Study on identification method of heat transfer deterioration of supercritical fluids in vertically heated tubes

Prediction of the heat transfer of supercritical fluids (SCFs), especially for heat transfer deterioration (HTD), is highly significant to the design and safe operation of supercritical boilers and advanced nuclear systems. To achieve higher predictive accuracy, heat transfer datasets of SCFs are usually classified as HTD cases and non-HTD cases using certain HTD identification methods, and prediction models, including empirical correlations and criteria of HTD occurrence, have been separately developed for HTD cases and non-HTD cases. Therefore, the rationality of HTD identification methods are crucial to the data classification and further development of high-precision prediction models but is seldom discussed in research. This paper first summarizes the existing identification methods of HTD to SCFs, and respective heat transfer datasets of supercritical water (SCW) and CO₂ (SCCO₂) are compiled. Based on these datasets, the accuracy of existing methods in identifying HTD cases and non-HTD cases is evaluated. The results show that, the most common identification method (Nu/Nu_db < 0.3) can mistake typical non-HTD case for HTD case and cannot reflect the actual location where HTD occurs. The Lokshin et al. method and Zhang et al. method (htc/htc₀ < 1.0) can accurately recognize HTD cases but are prone to error in detecting non-HTD cases. It is believed that the reference value representing normal heat transfer is the key to judging the heat transfer state of SCFs, and should be considered specifically in the pseudo-phase-change influenced region. Finally, an improved identification method of HTD suitable for SCW and SCCO₂ is proposed. Compared with previous methods, the new method shows favorable accuracy in discerning both HTD cases and non-HTD cases, and is helpful for classifying heat transfer cases and developing precise heat transfer correlations and HTD criteria of SCFs.