On the Relationship Between Large-Scale Vortical Structures and Scalar Transport Processes in a Controlled Confined Coaxial Jet (original) (raw)
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Near-field development of large-scale vortical structures in a controlled confined coaxial jet
Journal of Turbulence, 2007
We carry out direct numerical simulation (DNS) of scalar transport and mixing in a coaxial round jet issued into a small model combustor. The Reynolds number based on the diameter and bulk mean velocity of the outer annular jet is 1320. The outer-to-inner bulk mean velocity ratio is fixed at 6.4. Analysis is made on the detailed mechanism of scalar transport modulated by an active control of the near-field large-scale vortical structure. The main interest lies in dynamics of the vortical structure created by the present active control method, growth of streamwise vorticity, and the associated scalar transport process downstream of the nozzle exit. The mixing enhancement is found to be due to three-dimensional breakdown of the primary vortex rings in the inner shear layer. This breakdown process is caused by the streamwise vortical structure. Budget analysis reveals different dynamic processes taking place in the evolution of streamwise structure in the inner and outer shear layers. The process in the outer shear layer is essentially similar to that in plane mixing layers, while the structure in the inner shear layer is convected toward the central axis before stretched axially by the inner vortex rings.
Mixing enhancement in coaxial jets through inflow forcing: A numerical study
Physics of Fluids, 2007
Direct numerical simulations are performed to analyze the flow dynamics and the mixing properties of natural (unforced) and excited coaxial jets at moderate Reynolds number. First, the study of the natural coaxial jet, with species injected in the outer jet alone, allows us to understand the role of the coherent vortices on the mixing process during the transition stage. It is observed that the global flow behavior is controlled by the dynamics of the outer shear layer during the transition. The streamwise vortices are shown to play a significant role in the mixing process since they initiate intense ejections from the seeding regions. Spots of pure (unmixed) species from the outer jet are seen to persist far downstream. Two different types of inflow forcing are then considered based on the information provided by the natural coaxial jet: first, a purely axisymmetric excitation and second, combined axisymmetric and azimuthal excitations all of moderate amplitude. These excitations a...
Study of the turbulent mixing of confined coaxial jets
2014
In the present work the turbulent mixing of coaxial confined jets was studied, using measurements that were compared with numerical solutions of the Navier-Stokes equations by the Large-Eddy simulation method (les). The passive scalar transport equation was solved, using flux limited schemes in the convective terms and dynamic modelling of the subgrid quantities. Numerical and modelling tests were performed, with the effects on the profiles compared against the measurements. The main contribution of this work is the focus on the passive scalar turbulent quantities, both in experimental measurements and in les simulations. The velocity ratio r, between the bulk velocities of the outer and inner jets, had an impact on the uncertainty. High velocity ratio conditions had lower uncertainty in turbulent scalar statistics and fair agreement with les for the scalar fluctuation. The turbulent scalar flux, rarely presented in les of complex flows, shows similar order of magnitude but different profile shapes. The experimental database, obtained previously, was organised and stored and can now be easily accessed in full. Post-processing software was developed to analyse the database, obtaining profiles of the various statistics with uncertainty estimates and curve-fits. The statistics uncertainty was estimated using bootstrap methods and the impact of noise on the measurements was studied. The lowest velocity ratio condition, r = 0.8, had the smallest uncertainty on the mean scalar, whereas the highest, r = 6.5, had the smallest uncertainty on the fluctuation and turbulent scalar flux. A good compromise between uncertainty in the mean and turbulent scalar quantities was the r = 3.2 condition. The pdf evolution had a marching behaviour for low velocity ratio, indicating mixing by smallscale structures, and non-marching behaviour for high-velocity ratio, indicating mixing by large-scale structures. The cylindrical geometry was simulated using a previously validated code. Non-orthogonal grids were used; corrections for non-orthogonality were implemented and validated in a turbulent pipe flow simulation. The scalar numerical schemes and dynamic model were tested against published les for the plane channel flow, with agreement. The les numerical and modelling tests were focused on velocity ratios r = 0.8 and r = 3.2. The budgets of the transport equation terms have shown that the molecular diffusivity (i.e., Sc, the Schmidt number) had negligible weight on the statistics.
Journal of Turbulence, 2007
Direct numerical simulations (DNS) are performed to investigate mixing in free round coaxial jets. A great attention has been put on the influence of upstream conditions upon the global flow structure and the mixing process. The mixing behavior is studied through the spatial and temporal development of the mixture fraction of the annular and the inner fluids, and examined by means of flow visualization and statistics. It is shown that the turbulent mixing process and the mixture fraction field in coaxial jets depend on the upstream conditions, even though a quasi self-similar state is reached. The mixing alterations are explained by the understanding of the flow dynamics modifications implied by the different upstream conditions. These alterations are mainly due to the intense generation of streamwise vortices, favored by high inlet velocity gradients and velocity ratios, as well as low ratios between the inner and the outer jet diameters. This is associated with a high quality of mixing, as far as global mixedness is concerned. It is also shown that the annular fluid reaches the inner fluid and mixes swiftly into it. Conversely, the latter remains confined. Additionally, spots of pure unmixed species are observed at the end of the computational domain, and shown to be due to the annular jet.
Near-field turbulent structures and the local extinction of jet diffusion flames
Symposium (International) on Combustion, 1992
The aerodynamic structure of turbulent jet diffusion flames of methane stabilized on a thick-walled fuel tube in coflowing air has been studied by using laser-Doppler velocimetry and Mie scattering techniques to elucidate the local flame extinction and subsequent lifting phenomena. Instantaneous planar flow visualization and real-time line measurements revealed large-scale vortical structures, constituting the intermittent mixing layer, and sporadic radial mass ejection from the jet-fluid core. By seeding the jet and coflow separately in the LDV measurement, conditioned joint probability-density functions (pdf's) of the axial and radial velocity components and their statistical moments, i.e., the conditioned (zone) averages, rootmean-square fluctuations, skewnesses, kurtoses, and Reynolds shear stress, were determined. The unconditioned pdf and, in turn, the statistical moments were obtained from the conditioned pdf's and intermittency of the jet fluid. The jet-fluid parcels had a higher mean axial velocity than that of the external-fluid parcels and a positive mean radial velocity in contrast to a negative value for the external-fluid parcels. These differences arc responsible, to a great extent, for creating highly-distorted turbulent structures in the intermittent layer adjacent to the flame zone. Since diffusion processes require significantly longer time than convective processes associated with the large-scale vortices and the radial mass ejection, the radial location of the flame zone is insensitive, at least in the near-jet field, to the highfrequency jet-fluid concentration fluctuation in the core. As a result, the ejected jet fluid, or the large-scale vortex itself, can reach the flame-zone location without disturbing the concentration field, thereby locally quenching the already strained diffusion flame.
Journal of Fluid Mechanics, 2000
The stirring and mixing properties of one-phase coaxial jets, with large outer (annular) to inner velocity ratio r u = u 2 /u 1 are investigated. Mixing is contemplated according to its geometrical, statistical and spectral facets with particular attention paid to determining the relevant timescales of the evolution of, for example, the interface area generation between the streams, the emergence of its scale-dependent (fractal) properties and of the mixture composition after the mixing transition. The two key quantities are the vorticity thickness of the outer, fast stream velocity profile which determines the primary shear instability wavelength and the initial size of the lamellar structures peeled-off from the slow jet, and the elongation rate γ = (u 2 − u 1 )/e constructed with the velocity difference between the streams and the gap thickness e of the annular jet. The kinetics of evolution of the interface corrugations, and the rate at which the mixture evolves from the initial segregation towards uniformity is prescribed by γ −1 . The mixing time t s , that is the time needed to bring the initial scalar lamellae down to a transverse size where molecular diffusion becomes effective, and the corresponding dissipation scale s(t s ) are
Control of Coaxial Jets by an Azimuthal Excitation: Vortex Dynamic and Mixing Properties
Proceeding of Fifth International Symposium on Turbulence and Shear Flow Phenomena
The goal of this work is to improve the mixing properties of a coaxial jet with moderate Reynolds number by active control. Two direct numerical simulations of coaxial jets are performed. First, studying a "natural" (without deterministic control) coaxial jet, we show that the appearance of counter-rotating pairs of streamwise vortices allows ejections from the seeding regions. This initiates the turbulent mixing. However spots of unmixed fluids persist at the end of the computational domain. We use then deterministic perturbation to allow an improvement of the mixing properties of the jet. The deterministic perturbation has an azimuthal part which forces the appearance of pairs of streamwise vortices. Finally, we found a real improvement of the mixing properties with a good homogeneity at the end of the computational domain due to a quicker appearance of small scales.
Flow Turbulence and Combustion, 2005
Numerical simulation results are presented for ‘Delft Flame III’, a piloted jet diffusion flame with strong turbulence–chemistry interaction. While pilot flames emerge from 12 separate holes in the experiments, the simulations are performed on a rectangular grid, under the assumption of axisymmetry. In the first part of the paper, flow and mixing field results are presented with a non-linear first order k–ε model, with the transport equation for ε based on a modeled enstrophy transport equation, for cold and reactive flows. For the latter, the turbulence model is applied in combination with pre-assumed β-PDF modeling for the turbulence–chemistry interaction. The mixture fraction serves as conserved scalar. Two chemistry models are considered: chemical equilibrium and a steady laminar flamelet model. The importance of the turbulence model is highlighted. The influence of the chemistry model is noticeable too. A procedure is described to construct appropriate inlet boundary conditions. Still, the generation of accurate inlet boundary conditions is shown to be far less important, their effect being local, close to the nozzle exit. In the second part of the paper, results are presented with the transported scalar PDF approach as turbulence–chemistry interaction model. A C1 skeletal scheme serves as chemistry model, while the EMST method is applied as micro-mixing model. For the transported PDF simulations, the model for the pilot flames, as an energy source term in the mean enthalpy transport equation, is important with respect to the accuracy of the flow field predictions. It is explained that the strong influence on the flow and mixing field is through the turbulent shear stress force in the region, close to the nozzle exit.
Mixing and coherent vortices in turbulent coaxial jets
Comptes Rendus Mécanique, 2005
Direct numerical simulations associated with mixing in constant-density round coaxial jets are performed. They are validated by comparison against laboratory experiments. The mixing process is studied by seeding a passive tracer first in the outer annular jet, then in the inner jet. We demonstrate the important role played by coherent vortices in the mixing mechanisms. The turbulent mixing exhibits an intermittent character as a consequence of fluid ejections caused by the counter-rotating streamwise vortices. We quantify also the domination of the outer jet and show that the fluid issuing from the central jet remains confined.