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Jam Jar Pulsejet

40 seconds of continuous running. Barilla "Olive" pasta sauce glass jar (approximately 70 mm diameter and 120 mm height), with 12 mm hole drilled into centre of lid using a step drill bit on a pillar drill in order to get a clean and round hole with sharp edge (though the hole quality does not appear to be critical, based on other videos). The tray beneath the jar contains water and its purpose is to catch any spill and fire should the jar shatter into pieces from the thermal stress. By the end of this run I measured over 200 degC temperature on the glass (the limit of my instrument!). The fuel was methylated spirits (93% ethanol, 7% methanol). It's not so easy to get started. I found the most important thing to do was to get fresh air into the jar after giving it a swirl so as to try to get enough oxygen in there to begin the process. Otherwise the inside gets saturated with fuel vapour and does not combust. The lid must be screwed on tightly so as to make a pressure seal. Also, having a bendy lighter made it a bit easier to light without getting burnt. The lighter flame must go into the hole, not just immediately above it. The process begins with an internal flame front propagating downward towards the fuel, rapidly heating up the air in the jar. This increases the pressure causing the air to rush out the hole (as the flame jet reveals). Due to the momentum of the escaping air, a slight vacuum is pulled in the jar (Kadenacy effect). This then causes fresh air to be drawn back in to the jar which comes into contact with residual flame from the previous cycle. This then ignites lingering vapour with the fresh air and the cycle continues at the resonant frequency of the system. Note that this frequency does not equal the Helmholtz resonance frequency of the jar and lid hole. It turns out to be lower due to the dynamics of the combustion and gas flux. For this particular jar, the Helmholtz frequency is around 275 Hz. Looking at the spectrum of the audio signal, the frequency of the pulses is around 100 Hz, rising to about 130 Hz by the end. The rise in frequency is likely due to the increase in temperature of the gas in the jar (related to the sqrt(T) term that appears in the Helmholtz resonance equation (contained in the speed of sound)).