PSI - Issue 60

Keshav Mohta et al. / Procedia Structural Integrity 60 (2024) 402–410 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Case Study: Estimation of Wear Rate in Pressure Tube With the above methodology, fretting wear in pressure tubes of a large Indian PHWR has been assessed for two cases. These included (a) normal operating conditions without any pressure/ flow pulsation in coolant, and (b) pulsating flow conditions (40 kPa amplitude pressure pulse over normal operating pressure conditions at the vane passing frequency of 150 Hz as per Norsworthy and Ditschum (1995)). These typical conditions are taken to assess the fretting damage under realistic conditions and the potential of pressure pulsation to enhance the fretting damage. As the primary focus here is on methodology aspects, only a brief description of CFD and FEA analyses has been provided. 3.1. CFD Analysis: Evaluation of Transient Hydrodynamic Forces Transient CFD analysis was carried out using general purpose CFD code CFD-ACE+ (2014) to assess the hydrodynamic forces acting on the fuel elements. The modeled flow domain consisted of an upstream region of liner tube/ fuel locator region to first one and half bundle as shown in Fig. 4. Inside reactor core, the major turbulent activities in coolant flow occur due to the flow obstruction caused by the obstacles such as end plate and appendages of fuel elements (bearing pads and spacers). Therefore, one and half fuel bundle domain was considered with angular misalignment of 11.8˚ between the two, which leads to maximum obstruction o f the flow.

Fig. 4. Computational domain for CFD analysis

Fixed mass flow rate at the inlet and constant pressure boundary at the outlet were taken as boundary conditions in the CFD analysis. The pressure pulsation case was simulated in terms of mass flow pulsation through user defined function. It has been reported by Suh and Lightstone (2003) that the k- ε turbulence model could not accurately predict the transient variation in radial forces on the bundle. Therefore, Large Eddy Simulation (LES) turbulence model with wall adapted sub grid scaling (WALE) was used, although it is computationally very expensive. A steady state solution was first sought using the k- ε method, and then the LES was launched to obtain the unsteady solution. The numerical schemes being used was validated based on the pressure drop calculations using the experimental results available from earlier experimental studies at Bhabha Atomic Research Centre. It was observed that the pulsation has insignificant effects on fluid forces working on the fuel bundle as a whole. However, its effect was remarkably clear on the hydrodynamic forces acting on a typical outer ring fuel pin, as shown in Fig. 5. It is noted that the average radial force both the conditions remain same, but the fluctuations are highly pronounced in case of pulsating flow. Moreover, the excitation frequency (150 Hz) is clearly reflected in fluid forces acting on the pin.