An Investigation of Surface and Crown Fire Dynamics in Shrub Fuels

Doctor of Philosophy, Graduate Program in Mechanical Engineering
University of California, Riverside, August 2011
Dr. Shankar Mahalingam, Chairperson

The  focus  of  this  study  is  on  spatially  segregated  multiple  crown  fuel  matrices that  model  the  crowns  of  discrete  shrubs.    The  influence  of  the  horizontal  crown separation distance between crown fuel matrices on the transition process from surface fires  to  crown  fires  and  on  the  rate  of spread  of  surface  fires  in  chaparral  fuels  is investigated  experimentally  and  numerically.    The  experiments  are  carried  out  in  a 1.20  m  width  ×  1.20  m  height  ×  7.4  m  length,  open-roof  wind  tunnel  to  ensure  that flame-generated buoyancy effects are not suppressed.  The surface fire is initiated in a fuel  bed  comprised  of  aspen  (Populus  tremuloides  Michx)  excelsior  that  is  evenly distributed over an area of 0.8 m width × 2.8 m length to a depth of 0.10 m.  Crown fuel matrices  comprised  of  live  chamise  (Adenostoma  fasciculatum)  held  in  place  by  two 0.6 m length × 0.3 m height × 0.8 m width wire mesh baskets, at a height of 0.35 m from the surface of the fuel bed.  Crown separation distances (CSD) investigated range from 0.1 m to 0.3 m.

At a CSD of 0.1 m, as the fire front approaches and ignites the downwind crown fuel  matrix,  the  surface  fire  and  upwind  crown  fires  are  merged  into  a  single  fire. Following  ignition  of  the  downwind  crown  fuel,  a  single  merged  fire,  comprising  the surface and two crown fires, results.  At larger crown separation distances of 0.2 m and 0.3 m, the surface and upwind crown fires are segregated, appearing as two distinct fire fronts, with the ignition of the downstream crown fuel matrix occurring earlier in time for the lower CSD case.  In addition, a Particle Image Velocimetry system is utilized to investigate the influence of CSD on the flow field between adjacent crown fuel matrices.