| The behavior of flames spreading over thin cellulosic fuels is determined by various physical and chemical mechanisms. These mechanisms include: (1) gas phase convection and diffusion of mass, momentum and energy, heat transfer by radiation, combustion reactions, ignition and flame propagation; (2) condensed phase heat and mass transfer, fuel sample pyrolysis reaction; (3) interfacial convection and diffusion of mass, momentum and energy, radiation heat loss and fuel and oxidizer transport. Three numerical models are examined. These include a one-dimensional model: a two-dimensional model; and a three-dimensional model. These models are employed to investigate ignition and subsequent flame spread over thin cellulosic fuel samples. The investigation is carried out with increasing complexity. It includes: (1) A one-dimensional model for predicting transient heat and mass transfer in thin cellulosic fuels using the Broido pyrolysis scheme. This model is developed in order to model one-dimensional heat and mass transfer during flame and flamelet spread; (2) A two-dimensional, time-dependent, variable property model was developed to describe ignition and subsequent transition to flame spread over cellulosic fuels; (3) A three-dimensional, time-dependent numerical model, adapted from the NIST-developed Fire Dynamics Simulator (FDS), was employed to investigate flame and flamelet behavior over thin cellulosic fuels. The influence of cellulose pyrolysis kinetics on the heat transfer, the release of volatile combustibles, and the formation of char from the original solid cellulose fuel were all evaluated using the one-dimensional numerical model, and were later used in the two-dimensional model. Numerical results obtained from the two-dimensional model were compared with both "macroscopic" and "microscopic" analytical predictions to study the field-variable distributions and also to attempt to derive useful flame scaling parameters for the flame microstructure. Comparisons of various parameters such as flame spread rate, heat loss to nearby surfaces and fuel burned fraction are made between the 3-D FDS numerical results and experimental results to examine flame spread in near-limit conditions.; The ultimate goal of this research is three-fold: (1) To understand the basic physical mechanisms of flame spread over thin fuels; (2) To predict various observed behaviors such as flame-to-flamelet transition; (3) To assess the viability of a simulated zero-gravity Narrow-Channel testing procedure for potential use in evaluating materials for zero-g environments. |
