UPCOMING DISSERTATION DEFENSE: MOHAMED AHMED

Author: Mohamed Mohsen Ahmed

Title of Dissertation: Development of a Lagrangian-Eulerian Modeling Framework to Describe Thermal

Degradation of Porous Fuel Particles in Simulations of Wildland Fire Behavior at Flame Scale

Date and time: April 14, 2023, at 8:30 AM

Location: Fire and Risk Alliance Conference Room, 3106 J.M. Patterson Building

Zoom link: https://umd.zoom.us/j/7685207098 | Zoom meeting ID: 768 520 7098

Committee Members:

  • Professor Arnaud Trouvé, Chair/Advisor
  • Professor James Baeder, Dean’s Representative
  • Dr. Mark Finney
  • Professor Johan Larsson
  • Professor Stanislav Stoliarov
  • Professor Peter Sunderland

Abstract:

Wildland fire is a multi-scale problem in which different length-scales are believed to play a role in fire behavior. These length-scales range from sub-millimeter representative of small vegetation particles, to several kilometers’ representative of meteorological scales. Computational Fluid Dynamics (CFD) models have the potential to describe wildland fire behavior at different scales. Our objective in the present study is to develop a computational tool to better describe the coupling between solid phase and gas phase processes that control the dynamics of flame spread in wildland fire problems. We focus on a modelling approach that resolves processes occurring at flame and vegetation scales, i.e., the formation of flammable vapors from the biomass vegetation due to pyrolysis, the subsequent combustion of these fuel vapors with ambient air, the establishment of a turbulent flow because of heat release and buoyant acceleration, and the thermal feedback to the solid biomass through radiative and convective heat transfer. The modelling capability is based on a general-purpose Computational Fluid Dynamics (CFD) library called OpenFOAM and an inhouse Lagrangian Particle Burning Rate (PBR) model that treats drying, thermal pyrolysis, oxidative pyrolysis and char oxidation using a one-dimensional porous medium formulation that allows descriptions of thermal degradation processes occurring during both flaming and smoldering combustion. We also introduce a novel diagnostic called Pseudo Incident Heat Flux (PIHF) to characterize the particle external heat loading.

The modelling capability is calibrated for cardboard and pine wood using available micro-and bench-scale experimental data obtained. The model is applied to simulations of the fire spread across idealized fuel beds made of laser-cut cardboard sticks that have been studied experimentally at the Missoula Fire Sciences Laboratory. The simulations are conducted at prescribed particle and environmental properties (i.e., fuel bed height, fuel bed packing, particle size, moisture content, and wind velocity) that match the experimental conditions. The model is first validated against experimental measurements and observations such as the rate of spread of the fire and the flame residence time. The modeling capability is then used to provide insights into local as well as global behavior at individual particle level and at the fuel bed level with the fuel packing. The modelling capability is also applied to simulations of fire spread across idealized vegetation beds corresponding to thin, mono-disperse or bi-disperse, cylindrical-shaped sticks of pine wood under prescribed wind conditions. Depending on the particle size distribution, the simulations feature complete fuel consumption with successful transition from flaming to smoldering combustion or partial fuel consumption with no or limited smoldering. These simulations show the existence of either a mixed mode of heat transfer through convection and radiation for small particles or a radiation dominant heat transfer mode for larger particles. The results are interpreted using maps that characterize single particle burning behavior as a function of intensity and duration of the thermal loading process.