Numerical simulation study on stratified flow boiling in rectangular mini-channels

Abstract

In this study, the flow boiling heat transfer in a rectangular mini-channel with stratified flow is investigated using the VOSET method. Different heat transfer performances are observed in stratified flow compared to fill liquid flow. The effects of merging, fluctuation, fragmentation, and lifting are observed in stratified flow boiling, which are absent in fill liquid flow. The positive fluctuation and merging effects reduce the dry patch area on the wall. Large bubbles merge into the upper vapor layer upon contacting the free surface. Meanwhile, fluctuations in the free surface reduce the detachment radius of the bubbles and increase their detachment frequency. These positive effects lead to a reduction in wall superheat by up to 27.86 %. However, at high heat fluxes, negative fragmentation effect emerges. Intense boiling can cause the thin liquid film to fragment, increasing the area of dry patches. Despite this, the positive effects of merging and fluctuation still greater than the negative impact of fragmentation, resulting in a slight enhancement in overall heat transfer performance. Additionally, the liquid height ratio (LHR), defined as the ratio of the liquid film height (H_L) to the channel height (H), significantly impacts stratified flow boiling. For thicker liquid films, the heating wall is isolated by the vapor because of the negative lifting effect. This lifting effect drastically reduces the rewetting of the heating wall, leading to a significant deterioration in overall heat transfer performance. However, this negative lifting effect can be mitigated by adjusting the LHR. In practical operations, adjusting the liquid level height can promote heat transfer performance in mini-channel flow boiling.