Direct simulation of turbulent swept flow over a wire in a channel

Abstract

Turbulent swept flow over a cylindrical wire placed on a wall of a channel is investigated using direct numerical simulations. This geometry is a model of the flow through the wire-wrapped fuel pins, the heat exchanger, typical of many nuclear reactor designs. Mean flow along and across the wire axis is imposed, leading to the formation of separated flow regions. The Reynolds number based on the bulk velocity along the wire axis direction and the channel half height is 5400 and four cases are simulated with different flowrates across the wire. This configuration is topologically similar to backward-facing steps or slots with swept flow, except that the dominant flow is along the obstacle axis in the present study and the crossflow is smaller than the axial flow, i.e. the sweep angle is large. Mean velocities, turbulence statistics, wall shear stress and instantaneous flow structures are investigated. Particular attention is devoted to the statistics of the shear stress on the walls of the channel and the wire in the recirculation zone. The flow around the mean reattachment region, at the termination of the recirculating bubble, does not exhibit the typical decay of the mean shear stress observed in classical backward-facing step flows owing to the presence of a strong axial flow. The evolution of the mean wall shear stress angle after reattachment indicates that the flow recovers towards equilibrium at a rather slow rate, which decreases with sweep angle. Finally, the database is analysed to estimate resolution requirements, in particular around the recirculation zones, for large-eddy simulations. This has implications in more complete geometrical models of a wire-wrapped assembly, involving hundreds of fuel pins, where only turbulence modelling can be afforded computationally.

Numerical simulation of the flow in wire-wrapped pin bundles: Effect of pin-wire contact modeling

“…We present the mathematical model and the relevant parameters chose, and we discuss the methodology used for the simulations. The direct numerical simulation (DNS) calculation of Ranjan et al (2010) was selected for its generality, its high resolution (it was computing over 100,000,000 collocation points), and its relatively low Reynolds number. The geometry investigated in that paper corresponds to a wire immersed in a channel flow with both streamwise (direction z) and spanwise (direction x) periodic boundary conditions (the boundary conditions are identical for Nek5000 and STAR-CCM+).…”

“…In the first case we keep the same contact modeling used by Ranjan et al (2010), where the wire and wall are considered in contact and merged with an appropriate fillet. In the second case the wire and wall are separated by a nominal gap g. For the present case No heat transfer calculation was performed in the work by Ranjan et al (2010). In the present simulations a heat source is specified within the walls while it is assumed that no that heat is generated in the wire.…”

“…which is chosen to be equal to 1,000 (consistently with Case C in Ranjan et al (2010), albeit slightly higher). bulk U is the bulk velocity in the spanwise direction.…”

“…The primary test case is the geometry proposed by Ranjan et al (2010), which consists of a simple channel flow with a wire embedded in one of the walls. Ranjan et al provided detailed and extremely accurate (i.e., obtained with highly resolved direct numerical simulation techniques) distributions for the velocity field and other turbulent statistics, thus allowing us to fully verify and validate our calculations.…”

“…After reproducing the results using the LES code Nek5000 (Fischer et al, 2008), we examined the conjugate heat-transfer problem (which was not studied by Ranjan et al, 2010). We also considered an additional modeling option for the wire: the introduction of a nominal gap between the wire and wall.…”