A soot deposition gauge for fire measurements

The goal of this exploratory project is to demonstrate the feasibility of a conductometric measurement to determine the time-resolved soot deposition on surfaces in fire environments. Quantitative soot deposition data enabled by this measurement method is lacking in the literature and would be useful to advance fire analysis and fire model development. Laminar flow through a thin rectangular channel with a transverse temperature gradient is used to generate thermophoretic deposition exposures on a target surface. Computational fluid dynamics (CFD) modeling is used to design the channel geometry to have well characterized laminar velocity and temperature profiles. The results predict that fully developed laminar flow and temperature profiles are established by the midpoint of the channel length. The linear temperature gradient between channel walls causes thermophoretic deposition of soot particles on the cold wall of the channel. The channel flow is also modeled with the National Institute of Standards and Technology’s Fire Dynamics Simulator (FDS) to generate predictions of soot deposition in the channel. In the experiments particles depositing on the target surface cause an increase in the conductance between the interdigitated electrodes. The change in conductance is measured intermittently before, during and after the exposure using a picoammeter and an applied voltage. At the end of the exposure the mass loading of deposited soot is determined by two separate measurement methods, including a gravimetric method and alight transmission method. The relationship between the amount of deposition and the conductometric response is evaluated for both types of mass loading measurements. The gravimetric method produced a more coherent correlation to gauge conductance with less scatter than the transmission method. The measured mass loadings are also combined with measurements of the incoming soot concentration to calculate the overall deposition velocity. The deposition velocity is compared to the theoretically predicted thermophoretic velocity, as determined from the measured temperatures and an estimate of particle size. The deposition velocities derived from the gravimetric method corresponded well with the thermophoretic velocities, but both the vertical and horizontal transmission-based deposition velocities generally were lower than the thermophoretic velocities.