scholarly journals A Numerical Investigation of the Effects of Fuel Composition on the Minimum Ignition Energy for Homogeneous Biogas-Air Mixtures

2020 ◽  
Author(s):  
Vassilios Papapostolou ◽  
Charles Turquand d’Auzay ◽  
Nilanjan Chakraborty
Keyword(s):  
Flame Propagation ◽  
Power Law ◽  
Turbulence Intensity ◽  
Thermal Runaway ◽  
Turbulent Velocity ◽  
Chemical Mechanism ◽  
Scaling Analysis ◽  
Ignition Energy ◽  
Minimum Ignition Energy ◽  
Wide Range

AbstractThe minimum ignition energy (MIE) requirements for ensuring successful thermal runaway and self-sustained flame propagation have been analysed for forced ignition of homogeneous stoichiometric biogas-air mixtures for a wide range of initial turbulence intensities and CO2 dilutions using three-dimensional Direct Numerical Simulations under decaying turbulence. The biogas is represented by a CH4 + CO2 mixture and a two-step chemical mechanism involving incomplete oxidation of CH4 to CO and H2O and an equilibrium between the CO oxidation and the CO2 dissociation has been used for simulating biogas-air combustion. It has been found that the MIE increases with increasing CO2 content in the biogas due to the detrimental effect of the CO2 dilution on the burning and heat release rates. The MIE for ensuring self-sustained flame propagation has been found to be greater than the MIE for ensuring only thermal runaway irrespective of its outcome for large root-mean-square (rms) values of turbulent velocity fluctuation, and the MIE values increase with increasing rms turbulent velocity for both cases. It has been found that the MIE values increase more steeply with increasing rms turbulent velocity beyond a critical turbulence intensity than in the case of smaller turbulence intensities. The variations of the normalised MIE (MIE normalised by the value for the quiescent laminar condition) with normalised turbulence intensity for biogas-air mixtures are found to be qualitatively similar to those obtained for the undiluted mixture. However, the critical turbulence intensity has been found to decrease with increasing CO2 dilution. It has been found that the normalised MIE for self-sustained flame propagation increases with increasing rms turbulent velocity following a power-law and the power-law exponent has been found not to vary much with the level of CO2 dilution. This behaviour has been explained using a scaling analysis and flame wrinkling statistics. The stochasticity of the ignition event has been analysed by using different realisations of statistically similar turbulent flow fields for the energy inputs corresponding to the MIE and it has been demonstrated that successful outcomes are obtained in most of the instances, justifying the accuracy of the MIE values identified by this analysis.

scholarly journals A versatile set-up to study plasma/microwave sources for liquid fuel ignition

2020 ◽  
Vol 92 (3) ◽  
pp. 30903
Author(s):  
Beatrice Fragge ◽  
Jérôme Sokoloff ◽  
Olivier Rouzaud ◽  
Olivier Pascal ◽  
Mikael Orain
Keyword(s):  
Resonant Cavity ◽  
High Energy ◽  
Pollutant Emissions ◽  
Ignition Energy ◽  
Minimum Ignition Energy ◽  
Wide Range ◽  
Droplet Stream ◽  
Set Up ◽  
Microwave Sources ◽  
Plasma Microwave

Motivated by the high demand for an alternative, more reliable, high energy ignition source to facilitate the re-ignition of lean-burn combustion chambers which are necessary to reduce pollutant emissions, a new set-up has been designed to study plasma/microwave sources. The use of a waveguide-based resonant cavity leads to very low power plasma ignition. An example in this paper shows that a plasma at atmospheric pressure can be maintained with less than 2 W input power. Such a performance is possible using the large variety of possible adjustments (resonance frequency, different kind of initiators, etc.) that this versatile set-up offers. To illustrate the wide range of possible studies, another example is given and discussed : minimum ignition energy for an ethanol droplet stream with aluminum and stainless steel initiators. The results show that the initiator material and its surface quality have an influence on the minimum ignition energy, especially for large gaps. Depending on the gap size we can get down to under 10 W entering the cavity to ignite the droplet stream.

Effect of Turbulence On Forced Ignition of Jet-A/Air Mixtures

2021 ◽  
Author(s):  
Kaz Teope ◽  
David L. Blunck
Keyword(s):  
Confidence Interval ◽  
Turbulence Intensity ◽  
Reasonable Agreement ◽  
Infrared Camera ◽  
Heat Diffusion ◽  
Prior Work ◽  
Ignition Energy ◽  
Minimum Ignition Energy ◽  
Ignition Probability

Abstract Consistent ignition of reactive mixtures in turbulent conditions continues to be a challenge, particularly for large, multi-component fuels. Prior work has shown that turbulence can affect ignition parameters such as flame speed, mixture temperature, and minimum ignition energy. However, these works have primarily considered small, single-component fuels. This work studies the effect of turbulence on forced ignition of jet-A/air mixtures with f between 0.3 and 0.7. The ignition probability of these mixtures was measured for bulk velocities between 5 and 7 m/s and turbulence intensities between 3% and 9%. A FLIR SC6700 infrared camera was used to measure the radiation intensity emitted by the flame kernels. Increases in turbulence intensity between 3% and 4% cause the probability of ignition to generally increase. This increase is attributed to the negative flame stretch that develops as a result of the turbulence. This observation is significant because it shows that turbulence can facilitate ignition for jet-A/air mixtures. In contrast, increasing turbulence beyond 5% causes ignition probabilities to decrease. This reduction occurs due to the increased role of heat diffusion and the associated reduction in kernel temperature. The sensitivities of ignition behavior to turbulence intensity and fuel chemistry are reasonably captured using the Peclet number. Further agreement in ignition behavior is achieved by considering Pe/TI2. Ignition probability data for two additional fuels were compared using Pe/TI2. Reasonable agreement within a 95% confidence interval was observed for CH4 mixtures but not for C3H8 mixtures.

scholarly journals A scaling analysis of ozone photochemistry: I Model development

2005 ◽  
Vol 5 (6) ◽  
pp. 12957-12983
Author(s):  
B. Ainslie ◽  
D. G. Steyn
Keyword(s):  
Irradiation Time ◽  
Model Development ◽  
Chemical Mechanism ◽  
Dimensionless Temperature ◽  
Scaling Analysis ◽  
Model Output ◽  
Line Segments ◽  
Photolysis Rate ◽  
Wide Range ◽  
Relevant Variables

Abstract. A scaling analysis has been used to capture the integrated behaviour of several photochemical mechanisms for a wide range of precursor concentrations and a variety of environmental conditions. The Buckingham Pi method of dimensional analysis was used to express the relevant variables in terms of dimensionless groups. These grouping show maximum ozone, initial NOx and initial VOC concentrations are made non-dimensional by the average NO2 photolysis rate (jav) and the rate constant for the NO-O3 titration reaction (kNO); temperature by the NO-O3 activation energy (ENO) and Boltzmann constant (k) and total irradiation time by the cumulative javΔt photolysis rate (π3). The analysis shows dimensionless maximum ozone concentration can be described by a product of powers of dimensionless initial NOx concentration, dimensionless temperature, and a similarity curve directly dependent on the ratio of initial VOC to NOx concentration and implicitly dependent on the cumulative NO2 photolysis rate. When Weibull transformed, the similarity relationship shows a scaling break with dimensionless model output clustering onto two straight line segments, parameterized using four variables: two describing the slopes of the line segments and two giving the location of their intersection. A fifth parameter is used to normalize the model output. The scaling analysis, similarity curve and parameterization appear to be independent of the details of the chemical mechanism, hold for a variety of VOC species and mixtures and a wide range of temperatures and actinic fluxes.

scholarly journals Research on Ignition Energy Characteristics and Explosion Propagation Law of Coal Dust Cloud under Different Conditions

2021 ◽  
Vol 2021 ◽  
pp. 1-8
Author(s):  
Tianqi Liu ◽  
Ruiheng Jia ◽  
Ruicheng Sun ◽  
Weiye Tian ◽  
Ning Wang ◽  
...  
Keyword(s):  
Flame Propagation ◽  
Ignition Temperature ◽  
Coal Dust ◽  
Dust Cloud ◽  
Propagation Distance ◽  
Experimental Result ◽  
Ignition Energy ◽  
Minimum Ignition Energy ◽  
Energy Characteristics ◽  
Propagation Law

To study the ignition energy characteristics and explosion propagation law of coal dust cloud, a kind of coal dust cloud is studied through experiment and numerical simulation under different conditions. The result indicated that ignition delay time and dust spray pressure have obvious effects on the minimum ignition energy of coal dust cloud. CFD theory is used to simulate the explosion flame propagation. It is found that the simulation error of flame propagation distance is acceptable and the simulation result is consistent with the experimental result. When the spray pressure is 0.06 MPa, the flame propagation distance is the farthest, indicating that the turbulence of coal dust cloud is the largest at this condition. As the ignition temperature increases, the flame propagation distance continues to increase, proving that ignition temperature has an obvious effect on the flame propagation process of coal dust cloud explosion.

Ignition and flame quenching in flowing gaseous mixtures

1977 ◽  
Vol 357 (1689) ◽  
pp. 163-181 ◽  
Keyword(s):  
Combustible Mixture ◽  
Inert Gases ◽  
Ignition Energy ◽  
Minimum Ignition Energy ◽  
Turbulence Scale ◽  
Gaseous Mixtures ◽  
Wide Range ◽  
Theoretical Predictions ◽  
Quenching Distance ◽  
Velocity Turbulence

The influence of pressure, velocity, turbulence intensity, turbulence scale and mixture composition on minimum ignition energy and quenching distance in flowing gaseous mixtures is examined experimentally for methane and propane fuels. In some experiments, the nitrogen in the air is replaced by various inert gases such as carbon dioxide, helium or argon, while in others the nitrogen is either partly or totally replaced by oxygen. The tests are conducted at room temperature in a 9 cm square working section through which the combustible mixture is arranged to flow at various levels of pressure, turbulence and velocity. At each test condition, the spark energy required to ignite the flowing mixture is measured for several gap widths in order to establish the optimum gap width corresponding to minimum ignition energy. From analysis of the relevant combustion and heat transfer processes involved, expressions for the prediction of quenching distance in flowing mixtures are derived. Support for the model employed in this analysis is demonstrated by a close level of agreement between theoretical predictions of quenching distance and corresponding values calculated from the experimental data on minimum ignition energy obtained over a wide range of mixture compositions and flow conditions.

scholarly journals Improvement of Calculations for Turbulent Premixed Flame Characteristics Determination using PDF Monte Carlo Simulation

2021 ◽  
Vol 5 (1) ◽  
pp. 1-8
Author(s):  
Lajili M
Keyword(s):  
Monte Carlo ◽  
Flame Propagation ◽  
Turbulence Intensity ◽  
Equivalence Ratio ◽  
Premixed Flame ◽  
Minimum Ignition Energy ◽  
Flammability Limit ◽  
Flame Propagation Velocity ◽  
Turbulent Premixed Flame ◽  
Turbulent Premixed

This study aims at simulating turbulent premixed flame in a constant-pressure vessel (P = 1 atm) where the turbulence is supposed to be homogeneous and isotropic. The mixture of gas is composed by iso-octane-air. The realized CFD were based on Lagrange approach in Monte Carlo simulations. We focused on calculations of; flame radii R F , the flame propagation velocity S t , flame-brush thick ness  t an d flammability limit. During the study, influencing crucial parameters such as, the equivalence ratio  and the turbulence intensity u’ were considered. Results show that the equivalence ratio enhances the flame propagation when passing from lean to stoichiometric flames. Also, the turbulence intensity yields a notable growth for the flame characteristics mentioned above. Moreover, we noticed that the flammability limit is strongly depending of the turbulence intensity and the equivalence ratio. More precisely, we remarked that the minimum ignition energy (MIE) was situated quite smaller than the stoichiometric condition. But, it increased with the turbulence intensity.

scholarly journals A scaling analysis of ozone photochemistry

2006 ◽  
Vol 6 (12) ◽  
pp. 4067-4077 ◽  
Author(s):  
B. Ainslie ◽  
D. G. Steyn
Keyword(s):  
Irradiation Time ◽  
Chemical Mechanism ◽  
Dimensionless Temperature ◽  
Scaling Analysis ◽  
Model Output ◽  
Line Segments ◽  
Photolysis Rate ◽  
Wide Range ◽  
Relevant Variables ◽  
Ozone Photochemistry

Abstract. A scaling analysis has been used to capture the integrated behaviour of several photochemical mechanisms for a wide range of precursor concentrations and a variety of environmental conditions. The Buckingham Pi method of dimensional analysis was used to express the relevant variables in terms of dimensionless groups. These grouping show maximum ozone, initial NOx and initial VOC concentrations are made non-dimensional by the average NO2 photolysis rate (jav) and the rate constant for the NO–O3 titration reaction (kNO); temperature by the NO–O3 activation energy (ENO) and Boltzmann constant (k) and total irradiation time by the cumulative javΔt photolysis rate. The analysis shows dimensionless maximum ozone concentration can be described by a product of powers of dimensionless initial NOx concentration, dimensionless temperature, and a similarity curve directly dependent on the ratio of initial VOC to NOx concentration and implicitly dependent on the cumulative NO2 photolysis rate. When Weibull transformed, the similarity relationship shows a scaling break with dimensionless model output clustering onto two straight line segments, parameterized using four variables: two describing the slopes of the line segments and two giving the location of their intersection. A fifth parameter is used to normalize the model output. The scaling analysis, similarity curve and parameterization appear to be independent of the details of the chemical mechanism, hold for a variety of VOC species and mixtures and a wide range of temperatures and actinic fluxes.

scholarly journals Aircraft measurements of gravity waves in the upper troposphere and lower stratosphere during the START08 field experiment

2015 ◽  
Vol 15 (13) ◽  
pp. 7667-7684 ◽  
Author(s):  
Fuqing Zhang ◽  
Junhong Wei ◽  
Meng Zhang ◽  
K. P. Bowman ◽  
L. L. Pan ◽  
...  
Keyword(s):  
Power Law ◽  
Gravity Waves ◽  
Lower Stratosphere ◽  
Potential Temperature ◽  
Shear Instability ◽  
Aircraft Measurements ◽  
Upper Troposphere ◽  
Airborne Measurements ◽  
Wide Range

Abstract. This study analyzes in situ airborne measurements from the 2008 Stratosphere–Troposphere Analyses of Regional Transport (START08) experiment to characterize gravity waves in the extratropical upper troposphere and lower stratosphere (ExUTLS). The focus is on the second research flight (RF02), which took place on 21–22 April 2008. This was the first airborne mission dedicated to probing gravity waves associated with strong upper-tropospheric jet–front systems. Based on spectral and wavelet analyses of the in situ observations, along with a diagnosis of the polarization relationships, clear signals of mesoscale variations with wavelengths ~ 50–500 km are found in almost every segment of the 8 h flight, which took place mostly in the lower stratosphere. The aircraft sampled a wide range of background conditions including the region near the jet core, the jet exit and over the Rocky Mountains with clear evidence of vertically propagating gravity waves of along-track wavelength between 100 and 120 km. The power spectra of the horizontal velocity components and potential temperature for the scale approximately between ~ 8 and ~ 256 km display an approximate −5/3 power law in agreement with past studies on aircraft measurements, while the fluctuations roll over to a −3 power law for the scale approximately between ~ 0.5 and ~ 8 km (except when this part of the spectrum is activated, as recorded clearly by one of the flight segments). However, at least part of the high-frequency signals with sampled periods of ~ 20–~ 60 s and wavelengths of ~ 5–~ 15 km might be due to intrinsic observational errors in the aircraft measurements, even though the possibilities that these fluctuations may be due to other physical phenomena (e.g., nonlinear dynamics, shear instability and/or turbulence) cannot be completely ruled out.

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