scholarly journals The influence of diffusion on flame propagation

1949 ◽  
Vol 196 (1044) ◽  
pp. 105-113 ◽  
Keyword(s):  
Reaction Zone ◽  
Flame Propagation ◽  
Exothermic Reaction ◽  
Flame Speed ◽  
Heating Zone ◽  
Hydrogen Atoms ◽  
Flame Thickness ◽  
Thermal Mechanism ◽  
Free Hydrogen ◽  
Experimental Values

In stationary pre-mixed flames of hydrocarbons at pressures between 1 atm. and a few mm. the thickness of the reaction zone varies approximately inversely with the pressure, and the flame speed is independent of pressure. Marked exothermic reaction begins around 700 to 800° C, and the time of passage through the pre-heating zone is very much less than the induction period for ignition at this temperature; a purely thermal mechanism of flame propagation is inadequate. The thickness of the reaction zone is related to the distance which free hydrogen atoms may diffuse against the gas stream. Calculations are made on the relation between rates of diffusion, flame thickness and flame speed, and are compared with experimental values. It appears that diffusion of free atoms or radicals can account for flame propagation satisfactorily; the correct relation that the flame speed is nearly independent of pressure is obtained.

The theory of flame phenomena with a chain reaction

1956 ◽  
Vol 249 (957) ◽  
pp. 1-25 ◽  
Keyword(s):  
Reaction Zone ◽  
Flame Propagation ◽  
Reaction Rate ◽  
Laminar Flame ◽  
Combustible Mixture ◽  
Flame Speed ◽  
Spontaneous Ignition ◽  
Chain Reaction ◽  
Hydrazine Decomposition ◽  
A Chain

For single-step reactions there is a unique relation between reaction rate and reactedness for a given combustible mixture at a specified pressure and initial temperature. This paper examines whether the relation is still unique when chain reactions are present, by considering three types of flame—spontaneous ignition, laminar-flame propagation, and the homogeneous steady-flow reaction zone—with a chain-reaction scheme proposed by Adams & Stocks for the decomposition of hydrazine. It is found that the relation is not unique but that similarities exist between the relation for laminar-flame propagation and the relation for the homogeneous reaction zone. Incidentally, a general method of calculating laminar-flame speeds with reaction schemes of arbitrary complexity is presented. When applied to the hydrazine decomposition flame the predictions of the theory are in fair agreement with experimental results. In particular, the variation of flame speed with temperature is correctly predicted. It is shown that the use of the Karman-Penner 'steady-state assumption' would lead to an overestimate of the flame speed. Consideration of the changes which would result if the chain reaction should branch shows that there would once again tend to be a unique reaction rate versus reactedness relation, and that the laminar-flame speed would be increased by a factor of about three for the hottest flame considered but by larger factors for cooler flames.

Optical and Numerical Investigations of Flame Propagation in a Heavy Duty Spark Ignited Natural Gas Engine

2021 ◽  
Author(s):  
Jinlong Liu ◽  
Christopher Ulishney ◽  
Cosmin E. Dumitrescu
Keyword(s):  
Natural Gas ◽  
Flame Propagation ◽  
Turbulence Intensity ◽  
Laminar Flame ◽  
Combustion Process ◽  
Combustion Efficiency ◽  
Flame Speed ◽  
Laminar Flame Speed ◽  
Heavy Duty ◽  
Piston Chamber

Abstract Increasing the natural gas (NG) use in heavy-duty engines is beneficial for reducing greenhouse-gas emissions from power generation and transportation. However, converting compression ignition (CI) engines to NG spark ignition operation can increase methane emissions without expensive aftertreatment, thereby defeating the purpose of utilizing a low carbon fuel. The widely accepted explanation for the low combustion efficiency in such retrofitted engines is the lower laminar flame speed of natural gas. In addition, diesel engine’s larger bowl size compared to the traditional gasoline engines increases the flame travel length inside the chamber and extends the combustion duration. However, optical measurements performed in this study suggested that a fast-propagating flame was developed inside the cylinder even at extremely lean operation. This was supported by a three-dimensional numerical simulation, which indicated that the squish region of the bowl-in-piston chamber generated a high turbulence intensity inside the bowl. However, the flame propagation experienced a sudden 2.25x reduction in speed when transiting from the bowl to the squish region. Such a phenomenon was caused by the large decrease in the turbulence intensity inside the squish region during the combustion process. Moreover, the squish volume trapped an important fuel fraction, and it is this fraction that experienced a slow and inefficient burning process during the expansion stroke. This resulted in increased methane emissions and reduced combustion efficiency. Overall, it was the specifics of the combustion process inside a bowl-in-piston chamber not the methane’s slow laminar flame speed that contributed to the low methane combustion efficiency for the retrofitted engine. The results suggest that optimizing the chamber shape is paramount to boost engine efficiency and decrease its emissions.

Photometric investigations of alkali metals in hydrogen flame gases - II. The study of excess concentrations of hydrogen atoms in burnt gas mixtures

1956 ◽  
Vol 235 (1200) ◽  
pp. 89-106 ◽  
Keyword(s):  
Hydroxyl Radicals ◽  
Alkali Metals ◽  
Equilibrium Constants ◽  
The Other ◽  
Large Excess ◽  
Hydrogen Atoms ◽  
Free Hydrogen ◽  
Hydrogen Flame ◽  
Photometric Measurements ◽  
Oxygen Flame

Photometric measurements on alkali metals in hydrogen-oxygen flame gases, diluted with various proportions of nitrogen, are interpreted as giving a measure of the concentration of free hydrogen atoms, which persist in these gases for several milliseconds after primary combustion. These concentrations are well in excess of those expected from thermodynamic equilibrium, especially towards the lower end of the range of temperatures studied (2400 to 1600°K). Two kinds of measurement have been made. (i) Comparison of the intensities of the Na D lines and the Li resonance doublets, for equal traces of the two elements present in the gases. The amount of free lithium is modified by the balanced process, Li + H 2 O ⇌ LiOH + H, whereas corresponding reactions for sodium are negligible. Using estimated equilibrium constants for these reactions, [H] can be obtained. (ii) Measurements of the change in intensity of the Na D lines when 0∙01 to 0∙5% of chlorine or its compounds (a large excess over the sodium) are added to the flame gases. NaCl is considered to be formed by the balanced reactions Na + HCl ⇌ NaCl + H. The concentration of HCl, the most important chlorine compound in the hydrogen-rich flame gases, may be obtained from the total chlorine added. Using estimated equilibrium constants for the above reaction, [H] can again be obtained. The agreement between the [H] values obtained by these two independent methods is good. The decrease of [H] with height in the gases is consistent with ternary recombination towards full equilibrium. A general discussion of excess radical concentrations in hydrogen flame gases is given, for hydroxyl radicals and oxygen atoms as well as hydrogen atoms. The conclusions reached are supported by experimental evidence. A further discussion of the way in which the amounts of chlorine used in the experiments may affect the other radical concentrations is given.

Hydrogen–Air–Steam Deflagration Experiment Simulated Using Different Turbulent Flame-Speed Closure Models

2018 ◽  
Vol 4 (3) ◽  
Author(s):  
Holler Tadej ◽  
Ed M. J. Komen ◽  
Kljenak Ivo
Keyword(s):  
Flame Propagation ◽  
Nuclear Power ◽  
Large Scale ◽  
Turbulent Flame ◽  
Flame Speed ◽  
Energy Output ◽  
Combustion Model ◽  
Turbulent Flame Speed ◽  
Modeling Approach ◽  
Combustion Models

The paper presents the computational fluid dynamics (CFD) combustion modeling approach based on two combustion models. This modeling approach was applied to a hydrogen deflagration experiment conducted in a large-scale confined experimental vessel. The used combustion models were Zimont's turbulent flame-speed closure (TFC) model and Lipatnikov's flame-speed closure (FSC) model. The conducted simulations are aimed to aid identifying and evaluating the potential hydrogen risks in nuclear power plant (NPP) containment. The simulation results show good agreement with experiment for axial flame propagation using the Lipatnikov combustion model. However, substantial overprediction in radial flame propagation is observed using both combustion models, which consequently results also in overprediction of the pressure increase rate and overall combustion energy output. As assumed for a large-scale experiment without any turbulence inducing structures, the combustion took place in low-turbulence regimes, where the Lipatnikov combustion model, due to its inclusion of quasi-laminar source term, has advantage over the Zimont model.

Effects of H2-Enrichment on the Propagation Characteristics of CH4-Air Flames

2006 ◽  
Author(s):  
Alejandro M. Briones ◽  
Suresh K. Aggarwal ◽  
Vishwanath R. Katta
Keyword(s):  
Flame Propagation ◽  
Laminar Flame ◽  
Mixture Fraction ◽  
Leading Edge ◽  
Tangential Component ◽  
Flame Speed ◽  
Axial Position ◽  
Stoichiometric Mixture ◽  
Jet Mixing ◽  
Flame Curvature

The propagation of H2-enriched CH4-air triple flames in a nonpremixed jet is investigated numerically. The flames are ignited in a nonuniform jet-mixing layer downstream of the burner. A comprehensive, time-dependent computational model is used to simulate the transient ignition and flame propagation phenomena. The model employs a detailed description of methane-air chemistry and transport properties. Following ignition a well-defined flame is formed that propagates upstream towards the burner along the stoichiometric mixture fraction line. As the flame propagates upstream, the flame speed, which is defined as the normal flamefront velocity at the leading edge with respect to the local gas velocity, increases above or decreases below to the corresponding unstretched laminar flame speed of the stoichiometric planar premixed flame. Although the flame curvature varies as a function of axial position, the flame curvature remains nearly constant for a given flame. As hydrogen is added to the fuel stream the flame curvature during flame propagation remains nearly constant. During the flame propagation process, the hydrodynamic stretch dominates over the curvature-induced stretch. Hydrogen increases the heat release and the component of the velocity perpendicular to the flame increases across the surface, whereas the tangential component remains unchanged. This jump in the perpendicular velocity component bends the velocity vector toward the stoichiometric mixture fraction line. This redirection of the flow is accommodated by the divergence of the streamlines ahead of the flame, resulting in the decrease of the velocity and increase in the hydrodynamic stretch.

Experimental determination of flame speed and flame stretch using an optically accessible, spark-ignition engine

2019 ◽  
pp. 146808741987771 ◽  
Author(s):  
Behdad Afkhami ◽  
Yanyu Wang ◽  
Scott A Miers ◽  
Jeffrey D Naber
Keyword(s):  
Flame Front ◽  
Flame Propagation ◽  
Engine Speed ◽  
Spark Ignition ◽  
Flame Speed ◽  
Dominant Factor ◽  
Lean Mixture ◽  
Flame Stretch ◽  
Flame Development ◽  
The Rich

The current research experimentally studied flame speed and stretch under engine in-cylinder conditions. A direct-injection, spark-ignition, and optically accessible engine was utilized to image the flame propagation, and E10 was selected as the fuel. Also, three fuel–air mixtures (stoichiometric, lean, and rich) were examined. The flame front was located by processing high-speed images. This study introduces a novel approach for calculation of equivalent spherical flame radius for analysis of flame speed and stretch. Flame front propagation analysis showed that during the flame propagation period, the stretch decreased until the flame front touched the piston surface. This was a common trend for stoichiometric, lean, and rich mixtures, which occurred because the flame radius was the dominant factor in the stretch calculation. In addition, the rich fuel–air mixture showed a lower flame stretch compared to stoichiometric or lean mixture. This was the result of a lower Markstein number for the rich fuel–air mixture. To study the sensitivity of different fuel–air mixtures to the flame stretch, the trajectory of the flame centroid was tracked until the flame front touched the piston surface. The results showed that the end centroid for the lean mixture deviated from the start point more than those of the rich and stoichiometric mixtures because the lean mixture had a higher flame stretch and lower flame speed. Furthermore, comparing the flame stretch at three different engine speeds revealed that increasing the engine speed increased the flame stretch, especially during the early flame development period. According to previous studies which discussed flame stretch as a flame extinguishment mechanism, the probability of flame extinction is higher when the engine speed is higher. Also, uncertainty analysis was conducted to determine the effect of camera setting on the flame stretch. Results showed that a maximum relative uncertainty of 4.5% occurred during the early flame development.

Flame Propagation in a Narrow Closed Channel: Effects of Aspect Ratios, Blockage Ratio, and Mixture Reactivity on Flame Speed and Pressure Dynamics

2019 ◽  
Vol 192 (6) ◽  
pp. 986-996
Author(s):  
David Escofet-Martin ◽  
Yu-Chien Chien ◽  
Derek Dunn-Rankin ◽  
Edyta Dzieminska ◽  
A. Koichi Hayashi ◽  
...  
Keyword(s):  
Flame Propagation ◽  
Flame Speed ◽  
Blockage Ratio ◽  
Closed Channel ◽  
Aspect Ratios ◽  
Channel Effects ◽  
Pressure Dynamics

scholarly journals The effects of hydrodynamic stretch on the flame propagation enhancement of ethylene by addition of ozone

2015 ◽  
Vol 373 (2048) ◽  
pp. 20140339 ◽  
Author(s):  
Matthew Pinchak ◽  
Timothy Ombrello ◽  
Campbell Carter ◽  
Ephraim Gutmark ◽  
Viswanath Katta
Keyword(s):  
Heat Release ◽  
Flame Propagation ◽  
Atmospheric Pressure ◽  
Flame Speed ◽  
Two Dimensional ◽  
Axial Stretch ◽  
Exit Velocity ◽  
One Dimensional ◽  
Burner Exit ◽  
Steady Laminar

The effect of O 3 on C 2 H 4 /synthetic-air flame propagation at sub-atmospheric pressure was investigated through detailed experiments and simulations. A Hencken burner provided an ideal platform to interrogate flame speed enhancement, producing a steady, laminar, nearly one-dimensional, minimally curved, weakly stretched, and nearly adiabatic flame that could be accurately compared with simulations. The experimental results showed enhancement of up to 7.5% in flame speed for 11 000 ppm of O 3 at stoichiometric conditions. Significantly, the axial stretch rate was also found to affect enhancement. Comparison of the flames for a given burner exit velocity resulted in the enhancement increasing almost 9% over the range of axial stretch rates that was investigated. Two-dimensional simulations agreed well with the experiments in terms of flame speed, as well as the trends of enhancement. Rate of production analysis showed that the primary pathway for O 3 consumption was through reaction with H, leading to early heat release and increased production of OH. Higher flame stretch rates resulted in increased flux through the H+O 3 reaction to provide increased enhancement, due to the thinning of the flame that accompanies higher stretch, and thus results in decreased distance for the H to diffuse before reacting with O 3 .

Hydrogen atoms in the presence of a homogeneous magnetic field: a variational approach

1978 ◽  
Vol 56 (12) ◽  
pp. 1545-1548 ◽  
Author(s):  
H. S. Brandi ◽  
Belita Koiller
Keyword(s):  
Magnetic Field ◽  
Hydrogen Atom ◽  
Excited States ◽  
Three Dimensional ◽  
Homogeneous Magnetic Field ◽  
Basis Functions ◽  
Variational Parameter ◽  
Hydrogen Atoms ◽  
Wide Range ◽  
Free Hydrogen

We propose a variational scheme to obtain the spectrum of the hydrogen atom in the presence of an external homogeneous magnetic field. We use two different sets of basis functions to diagonalize the Hamiltonian describing the system, namely, the eigenfunctions of the free hydrogen atom and of the three-dimensional harmonic oscillator, both having their radial coordinates properly scaled by a variational parameter. Because of its characteristics, the present approach is suited to describe the ground state as well as an infinite number of excited states for a wide range of magnetic field strengths.

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