Ji Hwan Jeong

Numerical Analysis of Experimental Observations for Heat Transfer Augmentation by Ultrasonic Vibration

Previous experimental measurements showed that heat transfer performance of heaters submerged in a water pool is enhanced when ultrasonic vibration is applied to the pool. The increase rate in CHF values varied depending on the inclination angle of the heated surface with the maximum when horizontal and facing downward. Despite the obvious enhancement of heat transfer performance, it was not clear how ultrasonic vibration influenced heat transfer in a pool. In order to understand the mechanism, the experimental conditions inside the water pool with and without ultrasonic vibration were numerically analyzed. A commercial CFD package that has a dynamic mesh model, FLUENT V.6.1, was used in the analysis. The results show that the standard deviation of temperature distribution in the pool is reduced when ultrasonic vibration is applied to the pool. The reduction rate of standard deviation of temperature is greatest when the heated surface faces downward and decreases as the inclination angle of heated surfaces increases. This trend coincides with the trend of CHF increase rate. Based on these results, it is concluded that an ultrasonic vibration enhances fluid mixing around the heater and thus provides heated surface with fresh water. This has an effect of lowering liquid temperature adjacent to heater surface, and in turn, leads to a heat transfer augmentation.

Review of Heat Exchanger Studies for High-Efficiency Gas Turbines

Air transportation has been being expanded remarkably, and its growth is expected to continue in the coming decades. Environmental issues and airlines require gas turbine manufacturers to produce environmentally friendly gas-turbine engines with lower emissions and improved specific fuel consumption. These requirements can be met by incorporating heat exchangers into gas turbines for intercooling and recuperation. Relevant research in such areas as the design of a heat exchanger matrix, materials selection, and manufacturing technology and optimization has been carried out by a variety of researchers. These works are reviewed in this paper. The recent advance in technologies appears to herald the development of intercoolers and recuperators for civil aeroplane gas turbines. Based on results reported in previous studies, potential heat exchanger designs for an aero gas turbine recuperator, intercooler, and cooling-air cooler are suggested.Copyright

Experimental Study on Critical Power in a Hemispherical Narrow Gap

Abstract An experimental study of critical heat flux in gap (CHFG) has been performed to investigate the inherent cooling mechanism in a hemispherical narrow gap. The objectives of the CHFG test are to measure critical power from a critical heat removal rate through the hemispherical narrow gap using distilled water with experimental parameters of system pressure and gap width. The CHFG test results have shown that a countercurrent flow limitation (CCFL) brings about local dryout at the small edge region of the upper part and finally global dryout in a hemispherical narrow gap. Increases in the gap width and pressure lead to an increase in critical power. The measured values of critical power are lower than the predictions made by other empirical CHF correlations applicable to flat plate, annuli, and small spherical gaps. The measured data on critical power in the hemispherical narrow gaps have been correlated using nondimensional parameters with a range of approximately ±20%. The developed correlation has been expanded to apply the spherical geometry using the Siemens/KWU correlation.

AN EVALUATION OF CONSTITUENT CORRELATIONS FOR PREDICTING REFRIGERANT CHARACTERISTICS IN ADIABATIC CAPILLARY TUBES

Capillary tubes are widely used as a refrigerant flow control device in small refrigeration systems. Since the flow behavior inside the capillary tube is complex, many physical models are necessary to predict the characteristics of the refrigerant flow in a capillary tube. In the present paper, refrigerant flow characteristics inside the capillary tube have been studied to find out recommended empirical correlations of influential parameters. A numerical capillary model is developed to predict the refrigerant characteristics. Various empirical correlations regarding single-phase friction factor, two-phase viscosity, two-phase frictional multiplier and metastable flow are examined using this numerical capillary model. Calculated results are compared with experimental data to examine the accuracy in terms of required capillary tube length and mass flow rate. Based on the comparison, recommended correlations are selected to be used for capillary flow analysis.

Effects of tube rupture modeling and the parameters on the analysis of multiple steam generator tube rupture event progression in APR1400

A multiple steam generator tube rupture (MSGTR) event in APR1400 has been investigated using the best estimate thermal hydraulic system code, MARS1.4. The effects of the parameters such as the number of ruptured tubes, rupture location, affected steam generator on the analysis of the MSGTR event in APR1400 are examined. In particular, tube rupture modeling methods, single tube modeling (STM) and double tube modeling (DTM), are compared. The APR1400 is found to have the capability of allowing more than 30 min to operators for the MSGTR event of five tubes. The effects of rupture location on the MSSV lift time is not significant in the case of STM, but the MSSV lift time for tube-top rupture is found to be 25.3% larger than that for rupture at the hot-leg side tube sheet in the case of DTM. The MSSV lift time for the cases that both steam generators are affected (4C5x, 4C23x) are found to be larger than that of the single steam generator cases (4A5x, 4B5x) due to a bifurcation of the primary leak flow. The discharge coefficient of Cd is found to affect the MSSV lift time only for a smaller value of 0.5. It is found that the most dominant parameter governing the MSSV lift time is the leak flow rate. Whether any modeling method is used, it gives the similar MSSV lift time if the leak flow rate is close, except in the case where both steam generators are affected. Therefore, the system performance and the MSSV lift time of the APR1400 are strongly dependent on the break flow model used in the best estimate system code.

SUCTION PIPE DESIGN CRITERION FOR R-134a REFRIGERATORS TO SECURE OIL RETURN TO COMPRESSOR

Polyol Ester oil–air two-phase counter current flow experiments were performed with small diameter tubes to measure gas velocities for the counter current flow limitation point and the flow reversal point. The test section was made of a Pyrex glass tube to allow visual observation. The geometry of the test section was designed to simulate various shapes of suction lines of refrigerators. The inner diameter of the test tube was 7 mm and the height was 1 m. The inclination of the test tubes varied from vertical to crank type with various horizontal lengths. An empirical oil return criterion was suggested based on the flow reversal points. This criterion was also verified using a refrigerator test apparatus and refrigerant.

Performance Analysis and Optimal Design of Heat Exchangers Used in High Temperature and High Pressure System

A computational study for the optimal design of heat exchangers (HX) used in a high temperature and high pressure system is presented. Two types of air to air HX are considered in this study. One is a single-pass cross-flow type with straight plain tubes and the other is a twopass cross-counter flow type with plain U-tubes. These two types of HX have the staggered arrangement of tubes. The design models are formulated using the number of transfer units (e - NTU method) and optimized using a genetic algorithm. In order to design compact light weight HX with the minimum pressure loss and the maximum heat exchange rate, the weight of HX core is chosen as the object function. Dimensions and tube pitch ratio of a HX are used as design variables. Demanded performance such as the pressure loss (Δ P) and the temperature drop (Δ T) are used as constraints. The performance of HX is discussed and their optimal designs are presented with an investigation of the effect of design variables and constraints.

Performance Comparison of Modified Offset Strip Fins Using a CFD Analysis

Offset strip fins (OSF) have been widely used in plate-fin heat exchangers. Conventional OSFs are constructed with plain fins. Various design modifications to improve the performance of OSFs are compared here. The modifications suggested include perforations, dimples, vortex generators, and a double-layer design. The performance capabilities are evaluated using a computational fluid dynamics (CFD) analysis. The performances are compared in terms of the j-factor, f-factor, area goodness factor, and volume goodness factor. The f-factors of all modified designs appeared to be higher than that of the plain OSF, while the j-factor was found to be higher or lower than that of the plain OSF depending on the design modification used. The plain OSF has the largest area goodness factor, while OSFs with multiple perforations and dimples show a higher volume goodness factor than the plain OSF.

Heat Transfer Performance Variation of Condenser due to Non-uniform Air Flow

Abstract Heat transfer performance variation of a condenser caused by non-uniform distribution of air flow was investigatedusing a numerical simulation method. A heat exchanger used for a outdoor unit of a commercial heat pump system and represented by a numerical model was selected. Non-uniform profile of air-velocity was constructed by measuring the airvelocity at various locations of the outdoor unit. Simulation was conducted for various refrigerant circuits and air flow conditions. Simulation results show that the heat transfer capacity was reduced depending on the air-flow rate and the refrigerantcircuit configuration. It is also shown that the capacity reduction rate is increased as the average air velocity decreases.. Key words Condenser(응축기), Air flow non-uniform distribution(공기유동 불균일분포), Thermal resistance(열저항), Heat transfer performance(열전달 성능)†Corresponding author, E-mail: jihwan@pusan.ac.kr 기호설명 ： 면적 [m 2 ]   ： 상수   ： 정압비열 [J/kg K] ： 직경 [m] ： 상대습도 [%] ： 대류열전달계수 [W/m

Head Loss Coefficient Evaluation Based on CFD Analysis for PWR Downcomer and Lower Plenum

Nuclear vendors and utilities perform numerous simulations and analyses in order to ensure the safe operation of nuclear power plants (NPPs). In general, the simulations are carried out using vendor-specific design codes and best-estimate system analysis codes, most of which were developed based on one-dimensional lumped parameter models. During the past decade, however, computers, parallel computation methods, and three-dimensional computational fluid dynamics (CFD) codes have been dramatically enhanced. The use of advanced commercial CFD codes is considered beneficial in the safety analysis and design of NPPs. The present work analyzes the flow distribution in the downcomer and lower plenum of Korean standard nuclear power plants using STAR-CD. The lower plenum geometry of a pressurized water reactor (PWR) is very complicated, as there are many reactor internals, which hinders CFD analysis for real reactor geometry up to now. The present work takes advantage of 3D CAD model so that real geometry of a PWR is used. The results give a clear figure about flow fields in the downcomer and lower plenum of a PWR, which is one of major safety concerns. The calculated pressure drop across the downcomer and lower plenum appears to be in good agreement with the data in engineering calculation. An algorithm that can evaluate the head loss coefficient, which is necessary for thermal-hydraulic system code running, was suggested based on these CFD analysis results.