Understanding of CHF enhancement mechanisms with millimeter-scale fin structures

It is necessary to develop an effective way of decay heat removal passively. However, the flow rate is limited when there is no pump working and CHF can occur threatening the heat removal efficiency drop. Thus, CHF enhancement by surface treatment application on the passive safety system can enhance the safety margin. To apply the surface treatment for CHF enhancement, it is necessary to have a strategy for surface treatment proper for nuclear power plant safety system and the amount of margin which can be secured by the surface treatment. In this study, we focus on the fin structured surface in various shapes and arrangement in millimeter scale. For CHF mechanism analysis, we propose a CHF mapping method with various limitations and validated it with experimental results. We performed pool boiling CHF experiment using millimeter-scale finned surface to observe the parameter effect of the surface structure. The result shows the peak value of CHF along the fin length change. To describe the complicated test results, we propose the CHF map describing the nucleate boiling failure by setting the liquid demand and supply limitation. The map is presenting our conclusion intuitionally that the CHF occurs when the superheat is too high to rewet the dry spot rapidly (spot dry-out) or liquid cannot be supplied to the dry spot (free volume or local dry-out)