A coupled CFD finite element analysis methodology in a bifurcation pipe in a nuclear plant heat exchanger

The accurate calculation of temperature distribution in key parts of a nuclear plant plays a crucial role in maximising the power output and the plant efficiency, whilst ensuring safe operation. The need of making the most profitable use of the available sources of energy to keep competitive in the energy business forces the companies to look for configurations that may compromise the safety of the components. It was found that increasing the power output in a nuclear plant may reduce the life of the welds in the pipes of a heat exchanger operating in very adverse conditions. Rolls-Royce was requested to come up with a suitable solution that protected the pipe welds to fail and allow this increase in power output. Part of the design process was an exhaustive thermal analysis of the installation. Traditionally, in the industrial world and in Rolls-Royce in particular, fluid and solid simulations are conducted separately or using conjugate analysis. The first and more common method relies on the application of boundary conditions applied to the wall surface which are commonly based on heat transfer coefficient correlations or approximate read across of the CFD results. Alternatively, in very specific applications, conjugate calculations are conducted, but the computational cost and meshing difficulties to match both grids making them un-affordable in terms of computational cost and analyst time. This paper presents the application of an alternative method to this standard approach, using a communication library between an in-house finite element (FE) code SC03 and the commercial computational fluid dynamics (CFD) code Fluent. The program couples the fluid and solid extracting the heat fluxes from the CFD and gives them to the FE code, which works out the metal temperature distribution to feedback iteratively the CFD code as wall temperatures. This paper describes the application of the method to a bifurcation pipe in a heat exchanger in a nuclear installation as well as to the proposed solution. The tube is surrounded by CO2 at high temperature and contains superheated steam at lower temperature. Both fluid domains were meshed using unstructured numerical grids consisted of nearly three million elements including a prismatic o-grid of about ten layers. The standard k-ε turbulence model with enhanced wall functions was used. The finite element model grid consists of around fifty thousand cells and was solved using the Rolls- Royce in-house software SC03. The models were run in 'stand alone' mode using the traditional method and were afterwards coupled providing a more accurate temperature distribution. The method has been validated using test data from a Perspex model, where heat transfer coefficients were measured using a liquid crystal technique. A description of the test facility is also included in this paper together with the validation results. (authors)