HOW WE BUILD FIRES SPARKS INTEREST
It’s the same all over the world, and a research engineer tells us why it works
This summer, Postmedia’s Tom Spears brings you the often-offbeat science behind the season that calls us to go outdoors. It’s all part of a series we call Science of Summer. Today: a mechanical engineer who researched how to build the perfect campfire.
Adrian Bejan likes to grill dinner on real charcoal, and one day he started wondering why he instinctively always heaps the coals the same way to light them.
Bejan holds a research chair in mechanical engineering at Duke University and is a major figure in thermodynamics research. He has students from many different continents, and he asked them all to draw the kind of fire they would build.
All the same shape, he found. His students have learned to build fires in the Saudi desert, Central Africa and Asia that are the same shape as the fires Bejan built while growing up in Romania.
The engineer in him decided there must be a good reason. And his philosopher side decided this says something about evolution, though that will have to wait a minute.
Bejan got to work on addressing the question of the perfect campfire as a problem of thermodynamics, and he published his results in a major science journal last year called Scientific Reports, by the publishers of Nature. Title: Why Humans Build Fires Shaped the Same Way.
His basic answer is that the hottest fire, the one that makes the most efficient use of available fuel, is as high as it is wide at the base, narrowing toward the top in a pyramid or cone shape. The ratio of height to base width is crucial.
There are two main factors, he says. First, a fire releases hot gases from the wood, and these rise through the spaces between the sticks or logs and ignite.
That requires oxygen, which comes from the side. When the gas rises, it leaves lower pressure behind, which sucks in fresh air from beside the fire.
But if the wood pile is too low and wide, it has difficulty drawing in air and the gases are starved of oxygen. The results are incomplete combustion and a cooler fire.
If the wood is in a tall, skinny pile, a lot of air rushes in easily, but it cools the inside of the pile. Again, it’s a cooler fire.
When the height and width are the same, he says, “everybody is happy.” (There’s a lot of math along the way, but he concludes simply in his paper: “This confirms common knowledge: the fire is hotter when it ‘breathes’ better.”) Which brings him to the nature of evolution.
“Why do people think evolution belongs in biology? It’s actually everywhere — in technology” and other fields, he said. It turned into an interview that ranged from Magna Carta to sports to the academic freedom that allows him, as a university professor, to investigate a question just because it made him curious as he was cooking dinner.
Science and technology evolve as people take good ideas and refine them, adding new understanding, he said. The way humans learned to work with fire is a simple example, with big implications: This early technology enabled human survival on widely separated continents.
Scientists publish a lot of papers that usually circulate among their close colleagues. Publish research on a gene associated with the common cold, and a lot of cold researchers will read the work but no one else will.
But there was a flare-up of international interest as soon as Bejan published his fire research (which, incidentally, applies to fires in a fireplace as well as those built outdoors).
“I’m actually shocked by all the interest in it,” he said. He thinks he knows why. “A good idea has to be interesting, it has to be short, and it has to be accessible to the reader. I’m not snowing people with quantum mechanics . ... This is about something that everybody knows.”
We thanked the professor for the free lecture.
“No, not a lecture,” he said. “Just a nice chat.”