Modelling And Experimental Study On Upslope Surface Fire Spread Of Wildland Fuels

The terrain slope is an important influence factor of forest surface fire spread.Developing fire spread model is one of the main research methods and its goal is to accurately predict the rate of spread(ROS).For the physical models of fire spread,predictions on ROS,temperature and heat flux are achieved by modelling the processes of combustion,heat transfer and fluid based on energy conservation.For this reason,physical models can be applied to a larger range of slope compared with empirical models.However,model predictions are still suffused with errors or discrepancies in comparison with the measured results,which may arise from uncertain,imprecise or improper determinations of the model parameters.In this work,sensitivity analysis on the parametric uncertainty was conducted on a typical model of upslope fire spread which considered flame radiation,natural convective cooling and radiation loss as the preheating mechanisms.The model parameters introduced by the initial and boundary conditions that close the model could be attributed to three categories according to the origin of their uncertainties,namely,thermophysical parameters,measured parameters and preset parameters.In the analysis,the variation ranges of the parameter were determined based on its physical property and experimental data,thereby the relative deviation of ROS was used as a target variable of model calculation to evaluate the response of the model to parametric uncertainties.The results of sensitivity analysis reveal that the empirically assumed values of the ignition temperature and the average flame temperature exert a significant impact on the predicted values of ROS.The values of the effective fuel consumption efficiency,flame emissivity and the heat convection coefficient should be adjusted to better characterize the variation of flame intensity and flow patterns under different slopes.As for the geometric parameters of the flame,the errors for evaluation of the flame tilt angle has a negligible effect on model predictions,while the errors in estimating the flame length have a remarkable influence.Especially,the flame length fluctuations due to the nature of flame pulsation have a remarkable effect on model prediction,which however is never taken into consideration so far.Previous studies on fire spread mainly focused on the steady-state while the fire acceleration has received scarce attention.Eruptive fire,which often occurs under high slope conditions or in canyons,is a kind of extreme behavior characterized by rapid fire acceleration.However,no common conclusions on the mechanisms that trigger an eruptive fire are reached.This paper presented a systematical experimental study on eruptive fire over a pine needle(Pinus sylvestris)fuel bed in trench configuration in still air.The trench was constructed by setting up tempered glass and an insulated fireproof board along the lateral edges of the fuel bed.During the tests,slope angles varied from 0° to 40° and aspect ratio(the ratio of the trench sidewall height to the trench width)varied from 0.1?0.4.ROS was calculated by the thermocouples along the centerline of the fuel bed.Cameras were used to monitor the fire spread(the flame attachment was recorded by the side-view cameras).Weighing sensors,Pitot tubes and heat flux meters were used to measure the mass loss rate,flow velocity and heat flux,respectively.The occurrence of eruptive fire was evidenced by a sudden increase in ROS obtained by linear fitting from the fire front position vs time plots.After the fire erupted,the effective fuel consumption coefficient dropped below 0.1.The fire front swept quickly over the upper layer of the fuel bed.Behind the fire front,flaming combustion occupied a large area,the length of which was defines as flame depth.With the existence of sidewalls,the lateral air entrainment was restricted.Under different conditions of slope and aspect ratio,the degree of restriction in lateral air entrainment would vary,which further exerted impacts on the flame attachment behaviours,the flow field ahead of fire front and the convective heating which became an important preheating mechanism.For smaller aspect ratios,the lateral entrainment was partly restricted and the flame below the sidewall intermittently attached to the fuel.With the increase of aspect ratio,the flame height and the flame pulsation intensity decreased.The fire front started to fully attach to the fuel bed,which promoted the forward flow ahead of the fire front and the influence distance of convective heating.For a certain slope,when the height of the sidewall was larger than the flame height,the increase of aspect ratio had little influence on the fire spread.For an aspect ratio of 0.1,only the lateral entrainment near the fire base was prevented.Increasing slope led to higher intensity of flame pulsation and flame height,resulting in stronger flame radiation.For larger aspect ratios,the velocity and the influencing range of forward flow both increased with slope due to strengthened buoyance effect and Trench effect,which further enhanced convective heating.The critical slope angle of eruptive fire for certain aspect ratio was determined by experiments.For slopes lower than the critical slope angle,the lateral air entrainment can be neglected since the fire with small flame depth could be considered as a line fire.Under the critical slope angle,the flame attachment occurred.With the increase of the flame depth,the fireline intensity continuously increased.After the fireline intensity reached a certain level,the ROS increased rapidly,in other words,the eruptive fire occurred.