A New Acoustic Model for Valveless Pulsejets and Its Application to Optimization Thrust

Abstract

Due to its simplicity, the valveless pulsejet may be an ideal low cost propulsion system. In this paper, a new acoustic model is described, which can accurately predict the operating frequency of a valveless pulsejet. Experimental and computational methods were used to investigate how the inlet and exhaust area and the freestream velocity affect the overall performance of a 50cm pulsejet. Pressure and temperature were measured at several axial locations for different fuel flow rates and different geometries. Computer simulations were performed for exactly the same geometries and fuel flow rates using a commercial CFD package (CFX) to develop further understanding of the factors that affect the performance of a valveless pulsejet. An acoustic model was developed to predict the frequency of these valveless pulsejets. The new model treats the valveless pulsejet engine as a combination of a Helmholtz resonator and a wave tube. This new model was shown to accurately predict geometries for maximum thrust. The model was further extended to account for the effect of freestream velocity. Evidence is provided that valveless pulsejet generates the highest thrust when the inherent inlet frequency matches the inherent exhaust frequency.

Study on velocity and pressure characteristics of self-excited oscillating nozzle

“…In conclusion, the simplified two-dimensional model in this study coincides with the traditional numerical methods, and the simulation results in the free jet of the self-oscillating nozzle were reliable. The pressure implicit solver is used in the numerical simulation [34], using realizable k-ε turbulence model and SIMPLE algorithm [14], the discrete formats of momentum and pressure are QUICK and PRESTO! [27], the discrete scheme of turbulent kinetic energy and dissipation rate is second-order upwind [32].…”

Acoustic Analysis of Valveless Pulsejet Engines

“…In 1987, Smith [52] suggested that a valveless pulsejet could be modeled as the combination of a Helmholtz resonator (the combustion chamber and inlet pipe) and a quarter-wave tube (the exhaust pipe) although no quantitative results based on this notion were presented. Zheng et al [53] [54] followed a similar line proposing that the frequency of a valveless pulsejet is the average of the natural frequency of a Helmholtz resonator (represented by the combustion chamber and the inlet tube, Geometry 1 in Fig. 1-17) and the natural frequency of the combustion chamber coupled with the tailpipe (Geometry 2 in Fig.…”

“…Figure 1-17: Geometries for frequency calculation method of Zheng et al [53] The expression for the frequency of Geometry 1 (f 1 ), a Helmholtz resonator, is wellknown [55]:…”

“…Zheng et al [53] conjectured that the frequency of operation of a valveless pulsejet is the mean of f 1 and f 2 . They then experimented with a 50-cm nominal length pulsejet changing the exhaust length, inlet length, and inlet diameter, to determine the effects of these geometrical parameters on operating frequency.…”

“…Smith [52] suggested that a valveless pulsejet could be analyzed as the marriage of a Helmholtz resonator (the combustion chamber and inlet pipe) to a quarter-wave tube (the exhaust pipe), although he did not proceed to present any quantitative results based on this notion. Along similar lines, in an important development, Zheng et al [53] proposed that the frequency of a valveless pulsejet is the average of the natural (fundamental) frequency of a Helmholtz resonator (represented by the combustion chamber and the short inlet tube) and the natural frequency of the combustion chamber coupled with the tailpipe (also similar to a Helmholtz resonator, but with a long neck in which wave motion is accounted for).…”