Showing papers on "Diffusion flame published in 2008"

Non-normality and nonlinearity in combustion-acoustic interaction in diffusion flames

Abstract: The role of non-normality and nonlinearity in flame–acoustic interaction in a ducted diffusion flame is investigated in this paper. The infinite rate chemistry model is employed to study unsteady diffusion flames in a Burke–Schumann type geometry. It has been observed that even in this simplified case, the combustion response to perturbations of velocity is non-normal and nonlinear. This flame model is then coupled with a linear model of the duct acoustic field to study the temporal evolution of acoustic perturbations. The one-dimensional acoustic field is simulated in the time domain using the Galerkin technique, treating the fluctuating heat release from the combustion zone as a compact acoustic source. It is shown that the coupled combustion–acoustic system is non-normal and nonlinear. Further, calculations showed the occurrence of triggering; i.e. the thermoacoustic oscillations decay for some initial conditions whereas they grow for some other initial conditions. It is shown that triggering occurs because of the combined effect of non-normality and nonlinearity. For such a non-normal system, resonance or ‘pseudoresonance’ may occur at frequencies far from its natural frequencies. Non-normal systems can be studied using pseudospectra, as eigenvalues alone are not sufficient to predict the behaviour of the system. Further, both necessary and sufficient conditions for the stability of a thermoacoustic system are presented in this paper.

A Model of AP/HTPB Composite Propellant Combustion in Rocket-Motor Environments

Abstract: A comprehensive theoretical/numerical model for treating AP/HTPB composite-propellant combustion in a rocket-motor environment is presented. The formulation takes into account the conservation equations in both the gas and condensed phases, and accommodates finite-rate chemical kinetics and variable thermophysical properties. The processes in the two phases are coupled at the surface to determine the propellant burning behavior. An asymptotic analysis based on a large activation-energy approximation for the condensed-phase decomposition is applied to help resolve the combustion wave structure in the interfacial layer. A simplified global reaction is employed to characterize the final diffusion flame between the decomposition products of AP and the pyrolysis products of HTPB. Only laminar flows are considered here, to avoid complications arising from turbulence. A detailed parametric study is conducted on the gas-phase flame structures of AP/HTPB composite propellants. The dependence of burning rate, flame...

Kinetic Ignition Enhancement of Diffusion Flames by Nonequilibrium Magnetic Gliding Arc Plasma

Abstract: Kinetic ignition enhancement of CH 4 -air and H 2 -air diffusion flames by a nonequilibrium plasma discharge of air was studied experimentally and numerically through the development of a well-defined counterflow system. Measurements of ignition temperatures and major species, as well as computations of rates of production and sensitivity analyses, were performed to understand the kinetic enhancement pathways for ignition by plasma discharge of air. It was found that plasma discharge of air led to significant kinetic ignition enhancement illustrated by large decreases in the ignition temperatures for a broad range of strain rates. Examination of the radical and NO x production in the plasma showed that the enhancement was caused primarily by the catalytic effect of NO x . The results of numerical simulations of the counterflow burner with preheated air and NO, addition showed the existence of different ignition regimes, which appeared due to the competition between radical production by NO x and other pathways, as well as heat release. There were two ignition regimes for small concentrations of NO, and three ignition regimes for large concentrations of NO,. Numerical simulations agreed well with the experimental measurements and suggested a new strategy for plasma-assisted ignition in supersonic flow, where a combination of thermal and nonthermal plasma would work more efficiently for ignition enhancement.

Analysis and flamelet modelling for spray combustion

Abstract: The validity of a steady-flamelet model and a flamelet/progress-variable approach for gaseous and spray combustion is investigated by a two-dimensional direct numerical simulation (DNS) of gaseous and spray jet flames, and the combustion characteristics are analysed. A modified flamelet/progress-variable approach, in which total enthalpy rather than product mass fraction is chosen as a progress variable, is also examined. DNS with an Arrhenius formation, in which the chemical reaction is directly solved in the physical flow field, is performed as a reference to validate the combustion models. The results show that the diffusion flame is dominant in the gaseous diffusion jet flame, whereas diffusion and premixed flames coexist in the spray jet flame. The characteristics of the spray flame change from premixed-diffusion coexistent to diffusion-dominant downstream. Comparisons among the results from DNS with various combustion models show the modified flamelet/progress-variable approach to be superior to the other combustion models, particularly for the spray flame. Where the behaviour of the gaseous total enthalpy is strongly affected by the energy transfer (i.e. heat transfer and mass transfer) from the dispersed droplet, and this effect can be accounted for only by solving the conservation equation of the total enthalpy. However, even the DNS with the modified flamelet/progress-variable approach tends to underestimate the gaseous temperature in the central region of the spray jet flame. To increase the prediction accuracy, a combustion model for the partially premixed flame for the spray flame is necessary.

Flame colour characterization in the visible and infrared spectrum using a digital camera and image processing

Abstract: An attempt has been made to characterize the colour spectrum of methane flame under various burning conditions using RGB and HSV colour models instead of resolving the real physical spectrum. The results demonstrate that each type of flame has its own characteristic distribution in both the RGB and HSV space. It has also been observed that the averaged B and G values in the RGB model represent well the CH* and C*2 emission of methane premixed flame. Theses features may be utilized for flame measurement and monitoring. The great advantage of using a conventional camera for monitoring flame properties based on the colour spectrum is that it is readily available, easy to interface with a computer, cost effective and has certain spatial resolution. Furthermore, it has been demonstrated that a conventional digital camera is able to image flame not only in the visible spectrum but also in the infrared. This feature is useful in avoiding the problem of image saturation typically encountered in capturing the very bright sooty flames. As a result, further digital imaging processing and quantitative information extraction is possible. It has been identified that an infrared image also has its own distribution in both the RGB and HSV colour space in comparison with a flame image in the visible spectrum.

Effect of Flame Structure on the Flame Transfer Function in a Premixed Gas Turbine Combustor

Abstract: The flame transfer function in a premixed gas turbine combustor is experimentally determined. The fuel (natural gas) is premixed with air upstream of a choked inlet to combustor. Therefore, the input to the flame transfer function is the imposed velocity fluctuations of the fuel/air mixture without equivalence ratio fluctuations. The inlet-velocity fluctuations are achieved by a variable-speed siren over the forcing frequency of 75–280 Hz and measured using a hot-wire-anemometer at the inlet to the combustor. The output function (heat release) is determined using chemiluminescence measurement from the whole flame. Flame images are recorded to understand how the flame structure plays a role in the global heat release response of flame to the inlet-velocity perturbation. The results show that the gain and phase of the flame transfer function depend on flame structure as well as the frequency and magnitude of inlet-velocity modulation and can be generalized in terms of the relative length scale of flame to convection length scale of inlet-velocity perturbation, which is represented by a Strouhal number. Non-linear flame response is characterized by a periodic vortex shedding from shear layer and the non-linearity occurs at lower magnitude of inlet-velocity fluctuation as the modulation frequency increases. However, for a given modulation frequency, the flame structure does not affect the magnitude of inlet-velocity fluctuation at which the non-linearity starts.Copyright © 2008 by ASME

Flame acceleration and DDT run-up distance for smooth and obstacles filled tubes

Abstract: The process of flame acceleration inside the tubes and channels depends on several parameters such as nature of the fuel involved, composition of the mixture and configuration of the enclosure itself. The wall roughness and the presence of obstacles in the flame path act as a turbulence generator causing continuous flame acceleration. In some situations the flame can reach a sufficiently high speed to allow the transition of the deflagration into a detonation. A considerably large amount of experimental data on flame speed and DDT run-up distance for several mixtures have been accumulated. Nevertheless simple relationships, based on the most relevant parameters governing the phenomenon, could be useful for design purpose and safety assessment. The present paper suggests some simplified formulas for the evaluation of flame speed and DDT run-up distance of flammable mixtures for both smooth and obstacles filled tubes.

Impact of volume viscosity on a shock–hydrogen-bubble interaction

Abstract: We investigate the influence of volume viscosity on a planar shock–hydrogen-bubble interaction. The numerical model is two dimensional and involves complex chemistry and detailed transport. All transport coefficients are evaluated using algorithms which provide accurate approximations rigourously derived from the kinetic theory of gases. Our numerical results show that volume viscosity has an important impact on the velocity distribution – through vorticity production – and therefore on the flame structure.

A Numerical Study on the Influence of CO2 Addition on Soot Formation in an Ethylene/Air Diffusion Flame

Abstract: Earlier studies have confirmed that the addition of CO2 to a diffusion flame suppresses the formation of soot. However, a consensus has not been reached on the fundamental mechanisms of the suppression. In this paper, the influence of CO2 addition on soot formation in an ethylene/air diffusion flame is investigated by numerical simulation, with focus on the fundamental mechanism of the suppression effect on soot formation. A special strategy is employed to separate the chemical effect from the thermal and dilution effects. The simulation results confirm that the addition of CO2 suppresses soot formation through not only the thermal and dilution effects, but also the chemical effect. The chemical effect of CO2 addition is primarily caused by the reduced concentration of radical H due to the reaction CO + OH ⇐ CO2 + H, which suppresses the soot inception and surface growth rate. The chemical effect of CO2 addition has negligible effect on soot oxidation process.

Large Eddy Simulations of Swirling Non-premixed Flames With Flamelet Models: A Comparison of Numerical Methods

Abstract: This work investigates the application of large eddy simulation (LES) to selected cases of the turbulent non-premixed Sydney swirl flames. Two research groups (Loughborough University, LU and Imperial College, IC) have simulated these cases for different parameter sets, using two different and independent LES methods. The simulations of the non-reactive turbulent flow predicted the experimental results with good agreement and both simulations captured the recirculation structures and the vortex breakdown without major difficulties. For the reactive cases, the LES predictions were less satisfactory, and using two independent simulations has helped to understand the shortcomings of each. Furthermore one of the flames (SMH2) was found to be exceptionally hard to predict, which was supported by the lower amount of turbulent kinetic energy that was resolved in this case. However, the LES has identified modes of flame instability that were similar to those observed in some of the experiments.