The Strouhal-Froude number scaling for wildland fire spread

Abstract

A study of wildland fires leading to a correct understanding of flame spread in wildland fire would find its foundation firmly situated on an understanding of the governing mechanisms, processes, and threshold of ignition. It is, therefore, very important for effective firefighting efforts and safety reasons to identify the roles of radiative and convective heating. Over the years, the United States Department of Agriculture (USDA) Missoula Fire Sciences Laboratory has conducted a series of experiments in their unique wind tunnel fire experimental facility. This rich database provides years of numerical data and video from burns conducted under a wide range of well-specified conditions. After identifying the need to explore the roles of both convective heat transfer and radiative heat transfer in the ignition process, the USDA’s well documented line fire data provided an opportunity to observe ignition and, subsequently fire spread phenomenon, through a uniform fuel bed of laser-cut cardboard combs under controlled conditions. The goal of our study is to identify features distinguishing radiative heat transfer from convective heat transfer. The team worked to explain scaling laws, determine key parameters to support the development of scaling laws, and begin a comparison of the scaling law predictions with USDA data. When the above scaling laws are validated, it would be reasonable to design medium scale prescription fire experiments, which fall between the USDA experiments and the full scale wildland fires. Our step-by-step approach using different size scale model experiments eventually allow us to understand the governing physics that control the mechanism of flame spread through and ignition on the wildland fuel bed.