Theoretical and experimental investigation of the pulsejet engine

Abstract: The motivation behind this master thesis is the renewed interest in pulsejet engines towards power or thrust generation for small UAV’s. As technologically simple as it may seem, pulsejet engines appear to be the subject of ongoing scientific debate and controversy with regard to their operating principle. The problematic which this work addresses could be stated as such: How do pulsejet engines really work and how to predict their performance? The first part of the study was an exhaustive literature review aimed at gathering knowledge about pulsejet engines. Upon completion, the need was felt to clear some confusion and come up with an own understanding of the principles of pulsejet engines. Two aspects were simultaneously investigated: gas dynamics and combustion theory. The gas dynamics study was based on a zero-dimensional numerical model on MATLAB which served to calculate the thrust and the frequency of operation depending on the pulsejet geometry. It was found that the frequency of operation of a pulsejet is determined very closely by its first acoustic mode frequency, which corresponds either to the Helmholtz frequency or the quarter wave frequency depending on the pulsejet geometry. The ability to sustain a pulsating behaviour and to generate thrust was connected to a coupling between the pressure disturbance and the heat released through combustion. A criterion based on energy conservation was derived in order to predict the condition under which the modelled pulsejet would sustain a pulsating combustion. This criterion was found to be a version of the Rayleigh’s criterion for thermo-acoustic instabilities. The second part of the study intended to explore the laws and principles of combustion and flame theory, trying to analyse how these known phenomena can explain the pulsejets’ operation. More interest was given to the thermo-acoustic instability and results of experiments on flame propagating in closed-open tube. A proposal was made to explain the engine operation. Further studies related to pulsejet flame transfer function and operability diagram were suggested. This fundamental investigation of pulsejet engines was followed by a more problem-oriented engineering task aimed at evaluating their performance in terms of thrust. A formula was derived based on the thermodynamic study of the Lenoir cycle in order to connect the thrust with the pressure disturbance. Experimentations were carried out on a Redhead pulsejet in order to measure thrust, temperature, pressure and frequency. The frequency of operation was predicted with a relative difference while the thrust was under predicted with a relative difference. Continuation of this work involves a more refined numerical model which would allow identifying and characterizing more accurately the mechanisms at stake during the pulsejet operation. Also, suggestions were made to investigate noise reduction through pulsejet coupling, which have been briefly mentioned at the end of the report.