A novel friction factor model for wire-wrapped rod bundles

Wire-wrapped rod bundles are often considered for fast spectrum nuclear reactors, especially liquid metal cooled fast reactors (LMFRs). Accurate calculation of the pressure drops of these bundles is key for proper thermal-hydraulic design of the reactor. Existing models, while able to accurately predict bundle friction factor, are complex and can be difficult to use without utilizing a numerical implementation. In this paper, a new model for bundle friction factor is proposed based on exact laminar solutions of the Navier-Stokes equations for a simplified rod bundle geometry. These exact solutions can then be adjusted via geometric and empirical parameters to the prototypic geometry. The proposed model is able to calculate the bundle friction factor with an average absolute percent difference of 10.8 % compared to the available experimental data. This compares favourably to existing correlations, offering similar performance while minimizing the number of empirical coefficients and the overall effort required. The model is also valuable for determining a theoretical lower bound for bundle friction factor for a given geometry, with some data found to be lower (>10 %) than this bound. The overall methodology can also in principal be applied to other geometries where similar simplifications could be made.