Three-dimensional bounded jets are important in a variety of engineering applications. In nuclear engineering they are present in critical parts of several types of reactors (e.g., high-temperature gas-cooled reactors and boiling water reactors). The simulation of parallel jets through steady-state computational fluid dynamics has often proved to be problematic, in particular, when identical jets are simulated. In the present work the simulation of parallel jet mixing by the unsteady Reynolds-averaged Navier-Stokes (URANS) methodology has been carried out. Such methodology has the potential to improve the results of steady-state simulations at a limited computational cost. The experimental setup of Kunz et al., consisting of five parallel pipe jets mixing in a rectangular confinement, has been chosen as a benchmark test because of its similarity to the geometry of the IRIS reactor.

The ensemble-averaged time-dependent Navier-Stokes equations have been solved through the finite volume code STAR-CD 4.06.

Several computational models, mesh types, and resolutions have been tried. The results confirm that steady-state calculations tend to underestimate the spreading (mixing) of the jets. In particular, the spreading is acceptable in the near inlet region, while a strong discrepancy is observed far from the inlet. The results of the transient simulations indicate a stable oscillatory behavior downstream from the jet inlets, and the results are in better agreement with the test data. Additional large-eddy simulation calculations performed with the code FLUENT 6.3.26 have also been carried out in order to provide further insight into the URANS methodology results.