Experiments, Modeling and Correlation for Smoldering of Incense Sticks

This paper is concerned with the modeling of smoldering characteristics of incense sticks under free convection. Experimental data on smoldering characteristics are presented on specifically designed compositions from biomass to char, involving measurements of smoldering rate, condensed phase temperature with time and hence the distance and shape of the steadily smoldering surface. The modeling effort invokes a steady surface heat balance, expressing it in dimensionless terms and relating the smoldering Reynolds number, Rer, to the heat transfer by convection through transfer number, B, Grashof number, Gr, and flow Reynolds number, Reu. Three constants in the model are determined by seeking to minimize the differences with the experimental data. The extinction criterion that emerges from the data analysis is expressed in terms of smoldering Reynolds number. The shape of the smoldering front is shown to be related to the stoichiometric ratio that varies from biomass to char. While the predictions are shown to be good in most cases, the lack of experimental details in select cases is argued as the cause of lack of better comparison. A correlation of the smoldering Reynolds number in terms of transfer number B that depends on compositional parameters, oxygen fraction and ambient pressure is shown to predict the smoldering rate to within ±7% as well as extinction.