Effects of confined space on the critical heat flux under the pool-boiling condition

The most popular mechanism of the critical heat flux (CHF) triggering is dry-out through bubble packing, which prevents the liquid inflow onto the heater. It is common knowledge that the generated upward bubble flow on the heater easily blocks a small flow path for the downward liquid inflow during pool boiling. This blocked hydrodynamic liquid flow path normally leads to dry-out and CHF on the heated surface. In the present study, we design a unique test section to assess the effect of a small flow path and the wall through the installation of a cylindrical wall close to the heater. We measure the CHF for various diameters and heights of the cylindrical wall and notice an interesting trend in the plots. Contrary to common knowledge, we find the CHF enhancement in a narrowed hydrodynamic flow path between the heater and cylindrical wall compared to the bare CHF performance. From this result, the enhancement mechanism is suggested based on the accelerated inflow bulk liquid and shortening period between growing and departing bubble by the confined wall effect. This is the first study to report on the relationship between the CHF and the diameter and height of the cylindrical wall.