Flow Systems:
Then and Now

Statue of Denis Papin (left) and photo of Sadi Carnot (right).

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ME 101, Duke University’s thermodynamics course in mechanical engineering, was created decades ago by professors who had the vision to build Duke. Today, I suspect, thermodynamics would get a less memorable course number.

Though times change, principles remain. Contrivances, gadgets, and fads are far less enduring. They evolve, but their impact on the “thin book” of fundamentals is fleeting.

Recently, I visited two colleagues at the National Conservatory of Arts and Professions (CNAM) in Paris. This historic university originated many enduring ideas: evening classes for co-op students, continuing education, and the world’s first technology museum.

CNAM’s 300-year-old edifice includes towers, an inner court, statues, clocks, and a history chiseled in stone. Its front court features a statue of Denis Papin with the first piston-and-cylinder machine that expanded steam to produce work in 1690—80 years before James Watt.

The National Institute of Standards and Technology (NIST) maintains standards for measuring flow.

Around the buildings and ceilings of the oldest classrooms are names of former CNAM professors and students, including Sadi Carnot. Without Carnot, we would have no thermodynamics, engineering, or standard of living as we know them today.

In the early 1800s, after graduating from the École Polytechnique, Paris, Carnot went to CNAM to study the steam engines on display there, which were built by British engineers. Britain had industrialized in the 1700s. Finally, a century later, the industrial revolution was reaching the Continent.

Denis Papin's 1690 piston-and-cylinder machine.

Freedom Through Steam

Why were steam engines proliferating? Because of their dramatic, empowering effect on people's lives. By liberating serfs, slaves, and animals, they were facilitating global movement of humanity.

The principle that Carnot saw in the steam engines is that everything flows one way: high to low. Water flows through pipe from high pressure to low pressure. Heat flows from high temperature to low temperature. Today, we know this as thermodynamics, the science of everything that kicks and moves. Carnot’s original observation became the “second law of thermodynamics.”

Carnot’s experiences at CNAM still spur builders of any machines, through the enduring principle that one individual sustains the crowd, and vice versa. The big river sustains its basin’s many streams. A river basin connects an entire area or volume to one point—low resistances allocated to areas and volumes, all over the world. This principle is the "constructal law.” (For more details, visit www.constructal.org.)

The footprints of both principles—the second law and the constructal law—persist, like river beds or well-beaten tracks. A river or caravan that does not follow its bed or worn track won’t get far.

No flow system is an island. No human settlement thrives without its farmland and open spaces. Everything that has flowed and survived is in optimal balance with its surrounding and sustaining flows. Airflow to our lungs is optimally matched to the blood flow through the vascularized tissue.

In fact, "vascularized" is an apt term for the energy systems of thermodynamics. The tissues of energy flows, like society’s fabric and all biological tissues, are optimized architectures. Transdisciplinary efforts promote a high level of performance: the balance between seemingly unrelated flows, territories, and disciplines. This balancing act—the optimal distribution of imperfection—generates the design of any process, power plant, city, geography, and economics.

Understanding the Whole System

The need to consider the whole—the macroscopic system—is universal. Even when we discover and understand small-scale phenomena or processes, we must still assemble invisible elements into palpable devices. The unseen grains have to be sustained by flows, which connect and enable them. The challenge is to construct, interweave, and optimize during the process of assembling. This challenge is increasingly difficult. As the smallest scales continuously shrink, the number of components and complexity of useful devices (always macroscopic) increases.

Consider the rush to nanotechnology. Technology encompasses more than new physical phenomena on the frontiers of progressively smaller scales. A new technology becomes useful only after it incorporates principles of constructing, connecting, and packing multiscale flow systems into macroscopic spaces, leading to new devices that improve our lives.

New gadgets are like the engines that Carnot contemplated, two hundred years ago. They’ll all be forgotten, unless a Sadi Carnot is watching, identifying a pattern, and immortalizing it in the thin book of principles. This happens rarely. Whenever it does, it reconfirms the constructal principle, that “one sustains the crowd.”

[Adapted from “The Many, The Few,” by Adrian Bejan, Distinguished Professor of Mechanical Engineering, Duke University, for Mechanical Engineering, July 2007.]

No flow system is an island. Everything that has flowed and survived is in optimal balance with its surrounding and sustaining flows.

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