A computational study of oscillatory extinction of spherical diffusion flames

<div class="htmlview paragraph">This study examines the types of hydrogen leaks that can support combustion. Hydrogen, methane, and propane diffusion flames on round burners and leaky compression fittings were observed. Measurements included limits of quenching and blowoff for round burners with diameters of 0.006 - 3.18 mm. The measured mass flow rates at the quenching limits were found to be generally independent of burner diameter at relatively large burner diameters. In terms of mass flow rate, hydrogen had the lowest quenching limit and the highest blowoff limit of the fuels considered, which means that there are high and low flow rates where hydrogen is able to support a flame while methane and propane are not able to. The quenching limits for hydrogen diffusion flames on round burners with thick walls were found to be higher than for thin walls. The limits were also found to be independent of burner orientation; leaks with low flow rates are able to support flames independent of their orientation. The minimum mass flow rate of hydrogen that can support combustion from a leaking compression fitting was found to be 0.028 mg/s. This flow was independent of pressure (up to 131 bar) and about an order of magnitude lower than the corresponding methane and propane flow rates. The implications for fire safety are discussed.</div>

Effect of H[formula omitted] addition on NH[formula omitted] flame structure and dynamics in a mixing layer

Chemical effects of hydrogen addition on soot formation in counterflow diffusion flames: Dependence on fuel type and oxidizer composition

The stability of a diffusion flame burning in products of combustion

Different chemical effect of hydrogen addition on soot formation in laminar coflow methane and ethylene diffusion flames

Rotational Raman scattering from premixed and diffusion flames

This article studies the generation of entropy disturbances by laminar premixed flames. The total entropy generation equals the integrated ratio of the local heat release rate and the local temperature, namely, [Formula: see text]. Due to this path dependency, evaluating this integral requires an understanding of how the heat release is distributed in the temperature space. Several studies evaluate the local entropy generation as [Formula: see text], where [Formula: see text] refers to the burned gas temperature, implicitly assuming all the heat release occurs at [Formula: see text]. Such an approximation is motivated by the high activation energy nature of combustion chemistry. This work evaluates this assumption by comparing it to results from one-dimensional premixed flame calculations for hydrogen, methane, and propane-air flames over a range of pressures, equivalence ratios, and preheat temperatures, quantified via the ratio [Formula: see text]. We show that this assumption is quite reasonable for methane and propane-air flames (with errors ranging from 5% to 25%) but deviates significantly from the exact results for hydrogen flames (where errors can be as high as 50%). In general, the peak heat release moves to lower temperatures as preheat temperature is increased. Noting that the temperature sensitivity of heat release is directly related to the activation energy, we use Law's approach to extract global activation energies and show that the deviations of [Formula: see text] from unity can be approximately correlated with [Formula: see text]. Finally, we show that significant improvements in entropy generation calculations can be obtained by estimating [Formula: see text] using [Formula: see text], where [Formula: see text] is the temperature at which the reaction rate peaks. This estimation leads to predictions of ∼5% within the exact value for the hydrocarbon cases but can still be in significant error for hydrogen at certain conditions.

Flame characteristics when a non-premixed n-butane jet is ejected into a coaxial cylindrical tube are investigated experimentally. Flame stability depends mainly on the characteristics of flame propagation as well as air entrainment which depend on the jet momentum and on the distance between the nozzle exit and the base of a confined tube. As flow rate increases, the flame lifts off from a nozzle attached diffusion flame and a stationary lifted flame can be stabilized. The liftoff height increases nearly linearly with the average velocity at the nozzle exit. The lifted flame has a tribrachial flame structure, which consists of a rich premixed flame, a lean premixed flame, and a diffusion flame, all extending from a single location. As flow rate further increases, periodically oscillating flames are observed inside the confined tube. Once flame oscillation occurs, the flame undergoes relatively stable oscillation such that it has nearly constant oscillation amplitude and frequency. The criteria of flame oscillation are mapped as functions of nozzle diameter, the distance between nozzle and tube, and jet velocity. This type of flame oscillation can be characterized by Strouhal number in terms of flame oscillation amplitude, frequency, and jet velocity. Buoyancy driven flame oscillation which is one of the viable mechanism for flame oscillation is modeled and the results agrees qualitatively with experimental results, suggesting that the oscillation is due to periodic blowoff and flashback under the influence of buoyancy.

The effect of sodium chloride on the nanoparticles observed in a laminar methane diffusion flame

The quenching of premixed turbulent flames of iso-octane, methane and hydrogen at high pressures

Effects of sinusoidal velocity oscillation of air flow on counterflow diffusion flames of nitrogendiluted methane against air has experimentally been investigated. The burner exit velocity was 0.7∼1.7m/s. The unsteady flow characteristics and the extinction limit were measured by varying the frequency (5∼500 Hz) and the amplitude (burner exit velocity×0.3, 0.5) of the flow oscillation. Furthermore, the behavior of diffusion layer was observed by using Laser Tomography method. The result shows that the extinction limit of unsteady flame is determined as a function of the maximum strain rate regardless of amplitude of velocity oscillation and it almost coincides with that of the steady state flame. The extinction limit of unsteady-state flame depends on the frequency of velocity oscillation as well. The strain rate at the extinction becomes minimum at the frequency where the amplitude of fluctuation of diffusion layer width becomes maximum. This suggests that the variation of the structure of diffusion flame zone plays a key role on the flame extinction.

Measurement of the dimensionless extinction coefficient of soot within laminar diffusion flames

Detailed kinetic modeling of C1 — C3 alkane diffusion flames

The increasing global demand for energy, the need to reduce green house gasses, and the depletion of fossil fuel resources have led for the need for renewable fuel sources such as biodiesel fuels. In the diesel engines, biodiesel fuels can also be used directly without comprehensive engine changes. Biodiesel relates to a diesel fuel that is based on vegetable oil or animal fat consisting of longchain of methyl, ethyl, or propyl esters. Methyl ester fuel burns more efficiently and has lower emissions of particulate matter, unburnt hydrocarbon, and carbon monoxide than fossil fuels. However, combustion of methyl ester fuel results in increased nitrogen oxides (NOx) emissions relative to fossil fuels. This study is concerned with characterizing the formation of NOx in the combustion of methyl formate under a counter diffusion flame. This was carried out in an Exhaust Gas Recirculation (EGR) system. Simulation of the process was done using Combustion Simulation Laboratory Software (COSILAB), and involved simulating the reactions of methyl formate fuel. The results obtained were compared to those of the methane/air diffusion flame, which is a well-characterized system. The extension validated the results obtained for the methyl formate/air diffusion flame. The reduction of NOx was found to be 26% and 14% in methane and methyl formate diffusion flame respectively from 0% to 29.5% of EGR. Increased EGR from 0% to 29.5% increased NOx reduction. Compared to methane/air diffusion flame, methyl formate/air diffusion flame with and without EGR had lower NOx emission. This was found to be true when examining the amount of other metrics viz. temperature, H, OH and N radicals associated with NOx. This showed that EGR system have an effect on NOx formation.

The paper presents results from three-dimensional Direct Numerical Simulations of turbulent lean premixed hydrogen, propane and methane flames. The three fuels studied have Lewis numbers which range from small values (hydrogen) through near-unity values (methane) up to values larger than unity (propane). The computations make use of reduced chemical mechanisms with species number ranging from 9 to 28. It has been found that for the present time-explicit numerical method, the CPU-time scales non-linearly – with the power of 1.6 – with the number of chemical species.A new method to produce turbulent vortexes directly in the computational domain is presented. The vortexes are produced through localized forcing in physical space. This allows the full control of the length-scale of the vortexes as well as of their spatial and time distribution. The parallelization of the method requires no programming efforts, i.e. the parallelization follows automatically. The set of vortexes created for this particular investigation is identical for all flames studied: two methane, one hydrogen and one propane flames. Despite the identical turbulent vortex set, the flames of the different fuels react quite differently to it, exhibiting qualitatively different combustion regimes as well as different growth of the flame front area and of the consumption flame speed with time.The computations are carried out on the CRAY XE6 HERMIT supercomputer at the High Performance Computing Center Stuttgart (HLRS). Numerical issues and performance results for the three chemical mechanisms, corresponding to the three fuels used, are presented and discussed.