Experiments to understand bubble base evaporation mechanisms and heat transfer on nano-coated surfaces of varying wettability under nucleate pool boiling regime

Absolute surface wettability effects on the characteristics of nucleate pool boiling heat transfer under saturated conditions are investigated with water as working fluid. Wettability of the base surface (ITO-coated sapphire) is modified by coating 60 nm thick SiO2 film using different thin film deposition techniques. In this way, two surfaces with modified wettability (30 o and 65 o static contact angles) have the same surface chemistry with negligible variation in surface roughness, while the base surface itself acts as the third surface with 90 o static contact angle. Experiments are conducted for various constant heat flux conditions. Bubble dynamics and temperature distributions of the substrate surface are mapped in-situ using high speed videography and IR thermography, respectively. Pool boiling curves and overall heat transfer coefficient curves, obtained from the recorded IR images, showed that the overall boiling performance increases with enhanced surface wettability. The observed enhancement has been explained on the basis of the modified bubble dynamics and the associated bubble base evaporation mechanism. The bubble dynamics parameters such as bubble departure diameter and departure frequency, and nucleation site density (NSD) were found to get altered so as to augment the total evaporative heat flux with increasing surface wettability. The heat transfer coefficients and heat transfer rates corresponding to bubble base evaporation for isolated vapor bubble revealed that higher wettability surfaces outperform the surfaces with lower wettability in heat transport through bubble base evaporation. The corresponding underlying mechanisms have been identified. In addition, it was found that the microlayer evaporation mechanism is more efficient as compared to that based on pure contact line evaporation in transporting the heat away from the surface.

Automated summary

This study investigates the effects of absolute surface wettability on nucleate pool boiling heat transfer under saturated conditions using water as the working fluid. The results show that enhancing surface wettability improves the overall boiling performance, which can be explained by the modified bubble dynamics and the associated bubble base evaporation mechanism.