of pure CO2. We developed the prediction model for the heat transfer coefficient of CO2 undergoing condensation process in microchannel by considering the effects of the liquid film thickness and of the interface shape between liquid and vapor phases on the heat transfer coefficient. The present prediction model estimated the experimental data within 18.9% of mean deviation. For the pressure drop in microchannel tubes, the existing models developed by Mishima and Hibiki, and Garimella showed the marginal predictability.

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.

Cooling and Heating Operation Characteristics of Raw-water Source Heat Pump and Air Source Heat Pump in Water Treatment Facility

The dynamic characteristics of both raw-water source and air source heat pump utilized in water treatment facilities were investigated by using TRNSYS simulator. The modeling of the raw water source heat pump was verified by the measured data at the Cheongju water treatment facility, and the modeling at the air source heat pump was verified by the data from the Siheung water treatment facility. The average heating and cooling COPs from the raw-water source heat pump were higher than those of the air source heat pump by 19% and 18%, respectively. The power consumptions of the air source heat pump for the cooling and the heating were higher than those of the raw water source heat pump by 28% and 26%, respectively.

Optimum Design of Middle-Sized CO 2 Water Heater

Middle-sized CO2 water heater having compressor power of 7.45 kW was designed, and its performances were experimentally tested. Besides, optimum design of the CO2 water heater was conducted by cycle simulation. When ambient temperature of 7℃ and hot water outlet temperature of 80℃ the CO2 water heater showed the COP of 3.2. As hot water temperature increased the COP is getting decreased due to significant increase of compressor power consumption compared to increasing rate of heating capacity. When ambient temperature increased from -3℃ to 12℃ the COP increased by 30%. The optimum components design of a gas cooler, an internal heat exchanger, and an evaporator were conducted, and the experimental correlation between amount of EEV opening and ambient temperature, and hot water temperature was suggested.

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 %.

Numerical Study on Two-Phase Flow of Sand and Water Mixture in Pipelines

Torrential rainfall in Korea causes the large amount of water and soil mixture flow into drainage pipes. Soil in the mixture is sedimented in drainage pipes and frequently blocks the pipes, results in flooding in a city. In the present study, the effects of inlet velocity, size of soil particle, and volume concentration of soil in the soil and water mixture on the flow characteristics and the soil distribution inside the pipe were numerically investigated by using the two-phase mixture model. The horizontally located pipe, whose length and diameter are 10 m and 0.6 m, was used in this simulation. With increase of inlet mixture velocity, velocity at the upper part of the tube is higher than that at lower part by 14.9%, and the volume fraction of soil from bottom to 0.2 m of tube decreased subsequently by 4.6%. The distribution of mixture velocity and soil volume fraction is getting uniform with increase of the inlet volume fraction of the soil in the mixture. With increase of size of the soil particle, the soil concentration increased subsequently by 6.6% from bottom to 0.2 m of the pipe, which make significant distortions in velocity distribution in tube.

Optimization of a Raw Water Source Heat Pump for a Vertical Water Treatment Building

The optimum operation conditions of a raw water source heat pump for a vertical water treatment building were derived by changing operation parameters, such as temperature of thermal storage tank, temperature and inlet air flow rate of the conditioned spaces, and circulating water flow rate between thermal storage tank and air handling unit (AHU) through dynamic simulator of a transient system simulation program (TRNSYS). Minimum electric power consumption was found at temperature of thermal storage tank, which was ranged 18–23°C for cooling season. In heating season, temperature 40–45°C brings the highest coefficient of performance (COP) and temperature range of 30–35°C brings the lowest power consumption. When the temperature of the conditioned spaces was controlled between 27–28°C for cooling season, and 18–20°C for heating season the minimum electric power consumption was obtained. Inlet air flow rate of 1.1 m3/h for the conditioned spaces shows the highest performance of the present system, and effects of circulating water flow rate between thermal storage tank and AHU on minimum electric power consumption of the system were negligible.