Multiplicity of Premixed Flames Under the Effect of Heat Loss

2017 ◽  
Vol 9 (3) ◽  
Author(s):  
Eman Al-Sarairah ◽  
Chaouki Ghenai ◽  
Ahmed Hachicha
Keyword(s):  
Strain Rate ◽  
Heat Loss ◽  
Combustion Wave ◽  
Premixed Flames ◽  
Lewis Number ◽  
The Other ◽  
Premixed Flame ◽  
Two Dimensional ◽  
Combustion Waves ◽  
Loss Parameter

We investigate numerically the effect of heat loss and strain rate on the premixed flame edges encountered in a two-dimensional counterflow configuration for Lewis number higher than one. Under nonadiabatic conditions, multiple flame edges and multiple propagation speeds (positive and negative) are discussed. Different regions of multiple propagation speeds have been revealed ranging from two to four, depending on the value of the heat loss parameter and Damkohler number, which is inversely proportional to the strain rate. A combustion wave is modeled by connecting a strongly burning flame on one side of the burner to a weakly burning flame on the other side. These combustion waves are changing with increasing Dam number into flame edges with the fact that the strongly burning flame is the dominant.

Travelling Waves in a Two-Step Chain Branching Model with Heat Loss

2009 ◽  
Vol 4 (3) ◽  
Author(s):  
Harvinder S Sidhu ◽  
V.V. Gubernov ◽  
Geoff Mercer ◽  
A.V. Kolobov ◽  
A.A. Polezhaev ◽  
...  
Keyword(s):  
Reaction Mechanism ◽  
Numerical Investigation ◽  
Heat Loss ◽  
Combustion Wave ◽  
Travelling Waves ◽  
Chain Branching ◽  
Combustion Waves ◽  
Extinction Limit ◽  
Branching Model ◽  
Loss Parameter

In this paper we undertake a numerical investigation of travelling nonadiabatic combustion waves for the case of a two-step chain branching reaction mechanism. For simplicity we have assumed equal diffusivity of the reactant, radicals and heat. The speed of the combustion wave is analysed for different values of the heat loss parameter. We also determine how the extinction limit depends on the heat loss parameter as well as properties of the fuel.

scholarly journals Non-adiabatic combustion waves for general Lewis numbers: wave speed and extinction conditions

2004 ◽  
Vol 46 (1) ◽  
pp. 1-16 ◽  
Author(s):  
A. C. McIntosh ◽  
R. O. Weber ◽  
G. N. Mercer
Keyword(s):  
Activation Energy ◽  
Heat Loss ◽  
Combustion Wave ◽  
Wave Speed ◽  
Energy Analysis ◽  
Lewis Number ◽  
Heat Losses ◽  
Combustion Waves ◽  
Wave Speeds ◽  
Oscillatory Instabilities

AbstractThis paper addresses the effect of general Lewis number and heat losses on the calculation of combustion wave speeds using an asymptotic technique based on the ratio of activation energy to heat release being considered large. As heat loss is increased twin flame speeds emerge (as in the classical large activation energy analysis) with an extinction heat loss. Formulae for the non-adiabatic wave speed and extinction heat loss are found which apply over a wider range of activation energies (because of the nature of the asymptotics) and these are explored for moderate and large Lewis number cases—the latter representing the combustion wave progress in a solid. Some of the oscillatory instabilities are investigated numerically for the case of a reactive solid.

A Numerical Simulation to the Influence of Unsteady Strain Flow on Twin Premixed Flames

2019 ◽  
Vol 11 (3) ◽  
Author(s):  
Faisal Al-Malki
Keyword(s):  
Numerical Simulation ◽  
Finite Elements ◽  
Strain Rate ◽  
Premixed Flames ◽  
Lewis Number ◽  
Critical Value ◽  
The Other ◽  
Constant Density ◽  
Nonmonotonic Dependence

The aim of this paper was to examine the response of twin premixed flames formed in a counterflow configuration to the presence of an unsteady straining flow. We began by describing the problem mathematically using the thermodiffusive model with constant density and then adopted a finite elements approach to solve the problem numerically. The study has shown that the role of flow on flame propagation is determined by three main parameters, namely, flow amplitude A, strain rate ε, and fuel Lewis number LeF. For LeF ≥ 1, the flow is seen to promote flame extinction, while LeF < 1 the flow clearly enhances the flame reactivity. Qualitatively, it has been shown that for LeF = 1, there exists a critical value of A (that varies with ε) below which the reactivity decreases monotonically with A. For small LeF < 1, on the other hand, the reactivity was seen to increase with A. For LeF > 1, however, a nonmonotonic dependence, especially for small ε, is predicted.

Numerical Simulation on the Instability of Cylindrically Expanding Premixed Flames With Radiative Heat Loss at Low Lewis Numbers

2011 ◽  
Author(s):  
Takafumi Kusakai ◽  
Satoshi Kadowaki
Keyword(s):  
Heat Loss ◽  
Thermal Instability ◽  
Radiative Heat ◽  
Premixed Flames ◽  
Lewis Number ◽  
Radiative Heat Loss ◽  
Low Gravity ◽  
Gravity Condition ◽  
Quenching Condition ◽  
Reactive Gases

The instability of cylindrically expanding premixed flames with radiative heat loss was studied by two-dimensional unsteady calculations of reactive gases, based on the diffusive-thermal model equation. When the Lewis number was unity, instability phenomena were not observed. When the Lewis number was sufficiently low, on the other hand, cellular-shaped fronts on adiabatic and non-adiabatic cylindrical flames were observed, which was due to diffusive-thermal instability. As radiative heat loss increased, the behavior of cellular fronts became more unstable. This indicated that the radiation promoted the unstable behavior of flame fronts at low Lewis numbers. When radiative heat loss was much large compared with the quenching condition of a planar flame, cylindrical flames were broken up and several small flames appeared. This was in qualitative agreement with the experimental results on the dynamic behavior of lean hydrogen-air premixed flames with radiative heat loss under the low gravity condition. Several small flames appeared on the grounds that large curvature of flame fronts was necessary to keep high temperature against radiative heat loss.

On the Stability of Planar Premixed Flames Under Nonadiabatic Conditions and Preferential Diffusion

2017 ◽  
Vol 9 (3) ◽  
Author(s):  
Eman Al-Sarairah ◽  
Bilal Al-Hasanat ◽  
Ahmed Hachicha
Keyword(s):  
Heat Loss ◽  
Numerical Study ◽  
Lewis Number ◽  
Premixed Flame ◽  
Constant Density ◽  
Damkohler Number ◽  
Damköhler Number ◽  
Number Ratio ◽  
Preferential Diffusion ◽  
The Stability

In this paper, we provide a numerical study of the stability analysis of a planar premixed flame. The interaction of preferential diffusion and heat loss for a planar premixed flame is investigated using a thermodiffusive (constant density) model. The flame is studied as a function of three nondimensional parameters, namely, Damköhler number (ratio of diffusion time to chemical time), Lewis number (ratio of thermal to species diffusivity), and heat loss. A maximum of four solutions are identified in some cases, two of which are stable. The behavior of the eigenvalues of the linearized system of stabilty is also discussed. For low Lewis number, the heat loss plays a major role in stabilizing the flame for some moderately high values of Damköhler number. The results show the effect of increasing or decreasing Lewis number on adiabatic and nonadiabatic flames temperature and reaction rate as well as the range of heat loss at which flames can survive.

scholarly journals Comparison of the Reactive Scalar Gradient Evolution between Homogeneous MILD Combustion and Premixed Turbulent Flames

Energies ◽  
2021 ◽  
Vol 14 (22) ◽  
pp. 7677
Author(s):  
Hazem S.A.M. Awad ◽  
Khalil Abo-Amsha ◽  
Umair Ahmed ◽  
Nilanjan Chakraborty
Keyword(s):  
Strain Rate ◽  
Turbulence Intensity ◽  
Premixed Flames ◽  
Premixed Flame ◽  
Normal Strain ◽  
Mean Values ◽  
Mild Combustion ◽  
The Mean ◽  
Displacement Speed ◽  
Reactive Scalar

Moderate or intense low-oxygen dilution (MILD) combustion is a novel combustion technique that can simultaneously improve thermal efficiency and reduce emissions. This paper focuses on the differences in statistical behaviours of the surface density function (SDF = magnitude of the reaction progress variable gradient) between conventional premixed flames and exhaust gas recirculation (EGR) type homogeneous-mixture combustion under MILD conditions using direct numerical simulations (DNS) data. The mean values of the SDF in the MILD combustion cases were found to be significantly smaller than those in the corresponding premixed flame cases. Moreover, the mean behaviour of the SDF in response to the variations of turbulence intensity were compared between MILD and premixed flame cases, and the differences are explained in terms of the strain rates induced by fluid motion and the ones arising from flame displacement speed. It was found that the effects of dilatation rate were much weaker in the MILD combustion cases than in the premixed flame cases, and the reactive scalar gradient in MILD combustion cases preferentially aligns with the most compressive principal strain-rate eigendirection. By contrast, the reactive scalar gradient preferentially aligned with the most extensive principal strain-rate eigendirection within the flame in the premixed flame cases considered here, but the extent of this alignment weakened with increasing turbulence intensity. This gave rise to a predominantly positive mean value of normal strain rate in the premixed flames, whereas the mean normal strain rate remained negative, and its magnitude increased with increasing turbulence intensity in the MILD combustion cases. The mean value of the reaction component of displacement speed assumed non-negligible values in the MILD combustion cases for a broader range of reaction progress variable, compared with the conventional premixed flames. Moreover, the mean displacement speed increased from the unburned gas side to the burned gas side in the conventional premixed flames, whereas the mean displacement speed in MILD combustion cases decreased from the unburned gas side to the middle of the flame before increasing mildly towards the burned gas side. These differences in the mean displacement speed gave rise to significant differences in the mean behaviour of the normal strain rate induced by the flame propagation and effective strain rate, which explains the differences in the SDF evolution and its response to the variation of turbulence intensity between the conventional premixed flames and MILD combustion cases. The tangential fluid-dynamic strain rate assumed positive mean values, but it was overcome by negative mean values of curvature stretch rate to yield negative mean values of stretch rate for both the premixed flames and MILD combustion cases. This behaviour is explained in terms of the curvature dependence of displacement speed. These findings suggest that the curvature dependence of displacement speed and the scalar gradient alignment with local principal strain rate eigendirections need to be addressed for modelling EGR-type homogeneous-mixture MILD combustion.

PULSATING INSTABILITIES OF COMBUSTION WAVES IN A CHAIN-BRANCHING REACTION MODEL

2009 ◽  
Vol 19 (03) ◽  
pp. 873-887 ◽  
Author(s):  
V. V. GUBERNOV ◽  
A. V. KOLOBOV ◽  
A. A. POLEZHAEV ◽  
H. S. SIDHU ◽  
G. N. MERCER
Keyword(s):  
Activation Energy ◽  
Hopf Bifurcation ◽  
Combustion Wave ◽  
Traveling Wave ◽  
Lewis Number ◽  
Flame Speed ◽  
Traveling Wave Solution ◽  
Solution Branch ◽  
Combustion Waves ◽  
Parameter Values

In this paper we investigate the properties and linear stability of traveling premixed combustion waves and the formation of pulsating combustion waves in a model with two-step chain-branching reaction mechanism. These calculations are undertaken in the adiabatic limit, in one spatial dimension and for the case of arbitrary Lewis numbers for fuel and radicals. It is shown that the Lewis number for fuel has a significant effect on the properties and stability of premixed flames, whereas varying the Lewis number for the radicals has only qualitative (but not qualitative) effect on the combustion waves. We demonstrate that when the Lewis number for fuel is less than unity, the flame speed is unique and is a monotonically decreasing function of the dimensionless activation energy. Moreover, in this case, the combustion wave is stable and exhibits extinction for finite values of activation energy as the flame speed decreases to zero. However, for the fuel Lewis number greater than unity, the flame speed is a C-shaped and double valued function. The linear stability of the traveling wave solution was determined using the Evans function method. The slow solution branch is shown to be unstable whereas the fast solution branch is stable or exhibits the onset of pulsating instabilities via a Hopf bifurcation. The critical parameter values for the Hopf bifurcation and extinction are found and the detailed map for the onset of pulsating instabilities is determined. We show that a Bogdanov—Takens bifurcation is responsible for both the change in the behavior of the traveling wave solution near the point of extinction from unique to double valued type as well as for the onset of pulsating instabilities. We investigate the properties of the Hopf bifurcation and the emerging pulsating combustion wave solutions. It is demonstrated that the Hopf bifurcation observed in our present study is of supercritical type. We show that the pulsating combustion wave propagates with the average speed smaller than the speed of the traveling combustion wave and at certain parameter values the pulsating wave exhibits a period doubling bifurcation.

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