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Low-temperature reactions and cool flames in an unstirred, static reactor at terrestrial and reduced-gravity
Dissertation   Open access

Low-temperature reactions and cool flames in an unstirred, static reactor at terrestrial and reduced-gravity

Michael Robert Foster
Doctor of Philosophy (Ph.D.), Drexel University
Oct 2007
DOI:
https://doi.org/10.17918/etd-1898
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Abstract

Low temperature engineering Combustion Mechanical Engineering
The temperature and compositional changes associated with low-temperature hydrocarbon oxidation reactions in unstirred reactors generate a buoyant flow at terrestrial conditions. The resulting temperature, species concentration, and velocity distributions are time-dependent, multi-dimensional, and complicated. However, nearly all computational studies disregard buoyant convection and assume that transport of heat and mass occurs solely by diffusive fluxes (e.g., classical thermal ignition theory), which are generally weaker and thus masked by the convective flow. The purpose of this thesis is to systematically vary the relative importance of buoyant convection to diffusive transport (i.e., the Rayleigh number) by using reduced-gravity facilities and studying the changes in the flame structure and ignition parameters. To compliment the experiments, a computational model that includes the essential chemistry and diffusive transport is developed and comparisons are made to the experimental data. The experimental results reported herein were conducted using premixtures of either equimolar n-C₄H₁₀ + O₂ or equimolar C₃H₈ + O₂ at subatmospheric pressures in a 10.2 cm i.d. spherical quartz vessel. The pressure and radial temperature histories were recorded and analyzed for different initial pressures and temperatures. In addition, the visible light emission owed to excited formaldehyde was recorded using intensified video cameras and was observed to be radially symmetric in all cases at 10⁻²g. Yet, the temperature distribution during (and after the passage of) the cool flames and ignitions did not decrease monotonically in all cases as predicted by pure conduction models. The intensified video records were also used to determine the flame radius and propagation speed as a function of time for different reactor wall temperatures and initial pressures based on the maximum light intensity. To compliment the experiments, a numerical model was developed based on a four-step global thermokinetic scheme augmented with diffusion of heat and species, which captured the empirical trends. Finally, the non-dimensional species and energy equations, which were previously developed based on the incompressible Navier-Stokes equations with the Boussinesq approximation, were compared to computations performed with the full compressible model to extend the model to include variable density and explore the effects of hot gas expansion on the dynamics of the reaction.

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