Critical supercritical-boiling-number to determine the onset of heat transfer deterioration for supercritical fluids. (1st January 2020)

• Record Type: Journal Article
• Authors: Xu, Jinliang; Zhang, Haisong; Zhu, Bingguo; Xie, Jian
• Is Part Of: Solar energy. Volume 195(2020)
• Page Start: 27
• Page End: 36
• Publication Date: 2020-01-01
• Subjects: Supercritical fluid -- Pseudo-boiling -- Supercritical-boiling-number -- Heat transfer deterioration -- Critical heat flux -- S-CO2; Solar energy -- Periodicals; Solar engines -- Periodicals; 621.47
• DOI: 10.1016/j.solener.2019.11.036
• Languages: English
• ISSNs: 0038-092X

Abstract

Graphical abstract: Using the pseudo-boiling concept, supercritical heat transfer is analogized to the boiling at subcritical pressures. The proposed supercritical-boiling-number SBO represents the competition between the bubble expansion induced momentum force and the inertia force. Sudden changes between normal heat transfer and heat transfer deterioration are found when crossing critical SBO, which is experimentally determined for CO2, H2 O, R134a and R22. The linear law of supercritical heat transfer is discovered. Our study supports the heterogeneous structure of supercritical fluids. Highlights: Supercritical-boiling-number SBO dominates supercritical heat transfer mechanism. Sudden changes in heat transfer behaviors are found when crossing critical SBO . Critical SBO values are experimentally determined for CO2, H2 O, R134a and R22. Our study supports the heterogeneous structure of supercritical fluids. Abstract: Supercritical fluids such as CO2, water and organic fluids are frequently applied in power systems. The accurate prediction of heat transfer deterioration (HTD) is important to keep the safe operation of advanced power systems such as solar driven supercritical carbon dioxide Brayton cycle. As described in textbooks, it is impossible to identify liquid from gas beyond the critical point, thus supercritical fluid is assumed to have homogeneous structure with a single-phase. The single-phase assumption cannot explain and predict supercritical heat transfer (SHT). Instead, we investigate SHT by the pseudo-boiling concept. Heat transfer is analogized between supercritical pressure and subcritical pressure to create a new non-dimensional supercritical-boiling-number SBO, representing the bubble expansion induced momentum force against the inertia force when it is coupled with the ratio of liquid density with respect to vapor density. Our study reveals sudden changes from normal heat transfer (NHT) to heat transfer deterioration (HTD) with obvious temperature peak when crossing a critical SBO, which is 5.126 × 10 −4, 2.018 × 10 −4, 1.653 × 10 −4 and 1.358 × 10 −4 for CO2, H2 O, R134a and R22, determined by a large quantity of database. Our work paves a new way to understand the SHT mechanism and supports the heterogeneous structure of liquid-like fluid and vapor-like fluid for supercritical fluids.