Doctoral Dissertations
Date of Award
8-1997
Degree Type
Dissertation
Degree Name
Doctor of Philosophy
Major
Mechanical Engineering
Major Professor
Frank Collins
Committee Members
Schulz, Vakili, McGregor, Lewis
Abstract
The standard theories of particle combustion rely upon continuum gas dynamic relationships. These theories predict that small reacting particles are always essentially in thermal equilibrium with the ambient gas phase. This is a consequence of three results from continuum theory: 1) the convective loss is inversely proportional to particle size, 2) the convective loss is independent of pressure, and 3) the convective loss depends upon the thermal conductivity of the ambient gas. Because soot particles are smaller than the mean free path of the ambient gas under typical combustion conditions, the author believed that rarefaction effects may he important in establishing the particle temperature relative to the gas temperature. Therefore, this dissertation began with the derivation of the energy and mass conservation equations assuming free molecular conditions about a single particle. Under these assumptions, the convective loss was shown to be independent of particle size, dependent on the ambient pressure, and independent of the gas phase thermal conductivity: precisely the opposite from the continuum case. These equations were solved numerically for a variety of O/O/N, gas mixtures, pressures, and temperatures. High O, mole fractions at low pressure resulted in significant gas/particle thermal nonequilibrium; the particle surface temperature was shown to elevate significantly above the gas temperature. This result contradicted the theoretical behavior of small reacting particles as predicted by continuum theory.
High ambient gas temperature was shown to favor thermal nonequilibrium by increasing the oxidation rate. The presence of small amounts of atomic oxygen also raised the particle temperature significantly. Once the theory for rarefaction effects on particle temperature was developed, and the numerical method tested, soot oxidation in flames by OH. O, and O, was evaluated with the new model. The model results were compared to experimental data from a premixed lean, low pressure, flat methane/air flame. The computed particle temperatures turned out the he low when compared to the measurements, which show a temperature overshoot behavior that the currently formulated model cannot capture. It was concluded that additional effects, such as exothermic surface catalyzed recombination chemistry. were responsible for the measured particle temperature behavior. Alternately, the measurements might be in error due to calibration or other problems. A simpler shock tube experiment was designed by the author to isolate and quantify the effect of rarefaction on the temperature of small combusting soot particles. This experiment, or others of a similar nature, are required to resolve the issues concerning rarefaction effects on small particle heat transfer and combustion.
Recommended Citation
Hiers, Robert S., "Rarefaction effects in small particle combustion. " PhD diss., University of Tennessee, 1997.
https://trace.tennessee.edu/utk_graddiss/9517