A new acoustic model for pulsejets with valves

Principles of valved pulsejet operation were examined using a commercially available, but fully instrumented, 50 cm pulsejet. A variety of fuels were used, but primarily ethanol. Measurements providing thrust, pressure, temperature, velocity, and acoustic information were carried out. These experimental results were used in the validation of a numerical model. The pulsejet operation cycle can be divided into eleven steps involving a complicated interaction between fluid mechanics, acoustics, and chemical kinetic phenomena. The cycle consists of a high-pressure portion and a sub-atmospheric portion caused by the reflected waves. Combustion chamber analysis indicates a strong vortex which enhances turbulence and thus increases the reaction rate. An acoustics laboratory was used to obtain pulsejet resonance frequencies experimentally. Acoustic models were developed analytically using the one- dimensional wave equation applied to vibrations in tubes, with appropriate boundary conditions selected at the open end and the combustion chamber end of the pulsejet. These models were compared to the experimental data, leading to a number of conclusions regarding valved pulsejet acoustic characteristics. The pulsejet's first resonance frequency was found to be best described not as a fourth-wave tube, as typically done, but as a sixth-wave tube, due to the combustion chamber boundary condition. Important in the understanding of the pulsejet is that when heat is added at the moment of greatest condensation or removed at the moment of greatest rarefaction, then the "vibration is encouraged" [1,5]. Thus, for the proper operation of the pulsejet with valves, there must be coincidence between the acoustic resonance frequency and the fluid dynamic characteristic frequency, which is a function of the heat release.