In recent decades particular interest in applications of nonequilibrium plasma for the problems of plasma-assisted ignition and plasma-assisted combustion has been observed. A great amount of experimental data has been accumulated during this period which provided the grounds for using low temperature plasma of nonequilibrium gas discharges for a number of applications at conditions of high speed flows and also at conditions similar to automotive engines. The paper is aimed at reviewing the data obtained and discusses their treatment. Basic possibilities of low temperature plasma to ignite gas mixtures are evaluated and historical references highlighting pioneering works in the area are presented. The first part of the review discusses plasmas applied to plasma-assisted ignition and combustion. The paper pays special attention to experimental and theoretical analysis of some plasma parameters, such as reduced electric field, electron density and energy branching for different gas discharges. Streamers, pulsed nanosecond discharges, dielectric barrier discharges, radio frequency discharges and atmospheric pressure glow discharges are considered. The second part depicts applications of discharges to reduce the ignition delay time of combustible mixtures, to ignite transonic and supersonic flows, to intensify ignition and to sustain combustion of lean mixtures. The results obtained by different authors are cited, and ways of numerical modelling are discussed. Finally, the paper draws some conclusions on the main achievements and prospects of future investigations in the field.

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Plasma assisted ignition and combustion

Combustion dynamics constitutes one of the most challenging areas in combustion research. Many facets of this subject have been investigated over the past few decades for their fundamental and practical implications. Substantial progress has been accomplished in understanding analysis, modeling, and simulation. Detailed laboratory experiments and numerical computations have provided a wealth of information on elementary dynamical processes such as the response of flames to variable strain, vortex rollup, coupling between flames and acoustic modulations, and perturbed flame collisions with boundaries. Much recent work has concerned the mechanisms driving instabilities in premixed combustion and the coupling between pressure waves and combustion with application to the problem of instability in modern low NO x heavyduty gas turbine combustors. Progress in numerical modeling has allowed simulations of dynamical flames interacting with pressure waves. On this basis, it has been possible to devise predictive methods for instabilities. Important efforts have also been directed at the development of the related subject of combustion control. Research has focused on methods, sensors, actuators, control algorithms, and systems integration. In recent years, scaling from laboratory experiments to practical devices has been achieved with some successebut limitations have also been revealed. Active control of combustion has also evolved in various directions. A number of experiments on laboratory-scale combustors have shown that the amplitude of combustion instabilities could be reduced by applying control principles. Full-scale terrestrial application to gas turbine systems have allowed an increase of the stability margin of these machines. Feedback principles are also being explored to control the point of operation of combustors and engines. Operating point control has special importance in the gas turbine field since it can be used to avoid operation in unstable regions near the lean blowoff limits. More generally, closed loop feedback concepts are useful if one wishes to improve the combustion process as demonstrated by applications to automotive engines. Many future developments of combustion will use such concepts for tuning, optimization, and emissions reduction. This article proposes a broad survey of these fast-moving areas of research.

Combustion dynamics and control: Progress and challenges

Elementary processes that can be involved in the development of combustion instabilities in gas turbine combustors are described. The premixed mode of combustion is considered more specie cally because it is used in most advanced gas turbine systems. The processes envisaged portray the combustion dynamics of real systems, but they are analyzed in simple laboratory cone gurations. Among the many possible interactions, the most relevant mechanisms are those that generate e uctuations in heat release or induce pressure perturbations. Some typical paths are highlighted to help in the understanding of the multiple links that can exist between elementary processes. Processes involving acoustic/e ame coupling, unsteady strain rates, e ame response to inhomogeneities, interactions of e ames with boundaries, and e ame/vortex interactions are specie cally examined. For each process, a driving or a coupling path is proposed relating heat release e uctuations to acoustic variables in certain cases or leading from acoustic variables to heat release e uctuations in other cases. Stress is also put on characteristic time lags, which are key parameters in the triggering and development of instabilities. Well-controlled experiments illustrate the many possibilities and can serve to guide the modeling effort and to validate computational tools for combustion dynamics.

Combustion Dynamics and Instabilities: Elementary Coupling and Driving Mechanisms

: This book has several purposes, including a broad historical summary of combustion instabilities in propulsion systems; a concise compilation of the main mechanisms for instabilities in liquid and solid fueled rockets, ramjets, thrust augmentors and gas turbines; development of a theoretical framework for investigating unsteady motions in combustion systems; and accessible surveys of the basic material required to understand the subject. Emphasis is placed throughout the book on observed behavior. For well-understood reasons, the best, and in many respects most useful quantitative data, have been obtained for solid propellant rockets. Hence that type of device occupies a special position in the subject. The book comprises nine chapters and eight annexes. Material in the annexes is not essential for reading and broadly understanding the main part of the book, but is likely interesting for those choosing to do research on the subject. The nine chapters divide into three parts.

Unsteady Motions in Combustion Chambers for Propulsion Systems

Molecular transport effects on turbulent flame propagation and structure

Investigations of heat release, extinction, and time evolution of the flame surface, for a nonpremixed flame interacting with a vortex

Flame-vortex interactions constitute a problem of fundamental interest for turbulent combustion modeling. Flames rolled up in vortices may also be found in a variety of practical applications. Results of a theoretical and experimental study concerning the interaction of vortical structures with a nonpremixed flame are reported. Numerical computations are performed using direct numerical simulation with detailed models for the chemistry, transport processes, and thermodynamical properties. The accuracy of the time-dependent computation has been carefully checked. Due to the cost of the simulations, the calculations were carried out on parallel vector supercomputers. The experimental results are obtained using a steady, nonpremixed counterflow flame with hydrogen as the fuel. A vortex ring, which is generated from a tube installed in one of the combustor nozzles, impinges on the flame. A detailed description of the numerical results is given, showing a global enhancement of the chemical reactions due to the interaction with the vortices. Qualitative comparison between theoretical and experimental results concerning the flame structure during the interaction process are also described. Numerical results are used to complement the experimental measurements and help explain typical features of the roll-up, while experimental results may be employed to assess the validity of the computational procedure.

Structure of a non-premixed flame interacting with counterrotating vortices

An experimental method of investigation is used to analyze some fundamental features of a double laminar premixed flame interacting with a single vortex ring. A steady double premixed counterflow flame of propane and air is first established between two opposed nozzles. A vortex ring is generated from a cylindrical tube installed in the lower combustor nozzle and impinges on one flame if the vortex is weak and on both flames if not. The interaction between the toroidal vortex and the lower flame provides complementary information on the fundamental interaction processes previously studied by Driscoll and coworkers. Comparisons with the numerical simulations of Poinsot et al, are made and confirm the theoretical predictions. The interaction between the vortex and the upper reactive layer is alos interesting as it gives information on the dynamics of two colliding premixed flames and can be used to investigate flamefront mutal interaction processes. The ability of the vortex to reach the second flame front is characterized by a new number Tt(r) , and flame merging processes are discussed.

Wrinkling, pocket formation, and double premixed flame interaction processes

A thorough understanding of turbulent reacting flows is essential to the continued development of practical combustion systems. Unfortunately, these studies represent a tremendous research challenge owing to the inherent complexity of such flows. In an effort to reduce the complexity of these systems while capturing the essential features that define the physics and chemistry of turbulent reacting flows, we have been studying the interaction of a vortex with a laminar flame. The experimental apparatus includes a piston-cylinder device configured to provide a controlled toroidal vortex. The generated vortex/jet interacts with a nonpremixed hydrogen-air flame supported in a counterflow burner. The counterflow configuration permits precise selection of the flame and the associated strain field. Vortex characterization is essential to interpreting the experimental observation and accomplishing numerical modeling of vortex-flame interactions. Two-color particle- image velocimetry (PIV) has been employed to characterize the vortex and to describe the underlying counterflow velocity field. The hydroxyl (OH) layer produced by the flame is imaged using planar laser-induced fluorescence (PLIF). The PIV and PLIF measurements of OH are performed simultaneously. A distinct annular extinction of the OH layer is observed, in good agreement with previous computational modeling predictions for the apparatus.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Paul-Henri Renard

Papers

Investigations of heat release, extinction, and time evolution of the flame surface, for a nonpremixed flame interacting with a vortex

Structure of a non-premixed flame interacting with counterrotating vortices

Wrinkling, pocket formation, and double premixed flame interaction processes

Application of imaging techniques to the study of vortex-flame interactions