The hottest pile of burning fuel occurs when the height of the pile is roughly the same as its base diameter, according to a new study authored by Prof Adrian Bejan of Duke University.
The best campfires are roughly as tall as they are wide. Image credit: Adrian Bejan, Duke University.
“The adoption of fire in human civilization was a design change of the same nature as the emergence of organs for vision in animal design, the emergence of turbulence in laminar flow, the emergence of terrestrial animal locomotion from swimming, and the emergence of flying later on,” Prof Bejan said.
“This step occurred in an unmistakable direction, from no fire to fire, not the other way around. Why?”
“The answer is the same for all the transitions: to facilitate movement and mixing on Earth. For us, fire means ultimately more movement for humanity on the landscape, in accord with the Constructal law.”
Almost two decades ago, in 1996, Prof Bejan penned the Constructal law that postulates that movement (flow) systems such as trees, rivers or air currents evolve into configurations that provide easier and easier access to flows. Now, the law is increasingly finding applications in improving design and maximizing efficiency of manmade systems.
In his new study, Prof Bejan shows that the best campfires are roughly as tall as they are wide. The shape is the most efficient for the flow of air and heat.
“Our bonfires are shaped as cones and pyramids, as tall as they are wide at the base,” he said.
“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. They look the same as the pile of charcoal we make to grill meat. And now we know why.”
The study results were published online recently in the journal Scientific Reports.
Prof Bejan added: “future studies may address the shape effect of wind, material type, and packing. Key is why humans of all eras have been relying on this design of fire ‘unwittingly.’ The reason is that the heat flow from fire facilitates the movement and spreading of human mass on the globe.”
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Bejan, A. 2015. Why humans build fires shaped the same way. Scientific Reports 5, 11270; doi: 10.1038/srep11270
