Experimental study of vorticity-strain interactions in turbulent premixed counterflow flames

In turbulent flows, the interaction between vorticity, ω, and strain rate, s, is considered a primary mechanism for the transfer of energy from large to small scales through vortex stretching. The ω-s coupling in turbulent jet flames is investigated using tomographic particle image velocimetry (TPIV). TPIV provides a direct measurement of the three-dimensional velocity field from which ω and s are determined. The effects of combustion and mean shear on the ω-s interaction are investigated in turbulent partially premixed methane/air jet flames with high and low probabilities of localized extinction as well as in a non-reacting isothermal air jet with Reynolds number of approximately 13 000. Results show that combustion causes structures of high vorticity and strain rate to agglomerate in highly correlated, elongated layers that span the height of the probe volume. In the non-reacting jet, these structures have a more varied morphology, greater fragmentation, and are not as well correlated. The enhanced spatiotemporal correlation of vorticity and strain rate in the stable flame results in stronger ω-s interaction characterized by increased enstrophy and strain-rate production rates via vortex stretching and straining, respectively. The probability of preferential local alignment between ω and the eigenvector of the intermediate principal strain rate, s2, which is intrinsic to the ω-s coupling in turbulent flows, is larger in the flames and increases with the flame stability. The larger mean shear in the flame imposes a preferential orientation of ω and s2 tangential to the shear layer. The extensive and compressive principal strain rates, s1 and s3, respectively, are preferentially oriented at approximately 45° with respect to the jet axis. The production rates of strain and vorticity tend to be dominated by instances in which ω is parallel to the s1¯-s2¯ plane and orthogonal to s3¯.

The effects of chemical heat release on the large-scale structure in a chemically reacting, turbulent mixing layer are investigated using direct numerical simulations. Three-dimensional, time-dependent simulations are performed for a binary, single-step chemical reaction occurring across a temporally developing turbulent mixing layer. It is found that moderate heat release slows the development of the large-scale structures and shifts their wavelengths to larger scales. The resulting entrainment of reactants is reduced, decreasing the overall chemical product formation rate. The simulation results are interpreted in terms of turbulence energetics, vorticity dynamics, and stability theory. The baroclinic torque and thermal expansion in the mixing layer produce changes in the flame vortex structure that result in more diffuse vortices than in the constant-density case, resulting in lower rotation rates of the large-scale structures. Previously unexplained anomalies observed in the mean velocity profiles of reacting jets and mixing layers are shown to result from vorticity generation by baroclinic torques.

Heat release effects on laminar flame propagation in partially premixed flows are studied. Data for analysis are obtained from direct numerical simulations of a laminar mixing layer with a uniformly approaching velocity field. The structure that evolves under such conditions is a triple flame, which consists of two premixed wings and a trailing diffusion flame. Heat release increases the flame speed over that of the corresponding planar premixed flame. In agreement with previous analytical work, reductions in the mixture fraction gradient also increase the flame speed. The effects of heat release and mixture fraction gradients on flame speed are not independent, however; heat release modifies the effective mixture fraction gradient in front of the flame. For very small mixture fraction gradients, scaling laws that determine the flame speed in terms of the density change are presented.

The objectives of this experimental program are two fold; first, to investigate, understand, and document the effects of heat release on the characteristics of a dump combustor flowfield; secondly, to provide a benchmark set of experimental data to aid the development of time averaged CFD codes. A limited number of velocity profiles are reported in this manuscript to illustrate the effects of combustion and heat release, at an equivalence ratio of 0.65, on dump combustor flows. High-speed photography and spectral analysis complemented velocity measurements and served to examine and characterize combustion instability limits. Results indicate that combustion altered the dump combustor flowfield and significantly reduced the length of the corner recirculation region.

This work presents a numerical investigation of the applicability of the product of formaldehyde (CH 2 O) and hydroxyl radicals (OH), denoted as CH 2 O ⋅ OH, as a marker for the local heat release rate (HRR) in CH 4 flames over a broad range of boundary conditions. Both laminar freely propagating one-dimensional and premixed counter-flow CH 4 flames were simulated using several detailed reaction mechanisms. The analysis systematically varied the oxidizer composition (from air to pure oxy-fuel), equivalence ratio ( Φ ), inlet temperature, pressure, and strain rate. The correlation between the CH 2 O ⋅ OH profiles and the HRR was quantified using the Pearson correlation coefficient. Overall, the study confirms CH 2 O ⋅ OH as a practical and reliable HRR marker in CH 4 flames, particularly under high-temperature and high-pressure oxy-fuel conditions. The results show that in slightly rich CH 4 -air flames (about Φ = 1.5 ), CH 2 O ⋅ OH provides an accurate estimate of the HRR. In oxygen-enriched and pure oxy-fuel flames, the strongest correlation shifts to ultra-rich conditions (about Φ ≈ 3.0 ). Increasing pressure enhances the correlation at lower equivalence ratios, whereas preheating weakens the correlation locally but expands the overall range of validity. Although increasing strain rate generally degrades the correlation, CH 2 O ⋅ OH remains a robust marker under technically relevant turbulent conditions (strain rates below 10,000 s − 1 ), maintaining Pearson correlation coefficients of approximately R ≈ 0.9 even close to extinction. This demonstrates the strong feasibility of using CH 2 O ⋅ OH for HRR estimation in highly turbulent flames. The demonstrated robustness of CH 2 O ⋅ OH highlights its suitability as an HRR marker, providing valuable insights for experimental diagnostics. • CH 2 O ⋅ OH confirms as robust HRR marker across a wide range of conditions including variations in oxidizer content, equivalence ratio, preheat temperature, pressure, and strain. • For CH 4 -air flames, the highest correlation between CH 2 O ⋅ OH and HRR occurs at slightly rich mixtures ( Φ ≈ 1.5), while in CH 4 /O 2 flames, the strongest correlation shifts to ultra-rich conditions ( Φ ≈ 3.0). • Elevated pressure generally improves the correlation at leaner mixtures but narrows the range of high correlation, whereas preheating shifts the optimal equivalence ratio to richer conditions and broadens the applicability range. • Even under high strain rates (up to 10,000 s − 1 , relevant for turbulent flames), CH 2 O ⋅ OH maintains a strong correlation (R ≈ 0.9) with HRR, highlighting its suitability for turbulent combustion diagnostics. • The results of this study are intended to support and guide experimental diagnostics by identifying reliable marker regimes.

ABSTRACTThe local and non-local statistics of vorticity (i.e. curl of velocity vector ) and pseudo-vorticity (i.e. curl of density-weighted velocity vector ) have been investigated in detail based on a direct numerical simulation (DNS) database of freely propagating statistically planar flames in different regimes of premixed turbulent combustion. It has been found that the relative orientation of can be significantly different to that of for small values of the Karlovitz number and large values of the Reynolds number. The vorticity shows, depending on the case, considerable alignment with either the most extensive or compressive principal strain rate. In contrast, has been found to align with the intermediate principal strain rate for all cases. The transport statistics of Reynolds averaged enstrophy (i.e. ) and pseudo-enstrophy (i.e. ) have been investigated in detail. The non-local vorticity statistics in turbulent premixed combustion have been investigated based on the vorticity line segment (VLS) structure, which is defined with respect to each grid point as the part of the (pseudo) vorticity line bounded by the two adjacent extremal points of the (pseudo) vorticity magnitude. VLS can be characterised by the segment length and the (pseudo) vorticity magnitude difference across the segment, i.e. (Δω*) Δω. Strong dilatation effects tend to increase VLS length, whereas turbulent convection disturbs the flow field resulting in shorter VLS. The joint PDFs between the segment length and (Δω) Δω* are found to be symmetric, whereas the joint PDFs between the segment length and the pseudo-vorticity magnitude difference across the VLS in the flame normal direction show significant asymmetry, which is primarily determined by density variation across the flame front and turbulence intensity.

Imaging of OH* or CH* chemiluminescence with intensified cameras is often employed for the determination of heat release in premixed flames. Proportionality is commonly assumed, but in the turbulent case this assumption is not justified. Substantial deviations from proportionality are observed, which are due to turbulence-chemistry interactions. In this study a model based correction method is presented to obtain a better approximation of the spatially resolved heat release rate of lean turbulent flames from OH* measurements. The correction method uses a statistical strain rate model to account for the turbulence influence. The strain rate model is evaluated with time-resolved velocity measurements of the turbulent flow. Additionally, one-dimensional simulations of strained counterflow flames are performed to consider the non-linear effect of turbulence on chemi-luminescence intensities. A detailed reaction mechanism, which includes all relevant chemiluminescence reactions and deactivation processes, is used. The result of the simulations is a lookup table of the ratio between heat release rate and OH* intensity with strain rate as parameter. This lookup table is linked with the statistical strain rate model to obtain a correction factor which accounts for the non-linear relationships between OH* intensity, heat release rate, and strain rate. The factor is then used to correct measured OH* intensities to obtain the local heat release rate. The corrected intensities are compared to heat release distributions which are measured with an alternative method. For all investigated flames in the lean, partially premixed regime the corrected OH* intensities are in very good agreement with the heat release rate distributions of the flames.

As industry transitions to a net-zero carbon future, turbulent premixed combustion will remain an integral process for power generating gas turbines, aviation engines, and high-speed propulsion due to their ability to minimize pollutant emissions. However, accurately predicting the behavior of a turbulent reacting flow field remains a challenge. To better understand the dynamics of premixed reacting flows, this study experimentally investigates the effects of combustion heat release and flame scales on the evolution of turbulence in a high-speed, confined bluff-body combustor. The combustor is operated across a range of equivalence ratios from 0.7 to 1 to isolate the role of chemical heat release, flame speed, and flame thickness on the evolution of turbulence as the flow progresses from reactants to products. High-speed particle image velocimetry and CH* chemiluminescence imaging systems are simultaneously employed to quantify turbulent flame and flow dynamics. The results notably demonstrate that the flame augments turbulence fluctuations as the flow evolves from reactants to products for all cases, which opposes most simulations of premixed turbulent reactions. Notably, turbulence fluctuations increase monotonically with the heat of combustion and corresponding turbulent flame speed. Spatial profiles of turbulence statistics are conditioned on the mean flame front, and nondimensionalizing the turbulence profiles using laminar flame properties is shown to collapse all conditions onto a single curve. The resulting nondimensional profile confirms that turbulence dynamics scales with the heat of combustion and was used to develop a novel correlation to predict the increase in turbulent fluctuations across the premixed flame. A Reynolds averaged Navier Stokes decomposition is also explored to further characterize the effects of combustion heat release on the dominant mechanisms of turbulent energy transport. The cumulative results can guide modeling capabilities to better predict flame and flow dynamics and accelerate design strategies for premixed turbines with carbon-free fuels.

In this work the interaction between turbulence and premixed combustion is studied by means of direct numerical simulations using a level set equation based on the scalar G that describes the motion of the flame front represented by an iso-surface G = G0. The G-equation contains terms accounting for flame propagation, flame curvature and gas expansion effects. The flame front is transported and wrinkled by the turbulent flow field and propagates in a direction normal to itself with its laminar burning velocity. The turbulent flow field is based on the constant density Navier-Stokes equations. Results are presented for cases with and without heat release at the flame front. In the former case heat release is modeled by volume sources located on the flame front. For the case without heat release the flame surface area ratio and the source terms appearing in its balance equation accounting for production, kinematic restoration and dissipation are evaluated from the DNS results. The flame surface area ratio shows a linear increase with increasing ratios of υ/sL in the corrugated flamelets regime but a bending in the thin reaction zones regime. This is consistent with the evaluation of the source terms where it is found that kinematic restoration is the main sink term in the corrugated flamelets regime, while scalar dissipation becomes the dominant sink term in the thin reaction zones regime. By evaluating the pdf of G-fluctuations the link between the flame surface area ratio and the flame surface density is established. Agreement with existing models for the latter is found. When gas expansion effects are added, the induced velocity at the flame front can be evaluated. This leads to a purely kinematic explanation for counter gradient diffusion. Effects of gas expansion on the flame surface area ratio are found to be significant only if the laminar burning velocity is of the same order of magnitude as the turbulence intensity or larger.

Closure modeling for the conditional Reynolds stresses in turbulent premixed combustion

The article describes a study on determination of control point’s optimal number and their location on protected object for electro-optical distributed system of flame front speed estimation. Purpose of the system is to determine (predict) the location of flame front with given error based on the data on flame front speed with the goal of timely activation of the explosion suppression devices. The tasks of determining of the control point’s optimal number and their spatial location are related. Initially, the distance between the control points (flame detectors) is determined to ensure the best parameters of the EOS operation. In turn, the optimal number of control points is determined by ratio of the protected object total length to the optimal distance between the flame detectors. The optimality criteria are formulated for two approaches for predicting of the flame front location: by signals from one flame sensor for speeds no more than 30 m/s; by signals from two flame sensors for speeds above 30 m/s. Appropriate research methods are developed for the proposed options. The studies were carried out on the basis of non-stationary combustion computer simulation for hydrocarbon-air mixtures in conditions typical for coal mines. The optimal distance between photo detectors in the flame detector is 0.05 m. However, the required error values for calculating of the flame front position (no more than 15%) are achieved for flame front speed of no more than 30 m/s using a single flame detector. The optimal distance between the flame detectors is 1.75 m at flame front speed of more than 30 m/s. In this case, the minimum relative error in predicting of the flame front location does not exceed 7.1%.

This study presents a numerical analysis of the impact of hydrogen addition on the consumption speed of premixed lean methane-air laminar flames exposed to combined strain and heat loss. Equivalence ratios of 0.9, 0.7, and 0.5 with fuel mixture composition ranging from pure methane to pure hydrogen are considered to cover a wide range of conditions in the lean region. The 1-D asymmetric counter-flow premixed laminar flame (aCFPF) with heat loss on the product side is considered as a flamelet configuration that represents an elementary unit of a turbulent flame and the consumption speed is used to characterise the effect of strain and heat loss. Due to the ambiguity in the definition of the consumption speed of multi-component mixtures, two definitions are compared. The first definition is based on a weighted combination of the consumption rate of the fuel species and the second one is based in the global heat release rate. The definition of the consumption speed based on the heat release results in lower values of the stretched flame speed and even an opposite response to strain rate for some methane-hydrogen-air mixtures compared to the definition based on the fuel consumption. Strain rate leads to an increase of the flame speed for the lean methane-hydrogen mixtures, reaching a maximum value after which the flame speed decreases with strain rate. Heat loss decreases the stretched flame speed and leads to a sooner extinction of the flamelet due to combined strain and heat loss. Hydrogen addition and equivalence ratio significantly impact the maximum consumption speed and the flame response to combined strain rate and heat loss. The effect of hydrogen on the thermo-diffusive properties of the mixture, characterised by the Zeldovich number and the effective Lewis number, are also analyzed and related to the effect on the consumption speed. Two definitions of the Lewis number of the multi-component fuel mixture are evaluated against the results from the aCFPF.

Influence of combustion on principal strain-rate transport in turbulent premixed flames

Characteristics of the process of entrainment in plane mixing layers, and the changes with compressibility and heat release, were studied using temporal DNS with simultaneous fluid packet tracking. Convective Mach numbers of the simulations are 0.15, 0.7 and 1.1. The Reynolds number is quite high (between 11 000 and 37 000 based on layer width and velocity difference), and is above the mixing transition. The study agrees with recent findings in round jets: first, engulfed fluid volume and its growth rate are both very small compared with the volume of the turbulent region and its growth rate, respectively. Secondly, most often, the process occurs close to the turbulent–nonturbulent boundaries. A new finding is that both compressibility and heat release retard the entrainment process so that it takes an O(1) time for vorticity or scalar levels to grow even after growth has been initiated. This delay is manifested as the fall in mixing layer growth rates as compressibility and heat release levels increase.

Direct numerical simulation of DME auto-ignition with temperature and composition stratification under HCCI engine conditions