Grupo de Investigación "Mecánica de Fluidos"
http://hdl.handle.net/10016/9434
2014-09-17T17:38:14ZCritical radius for hot-jet ignition of hydrogen-air mixtures
http://hdl.handle.net/10016/18522
Critical radius for hot-jet ignition of hydrogen-air mixtures
Carpio, Jaime; Iglesias, Immaculada; Vera, Marcos; Sánchez, Antonio L.; Liñán, Amable
This study addresses deflagration initiation of lean and stoichiometric hydrogen–air mixtures by the sudden discharge of a hot jet of their adiabatic combustion products. The objective is to compute the minimum jet radius required for ignition, a relevant quantity of interest for safety and technological applications. For sufficiently small discharge velocities, the numerical solution of the problem requires integration of the axisymmetric Navier–Stokes equations for chemically reacting ideal-gas mixtures, supplemented by standard descriptions of the molecular transport terms and a suitably reduced chemical-kinetic mechanism for the chemistry description. The computations provide the variation of the critical radius for hot-jet ignition with both the jet velocity and the equivalence ratio of the mixture, giving values that vary between a few tens microns to a few hundred microns in the range of conditions explored. For a given equivalence ratio, the critical radius is found to increase with increasing injection velocities, although the increase is only moderately large. On the other hand, for a given injection velocity, the smallest critical radius is found at stoichiometric conditions.
2013-03-07T00:00:00ZHydrogen-air mixing-layer ignition at temperatures below crossover
http://hdl.handle.net/10016/18350
Hydrogen-air mixing-layer ignition at temperatures below crossover
Williams, Forman Arthur; Fernández-Tarrazo, Eduardo; Sánchez, Antonio L.
This paper addresses ignition histories of diffusion flames in unstrained hydrogen-air mixing layers for initial conditions of temperature and pressure that place the system below the crossover temperature associated with the second explosion limit of hydrogen–oxygen mixtures. It is seen that a two-step reduced chemical-kinetic mechanism involving as main species H₂, O₂, H₂O, and H₂O₂, derived previously from a detailed mechanism by assuming all radicals to follow a steady-state approximation, suffices to describe accurately the ignition process. The strong temperature sensitivity of the corresponding overall rates enables activation-energy asymptotics to be employed for the analysis, following the ideas developed for mixing-layer ignition by Liñán and Crespo in 1976 on the basis of one-step Arrhenius model chemistry. When the initial temperatures of both reactants differ by a relative amount that is of the order of or smaller than the ratio of this temperature to the effective activation temperature, the chemical reaction is seen to occur at a significant rate all across the mixing layer. The ignition time is then determined as a thermal runaway in a parabolic problem describing the evolution of the temperature increment and the H₂O₂ concentration, with local accumulation, chemical reaction, and transverse convection and diffusion, all being important. By way of contrast, when the air side is sufficiently hotter than the hydrogen side, as often occurs in applications, ignition occurs in a thin layer close to the air-side boundary, enabling a simplified description to be developed in which the ignition time is determined by analyzing the existence of solutions to a two-point boundary-value problem involving quasi-steady diffusion–reaction ordinary differential equations.
2013-10-01T00:00:00ZViscous stability analysis of jets with discontinuous base profiles
http://hdl.handle.net/10016/18349
Viscous stability analysis of jets with discontinuous base profiles
Coenen, Wilfried; Sevilla, Alejandro; Sánchez, Antonio L.
The viscous linear stability of parallel gaseous jets with piecewise constant base profiles is considered in the limit of low Mach numbers. Our results generalise those of Drazin [P.G. Drazin, Discontinuous velocity profiles for the Orr–Sommerfeld equation J. Fluid Mech. 10 (1961) 571–583], by contemplating the possibility of arbitrary jumps in density and transport properties between two uniform streams separated by a vortex sheet. The eigenfunctions, obtained analytically in the regions of uniform flow, are matched through an appropriate set of jump conditions at the discontinuity of the basic flow, which are derived by repeated integration of the linearised conservation equations in their primitive variable form. The development leads to an algebraic dispersion relation of ample validity that explicitly accounts for the parametric dependence of the stability properties on the jet-to-ambient density ratio, the Reynolds number, the Prandtl number, and the exponent of the presumed power-law dependence of viscosity and thermal conductivity on temperature. The dispersion relation is validated through comparisons with stability calculations performed with continuous profiles and is applied, in particular, to study the effects of molecular transport on the spatiotemporal stability of parallel non-isothermal gaseous jets with very thin shear layers. The eigenvalue computations performed by using the vortex-sheet model are shown to be several orders of magnitude faster than those associated with continuous profiles with thin shear layers.
2012-11-01T00:00:00ZNumerical analyses of deflagration initiation by a hot jet
http://hdl.handle.net/10016/18347
Numerical analyses of deflagration initiation by a hot jet
Iglesias, Immaculada; Vera, Marcos; Sánchez, Antonio L.; Liñan, Amable
Numerical simulations of axisymmetric reactive jets with one-step Arrhenius kinetics
are used to investigate the problem of deflagration initiation in a premixed fuel–air
mixture by the sudden discharge of a hot jet of its adiabatic reaction products. For the
moderately large values of the jet Reynolds number considered in the computations,
chemical reaction is seen to occur initially in the thin mixing layer that separates the hot
products from the cold reactants. This mixing layer is wrapped around by the starting
vortex, thereby enhancingmixing at the jet head, which is followed by an annular mixing
layer that trails behind, connecting the leading vortex with the orifice rim. A successful
deflagration is seen to develop for values of the orifice radius larger than a critical
value aϲ in the order of the flame thickness of the planar deflagration δL. Introduction
of appropriate scales provides the dimensionless formulation of the problem, with
flame initiation characterised in terms of a critical Damk¨ohler number ∆ϲ = (aϲ/δL)²,
whose parametric dependence is investigated. The numerical computations reveal that,
while the jet Reynolds number exerts a limited influence on the criticality conditions,
the effect of the reactant diffusivity on ignition is much more pronounced, with the
value of ∆ϲ increasing significantly with increasing Lewis numbers Le. The reactant
diffusivity affects also the way ignition takes place, so that for reactants with Le ≳ 1 the
flame develops as a result of ignition in the annular mixing layer surrounding the developing
jet stem, whereas for highly diffusive reactants with Lewis numbers sufficiently
smaller than unity combustion is initiated in the mixed core formed around the starting
vortex. The analysis provides increased understanding of deflagration initiation processes,
including the effects of differential diffusion, and points to the need for further
investigations incorporating detailed chemistry models for specific fuel–air mixtures.
2012-08-07T00:00:00Z