Experimental Study On The Effect Of Fuel Moisture Content On Smoldering And Its Transition To Flaming

Smoldering combustion involves heterogeneous reactions between solid fuel and oxygen.It often occurs in porous fuels,such as polyurethane foam,peat,sawdust,etc.Smoldering combustion often happens in forest fires.Although it spreads slowly,it can sustain long time and is difficult to put out once occurrence.Under certain conditions,smoldering combustion can transit to flaming combustion(StF),leading to higher fire risk.The moisture content(MC)of the fuel affects its smoldering propagation process,and sudden change of MC may cause the transition from smoldering to flaming.The MC of forest fuels is affected by hydrological conditions,climatic conditions and human factors,thus fuels with discontinuous moisture contents are common in forests.Therefore,it is necessary to conduct experimental research on the effect of MC on smoldering propagation and the transition from smoldering to flaming.In this thesis,red pine sawdust from Northeast China was used as experimental material,and the influence of MC on smoldering and transition from smoldering to flaming was investigated under natural convection condition.Firstly,a combustion chamber with open ends and closed sides was designed and built for smoldering experiments,and a thermogravimetric analyzer was used to obtain pyrolysis characteristic parameters for subsequent analysis of smoldering experiment results.Smoldering ignition experiments were carried out under different heating conditions to select appropriate heating conditions to maintain sample integrity and initiate self-sustaining smoldering.Secondly,the influence of MC(from 2.29%to 60.88%,dry-based)on smoldering propagation process of sawdust was studied under the selected heating conditions.The smoldering temperatures,propagation velocity and gas concentration generated from smoldering were quantitatively analyzed.It is found that with the increase of MC,the peak temperature and propagation velocity of smoldering process decreased,and the amount of CO2 and CO released also decreased.The forward theoretical smoldering model was used to calculate the smoldering velocity with different MC and explained the experimental results.Finally,the smoldering experiments of the sample with discontinuous moisture contents under natural convection were conducted.The first half of the sample was dry,while the second half was wet(dry based MC:11.83%～60.35%).The results showed that when the dry based MC of the second half was less than 19.30%,smoldering propagated self-sustainably;when the dry based MC of the second half was between 20.40%and 51.69%,a crevice conducive to gas accumulation formed at the MC mutation,and smoldering transited to flaming;when the dry based MC of the second half was higher than 53.57%,no StF occurred.From the evolutions of temperatures and combustible gases,it can be concluded that transition from smoldering to flaming was caused by high temperature,the formed crevice and the combustible gases concentrated in the crevice at the same time.