Improvement of bubble distribution characteristics through multi-objective optimization of flow characteristics of a swirling flow type microbubble generator with fixed blades

Abstract

Swirling flow type microbubble generators with fixed blades (hereafter referred to as "bubblers") can be easily downsized and upsized and can be easily integrated into existing facilities. For this reason, bubblers have been used for mercury targets; however, there is a need to improve the bubble distribution characteristics. The objective of this study is to establish a technique to rapidly improve bubble distribution characteristics through multi-objective optimization of the flow characteristics of this bubbler using single-phase flow computational fluid dynamics (CFD) analysis. Therefore, design variables for the bubbler blades were defined by applying turbomachinery design methods, and four flow characteristics were subjected to multi-objective optimization using single-phase flow CFD analysis and the response surface methods. While the resulting optimized bubbler increased the pressure loss coefficient by 0.9 %, it improved the wall shear stress by 3.4 %, the swirl number by 10.0 %, and the pressure reduction coefficient by 14.3 % compared to the original bubbler. Furthermore, experiments showed that the optimized bubbler produced smaller bubbles than the original bubbler, demonstrating that the multi-objective optimization design method can significantly improve the bubble distribution characteristics while maintaining the same pressure loss coefficient.