People often think that size matters, but it doesn’t. It’s the shape that really counts.
No matter what the fuel – charcoal briquettes, split hardwood logs, rolled-up newspapers – a fire performs better when the height and width of its fuel-source pile match numbers.
It tends to looks like a pyramid or cone shape, and it’s something we’ve instinctively known and repeated ever since the first campfire flickered outside the cave. But no one has ever explained why until Adrian Bejan, professor of mechanical engineering at Duke University, found inspiration one evening while grilling dinner in his backyard.
“I asked myself, ‘Why am I making this pile of charcoal always the same way?’” said Bejan, whose findings appeared in the Nature.com journal Scientific Reports last month. “It was a question that was waiting to be asked.”
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It’s all about the flow
In its most basic sense, fire is nothing more than the result of a chemical reaction between fuel, heat and oxygen. Tampering with any one of those components will prevent the chemical reaction from happening.
Air needs to be able to flow through the fuel pile in order to create combustion. As the air heats, natural convection occurs: Warmer air rises and cooler air fills its vacancy until the warmer air cools off and drifts back down, trading places with new warm air on the rise. It’s a continuous cycle.
Bejan noticed that the shape of the fuel pile dictated the success of the process, and that not every shape is up for the job.
“The shape tells the story,” he said.
Why pyramid and cone shapes work best
Air and heat don’t flow the same way through all shapes. Tall, skinny fuel piles, where the height measures longer than the base width, won’t muster much warmth. “If the pile is slender, it will burn, but it will be ‘cold’ because it is so slender that the cold air from all sides is breathed in, and will cool the burning volume of the fuel,” said Bejan.
Short, stout piles, where the base width is longer than the height, don’t fare much better. “If the pile is flat like a pancake, it will not breathe,” said Bejan. “It will burn. It will smolder, but it will not be ‘hot.’”
Simply put, pyramid and cone shapes provide the optimal conditions for air and heat to flow.
“The compromise between two bad extremes is what is good, and that is what is kept,” said Bejan. “That is how the design of anything really moves forward in our evolutionary story of life and survival.”
Size doesn’t factor in
The amount of fuel in the heap doesn’t play into it. A good roaring fire can be as large as a bonfire in a farmer’s field or as small as a flickering campfire shared by a few friends in the woods – as long as it takes on a shape where its height and width are nearly identical.
Early man was brilliant. Even if he didn’t know why it worked, he figured out the perfect shape for us. So next time you skewer a marshmallow and jab it in the campfire or tease a steak until it’s medium rare over a charcoal grill, think of him and his job well done.