s-Flame

A series of experiments is proposed with the following specific objectives, in both soot-free and sooty flames, to compare with detailed computational simulation to assess transport properties and detailed chemical kinetics (including chemiluminescence): Objective A. Measure and characterize the transient structure (such as temperature and species distribution) of diffusion flames established at initially non-steady, spherical flame structures for mixtures of different H2/CH4/diluent ratios (soot-free) and different C2H4/diluent ratios (sooty), for different flow rates. Objective B. Determine the quasi-steady convective/chemical-kinetic extinction limits (low system Damköhler numbers) and radiative/chemical-kinetic extinction limits (high system Damköhler numbers) for mixtures of different H2/CH4/diluent ratios (soot-free) and different C2H4/diluent ratios (sooty). Objective C. Determine the existence (for different Zeldovich number, Ze), onset (as a function Da), and nature (i.e. mode, frequency, amplitude) of pulsating instabilities theoretically predicted to occur in transient spherical diffusion flames using fuel/diluent mixtures that are above a critical Lewis number.

A series of experiments is proposed in which mixtures of various gaseous fuels and diluents will be issued into an oxidizing environment through a spherical porous burner and ignited to obtain, nominally, a spherical, one-dimensional stabilized flame. Synchronized measurements are to be made including spectrally resolved images, flame and burner temperatures, radiative flux measurements, and system operating conditions (including reactant flow rates, ambient temperatures, pressures and acceleration levels, etc.). These data are to be used for comparisons of flame behavior and structure with detailed numerical simulations of the flames. Conventional gaseous fuels including hydrogen, methane, and ethylene will be used. Hydrogen is the simplest fuel and its reaction mechanism is also reasonably established. Methane is the simplest hydrocarbon and extensive studies have been conducted on its combustion chemistry. Ethylene is a typical sooty fuel and is also an important intermediate for the oxidation of higher hydrocarbons. Diluents mixed with fuel are to include nitrogen and helium to adjust mixture diffusivities, and carbon dioxide to influence flame radiation characteristics.

The transient numerical simulation of spherical flames developed as part of this program includes detailed chemistry and transport models, especially radiation mechanisms. Each description of the flames obtained from the experiment, including measurements of aspects of the flame structure, growth or flame front velocities, and the specific test conditions where steady state flames, flame extinctions, flame front instabilities, soot suppression limits, etc., comprise a rigorous test of the detailed model. Together they provide a measure of the comprehensiveness of the chemical kinetic and transport models. The numerical simulation is also being used as a predictive tool to develop the science requirements for this experiment, especially the collection of test conditions and most aspects of the measurement systems. Due to time and g-jitter limitations, little of the planned test matrix can be adequately explored in the ground based test facilities, and the flight experiment definition will rely on the best effort in the modeling work.

The impact of this work consists of both practical and fundamental aspects. Practically, with diffusion flame burning as the basis in many combustion engines and devices, understanding the underlying dynamics and chemistry of diffusion flames is important in improving performance in terms of power, efficiency, pollutant emissions, and reliability. Fundamentally, the unique buoyancy-free environment offered by microgravity affords simple aerodynamic flow fields to acquire diffusion flame data of high fidelity, with the prospect of understanding unit thermochemical processes as well as quantifying the associated physico-chemical parameters.