What is it about?
Pulsejets are among the simplest jet engines. Instead of the heavy compressors and turbines of a conventional jet, they rely on intermittent combustion driven by sound waves. Fuel burns in short bursts, pushing hot gas out of a tailpipe. Pressure waves then bounce back and forth inside the engine, pulling in fresh air and triggering the next burst, hundreds of times per second. The valveless pulsejet takes this simplicity even further. It has no moving parts at all, not even the small flapping reed valves that wear out in ordinary pulsejets. This makes it an attractive, low-cost and rugged way to power small, high-speed drones. In this paper we studied a 50 cm valveless pulsejet using both laboratory experiments and computer simulations. We tested inlets of different diameters and lengths and exhaust pipes of different lengths, measuring pressure, temperature and operating frequency. We then developed a new acoustic model that treats the short inlet like the neck of a bottle when you blow across it (a Helmholtz resonator). It treats the long exhaust like an organ pipe. The engine's running frequency turns out to be close to the average of these two "notes," and our simple formula predicts the measured frequency to within 5%.
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Why is it important?
The key finding is that a valveless pulsejet produces the most thrust when its inlet and exhaust are "in tune," meaning they resonate at the same frequency. When the two frequencies drift too far apart, the pressure oscillations die away and the engine simply stops running. This gives designers a clear, practical rule for sizing the inlet and exhaust without lengthy trial and error. We also found a counterintuitive result: a wider inlet actually lets fewer hot combustion products escape backwards through the front of the engine. Because drones fly rather than sit on a test stand, we also examined how forward flight speed changes the picture. The incoming airstream shifts the inlet's natural frequency. We captured this with a simple correction based on flight Mach number. The corrected model correctly predicted the best inlet diameter at flight speeds of 30 m/s and 120 m/s. Together, these results turn the design of a valveless pulsejet from an art into a predictable engineering process. This is an important step toward simple, inexpensive engines for fast unmanned aircraft.
Perspectives
What I find most appealing about this work is how much of the engine's behavior can be explained by ideas familiar from musical acoustics. The same physics that governs a bottle or an organ pipe tells us how to get the most thrust from an engine with no moving parts. It was very rewarding to see a model simple enough to fit on a single line agree so closely with both our experiments and our detailed simulations. It was also a pleasure to work with my graduate students Fei Zheng and Robert Ordon. Our colleagues Tom Scharton, whose acoustics expertise shaped the model, and Will Roberts contributed greatly to this paper. Pulsejets were largely set aside in the 1960s in favor of turbojets. However, turbine engines become much less efficient as they are scaled down, while pulsejets remain simple, light and cheap at small sizes. I hope this paper helps renew interest in valveless pulsejets as a practical option for small high-speed drones. I also hope the "tune the inlet to the exhaust" rule proves useful to engineers and enthusiasts alike.
Andrey V Kuznetsov
North Carolina State University
Read the Original
This page is a summary of: A New Acoustic Model for Valveless Pulsejets and Its Application to Optimization Thrust, Journal of Engineering for Gas Turbines and Power, April 2008, ASME International,
DOI: 10.1115/1.2900730.
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