ScholarWorks@성균관대학교

초록

While pool boiling has been extensively studied due to its high heat transfer efficiency and structural simplicity, previously proposed critical heat flux prediction models remain empirical and show limited applicability across different working fluids and surface orientations. This study presents a theoretical critical heat flux model that comprehensively incorporates the effect of surface orientation, especially for downward-facing surfaces. Pool boiling experiments with water are conducted under saturated conditions on a rotatable heated surface, and high-speed visualizations are performed to observe vapor bubble behavior at varying heat fluxes. The results reveal two distinct regimes: on upward-facing surfaces with orientation angles less than 90°, bubbles detach due to buoyancy; in contrast, on downward-facing surfaces with angles greater than or equal to 90°, coalesced bubbles slide along the surface, forming a wavy liquid-vapor interface. Focusing on the more challenging downward-facing regime, a new critical heat flux model is developed by combining the interfacial lift-off mechanism with key findings from the visualizations, including the time-varying behavior of wetting fronts and a modified critical wavelength that accounts for the tangential component of buoyancy. Additionally, a functional relationship is introduced between surface orientation and the ratio of intermittent wetting front length to the distance between the centers of consecutive intermittent wetting fronts, enabling the model to more effectively capture orientation effects. The proposed model shows strong agreement with critical heat flux data across various working fluids and orientations, outperforming the previous model, particularly for downward-facing surfaces at high orientation angles.

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