대기압 사각 채널에서의 핵비등 현상의 실험적 연구

Abstract

Onset of Nucleate Boiling (ONB) is the point where a transition from single-phase flow to two-phase boiling occurs and is used as a criterion to determine a steady-state operational safety margin of Limited power for Safety System Settings (LSSS) in research type reactors. Although a lot of studies have been made on the ONB phenomenon, little is known whether existing correlations can reasonably predict the ONB heat flux and wall temperature, which are also related to the bubble dynamics at the ONB. Popular ONB correlation is known as Bergles-Rohsenow correlation and there have been some arguments whether the Bergles-Rohsenow correlation is to predict the ONB heat flux. Recent studies have shown that the Bergles-Rohsenow correlation over-predicted heat flux compared to the experimental value. Recent empirical works suggest that the ONB prediction is strongly influenced by geometry and thermodynamic inlet condition and still the issues remained unanswered. Thus this work investigates the prediction of the ONB heat flux with Aluminum (Al) and Stainless steel 316 (SUS316) surface conditions at a narrow channel, which is typical geometry adopted in research type reactors. Few attempts have been made for the ONB prediction at the realistic condition of research reactors.

Thus the purpose of this investigation is to obtain the ONB heat flux and surface temperature with Al and SUS316 at the research reactor flow condition. The reference research reactor was selected as the MITR-II. The experimental results were compared with a combinational empirical correlation of the Bergles-Rohsenow correlation. This thesis work takes two approaches of numerical and experimental. For the ONB prediction, intercepting method of single-phase forced convective correlation and Bergles-Rohsenow correlation was introduced. As far as the single-phase forced convective correlations, Dittus-Boelter and Petukhov-Gnielinski correlation were introduced. The experiment conditions include heat flux ranges 800 ~ 1400 kW/m2 in the flow condition of 2400 ~ 3400 kg/m2sec. Channel geometries were selected from the MIT research reactor (MITR). Hydraulic diameter is 2.0 mm and the heater is plate type with deionize water (DI water). Using the sputtering process, Al surface was engineered on the stainless steel substrate. The ONB phenomenon was identified using a high speed imaging technique. The test heaters were heated using Joule heating method with a high current DC power supply.

The experimental results indicated that the ONB is detected at 823.26 kW/m2 and 382 K (-1 K ±0.14%) with SUS316 heater and 713.44 kW/m2, 377 K (-1 K ±0.14%) with Al heater under 2400kg/m2sec. Bergles-Rohsenow correlation over-predicted the ONB heat flux with narrow channel, which are 1015 kW/m2 and 1036 kW/m2. This study is expected to contribute on obtaining the ONB heat flux and wall temperature data.