Inhomogeneous Inlets Research Articles

Measurements versus Numerical Simulations for Slotted Synthetic Jet Actuator

In many studies concerning synthetic jet flow fields the analysis is usually restricted to simple configurations, such as a single diaphragm oscillating in a cylindrical cavity, which is linked to the external environment with only one orifice/slot. Nonetheless, in many applications the requirement of small sizes and weights leads to many implementation issues, such as asymmetric actuator geometries, presence of several slots and diaphragms and irregular cavity shapes. Therefore, the design of a synthetic jet actuator for a specific flow control problem requires a dedicated study in order to characterize its behavior even in quiescent conditions. The aim of this work is to investigate the behavior of a novel synthetic jet actuator, composed of three independent diaphragms, acting on a single cavity, and linked to the external environment through four slots per diaphragm. The device has been studied in quiescent conditions, both numerically and experimentally. The experimental investigation has been carried out by means of hot-wire measurements. In particular, the distribution of the phase-averaged streamwise velocity along the slot spanwise direction has been detected near the slot exit plane. From the computational side, incompressible direct numerical simulations have been carried out using the open-source OpenFOAM code. The diaphragm motion is mimicked by a inhomogeneous inlet boundary condition, whose amplitude is chosen to match the experimental velocity at the exit plane. A fair agreement between the numerical and the experimental results is achieved for both the velocity field at the slot exit and the main non-dimensional parameters of the synthetic jet. After the validation, the numerical results are finally processed, to obtain information about the vortex motion in the external environment.

Effect of multiscalar subfilter PDF models in LES of turbulent flames with inhomogeneous inlets

To apply reduced-order manifold combustion models in Large Eddy Simulations (LES) of systems with multiple inlets, it is necessary to incorporate more than one mixture fraction. As a result, the joint subfilter Probability Density Function (PDF) of the mixture fractions must be modeled. To determine the sensitivity of the predicted turbulent flame structure to these models, LES calculations of the Sydney piloted jet burner with inhomogeneous inlets have been performed using the Dirichlet, Connor-Mosimann (CM), and Beta-Delta (BD) distributions. A memory-efficient convolution-on-the-fly approach was applied to enable use of more complex subfilter PDF models, such as the CM distribution. Strong sensitivities in the LES predictions were observed to the choice of subfilter PDF model. The Dirichlet distribution was found to be ill-suited to this partially premixed turbulent flame because as a subfilter PDF model it implicitly assumes symmetric mixing in mixture fraction space, which is inconsistent with partial premixing. Good agreement with the experimental data was obtained using both the CM and BD distributions, which are mathematically consistent with partial premixing. The CM distribution is the most general of the considered distributions and gives the best predictions, but this comes with increased complexity and computational cost.

Statistics of scalar dissipation and reaction progress in turbulent flames with compositional inhomogeneities

This paper presents detailed measurements of three-dimensional (3D) scalar dissipation rates collected in turbulent, piloted flames with varying degree of compositional inhomogeneity. Joints statistics of mixture fraction and reaction progress variable are also shown for a range of conditions. These measurements complement the already existing substantial data set for mixed-mode flames stabilized on the Sydney piloted burner with compositionally inhomogeneous inlets. It is found that the difference between 2D (χr) and 3D (χ) scalar dissipation increases with axial distance along the flame such that the ratio χ/χr may be as high as 2.6. The effects of spatial resolution become more significant as compositional inhomogeneity increases. Mixture fraction, ξ is well-correlated with reaction progress variable, c, in turbulent homogenous flames but the correlations deteriorates significantly as the compositional inhomogeneity increases. This transition should clearly be accounted for in modeling mixed-mode combustion. A new mode of conditioning the scalar dissipation data with respect to burnt and unburnt fluid samples reveal separate trends: while the levels of χ remain similar for burnt samples, unburnt fluid experiences increasing levels of χ as the flames approach blow-off.

A new post-processing technique for analyzing high-dimensional combustion data

This paper introduces a novel post-processing technique for analyzing high dimensional combustion data. In this technique, t-Distributed Stochastic Neighbor Embedding (t-SNE) is used to reduce the dimensionality of the combustion data with no prior knowledge while preserving the similarity of the original data. Multidimensional combustion datasets are from premixed and non-premixed laminar flame simulations and measurements of a series of well documented piloted flames with inhomogeneous inlets. The resulting reduced manifold is visualized by scatter plots to reveal the global and local structure of the data (manual labeling). Unsupervised clustering algorithms are then utilized for post-processing the t-SNE manifold in order to develop an automatic labeling process.

A general probabilistic approach for the quantitative assessment of LES combustion models

The Wasserstein metric is introduced as a probabilistic method to enable quantitative evaluations of LES combustion models. The Wasserstein metric can directly be evaluated from scatter data or statistical results through probabilistic reconstruction. The method is derived and generalized for turbulent reacting flows, and applied to validation tests involving a piloted turbulent jet flame with inhomogeneous inlets. It is shown that the Wasserstein metric is an effective validation tool that extends to multiple scalar quantities, providing an objective and quantitative evaluation of model deficiencies and boundary conditions on the simulation accuracy. Several test cases are considered, beginning with a comparison of mixture-fraction results, and the subsequent extension to reactive scalars, including temperature and species mass fractions of CO and CO2. To demonstrate the versatility of the proposed method in application to multiple datasets, the Wasserstein metric is applied to a series of different simulations that were contributed to the TNF-workshop. Analysis of the results allows to identify competing contributions to model deviations, arising from uncertainties in the boundary conditions and model deficiencies. These applications demonstrate that the Wasserstein metric constitutes an easily applicable mathematical tool that reduce multiscalar combustion data and large datasets into a scalar-valued quantitative measure.

Multi-environment probability density function approach for turbulent partially-premixed methane/air flame with inhomogeneous inlets

The present study aims to systematically evaluate the capability of a multi-environment PDF approach using non-premixed and premixed tabulated chemistry to predict the fundamental characteristics of turbulent partially-premixed piloted flames with near-homogeneous and inhomogeneous inlets. For the near-homogeneous case, the non-premixed manifold yields better agreement with measurements in terms of conditional mean, unconditional mean, and rms scalars; however, the premixed manifold generates a narrower hot flame zone, which is mainly attributed to its inability to account for diffusion in the mixture fraction space. For the inhomogeneous flame, the premixed and non-premixed manifolds are limited in their ability to reproduce the measured flame at upstream locations with multiple combustion modes, while the non-premixed manifold is reasonably good at predicting the unconditional and conditional profiles of all scalars in the downstream regions with dominant non-premixed combustion. In terms of the predicted environment-conditioned scalar profiles, the present multi-environment PDF approach demonstrated the ability to realistically predict the near-vertical transition from the fuel-rich flammable mixture condition to the stoichiometric pilot condition. Unlike the near-homogeneous case, the inhomogeneous case generates a distinctly different distribution of CO conditional means, where the peak conditional CO level is leaning to the richer side along the downstream region. This tendency is a clear indication of the flame transition from a premixed-dominated flame to a diffusion-dominated flame. Based on our numerical results, detailed discussions about the essential features of turbulent partially-premixed flames with multiple combustion modes are presented, as well as the limitations of the proposed approach.

Computational study of flame characteristics of a turbulent piloted jet burner with inhomogeneous inlets

Recent experimental studies of a piloted turbulent jet burner at Sydney and Sandia with inhomogeneous mixture composition at the inlet have revealed that the flame stability of a partially premixed flame can be significantly increased with a tailored mixture fraction profile at the burner exit. In the present work, large-eddy simulations of this burner are performed with three different levels of mixture inhomogeneity. A multi-regime flamelet model is employed, which accounts for the range of combustion modes found in these flames. The results of the simulations are validated against experimental data for the case with the highest blowoff velocity. Good agreement is observed for velocity, mixture fraction, and temperature fields. The simulations with the multi-regime model provide detailed data of the prevalent combustion regime as well as the heat release. The location of heat release and differences in the combustion modes are analyzed for three cases with different mixture inhomogeneities. The progress variable source term is split up into the individual contributions of the combustion regimes. Substantial differences are found for the contributions of premixed and non-premixed combustion in the different flames as well as the location of the heat release. These results are used to explain the respective flame stabilities. For the inhomogeneous case, which features substantially increased flame stability, hot pilot gases are in contact with reactive mixture directly at the nozzle. Therefore a predominantly premixed zone of strong heat release develops at the jet exit and stabilizes the flame. For both other cases, the heat release at the nozzle is lower leading to smaller blowoff velocities. For the non-premixed case, this is due to air shielding the pilot from the reactive mixture. For the premixed case, the homogeneous mixture issuing from the jet is above the flammability limit and consequently the heat release is diffusion dominated.

Turbulent flames with compositionally inhomogeneous inlets: Resolved measurements of scalar dissipation rates

Highly resolved measurements of scalar dissipation rates in turbulent piloted CH4/air flames with compositionally inhomogeneous inlets are presented. These were performed using Sandia's Raman–Rayleigh–LIF system but with data acquisition and processing strategies that result in enhanced spatial resolution and reduced noise. They complement earlier measurements with coarser resolution. The burner stabilising these flames enables variability in the mixture fraction profile at the exit plane. Earlier studies have shown enhanced stability at an optimal compositional inlet profile that leads to multiple modes of combustion, with premixed-stratified flames close to the jet exit but transitioning to diffusion-dominated burning downstream. It is found that at upstream locations, for jets with homogeneous inlets as well as for the high-temperature regions of flames with inhomogeneous inlets, both fine and coarse measurements of scalar dissipation rates yield similar results, giving confidence that measurements resolve the local dissipation scales. Downstream locations in homogeneous mixtures also show similar results for both coarse and fine measurements across all mixtures. Differences arise in the rich, inner regions of turbulent flames with inhomogeneous inlets, where the fine resolution measurements are more reliable due to the existence of steep gradients in composition. Both data sets provide a comprehensive platform to enhance the modelling of turbulent flames in the presence of multi-modes of combustion.

Fuel effects on the stability of turbulent flames with compositionally inhomogeneous inlets

This paper reports an analysis of the influence of fuels on the stabilization of turbulent piloted jet flames with inhomogeneous inlets. The burner is identical to that used earlier by the Sydney Group and employs two concentric tubes within the pilot stream. The inner tube, carrying fuel, can be recessed, leading to a varying degree of inhomogeneity in mixing with the outer air stream. Three fuels are tested: dimethyl ether (DME), liquefied petroleum gas (LPG), and compressed natural gas (CNG). It is found that improvement in flame stability at the optimal compositional inhomogeneity is highest for CNG and lowest for DME. Three possible reasons for this different enhancement in stability are investigated: mixing patterns, pilot effects, and fuel chemistry. Numerical simulations realized in the injection tube highlight similarities and differences in the mixing patterns for all three fuels and demonstrate that mixing cannot explain the different stability gains. Changing the heat release rates from the pilot affects the three fuels in similar ways and this also implies that the pilot stream is unlikely to be responsible for the observed differences. Fuel reactivity is identified as a key factor in enhancing stability at some optimal compositional inhomogeneity. This is confirmed by inference from joint images of PLIF-OH and PLIF-CH2O, collected at a repetition rate of 10kHz in turbulent flames of DME, and from one-dimensional calculations of laminar flames using detailed chemistry for DME, CNG, and LPG.

Compliance of combustion models for turbulent reacting flow simulations

The utilization of low-dimensional manifold combustion models for large-eddy simulation (LES) of turbulent reactive flows introduces model simplifications that represent sources of uncertainties in addition to those arising from turbulent closure models and numerical discretization. The ability to quantitatively assess these uncertainties in the absence of measurements or reference results is vital for reliable and predictive simulations of practical combustion devices. This paper is concerned with the extension of the manifold drift term to LES to examine the compliance of a particular combustion model in describing a quantity of interest (QoI) with respect to the underlying flow-field representation. This drift term was previously introduced as a key component of the Pareto-efficient combustion (PEC) framework. The behavior of the drift term is examined in a series of test cases. To this end, large-eddy simulations of a partially-premixed turbulent pilot flame with inhomogeneous inlet streams are performed, in which the non-premixed flamelet/progress-variable (FPV) model and the premixed filtered tabulated chemistry LES (F-TACLES) formulation are employed. The drift term is shown to be capable of identifying chemically sensitive regions with respect to user-specific QoIs. With this, a species-specific combustion regime indicator is derived by computing the relative magnitude of the drift terms for different combustion models. Comparisons with commonly employed flame indicators suggests that the flame index and other indicators that are solely based on major species and flame topology are insufficient in describing complex physical processes in multi-regime combustion.

A two mixture fraction flamelet model for large eddy simulation of turbulent flames with inhomogeneous inlets

A revised flamelet/progress variable (FPV) model in which two mixture fractions are defined has been developed to address the limitations of single mixture fraction FPV models that presume a single, compositionally uniform fuel stream. The revised model is applied in Large Eddy Simulation of the modified University of Sydney turbulent jet burner with compositionally inhomogeneous inlet conditions. The first mixture fraction (Z) characterizes the mixing between the methane/air mixture issuing from the burner and the surrounding coflow air. The second mixture fraction (Z*) tracks mixing of methane and air without regard to the point of origin of the air. Additionally, a fuel premixing fraction (F) has been defined that corresponds to the fuel side boundary condition for the solution of the 1D flamelet equations in terms of the first mixture fraction. Two methods of characterizing the thermochemical state are considered, one using Z and Z* and the other using Z and F. Comparison of temperature and species predictions with experimental data shows that the structure of the flame with inhomogeneous inlets cannot be predicted by a single mixture fraction model while this is successfully achieved with the two mixture fraction models. Even for a near-homogeneous inlet with only minor variations in composition, the two mixture fraction model predictions provide significant improvements over a single mixture fraction model.

A unified view of pilot stabilized turbulent jet flames for model assessment across different combustion regimes

Single-regime turbulent combustion has been the main focus in previous studies. Significant limitations exist in those studies since most practical combustion applications involve multi-regime combustion. Developing and validating multi-regime turbulent combustion models are expected to be an emerging area with significant challenges. To facilitate model assessment across different combustion regimes, we develop a model validation framework by unifying several existing pilot stabilized turbulent jet flames in different combustion regimes. The characteristic similarity and difference of the employed piloted flames are examined, including the Sydney piloted flames L, B, and M, the Sandia piloted flames D, E, and F, a series of piloted premixed Bunsen flames, and the Sydney/Sandia inhomogeneous inlet piloted jet flames. Proper parameterization and a regime diagram are introduced to characterize the pilot stabilized flames covering non-premixed, partially premixed, and premixed flames. A preliminary model assessment is carried out to examine the simultaneous model performance of the large-eddy simulations (LES)/probability density function (PDF) method for the piloted jet flames across different combustion regimes.

Local extinction and near-field structure in piloted turbulent CH4/air jet flames with inhomogeneous inlets

Line-imaged measurements of temperature and major species, based on well-established Raman/Rayleigh/CO-LIF techniques, are used to better understand the scalar structure of piloted CH4/air jet flames with inhomogeneous inlets. Recent studies using a variant of the Sydney piloted burner have demonstrated that the blowoff velocity of a partially-premixed jet flame of CNG or CH4 and air can be increased significantly by tailoring the mixture fraction profile at the burner exit. This is done by adding a small, retractable central tube within the main tube of the burner and separately supplying fuel and air for partial premixing. Both tubes are located within the pilot annulus. When the central tube is retracted far upstream of the burner exit, the flame has the same stability as that of the original burner with homogeneous fuel–air composition at the jet exit. However, when the inner tube supplies fuel and is retracted an optimal distance (10–13 times the main tube diameter) the blowoff velocity is increased by nearly 40%. Previous results indicated that combustion very close to the burner exit occurs in a stratified-premixed mode, augmenting the stabilizing effect of the pilot. This is followed by transition to a diffusion-dominated mode of burning within the first ten main tube diameters.The present paper provides a detailed examination of a series of piloted CH4/air jet flames with different inlet conditions and with a pilot flame that matches the composition and adiabatic flame temperature of CH4/air. It addresses trends in local extinction as well as differences in near-field flame structure. The evolution in the local mode of combustion is traced using instantaneous line-imaged realizations where the change in mixture fraction across a 1000 K interval is used as a conditioning variable. Doubly conditioned means of selected species mass fractions confirm that the flames with inhomogeneous inlet profiles undergo transition from a stratified-premixed mode of combustion to a diffusion-dominated mode of combustion within the first ten jet diameters. Calculations of strained laminar partially-premixed flames are used to gain insight on the effects of strain rate and fuel-side equivalence ratio on flame structure and the rate of heat release. These turbulent jet flames may serve as interesting test cases for models aimed at predicting the performance of practical burners that operate with mixed combustion modes.

Partial premixing and stratification in turbulent flames

This paper reviews recent advances in understanding the structure of turbulent partially premixed and stratified flames. The term “partially premixed” refers here to compositionally inhomogeneous mixtures that include flammable and non-flammable fluid while “stratified” combustion refers to a reacting front propagating through a range of compositions within the flammable limits. An overview of relevant laminar flame concepts is first introduced. In laminar partially premixed flames, the interaction between rich and lean mixtures is significant leading to improvement in the flame’s resistance to extinction by straining. In lean back-supported laminar stratified flames, the flux of excess heat and radicals into the lean fluid results in higher flame speeds, broader reaction zones, and extended flammability limits compared to homogeneous counterparts. Rich stratified flames are more complex due to the combined fluxes of heat as well as reactive species such as H2 and CO.Recent research in turbulent partially premixed as well as stratified flames is reviewed. Detailed measurements in burners representative of those found in gas turbine combustors show that partial premixing at the lifted flame base increases with instability. Well-characterised laboratory burners where different fuel concentration gradients may be imposed at the jet exit plane show improved flame stability due to mixed-mode combustion. Maximum stability is reached at some optimum level of compositional inhomogeneity. Highly resolved measurements in turbulent stratified flames show that the mass fractions of CO and H2 increase with stratification; a result that is consistent with laminar flame studies. Such experiments are, however, very difficult and require multi-level conditioning of the data. The paper concludes with a brief review of potential numerical approaches employed in the calculations of turbulent flames with inhomogeneous inlet conditions. A key challenge here is to reproduce the effects of increasing levels of stratification and/or inhomogeneity on the compositional structure of turbulent flames.

Stabilization of piloted turbulent flames with inhomogeneous inlets

This paper investigates the stabilization mechanism of turbulent jet flames with highly inhomogeneous inlet conditions. A modification of the standard piloted burner is employed here with the addition of a central tube (inner) carrying fuel that can slide within the existing (outer) tube carrying air. Both tubes are located within the pilot annulus and inhomogeneity is varied by translating the inner tube upstream of the jet exit plane. Two flames with identical overall air/fuel ratios, bulk jet velocities, and pilot conditions but different levels of homogeneity in the fuel/air mixture are selected for detailed investigations. Measurements are performed using Sandia’s Raman–Rayleigh–LIF line facility, and Large Eddy Simulation (LES) using the stochastic fields approach are also conducted for the same flames. Results reported here focus on the early stabilization region.The flame with inhomogeneous inlet conditions is more stable being at 57% of blow-off compared to the homogeneous counterpart, which is at 78% of blow-off. It is found that, very close to the jet exit plane, premixed combustion dominates the flame with an inhomogeneous profile. This is in contrast to the homogeneous case, which behaves like a diffusion flame. Further downstream, but still within the pilot region, partial mixing starts to occur between richer samples and hot combustion products. A comparison of the relative conditional scalar dissipation rates, χr shows that in the upstream region, and within the reactive limits, the homogeneous case has higher values of χr. Premixed combustion with higher rates of heat release and lower scalar dissipation rates in the near field are therefore key reasons for the improved stability of the flames with inhomogeneous inlets. These findings are corroborated by results from LES.

CFD simulation of flow and dispersion around an isolated building: Effect of inhomogeneous ABL and near-wall treatment

This paper presents a study of the effect of the inhomogeneous atmospheric boundary layer (ABL) and near-wall treatment on a computational fluid dynamics (CFD) simulation of the flow and dispersion around an isolated building. Based on a revised k−ɛ model (MMK), the effect of the inhomogeneous ABL is studied by comparing several sets of inhomogeneous inlet conditions with a homogeneous one, while the roughness-adapted standard wall functions and two-layer model are compared to examine the effect of the near-wall treatment of rough ground. Wind tunnel experimental data from previous investigations are used to validate the numerical simulations. The inhomogeneous ABL has a significant effect on the prediction of the flow and dispersion fields, depending on the percentage deviation of the incident from the inlet conditions. A decreased incident k profile results in a longer reattachment length on the roof and a larger recirculation region in the wake, which in turn produce decreased and increased nondimensional velocity and concentration fields, respectively. An increased incident U profile enlarges the reattachment lengths and raises the k field, which has a similar effect on the nondimensional velocity and concentration fields as decreased k. It is observed that the use of the two-layer model to solve the near-wall viscous sublayer clearly improves the prediction of flow and dispersion, depending on the role of the low-Reynolds-number effect of the near-wall region on the entire wall-bounded turbulent flow.

Experimental and numerical studies on the flame instabilities in porous media

Flame instabilities of inclination and breakup for lean CH4/air combustion in porous media are studied experimentally and numerically. Inclination and breakup are observed in the quartz tube filled with alumina pellets of 3mm in diameter for equivalence ratio range of 0.435–0.490. The effect of the equivalence ratio on the flame inclination and breakup are discussed and the results show that the equivalence ratio plays an important role on the breakup and flame inclination. Breakups always happen when the inclinational angle reaches 54–60°. In order to explain the motivities of the inclination, a two-dimensional one-step chemical reaction, two-temperature model without initial angles are carried out to simulate the experimental cases. In order to check the possible motivities of inclination, we assumed profiles (not uniform) of inlet velocity of premixed gas and equivalence ratio at inlet, different porosities in the tube, half the same on the right, half on the left, in the simulations. The results show that the inhomogeneous inlet velocity gives slight influence on the inclination, but the different distribution of porosity and the inhomogeneous equivalence ratio at inlet give great influences on the inclination. Calculation results show that the heat loss have a significant influence in determining the flame shape at the initial moment and lead to an increase in the development of the inclination instability. At a low mixture velocity, hydrodynamic effect on the inclination instability is ignored, while the inclination angle of the flame front continually increases as the mixture velocity increase.

Time Resolved Full-Annulus Computations of a Turbine with Inhomogeneous Inlet Conditions

Time Resolved Full-Annulus Computations of a Turbine with Inhomogeneous Inlet Conditions