The physics of embers

Dan Cowan collecting data from a controlled forest burn.

Graduate student Dan Cowan collects data from a controlled burn conducted by the Nature Conservancy. (Photo: Tyler Hudson)

On Sept. 2, 2017, a fire, reportedly caused by teenagers throwing fireworks, ignited in the Eagle Creek Canyon near the Columbia River Gorge in Oregon. Three days later, the fire leapt the Columbia River into Washington State—a distance of about 2 miles.

Unfortunately, instances of fires crossing miles in distance is not unprecedented.

David Blunck, assistant professor of mechanical engineering at Oregon State, is trying to better understand how wildfires spread and to help build a predictive model to assist those fighting to put them out. 

In a wind tunnel on the Oregon State campus, Blunck and his students set sticks on fire and measure the size and time to formation of the embers that fly off the burning wood. They fasten dowels of known species — white oak, Douglas fir, ponderosa pine, white fir — into a fireproof container, apply a propane flame and turn on a fan. The air stream carries the glowing embers into a metal pan at the end of the tunnel.

Blunck is investigating one of the processes that spread the Eagle Creek fire and others like it: ember generation. It’s a phenomenon familiar to anyone who has sat around a campfire and watched sparks fly into the night. However, the violent winds generated by a wildfire have been known to loft firebrands for miles, setting new spot fires and threatening homes and other structures.

“We know most embers come from crown fires. People have primarily focused on studying ember transport and ignition, but we’re looking at what controls the formation of them,” says Blunck, the Welty Faculty Fellow who previously studied combustion in gas turbine engines for the U.S. Air Force. “Knowledge regarding the physical process that controls ember formation is lacking.”

Blunck knows that the highly controlled environment in the laboratory hardly compares to the blustery maelstrom of a burning forest. But he notes that understanding the controlling factors — wood size, species, moisture, temperature, wind speed — can ultimately be used to help fire managers anticipate one of the most unpredictable and difficult to contain features of spreading wildfire.

In addition to wind tunnel studies, Blunck and his team are participating in controlled burns in open air. Comparing the results from such experiments allows the researchers to understand how embers are generated at different scales. They determine the ember generation rate by using infrared imaging and by tracking the number of particles that pass a specified distance from the flame, using image-tracking software.

A major goal of the work is that correlations and data determined in this study will be integrated with an ember transport model being developed with collaborators in the USDA Forest Service and shared with the fire suppression community. The model will simulate ember transport as a function of wind speed, direction and ember size, and shape. 

The coupled experimental and computational approach could prove to be a valuable tool to support management decisions. 

“If I am a fire manager, I need to make decisions about what type of resources to allocate or how best to protect an area,” Blunck explains. Ultimately, he hopes his research will allow those fire managers to better predict where embers will be transported and, therefore, where they should marshal their resources to limit the fire’s spread.

Oct. 16, 2017