A premixed Green Propellant consisting of N2O and C2H4: Experimental analysis of quenching diameters to design flashback arresters

Across the world several so called green propellants for in-space and orbital propulsion are under investigation to replace the highly toxic hydrazine . Aside with green alternatives based on ADN and H2O2 the DLR Institute of Space Propulsion examines a premixed propellant consisting of dinitrogen monoxide (N2O) and ethene (C2H4). The mixture called HyNOx (hydrocarbons mixed with nitrous oxide) offers high performance and low toxicity, but due to the premixed state the danger of flame flashback across the injections system is a major issue. To avoid flashback during operation of a future engine suitable flashback arresters are needed, thus the quenching diameters and corresponding critical Peclet numbers have to be derived. To analyze the quenching diameters, DLR built an ignition and flashback test setup. The Setup consists of two chambers separated by a Fitting for capillaries which serve as flashback arresters. Capillaries with diameters in between 0.1 and 0.5 mm were tested on their ability to quench the flame. Furthermore one chamber can be equipped with glass windows to record high speed videos of the flame propagation process. To obtain the critical quenching Peclet number the ignition pressure was consecutively raised until a flame flashback for a given capillary was observed. For the investigated capillaries the quenching Peclet numbers were in between 30 - 40 related to the conditions at ignition. Additionally high speed videos were recorded to analyze the flame propagation speed during the combustion tests. The average flame Propagation speed was in between 33 m/s for an ignition pressure of 0.49 bar to 40 m/s for an ignition pressure of 1 bar