- Author
-
Yuen, R. K. K.
- Title
- Pyrolysis and Combustion of Wood in a Cone Calorimeter.
- Coporate
- University of New South Wales, Australia
- Report
-
Thesis
June 1998
392 p.
- Keywords
-
wood
|
pyrolysis
|
combustion
|
cone calorimeters
|
moisture content
|
thermal properties
|
pilot ignition
|
mathematical models
|
evaporation
|
computational fluid dynamics
|
equations
|
thermophysical properties
|
vapor phases
|
formulation
|
permeability
|
timber
|
walls
- Abstract
- The effects of the moisture contents and anisotropic, variable thermal properties of wood on the pyrolysis, piloted ignition and burning of wood have been extensively studied. A three-dimensional mathematical model for the pyrolysis of wet wood which includes the complicated chemical and physical processes involved in pyrolysis has been developed. The model comprises detailed considerations of the evaporation of moisture, anisotropic and variable properties, and pressure-driven internal convection of gases in wood. The pyrolysis model has been coupled with a gas phase combustion model to give a CFD (field) model for the prediction of pilot ignition, flame spread and combustion of wood. The governing equations of the pyrolysis/gas phase combustion model are transformed into non-orthogonal coordinates making the model applicable to a wide range of geometries. A method for the determination of the permeability of partially charred wood, which involves experimental and pyrolysis modelling techniques with a newly designed permeability apparatus, has been developed. A new expression for the permeability as a function of the char content in the pyrolysing wood has been proposed. The computed permeabilities of partially charred wood using the new expression compare extremely well to the measured values. Numerical solutions have been obtained for the pyrolysis of a beech wood cube with different initial moisture contents heated at various furnace temperatures up to 1000 deg C. The heat and mass transfers in pyrolysing wood are found to be strongly influenced by the anisotropic properties due to the grain structure. A sensitivity analysis of the activation energy and pre-exponential factor has been performed. The kinetic data for beech wood pyrolysis have been determined by achieving a best fit to the measured mass loss history. Numerical studies have been performed to predict piloted ignition and subsequent combustion of wood with various initial moisture contents in a cone calorimeter at various irradiances of 20 to 70 kW m". The pilot has been simulated numerically and the ignition criteria such as critical surface temperature and critical mass flux are not required. The computed results indicate that the ignition time of wood increases with increase in initial moisture content. The predicted time to piloted ignitions and flame shapes compared with very good agreement to the measured results. The effectiveness of the model has been confirmed. Finally, a preliminary extension of the model for the simulation of enclosure fires, incorporating improved radiation modelling, has been made. In the model, the governing equations for wood pyrolysis have been simplified and coupled with the equations for the turbulent diffusion flame and participating gas radiation. Numerical solutions have been obtained for the enclosure fires involving a timber wall lining. The time to flashover is predicted. The computed results compare extremely well with measurements from a series of fire tests.