Archivo Digital UPM: No conditions. Results ordered -Date Deposited. 2020-02-16T18:35:28ZEPrintshttp://oa.upm.es/style/images/logo-archivo-digital.pnghttp://oa.upm.es/2015-01-19T11:03:14Z2015-01-19T11:03:14Zhttp://oa.upm.es/id/eprint/33570This item is in the repository with the URL: http://oa.upm.es/id/eprint/335702015-01-19T11:03:14ZFlammability conditions for ultra-lean hydrogen premixed
combustion based on flame-ball analysesIt has been reasoned that the structures of strongly cellular flames in very lean mixtures approach an array of flame balls, each burning as if it were isolated, thereby indicating a connection between the critical conditions required for existence of steady flame balls and
those necessary for occurrence of self-sustained premixed combustion. This is the starting assumption of the present study, in which structures of near-limit steady sphericosym-metrical flame balls are investigated with the objective of providing analytic expressions for critical combustion conditions in ultra-lean hydrogen-oxygen mixtures diluted with N2 and water vapor. If attention were restricted to planar premixed flames, then the lean-limit mole
fraction of H2 would be found to be roughly ten percent, more than twice the observed flammability limits, thereby emphasizing the relevance of the flame-ball phenomena.
Numerical integrations using detailed models for chemistry and radiation show that a onestep chemical-kinetic reduced mechanism based on steady-state assumptions for all
chemical intermediates, together with a simple, optically thin approximation for water-vapor radiation, can be used to compute near-limit fuel-lean flame balls with excellent accuracy. The previously developed one-step reaction rate includes a crossover temperature that determines in the first approximation a chemical-kinetic lean limit below which combustión cannot occur, with critical conditions achieved when the diffusion-controlled radiation-free
peak temperature, computed with account taken of hydrogen Soret diffusion, is equal to the crossover temperature. First-order corrections are found by activation-energy asymptotics in a solution that involves a near-field radiation-free zone surrounding a spherical flame sheet,
together with a far-field radiation-conduction balance for the temperature profile. Different scalings are found depending on whether or not the surrounding atmosphere contains wáter vapor, leading to different analytic expressions for the critical conditions for flame-ball
existence, which give results in very good agreement with those obtained by detailed numerical computations.Eduardo Fernández TarrazoAntonio Luis Sánchez PérezAmable Liñán MartínezF.A. Williams2014-10-31T07:37:18Z2014-10-31T07:37:18Zhttp://oa.upm.es/id/eprint/32544This item is in the repository with the URL: http://oa.upm.es/id/eprint/325442014-10-31T07:37:18ZDynamics of thermal ignition of spray flames in mixing layersConditions are identified under which analyses of laminar mixing layers can shed light on aspects of turbulent spray combustion. With this in mind, laminar spray-combustion models are formulated for both non-premixed and partially premixed systems. The laminar mixing layer separating a hot-air stream from a monodisperse spray carried by either an inert gas or air is investigated numerically and analytically in an effort to increase understanding of the ignition process leading to stabilization of high-speed spray combustion. The problem is formulated in an Eulerian framework, with the conservation equations written in the boundary-layer approximation and with a one-step Arrhenius model adopted for the chemistry description. The numerical integrations unveil two different types of ignition behaviour depending on the fuel availability in the reaction kernel, which in turn depends on the rates of droplet vaporization and fuel-vapour diffusion. When sufficient fuel is available near the hot boundary, as occurs when the thermochemical properties of heptane are employed for the fuel in the integrations, combustion is established through a precipitous temperature increase at a well-defined thermal-runaway location, a phenomenon that is amenable to a theoretical analysis based on activation-energy asymptotics, presented here, following earlier ideas developed in describing unsteady gaseous ignition in mixing layers. By way of contrast, when the amount of fuel vapour reaching the hot boundary is small, as is observed in the computations employing the thermochemical properties of methanol, the incipient chemical reaction gives rise to a slowly developing lean deflagration that consumes the available fuel as it propagates across the mixing layer towards the spray. The flame structure that develops downstream from the ignition point depends on the fuel considered and also on the spray carrier gas, with fuel sprays carried by air displaying either a lean deflagration bounding a region of distributed reaction or a distinct double-flame structure with a rich premixed flame on the spray side and a diffusion flame on the air side. Results are calculated for the distributions of mixture fraction and scalar dissipation rate across the mixing layer that reveal complexities that serve to identify differences between spray-flamelet and gaseous-flamelet problems.D. Martínez-RuizJ. UrzayAntonio Luis Sánchez PérezAmable Liñán MartínezF.A. Williams2014-10-31T07:31:30Z2014-10-31T07:31:30Zhttp://oa.upm.es/id/eprint/32540This item is in the repository with the URL: http://oa.upm.es/id/eprint/325402014-10-31T07:31:30ZSheath vaporization of a monodisperse fuel-spray jetThe group vaporization of a monodisperse fuel-spray jet discharging into a hot coflowing gaseous stream is investigated for steady flow by numerical and asymptotic
methods with a two-continua formulation used for the description of the gas and liquid phases. The jet is assumed to be slender and laminar, as occurs when the Reynolds number is moderately large, so that the boundary-layer form of the conservation equations can be employed in the analysis. Two dimensionless parameters are found
to control the flow structure, namely the spray dilution parameter 1, defined as the mass of liquid fuel per unit mass of gas in the spray stream, and the group vaporization
parameter e, defined as the ratio of the characteristic time of spray evolution due to droplet vaporization to the characteristic diffusion time across the jet. It is observed
that, for the small values of e often encountered in applications, vaporization occurs only in a thin layer separating the spray from the outer droplet-free stream. This regime of sheath vaporization, which is controlled by heat conduction, is amenable to a simplified asymptotic description, independent of ε,in which the location of
the vaporization layer is determined numerically as a free boundary in a parabolic problem involving matching of the separate solutions in the external streams, with
appropriate jump conditions obtained from analysis of the quasi-steady vaporization front. Separate consideration of dilute and dense sprays, corresponding, respectively,
to the asymptotic limits λ<<1 and λ>>1, enables simplified descriptions to be obtained for the different flow variables, including explicit analytic expressions for the spray penetration distance.A. Arrieta San AgustínAntonio Luis Sánchez PérezAmable Liñán MartínezF.A. Williams2014-10-31T07:24:39Z2014-10-31T07:24:39Zhttp://oa.upm.es/id/eprint/32538This item is in the repository with the URL: http://oa.upm.es/id/eprint/325382014-10-31T07:24:39ZNumerical analysis of deflagration initation by a hot jetNumerical 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 enhancing mixing 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 a c 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 Damköhler number Δc=(a d/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 Δc increasing significantly with increasing Lewis numbers. The reactant diffusivity affects also the way ignition takes place, so that for reactants with 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 corporating detailed chemistry models for specific fuel–air mixtures.Inmaculada Iglesias EstradéMarcos Vera CoelloAntonio Luis Sánchez PérezAmable Liñán Martínez2010-01-29T12:18:45Z2016-04-20T11:53:19Zhttp://oa.upm.es/id/eprint/2111This item is in the repository with the URL: http://oa.upm.es/id/eprint/21112010-01-29T12:18:45ZThe reduced kinetic description of lean premixed combustionLean premixed methane-air flames are investigated
in an effort to facilitate the numerical description
of CO and NO emissions in LPP (lean premixed
prevaporized) combustion systems. As an initial
step, the detailed mechanism describing the fuel
oxidation process is reduced to a four-step reduced
description that employs CO, H2 and OH as intermediates
not following a steady-state approximation.
It is seen that, under conditions typical
of LPP combustion, the mechanism can be further
simplified to give a two-step description, in which
fuel is consumed and CO is produced according to
the fast overall step CH4 + f 0 2 -*• CO + 2H20,
while CO is slowly oxidized according to the overall
step CO + | 0 a -*• CO2- Because of its associated
fast rate, fuel consumption takes place in thin layers
where CO, H2 and OH are all out of steady
state, while CO oxidation occurs downstream in a
distributed manner in a region where CO is the only
intermediate not in steady state. In the proposed
description, the rate of fuel consumption is assigned
a heuristic Arrhenius dependence that adequately
reproduces laminar burning velocities, whereas the
rate of CO oxidation is extracted from the reduced
chemistry analysis. Comparisons with results obtained
with detailed chemistry indicate that the
proposed kinetic description, not only reproduces
well the structure of one-dimensionai flames, including
profiles of CO, temperature and radicals,
but can also be used to calculate NO emissions by
appending an appropriate reduced chemistry description
that includes both the thermal and the
N20 production paths. Although methane is employed
in the present study as a model fuel, the
universal structure of the resulting CO oxidation
region, independent of the fuel considered, enables
the proposed formulation to be readily extended to
other hydrocarbons.Amable Liñán MartínezAntonio Luis Sánchez PérezAlain LépinetteM. BolligBenigno Lázaro Gómez2009-12-04T09:06:38Z2016-12-16T09:11:50Zhttp://oa.upm.es/id/eprint/1970This item is in the repository with the URL: http://oa.upm.es/id/eprint/19702009-12-04T09:06:38ZOne-step reduced kinetics for lean hydrogen–air deflagrationA short mechanism consisting of seven elementary reactions, of which only three are reversible, is shown
to provide good predictions of hydrogen–air lean-flame burning velocities. This mechanism is further
simplified by noting that over a range of conditions of practical interest, near the lean flammability
limit all reaction intermediaries have small concentrations in the important thin reaction zone that
controls the hydrogen–air laminar burning velocity and therefore follow a steady state approximation,
while the main species react according to the global irreversible reaction 2H2 + O2 → 2H2O. An
explicit expression for the non-Arrhenius rate of this one-step overall reaction for hydrogen oxidation
is derived from the seven-step detailed mechanism, for application near the flammability limit. The
one-step results are used to calculate flammability limits and burning velocities of planar deflagrations.
Furthermore, implications concerning radical profiles in the deflagration and reasons for the success
of the approximations are clarified. It is also demonstrated that adding only two irreversible direct
recombination steps to the seven-step mechanism accurately reproduces burning velocities of the full
detailed mechanism for all equivalence ratios at normal atmospheric conditions and that an eight-step
detailed mechanism, constructed from the seven-step mechanism by adding to it the fourth reversible
shuffle reaction, improves predictions of O and OH profiles. The new reduced-chemistry descriptions can
be useful for both analytical and computational studies of lean hydrogen–air flames, decreasing required
computation times.Daniel Fernández GalisteoAntonio Luis Sánchez PérezAmable Liñán MartínezF.A. Williams2009-12-04T07:47:25Z2016-04-20T11:46:52Zhttp://oa.upm.es/id/eprint/1973This item is in the repository with the URL: http://oa.upm.es/id/eprint/19732009-12-04T07:47:25ZThe hydrogen–air burning rate near the lean flammability limitThis paper investigates the inner structure of the thin reactive layer of hydrogen–air fuellean deflagrations close to the flammability limit. The analysis, which employs seven
elementary reactions for the chemistry description, uses the ratio of the characteristic radical and fuel concentrations as a small asymptotic parameter, enabling an accurate analytic expression for the resulting burning rate to be derived. The analysis reveals that the steady-state assumption for chemical intermediaries, applicable on the hot side of the reactive layer, fails, however, as the crossover temperature is approached, providing
a nonnegligible higher-order correction to the burning rate. The results can be useful, for instance, in future investigations of hydrogen deflagration instabilities near the lean flammability limit.Daniel Fernández GalisteoAntonio Luis Sánchez PérezAmable Liñán MartínezF.A. Williams2009-12-04T07:36:12Z2016-04-20T11:46:57Zhttp://oa.upm.es/id/eprint/1975This item is in the repository with the URL: http://oa.upm.es/id/eprint/19752009-12-04T07:36:12ZLaminar gas jets in high-temperature atmospheresNumerical and asymptotic methods are used to describe the structure of low-temperature laminar gas jets discharging into a hot atmosphere of the same gas in the limit of small jet-to-ambient temperature ratios " = Tj/To 1. In the limit " ! 0, heat conduction cannot modify significantly the temperature in the cold gas, leading to a two-region flow structure consisting of a neatly defined unperturbed cold jet for r < rf (x), where T = T0/To = " and u = U0/Uj = 1, surrounded by a hot gas. These two region are separated by a transition layer where T − " " and 1 − u 1. In planar jets the front thickens with distance achieving thickness of order unity at axial distances x "−(1+)Rea forcing the near-axis fluid to change slowly its velocity and temperature, being necessary distances x "−2Rea to reach values of T and 1−u of order unity. In round jets the front remains at r = a up to distances x "−(1−)(log "−1)2 where the front is forced to move radially towards the axis of the jet, reaching r = 0 at x "−1 log "−1Rea. The arrival of the front forces the change of the velocity and temperature in the near-axis region, reaching values of order unity in a far field region of characteristic length x "−1 log "−1Rea, distance comparable to that needed by the front to achieve the axis. In both geometries, the distance necessary for the fully development of the cold jet is considerably longer than that required by the isothermal jet x Rea.Mario Sánchez SanzAntonio Luis Sánchez PérezAmable Liñán Martínez2009-05-20Z2016-01-14T10:01:16Zhttp://oa.upm.es/id/eprint/1483This item is in the repository with the URL: http://oa.upm.es/id/eprint/14832009-05-20ZReduced kinetic mechanisms for modelling LPP combustión in gas turbinesReduced kinetic mechanisms for modelling LPP combustión in gas turbinesAmable Liñán MartínezM. BolligAntonio Luis Sánchez PérezBenigno Lázaro Gómez2009-04-24Z2016-04-20T06:33:33Zhttp://oa.upm.es/id/eprint/841This item is in the repository with the URL: http://oa.upm.es/id/eprint/8412009-04-24ZThe reduced kinetic description of lean premixed combustionLean premixed methane-air flames are investigated in an effort to facilitate the numerical description of CO and NO emissions in LP (lean premixed) and LPP (lean premixed prevaporized) combustion systems. As an initial step, the detailed mechanism describing the fuel oxidation process is reduced to a four-step description that employs CO, H2, and OH as intermediates not following a steady-state approximation. It is seen that, under conditions typical of gas-turbine combustion, this mechanism can be further simplified to give a two-step reduced description, in which fuel is consumed and CO is produced according to the fast overall step CH4 + 3/2 O2 CO + 2H2O, while CO is slowly oxidized according to the overall step CO + 1/2 O2 CO2. Because of its associated fast rate, fuel consumption takes place in a thin layer where CO, H2, and OH are all out of steady state, while CO oxidation occurs downstream in a distributed manner in a region where CO is the only intermediate not in steady state. In the proposed description, the rate of fuel consumption is assigned a heuristic Arrhenius dependence that adequately reproduces laminar burning velocities, whereas the rate of CO oxidation is extracted from the reduced chemistry analysis. Comparisons with results obtained with detailed chemistry indicate that the proposed kinetic description not only reproduces well the structure of one-dimensional unstrained and strained flames, including profiles of CO, temperature, and radicals, but can also be used to calculate NO emissions by appending an appropriate one-step reduced chemistry description that includes both the thermal and the N2O production paths. Although methane is employed in the present study as a model fuel, the universal structure of the resulting CO oxidation region, independent of the fuel considered, enables the proposed formulation to be readily extended to other hydrocarbons.Antonio Luis Sánchez PérezAlain LépinetteM. BolligAmable Liñán MartínezBenigno Lázaro Gómez2009-04-24Z2016-04-20T06:47:46Zhttp://oa.upm.es/id/eprint/1346This item is in the repository with the URL: http://oa.upm.es/id/eprint/13462009-04-24ZTheory of structures of hydrogen-air diffusion flamesThe structure of the hydrogen-air counterflow diffusion flame is investigated by Damkohler-number asymptotics. Attention is restricted to flowiield strain times smaller than dissociation times but larger than the characteristic chemical time of three-body recombination reactions, thereby placing the system on the upper, vigorously burning branch of the characteristic 5-curve of peak 'temperature as a function of strain time without equilibrium broadening. Under these conditions, it is shown that the reactants can coexist only within a thin reaction zone separating two radical-free equilibrium regions. First, the equations for the counterflow mixing layer in the two outer equilibrium regions are solved in the classical Burke-Schumann limit by the introduction of appropriate conserved scalars that account for variable transport coefficients and variable Schmidt numbers, different for different species. Then, the reaction zone is investigated with scaling that identifies the relevant reduced Darnkohler number. While the solutions in the outer zones are peculiar to the counterflow configuration because of their convective-difTusive character, the reactive-diflusive nature of the reaction zone at leading order enables its structure in transformed coordinates to be expressed independently of the geometrical configuration. Matching the solutions from the inner reaction region with those from the outer equilibrium regions yields the first-order asymptotic solution to the problem, which compares favorably with results obtained by numerical integration of the full conservation equations with detailed chemistry and transport descriptions. In particular, the reason that the maximum radical concentration and temperature deficit vary linearly with the one-third power of the strain rate is explained.Antonio Luis Sánchez PérezAmable Liñán MartínezF.A. WilliamsG. Balakrishnan2009-04-24Z2016-04-20T06:49:50Zhttp://oa.upm.es/id/eprint/1440This item is in the repository with the URL: http://oa.upm.es/id/eprint/14402009-04-24ZA WKB analysis of radical growth in the hydrogen-air mixing layerThe chain-branching process leading to ignition in the hydrogen-air mixing layer is studied by application of a novel WKB-like method with a four-step reduced scheme adopted for the chemistry description. Attention is restricted to initial free-stream temperatures above the crossover temperature corresponding to the second explosion limit of H2-O2 mixtures, thereby causing three-body recombination reactions to be negligible in the ignition process. It is shown that the initiation reactions, responsible for the early radical buildup, cease being
important when the radical mass fractions reach values of the order of the ratio of the characteristic branching time
to the characteristic initiation time, a very small quantity at temperatures of practical interest. The autocatalytic character of the chain-branching reactions causes the radical concentrations to growexponentially with downstream distance in the process that follows. It is shown that, because of the effect of radical diffusion, the radical growth rate is uniform across the mixing layer in the first approximation, with an exponent given by that of a premixed branching explosion evaluated at the location where the effective Damk¨ohler number based on the flow velocity is maximum. This exponent, as well as the leading-order representation of the radical profiles, are easily obtained by the imposition of a bounded, nonoscillatory behavior on the solution.Antonio Luis Sánchez PérezAmable Liñán MartínezF.A. Williams2009-04-22Z2016-04-20T06:34:13Zhttp://oa.upm.es/id/eprint/863This item is in the repository with the URL: http://oa.upm.es/id/eprint/8632009-04-22ZConfined swirling jets with large expansion ratiosThis paper presents the extension of our previous investigation of confined round jets with large Reynolds numbers and large expansion ratios (Revuelta, Sanchez & Linan 2002a) to the case of swirling jets with swirl numbers of order unity. In the absence of vortex breakdown, we encounter the four-region asymptotic structure identified earlier for the non-swirling jet, including a region of jet development where the azimuthal and axial velocity components are comparable. For the flow in the long recirculating eddy that forms downstream, where the pressure differences associated with the azimuthal motion become negligible, the jet is found to act as a point source with momentum flux equal to the flow force of the incoming jet, and angular momentum flux equal to that of the jet at the orifice. The solution for the weak circulation in this slender region, including the parameter-free leading-order description and the first-order corrections, is determined by integrating the azimuthal component of the momentum equation written in the boundary-layer approximation. The results are validated through comparisons with numerical integrations of the steady axisymmetric Navier-Stokes equations, which are also used to evaluate critical conditions for vortex breakdownAntonio Revuelta BayodAntonio Luis Sánchez PérezAmable Liñán Martínez2009-02-07Z2016-04-20T06:48:26Zhttp://oa.upm.es/id/eprint/1356This item is in the repository with the URL: http://oa.upm.es/id/eprint/13562009-02-07ZLifted laminar jet diffusion flamesThis paper addresses the numerical description of lifted flames in axisymmetric laminar coflow jets. The analysis considers moderately large values of the Reynolds number, when the boundary-layer approximation can be used to describe the slender mixing region that extends between the jet exit and the flame, providing the profiles of velocity and mixture fraction that exist immediately upstream from the flame front region. The description of the nonslender flame front region, which provides the front propagation velocity as an eigenvalue, requires integration of the Navier-Stokes equations with account taken of the reaction and thermal expansion effects. The limiting formulations corresponding to cases of practical interest, including large values of the air-to-fuel stoichiometric ratio, are
briefly discussed. Illustrative numerical results are given for flames lifted or propagating at distances small compared with the jet development length, where the mixing layer is nearly planar.Amable Liñán MartínezEduardo Fernández TarrazoMarcos Vera CoelloAntonio Luis Sánchez Pérez2009-02-02Z2016-04-20T06:48:33Zhttp://oa.upm.es/id/eprint/1360This item is in the repository with the URL: http://oa.upm.es/id/eprint/13602009-02-02ZReduced kinetics and coupling functions for calculating CO and NO emissions in gas-turbine combustionA reduced chemical-kinetic mechanism consisting of two global steps for fuel oxidation and an additional step for NO production is proposed as the minimal chemistry description for calculating CO and NO emissions in gas-turbine combustion. Carbon monoxide is seen to emerge as the main intermediate during the fuel-oxidation process, which takes place in two steps: fast partial hydrocarbon oxidation to give CO and H2O in a relatively thin fuel-consumption layer and slow CO oxidation to CO2 in a much larger region. All relevant intermediates but CO follow a steadystate approximation in the CO-oxidation region, so that the associated steady-state expressions can be employed to accurately compute the CO-oxidation rate. Since steady states for radicals and H2 fail in the fuel-consumption layer, fuel consumption cannot be computed with acceptable accuracy from the reduced kinetics, a
limitation that motivates the introduction of a heuristic Arrhenius law for the fuel-consumption rate. Production of oxides of nitrogen is represented by a single overall step that considers both the thermal and the nitrous oxide mechanisms but neglects the effects of the Fenimore and reburn contributions. As a preliminary step to facilitate computations, the conservation equations corresponding
to the resulting three-step mechanism are written in terms of appropriate coupling functions, different in premixed and nonpremixed systems. Preliminary calculations of methane-air flames, including both freely propagating premixed flames as well as counterflow nonpremixed flames, indicate that the proposed reduced kinetics produces good accuracy over a wide range of conditions.Alain LépinetteAmable Liñán MartínezBenigno Lázaro GómezAntonio Luis Sánchez Pérez2009-01-31Z2016-04-20T06:48:24Zhttp://oa.upm.es/id/eprint/1355This item is in the repository with the URL: http://oa.upm.es/id/eprint/13552009-01-31ZA generalized burke-schumann formulation for hydrogen-oxygen diffusion flames maintaining partial equilibrium of the shuffle reactionsUnder a wide range of conditions of ambient pressures, temperatures, dilutions and strain rates, nonpremixed combustion in hydrogen-oxygen systems maintains partial equilibrium of the four two-body chain-carrying reactions while experiencing finite rates of the three-body radical-recombination reactions H + 03 + M -+ HOa + M and H + H + M -+ H2 + M. There then exists a three-step reduced mechanism, with H as the only intermediate species and concentrations of the radicals O, OH and H02 related to that of H through steady states. The conservation equations corresponding to this chemical description are formulated here in terms of generalized coupling functions that account for species diffusivities that differ from the thermal diffusivity, providing a set of equations that describe the flame structure for strain conditions ranging from near extinction to weakly strained flames. As a model example, the formulation is applied to the analysis of flame development in the hydrogen-air laminar mixing layer with free-stream temperatures above the crossover temperature corresponding to the second explosion limit. The formulation can be used for many other model problems as well as for computational studies of nonpremixed combustion in complex configurations involving both laminar and turbulent flows.Antonio Luis Sánchez PérezAmable Liñán MartínezF.A. Williams2009-01-15Z2016-04-20T06:46:15Zhttp://oa.upm.es/id/eprint/1290This item is in the repository with the URL: http://oa.upm.es/id/eprint/12902009-01-15ZAn asymptotic analysis of chain-branching ignition in the
laminar wake of a splitter plate separating streams of
hydrogen and oxygenAbstract. The chain-branching process leading to ignition in the high-temperature laminar wake that forms at the trailing edge of a thin splitter plate separating a stream of hydrogen from a stream of oxygen is investigated with a reduced chemistry description that employs H as the only chain-branching radical not in steady state. The analysis presented covers ignition events occurring in the Rott-Hakkinen and Goldstein regions, where self-similar solutions for the different flow variables are available. It is found that the initiation reactions, which create the first radicals, are only important in a relatively small initial region, becoming negligible downstream as the radical mole fractions increase to values larger than the ratio of the characteristic branching time to the characteristic initiation time, a very small quantity at temperatures of practical interest. As a result, most of the ignition history is controlled by the autocatalytic branching reactions, giving rise to a radical pool that increases exponentially with distance in a process that is described by using as a large parameter the ratio of the streamwise distance to the downstream extent of the initial region where initiation reactions are significant. Comparisons of the asymptotic results with numerical integrations of the conservation equations reveal that
a three-term expansion for the H-atom profile is necessary in this case to provide an accurate prediction for the ignition distance.Antonio Luis Sánchez PérezInmaculada Iglesias EstradéAmable Liñán Martínez2009-01-15Z2016-04-20T06:46:45Zhttp://oa.upm.es/id/eprint/1308This item is in the repository with the URL: http://oa.upm.es/id/eprint/13082009-01-15ZBranched-chain ignition in strained mixing layersAbstract. The time-dependent evolution of the radical pool in an initially inert hydrogen-air counterflow mixing layer subject to variable strain is investigated analytically. Although the initial chemistry description contains three different chain carriers, namely, H, O and OH, it is shown
that the ignition problem can be accurately described in terms of a single radical-pool variable that incorporates steady-state assumptions for the radicals O and OH. Use of this non-standard procedure reduces the problem to the integration of a single conservation equation, whose solution depends on the existing Damkohler number A, defined as the ratio of the diffusion time across the mixing
layer to the characteristic branching time. Ignition takes place when A remains predominantly above a critical value of the order of unity. The exponentially growing radical pool, which extends across the mixing layer, can be described analytically by separation of variables in configurations with a slowly varying strain rate, providing a solution that is used to investigate the parametric
dependences of the ignition time. Weakly strained solutions are studied separately by addressing the asymptotic limit of large Damkohler numbers. It is seen that the reaction zone then becomes a thin layer of relative thickness A- 1 ' 4 centred at the location where the branching rate is maximum.
The analysis employs asymptotic expansions in decreasing powers of A for the shape and for the exponential growth rate of the radical pool. The accurate description of the solution necessitates computation of three terms in the asymptotic expansion for the growth rate, yielding predictions for the ignition time that remain accurate even for values of A of the order of unity.José Damián Mellado RamírezAntonio Luis Sánchez PérezJ.S. KimAmable Liñán Martínez2009-01-12Z2016-04-20T06:46:22Zhttp://oa.upm.es/id/eprint/1295This item is in the repository with the URL: http://oa.upm.es/id/eprint/12952009-01-12ZA bifurcation analysis of high-temperature ignition of H2-O2 diffusion flamesThe form of the ignition branch for steady, counterflow, hydrogen-oxygen diffusion flames, with dilution
permitted in both streams, is investigated for two-step reduced chemistry by methods of bifurcation theory.
Attention is restricted to fuel-stream temperatures less than or equal to the oxidizer-stream temperature
Tx and to T„ larger than or of the order of the crossover temperature Tc at which the rates of production
and consumption of H atoms are equal. Two types of solutions are identified, a frozen solution that always
exists in this kinetic approximation because all rates are proportional to the concentration of the intermedíate
H atom, and an ignited solution, represented by a branch of the curve giving the máximum H
concentration in terms of a Damkohler number constructed from the strain rate and the rate of the
branching step H + Os - OH + O. For T„ > T„ the latter bifurcates from the frozen solution if the
Damkohler number is increased to a critical valué. For T„ larger than a valué Ts > Tc, the effeets of
chemical heat reléase are small, and ignition is always gradual in the sense that the limiting ignited-branch
slope is positive (supercritical bifurcation) and there is no S curve. For T„ in the range Tc< T„ < T„ the
heat reléase associated with the radical-consumption step causes the limiting ignition-branch slope to
become negative (subcritical bifurcation), producing abrupt ignition which leads to an S curve. For valúes
of Tx below crossover, the ignited branch appears as a C-shaped curve unconnected to the frozen solution.
The method of analysis introduced here offers a first step toward analytical description of nonpremixed
H2-02 autoignition.Antonio Luis Sánchez PérezAmable Liñán MartínezF.A. Williams2008-03-03Z2016-04-20T06:35:07Zhttp://oa.upm.es/id/eprint/889This item is in the repository with the URL: http://oa.upm.es/id/eprint/8892008-03-03ZSimplified approach to the numerical description of methane-air diffusion flamesStarting with a three-step reduced chemistry description that employes H2 and CO as the only intermediates not in steady state, a simplified formulation aimed at facilitating numerical computations of non-premixed methane-air systems is developed. The analysis retains finite rates for radical recombination and CO oxidation but assumes infinitely fast fuel consumption taking place in a diffusion-controlled manner in an infinitely thin reaction sheet. To remove stiffness associated with the fast fuel consumption, the conservation equations for the major species and the temperature are written in terms of generalized coupling functions that for predictive accuracy permit species diffusivities that differ from the thermal diffusivity. The resulting formulation, which automatically determines the position of the fuel-consumption layer without necessity of front tracking or further interface approximations, can be used for analytical, computational, and modeling studies of both laminar and turbulent flows, removing stiffness difficulties associated with highly disparate chemical time scales. Comparisons of results of the simplified formulation in the counterflow mixing layer with those obtained with detailed chemistry and transport descriptions indicate that the proposed formulation applies with good accuracy to strain conditions ranging from weakly strained, robust flames to near-extinction flames.M. BolligAmable Liñán MartínezAntonio Luis Sánchez PérezF.A. Williams2008-02-22Z2016-04-20T06:34:24Zhttp://oa.upm.es/id/eprint/871This item is in the repository with the URL: http://oa.upm.es/id/eprint/8712008-02-22ZLaminar Craya-Curtet jetsThis Brief Communication investigates laminar Craya-Curtet flows, formed when a jet with moderately large Reynolds number discharges into a coaxial ducted flow of much larger radius. It is seen that the Craya-Curtet number, C=(J/sub c//J/sub j/)/sup 1/2/, defined as the square root of the ratio of the momentum flux of the coflowing stream to that of the central jet, arises as the single governing parameter when the boundary-layer approximation is used to describe the resulting steady slender jet. The numerical integrations show that for C above a critical value C/sub c/ the resulting streamlines remain aligned with the axis, while for C<C/sub c/ the entrainment demands of the jet cannot be satisfied by the coflow, and a toroidal recirculation region forms. The critical Craya-Curtet number is determined for both uniform and parabolic coflow, yielding C/sub c/=0.65 and C/sub c/=0.77, respectively. The streamlines determined numerically are compared with those obtained experimentally by flow visualizations, yielding good agreement in the resulting flow structure and also in the value of C/sub c/Antonio Revuelta BayodCarlos Martínez BazánAntonio Luis Sánchez PérezAmable Liñán Martínez2008-02-22Z2016-04-20T06:34:26Zhttp://oa.upm.es/id/eprint/872This item is in the repository with the URL: http://oa.upm.es/id/eprint/8722008-02-22ZThe quasi-cylindrical description of submerged laminar swirling jetsTThe quasi-cylindrical approximation is used to describe numerically the structure of a submerged swirling jet for subcritical values of the swirl ratio S<Sc . The emerging flow structure is affected by the swirling motion, which enhances the entrainment rate of the jet and induces an adverse pressure gradient that reduces its momentum flux. The effect is more pronounced as the swirl ratio S is increased, yielding for sufficiently large values of S a jet with an annular structure. The integration describes the smooth transition towards the far-field self-similar solution for all values of S smaller than a critical value S5Sc , at which the numerical integration fails to converge at a given downstream location. The comparisons with previous experimental results confirm the correspondence between the onset of vortex breakdown and the failure of the quasi-cylindrical approximation.Antonio Revuelta BayodAntonio Luis Sánchez PérezAmable Liñán Martínez2008-02-22Z2016-04-20T06:34:30Zhttp://oa.upm.es/id/eprint/874This item is in the repository with the URL: http://oa.upm.es/id/eprint/8742008-02-22ZSimulations of starting gas jets at low mach numbersThe starting jet produced by the impulsively started discharge of a submerged gas stream of constant velocity through a circular orifice in a plane wall is investigated by integrating numerically the axisymmetric Navier-Stokes equations for moderately large values of the jet Reynolds number. The analysis is restricted to low-Mach-number jets, for which the jet-to-ambient temperature ratio gamma=T/sub j//T/sub o/ emerges as the most relevant parameter. It is seen that the leading vortex approaches a quasisteady structure propagating at an almost constant velocity, which is larger for smaller values of gamma. The action of the baroclinic torque in regions of nonuniform temperature leads to significant vorticity production, with a constant overall rate equal to that of an inviscid starting jetInmaculada Iglesias EstradéMarcos Vera CoelloAntonio Luis Sánchez PérezAmable Liñán Martínez2008-02-22Z2016-04-20T06:34:34Zhttp://oa.upm.es/id/eprint/876This item is in the repository with the URL: http://oa.upm.es/id/eprint/8762008-02-22ZExact solutions for transient mixing of two gases of different densitiesThis Brief Communication presents a number of exact solutions describing the transient mixing of two gases of different molecular weights. Descriptions are given for both the concentration field and the associated induced motion in one-dimensional spherical, cylindrical, and planar configurations, including mixing layers, pockets, coflow jets, and concentrated mass sources.Antonio Luis Sánchez PérezMarcos Vera CoelloAmable Liñán Martínez2008-02-19Z2016-04-20T06:34:17Zhttp://oa.upm.es/id/eprint/865This item is in the repository with the URL: http://oa.upm.es/id/eprint/8652008-02-19ZThe virtual origin as a first-order correction for the far-field description of laminar jetsThe far-field velocity and composition fields of a submerged laminar jet are known to approach a self-similar solution corresponding to the flow induced by a point source of momentum and scalar. Previous efforts to improve this far-field description have introduced a virtual origin for the streamwise coordinate to remedy the singular behavior of the self-similar solution near the jet origin. The purpose of this note is to show, by means of a perturbative analysis of the point-source solution, that this virtual origin is in fact the first-order correction to the leading-order description. The perturbative analysis, which uses the ratio x of the streamwise distance to the length of jet development as an asymptotically large quantity, also indicates that the displaced point source provides the description in the far field with small relative errors of order x-3 for the round jet and of order x-10/3 for the plane jet. The values of the virtual origin are obtained by numerical integration of the boundary-layer equations in the region of jet development, giving values that depend on the shape of the jet velocity profile at the exit.Antonio Revuelta BayodAntonio Luis Sánchez PérezAmable Liñán Martínez2008-02-18Z2016-04-20T06:34:11Zhttp://oa.upm.es/id/eprint/862This item is in the repository with the URL: http://oa.upm.es/id/eprint/8622008-02-18ZHeat propagation from a concentrated energy source in a gasThis paper investigates the heat propagation process in a gas from concentrated energy sources with deposition times, t/sub d/', of the order of the characteristic acoustic time, t/sub a/', across the region where the temperature will be increased by a factor of order of unity. Heat propagation takes place by two different neatly defined spatial regions of comparable size. Around the source, we find a conductive region of very high temperature where the spatial pressure variations are negligible. The edge of the resulting strongly heated low- density region appears as a contact surface that acts as a piston for the outer flow, where the pressure disturbances, of order of the ambient pressure in the distinguished regime t/sub d/' ~ t/sub a/' considered here, generate a shock wave that heats up the outer gas as it propagates outwards. The mass and energy balances for the conductive region provide a differential equation linking its pressure with the velocity of its bounding contact surface, which is used, together with the jump conditions across the shock, when integrating the Euler equations for the outer compressible flow. Solutions for the heating history are obtained for point, line and planar sources for different values of the ratio t/sub d/'/t/sub a/', including weak sources with t/sub d/' Gt t/sub a/' and very intense sources with t/sub d/' Lt t/sub a/'. The solution determines in particular the temperature profile emerging as the pressure perturbations become negligible for times much larger than the acoustic timeV. KurdyumovAntonio Luis Sánchez PérezAmable Liñán Martínez2008-02-18Z2016-04-20T06:34:15Zhttp://oa.upm.es/id/eprint/864This item is in the repository with the URL: http://oa.upm.es/id/eprint/8642008-02-18ZFronts in high-temperature laminar gas jetsThis paper addresses the slender laminar flow resulting from the discharge of a low-Mach-number hot gas jet of radius a and moderately large Reynolds number Rj into a cold atmosphere of the same gas. We give the boundary-layer solution for plane and round jets with very small values of the ambient-to-jet temperature ratio ε accounting for the temperature dependence of the viscosity and conductivity typical of real gases. It is seen that the leading-order description of the jet in the limit ε → 0 exhibits a front-like structure, including a precisely defined separating boundary at which heat conduction and viscous shear stresses vanish in the first approximation, so that the temperature and axial velocity remain unperturbed outside the jet. Separate analyses are given for the jet discharging into a stagnant atmosphere, when the jet boundary is a conductive front, and for the jet discharging into a coflowing stream, when the jet boundary appears as a contact surface. We provide in particular the numerical description of the jet development region corresponding to axial distances of order Rja for buoyant and non-buoyant jets, as well as the self-similar solutions that emerge both in the near field and in the far field. In all cases considered, comparisons with numerical integrations of the boundary-layer problem for moderately small values of ε indicate that these front descriptions give excellent predictions for the temperature and velocity fields in the near-axis region.Mario Sánchez SanzAntonio Luis Sánchez PérezAmable Liñán Martínez2008-02-16Z2016-04-20T06:34:09Zhttp://oa.upm.es/id/eprint/861This item is in the repository with the URL: http://oa.upm.es/id/eprint/8612008-02-16ZConfined axisymmetric laminar jets with large expansion ratiosThis paper investigates the steady round laminar jet discharging into a coaxial duct when the jet Reynolds number, Re/sub j/, is large and the ratio of the jet radius to the duct radius, epsiv, is small. The analysis considers the distinguished double limit in which the Reynolds number Re/sub a/=Re/sub j/epsiv for the final downstream flow is of order unity, when four different regions can be identified in the flow field. Near the entrance, the outer confinement exerts a negligible influence on the incoming jet, which develops as a slender unconfined jet with constant momentum flux. The jet entrains outer fluid, inducing a slow backflow motion of the surrounding fluid near the backstep. Further downstream, the jet grows to fill the duct, exchanging momentum with the surrounding recirculating flow in a slender region where the Reynolds number is still of the order of Re/sub j/. The streamsurface bounding the toroidal vortex eventually intersects the outer wall, in a non-slender transition zone to the final downstream region of parallel streamlines. In the region of jet development, and also in the main region of recirculating flow, the boundary-layer approximation can be used to describe the flow, while the full Navier-Stokes equations are needed to describe the outer region surrounding the jet and the final transition region, with Re/sub a/=Re/sub j/epsiv entering as the relevant parameter to characterize the resulting non-slender flows.Antonio Revuelta BayodAntonio Luis Sánchez PérezAmable Liñán Martínez2008-02-13Z2016-04-20T06:34:00Zhttp://oa.upm.es/id/eprint/856This item is in the repository with the URL: http://oa.upm.es/id/eprint/8562008-02-13ZA simple one-step chemistry model for partially premixed hydrocarbon combustionThis work explores the applicability of one-step irreversible Arrhenius kinetics with unity reaction order to the numerical description of partially premixed hydrocarbon combustion. Computations of planar premixed flames are used in the selection of the three model parameters: the heat of reaction q, the activation temperature Ta, and the preexponential factor B. It is seen that changes in q with equivalence ratio φ{symbol} need to be introduced in fuel-rich combustion to describe the effect of partial fuel oxidation on the amount of heat released, leading to a universal linear variation q (φ{symbol}) for φ{symbol} > 1 for all hydrocarbons. The model also employs a variable activation temperature Ta (φ{symbol}) to mimic changes in the underlying chemistry in rich and very lean flames. The resulting chemistry description is able to reproduce propagation velocities of diluted and undiluted flames accurately over the whole flammability limit. Furthermore, computations of methane-air counterflow diffusion flames are used to test the proposed chemistry under nonpremixed conditions. The model not only predicts the critical strain rate at extinction accurately but also gives near-extinction flames with oxygen leakage, thereby overcoming known predictive limitations of one-step Arrhenius kinetics.Eduardo Fernández TarrazoAntonio Luis Sánchez PérezAmable Liñán MartínezF.A. Williams2008-02-10Z2016-04-20T06:32:39Zhttp://oa.upm.es/id/eprint/816This item is in the repository with the URL: http://oa.upm.es/id/eprint/8162008-02-10ZRelationships between bifurcation and numerical analyses for ignition of hydrogen-air diffusion flamesLinear bifurcation and numerical techniques are employed to determine critical conditions for ignition in steady, counterflow, nonpremixed hydrogen-air systems, with varying degrees of nitrogen dilution of the fuel, at temperatures larger than the crossover temperature associated with the second explosion limit for hydrogen. Analysis of profiles of the radical pool at ignition reveals that, irrespective of the degree of dilution of the fuel or oxidizer streams, the O-atom steady state fails on the oxidizer side of the mixing layer. Therefore, at least three overall steps, with O and H atoms as the chain-branching species, are necessary to describe the ignition process. A simplified model with variable density, specific heat and transport properties, and with Stefan-Maxwell approximations for the diffusion velocities, is proposed to describe the structure of the H2-O2- N2 weakly reactive mixing layer. Results of bifurcation analysis with this flow-field model and a three-step reduced chemical-kinetic scheme show excellent agreement with results of numerical integration of the full conservation equations with detailed chemistry for all degrees of dilution of the fuel feed.Antonio Luis Sánchez PérezG. BalakrishnanAmable Liñán MartínezF.A. Williams2008-02-10Z2016-04-20T06:33:53Zhttp://oa.upm.es/id/eprint/853This item is in the repository with the URL: http://oa.upm.es/id/eprint/8532008-02-10ZOn the calculation of the minimum ignition energyThe success of the initiation of a deflagration by an external energy source depends on the competition of heat conduction and radical diffusion with chemical heat release. As an order-of-magnitude criterion, one may state that the minimum ignition energyV. KurdyumovJ. BlascoAntonio Luis Sánchez PérezAmable Liñán Martínez2008-02-09Z2016-04-20T06:33:52Zhttp://oa.upm.es/id/eprint/852This item is in the repository with the URL: http://oa.upm.es/id/eprint/8522008-02-09ZLaminar mixing in diluted and undiluted fuel jets upstream from lifted flamesThe boundary-layer approximation is used to describe the frozen mixing process taking place when a fuel jet of radius a discharges into stagnant air. The results are applicable to the calculation of lifted flames stabilized in round laminar jets with relatively large Reynolds numbers, Re, for which the proposed formulation provides a detailed description for the velocity and composition fields encountered by the propagating triple flame formed at the base of the lifted flame. The problem is integrated for relevant values of the flow parameters, including values of the stoichiometric air-to-fuel mass ratio S of order unity, when the lifted flame is located in the region of jet development, corresponding to distances from the injector of order Re a. Further attention is given to the relevant case S ≫ 1, corresponding to typical conditions of undilute hydrocarbon-air flames, for which the resulting lifted flames are stabilized at relatively large distances from the injector, of order S Re a. It is seen that Schlichting asymptotic solution, which corresponds to a point source of momentum, is then applicable to describe the mixing process upstream from the lifted flame. Improved accuracy is sought by introducing expansions for the velocity components and for the reactant mass fractions in powers of S-1. The resulting development shows in particular that the first-order correction to the leading-order solution is equivalent to the introduction of a virtual origin for the axial coordinate. It is shown that the magnitude of the required translation, which is equal for the velocity and composition fields, must be determined from continuity considerations. As an illustrative example, the resulting description is used to calculate flame fronts with S ≫ 1 in the thermal-diffusive approximation.Antonio Revuelta BayodAntonio Luis Sánchez PérezAmable Liñán Martínez2008-01-04Z2016-04-20T06:32:08Zhttp://oa.upm.es/id/eprint/805This item is in the repository with the URL: http://oa.upm.es/id/eprint/8052008-01-04ZThe coupling of motion and conductive heating of a gas by localized energy sourcesThis paper investigates the time evolution of the near-isobaric flow field produced in a gas after the sudden application of a constant heat flux from a localized energy source. The problems of plane, line, and point heat sources are all investigated, with a power law for the temperature dependence of the thermal conductivity, after reduction to a quasi-linear heat equation for the temperature. In the planar and spherical cases, the constant heat flux defines scales for the length and time, which are used to nondimensionalize the problem. Numerical integration is used to provide the evolution of the temperature and velocity, and limiting solutions corresponding to small and large rescaled times are obtained. In the axisymmetric case, due to the absence of characteristic length and time scales, the solution is seen to admit a self-similar description in terms of the nondimensional heat flux. Profiles of temperature and radial velocity are provided for different values of this parameter, and the asymptotic limits of both small and large heating rates are addressed separately. The analysis reveals, in particular, the existence of front solutions when the resulting temperatures become much larger than the initial temperature, as occurs for sufficiently large times for the planar source, for sufficiently small times for the point source, and for sufficiently large heating rates for the line source.Antonio Luis Sánchez PérezJose Luis Jiménez AlvarezAmable Liñán Martínez2008-01-03Z2016-04-20T06:32:06Zhttp://oa.upm.es/id/eprint/804This item is in the repository with the URL: http://oa.upm.es/id/eprint/8042008-01-03ZChain-branching explosions in mixing layersThe chain-branching process leading to ignition in the high-temperature hydrogen-oxygen mixing layer is studied by application of a novel WKB-like method when, as is typically the case, two branching radicals cannot be assumed to maintain steady state. It is shown that the initiation reactions, responsible for the early radical buildup, cease being important when the radical mass fractions reach values of the order of the ratio of the characteristic branching time to the characteristic initiation time, a very small quantity at temperatures of practical interest. The autocatalytic character of the chain-branching reactions causes the radical concentrations to grow exponentially with downstream distance in the process that follows. It is shown that the transverse radical profiles that emerge can be described by exponential series of the WKB type in inverse powers of the streamwise coordinate. The analysis reveals that, because of the effect of radical diffusion, the rate of radical growth is uniform across the mixing layer in the first approximation, with the exponential growth in distance having the same nondimensional streamwise variation as that of a premixed branching explosion evaluated at the transverse location where the effective Damkoher number based on the flow velocity and branching rate is maximum. This functional streamwise variation, as well as the leading-order representation of the radical profiles, is obtained by imposing a condition of bounded, nonoscillatory behavior on the solution. The resulting radical profiles peak at the location of maximum local Damkohler number and decay exponentially to the sides. Analysis of the solution in the vicinity of the maximum, which is a turning point of second order in the WKB expansion, yields the second-order correction to the growth rate as an eigenvalue in a linear eigenvalue problem. The method developed can be extended to the analysis of chain-branching explosions in laminar, self-similar mixing layers with an arbitrary number of branching steps adopted for describing the chemistry.Antonio Luis Sánchez PérezAmable Liñán MartínezF.A. Williams