Combustion Science and Technology

Dunn-Rankin, Derek; Hayashi, A. Koichi

doi: 10.1080/00102202.2014.949099pmid: N/A

Dagaut, Philippe; Dayma, Guillaume; Diévart, Pascal; Hadj-Ali, Kamal; Mzé-Ahmed, Amir

doi: 10.1080/00102202.2014.934564pmid: N/A

The kinetics of oxidation of blends of gas-to-liquid (GtL) jet fuel with 20% vol. of 1-hexanol was studied. New experimental results were obtained using a jet-stirred reactor (p = 10 bar, constant mean residence time of 1 s, over the temperature range 550–1150 K, and for equivalence ratios of 0.5, 1, and 2). Concentration profiles of reactants, stable intermediates, and final products were obtained by probe sampling followed by on-line and off-line gas chromatography analyses and on-line Fourier transformed infra-red spectrometry. A comparison with corresponding results for Jet A-1 and pure GtL showed these fuels have similar combustion properties. The oxidation of the GtL-hexanol fuel under these conditions was modeled using a detailed kinetic reaction mechanism consisting of 8217 reactions and 2185 species and a 4-component surrogate fuel mixture (n-decane, iso-octane, n-propyl cyclohexane, and 1-hexanol). The proposed kinetic model showed good agreement with the presently reported jet-stirred reactor concentration profiles.

Coudour, B.; Chetehouna, K.; Courty, L.; Garo, J. P.; Lemée, L.; Mounaïm-Rousselle, C.; Halter, F.

doi: 10.1080/00102202.2014.934568pmid: N/A

This article deals with the emissions and combustion of l-fenchone and 3-hexen-1-ol. These biogenic volatile organic compounds (BVOCs) are the main compounds emitted respectively by two vegetal species often involved in forest fires: Lavandula stœchas and Cistus albidus. Two types of experimental setups are used: a flash-pyrolysis device to study the emissions at elevated temperatures and a spherical chamber to study the combustion characteristics. Emission results are compared to literature data for the same vegetal species at ambient temperature. Laminar burning speeds and Markstein lengths of these two BVOCs are determined at 453 K for different equivalence ratios. Results are compared to the ones obtained in our previous works for two other BVOCs at the same temperature.

Birouk, Madjid

doi: 10.1080/00102202.2014.934577pmid: N/A

This study presents new experimental results on the vaporization process of hydrocarbon droplet in a turbulent environment at elevated ambient pressure and temperature conditions. n-Heptane and n-decane, which provide a wide range of hydrocarbons properties, were tested. The initial droplet diameter was on the order of 1 mm, and its surrounding ambient consisted of varying turbulence intensity up to 3.10 m/s, pressure up to 16 bar, and temperature up to 150°C. The results revealed that the hydrocarbon droplet followed the d2-law throughout its entire lifetime under all ambient conditions explored here. Increasing the ambient pressure increases the droplet vaporization lifetime, whereas increasing the ambient temperature reduces the droplet lifetime. More importantly, turbulence becomes more effective as ambient pressure increases, whereas it diminishes with increasing the ambient temperature. The experimental data were used to develop a more comprehensive hydrocarbons droplet mass transfer correlation.

Gao, Y.; Chakraborty, N.; Swaminathan, N.

doi: 10.1080/00102202.2014.934581pmid: N/A

An a-priori analysis of the filtered reaction rate closure based on Favre-filtered scalar dissipation rate (SDR) for large eddy simulations (LES) of turbulent premixed combustion has been conducted using a direct numerical simulation (DNS) database of freely propagating statistically planar flames for a range of different values of heat release parameter τ, global Lewis number Le, and turbulent Reynolds number Ret. It has been found that an existing SDR based reaction rate closure for Reynolds averaged Navier–Stokes (RANS) simulations is also valid for LES, when the filter width is larger than the flame thickness. This RANS-based reaction rate closure has been extended here for LES, and a satisfactory performance of this LES closure is observed for the values of filter widths, τ, Le, and Ret investigated here. A-priori DNS assessment of the SDR closures based on passive scalar mixing model and a power-law has been conducted. Moreover, an existing algebraic model of Favre-averaged SDR for RANS simulations has been extended here for LES. The performances of the algebraic closures of have been assessed with respect to Favre-filtered SDR extracted from the DNS data. It has been found that the newly proposed model for , which was extended here for LES from an existing RANS-based closure, predicts both local and volume-averaged behaviors of SDR satisfactorily for a range of for flames with different values of τ, Le, and Ret. The sensitivity of sub-grid turbulent velocity fluctuation modeling on the newly developed algebraic SDR closure has also been analyzed and it has been observed that the modeling does not significantly affect the performance of the algebraic SDR model proposed in this study.

Katragadda, Mohit; Gao, Yuan; Chakraborty, Nilanjan

doi: 10.1080/00102202.2014.934584pmid: N/A

The strain rate contribution in the generalized flame surface density (FSD) transport equation remains a leading order unclosed source term, which plays a pivotal role in the modeling of transport for all filter widths in the context of large eddy simulations (LES). To date, most FSD-based closures have been proposed for flames without differential diffusion effects of heat and mass, characterized by a global Lewis number equal to unity (i.e., ). The effects of differential diffusion arising due to non-unity Lewis number on the FSD transport have rarely been analyzed in existing literature. In the present analysis, the statistical behaviors of the strain rate term of the FSD transport equation have been analyzed using a DNS database of freely propagating statistically planar turbulent premixed flames with a global Lewis number ranging from 0.34 to 1.2 (i.e., = 0.34–1.2). The FSD strain rate term has been split into components originating from the gradients of Favre-filtered velocity components (i.e., ), strain rate contribution due to chemical heat release (i.e., ) and sub-grid processes (i.e., ). The contributions of and assume positive values throughout the flame brush for all values of . The performances of the existing models for and have been assessed for flames with different values of global Lewis number . The contribution of remains positive throughout the flame brush for the = 0.6, 0.8, 1.0, and 1.2 flames but the variation of towards the burned gas side of the flame brush for the flame remains qualitatively different in comparison to the other cases considered here. The effects of on the local alignment of reaction progress variable gradient with principal strain rates are responsible for the observed influences of Lewis number on the sub-grid strain rate term . The existing models have been found to be inadequate for the purpose of capturing the qualitative behaviors of the sub-grid strain rate term for flames. Here a new model has been proposed based on a-priori analysis of explicitly filtered DNS data, which has been demonstrated to capture both the qualitative and quantitative behaviors of for all values of for flames with ranging from 0.34 to 1.2.

Serra, Sylvain; Robin, Vincent; Mura, Arnaud; Champion, Michel

doi: 10.1080/00102202.2014.934605pmid: N/A

Unresolved fluxes in turbulent diffusion flames are investigated by introducing the specific volume to analyze the effects of density variations. Unresolved fluxes are found to be related to scalar correlations involving this specific quantity. The algebraic models proposed for the turbulent scalar and momentum fluxes allow to recover and generalize well-known previously established relations and highlight the possible occurrence of non-gradient diffusion. These correlations are evaluated from the consideration of strained laminar diffusion flames and chemical equilibrium conditions. Finally, the calculations performed confirm that such non-premixed flames may exhibit a strong production of turbulence near stoichiometric conditions.

Schrödinger, C.; Paschereit, C. O.; Oevermann, M.

doi: 10.1080/00102202.2014.934610pmid: N/A

Laminar and turbulent one-dimensional lean premixed methane-air flames subject to equivalence ratio oscillations are investigated numerically. For the turbulent simulations, the Linear Eddy Model is employed. Harmonic perturbations at various frequencies and with various amplitudes are considered and their influence on CO and NO emissions, heat release rate fluctuations, and burning velocity is evaluated. The results indicate a strongly nonlinear behavior of the flame response for high forcing amplitudes attributed to nonlinear effects due to the interaction of burning velocity and equivalence ratio oscillations leading to nonsinusoidal oscillations at the flame front. Furthermore, the turbulent cases reveal decreasing mean burning velocities and heat release rates with increasing amplitudes due to damping of turbulent fluctuations induced by the oscillations.

Lutsenko, Nickolay A.; Levin, Vladimir A.

doi: 10.1080/00102202.2014.934611pmid: N/A

Gravity is known to affect heterogeneous combustion in porous media; it is especially appreciable at very low air (oxidizer) flow rates. Effect of gravity on the combustion of porous objects includes not only the force of gravity (gravity field inside the porous object), but also the pressure difference at boundaries of the porous object, which is caused by the action of gravity on the ambient air if the open borders of the object are located at different heights. In this work, it has been studied numerically how the gravity field inside the porous object and the above-mentioned pressure difference at the object boundaries affect the combustion process in the object. It is revealed that the gravity field inside the porous object and the pressure difference at object boundaries lead to opposite effects: the pressure difference on object borders leads to the propagation of the combustion wave in the same direction and the same mode as in the vertical porous object, but the gravity field leads to the spread of the wave in the opposite direction or in a different mode. In the vertical porous object the pressure difference at object boundaries defeats the effect of gravity field inside the porous object, but the gravity force leads to reducing the velocity of combustion wave.

Sarh, B.; Gillon, P.; Gilard, V.; Bodele, E.

doi: 10.1080/00102202.2014.934612pmid: N/A

The effect of a vertical magnetic field gradient on stoichiometric lengths of methane-air diffusion lifted flames was numerically investigated. The study was developed for understanding the evolution of these lengths when coaxial jet air velocity was varied keeping constant fuel velocity. As the air jet velocity is increased, gradient in mixture fraction and liftoff height increase and flame length decreases. Calculated reacting flow results of liftoff and flame length are presented for two cases, with and without magnetic field. For the same initial flow conditions, the application of the negative magnetic field gradient results in an increase of the stoichiometric flame lengths and a reduction of liftoff heights. For the lifted flame under magnetic field gradient, the Froude number was modified to take into account the magnetic buoyancy effect and it is shown that the correlation proposed by Altenkirch et al. (1976) can still be applied to represent lengths of lifted flames.