ABSTRACT

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.

摘要

临界热通量 (CHF) 是核、化工和电力行业两相流中遇到的灾难性故障模式之一。近几十年来，人们对水平、倾斜和垂直向上流动的 CHF 进行了大量研究。只有有限的信息，主要是在大气压力条件下，包括 CHF 增强器，如入口节流，可用于垂直管道中的 CHF，向下流动。由于流动不稳定性和管过早烧毁，与垂直向下两相流相关的复杂性要求严格的设计考虑，尤其是从 CHF 的角度来看。因此，准确估计 CHF 不仅是性能方面的要求，而且从安全角度来看也更为重要。本文的目的是开发流体向下流动的垂直管中 CHF 的相关性。作为这些调查的一部分，设计和开发了多功能测试台。实验是在 1 到 5 巴的压力范围内进行的，在没有入口节流或其他类似影响的情况下，建议将 CHF 相关性作为入口流体温度、质量流量和压力的函数。开发的新相关性与当前实验的数据非常吻合，平均偏差为 13.87%，标准偏差为 18.71%。由于输入变量的变化，不确定性分析揭示了 CHF 估计值的 90% 置信水平。实验是在 1 到 5 巴的压力范围内进行的，在没有入口节流或其他类似影响的情况下，建议将 CHF 相关性作为入口流体温度、质量流量和压力的函数。新的相关性与当前实验的数据非常吻合，平均偏差为 13.87%，标准偏差为 18.71%。由于输入变量的变化，不确定性分析揭示了 CHF 估计值的 90% 置信水平。实验是在 1 到 5 巴的压力范围内进行的，在没有入口节流或其他类似影响的情况下，建议将 CHF 相关性作为入口流体温度、质量流量和压力的函数。开发的新相关性与当前实验的数据非常吻合，平均偏差为 13.87%，标准偏差为 18.71%。由于输入变量的变化，不确定性分析揭示了 CHF 估计值的 90% 置信水平。