A heat flux model for nucleate boiling on micro-cavity surfaces

Abstract

This study investigates the influence of spacing (500-1000 mu m), cavity size (100-300 mu m), and subcooling (0-10 K) on nucleate pool boiling heat transfer performance using selectively micro-caved copper surfaces. The cavity spacings (S-p approximate to S/D-b <0.4 for saturated and S/D-b <0.5 for subcooled conditions) were optimized to minimize bubble interference, while cavity sizes were chosen based on active nucleation ranges derived from theoretical calculations. Micro-drilled surfaces generally demonstrated up to 69% higher critical heat flux (CHF) and significantly improved heat transfer coefficients (HTC) up to 10.1 x 104 W/m2K compared to plain surfaces. Subcooling enhanced the condensation rates of departing bubbles near the heating surface and delayed CHF, resulting in up to 46% higher CHF compared to saturated condition and steeper boiling curves. The larger spacing (1000 mu m) reduced thermal and hydrodynamic interactions, facilitating stable bubble detachment and efficient surface rewetting. In contrast, smaller cavities (100 mu m) increased the frequency of bubble departure and HTCs, while larger cavities (300 mu m) led to premature coalescence and persistent vapor layers, reducing heat transfer performance and approximating the results of plain surfaces with slightly smaller superheats. High-speed imaging provided detailed insights into isolated bubble departure dynamics and validated empirical models, while also verifying the predictive accuracy of the proposed model. The proposed heat flux model exhibits quite satisfactory reliability, with CHF predictions for micro-drilled surfaces within +/- 15% error. These findings highlight the complex interplay of geometric and thermal parameters in boiling heat transfer and offer a robust framework for optimizing surface designs and operating conditions in high-flux boiling applications.Istanbul Teknik Üniversites