Experimental study of subcooled boiling pool heat transfer and its “hook back” phenomenon on micro/nanostructured surfaces

This study presents the pool boiling heat transfer of gas dissolved FC-72 on micro/nanostructured surfaces. The micro/nanostructured surfaces, named LS30, LS70, LS100, LS200, LS400 and LS800 were fabricated by femtosecond laser processing (the number after LS specifies the spacing in μm). The experiments were conducted at different subcooled degrees (ΔT _sub ) of 1 K, 15 K, 25 K and 35 K. For comparison, the pool boiling heat transfer performance of a smooth surface was also tested. It was found that the critical heat flux (CHF) enhancement of micro/nanostructured surfaces increases with increase in subcooling. The maximum CHF of micro/nanostructured surfaces are 1.67, 1.82, 1.95 and 2.06 times larger than that of SS at ΔT _sub = 1, 15, 25 and 35 K, respectively. The processing spacing has pronounced effects on the sensitivity of CHF to subcooling. The CHF of LS30 is enhanced 163% at ΔT _sub = 35 K, while the only a 90% enhancement was obtained for LS800. The “hook back” phenomenon characterized as wall superheat decrease with increasing heat flux on boiling curves at high heat fluxes can be observed on LS30, LS70, LS100, LS200 and LS400. This phenomenon is more clear at the lower subcooling and the lager nanoporous structure covered area.