# How to build the perfect campfire

A Duke University engineer applies a theory of physics to understand what makes the hottest fires.

Clearly when the earliest users of flame were building their fires they weren’t considering laws of physics, yet they built them hot and raging. From cavemen to Girl Scouts, for as long as we’ve been building fires we’ve made them in the same general shape. Now we know why, thanks to a study published by Adrian Bejan, a professor of mechanical engineering at Duke University.

Bejan is responsible for penning the Constructal Law, a theory from 1996 that explores flow systems and how they evolve and survive. (“For a finite-size flow system to persist in time (to survive) its configuration must evolve (morph) in time in such a way that it provides easier flow access,” is how Bejan describes it.)

Applying the law to how fires burn, Bejan concludes that the hottest pile of burning fuel occurs when the height of the pile is roughly the same as its base diameter. The best fires are built to be as tall as they are wide. It’s that simple. The pile of fuel is permeable, air invades it by natural convection and drives the combustion.

“Our bonfires are shaped as cones and pyramids, as tall as they are wide at the base. They look the same in all sizes, from the firewood in the chimney, to the tree logs and wooden benches in the center of the university campus after the big game,” he says. “They look the same as the pile of charcoal we make to grill meat. And now we know why.”

“Humans from all eras have been relying on this design,” Bejan adds. “The reason is that this shape is the most efficient for air and heat flow. Our success in building fires in turn made it possible for humans to migrate and spread across the globe heat flow from fire facilitates the movement and spreading of human mass on the globe, which is a direct prediction of the Constructal Law.”

So there you have it. Next time you're in the woods camping and need to concoct the perfect fire, all you have to do is remember the Constructal Law ... or, just build a fire that is as wide as it is tall.

How to build the perfect campfire
A Duke University engineer applies a theory of physics to understand what makes the hottest flames.