Keun Sun Chang

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.

An Experimental Study on the Thermal Performance Evaluation of SCW Ground Heat Exchanger

In recent years, application of the standing column well (SCW) ground heat exchanger (GHX) has been noticeably increased as a heat transfer mechanism of ground source heat pump (GSHP) systems with its high heat capacity and efficiency. Determination of the ground thermal properties is an important task for sizing and estimating cost of the GHX. In this study, an in situ thermal response test (TRT) is applied to the thermal performance evaluation of SCW. Two SCWs with different design configurations are installed in sequence to evaluate their effects on the thermal performance of SCW using a single borehole. A line source method is used to derive the effective thermal conductivity and borehole thermal resistance. Effects of operating parameters are also investigated including bleed, heat injection rate, flow rate and filler height. Results show that the effective thermal conductivity of top drawn SCW (Type A) is 11.7% higher than that of bottom drawn SCW (Type B) and of operating parameters tested bleed is the most significant one for the improvement of the thermal performance (40.4% enhanced in thermal conductivity with 10.9% bleed).

An analytical study on the thermal performance of multi-tube CO 2 water heater

In this study, the heat transfer and pressure drop characteristics were evaluated for multi-tube CO2 water heaters with lengths of 4.5 m and 7.5 m. The evaluation was done using the -NTU method, and the results were compared with experimental data. Water flows through the shell side of the water heater, while CO2 flows through 8 inner tubes. The heater uses a counter-current design to maximize the heat transfer efficiency. The energy balance equation describing the flows of CO2 and water for each node is set up using the section-by-section method. The calculated heat transfer rates agree well with the experimental data within ±5 % error. The outlet water temperature decreased linearly with the increase of the water flow rate. The calculated heat transfer rates agreed well with the experimental data within ±3 % error. The results show that the heat transfer rate increases almost linearly with the increase of water flow rate or CO2 inlet temperature in both the 4.5-m and 7.5-m water heaters, whereas the water outlet temperature linearly decreases with the increase of the water flow rate. The comparison of the CO2 pressure drop between the calculation and experiment results shows good agreement at the high CO2 flow rate within 5 % error, but the value is about 20 % higher in the experimental pressure drop at the low CO2 flow rate.

A Study on the Thermal Characteristics of Horizontal Ground Heat Exchanger using Thermal Response Test

.Abstract Vertical and standing column well ground heat exchangers have mostly been installed for ground sourceheat pump systems (GSHP) and thermal response tests (TRT) have been applied to evaluate the thermal characteristicsfor these heat exchangers. In this paper, the TRT coupled with a line source method was applied to evaluate the thermal characteristics of the horizontal ground heat exchanger (HGHX). Load tests of a HGHX were also performedto examine the daily variations of the ground and fluid temperatures associated with the daily intermittent operationof GSHP. For this test, the straight HGHX (depth 2 m, length 50 m, 8 line) was installed in Ansan city.The results showed that the variations of ground thermal conductivity of HGHX during one year were relatively smallwith the range of 1.41 ~ 1.64 W/m?K, and the maximum and minimum values appeared in December and May, respectively. Load tests with heat injection rate of 6.0 kW for 10 hours per day to HGHX during twelve days were performed in June, September and December, and resulted in a ground initial temperature rise of 4.31

Effects of Tube Rupture Modeling Methods on MSSV Lift Time Following a MSGTR Event in PWR

A multiple steam generator tube rupture (MSGTR) event in APR1400, an advanced pressurized water reactor, is investigated using the best estimate thermal hydraulic system code, MARS1.4. The effects of parameters such as the number of ruptured tubes, rupture location, affected steam generator on analysis of the MSGTR event in APR1400 are taken into account. In particular, the effects of tube rupture modeling are compared. In the present study, single tube (STM) and double tube modeling (DTM) are examined for assessment on the main steam safety valve (MSSV) lift time. Nuclear steam supply system (NSSS) and several safety systems that are relevant to the APR1400 are modeled. Automatic safety systems are assumed to mitigate the MSGTR events including the reactor protection trip, reactor coolant pump trip, the pressurizer heaters, high-pressure safety injection (HPSI) pumps, and the valves for atmospheric dump, main steam safety, main steam isolation, and turbine stop and bypass. When five tubes are ruptured, the STM permits the operator response time of 2085 seconds before lifting of MSSVs. The effects of rupture location on the MSSV lift time is not significant in case of STM, while the MSSV lift time for tube-top rupture is found to be 25.3% larger than that for rupture at hog-leg side tube sheet in 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 for 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 smaller value of Cd below 0.5. For larger values of Cd than 0.5, its effect on the leak flow rates as well as the MSSV lift time become negligible. It is found that the most dominant parameter governing the MSSV lift time is the leak flow rate. Whichever modeling method is used, it gives the similar MSSV lift time if the leak flow rate is similar, except the case of 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.Copyright

Study on the Air-Side Heat Transfer and Pressure Drop for Fin-Tube Heat Exchangers

Experimental measurements and numerical analysis have been carried out in order to investigate performances of air-side heat transfer and pressure drop for six types of heat exhcangers with various fin shapes. An air-enthalpy calorimeter is used in this work. Numerical analysis shows a good agreement with experimental measurements. Measurements for six types of heat exchangers at various air-velocity are compared with each other. Heat transfer per unit area appears to be nearly the same in the range of 0.7~0.9 m/s while it varies within 3% at 1.5 m/s. The numerical results show that most of heat transfer takes place through fins for all types and majority of heat transfer happens from the 1st row.Copyright

Numerical prediction of flow and mixing characteristics in CVCS chemical addition system

Abstract A numerical result of the flow and mixing characteristics is presented for the flow field created by water injected into a cylindrical tank with an initially stationary fluid. The flow is relevant to the operation of the chemical addition system in the chemical and volume control system (CVCS) of nuclear power plants. This study was undertaken to provide a basis for modification of the previous design which gave a number of difficulties in installation and operation of the chemical addition system because it needs a special reciprocating pump with a high actual head. For the tank of length-to-diameter ratios ( L / D ) of 1, 2 and 3, each with and without a baffle inside, calculation results were obtained by solving the unsteady laminar two-dimensional elliptic forms of governing equations for the mass, momentum and species concentration. Finite-difference method was used to obtain discretization equations, and the SIMPLER solution algorithm was employed for the calculation procedure. Results showed that the baffle was very effective in enhancing the mixing in the tank, and that a baffle should be installed near the tank entrance in order to inject chemicals into the reactor coolant system (RCS) within the operating time required.