Regulated transient pool boiling of water during quenching on nanostructured surfaces with modified wettability from superhydrophilic to superhydrophobic

The effects of surface wettability, from superhydrophilic to superhydrophobic, on transient pool boiling of water under atmospheric pressure were experimentally examined by means of the quenching method with hot stainless steel spheres. The wettability changes, with a contact angle ranging from nearly 0° to more than 160°, were realized by nanostructured coating on the spheres. The quenching cooling rate was shown to slow down with increasing the contact angle as the vapor film was stabilized and retained by surface hydrophobicity even at very low wall superheats. Remarkable boiling heat transfer enhancement, with critical heat flux (CHF) increase up to nearly 70%, was achieved for the superhydrophilic case as compared to the original hydrophilic baseline case. As observed by high-speed imaging on the dynamic quenching processes, violent rewetting that stems from superhydrophilicity facilitated early collapse of the vapor film, thus leading to the great increase in film boiling heat transfer and CHF. The observations suggested an active means of regulation of transient pool boiling by employing the nanostructured surfaces with modified wettability to the extremes, toward either enhanced boiling heat transfer or retention of stable film boiling phase.