2,342 research outputs found
Aerodynamic properties of turbulent combustion fields
Flow fields involving turbulent flames in premixed gases under a variety of conditions are modeled by the use of a numerical technique based on the random vortex method to solve the Navier-Stokes equations and a flame propagation algorithm to trace the motion of the front and implement the Huygens principle, both due to Chorin. A successive over-relaxation hybrid method is applied to solve the Euler equation for flows in an arbitrarily shaped domain. The method of images, conformal transformation, and the integral-equation technique are also used to treat flows in special cases, according to their particular requirements. Salient features of turbulent flame propagation in premixed gases are interpreted by relating them to the aerodynamic properties of the flow field. Included among them is the well-known cellular structure of flames stabilized by bluff bodies, as well as the formation of the characteristic tulip shape of flames propagating in ducts. In its rudimentary form, the mechanism of propagation of a turbulent flame is shown to consist of: (1) rotary motion of eddies at the flame front, (2) self-advancement of the front at an appropriate normal burning speed, and (3) dynamic effects of expansion due to exothermicity of the combustion reaction. An idealized model is used to illustrate these fundamental mechanisms and to investigate basic aerodynamic features of flames in premixed gases. The case of a confined flame stabilized behind a rearward-facing step is given particular care and attention. Solutions are shown to be in satisfactory agreement with experimental results, especially with respect to global properties such as the average velocity profiles and reattachment length
Time-dependent computational studies of flames in microgravity
The research performed at the Center for Reactive Flow and Dynamical Systems in the Laboratory for Computational Physics and Fluid Dynamics, at the Naval Research Laboratory, in support of the NASA Microgravity Science and Applications Program is described. The primary focus was on investigating fundamental questions concerning the propagation and extinction of premixed flames in Earth gravity and in microgravity environments. The approach was to use detailed time-dependent, multispecies, numerical models as tools to simulate flames in different gravity environments. The models include a detailed chemical kinetics mechanism consisting of elementary reactions among the eight reactive species involved in hydrogen combustion, coupled to algorithms for convection, thermal conduction, viscosity, molecular and thermal diffusion, and external forces. The external force, gravity, can be put in any direction relative to flame propagation and can have a range of values. A combination of one-dimensional and two-dimensional simulations was used to investigate the effects of curvature and dilution on ignition and propagation of flames, to help resolve fundamental questions on the existence of flammability limits when there are no external losses or buoyancy forces in the system, to understand the mechanism leading to cellular instability, and to study the effects of gravity on the transition to cellular structure. A flame in a microgravity environment can be extinguished without external losses, and the mechanism leading to cellular structure is not preferential diffusion but a thermo-diffusive instability. The simulations have also lead to a better understanding of the interactions between buoyancy forces and the processes leading to thermo-diffusive instability
Time-dependent Computational Studies of Premixed Flames in Microgravity
This report describes the research performed at the Center for Reactive Flow and Dynamical Systems in the Laboratory for Computational Physics and Fluid Dynamics, at the Naval Research Laboratory, in support of NASA Microgravity Science and Applications Program. The primary focus of this research is on investigating fundamental questions concerning the propagation and extinction of premixed flames in earth gravity and in microgravity environments. Our approach is to use detailed time-dependent, multispecies, numerical models as tools to simulate flames in different gravity environments. The models include a detailed chemical kinetics mechanism consisting of elementary reactions among the eight reactive species involved in hydrogen combustion, coupled to algorithms for convection, thermal conduction, viscosity, molecular and thermal diffusion, and external forces. The external force, gravity, can be put in any direction relative to flame propagation and can have a range of values. Recently more advanced wall boundary conditions such as isothermal and no-slip have been added to the model. This enables the simulation of flames propagating in more practical systems than before. We have used the numerical simulations to investigate the effects of heat losses and buoyancy forces on the structure and stability of flames, to help resolve fundamental questions on the existence of flammability limits when there are no external losses or buoyancy forces in the system, to understand the interaction between the various processes leading to flame instabilities and extinguishment, and to study the dynamics of cell formation and splitting. Our studies have been able to bring out the differences between upward- and downward-propagating flames and predict the zero-gravity behavior of these flames. The simulations have also highlighted the dominant role of wall heat losses in the case of downward-propagating flames. The simulations have been able to qualitatively predict the formation of multiple cells and the cessation of cell-splitting. Our studies have also shown that some flames in a microgravity environment can be extinguished due to a chemical instability and without any external losses. However, further simulations are needed to more completely understand upward-propagating and zero-gravity flames as well as to understand the potential effect of radiative heat losses
A spectral deferred correction strategy for low Mach number reacting flows subject to electric fields
We propose an algorithm for low Mach number reacting flows subjected to
electric field that includes the chemical production and transport of charged
species. This work is an extension of a multi-implicit spectral deferred
correction (MISDC) algorithm designed to advance the conservation equations in
time at scales associated with advective transport. The fast and nontrivial
interactions of electrons with the electric field are treated implicitly using
a Jacobian-Free Newton Krylov approach for which a preconditioning strategy is
developed. Within the MISDC framework, this enables a close and stable coupling
of diffusion, reactions and dielectric relaxation terms with advective
transport and is shown to exhibit second-order convergence in space and time.
The algorithm is then applied to a series of steady and unsteady problems to
demonstrate its capability and stability. Although developed in a
one-dimensional case, the algorithmic ingredients are carefully designed to be
amenable to multidimensional applications
Triple flame structure and diffusion flame stabilization
The stabilization of diffusion ñames is studied using asymptotic techniques and numerical tools. The configuration studied corresponda to parallel streams of cold oxidizer and fuel initially separated by a splitter píate. It is shown that stabilization of a diffusion flame may only occur in this situation by two processes. First, the flame may be stabilized behind the flame holder in the wake of the splitter píate. For this case, numerical simulations confirm scalings previously predicted by asymptotic analysis. Second, the flame may be lifted. In this case a triple flame is found at longer distanees downstream of the flame holder. The structure and propagation speed of this flame are studied by using an actively controlled numerical technique in which the triple flame is tracked in its own reference frame. It is then possible to investigate the triple flame structure and velocity. It is shown, as suggested from asymptotic analysis, that heat reléase may induce displacement speeds of the triple flame larger than the laminar flame speed corresponding to the stoichiometric conditions prevailing in the mixture approaching the triple flame. In addition to studying the characteristics of triple flames in a uniform flow, their re-sistance to turbulence is investigated by subjecting triple flames to different vortical configurations
A fast, low-memory, and stable algorithm for implementing multicomponent transport in direct numerical simulations
Implementing multicomponent diffusion models in reacting-flow simulations is
computationally expensive due to the challenges involved in calculating
diffusion coefficients. Instead, mixture-averaged diffusion treatments are
typically used to avoid these costs. However, to our knowledge, the accuracy
and appropriateness of the mixture-averaged diffusion models has not been
verified for three-dimensional turbulent premixed flames. In this study we
propose a fast,efficient, low-memory algorithm and use that to evaluate the
role of multicomponent mass diffusion in reacting-flow simulations. Direct
numerical simulation of these flames is performed by implementing the
Stefan-Maxwell equations in NGA. A semi-implicit algorithm decreases the
computational expense of inverting the full multicomponent ordinary diffusion
array while maintaining accuracy and fidelity. We first verify the method by
performing one-dimensional simulations of premixed hydrogen flames and compare
with matching cases in Cantera. We demonstrate the algorithm to be stable, and
its performance scales approximately with the number of species squared. Then,
as an initial study of multicomponent diffusion, we simulate premixed,
three-dimensional turbulent hydrogen flames, neglecting secondary Soret and
Dufour effects. Simulation conditions are carefully selected to match
previously published results and ensure valid comparison. Our results show that
using the mixture-averaged diffusion assumption leads to a 15% under-prediction
of the normalized turbulent flame speed for a premixed hydrogen-air flame. This
difference in the turbulent flame speed motivates further study into using the
mixture-averaged diffusion assumption for DNS of moderate-to-high Karlovitz
number flames.Comment: 36 pages, 14 figure
Low-dimensional modelling of flame dynamics in heated microchannels
This paper presents simulations of stoichiometric methane/air premixed flames
into a microchannel at atmospheric pressure. These simulations result from
numerical resolutions of reduced-order models. Indeed, combustion control into
microchannels would be allowed by fast simulations that in turn enable
real-time adjustments of the device's parameters. Former experimental studies
reported the occurrence of a Flame Repetitive Extinction/Ignition (FREI)
phenomenon provided that a temperature gradient is sustained at the channel's
walls. Conducting unsteady one-dimensional simulations including complex
chemistry, a late numerical study tried to explain the occurrence of this
phenomenon. The present study therefore explores low-dimensional models that
potentially reproduce the FREI phenomenon. Provided a calibration of some
empirical constants, an unsteady two-dimensional model including one-step
chemical reaction is shown to decently reproduce the FREI regime all along the
range of mixture flow rates investigated by the experimental studies.
Complementing the aforementioned numerical study, furthermore, when the
channel's diameter is varied, the two-dimensional model unveils an unstable
regime that a one-dimensional model cannot capture. As two-dimensional
hydrodynamics appears to play a key role into the flame's dynamics, therefore
the heat rate released by the microcombustor, one-dimensional models are not
believed to deliver an adequate strategy of combustion control into such
microchannels.Comment: 37 pages, 12 figure
Front dynamics in turbulent media
A study of a stable front propagating in a turbulent medium is presented. The
front is generated through a reaction-diffusion equation, and the turbulent
medium is statistically modeled using a Langevin equation. Numerical
simulations indicate the presence of two different dynamical regimes. These
regimes appear when the turbulent flow either wrinkles a still rather sharp
propagating interfase or broadens it. Specific dependences of the propagating
velocities on stirring intensities appropriate to each case are found and
fitted when possible according to theoretically predicted laws. Different
turbulent spectra are considered.Comment: 8 pages, REVTEX, 6 postscript figures included. To appear in Phys.
Fluids (1997
Modeling Interface Motion Of Combustion (MINOC). A computer code for two-dimensional, unsteady turbulent combustion
A computer code for calculating the flow field and flame propagation in a turbulent combustion tunnel is described. The model used in the analysis is the random vortex model, which allows the turbulent field to evolve as a fundamental solution to the Navier-Stokes equations without averaging or closure modeling. The program was used to study the flow field in a model combustor, formed by a rearward-facing step in a channel, in terms of the vorticity field, the turbulent shear stresses, the flame contours, and the concentration field. Results for the vorticity field reveal the formation of large-scale eddy structures in the turbulent flow downstream from the step. The concentration field contours indicate that most burning occurred around the outer edges of the large eddies of the shear layer
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