Front propagation in laminar flows

The problem of front propagation in flowing media is addressed for laminar velocity fields in two dimensions. Three representative cases are discussed: stationary cellular flow, stationary shear flow, and percolating flow. Production terms of Fisher-Kolmogorov-Petrovskii-Piskunov type and of Arrhenius type are considered under the assumption of no feedback of the concentration on the velocity. Numerical simulations of advection-reaction-diffusion equations have been performed by an algorithm based on discrete-time maps. The results show a generic enhancement of the speed of front propagation by the underlying flow. For small molecular diffusivity, the front speed $V_f$ depends on the typical flow velocity $U$ as a power law with an exponent depending on the topological properties of the flow, and on the ratio of reactive and advective time-scales. For open-streamline flows we find always $V_f \sim U$, whereas for cellular flows we observe $V_f \sim U^{1/4}$ for fast advection, and $V_f \sim U^{3/4}$ for slow advection.Comment: Enlarged, revised version, 37 pages, 14 figure

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

Modelling extinction and reignition in turbulent flames

The presented work attempts to extend the conditional moment closure method for noon-premixed. turbulent combustion to predict extinction and reignition phenomena in turbulent flames. The conditional moment closure method is one of a????class of conserved scalar modelling approaches in turbulent non-premixed combustion. where chemistry is treated as mainly dependend on the mixing of oxidizer and fuel. However. as designers of combustion devices aim for higher turbulence rates to enhance mixing and promote combustion, chemical conversion is not solely determined by the rate at which fuel and oxidizer are mixed, but kinetic effects become important. Therefore it is necessary in these cases. to consider a second variable to govern the evolution of the chemical system. This variable will parameterize the chemical conversion process from cold. mixed reactants at fixed eguivalence ratio to an eguilibrium state. Equations describing the chemical system as a function of these two variables, the conserved scalar, commonly referred to as mixture fraction and the progress variable. can be derived and constitute the doubly conditioned moment closure equations. However, solution of this set of equations is computationally expensive and key parameters describing the rate of dissipation of the progress variable, which is a reactive scalar, are not yet fully understood. By considering conditional fluctuations of the progress variable, applying simple relationships for scalar dissipation and using a pre-computed functional dependence of conditional moments on the progress variable, the effect of double conditioning on the chemical source term and on the overall chemistry predictions can be examined. The methodology is tested for its capability to predict the turbulent. piloted flames of the Sandia D-F series. These laboratory flames show an increasing degree of local extinction and reignition due to varying turbulence levels. Hence they provide an ideal benchmark for the study of models trying to predict these phenomena.Imperial Users onl

The measurement of flame propagation within an electrostatic particulate system

A variant of the Electrostatic Particulate Chamber was developed to directly measure flame propagation velocity of combustible gas and powder mixtures. Flame propagation velocity was measured by tracking the position of the flame front utilizing the ionization effect and current due to a high intensity applied electric field. The position of the front was determined with respect to time by monitoring the voltage spike and current flow through spatially separated resistors.;With this technique, the flame propagation velocity was initially tested in gas mixtures of propane/air and natural gas/air. Measured velocities were compared to published values for burning velocities of laminar premixed flames. Although the data trends compared well to expected behaviors, the values measured were higher than expected, likely due to turbulent effects.;Additionally, burning measurements were taken in propane/air mixtures with various concentrations of 53-63mum copper particles acting to quench the flame. At low copper concentrations the burning velocity was increased. However, as copper particle concentration was increased the measured burning velocity quickly dropped below the burning velocity of similar mixtures without copper particles.;The propagation velocity of 30-35mum aluminum particles in a natural gas/air mixture was also measured. The measured velocity was found to be higher than the measured propagation velocity of the natural gas/air mixture without the aluminum particles. The brightness of the flame also indicated ignition of the aluminum particles

Front Propagation in Random Media

This PhD thesis deals with the problem of the propagation of fronts under random circumstances. A statistical model to represent the motion of fronts when are evolving in a media characterized by microscopical randomness is discussed and expanded, in order to cope with three distinct applications: wild-land fire simulation, turbulent premixed combustion, biofilm modeling. In the studied formalism, the position of the average front is computed by making use of a sharp-front evolution method, such as the level set method. The microscopical spread of particles which takes place around the average front is given by the probability density function linked to the underlying diffusive process, that is supposedly known in advance. The adopted statistical front propagation framework allowed a deeper understanding of any studied field of application. The application of this model introduced eventually parameters whose impact on the physical observables of the front spread have been studied with Uncertainty Quantification and Sensitivity Analysis tools. In particular, metamodels for the front propagation system have been constructed in a non intrusive way, by making use of generalized Polynomial Chaos expansions and Gaussian Processes.The Thesis received funding from Basque Government through the BERC 2014-2017 program. It was also funded by the Spanish Ministry of Economy and Competitiveness MINECO via the BCAM Severo Ochoa SEV-2013-0323 accreditation. The PhD is fundend by La Caixa Foundation through the PhD grant “La Caixa 2014”. Funding from “Programma Operativo Nazionale Ricerca e Innovazione” (PONRI 2014-2020) , “Innotavive PhDs with Industrial Characterization” is kindly acknowledged for a research visit at the department of Mathematics and Applications “Renato Caccioppoli” of University “Federico II” of Naples

Definition of smolder experiments for Spacelab

The feasibility of conducting experiments in space on smoldering combustion was studied to conceptually design specific smoldering experiments to be conducted in the Shuttle/Spacelab System. Design information for identified experiment critical components is provided. The analytical and experimental basis for conducting research on smoldering phenomena in space was established. Physical descriptions of the various competing processes pertaining to smoldering combustion were identified. The need for space research was defined based on limitations of existing knowledge and limitations of ground-based reduced-gravity experimental facilities