OBSERVATION OF CHF PHENOMENA BASED ON A VISUALIZATION OF NEAR WALL BOILING STRUCTURE IN A VERTICAL NARROW CHANNEL SUBMERGED IN A POOL

摘要

We visualized and identified a high heat flux boiling structure in a vertical narrow channel submerged in a pool of saturated R-113. A transparent ITO (Indium Tin Oxide) sputtered sapphire substrate was used as a heating surface and a Pyrex glass plate was placed in parallel to the sapphire substrate to form a gap of 2.25 mm between two vertical plates. Synchronized visualizations were applied to capture the complicated bubble motion and the corresponding dynamic behavior of dry areas on the boiling surface. A total reflection was used to visualize the dry pattern of the boiling surface, and a shadowgraph was used to visualize the bubble motion in a narrow channel. The experiments covered the heat flux condition ranging from 30.9% CHF (critical heat flux) to CHF condition. Above the heat flux of about 77% CHF condition, a slug bubble was periodically formed and the base of the slug bubble became mostly dry. However, the large dry area under the slug bubble was efficiently rewetted by the slug tail region as the slug bubble moved upward. The cooling of the heating surface was governed by the rewetting efficiency of the slug tail region which deteriorated gradually with an increase of the heat flux. CHF occurred when the dry area formation capability in the slug tail region overwhelmed the rewetting capability of the slug tail region.