Experimental and theoretical study of pool boiling heat transfer and its CHF mechanism on femtosecond laser processed surfaces

In the present study, the pool boiling heat transfer of micro/nano hierarchically structured surfaces, as well as that of a smooth surface in gas dissolved FC-72 (the subcooling is 1 K) was studied. Femtosecond laser processing was used to fabricate the structured surfaces. It was found that for the surfaces with small processing spacing (LS30 and LS70, where the number after LS specifies the spacing in μm), the critical heat flux (CHF) showed almost no increase, while the heat transfer coefficient (HTC) was enhanced noticeably compared to that of a smooth surface (SS). For LS100, LS200, LS200-2 (compared to LS200, LS200-2 has the same processing spacing but a much higher peak-to-valley height), LS400 and LS800, both the CHF and HTC were enhanced remarkably compared to those of SS. The maximum HTC enhancement was obtained for LS70, with the HTC being 5.87 times larger than that of SS. The most remarkable increase in the CHF was achieved for LS200-2, with an improvement of 91% relative to that of SS. The liquid supply mechanism at the CHF of the micro/nano hierarchically structured surfaces was investigated. A modified model taking into account the coalesced bubble departure frequency, Jakob number and capillary wicking effects was proposed for CHF prediction. The CHF data from this study and the literature were used to validate the model, and it was found that the predicted results agree quite well with the experimental data within ±8%.