Effects of Size and Prestressing of Aluminum Particles on the Oxidation of Levitated exo-Tetrahydrodicyclopentadiene Droplets

Abstract

Addition of high-energy-density materials such as aluminum (Al) microparticles or nanoparticles to liquid propellants potentially improves performance of the fuel. We report on the effects of untreated, prestressed, and superquenched aluminum particles with diameters of 100 nm, 250 nm, 500 nm, 1.6 μm, and 8.8 μm on the combustion of JP-10 droplets acoustically levitated in an oxygen–argon atmosphere. Ignition was initiated by a carbon dioxide laser, and the resulting oxidation processes were traced by Raman, Fourier-transform infrared (FTIR), and ultraviolet–visible (UV–vis) spectroscopies together with high-speed optical and IR thermal-imaging cameras. The UV–vis emission spectra reveal that the key reactive radical intermediates hydroxyl (OH), methylidyne (CH), dicarbon (C2), aluminum monoxide (AlO), and aluminum monohydride (AlH) were formed in addition to atomic aluminum (Al) and the final oxidation products of JP-10, namely, water (H2O) and carbon dioxide (CO2). The Al particles facilitated ignition of the JP-10 droplets and produced higher temperatures in the combustion process of up to typically 2600 K. The effect of the Al particles on the ignition and maximum flame temperatures increased as the diameters reduced. The different stress treatments did not produce observable changes for the ignition or combustion of the droplets, which indicates that the liquid propellant was not significantly affected by manipulating the mechanical properties of the fuel particle additive. The initiation and enhancement of the combustion were a consequence of forming highly reactive atomic oxygen (O) and aluminum monoxide (AlO) radicals in the reaction of aluminum atoms with molecular oxygen in the gas phase. These radicals initiate the degradation of JP-10 via atomic hydrogen abstraction forming the hydroxyl (OH) and aluminum hydroxide (AlOH) radicals in reactions which are mainly exothermic by up to 68 kJ mol–1. In contrast, hydrogen abstractions from JP-10 by molecular oxygen or atomic aluminum are strongly endothermic by up to 236 kJ mol–1, thus making these reactions less competitive. The generation of C10H15 hydrocarbon radicals from the JP-10 initiates successive oxidations and chain reactions with molecular oxygen leading eventually to carbon dioxide and water. These combined experimental results provide insight into how aluminum particles facilitate the oxidation and reaction mechanisms of JP-10 droplets.

Catalytic Effects of Zeolite Socony Mobil-5 (ZSM-5) on the Oxidation of Acoustically Levitated exo-Tetrahydrodicyclopentadiene (JP-10) Droplets

“…In the current study, the droplets were levitated in an environment of 60% oxygen (O 2 , 99.9999%, Airgas) and 40% argon (Ar, 99.9999%, Airgas) at a total pressure of 936 ± 1 Torr to overcome the minimum ignition conditions of the present droplets. 49 The droplets were oblate spheroids in shape with mean horizonal and vertical diameters of 2.4 ± 0.3 and 1.4 ± 0.2 mm, respectively (Figure 1). The droplets were introduced into the pressure node with the help of a homemade droplet deposition system.…”

“…The laser output can be tuned in between 1 and 40 W; the present work was performed at an output power of 32 W, which is achieved by setting the duty cycle of the CO 2 laser to 80%; this laser power overcame the minimum ignition conditions of the present droplets. 49 The ignition process of JP-10 in the presence of the zeolite samples was characterized using ultraviolet−visible (UV−vis) and Fourier-transform infrared (FTIR) spectroscopies along with an IR thermal-imaging and a high-speed optical camera. Raman spectroscopy of the droplets was attempted, but the spectra exhibited a smooth structureless continuum with no discrete peaks.…”

Controlled Chemistry via Contactless Manipulation and Merging of Droplets in an Acoustic Levitator

“…The present approach has the potential for a diverse range of additional fundamental and applied research. For example, laser-heating of the individual or merged droplets will enable reactions to be investigated over an extended temperature range relevant to combustion processes . Furthermore, the incorporation of a closed-cycle helium refrigerator or liquid nitrogen-based cooling system would reduce the temperatures of the buffer gas in the process chamber to those found on the planets and their moons, which would help us explore merging processes of astrochemical relevance including of hydrocarbon droplets in hydrocarbon rich atmospheres of planets and their moons such as Titan .…”

“…Any part of the IR beam incident on the sample is either absorbed or scattered away from the detector, so that only gases in the region of the droplet are detected by transmission FTIR spectroscopy. The number density of the absorber, n , and, hence, the moles of gases produced in the process chamber can be calculated using the Beer–Lambert law: where A is the absorbance from our spectrum, σ is the reference absorption cross section from the HITRAN database, and L is the path length of 37.6 cm.…”

“…For example, laser-heating of the individual or merged droplets will enable reactions to be investigated over an extended temperature range relevant to combustion processes. 22 Furthermore, the incorporation of a closed-cycle helium refrigerator or liquid nitrogen-based cooling system would reduce the temperatures of the buffer gas in the process chamber to those found on the planets and their moons, which would help us explore merging processes of astrochemical relevance including of hydrocarbon droplets in hydrocarbon rich atmospheres of planets and their moons such as Titan. 45 The possibility of mixing without biological contamination or a loss of reagents owing to contact with supports makes our approach suitable for biological applications, such as studying DNA transfection in which small volumes of biological suspensions of living mammalian cells are merged with plasmids.…”