Departure from nucleate boiling and pressure drop prediction for tubes containing multiple short-length twisted-tape swirl promoters

Abstract

Previous studies conducted at MIT showed that the power performance of an inverted pressurized water reactor (IPWR) conceptual design, i.e. the coolant and moderator are inverted such that the fuel is the continuous medium and the moderator flows through coolant channels, has potential to outperform a traditional pressurized water reactor (PWR). Similar to the traditional PWR, the IPWR design involves a tradeoff between core pressure drop and the minimum departure from nucleate boiling ratio (MDNBR). In order to increase the power density of the IPWR, Ferroni [231 examined the possibility of inserting multiple short-length twisted-tapes (MSLTTs) in the cooling channels. For a fixed coolant mass flow rate, the swirling flow produced by the MSLTTs allows the IPWR to have a higher operating heat flux while maintaining the design criteria of MDNBR as compared to either the traditional PWR or IPWR without swirl promoters. However, the addition of each twisted-tape increases the core pressure drop which limits the coolant flow rate due to pumping power limitations of existing reactor coolant pumps (RCPs). In order to better characterize the critical heat flux (CHF) enhancement caused by the addition of MSLTTs, this study performed a critical analysis of existing CHF correlations and models. Initially a phenomenological model was sought to describe the mechanisms of CHF for tubes containing MSLTTs; however, the full-length twisted-tape (FLTT) model that was selected for modification was found to have terms that could not be reconciled for the transition from fully developed swirl to decaying swirl. The existing CHF correlations for swirling flow were also found to be unsatisfactory. Therefore, the insights gained through working with the phenomenological model were used to develop a new empirical correlation to describe the departure from nucleate boiling (DNB) using existing swirling flow DNB data as well as an existing swirl decay model. In order to allow for more flexibility in the placement of the MSLThs, an existing FLTT pressure drop correlation was modified to account for the form pressure drop at the entrance to each twisted-tape insert as well as the friction pressure drop in the decaying swirl region downstream from the exit of each MSLTT. A sensitivity analysis of the new pressure drop correlation was also performed to determine if the complete methodology could be simplified. Design insights were presented that help to narrow the design space for the IPWR. These steps should be followed in order to find the maximum power density possible by the IPWR design. Finally, the existing swirl flow CHF data and correlations are presented in the appendices of this thesis.