Experimental and numerical investigation of an 8-cm valveless pulsejet

https://doi.org/10.1016/j.expthermflusci.2006.06.005Get rights and content

Abstract

This paper investigates the performance of a small scale pulsejet whose overall length is approximately 8 cm, the smallest pulsejet ever reported to the author’s knowledge. Gas dynamics, acoustics and chemical kinetics were modeled to gain an understanding of various physical phenomena affecting pulsejet operation, scalability, and efficiency. Numerical simulations were performed utilizing CFX to model 3-D compressible vicious flow in the pulsejet using the integrated Westbrook–Dryer single step combustion model. The simulation results were validated with experimental data and provide physical insight into the pulsejet operation. The pulsejet was run in valveless mode on hydrogen fuel with either a forward-facing inlet or a pair of rearward-facing inlets. Pressure, temperature, thrust, and frequency were measured as a function of valveless inlet and exit lengths and different geometries. As expected, the rearward-facing inlet produced considerably more net thrust, although still not very efficient, with a TSFC of 0.02 kg/N-h. The operating frequency was found to scale with inlet length to the negative 0.22 power, in addition to the inverse of the overall length for valved pulsejet.

Introduction

The pulsejet is one of the simplest propulsion devices, requiring no turbomachinery, or moving parts in some cases. The pulsejet was originally conceived in the early 1900s and developed into a successful propulsion system by the Germans in WWII for the V-1 ‘buzz bomb’, the name being derived from the impressive acoustic emission at 50 Hz from these engines. Their simple structure and light weight make them an ideal thrust-generation device, but their thermodynamic efficiency is low compared to gas turbine engines due to the lack of mechanical compression, which results in low peak pressure. Due to this low efficiency, the pulsejet received little attention after the late 1950s. However, pulsejets with no moving parts may be advantageous for building smaller propulsion devices. The thermodynamic efficiency of conventional engines (such as gas turbines and both SI and CI IC engines) decreases non-linearly with decreasing characteristic engine length scale. Also, small scale engines with moving parts are more prone to breakdown due to fatigue of the moving components [1]. Pulsejets, especially valveless pulsejets, are attractive as candidates for miniaturization due to their extremely simple design.

The general pulsejet cycle (forward-facing inlet) can be illustrated as follows. The combustion event begins when the combustion chamber pressure is above atmospheric and the temperature of the fuel/air mixture increases, due to mixing with residual products, to the auto-ignition temperature. A compression wave is generated and combustion increases both temperature and pressure in the combustion chamber, driving the flow toward the exit and inlet at gradually increasing velocity. The relatively short combustion event ends and when the compression wave reaches either the pulsejet inlet or the exit, an expansion wave is generated due to overexpansion and travels back into the combustion chamber. Flow velocity reaches its positive maximum at the exit at this time. The expansion wave decreases the pressure in the exhaust tube and the combustion chamber to sub-atmosphere, resulting in backflow at both the inlet and exit. The next charge of fuel/air mixture enters into the chamber due to this backflow at the inlet. The mass addition increases the combustion chamber pressure. When the pressure in the combustion chamber approaches the atmosphere pressure, the next cycle begins.

One of the most significant and technically challenging aspects of the micro-propulsion device is its limited residence time. Once the combustion chamber size becomes 2–3 orders of magnitude smaller than that of a large scale jet engine, the residence time within the combustion chamber approaches the characteristic chemical kinetic time scale for hydrocarbon–air reactions [2]. The chemical time scale can be shortened by using hydrogen as fuel, which was necessary in this work. It may even be necessary to premix and preheat the fuel and air to decrease the chemical kinetic time scale. It has also been suggested that increasing the combustion chamber size relative to the engine size will help with the very short residence times [3]. In a review paper, Roy et al. [4] reported the typical length for a pulse detonation engine is 0.3–3 m. We believe this 8 cm pulsejet is the smallest operational pulsejet reported [5], [6], [7], [8]. The fuel injection system, combustion chamber, and the inlet geometry must be carefully designed to create a fast mixing process and the necessary fluid dynamic and acoustic time scales to permit pulsejet operation.

Another challenge is the heat loss to the walls due to the high surface-area-to-volume ratio. Large thermal losses have a direct impact on overall combustor efficiency and they can increase kinetic times and narrow flammability limits through suppression of the reaction temperatures [3]. For the oscillating combustion process to be self-sustaining, excessive heat loss, which lowers the temperature of the walls and the residual gas, must be prevented.

Section snippets

Experimental setup

Hobby scale pulsejets on the order of 50 cm total length have been used for many years for RC aircraft and hydroplane propulsion applications. This hobby scale version is typically run in a valved mode, with reed valves opening on the low pressure ingestion stroke and closing as the pressure increases due to heat release from combustion.

Previously, a 15 cm total length pulsejet was investigated numerically and experimentally [9]. In the 15 cm version, the traditional valved inlet was replaced with

Numerical simulation

The current study models the unsteady, three-dimensional, compressible, viscous flow with heat transfer and radiation, utilizing the CFX5.7 commercial software package. A second order transient scheme and high-resolution advection scheme were used to capture the compression/expansion waves. Governing equations for the fluid flow are given in [10].

The computations were performed on a single 3.0 GHz Intel Xeon processor, with typical computational time for one cycle of the pulsejet being 20 CPU

Results and discussion

From the computational results, the pulsejet cycle can be described by the following 10 steps:

  • 1.

    Combustion event begins when hydrogen and air mix and are brought to their auto-ignition temperature through mixing with residual hot products from the previous cycle. The pressure and temperature begin to increase in the combustion chamber. Air continues entering the combustion chamber through the inlet with reduced velocity.

  • 2.

    Combustion continues, and peak pressure and temperature are reached in the

Conclusions

Small-scale pulsejets may be good candidates for micro-propulsion devices due to their simple designs. A combined experimental and numerical approach was used to investigate the performance of a hydrogen fueled 8 cm valveless pulsejet with two inlet configurations: forward-facing and rearward-facing. To the author’s knowledge, this is the smallest operational pulsejet reported. This work showed that:

  • 1.

    The simulation provided physical insight into the pulsejet operation. The simulated operation

Acknowledgements

This project is sponsored by the Defense Advanced Research Projects Agency (DARPA) under the supervision of Dr. R L. Rosenfeld, Grant No. HR0011-0-1-0036. The content of the information does not necessarily reflect the position or policy of the Government and no official endorsement should be inferred. The authors would also like to thank Dr. Vincent Castelli and Dr. Terry Scharton for their helpful comments and suggestions.

References (12)

  • G.D. Roy et al.

    Prog. Energy Combust. Sci.

    (2004)
  • W. Fan et al.

    Combust. Flame

    (2003)
  • A. Majumdar et al.

    Microscale Thermophys. Eng.

    (1998)
  • I.A. Waitz et al.

    Fluids Eng.

    (1998)
  • C.M. Spadaccini et al.

    Eng. Gas Turbines Power

    (2003)
  • S. Eidelman et al.

    J. Propulsion Power

    (1991)
There are more references available in the full text version of this article.

Cited by (23)

  • Revisiting the Argus pulsejet engine of V-1 buzz bombs: An experimental investigation of the first mass-produced pressure gain combustion device

    2019, Experimental Thermal and Fluid Science
    Citation Excerpt :

    These factors contribute to the utilitarian benefits of this type of combustor. It is generally agreed upon that pulsejets function based on a Rayleigh criterion-type combustion process where there is in-phase pressure increase and heat addition [10–13] While the exact mechanism of the combustion is not yet deciphered, several possibilities such as conventional turbulent flame ignition [8], auto-ignition from the heated wall [11] and shockless explosion combustion [12] have been proposed. Similarly, there is some contention on the nature of the acoustic oscillations happening inside pulsejets.

  • Micro Newcomen steam engine using two-phase working fluid

    2011, Energy
    Citation Excerpt :

    Homogeneous Charge Compression Ignition (HCCI) was also proposed to improve the micro engine’s performance [11–13]. To overcome the serious problems of friction and leakage between mechanical parts in micro engine [14], T. Geng fabricated a micro pulsejet [15]. S. Whalen developed a novel P3 engine, made of an elastic membrane [16,17].

  • Stability Characteristics of an Actively Valved Resonant Pulse Combustor

    2023, Journal of Engineering for Gas Turbines and Power
  • A review of pollutants emissions in various pressure gain combustors

    2019, International Journal of Spray and Combustion Dynamics
  • Influence of fuel on a valveless pulsejet engine performance and pollutant emissions

    2019, ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)
View all citing articles on Scopus
View full text