Unsteady Combustion Regimes Research Articles

Investigation of Unsteady Combustion Regimes in a Kerosene-Fueled Scramjet with Air Throttling

This paper describes an experimental study investigating unsteady combustion regimes in a kerosene-fueled scramjet. The results are obtained under inflow conditions of a 2.9 MPa stagnation pressure, 1900 K stagnation temperature, and a Mach number of 3.0. The air throttling position is 240 mm downstream of the combustor entrance, with an air throttling flow rate (ratio of air throttling mass flux to inflow mass flux) of 38% and a fuel equivalence ratio of 0.37. Combustion is relatively stable when air throttling is applied and is dominated by auto-ignition. When air throttling is turned off, the combustion becomes more unsteady and is dominated by flame propagation. At the same time, the combustion mode changes, and the frequency of the combustion mode transition is 286 Hz. Schlieren images and one-dimension analysis show that the effect of air throttling is the coupling of cold throat (aerodynamic throat) and hot throat (thermal throat). The proper orthogonal decomposition and dynamic mode decomposition analysis present that when air throttling is applied or removed, the frequencies of injector–flame feedback are almost the same, while the frequencies of shock–flame feedback exhibit considerable variation, which is caused by the location of the precombustion shock affected by air throttling.

Flow Instability Control in a Model Swirl-Stabilized Combustor with Central Jet Injection

Swirling flows often occur in nature and industrial applications. With an increase in swirl intensity, such rotating flows are known to become unstable and undergo a sudden breakdown of the vortex core, resulting in unsteady flow dynamics with intensive pressure fluctuations. In particular, swirling flows are organized in combustion chambers to stabilize the flame around the central recirculation zone, formed due to the vortex core breakdown. However, the impact of large-scale vortex structures, including the precessing vortex core and secondary helical vortices, on unsteady combustion regimes is still unclear. The present paper demonstrates experimentally that for the swirling flow of a model swirl combustor, the injection of a central jet may be used to alter the configuration of coherent flow structures, including helical vortices. In particular, the asymmetric hydrodynamics mode, associated with the precessing vortex core, is suppressed, whereas the symmetrical one becomes dominant. This effect demonstrates the importance of central jet injection to control the dominant mode of flow instability for the design of swirl combustors.

Numerical modeling of steady and unsteady combustion regimes of methane-air mixture in research combustion chamber with step

Numerical research of non-reactive and reactive flow in research combustion chamber with step are performed. Different approaches for inert and reactive flow modeling, RANS/DES/LES, are compared and validated on extensive experimental data, profiles of velocity and temperatures are among this data. Unsteady combustion phenomena, such as flashback and flamefront oscillations, are investigated and their quantitative characteristics for experimental conditions are determined. Main flow features, such as mixing layer in stabilization zone and induced by cooling system thermal boundary layer in post-combustion zone are extracted and analyzed, influence of this features on flow unsteadiness are considered.

Overview of Al-based nanoenergetic ingredients for solid rocket propulsion

Abstract The introduction of nano-sized energetic ingredients first occurred in Russia about 60 years ago and arose great expectations in the rocket propulsion community, thanks to the higher energy densities and faster energy release rates exhibited with respect to conventional ingredients. But, despite intense worldwide research programs, still today mostly laboratory level applications are reported and often for scientific purposes only. A number of practical reasons prevent the applications at industrial level: inert native coating of the energetic particles, nonuniform dispersion, aging, excessive viscosity of the slurry propellant, possible limitations in mechanical properties, more demanding safety issues, cost, and so on. This paper describes the main features in terms of performance of solid rocket propellants loaded with nanometals and intends to emphasize the unique properties or operating conditions made possible by the addition of the nano-sized energetic ingredients. Steady and unsteady combustion regimes are examined.

Linear analysis of hotspot-pulsating mode of propellant combustion

A model of the combustion of a condensed substance burning in the gasification mode is proposed, which makes it possible to examine non-one-dimensional unsteady combustion regimes. For an insulated cylindrical sample, the condition of steady burning stability with respect to flat and dimensional perturbations is determined. The spectrum of possible elementary perturbations, each of which is a set of oscillating hotspots, is obtained. In contrast to stable modes, which decay with time, an analysis of the time evolution of systems of unstable hotspots requires taking into account nonlinear effects. However, even linear analysis gives a number of results that can be compared qualitatively with experimental data.

Diffusion-thermal stability of gas filtration combustion waves in an inert porous medium

A mathematical model for the filtration gas combustion taking into account thermal conductivity, diffusion, and intense interfacial heat transfer is presented. The temperature dependence of the chemical reaction rate is approximated by a δ-function and the thermal-expansion coefficient of gases behind the combustion front is taken into account. Unsteady combustion regimes are analyzed using the method of small perturbations. The boundaries of the longitudinal and spatial stability for steady regimes of the filtration combustion wave are obtained. The dependence of the Lewis number on the thermal-expansion coefficient of the gas mixture along the boundary of stability is derived, along with other relations.

Investigation of the Hierarchical Structure of Kinetic Models in Ignition Problems

Abstract In this paper a novel method for global analysis of chemical kinetic models is discussed and applied to auto ignition of high hydrocarbons. It is mainly based on the concept of decomposition of motions and follows major steps of the ILDM method. However, a very important difference of the suggested method is its ability of an explicit and global representation of the system decomposition as a standard singular perturbed system (SPS). The fact that the ILDM provides local information about the decomposition makes the ILDM quite accurate in description of the slow system dynamics, but for fast motions, which become very important in the context of ignition/extinction problems, there are up to now no reliable methods based on ILDM. Therefore, the current work is devoted to developing such a method which can be used efficiently for global analysis of the reaction mechanisms with subsequent formulation of explicit reduced models for unsteady combustion regimes like ignition processes. The suggested method is illustrated by a simple Lindemann kinetic model and then applied successfully to the auto ignition of a homogeneous n-heptane/air system.

Joint RANS/LES Approach to Premixed Flame Modelling in the Context of the TFC Combustion Model

We present an original timesaving joint RANS/LES approach to simulate turbulent premixed combustion. It is intended mainly for industrial applications where LES may not be practical. It is based on successive RANS/LES numerical modelling, where turbulent characteristics determined from RANS simulations are used in LES equations for estimation of the subgrid chemical source and viscosity. This approach has been developed using our TFC premixed combustion model, which is based on a generalization of the Kolmogorov’s ideas. We assume existence of small-scale statistically equilibrium structures not only of turbulence but also of the reaction zones. At the same time, non-equilibrium large-scale structures of reaction sheets and turbulent eddies are described statistically by model combustion and turbulence equations in RANS simulations or follow directly without modelling in LES. Assumption of small-scale equilibrium gives an opportunity to express the mean combustion rate (controlled by small-scale coupling of turbulence and chemistry) in the RANS and LES sub-problems in terms of integral or subgrid parameters of turbulence and the chemical time, i.e. the definition of the reaction rate is similar to that of the mean dissipation rate in turbulence models where it is expressed in terms of integral or subgrid turbulent parameters. Our approach therefore renders compatible the combustion and turbulent parts of the RANS and LES sub-problems and yields reasonable agreement between the RANS and averaged LES results. Combining RANS simulations of averaged fields with LES method (and especially coupled and acoustic codes) for simulation of corresponding nonstationary process (and unsteady combustion regimes) is a promising strategy for industrial applications. In this work we present results of simulations carried out employing the joint RANS/LES approach for three examples: High velocity premixed combustion in a channel, combustion in the shear flow behind an obstacle and the impinging flame (a premixed flame attached to an obstacle).

Unsteady Combustion Regimes of Gasless Systems with a Low-Melting Inert Component

The paper studied the effect of temperature and heats of melting of an inert low-melting component of a mixture and its quantity on the regularities of ignition and unsteady combustion regimes of gasless systems.

Modelling of unsteady combustion regimes for polydisperse fuels—I. Instability and auto-oscillations

A general theory of instability and self-oscillations accompanying combustion of polydisperse assemblages of particles is elaborated on the basis of a model including heat and mass balance equations and the kinetic equation governing the particle size distribution. At arbitrary burning kinetics of particles and the particle influx rate they are reduced; under the condition of ideal mixing, to only two functional integrodifferential equations for the dimensionless temperature and oxidant concentration. Steady combustion processes are shown to be unstable in many important situations. Depending on physical and regime parameters, the break of stability can be realized in either ‘soft’ or ‘hard’ manner giving rise to establishing a regular auto-oscillating or a chaotic pulsating regime, respectively. The properties of possible slightly non-linear auto-oscillations are studied in greater detail.

Modelling of unsteady combustion regimes for polydisperse fuels—II. Parametrically controlled combustion

Results of computer calculations of essentially non-linear periodic combustion regimes originating far from the stability boundary are presented. The influence of the parametrical modulation of relevant processing parameters upon the neutral stability and upon the properties of oscillating combustion regimes is investigated. Modulation characteristics which cause either stabilization or destabilization of combustion are determined. Harmonic, ultra- and subharmonic frequency locking phenomena appearing in new regions of instability of the parametrically influenced combustion regimes are revealed. Quasiperiodic combustion states forming outside the regions of frequency locking are studied.