One dimensional downward-facing critical heat flux concerning surface orientation and gap size effects

As part of a joint Korean-U.S. International Nuclear Energy Research Initiative (INERI) project, the GAMMA 1D (Gap-cooling Apparatus against Molten Material Attack One-Dimensional) experiments are conducted to investigate general boiling phenomena, and the critical heat flux (CHF) triggering mechanism in a narrow gap using the one-dimensional crevice type heater assembly and de-mineralized water. Needless to say, the vapor plays an important role in boiling heat transfer. Hence, it is pivotal to investigate the bubble behavior in the vicinity of the heated surface depending on such geometric and flow parameters as surface inclination, gap size, pressure, mass flux, local enthalpy, channel length, and surface roughness. Consideration of all the above parameters, especially controlling the bubble behavior in the narrow gap, will tend to render the CHF mechanism in the GAMMA 1D too complicated to paraphrase into engineering correlations. In this situation, it is adequate to treat only a few dominant parameters affecting the pool boiling CHF employing the GAMMA 1D experiments. The current experimental study is performed to examine the pool boiling CHF on one-dimensional downward heating rectangular channel having a narrow gap by changing the orientation of the copper test heater assembly. The test parameters include both the gap sizes of 1, 2, 5, and 10mm, and the surface orientation angles from the downward facing position (180°) to a vertical position (90°), respectively. Most of all, photographs are taken of the bubble behavior in the narrow gap by using a high-speed digital camera with 105mm micro-lens. When taking photographs, a high-intensity light source is used to accompany the high shutter speed of 4000 frames/sec. The narrow gaps of precise dimensions are formed by attaching a pyrex-glass spacer, which also helps visualize bubble behavior in the confined space. Considering the heater surface orientation effect, the CHF decreases as the surface inclination angle increases and as the gap size decreases. However, results obtained from this study appear to be contradicting to the general findings in the subject area at certain surface orientation angles. In particular, the CHF in the gap size of 10mm is smaller than that in any other gap sizes at the fully downward-facing position (180°). At the vertical position (90°), the CHF decreases with the gap size in consistency with other results available in the literature. In concurrence with several studies reported in the literature, it is also found that there exists a transition angle above which the CHF changes with a rapid slope in concurrence with several studies reported in the literature.