Lean Operation of a Pulse Detonation Combustor by Fuel Stratification

Pressure gain combustion is a promising concept to substantially increase the thermal efficiency of gas turbines. One possible implementation that has been frequently investigated is pulse detonation combustors (PDCs), as they permit stable and reliable operation. At the same time, the need for part-load operation and low NO_x emissions requires combustion concepts in the lean regime. However, realizing lean combustion is still very challenging in PDCs since the deflagration to detonation transition (DDT) is very sensitive to the reactant composition. This work investigates an approach to realize lean combustion in PDC by applying fuel stratification experimentally. The scope is to find the necessary increase of fuel concentration inside the predetonation chamber to provide reliable DDT with respect to the overall equivalence ratio. Emission measurements in the exhaust of the PDC allow for a quantification of the NO_x emissions as a function of the injected fuel profile. A valveless PDC test rig is used, which contains a shock focusing geometry for detonation initiation and is ignited by a spark plug close to the upstream end wall. The subsequent expansion of the burned gas and interaction of the flame front with turbulence leads to the formation of a leading shock inside the predetonation chamber, which is then focused inside a converging-diverging geometry. The successful initiation of a detonation wave by shock focusing is very sensitive to the pressure ratio across the leading shock, which can be influenced by initial pressure, reactant composition and flow velocity. Results reveal that fuel stratification allows for reliable detonation initiation at a global equivalence ratio of $φglob=0.65$⁠, whereas repeatable successful operation with nonstratified fuel injection is limited to $φglob≥0.85$⁠.