Visual observations of bubble growth and nucleation site density on microstructured surfaces during flow boiling in a single microchannel

Recent studies have shown that modified microstructures can significantly enhance boiling heat transfer performance. However, the unclear mechanisms of boiling on microstructured surfaces present challenges in optimizing heat transfer through microstructural modulation. To investigate the influence of the microstructure on bubble dynamics, visual observations of bubble behavior on microstructured surfaces during flow boiling were carried out with a metallurgical microscope and high-speed photography. The experiments focused on bubble growth on a smooth surface with nine microcavities and nucleation site density on a microstructured surface during flow boiling under various conditions (Reynolds numbers of 17.5, 34.9, and 87.4, and heat fluxes of 50, 100, and 150 kW/m2). The observations reveal that heat flux strongly promotes bubble growth and nucleation site density when bubble sizes remain below 200 μm. Based on 114 experimental data points, ten existing bubble growth models were assessed and found to inadequately predict the growth of micrometer-scale bubbles. A new dimensionless model, with a mean absolute deviation of 9.65%, was developed by integrating heat transfer, fluid flow, interface dynamics, and temporal variations, and it can be adapted for similar experiments. Additionally, bubble numbers peak with active nucleation and decrease due to coalescence, influenced by velocity and heat flux. Higher heat fluxes promote bubble growth and coalescence, whereas the flow velocity has a dual effect on bubble growth and nucleation site density. The results suggest that optimizing the flow velocity on microstructured surfaces based on bubble behavior can effectively enhance the heat transfer performance, providing guidance for future thermal management innovations.