C. Thomas Avedisian’s research while affiliated with Cornell University and other places

An experimental and numerical study of combustion of a gasoline certification fuel (‘indolene’), and four (S4) and five (S5) component surrogates for it, is reported for the configurations of an isolated droplet burning with near spherical symmetry in the standard atmosphere, and a single cylinder engine designed for advanced compression ignition of pre-vaporized fuel. The intent was to compare performance of the surrogate for these different combustion configurations and to assess the broader applicability of the kinetic mechanism and property database for the simulations. A kinetic mechanism comprised of 297 species and 16,797 reactions was used in the simulations that included soot formation and evolution, and accounted for unsteady transport, liquid diffusion inside the droplet, radiative heat transfer, and variable properties. The droplet data showed a clear preference for the S5 surrogate in terms of burning rate. The simulations showed generally very good agreement with measured droplet, flame, and soot shell diameters. Measurements of combustion timing, in-cylinder pressure, and mass-averaged gas temperature were also well predicted with a slight preference for the S5 surrogate. Preferential vaporization was not evidenced from the evolution of droplet diameter but was clearly revealed in simulations of the evolution of mixture fractions inside the droplets. The influence of initial droplet diameter (Do) on droplet burning was strong, with S5 burning rates decreasing with increasing Do due to increasing radiation losses from the flame. Flame extinction was predicted for Do =3.0 mm as a radiative loss mechanism but not predicted for smaller Do for the conditions of the simulations.

Dynamic measurement precision assessment has been achieved for a differential circle measurement application. Differential circle diameter measurement, in image analysis, typically requires fitting a circle model that optimizes for image distortions, defects or occlusions. The differential task occurs when precise measurements of diameter change are required given object size variation with time. An automated system was designed to provide diameter measurements and associated measurement precision of images of a fuel droplet undergoing combustion in zero gravity for the FLEX-2 dataset. An image gradient-based, least-squares boundary point fitting method to a circle or ellipse model is used for diameter measurement. The presence of soot aggregates poses significant challenges for diameter measurements when it occludes part of the droplet boundary. The precision of the diameter measurements depends upon the image quality. Using synthetic image simulations that model the soot behavior, we developed a model based on image quality measures that assesses the measurement precision for each individual diameter measurement. Thus, diameter measurements with precision assessments were made available for follow-up scientific analysis. The algorithms success rate for measurable runs was 98%. In cases of limited occlusion, a measurement precision of 0.2 pixels for the FLEX-2 dataset was achieved.

This paper reports a study of the combustion dynamics of n-butyl acetate (BA) using the configuration of a burning droplet. Two grades of BA were examined: one (SBA) synthesized by a new process described in the paper that uses a metabolically engineered solventogenic Clostridium strain through an extractive fermentation process using n-hexadecane as the extractant; and one commercially available as a high-purity (99.9%) ‘neat’ BA grade produced by conventional Fischer esterification (NBA). The initial droplet diameter was primarily 0.6 mm with some limited experiments carried out for 0.4 mm droplets to show the influence of convection. Experiments were performed in the standard atmosphere and ignition was by spark discharge. The results showed the presence of impurities in the SBA at mass concentrations totaling about 6% which included n-butanol, n-hexadecane, iso-propyl alcohol and ethyl acetate. Droplet burning rates and flame structures were not influenced by these impurities at this concentration level. In the presence of convection created by buoyancy, droplets burned faster with stretched flames and a luminosity revealing the presence of soot by incandescence at the flame tips. Reducing the initial droplet diameter to 0.4 mm eliminated the convective effect and resulted in near spherical flames. The results presented show that the new synthesis process is a sustainable alternative for BA production with burning characteristics identical to NBA in both convective and stagnant gas transport fields.

Combustion of a seven-component surrogate for a research grade 87 octane gasoline mixed with 10% ethanol is investigated experimentally and numerically from the perspective of an isolated droplet burning under conditions that promote one-dimensional gas transport. The numerical analysis included a kinetic mechanism comprised of 398 species and 24,814 reactions and a soot model that accounted for nucleation, surface growth, coalescence/aggregation of soot particles, and luminous flame radiation. Measurements of droplet and flame diameters were made for an initial droplet diameter (Do) of approximately 0.63 mm. The simulations agreed well with the measurements including the location of the soot shell. Preferential vaporization was revealed by simulations of the liquid concentrations in the droplet. Predicted peak soot volume fractions coincided with temperatures between 1300 K and 1400 K as a soot inception temperature. Simulations were also carried out for Do between 0.25 mm and 5 mm to explore the effect of radiation and Do on burning. Below 0.25 mm radiation was negligible and burning rates and flame temperatures converged to a single value. Increasing Do up to 1.8 mm lowered the burning rate with luminous radiation having a strong effect. When radiation was entirely removed from the model the burning rate was nearly constant. Above Do = 2 mm droplets extinguished almost immediately after ignition. The flame temperature decreased with increasing Do while it increased when radiation was omitted. The simulations show that soot precursors including polyaromatic hydrocarbons were concentrated around the soot shell.

This study reports an experimental and numerical investigation of droplet combustion of a miscible n-heptane/iso-octane mixture at a fixed mixture fraction (equi-volume) for initial diameters (Do) in the range of 0.8 mm ≤ Do < 5 mm. This range encompasses burning transitions from hot flame (HF) combustion to the cool flame (CF) regime where radiative extinction can occur. The simulations assume spherically symmetric gas transport which was promoted in the experiments by a low gravity environment and relatively stationary droplets. Unsupported or free-floating droplets were deployed and ignited in a sealed chamber on the International Space Station to provide a low gravity condition and to accommodate the anticipated long droplet burning times (tens of seconds) for the droplet sizes investigated. The simulations incorporated multistep combustion kinetics with an embedded low temperature kinetic mechanism, non-luminous flame radiation, a model for phase equilibrium of the mixture, variable properties, unsteady gas and liquid transport, and species diffusion in the liquid. The results showed no evidence of preferential vaporization because of the close boiling points of n-heptane and iso-octane. For Do < 3 mm, the mixture droplets remained in the initial HF burning regime. For larger Do, a transition to extinction-like behavior occurred. Measured flame radiances confirmed the importance of radiation as a controlling mechanism for driving radiative extinction and transitioning to CF burning. Radiative extinction diameters exhibited a linear relationship with Do which agreed very well with simulations. Mixture radiative extinction diameters were also consistent with literature values for n-decane, n-heptane, and iso-octane. Simulated droplet and flame diameters, burning rates, and flame radiances were also in good agreement with experiments.