Numerical Study on flow boiling in micro/mini channels

kontakt: Qingming Liu

The study of micro-channels has attracted more and more attention during the last few decades. There are many reasons for this trend: advancements of MEMS (micro-electro -mechanical systems), fuel cells, cooling of micro-electronic chips, and also, compact heat exchangers.

While single phase flow in micro-channels has been comprehensively studied during the past three decades, the study of flow boiling is relatively rare and there are still many unsolved questions. Additionally, the mechanisms are much more complicated than in single phase flow due to the many reasons: interface phenomena, turbulence induced by bubble generation and moving. 

Previous studies of two-phase flow in macro-channels has indicated that there are several kinds of flow patterns, of which the most important are: bubbly flow, slug flow, annular flow and mist flow. However, the study of the flow pattern in micro channel has shown that two new flow patterns, confined bubbly flow and elongated bubbly/slug flow, appear besides the conventional four flow patterns (Figure 1- right). These new flow patterns and the downstream annular flow play an important role in the heat transfer process.

In order to investigate these new flow regimes, Mukejee and Kandlikar simulated the growth of a confined bubble in a rectangle micro channel. Special attention had also been put on elongated bubblesas an experiment  was implemented to study  the velocity of elongated bubble in diabatic micro channels and a predicative model was proposed by researchers(Agostini and et al). 

In spite of these efforts, the full image of flow boiling has not been drawn numerically. This project would study the whole process of flow boiling from the start to end which including nucleate onset, isolated bubbly flow, confined bubbly flow, elongated bubbly flow and annular flow. The transitions between these flow regimes would be studied as well.