Hypergolic fuel impacting a gelled oxidizer wall: Droplet dynamics, heat release, ignition, and flame analysis

Abstract

The combination of high-concentration solutions of hydrogen peroxide, known as High Test Peroxide (HTP), with the green fuel composed of tetramethylethylenediamine, dimethylaminoethanol, and methanol (TMEDA/DMEA/MeOH, 1:1:1 vol%), catalyzed by 1 wt% of copper chloride dihydrate, has significant potential for space hypergolic propulsion applications in terms of performance and safe operation. Besides the well-known and mature propulsion systems adopting liquid and solid propellants, gel propellants also present interesting characteristics that can be better explored to enable the creation of alternative propulsive systems. The present work employed HTP at 98 wt% with 6 wt% of fumed silica as a gelling agent to create gelled HTP (GHTP). Droplets of the green fuel impinged on a layer of GHTP placed over a glass slide, acting as a solid oxidizer, mimicked an element of a new hybrid gel/liquid hypergolic propulsion system. Data from infrared and visible light cameras enabled a detailed analysis of the dynamics of a reactive droplet impinging on a gelled oxidizer wall, as well as the heat release, ignition, and flame spread of the hypergolic GHTP/green fuel pair under open-air conditions. The heat release was observed to be distributed across concentric annular areas for both the fuel surrogate and the fuel with catalyst, before and after ignition, demonstrating a strong correlation with non-reactive droplet fluid dynamics, which was corroborated by spread diameter analysis processed from visible image data. Vapor and Ignition Delay Times (VDT and IDT) were found to be as low as 4 and 17 ms, respectively, for the highest droplet impact velocity and catalyst concentration. The reaction rate in the gas phase, considering the flame spread area, showed a dependency of both impact velocity and catalyst concentration, with the latter exhibiting a more pronounced and clear effect. The surface temperature ranges where first vaporization and ignition occurred were 60 °C $\sim$ 70 °C and 120 $\sim$ 170 °C, respectively, which is close to the boiling point of methanol and the auto-ignition temperature of TMEDA. This finding, along with the chemical mechanisms in the gas phase related to the presence of a catalyst in the fuel, may be important for hypergolic ignition. The relatively low ignition temperatures and the short ignition times represent additional advantages of the present hypergolic combination for propulsion applications.