Numerical investigation of bent pipe flows at transitional Reynolds number

The correct evaluation of flows at transitional Reynolds number in nuclear reactors is gaining higher importance in relation to the accident analysis for buoyancy-driven flows which dominate the heat decay removal process. In the present paper a comparative study of different turbulence modeling and wall treatment for the evaluation of a fluid flow in transitional Reynolds number, is presented employing computational fluid dynamics (CFD). The relative performance of the models is assessed through benchmarking of fully developed pipe flow at Reynolds number 4900 and of a 90° bend pipe at Reynolds number 5000. Predictions of velocity profiles at different locations are compared to both experimental and accurate numerical simulations.It has been found that the predictions between the models can vary considerably in particular in relation to the different wall treatment employed on the wall. The results show the concerns about the employment of the available turbulence models and wall treatments in low Reynolds number flow regimes and explanation is provided in relation to their formulation.

► Flows at transitional Reynolds number were studied in order to evaluate the behavior of coolant during protected accidents in nuclear reactors.
►  Both straight pipes and highly curved pipes were studied. The issues were found in evaluation of turbulent velocity profile for the straight pipe and velocity profiles downstream the sharp bent pipe.
► The study is a comparative work and the results refer to comparisons of physical characteristics between the most employed turbulence models and wall treatments.
► Results show that k-eps model with cubic formulation and two-layer wall treatment is the most suitable for the evaluation of flow into piping systems in nuclear reactors after accident onset.
► The employment of low-Re number models, even though considered more general, shows some concerns depending on the behavior of the adopted damping functions.