A Study of Detonations, DDT and Deflagrations in High Pressure Ethylene-Nitrous Oxide Mixtures

Nitrous oxide (N₂O) has gained popularity as a unique oxidizer due to its ability to decompose exothermically, producing nitrogen and oxygen. Additionally, there are concerns about the safety of nitrous oxide in the nuclear industry where it has been observed that stored nuclear waste generates and retains large amounts of flammable

gases such as hydrogen and ammonia along with nitrous oxide. These gases are at risk of explosion even in the presence of a weak ignition source which can result in detonations more violent than those initiated directly. Nitrous oxide is also finding an application in the geothermal industry where it is being tested in combination with ethylene as a stimulant mixture to fracture rock. The detonations initiated in this mixture have the ability to produced a network of fractures in the rock formation. In the rocket industry, nitrous oxide has been used for propulsion in multiple systems, but never in a detonative mode. In order to use nitrous oxide in these areas, its detonation properties in combination with a fuel require quantification. Available literature on nitrous oxide-hydrocarbon detonations is mainly restricted to initial pressures below one atm or with dilution. Therefore, detonations with nitrous oxide as the oxidizer are far from being completely characterized. In addition to this lack of general knowledge, understanding of nitrous oxide-fuel detonations at higher pressures, more typical of practical combustion systems is either extremely limited or

non-existent.

In the current work, the flame acceleration, deflagration-to-detonation transition (DDT), and detonation properties of a bipropellant mixture of ethylene (C₂H₄) and N₂O are studied as a function of initial pressures. These properties are compared to those in mixtures of ethylene-oxygen (O₂). these detonations are investigated in a combustion tube designed and fabricated in-house. The performances of these two mixtures are also investigated using theoretical Chapman-Jouguet detonation calculations as a basis of comparison with the measured properties. Additionally, detonation properties in a mixture of acetylene (C₂H₂) and nitrous oxide are also investigated to compare the two fuels. While C₂H₂ is a highly energetic fuel with theoretically high performance, it presents serious practical storage concerns when considered for propulsion applications. These practical issues motivates the investigation of C₂H₄ as a potential alternative fuel, which is relatively easy to manage.

A critical requirement for the application of bipropellant mixtures to detonation systems is rapid flame acceleration to achieve significant chamber pressure rise in a short distance with the potential for a prompt transition to detonation. This DDT behavior of mixtures using N₂O and O₂ with C₂H₄ and C₂H₂ is investigated for increasing initial pressures in the experimental portion of this work. This behavior is quantified by measuring the run-up distances leading to DDT. The pre-compression of the bipropellant mixtures during flame acceleration caused by the accelerating flame is also estimated and directly measured using appropriate instrumentation. These direct measurements of pre-compression are further used to estimate the path of the

accelerating flame in the combustion tube. These estimates are compared with the flame tracked by high-speed imaging in an optically accessible combustion tube.