Synchronized visualization of nucleate boiling: integrating spatiotemporal data on nucleation, microlayer behavior, and surface thermal patterns

In nucleate boiling, the microlayer forms beneath the bubble and plays a vital role in bubble nucleation. Quite different contributions of microlayer evaporation to bubble growth have been reported, ranging from 15% to 100%. Characterization of the microlayer dynamics and the associated heat transfer mechanism is desired to understand the physics behind bubble nucleation and model the heat flux partitioning. This study synchronized laser interferometry (LI), infrared thermometry (IR), and high-speed photography to measure the microlayer thickness, surface temperature and bubble size during bubble nucleation in saturated and subcooled pool boiling, 0 K ≤ ∆ T sub ≤ 8 K. Surface heat flux was derived based on coupled analysis of thermal radiation and conduction in the sapphire substrate. Appreciably high surface heat flux was observed near the contact line. The measured high surface heat flux region is in good consistency with the microlayer coverage. Based on the synchronized measurement, boiling heat transfer was investigated, especially associated with microlayer behaviors. Synthesized analysis of LI and IR measurements showed that significant discrepancies between the LI and IR results arise near the contact line and at the microlayer periphery, which may induce systematic biases in the calculated microlayer contribution and initial microlayer thickness. The contribution of microlayer evaporation to bubble growth was analyzed based on the measured wall heat flux in the microlayer coverage. The contribution of microlayer under saturation conditions is 40%–60%. The analysis showed that the microlayer contribution increases with subcooling and can even exceed the total energy of departure bubble under subcooled conditions.