Figure 13 - uploaded by Ponnuthurai Gokulakrishnan
Content may be subject to copyright.
Instantaneous vorticity profile for the skeletal mechanism for the following cases (a) Phi = 0.6 & EDC model ; (b) Phi = 0.6 & LC model ; (c) Phi = 0.45 & EDC model ; (d) Phi = 0.45 & LC model
Source publication
Large Eddy Simulations (LES) were performed to investigate the effect of turbulence - chemistry interaction on flame instability and flame-vortex interactions in bluff body stabilized premixed flames. A semi-global reduced kinetics mechanism and a skeletal mechanism were developed and implemented with a Laminar Chemistry (LC) model and an Eddy Diss...
Similar publications
Large Eddy Simulations (LES) with Smagorinsky subgrid scale model have been performed for the flow past two circular cylinders in tandem placed in the vicinity of a horizontal plane wall at very small gap ratios, namely G/D = 0.1, 0.3 and 0.5, in three-dimension (3D). The ratio of cylinder center-to-center distance to cylinder diameter, or the pitc...
The actuator line model (ALM) is a commonly used method to represent lifting
surfaces such as wind turbine blades within Large-Eddy Simulations (LES). In
ALM the lift and drag forces are replaced by an imposed body force which is
typically smoothed over several grid points using a Gaussian kernel with some
prescribed smoothing width $\epsilon$. To...
Citations
... In LES, turbulence scales larger than the grid spacing are resolved, while subgrid scales and their effects on the large scales are modeled [14]. LES has been shown [15][16][17][18][19] to be computationally practical for modeling unsteady phenomena, including transient combustion events. One of the critical aspects of modeling unsteady turbulent combustion in high-speed jets is appropriately accounting for the influence of turbulence on the rate of chemical reactions. ...
An approach based on the OpenFOAM library has been developed to solve a high-speed, multicomponent mixture of a reacting, compressible flow. This work presents comprehensive validation of the newly developed solver, called compressibleCentralReactingFoam, with different supersonic flows, including shocks, expansion waves, and turbulence–combustion interaction. The comparisons of the simulation results with experimental and computational data confirm the fidelity of this solver for problems involving multicomponent high-speed reactive flows. The gas dynamics of turbulence–chemistry interaction are modeled using a partially stirred reactor formulation and provide promising results to better understand the complex physics involved in supersonic combustors. A time-scale analysis based on local Damköhler numbers reveals different regimes of turbulent combustion. In the core of the jet flow, the Damköhler number is relatively high, indicating that the reaction time scale is smaller than the turbulent mixing time scale. This means that the combustion is controlled by turbulent mixing. In the shear layer, where the heat release rate and the scalar dissipation rate have the highest value, the flame is stabilized due to finite rate chemistry with small Damköhler numbers and a limited fraction of fine structure. This solver allows three-dimensional gas dynamic simulation of high-speed multicomponent reactive flows relevant to practical combustion applications.
... In LES, turbulence scales larger than the grid spacing are resolved, while subgrid scales and their effects on the large scales are modeled [14]. LES has been shown [15][16][17][18][19] to be computationally practical for modeling unsteady phenomena, including transient combustion events. One of the critical aspects of modeling unsteady turbulent combustion in high-speed jets is appropriately accounting for the influence of turbulence on the rate of chemical reactions. ...
An approach based on the OpenFOAM library has been developed to solve a high-speed, multicomponent mixture of a reacting, compressible flow. This work presents comprehensive validation of the newly developed solver called compressibleCentralReactingFoam with different supersonic flows, including shocks, expansion waves, and turbulence-combustion interaction. The comparisons of the simulation results with experimental and computational data confirm the fidelity of this solver for problems involving multicomponent high-speed reactive flows. The gas dynamics of turbulence-chemistry interaction are modeled using a partially-stirred reactor formulation and provide promising results to better understand the complex physics involved in supersonic combustors. A time-scale analysis based on local Damköhler numbers reveals different regimes of turbulent combustion. In the core of the jet flow, the Damköhler number is relatively high indicating that the reaction time scale is smaller than the turbulent mixing time scale. This means that the combustion is controlled by turbulent mixing. In the shear layer, where the heat release rate and the scalar dissipation rate have the highest value, the flame is stabilized due to finite rate chemistry with small Damköhler numbers and a high fraction of fine structure. This solver allows three-dimensional gas dynamic simulation of high-speed multicomponent reactive flows relevant to practical combustion applications.
... 9 With the development of computational technique, high spatial/temporal accuracy numerical accuracy simulation has provided researchers with powerful tools in flames dynamic analysis close to LBO. In the study of flame dynamics near LBO, LES-CMC (Large Eddy Simulation-Conditional Moment Closure) model was employed by Tyliszczak et al. 10 LES-PDF (Probability Density Function) model by Black and Smith 11 and Gokulakrishnan et al. 12 LES-EBU (Eddy Break-Up) model by Kim et al. 13 LES-EDC (Eddy Dissipation Concept) model by Gokulakrishnan et al. 14 LES-LEM (Linear Eddy Mixing) model by Menon et al. 15 and LES-LTEM (Lagrangian Transport Element Method) model 16 by Erickson et al. When the combustion was close to LBO, similar flame behavior was detected in these simulation works. ...
Lean Blow-Off (LBO) prediction is important to propulsion system design. In this paper, a hybrid method combining numerical simulation and Da (Damköhler) model is proposed based on bluffbody stabilized flames. In the simulated reacting flow, Practical Reaction Zone (PRZ) is built based on OH radical concentration, and it is considered to be the critical zone that controls LBO. Da number is obtained based on the volume-averaged parameters of PRZ. The flow time scale (τf) indicates the residence time of the fresh mixture flowing through the PRZ. It is obtained based on the characteristic length and volume-averaged axial velocity of the PRZ. The chemical time scale (τc) indicates the shortest time needed to trigger the reaction of the mixture. It is obtained by commercial software CHEMKIN through monitoring the transient variation of the reactor temperature. The result shows that the average Da number under different LBO conditions is 1.135 (the Da number under each LBO condition ranges from 0.673 to 1.351). This indicates that the flow time scale and chemical time scale are comparable. The combustion is in a critical state where LBO is easy to occur. With the increase of the fuel mass flow rate (the global fuel/air ratio increases from 0.004761 to 0.01095), τf increases from 0.001268 s to 0.007249 s, and τc decreases from 0.00124 s to 0.00089 s. Accordingly, Da number increases from 1.023 to 8.145, which shows that the combustion becomes more stable. The above results show that the method proposed in the present study can properly predict the LBO limits of combustors, which provides important technical supports for combustor design and optimization.
... A similar conclusion was given for example by Petrova [54] showing that it was necessary to take into account the turbulence-chemistry interaction in order to spread the temperature profiles. Gokulakrishnan [55] also pointed out that the unsteady instability characteristics of the reaction process were more pronounced when the turbulent-chemistry interaction model was activated compared to the laminar model that assumed zero fluctuations. It shall be noted that the Navier-Stokes equations are used for the turbulent flow subjected to decomposition and Favre averaging [56]. ...
The hydrodynamics of non-reactive and reactive distillation sieve tray columns are analyzed in this study. The first mentioned task was performed in order to verify the accuracy of Computational Fluid Dynamics (CFD) simulation in modeling the flow behavior of a sieve tray system. For the reactive distillation system, the studies were performed for the 25th reactive tray in the methyl acetate (MeAc) production process. Aspen Plus V9.0 simulation was carried out to determine the clear liquid height on the tray as a function of tray geometry and operating condition. The transient vapor-liquid hydrodynamics on the 25th reactive tray was simulated using CFD techniques involving the turbulence-chemistry interaction (TCI) effects. The predicted chemical products in liquid and vapor phases by Aspen Plus and CFD are of reasonable agreement. Furthermore, the CFD results can deliver more physics in determining localized distributions of the hydrodynamic parameters in the MeAc production process. Information obtained about the location and distribution of the vortex flow can be used to optimize the heat transfer and turbulence mixing.
... This approach is often limited to a narrow range of conditions by its construction and parameterization. One example of such a mechanism is the propane-air mechanism of Gokulakrish-nan et al. [38] , used by Kim and Pope [9] . Finally, a skeletal reaction mechanism is comparable to a detailed reaction mechanism but only using the major reaction steps. ...
... These ignitiondelay time, laminar flame-speed, and extinction strain-rate computations were performed using CANTERA [56] , and CHEMKIN [57] . The short-listening of potentially interesting global, reduced and skeletal propane-air mechanisms include the well-known 1-and 2-step global mechanisms of Westbrook and Dryer [36] , (hereafter referred to as WD1 and WD2, respectively), the 4-step global mechanism of Jones and Lindstedt [37] , (JL4), the 44-step quasiglobal mechanism of Gokulakrishnan et al. [38] , (G44), the 73-step skeletal mechanism of Haworth et al. [58] , (H73) and a new 66step skeletal mechanism, Z66, described in Appendix 1 . The motivation for developing a new skeletal mechanism for C 3 H 8 -air combustion is the shortage of accurate, reliable and well-documented C 3 H 8 -air mechanisms appropriate for LES modeling in the open literature. ...
... For s u and T flame at standard conditions the global [36,37] , quasi-global [38] , skeletal [58] , detailed [33,34,57] , and new skeletal Z66 mechanism are compared to experimental data [58,59] in Fig. 2 a and b. The three global mechanisms overpredict T flame and s u at rich conditions, mainly due to the absence of intermediate species, comprising several C-based species. ...
The increasing computational capacity in recent years has spurred the growing use of combustion Large Eddy Simulation (LES) for engineering applications. The modeling of the subgrid stress and flux terms is well-established in LES, whereas the modeling of the filtered reaction rate terms is under intense development. The significance of the reaction mechanism is well documented, but only a few computational studies have so far been conducted with the aim of studying the influence of the reaction mechanism on the predicted flow and flame. Such an investigation requires the availability of well documented, thoroughly tested, and accurate reaction mechanisms suitable for use in practical engineering simulations. Global and detailed reaction mechanisms are available for many fuel mixtures, whereas skeletal reaction mechanisms suitable for LES are in rather short supply. This research attempts to close this gap by using combustion LES to examine a well-known bluff-body stabilized premixed propane–air flame using two well-known global reaction mechanisms and a novel skeletal reaction mechanism, developed as part of this study. These reaction mechanisms are studied for laminar flames, and comparison with experimental data and detailed reaction mechanisms demonstrates that the skeletal mechanism shows improved agreement with respect to all parameters studied, in particular the laminar flame speed and the extinction strain rate. The LES results reveal that the choice of the reaction mechanism does not significantly influence the instantaneous or time-averaged velocity, whereas the instantaneous and time-averaged species and temperature are influenced. The agreement with the experimental data increases with increased fidelity of the reaction mechanism, and the skeletal reaction mechanism provides a more realistic basis for e.g. emission predictions.
... Numerical methods, especially Large Eddy Simulations (LES), have made it possible to model combustion transients' phenomena with high temporal and spatial resolution. During the past decade, LES has developed in a variety of directions: a linear eddy model (LEM) (14)(15)(16)(17) , an Eddy Break-up Model (EBM) (18) , an eddy dissipation 240 February 2017 The Aeronautical Journal concept (EDC) (19) , and probability density functions (PDF) (20) . Menon, based on LES-LEM (16,17,21) , attained the flame distortion and variation in flame spread velocity but did not succeed in capturing eddies that mostly effect the local extinction. ...
Lean Blow-Out (LBO) limits are critically important in the operation of aero engines. Previously, Lefebvre's LBO empirical correlation has been extended to the flame volume concept by the authors. Flame volume takes into account the effects of geometric configuration, spatial interaction of mixing jets, turbulence, heat transfer and combustion processes inside the gas turbine combustion chamber. For these reasons, LBO predictions based on flame volume are more accurate. Although LBO prediction accuracy has improved, it poses a challenge associated with V f estimation in real gas turbine combustors. This work extends the approach of flame volume prediction based on fuel iterative approximation with cold flow simulations to reactive flow simulations. Flame volume for 11 combustor configurations were simulated and validated against experimental data. To make prediction methodology robust, as required in preliminary design stage, reactive flow simulations were carried out with the combination of presumed Probability Density Function (PDF) and discrete phase model (DPM) in Fluent 15.0 The criterion for flame identification was defined. Two important parameters—critical injection diameter (D p,crit ) and critical temperature (T crit )—were identified and their influence on reactive flow simulation was studied for V f estimation. Results exhibit ±15% error in Vf estimation with experimental data.
... Two different propane mechanisms are investigated: a 30-species, 114-step skeletal mechanism and a 15-species, 44-step quasi-global reduced mechanism. The skeletal mechanism is a subset of the detailed propane mechanism [55] and the quasi-global model was modified for fuel-lean conditions [56]. Both propane mechanisms have previously been used to predict bluff-body stabilised flames [56]. ...
... The skeletal mechanism is a subset of the detailed propane mechanism [55] and the quasi-global model was modified for fuel-lean conditions [56]. Both propane mechanisms have previously been used to predict bluff-body stabilised flames [56]. FlameMaster is used to determine the properties of a freely-propagating laminar premixed flame predicted by each mechanism, for the same unburnt mixture properties as in the Volvo experiment. ...
A turbulent lean-premixed propane–air flame stabilised by a triangular cylinder as a flame-holder is simulated to assess the accuracy and computational efficiency of combined dimension reduction and tabulation of chemistry. The computational condition matches the Volvo rig experiments. For the reactive simulation, the Lagrangian Large-Eddy Simulation/Probability Density Function (LES/PDF) formulation is used. A novel two-way coupling approach between LES and PDF is applied to obtain resolved density to reduce its statistical fluctuations. Composition mixing is evaluated by the modified Interaction-by-Exchange with the Mean (IEM) model. A baseline case uses In Situ Adaptive Tabulation (ISAT) to calculate chemical reactions efficiently. Its results demonstrate good agreement with the experimental measurements in turbulence statistics, temperature, and minor species mass fractions. For dimension reduction, 11 and 16 represented species are chosen and a variant of Rate Controlled Constrained Equilibrium (RCCE) is applied in conjunction with ISAT to each case. All the quantities in the comparison are indistinguishable from the baseline results using ISAT only. The combined use of RCCE/ISAT reduces the computational time for chemical reaction by more than 50%. However, for the current turbulent premixed flame, chemical reaction takes only a minor portion of the overall computational cost, in contrast to non-premixed flame simulations using LES/PDF, presumably due to the restricted manifold of purely premixed flame in the composition space. Instead, composition mixing is the major contributor to cost reduction since the mean-drift term, which is computationally expensive, is computed for the reduced representation. Overall, a reduction of more than 15% in the computational cost is obtained.
... How to predict the LBO limit of a combustor efficiently and accurately based on numerical simulations has become a research focus in the application of combustion engineering. Currently, lots of numerical studies were conducted on the flame dynamics near LBO, such as Menon (LES-LEM [12][13][14][15]), Kim (LES-EBU [16]), Smith (LES [17][18][19]), Gokulakrishnam (LES-EDC [20], LES-PDF [21]), etc. An agreement of these studies is reached: (1) local extinction is the necessary stage which the flame has to go through before the entire extinction, (2) the development of the local extinction has direct impacts on the entire extinction. ...
According to the Lefebvre's model and flame volume (FV) concept, an FV model about lean blow-out (LBO) was proposed by authors in early study. On the other hand, due to the model parameter (FV) contained in FV model is obtained based on the experimental data, FV model could only be used in LBO analysis instead of prediction. In view of this, a hybrid FV model is proposed that combines the FV model with numerical simulation in the present study. The model parameters contained in the FV model are all estimated from the simulated nonreacting flows. Comparing with the experimental data for 11 combustors, the maximum and average uncertainties of hybrid FV model are +/- 16% and +/- 10%.
... Zhang et al [14] recommended the Standard k-ε turbulence model for bluff-body flames. Kiel et al [15] used the Eddy Dissipation Concept (EDC) combustion model to account for turbulent-chemistry interactions for turbulent bluff-body stabilized flames. In this paper the Standard k-ε turbulence and EDC combustion models are both applied. ...
The objective of this work is to investigate the flame stabilization mechanism and the impact of the operating conditions on the characteristics of the steady, lean premixed flames. It’s well known that the flame base is very important to the existence of a flame, such as the flame after a V-gutter, which is typically used in ramjet and turbojet or turbofan afterburners and laboratory experiments. We performed two-dimensional simulations of turbulent premixed flames anchored downstream of the heat-conducting V-gutters in a confined passage for kerosene-air combustion. The flame bases are symmetrically located in the shear layers of the recirculation zone immediately after the V-gutter’s trailing edge. The effects of equivalence ratio of inlet mixture, inlet temperature, V-gutter’s thermal conductivity and inlet velocity on the flame base movements are investigated. When the equivalence ratio is raised, the flame base moves upstream slightly and the temperature gradient dT/dx near the flame base increases, so the flame base is strengthened. When the inlet temperature is raised, the flame base moves upstream very slightly, and near the flame base dT/dx increases and dT/dy decreases, so the flame base is strengthened. As the V-gutter’s thermal conductivity increases, the flame base moves downstream, and the temperature gradient dT/dx near the flame base decreases, so the flame base is weakened. When the inlet velocity is raised, the flame base moves upstream, and the convection heat loss with inlet mixture increases, so the flame base is weakened.
... Zhang and Guo [14] recommended the Realizable k-ε turbulence model. Kiel et al. [15] recommended the EDC combustion model. In this paper the Realizable k-ε turbulence and EDC combustion models are both applied. ...
In order to control the NOx pollutant emissions and reduce fuel consumption, lean combustion becomes an important subject. A bluff body is used in this paper to study the flame-holding mechanism by numerical simulation. After the validation of the numerical method, 6 cases were simulated with different inlet velocity of the premixed mixture of kerosene vapor and air. The responses of reaction rate near blowout to inlet velocity variation were analyzed. The results show that with the increase of the inlet flow velocity, the anchoring points move upstream to the bluff body, while nearly no movement along the cross direction, and the flame will finally be blown out if the anchoring points are almost attached to the bluff body. It is implied that the quenching effect caused by heat transfer between flame fronts and bluff body is the key factor of flame blowout.
