Document


Title

Thermal degradation modeling of live vegetation for fire dynamic simulator
Document Type: Conference Paper
Author(s): Mark A. Dietenberger; Charles R. Boardman; Babak Shotorban; William E. Mell; David R. Weise
Publication Year: 2020

Cataloging Information

Keyword(s):
  • CFD - Computational Fluid Dynamics
  • FDS - Fire Dynamics Simulator
  • leaf components
  • live vegetation
  • pyrolysis
  • thermal degradation
  • transport properties
Topic(s):
Record Maintained By:
Record Last Modified: October 16, 2020
FRAMES Record Number: 62118

Description

Fundamental pyrolysis thermal and kinetics properties for live vegetation that are essential to CFD modeling of pyrolysis and flammability were obtained from a series of small-scale tests for live and dead vegetation. The complexity of live leaf composition was recently documented allowing analysis of 12 crude compounds. Model simplification required a practical grouping into contents of (0) moisture, (1) lipids, (2) digestives (glucose, fructose, and protein), (3) hemicellulose (xylan and pectin), (4) glucan (cellulose and starch), (5) phenolic (lignin and tannins), and (6) inert (silicate and mineral), along with the use of higher reaction orders to accommodate the wide pyrolysis peaks, and of bi-production of volatiles (tar/gas) and char from each pyrolysis group. Extensive TGA tests in nitrogen and air were used to derive oxidative pyrolysis kinetics of the celluloses and of the char, which is also suitable for smolder modeling. Moisture desorption was based on phase change kinetics coupled with the moisture isotherm relationship and heat of desorption for bound water to add generality over that of the common first-order Arrhenius kinetics relationship. Extensive DSC tests in nitrogen flow provided leaf heat capacity and estimates of the exothermic heat of pyrolysis (primarily charring) reactions over a range of temperatures. The heat of combustion remains established as a correlation based on the oxygen consumption principle whether as a volatile or a char. Finally, the transport properties of leaf surface emissivity and convective heat transfer coefficient, in conjunction with a semi-theoretical expression for the leaf thermal conductivity varying with composition, temperature, moisture content, and material degradation was obtained via an inverse heat conduction approximation method with a specialized vegetation test using leaf surface thermocouples in the cone calorimeter. This paper provides the summary of such formulations and properties that could replace or supplement existing formulations in the vegetation module in the CFD modeling of wildland fires, e.g., Fire Dynamics Simulator (FDS).

Online Link(s):
Citation:
Dietenberger, Mark A.; Boardman, Charles R.; Shotorban, Babak; Mell, William; Weise, David R. 2020. Thermal degradation modeling of live vegetation for fire dynamic simulator. 20 p. In: Proceedings, 2020 Spring Technical Meeting, Central States Section of the Combustion Institute.