A Near-Wall Interfacial Area Concentration Model to Predict Departure From Nucleate Boiling Critical Heat Flux Based on High Speed Video From Boiling Water Flows

In nucleate boiling as the heat flux from the wall to the fluid is increased the heat transfer coefficient initially increases. At a sufficiently high heat flux called the critical heat flux (CHF) the heat transfer mechanism suddenly becomes less effective resulting in a rapid jump in wall temperature. In bubbly subcooled (or near-subcooled) conditions the CHF mechanism is referred to as departure from nucleate boiling. Departure from nucleate boiling (DNB) refers to the transition from nucleate boiling where liquid contacts the wall to film boiling in which a vapor layer contacts the wall. Various hypotheses have been used in modeling and predicting CHF. High speed video images of boiling water flows taken at Bettis Laboratory at the critical heat flux visually captured sufficient evidence of the DNB mechanism that improved insight into DNB modeling may be possible. This paper summarizes high speed video image analysis and the development of a new DNB critical heat flux model based on the image analysis findings. Using short window averages of image data, a significant increase in transmitted light intensity is seen near the wall just prior to CHF. The increase suggests that at CHF there is a transient reduction in the interfacial area concentration, ai, or bubble number density near the wall. This is believed to be the result of a sudden increase in bubble coalescence rates near the wall. The increase in coalescence rates results in a reduction in the interfacial area concentration causing it to reach a maximum at CHF. This near-wall maximum in ai at CHF under flow boiling conditions is consistent with recent pool boiling data in the literature. The image based observations motivated development of an interfacial area based CHF model to predict the maximum in the interfacial area concentration at CHF. The model predicts that a critical nucleation site density or a near-wall critical void fraction can be used as a DNB CHF criterion. This is a valuable simplification that can be directly implemented in three-dimensional thermal hydraulic codes. The critical nucleation site density result was used as an input to a simple wall heat transfer partition model to predict the critical heat flux. The model relies on correlation based estimates for the superheat temperature, bubble departure diameter, and bubble departure frequency. Model predictions are compared to CHF values taken from Groeneveld’s 2006 CHF look-up table.