Ratio pyrometry of emulated firebrand streaks

Abstract

Firebrands can dramatically increase the spread rates of large outdoor fires. Although much is known about other aspects of firebrand behavior, their temperatures are poorly understood. In particular, the temperatures of burning airborne firebrands have never been measured. In a novel configuration, airborne firebrand motion is emulated here by rotating a camera while imaging a tethered maple ember as a streaking ember. Ratio pyrometry is performed using a color digital camera calibrated with a blackbody furnace at 600–1200 °C. Over 1600 images of 55 stationary and streaking embers were recorded and analyzed. The same ember temperatures were obtained for stationary embers as for streaking embers with either a horizontal or a vertical axis of camera rotation. The mean ember temperatures were 918 ad 955 °C for 1 and 2 m/s wind velocities. These were independent of ember distance and velocity, but generally increased with time after ignition at a mean rate of 0.77 °C/s. The diagnostic has an estimated measurement uncertainty of ±20 °C and similar accuracy for stationary and streaking embers.