What is it about?
Pulsejets are among the simplest jet engines ever built. Instead of the heavy compressors and turbines of a conventional jet, they rely on intermittent combustion driven by sound waves. A burst of combustion sends a pressure wave racing down a tube. When it reaches the open end, it bounces back as a "suction" wave that lowers the pressure in the combustion chamber, opens small flap valves, and draws in fresh fuel and air for the next burst. Because the design is so simple, with only the thin valve flaps moving, pulsejets are an attractive candidate for shrinking down to power very small, fast drones, where turbines and piston engines become complicated, fragile and inefficient. In this paper we used computer simulations to ask whether this cycle would still work in a pulsejet only 2 cm long. We compared it with a hobby-sized pulsejet about 46 cm long, following how the pressure and suction waves travel through each engine after a combustion event. We focused on three questions: whether the suction wave would still lower the pressure enough to open the valves, how fast the tiny engine would cycle, and whether the friction of the air against the walls would choke the waves in such a narrow tube.
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Why is it important?
The simulations gave encouraging answers. In both the large and the tiny engine, the suction wave lowered the chamber pressure to about 62% of the surrounding air pressure, which is enough to open the valves and refill the chamber. The chamber also stayed hot enough, three to four times the ambient temperature, to ignite the next charge, provided the engine is well insulated. The operating frequency scaled inversely with engine length: the large engine cycled about 390 times per second, while the 2 cm engine cycled nearly 9,000 times per second. In other words, the basic acoustic cycle of a pulsejet survives miniaturization. We also identified the main challenge. In the tiny engine, the slow-moving layer of gas along the walls fills up to about 60% of the tube's diameter, compared with about 12% in the large engine. The waves can still travel through the central core of the tube, but wall friction becomes far more important and is likely to reduce thrust. These findings gave the first clear signal that micro-pulsejets are physically feasible and pointed to what must be solved next. They opened the door to experiments on real miniature pulsejets as simple, low-cost engines for micro air vehicles and portable power generation.
Perspectives
This short paper holds a special place for me because it was where our pulsejet research began. The questions were simple, but no one had really answered them: if you shrink a pulsejet dramatically, does the physics that makes it run still work? Seeing the simulations show that the suction wave was just as strong in a 2 cm engine as in a hobby engine was the result that convinced us to pursue this direction experimentally. I am grateful to my student Qun Wan, who carried out the simulations, and to my long-time colleague Will Roberts, with whom I shared many years of pulsejet research. Looking back, the predictions in this paper were a useful guide. In the years that followed, our group built and tested real valveless pulsejets of 15 cm and then 8 cm, and we confronted exactly the challenges this study anticipated: heat loss, wall friction and the race between chemistry and acoustics. I hope this paper reminds readers that a simple model, asked the right questions, can tell us a great deal about whether a bold idea is worth pursuing. Tiny, low-cost pulsejets for small, fast drones remain, in my view, an idea well worth pursuing.
Andrey V Kuznetsov
North Carolina State University
Read the Original
This page is a summary of: Computational analysis of the feasibility of a micro-pulsejet, International Communications in Heat and Mass Transfer, January 2005, Elsevier,
DOI: 10.1016/j.icheatmasstransfer.2004.05.020.
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