Flame spray pyrolysis is an established technique for synthesizing nanoparticles in the gas phase through aerosol combustion of precursor/solvent droplets. The combustion characteristics of isolated micron-sized precursor/solvent droplets are investigated experimentally. Pure solvent droplets burn uniformly and classically quasisteady, whereas precursor/solvent droplets manifest disruptive combustion behavior. The fast onset of droplet disruption, which occurs only for solutions with dissolved metal precursors, is not due to solid-particle precipitation within the droplet. Instead, the mechanism of disruptive droplet burning is similar to that of slurry droplets, consisting of three main steps: (1) diffusion-controlled burning of the high-volatile solvent, (2) viscous-shell formation due to decomposition of the low-volatile metal precursor, and (3) subsequent disruption due to heterogeneous nucleation. The time sequence of the three steps depends on the concentration and decomposition characteristics of the metal precursor, shortening with increased concentration and higher incremental decomposition temperature. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4553–4566, 2013

Disruptive burning of precursor/solvent droplets in flame‐spray synthesis of nanoparticles

Experimental investigation of burning rates of pure ethanol and ethanol blended fuels

Flame shapes and burning rates of spherical fuel particles in a mixed convective environment

This work analyses the classical Emmons (1956) solution of flat plate laminar flame combustion on a film of liquid fuel A two-dimensional (2D) numerical model developed for this purpose has been benchmarked with experimental results available in the literature for methanol In the parametric study, numerical predictions have been compared with Emmons classical solution The study shows that the Emmons solution is valid in a range of Reynolds numbers where flame anchors near the leading edge of the methanol pool and the combustion zone is confined around the hydrodynamic and thermal boundary layers However, in cases of low free stream velocities the combustion zone is beyond the boundary layer zone and the Emmons solution deviates In cases of very high free stream velocities, the flame moves away from the leading edge and anchors at a location downstream The Emmons solution is not applicable in this case as well For the fuel considered in this study (methanol), accounting for thermal radiation, employ

Laminar flame propagation on a horizontal fuel surface: Verification of classical Emmons solution

Experimental investigation of an isolated droplet burning in a convective flow is reported. Acetone droplets were injected in a steady laminar diffusion counterflow flame operating with methane. Planar laser-induced fluorescence measurements applied to OH radical and acetone was used to measure the spatial distribution of fuel vapour and the structure of the flame front around the droplet. High-magnification optics was used in order to image flow areas with a ratio of 1:1.2. The different combustion regimes of an isolated droplet could be observed from the configuration of the envelope flame to that of the boundary-layer flame, and occurrence of these regimes was found to depend on the droplet Reynolds number. Experimental results were compared with 1D numerical simulations using detailed chemistry for the configuration of the envelope flame. Good agreement was obtained for the radial profile of both OH radical and fuel vapour. Influence of droplet dynamics on the counterflow flame front was also investigated. Results show that the flame front could be strongly distorted by the droplet crossing. In particular, droplets with high velocity led to local extinction of the flame front whereas droplets with low velocity could ignite within the flame front and burn on the oxidiser side.

Investigation of droplet combustion in strained counterflow diffusion flames using planar laser-induced fluorescence

The combustion of a fuel droplet in a mixed convective environment has been simulated both theoretically and experimentally. In the theoretical model, the flow and transport equations have been solved subject to the assumptions of constant properties (except for density) and infinite rate kinetics, using the finite element method. The gas density has been treated as a variable, only to determine the buoyancy force contribution, and it has been evaluated assuming an ideal gas mixture. A porous-sphere facility has been employed for simulating the burning characteristics of a fuel droplet experimentally. The effects of airflow rate and droplet size have been studied for both upward and downward airflow configurations. Theoretical predictions for the mass burning rate and flame shape are in excellent agreement with the experimental results of the present study and also those reported in the literature. For upward airflow configuration, the mass burning rate is under-predicted by 15 to 20 percent when...

Combustion of a Fuel Droplet in a Mixed Convective Environment

The interference effects during the burning of a binary tandem droplet array in a mixed convective environment have been studied both theoretically and experimentally for the upward airflow configuration. In the theoretical model, the flow and transport equations have been solved subject to the assumptions of constant properties (except for density) and infinite rate kinetics, by the use of the finite element method. The gas density has been treated as a variable, only for evaluating the buoyancy force contribution, and it has been determined through the ideal gas mixture assumption. A porous-sphere facility has been employed for simulating experimentally the burning characteristics of a fuel droplet array with fixed separation distance. The measured mass burning rates for both the spheres and the observed flame shapes are in excellent agreement with the corresponding theoretical predictions. The burning rate of the rear droplet is considerably less than that of the front droplet or an isolated droplet, for smaller separation distances. For larger separation distances, the burning rate of the rear droplet is of the same order as the front droplet, and it is sometimes even larger than that of the isolated droplet. The rate of burning of the front droplet mildly varies with separation distance, and it is approximately equal to that of an isolated droplet. The flame of a tandem binary droplet system is observed to be pushed away from the axis due to the presence of the second droplet.

P. Balakrishnan

Papers

Combustion of a Fuel Droplet in a Mixed Convective Environment

Interference Effects During Burning of Tandem Porous Spheres in Mixed Convective Environment