Design, fabrication and analysis of stagnation flow microreactors used to study hypergolic reactions

A novel method to study the condensed phase reactions that occur during the ignition of hypergolic propellants (very fast liquid reactions) using microreactors is presented. Planar counterflow microreactors are used to isolate liquid-phase reactions and diffusion from secondary gas-phase chemical and transport processes that often occur concurrently during the overall ignition process. The counterflow microreactor has made it possible to achieve valuable insight into the preignition mechanisms of hypergolic propellants hitherto not possible using conventional drop or impinging jet tests. In the present paper, the microreactor fabrication, flow field characterization, and reactivity of 2-dimethylaminoethylazide and nitric acid as hypergols are presented. Particle image velocimetry and numerical simulations were conducted to characterize the laminar velocity flow-field from which stagnation point strain rates and contact residence times along the centerline of the microreactor were evaluated. Temperature measurements at the exit of the reactor (as well as at the stagnation point) were used as a measure for the extent of the reaction or the heat released from the reaction. For the hypergols, an increase in reactant flow (or equivalently strain rate at the stagnation point) was found to initially increase reactivity, but eventually resulted in a decrease in temperature, revealing a maxima in temperature and reactivity. The trends indicated a reaction that was initially diffusion or heat loss controlled, which transitioned towards kinetic control at higher strain (flow) rates. This paper details the first comprehensive measurements and analysis of the effects of diffusion based mixing on the interfacial reactions occurring between hypergols.