A DNS study of hydrogen/air swirling premixed flames with different equivalence ratios

Hydrogen/air swirling premixed flames with different equivalence ratios are studied using direct numerical simulation. A fourth-order explicit Runge–Kutta method for time integration and an eighth-order central differencing scheme for spatial discretization are used to solve the full Navier–Stokes (N–S) equation system. A 9 species 19-step reduced mechanism for hydrogen/air combustion is adopted. The flames are stabilized with the help of a recirculation zone characterizing a high swirling flow. The vortex structures of the swirling premixed flames are presented. The flame structures are investigated in terms of the flame front curvature and tangential strain rate probability density functions (pdfs). The local flamelet temperature profiles are also extracted randomly along the flame front and compared with the corresponding laminar flame temperature profile. In order to study preferential diffusion effects, direct numerical simulation of two additional freely propagating planar flames in isotropic turbulence is conducted. Preferential diffusion effects observed in the planar flames are suppressed in the swirling flames. Further analysis confirms that the coherent small-scale eddies play important roles in the interactions between turbulence and the flame front. They are able to change the dynamic properties of the flame font and lead to enhanced burning intensity in the flame front with negative curvature for both stoichiometric and fuel-lean flames.     

Hydrogen-enriched nonpremixed jet flames: Effects of preferential diffusion

Influence of preferential diffusion on the dynamics of hydrogen and syngas nonpremixed impinging jet flames was studied using direct numerical simulation and flamelet generated manifolds based on detailed chemical kinetics. The results presented in this study were obtained from a uniform Cartesian grid with 768 × 768 × 768 points. Reynolds number used was Re = 2000, based on the reference quantities. Results reported here indicate that the preferential diffusion significantly affects the structures and the maximum temperature of the hydrogen flame, which deviates significantly from the results obtained without considering the preferential diffusion. The preferential diffusion results in a shift in the equivalence ratio in the reaction zone to leaner conditions. Moreover, the numerical results suggest that the preferential diffusion influences the flame–wall interaction and thus wall heat transfer, which is critical for the design of combustion equipment for clean combustion applications with high hydrogen contents in the fuel.     

Evaluation of the laminar diffusion flamelet model in the calculation of an axisymmetric coflow laminar ethylene-air diffusion flame

A numerical study of an axisymmetric coflow laminar ethylene-air diffusion flame at atmospheric pressure was conducted using detailed chemistry and complex thermal and transport properties and two different methodologies: (1) the direct simulation method of solving the two-dimensional axisymmetric elliptic governing equations, and (2) the steady-state stretched diffusion flamelet model. Soot formation and radiative heat transfer were not taken into account in these calculations, both for simplicity and to avoid the complications associated with the issues of how to incorporate these chemical and physical processes into the flamelet model. The same reaction mechanism and thermal and transport properties were used in the 2D direct simulation and the generation of the flamelet library. The flamelet library was generated from the solutions of counterflow ethylene-air diffusion flames at a series of stretch rates. Results of the 2D direct simulation and the flamelet model are compared in physical space. Although the overall results of the flamelet model are qualitatively similar to those of the direct simulation, significant differences exist between the results of the two methods even for temperature and major species. The direct simulation method predicts that the peak concentrations of CO₂ and H₂O occur in different regions in the flame, while the flamelet model results show that the peak concentrations of CO₂ and H₂O occur in the same region. The flamelet model predicts an overly rapid approach to the equilibrium structure in the downstream region, leading to significantly higher flame temperatures. The main reason for the failure of the flamelet model in the downstream region is due to the neglect of the effects of multidimensional convection and diffusion and the fundamental difference in the chemical structure between a coflow diffusion flame and a counterflow diffusion flame. The findings of this paper are highly relevant to understanding the flamelet model results in the calculations of multidimensional turbulent diffusion flames.     

Progress in knowledge of flamelet structure and extinction

In the past 25 years there has been a considerable amount of progress in studying flamelets, their structures and their responses to various perturbations. The term “flamelet” as used here really would mean “laminar flame” to most readers and is employed only because a major motivation is for ultimate use in connection with more complex flows, mainly turbulent. There is, however, no consideration here of how the knowledge reviewed may be employed in flamelet modeling of turbulent combustion. Not even time-dependent flamelets are addressed, although a few related references are provided. The focus is narrower, namely on quasisteady flamelets, and therefore the considerations of flamelet extinction that are presented concern quasisteady extinction, that is, not the dynamics of extinction. Even in this narrow context, it will be seen that a great deal has been accomplished. When such a long-term view is taken, it is found remarkable how much progress has been made. The progress is addressed separately for premixed, nonpremixed and partially premixed systems. Suggested directions of future research also are indicated. Despite the limited scope of the topic and the extensive advancement that has occurred, much more research remains to be done.     

The anchoring mechanism of a bluff-body stabilized laminar premixed flame

The objective of this work is to investigate the mechanism of the laminar premixed flame anchoring near a heat-conducting bluff-body. We use unsteady, fully resolved, two-dimensional simulations with detailed chemical kinetics and species transport for methane–air combustion. No artificial flame anchoring boundary conditions were imposed. Simulations show a shear-layer stabilized flame just downstream of the bluff-body, with a recirculation zone formed by the products of combustion. A steel bluff-body resulted in a slightly larger recirculation zone than a ceramic bluff-body; the size of which grew as the equivalence ratio was decreased. A significant departure from the conventional two-zone flame-structure is shown in the anchoring region. In this region, the reaction zone is associated with a large negative energy convection (directed from products to reactants) resulting in a negative flame-displacement speed. It is shown that the premixed flame anchors at an immediate downstream location near the bluff-body where favorable ignition conditions are established; a region associated with (1) a sufficiently high temperature impacted by the conjugate heat exchange between the heat-conducting bluff-body and the hot reacting flow and (2) a locally maximum stoichiometry characterized by the preferential diffusion effects.     

Molecular diffusion effects in LES of a piloted methane–air flame

Molecular diffusion effects in LES of a piloted methane–air (Sandia D) flame are investigated on a series of grids with progressively increased resolution. The reacting density, temperature and chemical composition are modeled based on the mixture fraction approach combined with a steady flamelet model. With a rationale to minimize interpolation uncertainties that are routinely introduced by a flamelet table look-up, quadratic splines relationships are employed to represent thermochemical variables. The role of molecular diffusivity in effecting spatial transport is studied by drawing a comparison with the turbulent diffusivity and analyzing their statistics conditioned on temperature. Statistical results demonstrate that the molecular diffusivity in the near-field almost always exceeds the turbulent diffusivity, except at low temperatures (less than 500 K). Thus, by altering the jet near-field, molecular transport plays an important role in the further downstream jet development. Molecular diffusivity continues to dominate in the centerline region throughout the flow field. Overall, the results suggest the strong necessity to represent molecular transport accurately in LES studies of turbulent reacting flows.     

Large-eddy simulation of a bluff-body-stabilized non-premixed flame using a recursive filter-refinement procedure

A large-eddy simulation (LES) of a bluff-body-stabilized flame has been carried out using a new strategy for LES grid generation. The recursive filter-refinement procedure (RFRP) has been used to generate optimized clustering for variable density combustion simulations. A methane-hydrogen fuel-based bluff-body-stabilized experimental configuration has been simulated using state-of-the-art LES algorithms and subfilter models. The combustion chemistry is described using a precomputed, laminar flamelet model-based look-up table. The GRI-2.11 mechanism is used to build the look-up table parameterized by mixture fraction and scalar dissipation rate. A beta function is used for the subfilter mixture fraction filtered density function (FDF). The simulations show good agreement with experimental data for the velocity field. Time-averaged profiles of major species and temperature are very well reproduced by the simulation. The mixture fraction profiles show excellent agreement at all locations, which helps in understanding the validity of flamelet assumption for this flame. The results indicate that LES computations are able to quantitatively predict the flame structure quite accurately using the laminar flamelet model. Simulations tend to corroborate experimental evidence that local extinction is not significant for this flame.     

LES of a premixed jet flame DNS using a strained flamelet model

Turbulent premixed flames in the thin and broken reaction zones regimes are difficult to model with Large Eddy Simulation (LES) because turbulence strongly perturbs subfilter scale flame structures. This study addresses the difficulty by proposing a strained flamelet model for LES of high Karlovitz number flames. The proposed model extends a previously developed premixed flamelet approach to account for turbulence’s perturbation of subfilter premixed flame structures. The model describes combustion processes by solving strained premixed flamelets, tabulating the results in terms of a progress variable and a hydrogen radical, and invoking a presumed PDF framework to account for subfilter physics. The model is validated using two dimensional laminar flame studies, and is then tested by performing an LES of a premixed slot-jet direct numerical simulation (DNS). In the premixed regime diagram this slot-jet is found at the edge of the broken reaction zones regime. Comparisons of the DNS, the strained flamelet model LES, and an unstrained flamelet model LES confirm that turbulence perturbs flame structure to leading order effect, and that the use of an unstrained flamelet LES model under-predicts flame height. It is shown that the strained flamelet model captures the physics characterizing interactions of mixing and chemistry in highly turbulent regimes.     

A consistent level set formulation for large-eddy simulation of premixed turbulent combustion

A consistent formulation of the G-equation approach for LES is developed. The unfiltered G equation is valid only at the instantaneous flame front location. Hence, in a filtering procedure applied to derive the appropriate LES equation, only the instantaneous unfiltered flame surface can be considered. A new filter kernel is provided, which averages along the flame surface. The filter kernel is used to derive the G equation for the filtered flame front location. This equation has two unclosed terms, involving a flame front conditional averaged flow velocity, and a filtered propagation term. A model for the conditional velocity is derived, expressing this quantity in terms of the Favre-filtered flow velocity, which is typically known from a flow solver. This model leads to the appearance of a density ratio in the propagation term of the G equation. LES of combustion in the thin reaction zones regime is discussed in the LES regime diagram. A new line is identified separating the thin reaction zones regime into two parts, where the broadened flame thickness is larger and smaller than the filter size, respectively. A model for the propagation term is provided. This leads to a term including the subfilter turbulent burning velocity and an additional term proportional to the resolved flame front curvature. For the former, an algebraic model is provided from an equation for the subfilter flame front wrinkling. The latter term depends on the inverse of the subfilter Damköhler number and disappears in the corrugated flamelets regime.     

Large-eddy simulation of a piloted premixed jet burner

A three-stream flamelet/progress variable model is applied to the Sydney piloted premixed jet burner (PPJB). Using experimental data, a prior model evaluation is performed to assess critical modeling assumptions regarding the applicability of this formulation to partially-premixed combustion, the statistical representation of the scalar mixing, and the joint PDF-closure. A Dirichlet distribution, as generalization of the beta distribution, is introduced to represent the interaction between the two mixture fractions that are associated with the fuel, pilot, and coflow streams. Comparisons with experimental data are performed to demonstrate the accuracy of this closure-model. Following this prior model evaluation, the three-stream combustion model is applied to large-eddy simulation, and calculations of all four burner configurations, designated as PM1-{50, 100, 150, 200}, are performed. Through comparisons with experimental data and equilibrium computations it was found that the flow-field is sensitive to the scalar inflow composition, and scalar boundary conditions consistent with experimental measurements were used for all simulations. The effect of wall heat-losses on the temperature and species profiles is assessed in an approximate way, suggesting that species profiles are unaffected by the heat-transfer between pilot and coflow streams. Comparisons of statistical results and thermo-chemical correlations show that the model is capable of predicting flow-field, temperature, and major species profiles. The simulations over-predict the fuel-consumption for PM1-150 and PM1-200, which has also been observed in previous investigations. Aspects regarding model extensions to account for heat-losses and transient extinction/re-ignition processes are discussed.     

Flame temperatures along a laminar premixed flame with a non-uniform stretch rate

We investigated experimentally the effects of a spatially non-uniform stretch rate on the flame temperature. A flame surface with a non-uniform stretch rate was formed by creating a wrinkled laminar premixed flame in a spatially periodic flow field of a lean propane/air mixture. The measured flame temperature was lower/higher than the adiabatic flame temperature at flame segments with positive/negative stretch rates. This was a result of the effects of flame stretch and preferential diffusion for Lewis number greater than unity. The flame temperature estimated using the conventional flame stretch theory, which is based on a uniform stretch rate along the flame surface, did not agree quantitatively with the measured temperature. Therefore, we revised the theory, taking into account heat transfer along the flame surface, and then produced estimates that agreed with the measured temperature. We found that the effect of flame stretch and preferential diffusion is changed along the flame surface which has spatially non-uniform stretch rate, causing a temperature gradient along the surface, which in turn transfers heat and changes the flame temperature. Thus, heat transfer along the flame surface is an important factor in estimating flame temperature. In addition, a second temperature gradient appears downstream just behind the flame, because the temperature of the burned gas is also non-uniform. Therefore, conductive heat transfer is believed to occur between the flame and the burned gas. The effect of the downstream heat transfer is not as large as that of the heat transfer along the flame surface.     

Counter-gradient transport in the combustion of a premixed CH4/air annular jet by combined PIV/OH-LIF

A combination of PIV/OH laser induced fluorescence technique is used to measure the conditional - burned and unburned - gas velocity in a turbulent premixed CH₄/air annular bluff-body stabilized burner. By changing the equivalence ratio from lean to almost stoichiometric, the energy budget of the recirculating region anchoring the flame is altered in such a way to increasingly lift the flame away from the jet exit. The overall turbulence intensity interacting with each flame is thus systematically varied in a significant range, allowing for a parametric study of its effect on turbulent scalar transport under well controlled conditions, always well within the flamelet regime. The component of the flux normal to the average front is found to reverse its direction, confirming the Bray number as a good indicator of gradient/counter-gradient behavior, once the actual incoming turbulence level felt locally by the flame is assumed as the proper control parameter.     

Characteristics of flamelets in spray flames formed in a laminar counterflow

A two-dimensional numerical simulation of a spray flame formed in a laminar counterflow is presented, and the flamelet characteristics are studied in detail. The effects of strain rate, equivalence ratio, and droplet size are examined in terms of mixture fraction and scalar dissipation rate. n-Decane (C₁₀H₂₂) is used as a liquid spray fuel, and the droplet motion is calculated by the Lagrangian method without the parcel model. A one-step global reaction is employed for the combustion reaction model. The results show that there appear large differences in the trends of gaseous temperature and mass fractions of chemical species in the mixture fraction space between the spray flame and the gaseous diffusion flame. The gas temperature in the spray flame is much higher than that in the gaseous diffusion flame. This is due to the much lower scalar dissipation rate and the coexistence of premixed and diffusion-limited combustion in the spray flame. For the spray flames, gas temperature and mass fractions of chemical species are not unique functions of the mixture fraction scalar dissipation rate. This is because the production rate of the mixture fraction, namely evaporation rate of the droplets, in the upstream region is not in proportion to its transport-diffusion rate in the downstream region. The behavior shows marked differences as the strain rate decreases, the equivalence ratio increases, or the droplet size decreases.     

Experimental study on the behaviors and shape changes of premixed hydrogen-air flames propagating in horizontal duct

The behaviors and shape changes of premixed hydrogen-air flames at various equivalence ratios propagating in half-open and closed horizontal ducts are experimentally investigated using high-speed schlieren imaging and pressure sensors. The study shows that the premixed hydrogen-air flame undergoes more complex shape changes and exhibits more distinct characteristics than that of other gaseous fuels. One of the outstanding findings is that obvious distortion happens to tulip flame after its full formation when equivalence ratio ranges from 0.84 to 4.22 in the closed duct. The salient tulip flame distortions are specially scrutinized and distinguished from the classical tulip collapse and disappearance. The dynamics of distorting tulip flame is different from that of classical tulip flame. The normal tulip flame can be reproduced after the first distortion followed by another distortion. The initiation of flame shape changes coincides with the deceleration both of pressure rise and flame front speed for flames with tulip distortions. And the formation and dynamics of tulip/distorting tulip flames depend on the mixture composition.