Doctoral Dissertations

Date of Award

5-1995

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Aerospace Engineering

Major Professor

John Steinhoff

Committee Members

Robert Roach, John Caruthers, Roy Schulz

Abstract

The fundamental processes governing fuel droplet ignition in an oxidizing environment have been studied using a comprehensive numerical model. The study started with an existing time-accurate one-dimensional droplet combustion model which was modified to become a specific model capable of assessing droplet ignition in a hot oxidizing gas or simple ignition as a result of energy deposition (e.g. a spark) within the gas-phase. The baseline numerical model solves the time dependent one-dimensional spherically symmetric governing equations of mass, momentum, and energy within the gas-phase and is based on the Arbitrary Lagrangian Eulerian (ALE) numerical algorithm for the numerical simulation. This model was modified to include finite droplet thermal conductivity, droplet incident radiation, and simplified energy addition representative of a spark source to investigate the physical processes occurring near ignition. An energy conservation equation for predicting the temperature distribution within the liquid droplet has been added to define the temporal and spatial temperature gradients inside the droplet including the droplet surface temperature. Gas-phase radiation heat transfer to the droplet has been included in the formulation. Ignition criteria have been defined and implemented in the assessment of droplet ignition. Model results have been compared with analytical theory. In addition, the model has been demonstrated by comparing its predicted results to experimental data, and the comparison is good. As a result, the model has been validated over a limited range of hydrocarbon fuels for various gas-phase ambient conditions, based on appropriate chemical kinetic reaction mechanisms. The effects of droplet finite thermal conductivity, incident radiation and chemical kinetics on ignition processes have been assessed. Results show that finite thermal conductivity and radiation heating of the droplet reduce ignition delay. In addition, the numerical results in the present study show that the assumption that droplets have infinite conductivity and no radiation effects may produce results which underestimate ignition delay. Physical flow field parameters have been defined which correlate very well with ignition. Chemical kinetics have a profound effect on ignition delay, which was shown numerically using a detailed chemical kinetic mechanism for methanol in air. This calculation showed that experimental pre-exponential and activation energy values must be better defined for use in modeling ignition of hydrocarbon diffusion flames. The potential usefulness of the model for spark and laser ignition are discussed.

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