Experimental study of flow boiling under subatmospheric pressures in a vertical square channel

critical heat flux;flow boiling;subatmospheric;departure from nucleate boiling

Colgan, Nathan Eamon ; Kozlowski, Tomasz ; Brooks ; Caleb S.

关键词: critical heat flux;    flow boiling;    subatmospheric;    departure from nucleate boiling;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/101554/COLGAN-THESIS-2018.pdf?sequence=1&isAllowed=y

美国|英语

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

【 摘 要 】

Experiments have been conducted in a vertical square channel to investigate wall nucleation characteristics and Critical Heat Flux (CHF) in subcooled boiling flows under subatmospheric pressure. Forced convection boiling is a widely used and effective means of heat transfer. An understanding of the mechanisms and limitations involved in forced convection boiling is important for the accurate prediction of the behavior of two phase systems under a wide range of conditions. The objective of this work is to provide a unique dataset to evaluate the scalability of existing models of nucleation characteristics and CHF for flow boiling conditions below atmospheric pressure as the performance of these models has not been investigated under these conditions.The experimental nucleation data are presented as a parametric study of the effects of important dimensionless groups on nucleation characteristics, specifically Boiling number, Jakob number, and density ratio. Bubble departure diameters and departure frequencies are measured via high-speed photography and compared to existing models. CHF data are presented as a parametric study of pressure from 20 kPa to 108 kPa, mass flux from 45 kg/m2s to 190 kg/m2s, and inlet subcooling from 0 to 10 K. Heat flux is gradually increased until an excursion of the wall temperature, signifying CHF, occurs. Several existing models that correlate departure characteristics with the relevant dimensionless groups enumerated above are found to accurately predict departure diameter and frequency. However, at the lowest pressure, departure diameters are of the same length scale as the channel so geometric effects may affect these departures. CHF is shown to vary directly with pressure but is only weakly affected by changes in mass flux and inlet subcooling for the observed conditions. Models of the Boiling number at CHF that incorporate the density ratio and Weber number are found to predict the experimental results well. A new correlation for CHF is developed that incorporates the characteristic bubble diameter into the Weber number and is found to predict CHF with an average error of ±15.62%.