Harsha K. Chelliah

Dynamics of Water Droplets in a Counterflow Field and their Effect on Flame Extinction

Abstract The effect of fine-water droplets in extinguishing steady, laminar counterflow methane–air nonpremixed flames is investigated here, using a numerical approach. A new two-phase model using a hybrid Eulerian–Lagrangian formulation for the gas-droplet flow is developed as part of this work. A key feature of the model developed is that it can avoid the singularity associated with the droplet number density equation in a consistent manner by using a Lagrangian equation for droplet flux fraction. The gas phase is described by a detailed model involving full chemical kinetics and transport, whereas droplet evaporation and heat transfer are modeled assuming quasisteady conditions. Application of the model to several monodisperse sizes of water droplets, ranging from 5–50 μm, revealed an interesting nonmonotonic dependence of the flame extinction strain rate on droplet size. This phenomenon is attributed to the droplet dynamics in the counterflow field considered here and to the resulting nonmonotonic heat sink associated with mass evaporation observed at the oxygen-consumption or radical production layer of the flame.

Extinction of nonpremixed flames with halogenated fire suppressants

Abstract An experimental, analytical, and numerical study was performed to elucidate the influence of eleven gaseous agents, considered to be substitutes for CF3Br, on the structure and critical conditions of extinction of diffusion flames burning liquid hydrocarbon fuels. The effectiveness of these agents in quenching flames was compared to those of CF3Br and an inert diluent such as nitrogen. Experiments were performed on diffusion flames stabilized in the counterflowing as well as in the coflowing configuration. The fuels tested were heptane in the counterflowing configuration, and heptane, the jet fuels JP-8, and JP-5, and hydraulic fluids (military specifications 5606 and 83282) in the coflowing configuration. The oxidizing gas was a mixture of air and the agent. On a mass and mole basis CF3Br was found to be most effective in quenching the flames and the mass-based effectiveness of the other eleven agents was found to be nearly the same as that of nitrogen. Experimental results were interpreted using one-step, activation-energy asymptotic theories and the results were used to provide a rough indication of the thermal and chemical influence of these agents on the flame structure. To understand in some detail the influence of CF3Br on the structure and mechanisms of extinction of the flame, numerical calculations using detailed chemistry were performed. The calculated structure of counterflow heptane-air diffusion flames inhibited with CF3Br was found to consist of three distinct zones including a CF3Br consumption zone which appears to act as a sink for radicals. The calculated values of the critical conditions of extinction of counterflow heptane-air diffusion flames inhibited with CF3Br were found to agree fairly well with measurements. The study suggests the need for refinement of the inhibition chemistry.

Modeling of graphite oxidation in a stagnation-point flow field using detailed homogeneous and semiglobal heterogeneous mechanisms with comparisons to experiments

Abstract Numerical simulation results are presented on the mass burning rate and the gas-phase flame structure of a heated cylindrical graphite rod in a stagnation-point flow field, using detailed homogeneous chemical kinetics and semiglobal heterogeneous chemical kinetics. Extensive comparisons with new experimental data and data from the literature are shown for various oxidizer compositions, pressures and strain rates. The relative importance of the carbon-radical reactions in the two semiglobal heterogeneous mechanisms employed is demonstrated, while the deficiencies and limitations of applying semiglobal heterogeneous mechanisms for graphite rod oxidations are identified. Under simplifying assumptions, a method for including the graphite porosity in the present quasi-one dimensional formulation is described. The need to develop elementary reaction mechanisms for the heterogeneous kinetics and the importance of accurate estimation of effective surface are is stressed.

EXTINCTION CONDITIONS OF NON-PREMIXED FLAMES WITH FINE DROPLETS OF WATER AND WATER/NaOH SOLUTIONS

Interactions of fine droplets of water and water/NaOH solutions with a steady, laminar counterflow methane/air nonpremixed flame are investigated experimentally and numerically. A water atomizer generating a polydisperse distribution of droplet sizes with a median diameter of 20 lm is used in experiments with steady feed rate. Comparisons of the measured flame extinction condition as a function of droplet mass fraction in the air stream indicate a trend similar to that predicted previously using 20 lm monodisperse water droplets. The hybrid Eulerian-Lagrangian numerical model previously developed is generalized to include polydisperse distribution of drop sizes; however, the differences seen between experiments and the numerical predictions at high water mass fractions could not be attributed to variation in size distribution alone. Present experiments support the conclusions of an earlier modeling work that on a mass basis, fine water mist can be as effective as the now-banned gaseous fire suppressant halon 1301. Inclusion of NaOH in water (up to 17.5% by mass) is shown to significantly enhance the fire suppression ability of water by complementing its thermal effects with chemical catalytic radical recombination effects of NaOH.

Inhibition of Premixed and Non-Premixed Flames With Fine Droplets of Water and Solutions

Inhibition/extinction of premixed and non-premixed methane/air flames with fine droplets of water and solutions containing several chemical agents has been investigated experimentally. While solutions allow delivery of much higher concentrations of chemical agent to the flame front than otherwise possible, the non-premixed flame extinction results indicate saturation (or condensation) of the agent at some effective temperature below the flame temperature. Based on the chemical additives considered, on a molar basis, the following order of effectiveness is observed: KOH>NaCl>NaOH. The inhibition of premixed flames by similar size droplets indicates insensitivity to NaOH mass fraction in the water. This insensitivity was related to the shorter residence time of the droplets (13 μm median diameter) through the premixed flame structure. Detailed comparison of the premixed and non-premixed flame inhibition/extinction with pure water droplets supports the importance of droplet residence time and optimum droplet size in controlling the interaction of droplets with the flame front.

Effect of sodium bicarbonate particle size on the extinction condition of non-premixed counterflow flames

Abstract The effect of chemically active sodium bicarbonate (NaHCO3) particles and inert silica particles on suppression of laminar, non-premixed counterflow methane-air flames is investigated. In experiments, the NaHCO3 particles are separated into size ranges of

Large Eddy Simulation of High-Speed, Premixed Ethylene Combustion

A large-eddy simulation / Reynolds-averaged Navier-Stokes (LES/RANS) methodology is used to simulate premixed ethylene-air combustion in a model scramjet designed for dual mode operation and equipped with a cavity for flameholding. A 22-species reduced mechanism for ethylene-air combustion is employed, and the calculations are performed on a mesh containing 93 million cells. Fuel plumes injected at the isolator entrance are processed by the isolator shock train, yielding a premixed fuel-air mixture at an equivalence ratio of 0.42 at the cavity entrance plane. A premixed flame is anchored within the cavity and propagates toward the opposite wall. Near complete combustion of ethylene is obtained. The combustor is highly dynamic, exhibiting a large-scale oscillation in global heat release and mass flow rate with a period of about 2.8 ms. Maximum heat release occurs when the flame front reaches its most downstream extent, as the flame surface area is larger. Minimum heat release is associated with flame propagation toward the cavity and occurs through a reduction in core flow velocity that is correlated with an upstream movement of the shock train. Reasonable agreement between simulation results and available wall pressure, particle image velocimetry, and OH-PLIF data is obtained, but it is not yet clear whether the system-level oscillations seen in the calculations are actually present in the experiment.

US National Center for Hypersonic Combined Cycle Propulsion: An Overview

A new research center has been funded by the AFOSR and the NASA Fundamental Aeronautics Program to study hypersonic combined cycle propulsion systems. The Center has identified three regimes of a TBCC as critical to the successful operation of the combined cycle system: the turbine to ramjet transition, the ramjet to scramjet (dual-mode) transition and the hypervelocity operating regime. Experiments will be conducted, using advanced flowfield diagnostics, in three separate Center facilities, to provide measurements to characterize these three flow regimes. Advanced models will be developed using data from the three experiments. The goal of the model development is to provide an advanced suite of computational tools for modeling the complex flow in hypersonic combined cycle propulsion systems.

The influence of heterogeneous kinetics and thermal radiation on the oxidation of graphite particles

Numerical integrations have been performed to study the influence of porosity and radiation on the combustion of spherical graphite particles in a quiescent atmosphere. Two sets of heterogeneous rate constants, one assuming no internal burning and another assuming partial internal burning, have been employed for this purpose. Based on the quasi-steady burning conditions, the numerical predictions have shown for nonporous particles the existence of a critical particle diameter below which strong burning conditions cannot be sustained. This critical particle diameter is found to be a strong function of the porosity. The influence of thermal radiation on the burning characteristics has also been examined and a complex interaction identified between surface kinetics, gas-phase kinetics, mass transport through the boundary layer and thermal radiation.

Numerical prediction of oblique detonation wave structures using detailed and reduced reaction mechanisms

Modelling of the structure and the limiting flow turning angles of an oblique detonation wave, established by a two-dimensional wedge, requires the implementation of detailed chemical kinetic models involving a large number of chemical species. In this paper, a method of reducing the computational effort involved in simulating such high-speed reacting flows by implementing a systematically reduced reaction mechanism is presented. For a hydrogen - air mixture, starting with an elementary mechanism having eight species in 12 reactions, three alternate four-step reduced reaction mechanisms are developed by introducing the steady-state approximation for the reaction intermediates HO2, O and OH, respectively. Additional reduction of the computational effort is achieved by introducing simplifications to the thermochemical data evaluations. The influence of the numerical grid used in predicting the induction process behind the shock is also investigated. Comparisons of the induction zone predicted by two-dimensi...