Reduction in the run-up distance for the deflagration-to-detonation transition and applications to pulse detonation combustion

Pressure-gain combustion has been a topic of research interest for several decades. Due to the potential of pressure-gain thermodynamic cycles of increasing gas turbine efficiency by more than 10%, they offer a strategy to combat the growing problem of continually scarcer resources by simultaneous enforcement of ever stricter emissions controls. Furthermore, when hydrogen is used as a fuel, emissions of CO2, a known greenhouse gas, are eliminated. Hydrogen can also be obtained using electrolysis powered by renewable energy sources. In times of less demand, excess energy can be used to produce hydrogen, which can then later be used for combustion-based energy generation when demand once again rises. Gas turbines offer an ideal platform for this technology, due to their fast response times when compared to other sources of combustion-based energy. One type of pressure-gain combustion is known as pulse detonation combustion. Using this cyclical concept, the fuel is combusted by means of a detonation wave propagating at around 2000 m/s. Because of the speed of propagation, there is no time for the gas to expand during the combustion process and almost the entirety of the energy release is directed towards increases in pressure and temperature. This cycle is known as the Fickett–Jacobs cycle. Due to energy considerations, a flame is typically ignited by a low-energy ignition source and accelerated until it transitions to a detonation. This process is called the deflagration-to-detonation transition (DDT) and is the main focus of this work. Reducing the run-up distance to detonation has a direct impact on efficiency. Thus, it is worthwhile to achieve this transition over as short a distance as possible. In this thesis, various methods of shortening the run-up distance to DDT using obstacles are investigated. Experiments to characterize the initial flame acceleration caused by a single obstacle concluded that the geometry of the obstacle plays only a minor role when compared to its blockage ratio. Furthermore, when multiple orifice plates are used, the optimal separation distance was found to be just over two tube diameters. Experiments on a separate test bench confirmed this finding also in regards to DDT and found that a tube diameter of around 40 mm is necessary to obtain reliable DDT over a reasonable run-up length using orifice plates. The results of these initial studies aided in designing a modular pulse detonation combustion test bench. Using oxygen enrichment to simulate the operating conditions of a micro gas turbine, DDT was achieved using only 2-3 orifice plates when a wave-reflecting geometry was used at the inlet of the detonation chamber to support initial flame acceleration. Further investigations on a shock-focusing nozzle were successful in producing reliable DDT over a length of just 158 mm. Using this nozzle, a local explosion is initiated ahead of a fast accelerating turbulent flame by reflection and focusing of the leading shock. The result is a pressure increase in the region of focus in excess of 50 bar. The process is also found to be very deterministic. Therefore, this geometry presents a very promising means of producing DDT for pulse detonation combustion applications.