Molecular Clusters in Mesoporous Materials as Precursors to Nanoparticles of a New Lacunar Ternary Compound PdxMoyP

Bimetallic clusters of composition Pd2Mo2(g5-C5H5)2(l3-CO)2(l2-CO)4 (PR3)2 (R = ethyl or phenyl) were incorporated by impregnation from solution into two different silica matrices, amorphous xerogels and ordered SBA-15, and a study of their thermal decomposition under a reducing atmosphere is reported. With both matrices, a suitable thermal treatment afforded nanoparticles of a new bimetallic phosphide. Although nanoparticles of composition PdxMoyP, isostructural with Mo3P, were formed in both matrices, they were more uniformly distributed in the SBA-15 framework and showed a narrower size distribution. The samples have been characterized by powder XRD, chemical analysis, FT-IR spectroscopy, TEM and electron tomography (3D TEM)

Prediction of heat release rate of shredded paper tapes based on profile burning surface

A series of shredded paper fire experiments were conducted by means of a calorimeter. The mass loss rate and heat release rate were measured. The flame spread process was recorded, which shows that the flame spread process can be divided into four typical stages, and the mean spread rates along different directions were obtained from the observed combustion process. Based on the mean flame spread rate, a mathematical model for predicting the burning surface as a function of time during the four stages is established. Combining this model with the effective heat of combustion calculated from measured mass loss rate and heat release rate, an improved model to predict the heat release rate as a function of time was developed. In this model, the linear relationship between heat release rate and burning surface is found, and the predicted result agrees well with the measured heat release rate

Autoignition of Dead Shrub Twigs: Influence of Diameter on Ignition

International audienceThe effect of the diameter of dead twigs of Cistus monspeliensis on their ignition was studied experimentally and theoretically. Autoignition experiments were carried out in a cone calorimeter. The ignition time, surface temperature before ignition, flame residence time, smoldering time and mass loss were measured. The particles were classified into two groups based on their ignitability. The first group contained the most flammable twigs, which had diameters smaller than or equal to 4 mm, along with leaves. The second one included twigs with diameters equal to or larger than 5 mm. For a radiant heat flux of 50 kW/m2, the 4-mm value appeared to be the upper limit for the size of the particles potentially involved in the spread dynamics of wildfires. However, bark detachment was observed on the thickest twigs, which greatly decreased their ignition time. Two ignition criteria were investigated: the ignition temperature and critical mass flux. The ignition temperature increased with the twig diameter, showing that this quantity should be carefully considered in ignition models. A thermal ignition model was proposed to determine the ignition time of twigs according to their diameter. The critical mass flux appeared to be fairly constant for any fuel diameter and could also be convenient for modeling the ignition of shrub fuels

Examination of WFDS in Modeling Spreading Fires in a Furniture Calorimeter

International audienceValidation of physics-based models of fire behavior requires comparing systematically and objectively simulated results and experimental observations in different scenarios, conditions and scales. Heat Release Rate (HRR) is a key parameter for understanding combustion processes in vegetation fires and a main output data of physics-based models. This paper addresses the validation of the Wildland-urban interface Fire Dynamics Simulator (WFDS) through the comparison of predicted and measured values of HRR from spreading fires in a furniture calorimeter. Experimental fuel beds were made up of Pinus pinaster needles and three different fuel loadings (i.e. 0.6, 0.9 and 1.2 kg/m2) were tested under no-slope and up-slope conditions (20°). An Arrhenius type model for solid-phase degradation including char oxidation was implemented in WFDS. To ensure the same experimental and numerical conditions, sensitivity analyses were carried out in order to determine the grid resolution to capture the flow dynamics within the hood of the experimental device and to assess the grid resolution’s influence on the outputs of the model. The comparison of experimental and predicted HRR values showed that WFDS calculates accurately the mean HRR values during the steady-state of fire propagation. It also reproduces correctly the duration of the flaming combustion phase, which is directly tied to the fire rate of spread