scholarly journals Effect of the equivalence ratio, Damköhler number, Lewis number and heat release on the stability of laminar premixed flames in microchannels

2014 ◽  
Vol 161 (5) ◽  
pp. 1282-1293 ◽  
Author(s):  
Mario Sánchez–Sanz ◽  
Daniel Fernández-Galisteo ◽  
Vadim N. Kurdyumov
Keyword(s):  
Heat Release ◽  
Equivalence Ratio ◽  
Premixed Flames ◽  
Lewis Number ◽  
Damkohler Number ◽  
Damköhler Number ◽  
The Stability

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.

A General Study of Counterflow Diffusion Flames for Supercritical CO2 Combustion

2019 ◽  
Vol 141 (12) ◽  
Author(s):  
K. R. V. Manikantachari ◽  
Scott Martin ◽  
Ramees K. Rahman ◽  
Carlos Velez ◽  
Subith Vasu
Keyword(s):  
Prandtl Number ◽  
Supercritical Co2 ◽  
Diffusion Flames ◽  
Lewis Number ◽  
Dilution Effect ◽  
Scalar Dissipation ◽  
Damkohler Number ◽  
Nonpremixed Combustion ◽  
Flame Thickness ◽  
Damköhler Number

Abstract A counterflow diffusion flame for supercritical CO2 combustion is investigated at various CO2 dilution levels and pressures by accounting for real gas effects into both thermal and transport properties. The UCF 1.1 24-species mechanism is used to account the chemistry. The nature of important nonpremixed combustion characteristics such as Prandtl number, thermal diffusivity, Lewis number, stoichiometric scalar dissipation rate, flame thickness, and Damköhler number are investigated with respect to CO2 dilution and pressure. The results show that the aforementioned parameters are influenced by both dilution and pressure; the dilution effect is more dominant. Further, the result shows that Prandtl number increases with CO2 dilution and at 90% CO2 dilution, the difference between the Prandtl number of the inlet jets and the flame is minimal. Also, the common assumption of unity Lewis number in the theory and modeling of nonpremixed combustion does not hold reasonable for sCO2 applications due to large difference of Lewis number across the flame and the Lewis number on the flame drop significantly with an increase in the CO2 dilution. An interesting relation between Lewis number and CO2 dilution is observed. The Lewis number of species drops by 15% when increasing the CO2 dilution by 30%. Increasing the CO2 dilution increases both the flow and chemical timescales; however, chemical timescale increases faster than the flow timescales. The magnitudes of the Damköhler number signify the need to consider finite rate chemistry for sCO2 applications. Further, the Damköhler numbers at 90% sCO2 dilution are very small; hence, laminar flamelet assumptions in turbulent combustion simulations are not physically correct for this application. Also, it is observed that the Damköhler number drops nonlinearly with increasing CO2 dilution in the oxidizer stream. This is a very important observation for the operation of sCO2 combustors. Further, the flame thickness is found to increase with CO2 dilution and reduce with pressure.

Effect of Damkohler Number and Non-Unity Lewis Number on a Laminar Diffusion Flame

2003 ◽  
Author(s):  
Bassem H. Ramadan
Keyword(s):  
Diffusion Flame ◽  
Rectangular Channel ◽  
Flame Structure ◽  
Lewis Number ◽  
Damkohler Number ◽  
Laminar Diffusion Flame ◽  
Damköhler Number ◽  
Laminar Diffusion ◽  
Adi Schemes ◽  
Steady Laminar

The effect of the Damkohler number (Da) and non-unity Lewis number on a two-dimensional, steady, laminar diffusion flame anchored by a dividing plate in a rectangular channel was considered. The governing equations were solved numerically, using the SIMPLE and ADI schemes. The results for non-unity Lewis number were compared with those for a unity Lewis number, and Da a was also varied in order to determine their effect on the flame structure. The results show that an increase in the Da causes the flame to exist closer to the trailing edge of the divider and to increase the reactivity. A non-unity Lewis number creates a non-symmetrical flame by forcing the flame to exist on the fuel side.

Lewis-number effects on edge-flame propagation

2002 ◽  
Vol 458 ◽  
pp. 219-228 ◽  
Author(s):  
VEDHA NAYAGAM ◽  
F. A. WILLIAMS
Keyword(s):  
Activation Energy ◽  
Propagation Velocity ◽  
Steady Motion ◽  
Lewis Number ◽  
Diffusion Time ◽  
Damkohler Number ◽  
Dimensional Model ◽  
One Dimensional ◽  
Damköhler Number ◽  
Propagation Velocities

Activation-energy asymptotics is employed to explore effects of the Lewis number, the ratio of thermal to fuel diffusivity, in a one-dimensional model of steady motion of edges of reaction sheets. The propagation velocity of the edge is obtained as a function of the relevant Damköhler number, the ratio of the diffusion time to the chemical time. The results show how Lewis numbers different from unity can increase or decrease propagation velocities. Increasing the Lewis number increases the propagation velocity at large Damköhler numbers and decreases it at small Damköhler numbers. Advancing-edge and retreating-edge solutions are shown to exist simultaneously, at the same Damköhler number, if the Lewis number is sufficiently large. This multiplicity of solutions has a bearing on potential edge-flame configurations in non-uniform flows.

Effect of Equivalence Ratio and Burner Geometry on the Characteristics of Laminar Premixed Flames at Moderate Coflow

2004 ◽  
Author(s):  
P. Hariharan ◽  
S. R. Gollahalli
Keyword(s):  
Aspect Ratio ◽  
Equivalence Ratio ◽  
Premixed Flames ◽  
Combustion Efficiency ◽  
Flame Height ◽  
No Production ◽  
Air Velocity ◽  
Burner Flame ◽  
Jet Velocity ◽  
The Stability

The importance of studying laminar premixed flames lies in applications such as gas ranges and ovens, heating appliances and Bunsen burners. With the current demand for large amounts of economical, clean power, there is a need for research in increasing the combustion efficiency. Laminar premixed Propane/Hydrogen/Air flames with 3 m/s coflow and without coflow, with a variation of jet equivalence ratio (JEQ) from 0.5 to 4 for 20 m/s jet velocity, have been studied experimentally to determine the interactions of burner geometry of premixed flames and coflow. Two different burner geometries (circular burner, and 3:1 aspect ratio (AR) burners) were used in the experiments. The stability tests indicated that for 20 m/s jet velocity both at quiescent and coflow conditions the circular burner was more stable than the 3:1AR elliptical burner. Flame height studies indicated that circular burner flames were taller than the 3:1AR elliptical burner flames. However, there was a reduction in flame height when coflow air velocity of 3 m/s was introduced. Temperature profile indicated a higher peak temperature for circular burners followed by elliptical burner, both at quiescent and coflow conditions. The introduction of moderate coflow showed a decrease in NO production rate. In order to explain the structure of the flame in detail and various mechanisms that lead to the explanation of global flame characteristics, inflame concentration measurements were taken in near burner (25% of flame height), mid burner (50% of flame height) and far burner (75% of flame height) regions of the flame.

Influence of the Combustor Aerodynamics on Combustion Instabilities From Equivalence Ratio Fluctuations

2002 ◽  
Vol 125 (1) ◽  
pp. 11-19 ◽  
Author(s):  
T. Sattelmayer
Keyword(s):  
Heat Release ◽  
Equivalence Ratio ◽  
Spatial Dispersion ◽  
Combustion System ◽  
Combustion Instabilities ◽  
Additional Mechanism ◽  
Wide Range ◽  
Unrealistic Assumption ◽  
Unstable Case ◽  
The Stability

Gas turbine combustors are often susceptible to self-excited oscillations, which lead to unacceptable levels of pressure, velocity, and heat release fluctuations. Although instabilities can occur in systems with locally constant equivalence ratio, it is very important to take into account the influence of equivalence ratio fluctuations, which are generated in the fuel air mixer in the unstable case. These fluctuations are convected into the flame and lead to an additional mechanism for the generation of heat release fluctuations. Moreover, entropy waves are produced in the flame, which travel through the combustor and generate additional pressure waves during the acceleration of the flow at the combustor exit. To date, available theories use the physically unrealistic assumption that the equivalence ratio waves as well as the entropy waves are convected downstream without any spatial dispersion due to the combustor aerodynamics. An analytical approach is presented, which allows us to take the spatial dispersion into consideration. For that purpose, the response of the burner and the combustor to an equivalence ratio impulse or an entropy impulse is calculated using the Laplace transformation and a more general transfer function for harmonic waves is derived. The obtained expression has three parameters, which represent the influence of the burner or the combustor aerodynamics, respectively. This equation can be used in numerical codes, which represent the combustion system through a network of acoustic multiports, if the equivalence ratio and the entropy are added to the vector of variables considered. The parameters required for the dynamic combustor model can be deduced from a detailed CFD analysis of the combustor flow in case of the application of the theory to a particular combustor design. As an example, a simple model combustor is used to demonstrate the application of the theory. It is highlighted how the spatial dispersion of the equivalence ratio and entropy fluctuations can be included in the stability analysis. The calculated examples reveal that the influence of both variables on the generation of instabilities is highly overpredicted if the spatial dispersion is not taken into account. Furthermore, it can be deduced from the study that burner and combustor designs with a wide range of convective time scales have advantages with respect to the stability of the combustor.

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