Experimental Study of Impingement and Reaction of Hypergolic Droplets

Abstract

Understanding hypergolic ignition is critical for the safe and successful operation of hypergolic engines. Presently, hypergolic ignition models do not capture the complex coupling of physical and chemical processes necessary to achieve ignition and rely on experimental results for validation. In some cases, the chemical kinetics of hypergolic propellants and the fluid dynamics of droplet collisions couple to produce unexpected phenomena. This research investigates contact between droplets and pools of liquid hypergolic propellants under various impact velocities and impact geometries in order to quantify the liquid-phase reactions and categorize the resulting interaction. A total 142 drop test experiments were performed. Three distinct types of impacts were identified: explosions, bounces, and splashes. The impact type was found to depend on the impact Weber number and impact angle. Splashes occurred above a critical Weber number of 250, regardless of impact angle. Explosions occurred for Weber numbers less than 250, and for impact angles less than seven degrees. If the impact angle was greater than seven degrees, the test resulted in a bounce. High-speed videos acquired at up to 100,000 frames per second provided a means to observe the liquid-phase chemical reactions in detail. From these videos, the delay between contact and first gas production was measured to be between 20 and 200 microseconds for monomethylhydrazine and red fuming nitric acid. This delay provides insight into the rates of the liquid-phase chemical reactions, and has helped calibrate liquid-based ignition models. Nomenclature k = rate constant of a chemical reaction A = Arrhenius pre-exponential factor E a = activation energy R = ideal gas constant T = temperature m,n = reaction partial order exponents

Hypergolic ignition by head-on collision of N,N,N′,N′ −tetramethylethylenediamine and white fuming nitric acid droplets

“…This apparatus can be used to simulate various temperature, pressure and concentration conditions met in rocket engine combustors. Figure 1 (c) schematizes the widespread drop test for prescreening potential hypergolic propellants [12,13] . In the test, a fuel drop is made to impact a small amount of oxidizer pool in either a glass cuvette or a flat tray.…”

Mass interminglement and hypergolic ignition of TMEDA and WFNA droplets by off-center collision

“…It is noted that the Weber numbers in the previous hypergolic ignition experiments are of for drop tests [17,19], and of for jet impinging tests [15,41]. The droplets used in prevailing droplet collision experiments [8,17,19,20] (including the present study) are within the range of , which are larger than the droplets in real engines.…”

“…It is noted that the Weber numbers in the previous hypergolic ignition experiments are of for drop tests [17,19], and of for jet impinging tests [15,41]. The droplets used in prevailing droplet collision experiments [8,17,19,20] (including the present study) are within the range of , which are larger than the droplets in real engines. However, according to the similarity law of fluid dynamics, the non-dimensional parameters, such as the Weber number, the size ratio, and the impact parameter, but not the dimensional droplet sizes, control the phenomena concerned in the present study.…”

“…Designed for variable testing conditions, the impinging jet apparatus [7,[14][15][16] often employs partially mixing reactants. In recent years, the drop test [8,17] has been widely used as a fast tool to prescreen potential hypergolic propellants. In a typical drop test, a free or suspended propellant drop is made to impact the liquid pool of another propellant [18,19], which is either confined in a container or placed on a solid surface.…”

“…In a typical drop test, a free or suspended propellant drop is made to impact the liquid pool of another propellant [18,19], which is either confined in a container or placed on a solid surface. Although these test systems help researchers obtain a lot of valuable understanding on hypergolic ignition [7,8,17,19], they have a common deficiencyit is generally difficult to quantify the system-dependent liquid-phase mixing owing to the unavoidable "wall effect" by the jet nozzle or the container or the surface.…”