Performance of RANS models for simulating turbulent swirling and free jet flows

The present work aims at assessing the perforrnances of RANS turbulence models for simulating turbulent swirling and free jet flows, using the commercial software FLIIENT version 6.I.22.The RANS turbulence models examined are grouped into two families: (l) the two-equation eddy-viscosity models, which are the k-e, RNG k-c, realizable k-t, and the ^9^SZ k-ø, and (ii) the Reynolds stress models, which are the standard .RSM and the ,S,SG.The first flow case simulated in this thesis is a turbulent swirling flow in a can- combustor, in which two inlet swirl intensities (i.e.,S:0.4 and ,5:0.81) are considered.The predictions compared against published experimental data revealed that the eddyviscosity models are unable to capture the central recirculation zone in the case of the weakly swirling flow (,S:0.4).However, although they revealed the existence of this feature for the strongly swirling flow (5:0.81),they were incapable of predicting its correct size.On the other hand, the Reynolds stress models were able to predict the corner and the central recirculation zones for both swirl intensities.The predictions of turbulence intensities by using the realizable k-e and the SST k-a were comparable to those of the Reynolds stress closures.The shear stresses were not well predicted by all the tested models.Both the eddy-viscosity and the Reynolds stress closures showed relatively less approximation errors in the weakly swirling flow.The second flow case examined is a turbulent free jet issuing from a sharp-edged equilateral triangular orifice in still air surrounding.The numerical simulations revealed that among the eddy-viscosity models, the performance of the reqlizctble k+ model is comparable to that of the Rel.nolds stress models with the exception of the predictions of iii the turbulence intensities.The vena contracta effect was predicted by all the tested models' The Æ-e and the RNG k+ models showed faster and slower mixing than that of the experiment, respectively.On the other hand, the Reynolds stress models, especially the standard RSM, appeared to produce better predictions than the eddy-viscosity models.However, the standard RSM seemed unable to capture accurately enough the flatness of the streamwise velocity profiles (top-hat profile) near the centreline in the near-field.The third and last flow case examined in the present thesis is a turbulent free jet issuing from a circular nozzle with a triangular collar.In this exercise, only the standard RSM and the standard Æ-e model are employed.This is because the trends seen in the combustor and the triangular orifice studies suggested that in these geometries the standard R,SM has the potential of producing results that agree remarkably well with experimental data.Whereas the standard /r-e model is used as a simple representative of the two-equation eddy-viscosity models.This flow case, a circular nozzle which is connected to a triangular collar with a step, may be regarded as a combination of the previous geometries.This is because it encompasses both confinement and sudden expansion.The predictions revealed that the standard, RSM, which requires more computational time than does the standard k-e model, is capable of reproducing remarkably well the experimental results.

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