Effects of flow obstacles on pool boiling critical heat flux of a downward-facing metal heater

Abstract

Pool boiling critical heat flux (CHF) experiments were performed to investigate the influences of the size (10, 15, 20, and 25 mm) and the shape (square, circular, and elliptic) of flow obstacles on the CHF trigger. Stainless steel grade 304 heaters with dimensions of 99 x 49 x 2 mm(3) were inclined at 170 degrees facing downward with a rectangular gap channel 10 mm in height. Pool boiling conditions with saturated deionized water at atmospheric pressure were applied for all experiments. A high speed video system was introduced to accurately capture the boiling phenomena under the explored test matrix. Compared to CHF without flow obstacles, the CHF decreased slightly as the obstacle size (i.e. blockage ratio) increased because the larger obstacle delayed separation of the flow edge from the trailing edge of the obstacles. Thus, the entrance of the following bubble flow was postponed, which increased the residence time of the dry region behind the obstacles. The shape effect suddenly intensified with a maximum blockage ratio of 0.51. For the square obstacles, the edge separation occurred at the right angle edge of the square obstacle with a direction parallel to the flow direction. So, the dry region remained stagnant approaching the CHF. In contrast, the edge stagnation of circular and elliptic obstacles occurred along the curved-obstacle boundary and eventually separated from the trailing edge of the obstacle. This induced interaction between the bubble flow and the stable vapor layer, which effectively delayed the CHF. Here, we revealed that the circular and elliptic obstacles having a curved-boundary along the flow direction were more beneficial in delaying the occurrence of the CHF compared to the right-angled obstacles.