Experimental Investigation of Bulk Density and its Role in Fire Behavior in Live Shrub Fuels

Department of Mechanical Engineering
Advisor: Professor Shankar Mahalingam

Wild  land  fires  occur  in  Southern  California  every  year  and  frequently  cause  damage in excess of millions of US dollars to urban household, farmland, forests and even wilderness areas. This thesis examines the  role  of bulk density on fire behaviour in live shrub fuels. This includes an investigation of shrub bulk density distribution in the vertical, effects of the influence of bulk density variation on fire behaviour, and the influence of wind and ignition method on fire dynamics. The results of the study will help improve the understanding of the effects of variation in  live  fuel  bulk  density  on  fire  behaviour  by  suggesting  models  to  investigate some features that have not been adequately considered in previous studies.

The approach used in this research is a combined experimental-numerical study. Results indicate that for relatively young Chamise and Manzanita (0-4 years), the bulk  density  appears  differently  distributed.  It  is  expected  that  fire  behaviour would change when varying bulk density, wind speed and ignition method. Three bulk density cases including low, mean, and high bulk density were investigated to study response to ignition method and resulting fire behaviour. Statistical T-test was  used  to  analyse  the  effects  these  parameters  and  variance  error.  Low  bulk density means the shrub contains less overall mass and fuel energy in a specific volume. However, experiments show that the  fire  spread rate  was not decreased when compared to the mean bulk density case where energy and mass contained is relatively higher than the low bulk density case. The flame propagates faster for the same wind speed and ignition method because of enhanced entrainment of air into the flame zone. Large eddy simulation (LES) simulation results show that the total burning time for the low bulk density case is significantly lower than mean and high bulk density cases.