Development of a new apparatus to measure flame spread through a free stratified fuel/air mixture: numerical modeling and experimental results

While flame spread through uniform fuel-air mixtures has been widely studied in combustion science, there has been relatively little attention given to the study of non-homogenous, or layered, fuel-air mixtures. However, these systems are common occurrences in such cases as terrestrial fuel spills and fuel leaks in both normal and microgravity. Conducting research on layered fuel-air mixtures and understanding the properties of flame propagation has potential implications for fire safety (both on earth and in space), as well as being of fundamental interest. The main objective behind this study is to determine flame speed, flammability regions, stability limits, and the shape of a flame propagating through a free, layered fuel-air mixture, as opposed to flame spread though layered mixtures over a solid surface, which had been previously studied. A free layer eliminates contact between the flame and the floor, which in turn reduces heat transfer and flow field effects. Such a system also simulates a fuel leak in microgravity conditions where the fuel vapor can be distributed by the slow ventilation flows, or a leak in normal gravity where a light fuel can ride in a plume. The system chosen for study consists of a 79 cm long, roughly 10 cm2 flow duct. A heated, porous bronze, fuel emitting airfoil is positioned 10 cm from the inlet along the centerline while a slow stream of air is blown parallel to the airfoil, creating the layered mixture in the laminar wake region. To design the flow duct geometry, a 2-D, multispecies, non-reacting numerical model of the system was developed using the FLUENT CFD software. This model accounts for diffusion and temperature of the fuel, which was ethanol in this study. The model provides a better understanding of the characteristics of the flow in the experimental apparatus, such as predicting velocity profiles, fuel concentration, and an estimated flame shape. Modeling results show that the flammable region in the duct is approximately 1 cm thick. The modeling results were used to position the igniter for the experimental runs, and to choose the inlet velocity and airfoil temperature. Analytical calculations were also performed to determine the conditions under which a stable, stationary (i.e. non-propagating) flame could exist in the wake of the airfoil. In this configuration, the velocity of the propagating flame is balanced by the convective velocity of the fuel/air mixture. The calculations also show the precise locations in the flow field wherein a stoichiometric fuel/air mixture exists. Once the geometry was characterized numerically, cold flow and combustion tests were performed. Cold flow testing included smoke tests which visualized the flow to ensure a steady, laminar quality, as well as hotwire anemometer and thermocouple scans to measure velocity and temperature profiles, respectively; all of these agreed with model predictions. Preliminary experimental results show that it is possible to obtain a propagating flame in a non-uniform free layer with flame spread rates of up to 180 cm/s in flame fixed coordinates. If conditions where optimal, a triple flame structure would form. Image sequences of the side view of the flame spread, along with spread rate, are presented in this thesis

Gravitational Influences on Flame Propagation Through Non-Uniform, Premixed Gas Systems

Flame propagation through non-uniformly premixed (or layered) gases has importance both in useful combustion systems and in unintentional fires. As summarized recently and in previous Microgravity Workshop papers, non-uniform premixed gas combustion receives scant attention compared to the more usual limiting cases of diffusion or uniformly premixed flames, especially regarding the role gravity plays. This paper summarizes our recent findings on gravitational effects on layered combustion along a floor, in which the fuel concentration gradient exists normal to the direction of flame spread. In an effort to understand the mechanism by which the flames spread faster in microgravity (and much faster, in laboratory coordinates, than the laminar burning velocity for uniform mixtures), we have begun making pressure measurements across the spreading flame front that are described here. Earlier researchers, testing in 1g, claimed that hydrostatic pressure differences could account for the rapid spread rates. Additionally, we present the development of a new apparatus to study flame spread in free (i.e., far from walls), non-homogeneous fuel layers formed in a flow tunnel behind an airfoil that has been tested in normal gravity