High-speed diagnostics of shock dynamics in pulse detonation combustion chambers

The use of gas turbines is indispensable for a stable energy supply and propulsion technology in aviation in the future. A possible concept for a comparatively drastic increase in efficiency starts with the combustion process, which is the most fundamental step in the energy conversion of a gas turbine. Within the framework of the Collaborative Research Center at the Technical University of Berlin (SFB1029), the classical constant-pressure combustion is to be replaced by an approximate constant-volume combustion process. Among other things, this can be realized by using pulsed detonation combustion (PDC).

The concept of PDC aims at achieving a pressure increase between compressor and turbine plenum by means of pulsed detonation. For the implementation of the PDC, it is essential that the flow exiting the PDC is adapted for the downstream turbine. This is necessary to allow efficient energy conversion from the pulsating and high pressure and temperature hot gas into shaft power. The primary goal of this project is to measure the flow exiting the PDC and to develop measures to reduce the flow fluctuations. This will be achieved, for example, by installing a plenum downstream of the PDC. Figure 1 schematically shows the main assemblies of a PDC-operated gas turbine with such a plenum.

In this project, primarily experimental methods, such as particle image velocimetry, schlieren as well as pressure measurements, are used. The experimentally obtained data are additionally used to validate numerical simulations performed by a partner project within the SFB1029 at the Freie Universität Berlin. The methods were first applied to record the outflow from the PDC.

Subsequently, methods, such as the plenum as well as shock distributors (divider), were developed and investigated to minimize the fluctuations from the PDC. The divider aims to distribute the strong transient shocks into weaker shocks before they enter the turbine. The essential goal of the plenum is not only to reduce the strength of the transient shocks, but to reduce the overall instationarity of the PDC outflow.

The lower figure (a) shows the PDCs and the downstream plenum with the sensor system. Figures (b) and (c) show examples of the experimentally and numerically obtained pressure data after passing a detonation wave in the plenum, which show good agreement between the numerical and experimental data. It could be shown that the plenum can bring about a minimization of the pressure peak up to 89%. In addition, a feasibility study showed that the divider can split the leading shock wave into several weaker shocks. Thus, methods have been devised that can facilitate efficient implementation of PDC in a gas turbine.

The video below shows an animation with numerically and experimentally obtained data to illustrate the formation of the detonation as well as the outflow from the PDC.