Session R22: DFD Minisymposium: Frontiers in Combustion Physics II

Mixing in combustion

Mixing of reactants represents an important element in both non-premixed and premixed turbulent combustion. In non-premixed combustion, molecular mixing is a necessary first step that brings reactants together. In premixed combustion with local flame extinction and reignition, turbulent mixing of hot products with as-yet unburnt fluid is important to combustion behavior. The discussion on mixing will cover the role of entrainment, effects of Reynolds number and the mixing transition, effects of Schmidt number and gas- vs. liquid-phase reacting flows, heat release, Damkoehler-number (finite kinetic-rate) effects, and Mach-number effects.   

Highly Turbulent Counterflow Flames: A Laboratory Scale Benchmark for Practical Combustion Systems

Since the pioneering work of Weinberg's group at Imperial College in the `60s, the counterflow system has been the workhorse of laminar flame studies. Recent developments have shown that it is also a promising benchmark for highly \textit{turbulent} (Re$_{t}$ $\sim$ 1000) nonpremixed and premixed flames of direct relevance to gasturbine combustion. Case studies will demonstrate the versatility of the system in mimicking real flame effects, such as heat loss and flame stratification in premixed flames, and the compactness of the combustion region. The system may offer significant advantages from a computational viewpoint, including: a) aerodynamic flame stabilization near the interface between the two opposed jets, with ensuing simplifications in the prescription of boundary conditions; b) a fiftyfold reduction of the domain of interest as compared to conventional nonpremixed jet flames at the same Reynolds number; and c) millisecond mean residence times, which is particularly useful for DNS/LES computational modeling, and for soot suppression in the combustion of practical fuels.\\[4pt] In collaboration with Bruno Coriton and Jonathan Frank, Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94551, USA.   

Supersonic combustion

Combustion in the supersonic regime presents several challenges over what the low-speed counterpart admits. Here we will review some of these challenges, and we will describe some of the key features of one of the canonical flow fields in supersonic combustion: the reacting transverse jet in a supersonic crossflow (JISCF). From a practical standpoint, the key challenges that limit our control of this combustion regime are fast mixing, robust flame holding and stability. In turn, these aspects are controlled by the complex effects introduced by chemistry, compressibility, shocks and shock/flow interactions, turbulence and the underlying coupling among them. Some of their properties will be discussed here. In particular, for a JISCF in a Mach 2.4 high enthalpy crossflow, the reaction zone structure, its dependence on near-wall events, boundary layer, and shock/boundary layer interaction will be described. We will demonstrate the paramount importance of the coupling between boundary layers and compressibility to provide mechanisms for flame stabilization at the wall. Mixing characteristics, overall structure, and the link to global parameters (momentum flux, velocity and density ratios) that characterize the JISCF, and possibly free shear supersonic flows in general, will also be highlighted from non-reacting experiments.   

Kinetic Modeling of Low-Temperature Plasma Assisted Combustion

Quantitative insight into kinetics of low-temperature plasma assisted fuel oxidation and ignition would be impossible without kinetic modeling. The principal challenges in development of a predictive kinetic model of nonequilibrium plasmas sustained in fuel-air mixtures include (i) lack of ``conventional'' chemical kinetics mechanisms validated at low temperatures, (ii) lack of data on rates and products of reactions of excited species generated in the plasma, some of which are not well understood, and their coupling with fuel-air plasma chemistry, and (iii) scarcity of data obtained in well-characterized plasma-assisted combustion experiments, which can be used for model validation. ``Conventional'' combustion chemistry mechanisms have been developed for relatively high temperature conditions. Their applicability at temperatures below ignition temperature, common in plasma assisted combustion environments, needs to be assessed to determine if they can be used as a basis for a plasma-assisted combustion chemistry mechanism. This requires time-resolved measurements of radical species concentrations during low-temperature fuel oxidation, when an initial pool of primary radicals (O, H, and OH) is generated in the plasma, such as in the late afterglow of an electric discharge. This allows isolating relatively slow ``conventional'' low-temperature fuel oxidation reactions triggered by the radicals from the reactions of excited species generated in the discharge, which decay relatively rapidly. Kinetic modeling calculations demonstrated that some of the existing combustion mechanisms provide good agreement with the experimental data taken in lean H$_{2}$-, CH$_{4}$-, and C$_2$H$_{4}$-air mixtures at low temperatures, while data taken in C$_3$H$_8$-air are not reproduced by any of the mechanisms tested. A complementary approach is to focus on kinetics of ``rapid'' reactions of electronically and vibrationally excited species in the electric discharge, as well as oxygen dissociation by electron impact, and their effect on production of radicals in the early afterglow. These experiments provide key data on coupling of molecular energy transfer processes in the plasma with ``conventional'' chemical reactions. Time-resolved and spatially-resolved measurements of temperature, vibrational and electronic levels populations, and radical species concentrations are critical for characterization of the nonequilibrium reacting mixture at these conditions. Kinetic modeling of recent experiments in a diffuse filament, nanosecond pulse electric discharges in air suggest that the role of electronically excited N$_2$* molecules on chemical reactions in the afterglow, such as NO generation reactions, has been significantly underestimated in the past. Further experiments in fuel-air mixtures are expected to provide additional data on the role of these excited species on low-temperature fuel-air chemistry.   

Manipulating Flames with AC Electric Fields

Time-oscillating electric fields applied to plasmas present in flames create steady flows of gas capable of shaping, directing, enhancing, or even extinguishing flames. Interestingly, electric winds induced by AC electric fields can be stronger that those due to static fields of comparable magnitude. Furthermore, unlike static fields, the electric force due to AC fields is localized near the surface of the flame. Consequently, the AC response depends only on the local field at the surface of the flame - not on the position of the electrodes used to generate the field. These results suggest that oscillating electric fields can be used to manipulate and control combustion processes at a distance. To characterize and explain these effects, we investigate a simple experimental system comprising a laminar methane-air flame positioned between two parallel-plate electrodes. We quantify both the electric and hydrodynamic response of the flame as a function of frequency and magnitude of the applied field. A theoretical model shows how steady gas flows emerge from the time-averaged electrical force due to the field-induced motion of ions generated within the flame and by their disappearance by recombination. These results provide useful insights into the application of AC fields to direct combustion processes.