Flashback in Lean Prevaporized Premixed Combustion: Nonswirling Turbulent Pipe Flow Study

A fundamental study has been performed on the upstream flame propagation of a turbulent kerosene flame, stabilized in a confined stagnation flow at atmospheric pressure. Besides temperature and equivalence ratio, mixture properties and fluid dynamic parameters have been varied. The flashback phenomenon is discussed in terms of critical mean velocities and additionally based on detailed LDV data at the outlet of the premixing duct. The largest critical velocities uc for flashback are found for the “perfectly” premixed case and equivalence ratios close to stoichiometric, which is in accordance with the theory on laminar flame propagation. In the case of a homogeneous mixture, flashback is determined by the velocity distribution at the outlet of the premixing section. In the undisturbed pipe flow the flame propagates through the wall boundary layer. The data for this case are compared with the theory of side-wall quenching in terms of a critical Peclet number and critical velocity gradients at the wall. Both are deduced from the experimental data. Reducing the velocity on the axis forces the flame to propagate through the center at a velocity predicted by correlations on turbulent flame velocity.

Download Full-text

Experimental Study of Flame Stretch Under Engine-Like Conditions

Volume 1: Large Bore Engines; Fuels; Advanced Combustion ◽

10.1115/icef2017-3636 ◽

2017 ◽

Cited By ~ 1

Author(s):

Behdad Afkhami ◽

Yanyu Wang ◽

Scott A. Miers ◽

Jeffrey D. Naber

Keyword(s):

Flame Front ◽

Flame Propagation ◽

High Speed ◽

Combustion Engine ◽

Turbulent Flame ◽

Experimental Studies ◽

Flame Velocity ◽

Turbulent Flame Speed ◽

Stretch Rate ◽

Flame Stretch

Since fossil fuels will remain the main source of energy for power generation and transportation in next decades, their combustion processes remain an important concern for the foreseeable future. For liquid or gaseous fuels, flame velocity that propagates normal to itself and relative to the flow into the unburned mixture is one of the most important quantities to study. In a non-uniform flow, a curved flame front area changes continually which is known as flame stretch. The concept becomes more important when it is realized that the stretch affects the turbulent flame speed. The current research empirically studies flame stretch under engine-like conditions since there has not been enough experimental studies in this area. For this reason, a one-cylinder, direct-injection, spark-ignition, naturally-aspirated optical engine was utilized to image the flame propagation process inside an internal combustion engine cylinder on the tumble plane. The flame front was found by processing high speed images which were taken from the flame inside the cylinder. Flame front propagation analysis showed that after the flame kernel was developed, during flame propagation period, the stretch rate decreased until the flame front touches the piston surface. This trend was common among stoichiometric, lean, and rich mixtures. In addition, the fuel-air mixture with λ = 0.85 showed lower stretch rate compared to stoichiometric or lean mixture with λ = 1.2. However, based on previous studies, further enrichment may result in the flame stretch rate become greater than that of the stretch rates for stoichiometric or lean mixtures. Also, comparing the stretch rate at two different engine speeds revealed that as the speed increased the stretch rate also increased; especially during the early flame development period. Therefore, according to previous studies which discussed flame stretch as a mechanism for flame extinguishment, the probability of the flame extinction is higher when the engine speed is higher.

Download Full-text

Turbulent flame propagation by large-scale wrinkling of a laminar flame front

Symposium (International) on Combustion ◽

10.1016/s0082-0784(58)80100-9 ◽

1958 ◽

Vol 7 (1) ◽

pp. 615-620 ◽

Cited By ~ 1

Author(s):

J.K. Richmond ◽

J. Grumer ◽

D.S. Burgess

Keyword(s):

Flame Front ◽

Flame Propagation ◽

Large Scale ◽

Laminar Flame ◽

Turbulent Flame ◽

Laminar Flame Front

Download Full-text

Explosion-Suppression Characteristics of Nonmetallic Spherical Spacers on Propane-Air Mixtures in Confined Space

Applied Sciences ◽

10.3390/app11199238 ◽

2021 ◽

Vol 11 (19) ◽

pp. 9238

Author(s):

Yangyang Yu ◽

Lehai Liu ◽

Junhong Zhang ◽

Jun Wang ◽

Xiangde Meng ◽

...

Keyword(s):

Flame Propagation ◽

High Speed ◽

Laminar Flame ◽

Turbulent Flame ◽

Confined Space ◽

Effective Prevention ◽

Jet Flame ◽

Explosion Suppression ◽

Explosion Overpressure ◽

Suppression Effects

The explosion-suppression effects of NSSs on overpressures, flame propagation and flame tip velocities were explored under the initial pressures of 0.2 MPa, 0.3 MPa and 0.4 MPa. All experiments tested in a constant volume combustion bomb (CVCB). Explosion reaction of premixed propane–air gas in a new designed CVCB filled with nonmetallic spherical spacers (NSSs) was analyzed. The results showed that overpressures decreased under the different initial pressures. With the increase of filling density, the overpressure decreased, the time to reach explosion overpressure decreased, and the decay rate of explosion overpressure increased. It was also found that the explosion-suppression effects of NSSs on pressures. Flame front could be captured by high-speed schlieren photography. Combustion phenomena were captured including flame propagation, corrugated laminar flame, jet flame, corrugated turbulent flame as well as tulip flame under different initial pressures. Flame tip velocities also were captured. The results demonstrate that flame tip velocities decreased with the increase of filling densities. However, compared with unfilled CVCB, flame tip velocities increased after filling NSSs in CVCB under different initial pressures. NSSs suppressed the explosion overpressure in the cylinder, and promoted the flame propagation. In both cases, NSSs played a dual role. The suppression effect of NSSs was affected by both its suppression and promotion effect on the explosion. This work provides a scientific basis for the effective prevention of explosion accidents with propane–air premixtures and the development of explosion-suppression products.

Download Full-text

Empirical investigation of spark-ignited flame-initiation cycle-to-cycle variability in a homogeneous charge reciprocating engine

International Journal of Engine Research ◽

10.1177/1468087417720558 ◽

2017 ◽

Vol 19 (5) ◽

pp. 491-508 ◽

Cited By ~ 24

Author(s):

Philipp Schiffmann ◽

David L Reuss ◽

Volker Sick

Keyword(s):

Mass Fraction ◽

Laminar Flame ◽

Turbulent Flame ◽

Operating Conditions ◽

Strain Rate Tensor ◽

Flame Velocity ◽

Fuel Mass ◽

Markstein Number ◽

Fuel Mass Fraction ◽

Homogeneous Charge

This experimental study investigates the flame-initiation period variability in the spark-ignited homogeneous charge third-generation transparent combustion chamber optical engine. The engine was operated with lean, rich, and stoichiometric, propane and methane, with and without nitrogen dilution. These operating conditions were chosen to systematically change the unstretched laminar flame velocity and the Markstein number. Traditional pressure measures, apparent heat release analysis, particle image velocimetry, and OH* flame imaging were used to generate over 400 metrics for 750 cycles at each of the 34 tests at 11 operating conditions. A multivariate statistical analysis was used to identify the parameters important to the variability of the crank angle at 10% fuel mass fraction burned but could not reveal physical mechanisms or cause and effect. The analysis here revealed that the combustion-phasing cycle-to-cycle variability is established by the time of the notional laminar-to-turbulent flame transition that occurs by 1% mass burn fraction, measured here from the flame image growth. Both the Markstein number and stretched laminar flame speed were found to be important. The velocity magnitude and direction were found to correlate with fast and slow 10% fuel mass fraction burned as found in early literature. It was also revealed that the shear strength, a property of the strain rate tensor at the scales resolved here (1 mm), deserves further investigation as a possible effect on 10% fuel mass fraction burned.

Download Full-text

Anisotropic enhancement of turbulence in large-scale, low-intensity turbulent premixed propane–air flames

Journal of Fluid Mechanics ◽

10.1017/s0022112002008650 ◽

2002 ◽

Vol 462 ◽

pp. 209-243 ◽

Cited By ~ 26

Author(s):

JUNICHI FURUKAWA ◽

YOSHIKI NOGUCHI ◽

TOSHISUKE HIRANO ◽

FORMAN A. WILLIAMS

Keyword(s):

Turbulent Combustion ◽

Large Scale ◽

Isotropic Turbulence ◽

Laminar Flame ◽

Turbulent Flame ◽

Vertical Plane ◽

Turbulent Pipe Flow ◽

Laser Doppler Velocimeter ◽

Arrival Times ◽

Flame Brush

The density change across premixed flames propagating in turbulent flows modifies the turbulence. The nature of that modification depends on the regime of turbulent combustion, the burner design, the orientation of the turbulent flame and the position within the flame. The present study addresses statistically stationary turbulent combustion in the flame-sheet regime, in which the laminar-flame thickness is less than the Kolmogorov scale, for flames stabilized on a vertically oriented cylindrical burner having fully developed upward turbulent pipe flow upstream from the exit. Under these conditions, rapidly moving wrinkled laminar flamelets form the axisymmetric turbulent flame brush that is attached to the burner exit. Predictions have been made of changes in turbulence properties across laminar flamelets in such situations, but very few measurements have been performed to test the predictions. The present work measures individual velocity changes and changes in turbulence across flamelets at different positions in the turbulent flame brush for three different equivalence ratios, for comparison with theory.The measurements employ a three-element electrostatic probe (EP) and a two-component laser-Doppler velocimeter (LDV). The LDV measures axial and radial components of the local gas velocity, while the EP, whose three sensors are located in a vertical plane that passes through the burner axis, containing the plane of the LDV velocity components, measures arrival times of flamelets at three points in that plane. From the arrival times, the projection of flamelet orientation and velocity on the plane are obtained. All of the EP and LDV sensors are located within a fixed volume element of about 1 mm diameter to provide local, time-resolved information. The technique has the EP advantages of rapid response and good sensitivity and the EP disadvantages of intrusiveness and complexity of interpretation, but it is well suited to the type of data sought here.Theory predicts that the component of velocity tangent to the surface of a locally planar flamelet remains constant in passing through the flamelet. The data are consistent with this prediction, within the accuracy of the measurement. The data also indicate that the component of velocity normal to the flamelet, measured with respect to the flamelet, tends to increase in passing through the flamelet, as expected. The flamelets thereby can generate anisotropy in initially isotropic turbulence. They also produce differences in turbulent spectra conditioned on unburnt or burnt gas. Local modifications of turbulence by flamelets thus are demonstrated experimentally. The modifications are quantitatively different at different locations in the turbulent flame brush but qualitatively similar in that the turbulence is enhanced more strongly in the radial direction than in the axial direction at all positions in these flames.

Download Full-text