Flow, Turbulence and Combustion最新文献

The validity of the usual laws of the wall for Favre mean values of the streamwise velocity component and temperature for non-reacting flows has been assessed for turbulent premixed flame-wall interaction using Direct Numerical Simulation (DNS) data. Two different DNS databases corresponding to friction velocity-based Reynolds number of 110 and 180 representing unsteady head-on quenching of statistically planar flames within turbulent boundary layers have been considered. The usual log-law based expressions for the Favre mean values of the streamwise velocity and temperature for the inertial layer have been found to be inadequate at capturing the corresponding variations obtained from DNS data. The underlying assumptions of constant shear stress and the equilibrium of production and dissipation of turbulent kinetic energy underpinning the derivation of the usual log-law for the mean streamwise velocity have been found to be rendered invalid within the usual inertial layer during flame-wall interaction for both cases considered here. The heat flux does not remain constant within the usual inertial layer, and the turbulent flux of temperature exhibits counter-gradient transport within the so-called inertial layer for the cases considered in this work. These render the assumptions behind the derivation of the usual log-law for temperature to be invalid for application to turbulent flame-wall interaction. It has been found that previously proposed empirical modifications to the existing laws of the wall, which account for density and kinematic viscosity variations with temperature, do not significantly improve the agreement with the corresponding DNS data in the inertial layer and the inaccurate approximations for the kinematic viscosity compensated wall normal distance and the density compensated streamwise velocity component contribute to this disagreement. The DNS data has been utilised here to propose new expressions for the kinematic viscosity compensated wall normal distance and the density compensated streamwise velocity component, which upon using in the empirically modified law of wall expressions have been demonstrated to provide reasonable agreement with DNS data.

A novel technique is presented to improve the initialization of compressible combustion LES, DNS or URANS by numerically turning the flame into a damper to quickly remove (artificial) pressure fluctuations and acoustic energy from the system. This is achieved by modifying the pressure dependency of the heat release rate, effectively modifying the Rayleigh Integral to achieve negative values, so that the acoustic energy is quickly removed from the system. The technique can (a) reduce the cost of simulations (by shortening the initialization), (b) contribute to stabilize the simulation, (c) help to avoid unrealistic thermoacoustic modes and, (d) potentially, be modified to compensate for excessive numerical dissipation of acoustic energy. Examples from LES of a thermoacoustic test case are presented to demonstrate the effective stabilization achieved.

Ignition and combustion behavior in the second stage of a sequential combustor are investigated numerically at atmospheric pressure for pure ({text{CH}}_{4}) fueling and for two ({text{CH}}_{4})-({text{H}}_{2}) fuel blends in 24:1 and 49:1 mass ratios , respectively, using Large Eddy Simulation (LES). Pure ({text{CH}}_{4}) fueling results in a turbulent propagating flame anchored by the hot gas recirculation zones developed near the inlet of the sequential combustion chamber. As the ({text{H}}_{2}) content increases, the combustion process changes drastically, with multiple auto-ignition kernels produced upstream of the main flame brush. Analysis of the explosive modes indicates that, for the highest ({text{H}}_{2}) amount investigated, flame stabilization in the combustion chamber is strongly supported by auto-ignition chemistry. The analysis of fuel decomposition pathways highlights that radicals advected from the first stage flame, in particular OH, induce a rapid fuel decomposition and cause the reactivity enhancement that leads to auto-ignition upstream of the sequential flame. This behavior is promoted by the relatively large mass fraction of OH radicals found in the flow reaching the second stage, which is approximately one order of magnitude greater than it would be at chemical equilibrium. The importance of the out-of-equilibrium vitiated air on the ignition behavior is proven via an additional LES that features weak auto-ignition kernel formation when equilibrium is artificially imposed. It is therefore concluded that parameters affecting the relaxation towards chemical equilibrium of the vitiated flow can have an important influence on the operability of sequential combustors fueled with varying fractions of ({text{H}}_{2}) blending.

A data-driven wall function estimation approach is proposed, aimed at accounting for non-equilibrium effects in turbulent boundary layers in RANS simulations of wall bounded flows. While keeping key simplifying hypothesis of standard wall functions and their general structure, the law-of-the-wall is replaced by a fully connected feed-forward neural network. The latter is trained to infer wall friction from the local flow state at the first of-wall nodes, described by an extended set of flow variables and gradients. For this purpose, the neural network is trained on high-fidelity wall resolved simulation data. It is then applied to formulate two different wall functions trained on high-fidelity data: a backward-facing step and a round jet impacting a flat wall. After integration into an industrial CFD code, they are applied to perform RANS simulations of the flow configurations they were trained for, and are shown to yield a largely improved prediction of wall friction as compared to standard wall functions. Finally, key issues related to the practical usability in RANS applications of the proposed data-driven approach are critically discussed.

We investigate the possibility of determining the local turbulent flame speed by measuring the individual terms in the balance of a mean progress of reaction variable for the case of a low turbulence methane-air Bunsen flame in the thin flame regime. Velocity distributions and flame edge positions were measured by particle image velocimetry techniques at 3 kHz for a flame stabilized by a surrounding pilot of the same stoichiometry, for a turbulent Reynolds number around 66 and Karlovitz numbers of the order of 4. The conservation equation for mean progress variable was analyzed along different streamlines as a balance of terms expressed as velocities, including terms for convection, turbulent diffusion, mean reaction, and turbulent and molecular diffusion. Each term was estimated from local velocities and flame locations using a thin flame approximation, and their uncertainty was evaluated based on propagation of experimentally measured statistical correlations. The largest terms were the convective and reaction terms, as expected, with smaller roles for turbulent and molecular diffusion across the flame brush. Countergradient diffusion and transition to gradient diffusion were observed across the flame brush. Closure of the balance of terms in the conservation equations using independently measured terms was not consistently achieved across the flame brush within the reckoned uncertainties, arriving at a balance within 20–30% of the absolute value. Testable hypotheses are offered for the possible reasons for the mismatch, including the role of spatial filtering and 3D effects on the reaction rate term. Finally, the experiments identify the inaccuracies in measuring a true local turbulent flame speed, and suggest a consistent methodology to reduce errors in such estimations. This is the first time such a detailed experimental closure is attempted for any configuration. The results suggest that the significant improvements in spatial resolution are necessary for a full closure.

Abstract

Three-dimensional Direct Numerical Simulations of Exhaust Gas Recirculation (EGR)-type Moderate or Intense Low Oxygen Dilution (MILD) combustion of homogeneous mixtures of methane- and n-heptane–air have been conducted with skeletal chemical mechanisms. The suitability of different choices of reaction progress variable (which is supposed to increase monotonically from zero in the unburned gas to one in fully burned products) based on the mass fractions of different major species and non-dimensional temperature have been analysed in detail. It has been found that reaction progress variable definitions based on oxygen mass fraction, and linear combination of CO, CO₂, H₂ and H₂O mass fractions (i.e. ({c}_{O2}) and ({c}_{c}) ) capture all the extreme values of the major species in the range between zero and one under MILD conditions. A reaction progress variable based on fuel mass fraction is found to be unsuitable for heavy hydrocarbons, such as n-heptane, since the fuel breaks down to smaller molecules before the major reactants (products) are completely consumed (formed). Moreover, it has been found that the reaction rates of ({c}_{O2}) and ({c}_{c}) exhibit approximate linear behaviours with the heat release rate in both methane and n-heptane MILD combustion. The interdependence of different mass fractions in the EGR-type homogeneous mixture combustion is considerably different from the corresponding 1D unstretched premixed flames. The current findings indicate that the tabulated chemistry approach based on premixed laminar flames may need to be modified to account for EGR-type MILD combustion. Furthermore, both the reaction rate and scalar dissipation rate of ({c}_{O2}) and ({c}_{c}) are found to be non-linearly related in both methane and n-heptane MILD combustion cases but the qualitative nature of this correlation for n-heptane is different from that in methane. This suggests that the range of validity of SDR-based turbulent combustion models can be different for homogeneous MILD combustion of different fuels.

This paper presents a feature-based adaptive mesh refinement (AMR) method for Large Eddy Simulation of propagating deflagrations, using massive-scale parallel unstructured AMR libraries. The proposed method, named turbulent flame propagation-AMR (TFP-AMR), is able to track the transient dynamics of both the turbulent flame and the vortical structures in the flow. To handle the interaction of the turbulent flame brush with the vortical structures of the flow, a vortex selection criterion is derived from flame/vortex interaction theory. The method is built with the general intent to prioritise conservatively estimated parameters, rather than to rely on user-dependent parameters. In particular, a specific mesh adaptation triggering strategy is constructed, adapted to the strongly transient physics found in deflagrations, to guarantee that the physics of interest consistently reside within a region of high accuracy throughout the transient process. The methodology is applied and validated on several elementary cases representing fundamental bricks of the full problem: (1) a laminar flame propagation, (2) the advection of a pair of non-reacting vortices, (3) a flame/vortex interaction. The method is then applied to three different configurations of a three-dimensional complex explosion scenario in an obstructed chamber. All cases demonstrate the TFP-AMR capability to recover accurate results at reduced computational cost without requiring any ad hoc tuning of the AMR method or its parameters, thus demonstrating its genericity and robustness.

As a zero-carbon fuel, hydrogen is considered a promising alternative fuel. Hydrogen flames can be greatly affected by intrinsic instabilities including the diffusional-thermal instability (DTI) and Darrieus-Landau instability (DLI). Therefore, it is important to understand their properties, especially for cryogenic flames that are related to the safe utilization of liquid hydrogen. In this work, we conduct two-dimensional simulations of unsteady hydrogen/air conical flames to assess the effects of intrinsic instabilities, DTI and DLI, on the response of premixed hydrogen/air conical flames to inlet flow perturbations. The equivalence ratio and initial temperature are changed to respectively achieve different Lewis numbers (related to DTI) and expansion ratios (related to DLI). It is found that under certain conditions flame pinch-off occurs, during which a separated flame pocket is formed by the strong amplification of flame wrinkles generated by the inlet flow perturbations. The underlying mechanism of flame pinch-off enhancement due to DTI and DLI is different. For fuel-lean hydrogen/air at normal temperature, the flame front wrinkling is enhanced by strong DTI and it is the stretch-chemistry interaction that leads to flame pinch-off. However, for stoichiometric hydrogen/air at cryogenic temperature, there is a strong effect of DLI and flame pinch-off is mainly induced by flame-flow interaction. Moreover, downstream flow and flame speed near the separated flame pocket for flames exhibiting strong DTI and DLI are compared and the difference is analyzed. The findings indicate that intrinsic flame instability can amplify flame wrinkling and fluctuations in heat release rate, thereby contributing to flame pinch-off.