Flame-assisted spray pyrolysis; Phase-selective LIBS; Transition from droplets to nanoparticles; Functional TiO2-based nanoparticle;INDUCED BREAKDOWN SPECTROSCOPY; PULVERIZED COAL COMBUSTION; NANOSTRUCTURED MATERIALS; AEROSOL SYNTHESIS; PARTICLES; OXIDATION; TIO2; COAGULATION; GROWTH; MODEL

Mechanisms involved in flame-assisted-spray-pyrolysis (FASP) synthesis of TiO2-based functional nanoparticles are investigated using in situ phase-selective laser-induced breakdown spectroscopy. Specifically, the transition from droplets to nanoparticles is examined, as well as the particle growth process in the post flame region. In contrast to vapor-fed flame synthesis, the existence of the precursor breakdown in sprayed droplets is observed by both Ti atomic signal and Bremsstrahlung emissions. The emission signal intensity variation reveals the transition from droplets to nanoparticles, indicating that the competition between precursor vaporization and nascent particle formation is the rate determining factor. For the ensuing particle growth regime, the prediction of the particle size evolution by a polydispersed population balance model is in good agreement with TEM and laser-diagnostics results, demonstrating that the particle growth is governed by the collision-coagulation mechanism. Besides, the transition from droplets to nanoparticles can be accelerated with sufficient OH radicles provided by combustible solvents. Furthermore, doping of TiO2 with V and Zr will lead to changes in band gap of the nanoparticle observed, i.e., a distinct strengthening by 23% with V doping and a weakening by 22% with Zr doping in the intensity of Ti signal, respectively. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.

Alkali metal release; Biomass; Pyrolysis; LIBS; In situ measurement;GAS-PHASE; COMBUSTION; CHLORINE; POTASSIUM; GASIFICATION; COAL; CL; BEHAVIOR; STRAW; WOOD

The release of alkali metals during biomass pyrolysis was numerically and experimentally studied. The concentration of sodium and potassium in the gas phase above a biomass particle was measured; quantitative and time resolved data were acquired by means of the Laser-Induced Breakdown Spectroscopy(LIBS) technique. LIBS made it possible to extend the measurements of alkali metal concentration to the sooty pyrolysis stage. Data from the measurements revealed a staged release of alkali metals from biomass. Two distinct peaks of concentrations were observed, one associated with the pyrolysis stage and the other with the gasification stage. Since during the pyrolysis stage a large temperature gradient exists inside the particle, numerical simulations were carried out to explain the experimental measurements and extract the kinetic data. Using a detailed particle model, the rates of potassium and sodium release from the particle during the pyrolysis stage were attained. For sodium release the activation energy was found to be in the range of 218 to 248 kJ/mole and for potassium release it was found to be between 168 and 198 kJ/mole. Furthermore, equilibrium calculations were performed to identify the stable sodium and potassium compounds and their phases during the pyrolysis stage of the particle. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.

Laser-induced breakdown; Scramjet; Cavity flameholder; Gated LIBS;REACTING FLOWS; INDUCED PLASMA; AIR; SPARK; CAVITY; MIXTURES; IGNITION; DENSITY; FLAMES; LIBS

Gas density and fuel mole fraction in a cavity flameholder of a supersonic combustor with ethylene (C2H4) fueling are simultaneously measured using nanosecond gated (10 ns) laser-induced breakdown spectroscopy (n-LIBS). Emission spectra from the laser-induced plasma over a range (550 nm-830 nm) containing multiple emission lines of O, H, N, and C are captured and used to quantify the gas conditions. A direct spectrum matching (DSM) process is employed for the gas property quantification in which an unknown spectrum is matched to a spectrum from a database, constructed through n-LIBS analysis of well-known gas conditions. A back-scattering collection method is implemented to allow for measurements using a single optical access point. Fuel mole fraction and gas density are mapped within a cavity flameholder on planes parallel to the Mach 2 freestream flow above the cavity. With C2H4 injection from the closeout ramp of the cavity toward the front step, a high fuel mole fraction region appears near the front step where density and flow velocity are low, and thus favorable for cavity flame ignition. The fuel mole fraction at a location in the cavity is found to be nearly proportional to fueling rate; the gas density, on the other hand, is not affected by the fueling rate, under the typical operating conditions. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.

Laser-induced breakdown; Ignition; Diffusion flame; Schlieren imaging;INDUCED BREAKDOWN SPECTROSCOPY; AIR MIXTURES; ENERGY

An experimental investigation was performed to study the ignition and flame propagation behaviors of a methane diffusion jet flame (Re = 5500) when dual pulse laser-induced spark discharges were introduced in a mixing layer. Time intervals (dt) of 50 ns, 100 mu s, and 600 mu s between two laser pulses were tested, and the results were compared to a single pulse discharge case with the same total laser energy (60 mJ). The laser-induced breakdown was introduced to the mixing layer at 9.5 jet diameters downstream from the jet exit. The chemical delay time scale under the flow condition was approximately 350 mu s (t(cd)). The disturbances in density fields generated by laser-induced breakdowns and chemical reactions were visualized using a high-speed schlieren imaging technique. The visualization at stationary air showed that interactions between two laser-induced breakdowns increased the surface area of hot plumes, but the effects of the interactions diminished when the breakdowns were introduced in a non-reacting air jet. The effects of the flow on the hot plumes prevailed the interaction effects between two breakdowns in non-reacting air jet condition. However, when the dual pulse laser-induced spark discharges (dt = 600 mu s, dt/t(cd) = 1.71) were generated in a mixing layer of a methane jet, a rapid propagation of the flame was observed since the second breakdownenlarged the ignition kernel surfaces generated by the first breakdown. The results showed that using the dual pulse with intervals shorter than the electron lifetime scale (under 200 ns) or longer than the chemical delay time scale could be beneficial in enhancing ignition and flame propagation processes due to the increased energy deposition or hot surface area, respectively. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.