Wildland fires continue to be one of the most prominent natural disasters, resulting in hundreds of billions of dollars in economic damage and many fatalities. One of the primary ways wildland fires ignite structures is through firebrands, which are small pieces of burning debris lofted by the fire to locations as far as 1-2 km from the main fire. Understanding the heat transfer from firebrands to the surfaces on which they land is a crucial step in engineering structures to be more resilient to wildland fires.

The heat transfer from smoldering firebrands is a highly dynamic phenomenon, with a constant interplay between the three modes of heat transfer. In order to understand how the firebrand’s surface contact and general shape change with time and impact heat transfer, it is necessary to be able to view the smoldering firebrand from all angles. The ability to view the underside of the burning specimens is even more crucial when analyzing heat transfer from piles of firebrands. An experimental method was developed in this work to quantify the spatially resolved and time dependent heat transfer from firebrands on a transparent quartz surface. This allows simultaneous visualization and heat transfer measurements of single firebrands and firebrand piles.  Heat transfer measurements were performed using both a stainless-steel plate and a quartz plate to demonstrate the quartz plate measurement technique.

High spatial resolution heat transfer measurements with time from fires to surfaces have been performed using IR thermography and inverse heat transfer (IHT) on a thin, black stainless-steel plate\[1\]. The unexposed side of the plate is monitored using infrared (IR) thermography to obtain a two-dimensional map of temperatures across the surface. The field of view of the IR camera is subdivided into small, discrete areas on the non-exposed surface, with each area corresponding to a pixel, and thus a known temperature. An energy balance with known ambient conditions is used to obtain the unknown heat flux on the exposed surface. Using this sensing method, heat fluxes can be resolved with sub 0.5mm resolution.

In this study, the stainless-steel plate was replaced by a quartz (silicon dioxide) plate to allow for visual access of the contact between firebrands and the surface. Quartz was selected since it is transparent in the visible (0.4-0.7μm) spectrum but opaque in the long-wavelength IR (7.5-14.0μm) spectrum. This allows visual cameras to track the shape of the underside of the firebrands while the IR camera measures the surface temperature of the quartz plate for use in the IHT analysis.

Due to the quartz plate being transparent in the visible and short wavelength IR, less radiation is absorbed by the quartz than would be absorbed by a more opaque metallic or cellulosic material. This was accounted for using the spectral emissivity of quartz, the emissivity of stainless-steel, and the expected emission spectrum of a firebrand. Previous experiments have shown the peak temperatures of firebrands range from 600-900 °C. Using Planck’s distribution and the spectral emission of quartz, the average absorptivity of quartz over this temperature range was determined to be *α=0.49*, with the rest of the energy from the firebrand either being reflected or transmitted through the medium. Heat fluxes measured using the quartz plate were used along with the calculated emissivity to determine the expected heat flux into a stainless-steel plate exposed to the same firebrand. A series of experiments using similar firebrands was conducted and the heat flux was measured using IHT with stainless-steel and quartz plates. After accounting for the difference in spectral response, the results between the peak measured heat fluxes were within 25% and the average heat flux during smoldering prior to decay was within 10%, which is within the repeatability of the experiment. These results show quartz is a viable material for IHT while offering the distinct advantage of visual access.

\[1\] C.M.Rippe;B.Y.Lattimer,“Full-field surface heat flux measurement using non-intrusive infrared thermography,”FireSaf.J., vol.78,pp.238–250,2015.


Measuring Heat Transfer From Firebrands to Surfaces

Division / Sponsors: 
Heat Transfer Division (HTD), Fluids Engineering Division (FED)

Heat Transfer Summer Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/HT/HT2020/volume/83709

18th International Conference on Nanochannels, Microchannels, and Minichannels
Paper Proceedings: https://asmedigitalcollection.asme.org/ICNMM/ICNMM2020/volume/83693

Fluids Engineering Division Summer Meeting
Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2020/volume/83716

HT-ICNMM2020 - Virtual Conference, SHTC - Virtual Conference, FEDSM2020 - Virtual Conference

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