scholarly journals Effect of equivalence ratio on premixed flame response to unsteady strain-rate and curvature

1998 ◽  
Author(s):  
H.N. Najm ◽  
P.S. Wyckoff ◽  
O.M. Knio
Keyword(s):  
Strain Rate ◽  
Equivalence Ratio ◽  
Premixed Flame ◽  
Flame Response

The Structure of Two-Stage Counterflow n-Heptane-Air Flames

2000 ◽  
Author(s):  
S. K. Aggarwal ◽  
H. S. Xue
Keyword(s):  
Strain Rate ◽  
Reaction Zone ◽  
Equivalence Ratio ◽  
Flame Structure ◽  
Premixed Combustion ◽  
Premixed Flame ◽  
Partially Premixed Combustion ◽  
Reaction Zones ◽  
Partially Premixed ◽  
Partially Premixed Flame

Partially premixed flames are formed by mixing air (in less than stoichiometric amounts) into the fuel stream prior to the reaction zone, where additional air is available for complete combustion. Such flames can occur in both laboratory and practical combustion systems. In advanced gas turbine combustor designs, such as a lean direct injection (LDI) combustor, partially premixed combustion represents an impotent mode of burning. Spray combustion often involves partially premixed combustion due to the locally fuel vapor-rich regions. In the present study, the detailed structure of n-heptane/air partially premixed flame in a counterflow configuration is investigated. The flame is computed by employing the Oppdif code and a detailed reaction mechanism consisting of 275 elementary reactions and 41 species. The partially premixed flame structure is characterized by two-stage burning or two distinct but synergistically coupled reaction zones, a rich premixed zone on the fuel side and a ‘nonpremixed zone on the air side. The fuel is completely consumed in the premixed zone with ethylene and acetylene being the major intermediate species. The reactions involving the consumption of these species are found to be the key rate-limiting reactions that characterize interactions between the two reaction zones, and determine the overall fuel consumption rate. The flame response to the variations in equivalence ratio and strain rate is examined. Increasing equivalence ratio and/or strain rate to a critical value leads to merging of the two reaction zones. The equivalence ratio variation affects the rich premixed reaction zone, while the variation in strain rate predominantly affects the nonpremixed reaction zone. The flame structure is also characterized in terms of a modified mixture fraction (conserved scalar), and laminar flamelet profiles are provided.

Premixed flame response to equivalence ratio perturbations

2010 ◽  
Vol 14 (5) ◽  
pp. 681-714 ◽  
Author(s):  
Shreekrishna ◽  
Santosh Hemchandra ◽  
Tim Lieuwen
Keyword(s):  
Equivalence Ratio ◽  
Premixed Flame ◽  
Flame Response

Laminar premixed flame response to equivalence ratio oscillations

2005 ◽  
Vol 140 (1-2) ◽  
pp. 116-129 ◽  
Author(s):  
Ju Hyeong Cho ◽  
Tim Lieuwen
Keyword(s):  
Equivalence Ratio ◽  
Premixed Flame ◽  
Flame Response

scholarly journals Premixed flame response to unsteady strain rate and curvature

1997 ◽  
Vol 110 (1-2) ◽  
pp. 92-112 ◽  
Author(s):  
Habib N. Najm ◽  
Peter S. Wyckoff
Keyword(s):  
Strain Rate ◽  
Premixed Flame ◽  
Flame Response

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.

Sign in / Sign up

Export Citation Format

Share Document