Contents lists available at ScienceDirect Fire Safety Journal journal homepage: www.elsevier.com/locate/ firesaf Measurement of instantaneous flame spread rate over solid fuels using image analysis Subrata Bhattacharjee ⁎ , Luca Carmignani, Gregory Celniker, Blake Rhoades Mechanical Engineering Department, San Diego State University, 5500, Campanile Blvd, San Diego, CA 92182, USA ARTICLE INFO Keywords: Fire spread Image analysis Spread rate measurement Downward spread ABSTRACT Spread rate is an overall property of flame propagation that characterizes the condition of a flame better than any other property. As a result, prediction and measurement of spread rate is central to flame spread studies over solid fuels. Significant amount of data have been collected over last four decades of research on flame spread over various fuels under different conditions. In most of these studies, however, only average spread rate is reported which is adequate for steady phenomena. Given that a flame may not face the same conditions during the spread, it is possible for the spread rate to change during the duration of the spread continually. In this work a methodology for image analysis is presented with the goal of evaluating instantaneous spread rate to study time-dependent phenomena. The parameters that control the error and time resolution of the flame spread history are identified, and a sensitivity study is carried out to validate the results of a scale analysis. A MATLAB-based Flame Image Analyzer (FIA) package is developed and applied to flame spread videos recorded in several experiments in different regimes of opposed-flow flame spread. An expression for the error in spread rate for a given time resolution is expressed in terms of the imaging parameters. The two parameters that are found most important are the pixel resolution and the frame rate. A non-dimensional imaging parameter is identified that is shown to govern the quality of imaging for spread rate measurement. Theoretical prediction from the error analysis is confirmed by doing various case studies using the Analyzer. 1. Introduction Flame spread rate plays a fundamental role in multiple areas of research, such as fire safety and combustion, since it is related to the flame growth and the research of flammability limits. It is well known that flames can be very different in shape and behavior on the basis of fuel properties and the surrounding environment. In general, ambient conditions may vary during the propagation of the flame, such as in a boundary layer region where the flow velocity profile depends on the location along the surface, and therefore we would expect a variation in the flame spread rate. Determining the correct value of flame spread rate in different burning conditions has always been challenging, and many different approaches have been developed in the last fifty years, thanks also to the evolution of technology. In early studies, slow flames (with velocity lower than about 2.5 mm/s) were tracked with stop-watch-measure- ments of the time needed by the flame itself to spread for few centimeters or regular marks [1,2]. This approach could work only for “steady flames”, i.e. flames that do not accelerate or decelerate during the experiment, and it is not very accurate. For faster flames it seemed necessary to calculate the spread rate using other methods, such as video analysis. During video analysis, the user was required to manually measure the distance covered by the flame on a monitor [1,3], or on printed pictures [4]. A good alternative to video analysis for relatively fast flames is the use of thermocouples; in their experiments, Fernandez-Pello et al. used arrays of thermocouples placed at regular intervals normal to the direction of propagation to calculate the flame spread rate [2]. Thermocouples were used also by Bhattacharjee et al., but in a completely different way: flames spreading downward were rendered stationary thanks to a PID control and a thermocouple placed close to the flame leading edge; the sample holder, connected to a motor, moves upward when a reference temperature is reached, and the velocity of the motor can be directly related to the flame spread rate [5]. Even though thermocouples are relatively cheap and reliable, they can make the experimental apparatus very complicated for small scales or particular conditions. An interesting alternative is the use of infrared sensors to obtain temperature profiles, like in the study of Arakawa et al., who measured two-dimensional flame spread rates over vertical solid fuel, showing good agreement with the results obtained with thermocouples [6]. The IR camera can give really accurate temperature http://dx.doi.org/10.1016/j.firesaf.2017.03.039 Received 31 January 2017; Accepted 15 March 2017 ⁎ Corresponding author. E-mail address: prof.bhattacharjee@gmail.com (S. Bhattacharjee). Fire Safety Journal 91 (2017) 123–129 Available online 06 April 2017 0379-7112/ © 2017 Elsevier Ltd. All rights reserved. MARK