Effects of heat flux scenarios on smoldering in cotton

Abstract

The onset of smoldering in cotton has been investigated under six different heating scenarios: A. High heat flux (12.8kW/m²) followed by cooling, B. Medium high heat flux (4.5kW/m²) followed by cooling, C. Medium low heat flux (2.2kW/m²) followed by cooling, D. Low constant heat flux, E. High heat flux (12.8kW/m²) followed by low constant heat flux, and F. Multiple heating and cooling of the same sample. In the experiments, cotton was exposed to an electrically heated hotplate. The cotton sample was 0.15m×0.15m×0.15m, and the density of the cotton was varied between 20 and 100kg/m3.Both density and heat flux affect the temperature for onset of smoldering. A difference of 19°C was found in the ignition temperature between high heat flux followed by cooling (Scenario A) and low constant heat flux (Scenario D) for cotton with density 100kg/m3. Furthermore, a difference of 36°C was found in the ignition temperature between cotton with density 40 and 100kg/m3 when heated with low constant heat flux (Scenario D). An ignition model has been developed that estimates the ignition temperature of cotton to ±7% of the experimental results. In combination with a one-dimensional heat transfer model, the ignition model is capable of determining the time to and temperature at onset of smoldering for a semi-infinite slab to, respectively, 24% and 2% of the experimental results.The work presented in this article indicates that both low constant heat flux and density should be included in material tests for onset of smoldering.