On the highly swirling flow through a confined bluff-body

Abstract

We address highly swirling, confined-bluff-body-flow evolving through a burner; particularly, contributions of the swirling motion to a central-recirculation-zone (CRZ) downstream the injector. Previous studies suggest that flame stability reduces in combustors lacking this zone; careful consideration of the CRZ is thus desirable. We use Reynolds-Averaged-Navier–Stokes (RANS) and Large-Eddy-Simulation (LES) to simulate the flow, and the influence of the swirl device was included in defining the inflow conditions for LES simulations. We use mean velocity profiles and turbulence statistics to test results. There is qualitative agreement between computed and reported experimental data, and we document quantitative differences obtained with the RANS models. LES velocity field results are mostly within 3% of the experimental data, better than the latest reported LES data, reinforcing the suitability of our approach. We took advantage of the quality of the LES mesh, which solves 95.6% of the resolved-turbulence-energy, to present the vorticity structures showing the precessing vortex motion on the CRZ boundaries. Anisotropic states of the Reynolds-stress were characterized with the aid of an anisotropy invariant map, a novelty for this type of burner; the turbulence states considerably vary inside the burner, behaving isotropically in the center of the CRZ, whereas axisymmetric turbulence is predominant in the other areas of the CRZ. The results reinforce the importance of applying appropriate turbulence models and inflow conditions for simulations involving confined-bluff-body-flows in order to capture the main flow fields and structures in the CRZ.

Effect of side dilution jets on the velocity field and mixing in swirl and bluff-body stabilised annular confined flows

“…This over estimation of peak axial velocity in k-ε based models is also reported in previous numerical attempts. 12,47 Compared to other models, RSM accurately captured axial velocity peaks and predicted jet decay excellently with close agreement to the experimental data particularly from x/D = 0.4 to 1.2. In terms of vortex breakdown modelling, its starting and end point, its overall length, the RSM completely outperforms all other RANS models.…”

“…The plot at x/D = 0.272 (upstream region) clearly shows that RSM perfectly predicts the peak axial velocity at the centre, shear region and over the annulus without any distinguishable uncertainty relative to experimental data. The work of Lopes et al 12 also manifests the accuracy of RSM in perfectly capturing the shear layer and associated peak velocities. Moving further downstream locations (up to x/D = 1.6), it can be seen that the modelling results shows very good accuracy in the prediction of centreline jet decay and dissipation of annular jet (Figure 8).…”

“…Practical gas turbines featuring swirl stabilised flame burners further include dilution (side) jets to affect downstream reactant mixing with cooler dilution air thereby helping to control combustion temperature and limit pollutant emissions (particularly oxides of nitrogen). [6][7][8][9] Whilst earlier works have looked into either BB jets [10][11][12] or dilution jets in crossflow regimes, 8,[13][14][15][16] no attempt has been made to study the influence of side dilution jets on the flow field and turbulent mixing of non-swirling and swirling BB stabilised jets in geometries similar to those in industrial combustors. With the above in mind, the present study numerically investigates the impact of side dilution jets on the flow features, turbulence field, and mixing of BB jets, both non-swirling and swirling confined flows.…”

“…Intense mixing (peak normal stresses) was reported in the recirculation zone and in the outer and inner shear layer of annular jet, but again with no study of the impact of side jets. Similarly, Lopes et al 12 attempted RANS and LES modelling of highly swirling confined BB flow and established that peak Turbulence Kinetic Energy (TKE) occurs at the inner and outer shear layers of the annular jet which is indicative of intense turbulent mixing. Such intense upstream turbulence and mixing in annular/BB jets warrants further investigation for the influence of turbulent side jets on the BB stabilized recirculation zone and shear layer, in geometries relevant to practical combustors.…”

Determination of single and double helical structures in a swirling jet by spectral proper orthogonal decomposition

“…fluid flow through a coaxial cylindrical passage, is extensively used in bluff-body combustors [5][6][7] . On the one hand, the wake behind the central body can enhance the fuel-air mixing rate, while on the other hand, the central bluff-body can serve as a fuel injector using cross-flow or co-flow injection [8][9][10] . In addition to the jet's outer shear layer, the existence of the central bluff-body also introduces an inner shear layer between the jet and the wake, which is characterized by strong anisotropic turbulence 11,12 .…”