LAUSR

Abstract The effect of spacer grids on bubble behavior and critical heat flux (CHF) has been investigated through visualization of a heated single-rod channel cooled with refrigerant R-134a over a wide range of conditions. The test section consisted of a heated rod inside a square channel, which was equipped with a viewing window. A high-speed video camera was used to record bubble-growth phenomena and evolution process in flow boiling. Several spacer grids were installed along the heated rod to maintain its location and minimize vibration. Two different configurations of spacer grid (the full span and half span structures) were used. The half-span arrangement of spacer grids intensified the “tearing effect” on bubbles and reduced the size of bubbles. The strip, rigid convex and spring have been shown to be the main pushers of the “tearing effect” of bubbles in addition to maintaining the position of the spacer grid. The mixing vane promoted the spiral rise of bubbles and thus enhanced the cross-flow mixing. “Tearing effect” and cross-flow mixing facilitated the heat transfer enhancement leading to improvement in CHF. A comparative study was performed for the experiment with and without mixing vane by quantitative analysis of bubble characteristics. The results could support the design of a spacer grid with mixing vanes.