EFFECTS OF CONVECTIVE HEAT TRANSFER COEFFICIENT IN PREDICTION OF MATERIALS PROPERTIES FROM CONE CALORIMETER TESTING

摘要

It is increasingly important to develop appropriate models for material behavior under fire,rnparticularly as structures and products use increasingly complex materials and computer simulationsrnof fire become more prevalent. Both large and small-scale fire test methods are used; however, largescalerntesting under realistic fire conditions is costly and often does not allow for examination ofrnmaterial properties across the full parameter space. Thus, full-scale work is being replaced by testingrnat reduced scales, combined with correlations and modeling of results to larger scales. A key to properrncharacterization of material properties in fire lies in understanding the key parameters in the scaledrntests and appropriately interpreting the data.rnThe cone calorimeter is a small-scale test used extensively to characterize ignition and flammabilityrnparameters of materials. Results are incorporated into physical models to extract values of thermalrninertia, kρc, and critical flux levels as a function of imposed heat flux. The method is based upon thernassumption that the convective heat-transfer coefficient, hc, within the cone-sampling region is arnknown, and typically constant, parameter that describes heat transfer between the material andrnsurrounding air under the prescribed radiative heat flux of the cone. In reality, the degree of scatter inrnvalues of hc reported in the literature is remarkable. For comparable test situations reported valuesrnspan between 7 and 34 W m~(-2)K~(-1); most are between 10 and 20 W m~(-2)K~(-1). This study examines therncurrent methods used to determine hc in cone calorimeter tests, and presents results on the impact ofrnhc values in the derivation of materials properties for even a well-known material such as PMMA.