Soot Volume Fraction Measurements by Auto-Compensating Laser-Induced Incandescence in Diffusion Flames Generated by Ethylene Pool Fire

The main characteristics of pool fire flames are flame height, air entrainment, pulsation of the flame, formation and properties of soot particles, mass burning rate, radiation feedback to the pool surface, and the amount of pollutants including soot released to the environment. In this type of buoyancy controlled flames, the soot content produced and their subsequent thermal radiation feedback to the pool surface are key to determine the self-sustainability of the flame, their mass burning rate and the heat release rate. The accurate characterization of these flames is an involved task, specially for modelers due to the difficulty of imposing adequate boundary conditions. For this reason, efforts are being made to design experimental campaigns with well-controlled conditions for their reliable repeatability, reproducibility and replicability. In this work, we characterized the production of soot in a surrogate pool fire. This is emulated by a bench-scale porous burner fueled with pure ethylene burning in still air. The flame stability was characterized with high temporal and spatial resolution by using a CMOS camera and a fast photodiode. The results show that the flame exhibit a time-varying propagation behavior with a periodic separation of the reactive zone. Soot volume fraction distributions were measured at nine locations along the flame centerline from 20 to 100 mm above the burner exit using the auto-compensating laser-induced incandescence (AC-LII) technique. The mean, standard deviation and probability density function of soot volume fraction were determined. Soot volume fraction presents an increasing tendency with the height above the burner, in spite of a local decrease at 90 mm which is approximately the position separating the lower and attached portion of the flame from the higher more intermittent one. The results of this work provide a valuable data set for validating soot production models in pool fire configurations.

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%.

Effects of aluminum composites on the regression rates of solid fuels

Innovative, mechanically activated Al–polytetrafluoroethylene (PTFE) composites and ammonium perchlorate (AP) coated nano-sized aluminum (C-ALEX) were produced, characterized, and tested as solid fuel additives. The ballistics of fuel formulations based on hydroxylterminated polybutadiene (HTPB) was investigated in a microburner by a time-resolved technique for regression rate (rf) data reduction. Both Al-composites show promising results in terms of rf and mass burning rate enhancement. In particular, the C-ALEX showed a percent rf increase over the baseline (HTPB) of 27% at an oxidizer mass flux of 350 kg/(m2s), without requiring dedicated dispersion procedures. This performance enhancement was nearly constant over the whole investigated range.

Experimental Study of Mass Burning Rate of Liquid Pool Fires Under Dynamic Pressure in an Altitude Chamber

To study the influence of dynamic pressure on the liquid combustion characteristics, two kind sizes of pool fires were studied under varied pressure rates, namely 100Pa/s, 200Pa/s, 300Pa/s from 90kPa to 38kPa in an altitude chamber which size is 2m*3m*4.65m. Combustion characteristics of n-heptane pool fires, such as mass burning rate, flame temperature, chamber pressure were measured in this research. Experiment results show that the mass burning rate of 20cm pool fires, decreases when the ambient pressure reduces, and the variation trend become more sharply when the dynamic pressure rate is increased, while 30cm pool fires at the beginning of the combustion stage almost remain constant, this is because fire heat feedback have a great influence on it. The results also show that compared to the radiation model, pressure model could be linear fitting better in a double logarithm coordinate, and oil pool fires under 300Pa/s of 20cm, 100Pa/s of 30cm the value of α obtained by the fitted curves were more closer to fixed pressure ones.