EXPERIMENTAL INVESTIGATION OF TWO-PHASE HEAT TRANSFER OVER PARTIALLY ROUGHED VERTICAL TUBE

Saturated pool boiling over vertical tubes occurs in various industries, particularly the nuclear industry, as passive residual heat removal systems, where nucleate pool boiling plays a crucial role in transferring heat. The enhanced heat transfer of this equipment is challenging, however, and thus passive enhancement is prevalent in this field. Enhancement through the use of rough tubes (i.e., tubes with a textured or uneven surface) is a well-established, proven technique. However, rough tubes have some drawbacks in terms of promoting corrosion. Thus, this study aimed to introduce a new technique to enhance the thermal performance of existing conventional tubes. The present experiment introduced two partially rough vertical tubes: tubes with only the lower part rough (LPR) and tubes with only the upper part rough (UPR). The performance of these two versions was compared with that of a fully smooth tube and a fully rough (FR) tube. All test tubes were composed of AISI 304 grade stainless steel. Smoothness and roughness were achieved through turning with different grades of grit paper. It was found that the LPR, FR, and UPR tubes exhibit enhancements in nucleate boiling heat transfer coefficient of approximately 37%, 25%, and 9%, respectively, compared with a smooth tube at the highest heat flux of 72.92 kW/m<sup>2</sup>. Compared with the FR tube, the LPR tube exhibited 5% to 10% enhanced performance. This study highlights the single bubble trajectory over these vertical tubes to explore the physics behind the enhancement.