Pool boiling heat transfer on a nanoscale roughness-enhanced superhydrophilic surface for accelerated quenching in water

A superhydrophilic surface was prepared on stainless steel spheres by spray coating of silica nanopartides to enhance the surface roughness at the nanoscale. The quenching performance of such surface was tested in both saturated and subcooled water. It was shown that quenching is accelerated on the super hydrophilic surface by a factor of 60%, due to the marked boiling heat transfer enhancement with the critical heat flux being increased by 78%. Under saturated condition, the initial temperature of 800 degrees C is not high enough to lead to stable film boiling on the superhydrophilic surface. Instead, quenching begins with an extended transitional film boiling sub-regime that is dominated by point-contact mode of liquid-solid contacts. The quenching time becomes shorter with increasing the subcooling degree, in agreement with the enhancement in quasi-film-boiling heat transfer and CHF. Bubble jetting phenomenon is observed at the liquid-vapor interface for the subcooled cases, which becomes more violent at a deeper subcooling. The quenching performance maintains upon consecutive tests for up to 100 times, because the superhydrophilicity is sustained on the surface. The spray coating method adopted in this work is not only fast and cost-effective, but is also scalable to large and curved metallic surfaces for a variety of applications in thermal engineering including quenching. (C) 2016 Elsevier Ltd. All rights reserved.