Abstract
This paper deals with the inaccuracy assessment of the friction pressure loss estimation based on Darcy formula combined with an equivalent hydraulic diameter and a friction factor valid for circular pipes when applied to a square rod bundle. The assessment has been done by comparing the analytical and semi-empirical predictions with two different CFD codes results: CFX and NEPTUNE_CFD. Two different analytical approaches have been considered: the whole-bundle and sub-channel approaches, both for laminar and turbulent flow conditions. Looking at results, it is reasonable to assume that an error in the range of 11% - 23% is likely when using equivalent diameter in the laminar regime. In the case of turbulent regime, the equivalent diameter works better and the error is in the range between a few percent and ~12%.
Highlights
The coolant pressure loss across the rod bundle, as well as across the reactor coolant circuits, is an important factor both for design and safety
It is well known that this way is approximated, i.e. in the rod bundle the flow passage has sharp corners and the flow in the corners is slower than in the core of the channel, so the friction calculated on the mean velocity and the hydraulic diameter is not a good representation of the true flow
The error estimation step aims at estimating the error done calculating the pressure drop due to friction across a square bundle using the Darcy formula combined with an equivalent hydraulic diameter and a friction factor valid for circular equivalent pipes
Summary
The coolant pressure loss across the rod bundle, as well as across the reactor coolant circuits, is an important factor both for design and safety. The friction pressure losses are often estimated using the equivalent hydraulic diameter (Dh = 4A/P, where A is the cross sectional area and P the wetted perimeter) and a friction factor (λ) valid for circular pipes, even for more complex geometries. It is well known that this way is approximated, i.e. in the rod bundle the flow passage has sharp corners and the flow in the corners is slower than in the core of the channel, so the friction calculated on the mean velocity and the hydraulic diameter is not a good representation of the true flow. Rehme [1] developed a method to predict the turbulent friction factor in non-circular channels if the geometry factor of laminar flow is known. The geometry factor (K) is defined as K=Reλ, where Re is
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