Direct demonstration of complete combustion of gas-suspended powder metal fuel combustion using bomb calorimetry

Off-the-shelf calorimeters are typically used for hydrocarbon-based fuels and not designed for simulating metal powder oxidation in gaseous environments. We have developed a method allowing a typical bomb calorimeter to accurately measure heat released during combustion and achieve nearly 100% of the reference heat of combustion from powder fuels such as aluminum. The modification uses a combustible organic dispersant to suspend the fuel particles and promote more complete combustion. The dispersant is a highly porous organic starch-based material (i.e., packing peanut) and allows the powder to burn as discrete particles thereby simulating dust-type combustion environments. The demonstrated closeness of measured Al heat of combustion to its reference value is evidence of complete metal combustion achieved in our experiment. Beyond calorific output under conditions simulating real reactive systems, we demonstrate that the calorimeter also allows characterization of the temporal heat release from the reacting material and this data can be extracted from the instrument. The rate of heat release is an important additional parameter characterizing the combustion process. The experimental approach described will impact future measurements of heat released during combustion from solid fuel powders and enable scientists to quantify the energetic performance of metal fuel more accurately as well as the transient thermal behavior from combusting metal powders.

ON THE ROLE OF HETEROGENEOUS PROCESSES BY COMBUSTION OF ALUMINUM NANOPOWDERS IN WATER VAPOR

The paper presents a numerical estimate of the impact of heterogeneous reactions on the combustion of aluminum nanoparticles in water vapor. The specific feature of metal combustion in oxidizing media (O2, H2O, CO2, etc.), including aluminum, is the formation of a condensed phase. Two mechanisms of formation of the condensed phase of aluminum oxide are considered: condensation of thermally unstable gas molecules Al2O3 on aluminum oxide particles; and heterogeneous reactions on their surface involving aluminum suboxides (AlO, AlO2, and Al2O2) and atomic oxygen. The influence of heterogeneous reactions on the surface of Al nanoparticles with H2 O molecules is estimated. These reactions are involved in the process of combustion initiation. It is shown that their role at initial temperatures of 2300 K and above is insignificant.

Combustion of Metals in Oxygen-Enriched Atmospheres

Metal combustion is one of the main issues threatening service safety in oxygen-enriched atmospheres, leading to unexpected explosions in rocket engines. This paper reviews the recent development of metals combustion in oxygen-enriched atmospheres. Test methods under three typical conditions and combustion behaviors of three typical metals are mainly discussed. The microstructures of the combustion areas of tested samples in stainless steels, nickel superalloys, and titanium alloys are similar, containing an oxide zone, a melting zone, and a heat-affected zone. The development trend of metal combustion in oxygen-enriched atmospheres in the future is also forecasted.

Nano-Sized and Mechanically Activated Composites: Perspectives for Enhanced Mass Burning Rate in Aluminized Solid Fuels for Hybrid Rocket Propulsion

This work provides a lab-scale investigation of the ballistics of solid fuel formulations based on hydroxyl-terminated polybutadiene and loaded with Al-based energetic additives. Tested metal-based fillers span from micron- to nano-sized powders and include oxidizer-containing fuel-rich composites. The latter are obtained by chemical and mechanical processes providing reduced diffusion distance between Al and the oxidizing species source. A thorough pre-burning characterization of the additives is performed. The combustion behaviors of the tested formulations are analyzed considering the solid fuel regression rate and the mass burning rate as the main parameters of interest. A non-metallized formulation is taken as baseline for the relative grading of the tested fuels. Instantaneous and time-average regression rate data are determined by an optical time-resolved technique. The ballistic responses of the fuels are analyzed together with high-speed visualizations of the regressing surface. The fuel formulation loaded with 10 wt.% nano-sized aluminum (ALEX-100) shows a mass burning rate enhancement over the baseline of 55% ± 11% for an oxygen mass flux of 325 ± 20 kg/(m2∙s), but this performance increase nearly disappears as combustion proceeds. Captured high-speed images of the regressing surface show the critical issue of aggregation affecting the ALEX-100-loaded formulation and hindering the metal combustion. The oxidizer-containing composite additives promote metal ignition and (partial) burning in the oxidizer-lean region of the reacting boundary layer. Fuels loaded with 10 wt.% fluoropolymer-coated nano-Al show mass burning rate enhancement over the baseline >40% for oxygen mass flux in the range 325 to 155 kg/(m2∙s). The regression rate data of the fuel composition loaded with nano-sized Al-ammonium perchlorate composite show similar results. In these formulations, the oxidizer content in the fuel grain is <2 wt.%, but it plays a key role in performance enhancement thanks to the reduced metal–oxidizer diffusion distance. Formulations loaded with mechanically activated ALEX-100–polytetrafluoroethylene composites show mass burning rate increases up to 140% ± 20% with metal mass fractions of 30%. This performance is achieved with the fluoropolymer mass fraction in the additive of 45%.

The choice of the composition of the metal transition for the combustion chamber of low-thrust liquid thruster

When replacing the metal combustion chambers of a low-thrust liquid rocket engine to combustion chambers made of carbon-ceramic composite material, a problem arises between the connection of metal and carbon-ceramic. To solve this problem, in this work presents the results of strength calculations of the connection of various materials in the form of a transition ring. Replacement of the material will avoid the effect of thermal coefficients of linear expansion on the operation of a low-trust liquid rocket engine to maintain the tightness of the joint node when exposed to operating temperatures.

A SPH Implementation with Ignition and Growth and Afterburning Models for Aluminized Explosives

Aluminized explosives have been applied in military industry since decades ago. Compared with ideal explosives such as TNT, HMX, RDX, aluminized explosives feature both fast detonation and slow metal combustion chemistry, generating a complex multi-phase reactive flow. Though aluminized explosives have been employed for a long time, the mechanism underneath the chemical process is still not thoroughly understood. In this paper, a smooth particle hydrodynamics (SPH) method incorporated ignition and growth model, and afterburning model has been proposed for the simulation of aluminized explosive. Ignition and growth model is currently the most popular model for the simulation of high explosives, which is capable of accurately reproducing arrival time of detonation front and pressure history of high explosives. It has been integrated in commercial software such as ANSYS-LS DYNA. In addition, an afterburning model has been integrated in the SPH code to simulate the combustion of aluminum particles. Simulation is compared with experiment and good agreement is observed. The proposed mathematical model can be used to study the detonation of aluminized explosives.