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High-speed diagnostics of shock dynamics in pulse detonation combustors

Contact: Mohammad Rezay Haghdoost

The PDC concept aims to achieve a pressure increase between the compressor and turbine plenum by means of pulsed detonation. It is desirable that the compressors, turbines and other gas turbine components used are largely identical in construction to those used in conventional gas turbines. To make this possible, the flow exiting the PDC must be adapted to the turbine installed downstream. This is necessary to enable efficient energy conversion from the pulsating flow into shaft power. To make this possible, the machine is to be equipped with a plenum. Figure 1 schematically shows the main assemblies of PDC based gas turbine.

Fig. 1: PDC gas turbine concept

The primary task of the plenum is the minimization of the expected pulsations of the flow variables in time and space. At the same time, the kinetic energy generated during pulsating combustion is to be transferred from the detonation tube as efficiently as possible into the turbine for work conversion. The control of the interactions and loss processes occurring in the plenum is of particular relevance for an efficient overall process.

The focus of this work is the development of the basic thermodynamic and fluid mechanical relationships for the design of such a plenum. This is to be achieved primarily by experimentally examining various design studies at a PDC. High speed Particle Image Velocimetry (PIV) , High speed schlieren and dynamic pressure measurements have already been successfully used for this purpose.  These data allow for the determination of the impact of various operating parameters as well as the identification of the fundamental physical phenomena in the exhaust of a PDC.

The experimental data were also used to validate numerical simulations carried out by our partners within the SFB1029 at the Freie Universität Berlin. For this purpose, the exhaust of a single tube configuration for one cycle was investigated in detail. A comprehensive data set has been collected experimentally. Spatially and temporally highly resolved PIV, Schlieren and pressure measurements provide a solid basis for the validation of the numerical simulations. Fig. 2 shows the development of the PDC exhaust based on a time sequence of Schlieren images. The images on the left side show the numerical and the images on the right side the experimentally recorded images. The comparison shows a very good agreement between the numerical and experimental data.

Fig. 2: Experimental and numerical schlieren and contour plots of the Mach number from the PDC exhaust.

After comprehensive investigations of the PDE single tube configuration, experiments were carried out on the PDC multi tube test rig. In order to reduce pulsations from the detonation tubes, a plenum was first developed, which is located downstream of the detonation tubes (see Fig. 3(a)). The impact of the plenum on the PDC exhaust flow is currently being investigated both experimentally and numerically. As an example, Fig. 3(b) shows the experimentally measured static pressure at the plenum wall for a single firing event. As shown in Fig. 3, the pressure peak is significantly reduced from the plenum inlet to its outlet. Hence, a significant minimization of pressure pulsation for the turbine inlet can be realized by using a plenum in a pulsed operation mode for a hybrid PDE. In addition, a very good agreement between the experimental (Fig. 3(b)) and numerical (Fig. 3(c)) obtained data has been achieved. The cooperation with the numerical partners should on the one hand result in well-founded proposals for experimental design studies and on the other hand the numerical data can provide more detailed information about the underlying gas-dynamic phenomena. Furthermore, for the upcoming experimental studies a downstream turbine is simulated by throttling the plenum outlet. For this purpose, orifices with different blockage ratios will be mounted at the plenum outlet to investigate the impact of a potential turbine.

Video:Detonation Initiation and Exhaust of a Pulse Detonation Combustor

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