Hydrodynamic and thermodiffusive instability effects on the evolution of laminar planar lean premixed hydrogen flames

Numerical simulations with single-step chemistry and detailed transport are used to study premixed hydrogen/air flames in two-dimensional channel-like domains with periodic boundary conditions along the horizontal boundaries as a function of the domain height. Both unity Lewis number, where only hydrodynamic instability appears, and subunity Lewis number, where the flame propagation is strongly affected by the combined effect of hydrodynamic and thermodiffusive instabilities are considered. The simulations aim at studying the initial linear growth of perturbations superimposed on the planar flame front as well as the long-term nonlinear evolution. The dispersion relation between the growth rate and the wavelength of the perturbation characterizing the linear regime is extracted from the simulations and compared with linear stability theory. The dynamics observed during the nonlinear evolution depend strongly on the domain size and on the Lewis number. As predicted by the theory, unity Lewis number flames are found to form a single cusp structure which propagates unchanged with constant speed. The long-term dynamics of the subunity Lewis number flames include steady cell propagation, lateral flame movement, oscillations and regular as well as chaotic cell splitting and mergin

Flame front/turbulence interaction for syngas fuels in the thin reaction zones regime: turbulent and stretched laminar flame speeds at elevated pressures and temperatures

Experiments were performed in dump-stabilized axisymmetric flames to assess turbulent flame speeds ( ${S}_{T}$ ) and mean flamelets speeds (stretched laminar flame speeds, ${S}_{L, k}$ ). Fuels with significantly different thermodiffusive properties have been investigated, ranging from pure methane to syngas ( {\mathrm{H} }_{2} \text{{\ndash}} \mathrm{CO} blends) and pure hydrogen, while the pressure was varied from 0.1 to 1.25MPa. Flame front corrugation was measured with planar laser-induced fluorescence (PLIF) of the OH radical, while turbulence quantities were determined with particle image velocimetry (PIV). Two different analyses based on mass balance were performed on the acquired flame images. The first method assessed absolute values of turbulent flame speeds and the second method, by means of an improved fractal methodology, provided normalized turbulent flame speeds ( ${S}_{T} / {S}_{L, k}$ ). Deduced average Markstein numbers exhibited a strong dependence on pressure and hydrogen content of the reactive mixture. It was shown that preferential-diffusive-thermal (PDT) effects acted primarily on enhancing the stretched laminar flame speeds rather than on increasing the flame front corrugations. Interaction between flame front and turbulent eddies measured by the fractal dimension was shown to correlate with the eddy temporal activit

Direct numerical simulation of the autoignition of a hydrogen plume in a turbulent coflow of hot air

The autoignition of an axisymmetric nitrogen-diluted hydrogen plume in a turbulent coflowing stream of high-temperature air was investigated in a laboratory-scale set-up using three-dimensional numerical simulations with detailed chemistry and transport. The plume was formed by releasing the fuel from an injector with bulk velocity equal to that of the surrounding air coflow. In the ‘random spots' regime, autoignition appeared randomly in space and time in the form of scattered localized spots from which post-ignition flamelets propagated outwards in the presence of strong advection. Autoignition spots were found to occur at a favourable mixture fraction close to the most reactive mixture fraction calculated a priori from considerations of homogeneous mixtures based on inert mixing of the fuel and oxidizer streams. The value of the favourable mixture fraction evolved in the domain subject to the effect of the scalar dissipation rate. The hydroperoxyl radical appeared as a precursor to the build-up of the radical pool and the ensuing thermal runaway at the autoignition spots. Subsequently, flamelets propagated in all directions with complex dynamics, without anchoring or forming a continuous flame sheet. These observations, as well as the frequency of and scatter in appearance of the spots, are in good agreement with experiments in a similar set-up. In agreement with experimental observations, an increase in turbulence intensity resulted in a downstream shift of autoignition. An attempt is made to understand the key processes that control the mean axial and radial locations of the spots, and are responsible for the observed scatter. The advection of the most reactive mixture through the domain, and hence the history of evolution of the developing radical pools were considered to this effec

Three-dimensional simulations of premixed hydrogen/air flames in microtubes

The dynamics of fuel-lean (equivalence ratio φ = 0.5) premixed hydrogen/air atmospheric pressure flames are investigated in open cylindrical tubes with diameters of d = 1.0 and 1.5 mm using three-dimensional numerical simulations with detailed chemistry and transport. In both cases, the inflow velocity is varied over the range where the flames can be stabilized inside the computational domain. Three axisymmetric combustion modes are observed in the narrow tube: steady mild combustion, oscillatory ignition/extinction and steady flames as the inflow velocity is varied in the range 0.5 ≤ UIN ≤ 500 cm s−1. In the wider tube, richer flame dynamics are observed in the form of steady mild combustion, oscillatory ignition/extinction, steady closed and open axisymmetric flames, steady non-axisymmetric flames and azimuthally spinning flames (0.5 ≤ UIN ≤ 600 cm s−1). Coexistence of the spinning and the axisymmetric modes is obtained over relatively wide ranges of UIN. Axisymmetric simulations are also performed in order to better understand the nature of the observed transitions in the wider tube. Fourier analysis during the transitions from the steady axisymmetric to the three-dimensional spinning mode and to the steady non-axisymmetric modes reveals that the m = 1 azimuthal mode plays a dominant role in the transition

Lattice Boltzmann method for direct numerical simulation of turbulent flows

We present three-dimensional direct numerical simulations (DNS) of the Kida vortex flow, a prototypical turbulent flow, using a novel high-order lattice Boltzmann (LB) model. Extensive comparisons of various global and local statistical quantities obtained with an incompressible-flow spectral element solver are reported. It is demonstrated that the LB method is a promising alternative for DNS as it quantitatively captures all the computed statistics of fluid turbulenc

A LES-CMC formulation for premixed flames including differential diffusion

© 2018 Informa UK Limited, trading as Taylor & Francis Group A finite volume large eddy simulation–conditional moment closure (LES-CMC) numerical framework for premixed combustion developed in a previous studyhas been extended to account for differential diffusion. The non-unity Lewis number CMC transport equation has an additional convective term in sample space proportional to the conditional diffusion of the progress variable, that in turn accounts for diffusion normal to the flame front and curvature-induced effects. Planar laminar simulations are first performed using a spatially homogeneous non-unity Lewis number CMC formulation and validated against physical-space fully resolved reference solutions. The same CMC formulation is subsequently used to numerically investigate the effects of curvature for laminar flames having different effective Lewis numbers: a lean methane–air flame with Le eff = 0.99 and a lean hydrogen–air flame with Le eff = 0.33. Results suggest that curvature does not affect the conditional heat release if the effective Lewis number tends to unity, so that curvature-induced transport may be neglected. Finally, the effect of turbulence on the flame structure is qualitatively analysed using LES-CMC simulations with and without differential diffusion for a turbulent premixed bluff body methane–air flame exhibiting local extinction behaviour. Overall, both the unity and the non-unity computations predict the characteristic M-shaped flame observed experimentally, although some minor differences are identified. The findings suggest that for the high Karlovitz number (from 1 to 10) flame considered, turbulent mixing within the flame weakens the differential transport contribution by reducing the conditional scalar dissipation rate and accordingly the conditional diffusion of the progress variable

Study of ignition delay time and generalization of auto-ignition for PRFs in a RCEM by means of natural chemiluminescence

An investigation of the effects of contour conditions and fuel properties on ignition delay time under Homogeneous Charge Compression Ignition (HCCI) conditions is presented in this study. A parametric variation of initial temperature, intake pressure, compression ratio, oxygen concentration and equivalence ratio has been carried out for Primary Reference Fuels (PRFs) in a Rapid Compression Expansion Machine (RCEM) while applying the optical technique of natural chemiluminescence along with a photo-multiplier. Additionally, the ignition delay time has been calculated from the pressure rise rate and also corresponding numerical simulations with CHEMKIN have been done. The results show that the ignition delay times from the chemical kinetic mechanisms agree with the trends obtained from the experiments. Moreover, the same mechanism proved to yield consistent results for both fuels at a wide range of conditions. On the other hand, the results from natural chemiluminescence also showed agreement with the ignition delay from the pressure signals. A 310 nm interference filter was used in order to detect the chemiluminescence of the OH* radical. In fact, the maximum area and peak intensity of the chemiluminescence measured during the combustion showed that the process of auto-ignition is generalized in the whole chamber. Moreover, the correlation of peak intensity, maximum area and ignition delay time demonstrated that natural chemiluminescence can also be used to calculate ignition delay times under different operating conditions. Finally, the area of chemiluminescence was proved to be more dependant on the fuel and ignition delay time than on the operating conditions. (C) 2015 Elsevier Ltd. All rights reserved.The authors would like to thank different members of the LAV team of the ETH-Zurich for their contribution to this work. The authors are grateful to the Universitat Politecnica de Valencia for financing the Ph.D. studies of Vera-Tudela (FPI SP1 Grant 30/05/2012) and his stay at ETH-Zurich (grant 30/12/2014). Finally, the authors would like to thank the Spanish Ministry of Education for financing the Ph.D. studies of Lopez-Pintor (Grant FPU13/02329) and his stay at ETH-Zurich (Grant EST14/00626).Desantes Fernández, JM.; García Oliver, JM.; Vera-Tudela-Fajardo, WM.; López Pintor, D.; Schneider, B.; Boulouchos, K. (2016). Study of ignition delay time and generalization of auto-ignition for PRFs in a RCEM by means of natural chemiluminescence. Energy Conversion and Management. 111:217-228. https://doi.org/10.1016/j.enconman.2015.12.052S21722811