Flame speed

Thesis by Alexander Machold, 2011 (Graz Technical University / AVL)
192 pages with auxiliary appendix

german keywords: Wankelmotor / Kreiskolbenmotor
English Keywords: Wankel Engine / Wankel Rotary Engine / Rotary Piston Engine / Rotary Combustion Engine

> Content » Inhaltsverzeichnis
> 1  Formelzeichen  4
  1.1  Roemische Zeichen  4
  1.2  Griechische Zeichen  5
  1.3  Allgemeine Abkuerzungen  6
> 2  Vorwort  7
> 3  Kurzfassung  9
> 4  Einleitung  10
> 5  Die Entwicklung des Rotationskolbenmotors  10
  5.1  Aufgabe der Kraftmaschinen  11
  5.2  Einteilung nach Bauart  11
  5.3  Chronologische Entwicklung (3)  13
> 6  Der Drehkolbenmotor  18
> 7  Funktionsweise  21
  7.1  Die Arbeitstakte des Wankelmotor  23
> 8  Vor und Nachteile des Wankelmotors  26
  8.1  Vorteile  26
  8.2  Nachteile  28
> 9  Der Kreiskolbenmotor (KKM)  30
  9.1  Trochoide  30
  9.2  Kolben  34
  9.3  Exzenterwelle  36
  9.4  Seitenplatten  37
> 10  Motorische Kenngroessen  38
  10.1  Gemisch Heizwert  41
> 11  Der Basismotor  44
  11.1  Basisdaten Wankelmotor GEN1A2  47
  11.1.1  Geometrische Daten  47
  11.1.2  Technische Daten  47
> 12  Verbrennungsanalyse und Bewertung im Kreiskolbenmotor  49
  12.1  Software AVL FIRE  49
  12.2  Software AVL BOOST  50
  12.3  Gittergenerierung  51
  12.4  Gittergenerierung der Kammer  52
  12.4.1  Motivation  52
  12.4.2  Hintergrund zu bewegten Gittern in FIRE  53
  12.4.3  Gittererstellung  54
  12.5  Erstellung des Einlass- und Auslasskanal Gitter  56
  12.5.1  Einlasskanal  56
  12.5.2  Auslasskanal  58
  12.5.3  Zuendkerzen  60
> 13  Erstellung der Berechnungsdomaene  61
  13.1  Ein-Kammer Domaene  61
  13.2  Drei-Kammer Domaene  61
  13.3  Verbindung zw. stehendem und bewegtem Gitter (10)  62
> 14  Definition der Berechnungsvarianten  64
> 15  Basis Variante, FIRE Einstellungen  65
  15.1  1D BOOST Randbedingung  65
  15.1.1  Einlass  65
  15.1.2  Auslass  66
  15.1.3  BOOST Diagramme  66
  15.1.4  uebersicht ueber das Berechnungssetup  67
  15.2  FIRE Solver  70
  15.2.1  k-e Turbulenzmodell  70
  15.2.2  Standard Wand Funktion  70
  15.2.3  Verbrennungsmodell ECFM (11)  71
  15.3  Verwendete Hardware  71
> 16  Ergebnisse der Basisvariante  73
  16.1  Bewertung von Druck und Temperatur  73
> 17  Flammenausbreitung  76
  17.1  Geometrische Lage der Schusskanalzuendkerze  78
> 18  Varianten Untersuchungen  80
  18.1  Zuendwinkel 6° v. OT Variantenvergleich  82
  18.2  Zuendwinkel 6° & 11° v. OT  89
  18.3  Zuendwinkel 6° & 16° v. OT  90
> 19  Drei-Kammer Modell  91
  19.1  Ergebnisse  91
> 20  Variation der Brennraummulde  99
  20.1  Fazit der Untersuchungen an den Rotormulden  105
> 21  Zusammenfassung und Ausblick  106
> 22  Abbildungsverzeichnis  108
> 23  Literaturverzeichnis  111
> 24  Anhang A Flammenfront der Basisrotormulde  112
  25  Anhang B Flammenfront der Rotormulde Variante 1  116
  26  Anhang C Flammenfront der Rotormulde Variante 2  120
  27  Anhang D Geschwindigkeitsfeld im 3 Kammermodell  124
  28  Anhang E Detail Geschwindigkeitsfeld Zuendkerzen  129
  29  Anhang F Druckfeld in Brennkammer  132

> auxiliary appendix ...
  » AT 509.555, filed 2010-11-18 (Side Seals)  141
  » Translation of AT 509.555 (English, Google)  152
  » AT 512.369, filed 2011-08-04 (Optimized Recess Shapes)  154
  » Translation of AT 512.369 (English, Google)  168
  » AT 512.319, filed 2011-12-19 (Optimized Ignition Channel)  173
  » US 2014.0.202.419 » English Version of AT 512.319  184

Nitrous oxide is a substance composed of two molecules of nitrogen linked to one of oxygen. Due to the high concentration of O2 in its composition in relation to air, the gas has a great oxidizing power when
exposed to high temperature. Therefore, it has a great applicability in the military area in WWII airplanes and today in many others issues. The goals of this work are: to discuss the thermodynamic properties of N2O, the reactions that occur in the combustion chamber and relate with the elevation of performance. The
development of methods for increasing power density becomes essential for the improvement of thermal
motors. A conventional motor (1.6 SIGMA Ti-VCT) was used with the installation of an N2O injection system to confirm in practice the increase in performance through graphics. The obtained result presents the increase of the expressive performance in the whole range of the engine with a margin of 63,7hp of power and 100Nm of torque without detonation or overheating. The use of N2O was very effective in the accomplishment of the goals of this study, however future work will need to be carried out to measure component wear, fuel economy and also simplify the N2O system.

Results of experiments and data analysis on turbulent flame propagation in obstructed channels are presented. The data cover a wide range of mixtures: H2/air, H2/air/steam (from lean to rich) at normal and elevated initial temperatures (from 298 to 650 K) and pressures (from 1 to 3 bar); and stoichiometric H2/O2 mixtures diluted with N2, Ar, He and CO2 at normal initial conditions. The dataset chosen also covers a wide range of scales exceeding two orders of magnitude. It is shown that basic flame parameters, such as mixture expansion ratio σ, Zeldovich number β and Lewis number Le, can be used to estimate a priori a potential for effective flame acceleration for a given mixture. Critical conditions for effective flame acceleration are suggested in the form of correlations of critical expansion ratio σ∗ versus dimensionless effective activation energy. On this basis, limits for effective flame acceleration for hydrogen combustibles can be estimated. Uncertainties in determination of critical σ∗ values are discussed.

Thesis by Guido A. Danieli, 1976 (MIT)
159 pages
german keywords: Wankelmotor / Kreiskolbenmotor
English Keywords: Wankel Engine / Wankel Rotary Engine / Rotary Piston Engine / Rotary Combustion Engine
based on Mazda 12A engine

> Table of Contents
  » Abstract  2
  » Acknowledgments  3
  » Table of Contents  5
  » Introduction  7
  » Chapter 1 - Theoretical Analysis  9
      » 1.1 Thermodynamic Model of the System  9
      » 1.2 Combustion Model  20
      » 1.3 Flame Propagation Model  20
      » 1.4 Heat Transfer Model  31
      » 1.5 Quenching Mechanism  36
      » 1.6 Leakage + Gas Exchange Mechanisms  37
      » 1.7 Geometrical Model  39
  » Chapter 2 - Experimental Results  42
      2.1 Experimental Apparatus + Techniques  42
      2.2 Results + their Analysis  46
  » Chapter 3 - Comparison between Theory + Experiments  52
      » 3.1 Motoring Conditions  53
      » 3.2 Firing Conditions  58
      » 3.3 Some parametric Studies on Engine Performance  72
  » Conclusions  76
  » References  80
  » Tables  82
  » Appendix A - Formulation of Momentum Equation to compute Pressure Differences within the Chamber of a Wankel Engine  95
  » Appendix B - Geometrical Notes  98
  » Appendix C - Mixing Gases at constant Pressure (Heat Transfer Model)  102
  » Appendix D - Simple Case of Flame Propagation  103
  » Appendix E - Determination of the Angle alpha which determines the Housing Coordinates of the Intercept between the perpendicular to the Rotor Surface from a given Point + the Housing  106
  » Appendix F - Possible Sources of Error in the Energy Balance  108
  » Appendix G - Meaning of pV during Combustion  113
  » Figures  115
  » Biographical Notes on Guido A. Danieli  159

Atomization, vaporization, turbulent mixing, and the chemical kinetics of a liquid fuel impact the combustion efficiency, stability, and emissions within IC engines. When fuels with varying physical properties are used in the same fuel injection system, they can exhibit very different atomization, represented by droplet size (SMD) and spray droplet distribution (SDD), which can then potentially influence the flame stability. In the current paper, the effect of the atomization process on flame stability is investigated by using a single nozzle size with three different hydrocarbon fuels, i.e., n-heptane, n-dodecane, and toluene. This study also investigates the flame stability for the same fuels using a unique approach in which nozzle sizes are selectively varied for each fuel to control the atomization process and minimize differences in the spray characteristics. The results show that the atomization process plays a major role in the flame liftoff heights and blowout limits. n-Dodecane, which produces the largest droplets and exhibits the lowest volatility, experiences highest flame liftoff heights and the greatest resistance to flame blowout compared to the other tested fuels. When the atomization process is controlled, more similar liftoff heights and blowout limits for all fuels were observed. The differences in liftoff height was shown to be a result of two-phase (spray) flame speed (i.e., pre-vaporized fuel reactivity (SL), and time scales associated with droplet evaporation). Differences in blowout was shown to be largely tied to fuel volatility, droplet size (i.e., the liquid loading into the flame), and the heat of combustion.

Carbon dioxide emissions are considered one of the most important factors that cause global warming. For this reason, it is suggested to burn fuels using an oxidizer mixture consisting of oxygen and carbon dioxide rather than the conventional burning in air. This combustion technique ensures that the resulting products consist mainly of carbon dioxide and water vapor which facilitates capturing and sequestration of the carbon dioxide gas and thus eliminates its release into the atmosphere. One of the main challenges
that faces this technique is the need to ensure the flame stability in a wide range of operating conditions because of  he adverse effects of adding carbon dioxide to the oxidizer mixture on chemical kinetics. This study experimentally examines some of the conditions that must be met to ensure stable flame operation when burning compressed natural gas in a controlled mixture of oxygen and carbon dioxide stabilized over a perforated-plate burner at different equivalence ratios and at different oxygen fractions. Two sets of experiments are carried-out in this study. In the first set, the study identifies the range of equivalence ratios at constant oxygen fractions necessary for stable flame operation. In the second set, the study identifies the range of oxygen fraction at constant equivalence ratios necessary for stable flame operation. The study also documents the visual flame length and color and identifies the extinction mechanism outside the flammability limits for both sets. The first set of experiments reveals that, in the range of Reynolds number considered and at an oxygen fraction of  6%, stable flame operation is observed with flammability limits that are approximately 80% of the corresponding air-fuel combustion under the same Reynolds number. This ratio decreases as the oxygen fraction decreases. The visual flame length is longer than that of air-fuel combustion by approximately 15%. Extinction occurs by blow-off outside the upper and lower flammability limits. The second set of experiments reveals that for the burner and the range of  Reynolds numbers considered, operation below an oxygen fraction of 29% is not possible at all equivalence ratios considered (namely 0.7, 0.85, 1 and 1.15) while the upper flammability limit is a function of the equivalence ratio. Extinction occurs by blow-off below the lower flammability limit, and by flash-back above the upper flammability limit.
Keywords: Oxy-fuel combustion, CNG/O2/CO2 combustion, flammability limit, extinction mechanism, visual flame appearance and perforated-plate burner.

Experimental data and numerical simulations on flame acceleration, shock-flame interaction and deflagration-to-detonation transition mechanism for stoichiometric hydrogen-oxygen mixtures at reduced pressure in relatively short tubes have been analysed. It was shown that the detonation occurs as a result of multiple reflections of precursor shock wave and its interaction with the flame. Adiabatic compression and heating of unreacted gas a front of the flame together with flame surface increase due to the Richtmyer-Meshkov instability provide preconditioning of the DDT process. Several times higher pressure, temperature and reaction rate within a preconditioning zone leads to significant decrease of run-up-distance to DDT in relatively short tubes. Results of the work will provide detailed information on multiple shock - flame interactions leading to the DDT process for numerical code validations.

This study investigates the impact of pressure and dilution on combustion behavior of of C2H6/air/N2 mixtures within
an externally heated micro-tube using both experimental and analytical techniques. Experiments are conducted for
pressures up to 10 bar at multiple dilution levels and document three flame regimes, i.e. weak flames, FREI and normal flames. Characteristic temperatures for flame stabilization, as well as and ignition and extinction are obtained
by applying a novel thin filament pyrometry method (TFP), which uses a thin silicon carbide (SiC) filament that enables temperature measurements for both reacting and non-reacting cases. Results show a decrease of characteristic
temperatures with increasing pressure, which can be which can be partially compensated by a decrease of O2 content. Further, regime transitions occur at lower velocity as the pressure increases, whereas weak flame positions are
insensitive to dilution at a given pressure. Predictions from the analytical model compare favorably with experimental
results. The most prominent finding is that as dilution increases, the transition from weak flame to FREI becomes less
abrupt and is completely lost for high dilution levels (i.e. low O2 content). Both experimental and analytical results
indicate that the unstable FREI combustion regime does not exist at high dilution levels, i.e. weak flame and normal
flame regimes are no longer clearly separated. As FREI are interpreted as a cyclic transition from an ignition zone to a
propagating flame, results for highly diluted conditions are interpreted as a combustion regime where classical flame
propagation ceases to exist.

Motivation - Edge detection has been a tricky issue since the 1960s advent of computer science. Edge detection serves two main purposes; for object identification, and manipulation in the realm of robotics, as well as intricate image rendering for other applications such as flame front identification.

The effect of acetone on the laminar flame speed of methane/air mixtures is investigated over a range of stoichiometries at atmospheric pressure and room temperature. The liquid acetone is vaporised and seeded into the methane/air mixture at 5%, 9% and 20% of the total fuel by mole. The experiment is performed using the jet-wall stagnation flame configuration and the particle imaging velocimetry (PIV) technique. Laminar flame speeds are derived by extrapolating the reference flame speed back to zero strain rate. Experimental results are compared to numerically calculated values using a base methane chemical kinetic mechanism (GRI-Mech 3.0) extended with acetone oxidation and pyrolysis reactions from the literature. The experimental results show that acetone addition does not affect the laminar flame speed of methane significantly within the range of concentrations considered, with a stronger effect on the rich range than under fuel-lean conditions, and that the peak laminar flame speed of acetone in air is ∼42.5 cm/s at ϕ = 1.2. Simulation results reveal that the most important reactions determining acetone laminar flame speeds are H + O2 → O + OH, OH + CO → H + CO2, HO2 + CH3 → OH + CH3O and H + O2 + H2O → HO2 + H2O. Comparison of the expected disappearance of acetone relative to methane shows that the former is a good fluorescent marker for the latter.

The stabilizing, amorphous alumina (Al 2 O 3) passivation layer surrounding aluminum (Al) particles participates in reactions that lower barriers to bulk Al oxidation. The behavior has been observed in thermites comprised of nanoscale Al particles (nano-Al) dispersed within a fluoropolymer matrix. Studies reported herein show the oxide passivation shell on nano-Al particles is affected by the polarity and hydrogen bonding properties of the solvent employed for thermite dispersal, resulting in enhanced thermal energy propagation during Al combustion in nano-Al + poly(tetrafluoroethylene) (PTFE) mixtures. Relative to conventional treatments that employ hexane for thermite dispersal, the speed of flame front movement measured in a Bockmon Tube apparatus under steady-state conditions increased more than 2-fold following treatments in acetone or 2-proponal. Differential scanning calorimetry and infrared spectroscopy measurements indicate contact with the polar solvents increases the amount and accessibility of hydroxyl species on the nano-Al oxide shell, which in turn participates in a preignition reaction (PIR) that activates PTFE and likely weakens the Al passivation layer. A molecular-scale mechanism is proposed for the PIR that derives from catalytic reactions of alumina and halocarbon fluorinating reagents. Additionally, infrared spectra show evidence for a greater fraction of disordered, liquid-like hydrogen-bonded water molecules within the alumina layer of nano-Al particles after treatment in the polar solvents studied. The O−H vibrational features suggest solvent treatment may affect the structure of nano-Al surface oxides and PIR kinetics. This study reveals potential strategies for optimizing fuel particle reactivity that include modification of the Al particle passivation shell using polar solvents to promote early preignition exothermic reaction.

Laminar flame speeds of lean premixed H 2 /CO/air mixtures were measured in the counterflow configuration over a wide range of H 2 content at lean conditions. The values were determined by extrapolating the referenced flame speed to zero stretch rate using the non-linear extrapolation method to reduce the systematic error. Detailed calculation of laminar flame speed was also conducted using PREMIX code coupled with three different kinetic models. In general, simulation results agreed well with the experimental data. Both the experimental and calculation results revealed that the laminar flame speeds of lean pre-mixed H 2 /CO/air mixtures increased with H 2 content significantly when H 2 content was small (615%) and gradually when H 2 content was large (>15%).

The hydrogen oxidation chemistry constitutes the foundation of the kinetics of all carbon- and hydrogen-containing fuels. The validation of rate constants of hydrogen-related reactions can be complicated by uncertainties associated with experimental data caused by the high reactivity and diffusivity of hydrogen. In the present investigation accurate experimental data on flame propagation and extinction were determined for premixed and non-premixed hydrogen flames at pressures between p = 1 and 7 atm. The experiments were designed to sensitize the three-body H + O2 + M → HO2 + M reaction, whose rate is subject to notable uncertainty. This was achieved by increasing the pressure and by adding to the reactants H2O and CO2 whose collision efficiencies are high compared to other species. In the present study, directly measured flame properties were compared against computed ones, in order to eliminate uncertainties associated with extrapolations, as is the case for laminar flame speeds. ...

The response of premixed hydrogen/air flames propagating outwardly through moderately inhomogeneous fuel distributions and subjected to stretch and simultaneous pressure and equivalence ratio oscillations was explored computationally using an implicit method (N.A. Malik, 2009a, 2009b). The method is second-order accurate in space and time
and solves the fully compressible Eulerian balance equations coupled to the detailed chemistry and transport properties. The impact of increasing curvature was investigated through
the use of planar, cylindrical, and spherical geometries. Individual chemical species show markedly different responses to the disturbances. Species formed in thin reaction layers
are generally not disturbed except with respect to their peak concentration. By contrast, species with broader profiles are generally significantly affected. For pressure oscillation
frequencies in the range of 200–1000Hz and amplitudes of 1
2% of atmospheric pressure, a spectral response is observed centred around 5 kHz and scaling approximately as x
2
under the persent conditions, thus indicating a coupling between the oscillations and reaction layers. The results appear to be correlated with the disruption of the thermochemical
structure of the flames. Although the flames adjust to the local conditions, some evidence of a time lag or memory effect between fuel consumption and the rate of heat release is
observed. The influence of stretch (curvature) is seen in the reduction in the flame relaxation times sR from 1.1 ms in a planar flame to 0.65 ms in a spherical flame; sR is the time
taken for the system to return to the prevalent mean conditions after a disturbance. Under unsteady random pressure fluctuations, the Markstein number shows large variations,
indicating a breakdown of the Markstein hypothesis.

A simple algebraic model for the Favre averaged scalar dissipation rate, c, in high Damkohler number premixed flames is obtained from its transport equation by balancing the leading order terms. Recently proposed models for the dominant terms in the transport equation are revisited and revised. The algebraic model incorporates essential physics of turbulent premixed flames, namely, dilatation rate, its influence

Sound waves are persistent in everyday life, although they are never seen. The particles of the average vibrations are parallel to the direction of propagation of the wave. In this article, the effect of changing the frequency of the sound wave on standing waves was investigated with different hertz frequencies to show technical images that can be visually translated to give fundamental predictions about the origin of the musical combinations, as it was proved that the sound is a pressure wave by giving the vibrations one side. At the same time, the gas is released on the other side. The variables were fixed in the experiment as the type of gas supplied for the same experiments to obtain accurate results. It is also installed so that the distance between the diaphragm and the amplifier does not differ. The results analyzed after numerous data collection and calculations verify that the generated wavelength and frequency are directly proportional. As the frequency specified in Hz increased, the number of inverse nodes and nodes also increased. Obtaining standing waves helps to understand the nature of sound as a pressure wave and give all the details about the experiment and evaluate it. Sources of error and possible solutions to overcome these problems are also mentioned.

Post injection is a multiple-injection strategy that is commonly used as a particulate matter control measure to reduce soot emissions, yet the mechanisms and the interactions between the main and post injections are only vaguely understood. For this work, experiments were performed to assess the effects of varying dwell time between the main and post injections in a compression-ignition (CI) engine environment simulated using a constant-volume combustion chamber. The ambient density, bulk temperature, and oxygen concentration used for this work were controlled at 19.4 kg/m3, 900 K, and 15 vol % O2, respectively. A canola oil-based biodiesel was tested and injected at a fixed injection pressure of 100 MPa into the simulated CI engine environment. A mass ratio of 80%–20% was maintained between the main and post injections, with the dwell time between the injections varied from 1.5 to 2.5 ms. Comparative measurements were performed using the same fuel and injection schedules, but at a higher ambient gas temperature condition of 1100 K. Optical diagnostics methods, including diffused-back illumination and high-speed flame luminosity imaging, were used to assess the spray and combustion processes of the post injection test case. Under the conditions of this work, it was found that the ignition delays, ignition locations, and flame lift-off lengths of the post injection flames are consistently shorter than those of the main injections, with the variations influenced by the extent of the interaction of the post injection with the combustion products from the main injection. A two-color pyrometry technique was also used to measure the soot temperature and soot concentration factor information on the main–post injection cases. The data revealed a greater interaction between the main and post injections resulted in a more rapid development of the soot zone of the post injection with higher temperature after ignition. The distribution of the most probable soot concentration factors of the post injection was also found to be narrower, with lower soot content values.

Measurement of curvature of the flamefront of premixed turbulent flames is important for the validation of numerical models for combustion. In this work, curvature is measured from contours that outline the flamefront, which are generated from laser-induced fluorescence images. The contours are inherently digitized, resulting in pixelation effects that lead to difficulties in computing curvature of the flamefront accurately. A common approach is to fit functions locally to short sections along the flame contour, and this approach is also followed in this work; the method helps smoothen the pixelation before curvature is measured. However, the length and degree of the polynomial, and hence the amount of smoothing, must be correctly set in order to maximize the precision and accuracy of the curvature measurements. Other researchers have applied polynomials of different orders and over different segment lengths to circles of known curvature as a test to determine the appropriate choice of polynomial; it is shown here that this method results in a sub-optimal choice of polynomial function. Here, we determine more suitable polynomial functions through use of a circle whose radius is sinusoidally modulated. We show that this leads to a more consistent and reliable choice for the local polynomial functions fitted to experimental data. A polynomial function thus determined is then applied to flame contour data to measure curvature of experimentally acquired flame contours. The results show that there is an enhancement in local flame speed at sections of the flamefront with a non-zero curvature, and this agrees with numerical models.
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https://doi.org/10.1088/0957-0233/19/12/125503
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A set of tribrachial flames of n-heptane/air is simulated with finite rate chemistry and detailed transport in a realistic laminar jet configuration for which experimental data are available. The flames differ by the temperature of the unburnt mixture and stabilization height, which controls the mixture fraction gradient ahead of the flame front. The simulations reproduce the lift-off heights in the experiments, showing that the flame stabilizes further downstream as the unburnt temperature decreases. For the lowest unburnt temperature , resulting in a weak mixture fraction gradient at the tribrachial point, positive stretch along the rich premixed wing leads to an increase in the rate of chemical reaction in the whole flame. The tribrachial flame burning velocity exceeds that in the unstretched, one-dimensional flame. For the highest temperature, the flame stabilizes closest to the nozzle. Large flame tilt, large mixture fraction gradient, and small radius of curvature lead to a reduction in the heat release rate and the flame propagates slower than its one-dimensional counterpart. The observed behavior is explained with a detailed analysis of the flame geometry, differential diffusion effects, flame stretch, and transport of heat and mass from the burnt gases to the flame front.