Novel measurement of receding wicked liquid responsible for critical heat flux enhancement

Recent advancement in technology has seen the rapid growth and use of micro and nanostructures in numerous fields. Generally, liquid-wicking provides an additional technique for supplying liquids along the wick channels which is key to improving the critical heat flux. Unfortunately, wicking dominant surfaces may result in excessive critical heat flux enhancement which is generally difficult to explain using the wettability effect. This has attracted significant attention of researchers over the past few years. To this end, researchers have been looking for effective alternatives to experimentally investigate wicked liquid in microstructures.

In a previously published literature, it is stated that critical heat flux improvement generally depends on the size of the dry area. However, due to the high heat boiling process in conjunction with the phase change heat transfer, distinguishing wicking structures is still a major challenge. Therefore, researchers have been looking for alternative and efficient methods for overcoming the aforementioned drawbacks and have identified characterization of the wicking momentum by decoupling heat transfer as a promising solution.

Recently, Hanyang University researchers led by Professor Sung Joong Kim at developed a new wicking experiment technique to investigate liquid wicking in solid structures. They looked at the behavior of the solid structures around expanding dry areas as well as determine its momentum quantity. The authors used four different surfaces to compare and contrast the behavior of the wicking corresponding to the changes in morphology and structural scale. The surfaces included smooth, nanoporous, nanostructure and microstructure. The contact lines between the wicked liquid and the dry area were continuously observed to determine the variation of the dry area size. Eventually, using the critical heat flux enhancement trend, the size of the dry area was analyzed to understand the role of wicked liquid. Their work is published in International Journal of Heat and Mass Transfer.

The research team observed that the wicked liquid receded from the expanding area in the three surfaces nanoporous, microporous nanostructure except for the smooth surface which had got no liquid wicking. Microstructure surface exhibited the lowest receding velocity followed by nanoporous nanostructure. This was attributed to the fact that the surfaces support hydrodynamic that majors in contact line length and smaller dry spot size in microscale as compared to nanoscale level. Furthermore, the dry area diameter depicted a linear relationship with the critical heat flux enhancement indicating that small dry area diameters are more effective in delating the irreversible expansion of dry spots as compared to the larger diameters.

The study by Hanyang University scientists is the first to investigate the preceding behavior of wicked liquid using the external source of pressure. Therefore, it will significantly advance the analysis of critical heat flux enhancement on surfaces including wickability and wettability as well. This is due to the similarity to the hydrodynamic behavior wicking liquid and dry spot in boiling.