Experimental investigation of variability between nucleation sites in flow boiling

Subcooled boiling;Departure diameter;Departure frequency;Wall nucleation;Active nucleation site density

Ooi, Zhiee Jhia ; Kozlowski, Tomasz ; Brooks ; Caleb Stephen

关键词: Subcooled boiling;    Departure diameter;    Departure frequency;    Wall nucleation;    Active nucleation site density;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/99348/OOI-THESIS-2017.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Wall nucleation characteristics are studied in terms of the bubble departure diameter and frequency in order to improve the prediction of bubble production and release in subcooled boiling flow. In the past, departure diameter and frequency models are shown to exhibit a wide range of accuracy depending on the experimental conditions. The variability of bubble departure diameter and frequency in an experimental condition is investigated in an effort to determine the possible causes behind the poor accuracy of existing wall nucleation characteristic models. Subcooled boiling flow experiments have been conducted in a vertical square channel where bubble departures from multiple sites are recorded and measured simultaneously with a high-speed camera for ten conditions. Existing bubble departure diameter models are benchmarked and are shown to be satisfactory in predicting condition-average bubble departure diameter. The probability density functions (PDF) of bubble departure diameters are observed to be normal at medium and elevated pressure but positive-skewed at low pressure.A skewed distribution suggests that the number mean bubble diameter (the sum of all departure diameters measured at an active nucleation site divided by the number of departures measured) is different from the surface area and volume mean diameters. Hence, the number mean diameter needs to be corrected with the distribution factors to ensure the accuracy of volume and surface area modeling. On the other hand, significant variations are observed in the bubble departure frequency across different nucleation sites of a given experimental condition largely due to intermittent periods of inactivity. A benchmark of the existing bubble departure frequency models shows that the models are generally applicable to an ‘active’ frequency but cannot account for the impact of these periods of inactivity. This finding highlights an important issue in the current modeling and understanding of the gas-phase boundary condition. This periodic inactivity, if not incorporated into the wall nucleation modeling, will result in a large overprediction of bubbles generated at the wall. A physical justification for this inactivity is discussed based on the modeling of the active nucleation site density which is observed to fluctuate strongly according to wall superheat. As the departure frequencies of multiple sites are considered, a new method to determine the condition-average departure frequency is proposed.