Engineers are always working to innovate, especially on big-ticket projects such as automotive engines, jets and rockets. For tech like these, design often depends not only on the potential applications, but on the physics fundamentals that govern the combustion processes that power them. This makes the discoveries in a recent study by the University of New Mexico intriguing—that centuries-old laws about the behavior of gas mixtures do not apply in the presence of shock waves.
What makes this specifically interesting are the potential applications for some of our most impressive technologies: automotive and jet engines which are based on mixtures of gases exposed to a shock wave, such as those that occur during combustion.
“Our study found that classical laws used to predict gas mixture properties fail to work in a fairly common and practically important situation,” said Peter Vorobieff of the Department of Mechanical Engineering at UNM.
This discovery also holds potential impacts for both conventional and nuclear explosions, supersonic jet engines, gas-cooled nuclear reactor plants and even inertially-confined fusion.
Breaking Classical Laws
The classical laws in question are Dalton’s Law, and Amagat’s Law.
French physicist Emile Hilaire Amagat’s developed his law of partial volumes in 1880. It states that the total volume of a gas mixture is equal to the sum of the partial volumes each gas would occupy if it existed alone at the temperature and pressure of the mixture.
Dalton’s Law was developed by scientist John Dalton earlier that century, in 1802. This law stated that the total pressure in a non-reactive gas mixture – at constant temperature and volume – is equal to the sum of the partial pressures of the component gases.
The UNM study involved pre-mixing two gases with dramatically different properties: light helium, and heavy and viscous sulfur hexafluoride. The team then characterized the properties of the resulting mixture, which agreed well with classical theory.
Next, a shock wave was introduced, and the temperature and pressure of the shock-accelerated medium were measured over several milliseconds. Though this sounds like a short time to think of in normal terms, it is a long interval compared to the time scales associated with the passage of a shock wave.
What the researchers found was that the temperature and pressure after the shock compression did not line up with what would have been expected from the predictions of either of the two classical theoretical laws – Dalton’s or Amagat’s.
Shocking Behavior Under Pressure
As Vorobieff described it, the reason for the disagreement between the UNM team’s results and the classical laws is that neither classical law can accurately describe what happens on the molecular level.
Time scales from kinetic molecular theory, and how they are affected by shock acceleration, appear to provide at least a qualitative explanation of the experimental observations. Vorobieff said that although this is a solid first step, the ultimate implications have not yet been determined, and much further study is required.
But the most noteworthy potential outcome would be the possibility of significant impacts and design changes for mechanisms such as engines, in order to take into account this new information on how shock waves affect the gas mixture properties.
“Our work has shown that classical gas mixture theory does not work in shock-accelerated and possibly other compressible flows,” Vorobieff said. “We must conduct experiments with more gas mixtures and a broader range of conditions to explore the scope of the problem and develop a theory explaining our observations.”
With simulation and computation fluid dynamics doing more and more of the development work in combustion engineering, this research could lead to better modelling of the notoriously complex combustion process—and to better, more efficient engines in the bargain.
For now, the results are published in the paper “Dalton's and Amagat's Laws Fail in Gas Mixtures with Shock Propagation” in the July 2019 edition of Science Advances.