Development of correlation for critical heat flux for vertically downward two-phase flows in round tubes

Critical Heat Flux (CHF) is one of the catastrophic failure modes encountered in two-phase flows in nuclear, chemical, and power industries. Considerable research was carried on CHF in last few decades in horizontal, inclined, and vertically upward flows. Only limited information, mostly at atmospheric pressure conditions and including CHF enhancers like inlet throttling, was available on CHF in vertical pipes with flow directed downwards. The complexity associated with vertically downward two-phase flows, due to flow instabilities and premature tube burnout, dictates for stringent design considerations, especially, from CHF perspective. As a result, accurate estimation of CHF is a requirement not only from performance, but also more significantly from safety perspective. The objective of this paper is to develop a correlation for CHF in vertical tubes in which the fluid flow was directed downward. As part of these investigations, a versatile test rig was designed and developed. Experiments were conducted in the pressure range of 1 to 5 bar and a CHF correlation is proposed as a function of inlet fluid temperature, mass flux and pressure in the absence of inlet throttling or other similar effects. The new correlation developed agrees well with the data from current experiments with a mean deviation of 13.87% and standard deviation of 18.71%. The uncertainty analysis revealed a 90% confidence level on the CHF estimate due to variation in the input variables.

Automated summary

CHF is a catastrophic failure mode encountered in various industries, with limited information available particularly in vertically downward two-phase flows. Accurate estimation of CHF is crucial for performance and safety, prompting the development of a new correlation for CHF in downward flow in vertical tubes with good agreement with experimental data.