Mitigation of PWR fuel assembly vibrations using bio-inspired nozzles

ABSTRACT: Jet flows injected in a transverse flow need rapid and effective mixing for various applications ranging from medicine injection into bloodstreams to nuclear pressurized water reactors (PWRs). Inspired by marine organisms, spiral snails, and sharks, bio-inspired nozzles are proposed and experimentally investigated to explore their advantages in suppressing nuclear fuel assembly vibrations. It has been observed that a combination of axial flow and jet cross-flow causes vibrations of fuel rods and potential wear at spacer grid supports. Marine biomimetics is used to improve the mixing between the jet flow and surrounding fluid flows. Inspired by the structure of gastropod shells, a variable whorl spacing nozzle is proposed to induce a swirling jet flow to enhance the mixing rate with the flow inside the reactor cores. In addition, the smooth maneuverability of the sharks highlights the importance to include gill slits structure into nozzles. This work focuses on mitigating PWR fuel assembly vibrations using two biomimetic nozzles, a snail nozzle and a shark nozzle. These two nozzles are proposed to improve the mixing rate between the injected flow and the primary coolant flow, resulting in a reduced jet flow effect on fuel rods. A single-span mock-up PWR array is designed, fabricated, and instrumented to mimic the real nuclear fuel assembly. The array is experimentally tested under combined axial flow and jet cross-flow to investigate its dynamical behavior. Three different nozzles, a basic circular nozzle, a snail nozzle, and a shark nozzle, are tested. The research investigates the ability of the proposed marine biomimetic nozzles to suppress the vibration of the rod bundle by comparing the results from the three tested nozzles. The obtained results suggest that the proposed snail-inspired biomimetic nozzle is significantly better than the circular nozzle since it reduces rod bundle vibration by increasing flow mixing. A 50% reduction was achieved by implementing it instead of the circular nozzle. More importantly, the shark-inspired nozzle delays the critical jet flow rate, at which the unstable vibration occurs in the rod bundle, by 20%. In addition to delaying instability, a vibration amplitude reduction of 87.5% was obtained using the proposed shark-inspired nozzle compared to the circular nozzle. The results are promising for various applications including gas burners, combustion chambers, and chemical reactors for providing efficient and rapid mixing between two fluid streams

On the Feasibility of Modeling Two-Phase Flow-Induced Fluidelastic Instability in Tube Bundles

In the 80’s a number of theoretical models were developed to model fluidelastic instability, primarily for single phase flows. The models ranged from purely analytical models to semi-empirical models requiring considerable experimental data as input. While these models were very successful in uncovering the nature of fluidelastic instability and the underlying mechanisms in single phase flow, this work seemed to stop short of getting to the next step of practical application to two-phase flows. During the same period, Connors formula became ‘entrenched’ in industry to the extent that the formula now forms part of the design norms against fluidelastic instability. In an ongoing research program the quasi-steady model has been chosen as a possible candidate for modeling fluidelastic instability in two-phase flows. This paper discusses the challenges associated with accurate modeling of fluidelastic instability in two phase flows using this and other models. The unsteady model is shown to have limitations when it comes to measuring accurately the necessary unsteady fluid force coefficients. A comparison of the stability analysis results with experimental measurements shows that the quasi-steady model can give a reasonable estimate of the instability velocity as well as the inter-tube dynamics. Finally, the remaining challenges, before the quasi-steady model and possibly other models can be fully implemented for prototypical conditions are discussed. In particular the need for more work to understand the flow itself is highlighted.</jats:p

Nonlinear dynamics of a loosely-supported cylinder in cross-flow

Loosely supported cylinders subjected to cross-flow may undergo fluidelastic instability in the support inactive mode resulting in cylinder/support impacting. The cylinder/support interaction forces and, in turn, the resulting cylinder wear rates are strongly dependent on the detailed dynamical response. This Thesis examines the response of a loosely supported cylinder located in the third row of an otherwise rigid rotated triangular array. The feasibility and potential of a modern nonlinear dynamics approach to the understanding of the underlying dynamics is investigated.A nonlinear quasi-steady model was formulated to model the dynamical behaviour. The steady fluid force field, required as input to the model, was measured experimentally for a cylinder within a rotated triangular array. A lither stability analysis showed the cylinder stability behaviour to be strongly dependent on cylinder position. This result serves as a possible explanation for the rare occurrence of, theoretically predicted, multiple instability regions in experimental measurements.The nonlinear analysis uncovered two important transition routes to chaos. The first, a switching mechanism prevalent at the onset of impacting. The second and most important is the intermittency route to chaos. The theoretical model showed good agreement with experiments in predicting the bifurcation sequences and transitions to chaos--comparisons were quantified via fractal dimensions and saddle orbit distributions.The identification of type I intermittency leads to a quantitative estimate of the probability distribution of the length of laminar phases. It is shown that the duration of laminar phases and the associated frequency may provide better estimates of integration time and frequency for wear-rate computation