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

- A nanoscale roughness-enhanced superhydrophilic surface was made by spray coating.
- Quenching in saturated water was shown to be accelerated by 60% using such surface.
- Changes in boiling behaviors were analyzed for both saturated and subcooled cases.
- The superhydrophilic surface exhibited good durability upon consecutive quenching.
- This coating method can be readily applied to scaled-up and curved metal surfaces.

A superhydrophilic surface was prepared on stainless steel spheres by spray coating of silica nanoparticles 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 superhydrophilic 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 °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.