Yen-Shu Chen

Investigations of the Thermal Spreading Effects of Rectangular Conduction Plates and Vapor Chamber

This study examines the spreading ability of rectangular plates numerically, analytically, and experimentally. The effect of aspect ratio, defined as an equivalent radius of a heater divided by that of a spreader plate, is investigated. The numerical results show a very good agreement with the analytical solutions. From the calculated results, the spreading resistance of the conduction plates with a small aspect ratio is higher than the one-dimensional conduction resistance. Calculated results also show that the spreading ability of a metal plate would be affected slightly by the external convective heat-transfer coefficient when the ratio of the longitudinal heat convection to the lateral heat spreading is less than 0.1. In addition to the numerical analysis, experimental comparisons between copper∕aluminum plates and a vapor chamber having the same thickness have been conducted. The experimental results show that the thermal resistance of the metal plates is independent of input power whereas that of the vapor chamber shows a noticeable drop with increased power. For the influence of concentrated heat source, the surface temperature distributions for the metal plates become concentrated with a reduced aspect ratio. However, the variations of the aspect ratio and the input power would yield minor effects to the surface temperature distribution of the vapor chamber. As compared with the conduction plates, the vapor chamber would offer a lower temperature rise and a more uniform temperature distribution. Thus, the vapor chamber provides a better choice as a heat spreader for concentrated heat sources.

Determination of Optimized Rectangular Spreader Thickness for Lower Thermal Spreading Resistance

This study presents an approximation for determining an optimized thickness of a concentric heated rectangular plate and derives an analytical solution for spreading resistance of a spreader having orthotropic conductivities. The solution for the orthotropic plate is obtained by separation of variables, and the optimized thickness is determined by taking the derivative of the thermal resistance with respect to the spreader thickness. According to the calculated results, an enhanced in-plane spreading effect can reduce the spreading resistance. The spreading resistance dominates the overall resistance of thin plates, whereas the one-dimensional conduction resistance becomes important for thick plates. However, the predicted optimized thickness from the approximation shows a disparity from the analytical results, while the aspect ratio between a spreader and heat source is less than 0.2. Even so, the thermal resistance corresponding to the predicted thickness is still in good agreement with the analytical solution. The proposed approximation will be useful for practical thermal design of heat sinks by predetermining the spreader thickness.

Loss of Cooling Thermal Analysis for the Spent Fuel Pool of the Chinshan Nuclear Power Plant

The Chinshan Nuclear Power Plant owned and operated by the Taiwan Power Company is a twin-unit BWR-4 plant. Unit 1 and unit 2 began their commercial operation in 1978 and 1979, respectively. Since commercial operation, all the fuels discharged from reactor core at each cycle are stored in the spent fuel pool (SFP). An engineering analysis is performed to predict the SFP water temperature and pool water level during a postulated loss of forced cooling accident. A full-core discharged loading is considered, and the fuel assemblies are moved to the SFP just after 7 days of cooling. The pool temperature and level are calculated using lumped energy and mass balances. Calculation results show that the water temperature reaches the saturation temperature at 9.4 hours after the onset of the accident, and the pool level drops to the top of the active fuels at 76.8 hours. After the pool level drops to the top of the active fuels, the cladding temperature increases dramatically because the convective heat transfer of steam is much weaker than that of liquid water. The peak cladding temperature after fuel uncovery is calculated by detailed CFD simulations, and the results show that the peak cladding temperature reaches 600°C in 3 hours and 1200°C in 9.5 hours after the fuels are uncovered. Additionally, the check-board arrangement for fuels is also investigated. Through enhanced the radiation heat transfer, the check-board fuel arrangement can have slower heating rate for the fuels. For the Chinshan SFP, extra 2.5 hours can be gained by employing such an arrangement for necessary actions.Copyright

Effects of the RHR Return Line Elevation to the Suppression Pool Temperature of the Lungmen ABWR Containment

Lungmen Nuclear Power Plant in Taiwan is a twin-unit Advanced Boiling Water Reactor (ABWR) plant. In this study, a long-term GOTHIC model for the Lungmen ABWR primary containment response analysis is established. The wetwell space is vertically divided into several volumes to catch the pool temperature stratification effect. The long-term containment responses for a double-ended feedwater line break (FWLB) accident are calculated. The fuel decay heat is absorbed by the reactor coolant, and the coolant flows to the containment via the broken line. The suppression pool is gradually heated up by the high-temperature gas-water mixture following through horizontal vents. To reduce the pool temperature, the Residual Heat Removal (RHR) system will be required to operate in the suppression pool cooling mode. The RHR pumps have suction flow from suppression pool and discharge it to the RHR heat exchangers for cooling. The cooled water then returns to the pool. An elevated RHR return line is desired to avoid the cooled water being directly sucked again. The wetwell temperature stratification associated with the RHR return line elevation is investigated in this study. Effects of the RHR return line elevation on the pool temperature can be determined since the whole wetwell space is not lumped as a node only. The calculated peak pool temperature is 92.6°C based on the plant piping configuration. The peak temperature can be reduced to 88.9°C by returning the water via the wetwell spray spargers located in the top of the wetwell. However, it should be noted that using the wetwell spray also pressurizes the wetwell because the pool water temperature is higher than that of airspace during the late period of the event. Returning the pool water via the wetwell spray spargers is not suggested because it causes long-term wetwell pressurization.Copyright