Investigating Isothermal Pressure Drop Characteristics in a 5x5 PWR Rod Bundle with Reverse Grid Configuration Using an Automated CHF Test Facility

Abstract

This study provides a comprehensive understanding of pressure drop characteristics across 5x5 nuclear fuel rod bundles under various flow rates and system pressures in nuclear reactor systems. Primarily, the isothermal pressure drop test was performed to investigate the phenomena, with system pressures ranging from 0 to 200 psig. The analysis of this data led to insights regarding the dependence of the pressure drop on the Reynolds number and the differences observed in forward and reverse-oriented grid structures for rod bundles. Interestingly, at higher Reynolds numbers, a significant disparity in pressure drops was seen, with the forward grid configuration exhibiting a considerably higher drop than the reverse grid.

Further analysis involved determining the spacer grid loss coefficients (K) as a function of the Reynolds number, revealing that K for N=6 configurations typically exceeded that for N=1 configuration. This trend is attributed to increased pipe friction due to the longer physical length of N=6. An inverse relationship was observed between K values and the Reynolds number, underscoring the transition from viscous to inertial flow as Reynolds numbers increase.

The study also delved into pressure drop behavior at low Reynolds numbers. It was discerned that both N=1 and N=6 grid configurations displayed characteristics of laminar flow up to a Reynolds number of about 2,000, with a transition zone from 2,000 to 10,000 Reynolds numbers, after which the system exhibited turbulent regime characteristics.

In essence, the study offers critical insights into pressure drop behavior, shedding light on the interplay of system parameters and their impact on system design and operation in nuclear engineering. The findings reinforce established correlations and contribute to future research and applications, improving our understanding of core backflows and pressure drops.