Study of Ejector Geometry on Thrust Augmentation for Pulse Detonation Engine Ejector Systems
Open Access
- Author:
- Shehadeh, Ra'fat
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 12, 2007
- Committee Members:
- Robert John Santoro, Committee Chair/Co-Chair
Vigor Yang, Committee Member
Richard A Yetter, Committee Member
Michael Matthew Micci, Committee Member - Keywords:
- PDE
Pulse Detonation Engine
Ejector - Abstract:
- Pulse detonation engine (PDE) technology is a novel form of propulsion that offers the potential of high efficiency combustion with reduced hardware complexity. Although the primary interest of the research in the pulse detonation engine field is directed towards overcoming the problems associated with operating a pure PDE system, there are other worthy options to be considered for these engines. The PDE driven ejector concept is one such option where the system would be part of a hybrid PD/Turbofan engine. This system offers the promise of replacing the high-pressure turbine sections of the core of a high bypass turbofan engine. The purpose of the current research is to investigate the thrust augmentation capabilities of a PDE driven ejector and provide experimental data that would assist in understanding the behavior of such a system. The major potential advantages of the PDE-ejector include reduced costs due to the reduced engine weight, along with improved specific fuel consumption and specific power inherent in the incorporation of a PDE component. However, for the PDE-ejector system to become viable, the effect that various ejector geometries have on the configuration has to be understood. There is currently insufficient data on the performance of such an ejector system to allow further development. Some of the key issues can be resolved if suitable measurements of the thrust augmentation were available over a suitable parameter space. To achieve the goal of this research, the thrust augmentation of a PDE driven ejector was characterized for a set of configurations. Two separate PDE’s were utilized in this study. The first PDE was capable of operating at a constant frequency of 10 Hz de to flow rate limitations, and another PDE built to have an operational frequency range of 10 Hz-70 Hz to test the effect of operational frequency on PDE-ejector systems. Optical diagnostics were employed at specific positions of interest to understand the physical behavior of the flow. The thrust of the PDE operating alone, i.e. the baseline case, was measured first. The baseline was the time averaged value of the thrust for the PDE operating at a various frequencies ranging from 10 Hz to 70 Hz and duration of 8 seconds with a propellant mixture comprising of ethylene/nitrogen/oxygen at 100% fills. Line-of-sight laser absorption measurements of the fuel at the PDE tube exit were used to define the 100% fill condition and evaluate the PDE injector performance. These baseline experimental results helped define and understand the operational characteristics of the PDE’s utilized in this study. Thrust measurements were then made for PDE driven ejector configurations. The parameters that were independently changed were the inlet geometry of a constant diameter ejector, as well as the overlap distance between the PDE tube exit and ejector tube inlet. Ejectors of different size diameters were also tested and their performance documented. It was found that increasing the ejector inner diameter provided better thrust augmentation for ejectors of the same length. Then further study was devoted into the effects of varying the length of the ejector with the diameter providing the most thrust augmentation. Along with the length, the inlet geometry of the ejector was also varied. A rounded inlet was mounted on the ejector and the inlet’s effect on thrust was recorded. From these previous studies it was found that with a relatively short ejector having a rounded inlet, thrust augmentation up to 40% was possible. Thus the effect of simply rounding the inlet caused a considerable increase in thrust production. Thus three different size rounded inlets are tested for thrust augmentation. To study the effect of frequency on PDE-ejector performance a PDE capable of operating at frequencies up to 70 Hz was assembled. The thrust output of the PDE served as the baseline case and was measured for the various operating frequencies under study. Thrust measurements were then made for PDE driven ejector configurations. Four different ejectors with the same constant diameter but varying lengths were utilized. The geometric parameters that were independently changed were the ejector inlet geometry, the overlap distance between the ejector and PDE tube exit, ejector tube length, and system operating frequency. Thrust augmentation levels of 120% were achieved with this system using the longest available ejector with a rounded inlet. A square ejector with plexi-glass sides allowing optical access into the ejector was designed. The ejector was designed to have the same hydraulic diameter as the round ejectors utilized. The ejector was first tested for thrust augmentation and proved to provide the same thrust augmentation trend as its round counterparts. Shadowgraph imaging was then used to observe the flow at various ejector locations for the duration of a single PDE cycle. These images provided insight into the behavior of the gas inside the ejector during a single detonation cycle. It was observed that the flow characteristics of the detonation cycle changes with the location of the ejector, where ejector position alters the blowdown process undergone by a PDE. It was also observed that the flow of secondary gas into the ejector tube starts earlier for the case of 25% ejector overlap position. Although a lot of work is still to be done to understand the factors behind thrust enhancement in a PDE-ejector system, this work serves to explain and characterize some of the phenomena displayed by such systems and lay the foundation for future work into the subject.