Effects of heat release on triple flames

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. © 1995 American Institute of Physics

Triple flame structure and diffusion flame stabilization

The stabilization of diffusion ñames is studied using asymptotic techniques and numerical tools. The configuration studied corresponda to parallel streams of cold oxidizer and fuel initially separated by a splitter píate. It is shown that stabilization of a diffusion flame may only occur in this situation by two processes. First, the flame may be stabilized behind the flame holder in the wake of the splitter píate. For this case, numerical simulations confirm scalings previously predicted by asymptotic analysis. Second, the flame may be lifted. In this case a triple flame is found at longer distanees downstream of the flame holder. The structure and propagation speed of this flame are studied by using an actively controlled numerical technique in which the triple flame is tracked in its own reference frame. It is then possible to investigate the triple flame structure and velocity. It is shown, as suggested from asymptotic analysis, that heat reléase may induce displacement speeds of the triple flame larger than the laminar flame speed corresponding to the stoichiometric conditions prevailing in the mixture approaching the triple flame. In addition to studying the characteristics of triple flames in a uniform flow, their re-sistance to turbulence is investigated by subjecting triple flames to different vortical configurations

Scalar flux modeling in turbulent flames using iterative deconvolution

In the context of Large Eddy Simulations, deconvolution is an attractive alternative for modelling the un-closed terms appearing in the filtered governing equations. Such methods have been used in a number of studies for non-reacting and incompressible flows, however their application in reacting flows is limited in comparison. Deconvolution methods originate from clearly defined operations, and in theory can be used in order to model any un-closed term in the filtered equations including the scalar flux. In this study, an iterative deconvolution algorithm is used in order to provide a closure for the scalar flux term in a turbulent premixed flame by explicitly filtering the deconvoluted fields. The assessment of the method is conducted a priori using a three-dimensional direct numerical simulation database of a turbulent freely-propagating premixed flame in a canonical configuration. In contrast to most classical a priori studies, the assessment is more stringent as it is performed on a much coarser LES mesh which is constructed using the filtered fields as obtained from the direct simulations. For the conditions tested in this study, deconvolution is found to provide good estimates both of the scalar flux and of its divergence

Model Equation for the Dynamics of Wrinkled Shockwaves: Comparison with DNS and Experiments

International audienceA model equation for the dynamics and the geometry of the wrinkled front of shock waves, obtained for strong shocks in the Newtonian limit, is tested by comparison with direct numerical simulations and a shock tube experiment

Structure of turbulent non-premixed flames modeled with two-step chemistry

Direct numerical simulations of turbulent diffusion flames modeled with finite-rate, two-step chemistry, A + B yields I, A + I yields P, were carried out. A detailed analysis of the turbulent flame structure reveals the complex nature of the penetration of various reactive species across two reaction zones in mixture fraction space. Due to this two zone structure, these flames were found to be robust, resisting extinction over the parameter ranges investigated. As in single-step computations, mixture fraction dissipation rate and the mixture fraction were found to be statistically correlated. Simulations involving unequal molecular diffusivities suggest that the small scale mixing process and, hence, the turbulent flame structure is sensitive to the Schmidt number

Effect of finite-rate chemistry and unequal Schmidt numbers on turbulent non-premixed flames modeled with single-step chemistry

The interaction between a quasi-laminar flame and a turbulent flowfield is investigated through direct numerical simulations (DNS) of reacting flow in two- and three-dimensional domains. Effects due to finite-rate chemistry are studied using a single step global reaction A (fuel) + B (oxidizer) yields P (product), and by varying a global Damkoehler number, as a result of which the turbulence-chemistry interaction in the flame is found to generate a wide variety of conditions, ranging from near-equilibrium to near-extinction. Differential diffusion effects are studied by changing the Schmidt number of one reactive species to one-half. It is observed that laminar flamelet response is followed within the turbulent flowfield, except in regions where transient effects seem to dominate

Alkali metal emissions in early stage of a pulverized-coal flame: DNS analysis of reacting layers and chemistry tabulation

The intricate coupling between coal pyrolysis, gas phase combustion and the emissions of alkali metal, such as sodium, is studied in early stage of a temporally evolving three-dimensional planar turbulent jet carrying pulverized-coal particles. Complex chemistry is used to account for both the combustion of volatile hydrocarbons and the sodium containing species. The response of the sodium chemistry is analyzed in the mixture fraction space, along with the topology of the reactions zones. Combustion is found to start preferentially in partially premixed flames, which then evolve toward di usion-like reactive layers and reach chemical equilibrium. From the direct numerical simulation (DNS) database, the possibility of modeling the dynamics of sodium species using one-dimensional premixed flamelet generated manifolds (FGM) is investigated. A chemical lookup table is constructed for the combustion of the partially premixed volatiles and an additional three-dimensional simulation is performed to compare the tabulated sodium species against their reference counterparts with complex chemistry. Quantitative analysis of the performance of the developed chemistry tabulation confirms the validity of the approach. Perspectives for the modeling of sodium emissions in pulverized-coal furnaces and boilers are finally drawn.National Natural Science Foundation of China and China Postdoctoral Science Foundatio

Numerical study of HCl and SO2 impact on potassium emissions in pulverized-biomass combustion

National Natural Science Foundation of China (51706200), the China Postdoctoral Science Foundation (2018M632460), the Fundamental Research Funds for the Central Universities (2018FZA4012), the Engineering and Physical Sciences Research Council (EPSRC) and the Royal Society of the UK

Acoustical Excitation for Damping Estimation in Rotating Machinery

In experimental modal analysis a structure is excited with a force in order to estimate the frequency response function. Typically, this force is generated by a shaker or a hammer impact. Both methods have proven their usefulness, but have some well-known disadvantages. A main disadvantage of the shaker is that it has to be fixed to the structure whereas with a hammer it is not possible to excite a specific frequency. To overcome these disadvantages, alternative non-contact methods can be used. There are several non-contact techniques, i.e. pressurized air, laser, acoustics, etc. By using acoustics as an excitation technique it is easy to select an excitation signal going from random noise to a simple sine. Also the equipment to produce the acoustic excitation is rather cheap. However, the state of the art does not offer a straightforward technique to estimate the excitation force, making it difficult for applications such as experimental modal analysis. In this research, acoustic excitation is compared with hammer excitation to estimate the frequency response function of two shafts. Especially a method to validate the force induced by the acoustics is derived. The final purpose of this research is to estimate the damping properties of rotating machinery