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Control of shock dynamics for pulse detonation engines

Contact: Bhavraj Thethy

Pulse-detonation engines (PDEs) are a promising development in power generation and propulsion, offering the potential to drastically increase efficiency of engines. They operate on the principle of pressure-gain combustion, where combustion of fuel and oxidiser is instigated by a detonation wave. One promising configuration is the hybrid pulse-detonation combustor (PDC), in which a PDC replaces the standard combustor inside a gas turbine. This configuration has been the focus of much work including the Technical University of Berlin Collaborative Research Centre (SFB1029). One of the many challenges associated with the PDC-turbine is the alleviation of the pressure fluctuations and impact of the supersonic PDC exhaust on the turbine. A method to achieve this is to place a plenum chamber between the exhaust of the combustor and the inlet of the turbine. This leads to a series of questions, including interaction of this plenum with the exhaust flow from the PDC, which is the subject of a related project. 

The focus of this work is to explore the methods of altering the shock dynamics of PDC exhaust flow. The complex dynamics of detonation-driven transient supersonic jets (such as those found in a PDC exhaust) have been linkedto the dynamics of shock-driven transient supersonic jets. The project began with the design and commissioning a new, portable shock tube facility at Monash University (see Figure 1). This facility allows for cheap and rapid experimental turnaround time of the canonical shock-driven supersonic starting jet process. The shock tube facility, after commissioning and initial experimentation at Monash University, has been transported to the Technical University of Berlin. Further experimentation will be conducted at TU Berlin. 

Shock tube facility, designed and commissioned at Monash University and transported to TU Berlin for further experimentation.

Traditionally, one method to alter the shock dynamics of supersonic flow has been to attach a nozzle to the outlet of a jet. Initial experiments have focused on the use of traditional converging, diverging and straight nozzles to examine the influence this has on the shock dynamics. Figure 2 shows an example of a time-resolved ultra-high speed schlieren sequence of a shock-driven transient supersonic jet emitted from a converging nozzle. The complex nature of the transient process is evident with the leading shock diffracting out of the open tube, followed by the roll up and axial translation of vortices at the nozzle exit. As the vortices translate further downstream, the typical Mach disk shape, associated with an underexpanded steady jet, appears. Further analysis of the influence of the nozzle geometry on this flow is currently underway. 

Ultra-high speed schlieren images of a shock-driven transient supersonic jet emitted from a converging nozzle. The images are taken at an acquisition speed of 250 kfps.
Photograph of one of the shock divider assemblies.

Alongside the use of traditional nozzles, a novel method of altering the shock dynamics of a supersonic jet is also being explored. Termed the shock divider (see Figure 3), the leading shock wave will be split into multiple waves before reaching the nozzle, in order to alleviate some of the pressure fluctuations associated with this leading shock. The divider study will look at a simplified planar square geometry (Figure 3) and a non-planar circular geometry which is more representative of the PDC exhaust. The planar geometry allows line of sight access to the internal motions of the shock wave. Using schlieren and shadowgraph techniques, this will provide experimental insight into the internal motions of the wave. The exit flow of both the planar and non-planar geometries will be investigated and compared. Finally, the experiments will be used to validate numerical results, in order to provide a more in-depth insight into the flow physics. The goal of the project is to determine an effective method to alter the shock dynamics of a transient supersonic jet and apply this method in the design of a PDC-turbine hybrid engine.   

This project is a collaboration with Monash University’s Laboratory for Turbulence Research in Aerospace and Combustion (LTRAC)and is supported by the DFGthrough SFB 1029Australian Research Councilthrough DP190102220and by the DAAD

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