Fernando F. Fachini

Transient effects in droplet ignition phenomenon

We discuss the transient effects of mass and energy accumulation processes and of an applied acoustic field interaction on the droplet ignition phenomenon. In order to observe simultaneously the influence of those processes on the ignition, we assume that the chemical reaction occurs in the droplet unsteady far field, where the transient accumulation processes and the acoustic field are as important as the other processes (i.e., the conduction process and the chemical reaction). We also consider the Lewis number as being constant and nonunity. The results show that the ignition time is modified by the acoustic field.

Theoretical analysis of ultra-lean premixed flames in porous inert media

The structure of stationary adiabatic premixed flames within porous inert media under intense interphase heat transfer is investigated using the asymptotic expansion method. For the pore sizes of interest for combustion in porous inert media, this condition is reached for extremely lean mixtures where lower flame velocities are found. The flame structure is analysed in three distinct regions. In the outer region (the solid-phase diffusion length scale), both phases are in local thermal equilibrium and the problem formulation is reduced to the one-equation model for the energy conservation. In the first inner region (the gas-phase diffusion length scale), there is local thermal non-equilibrium and two equations for the energy conservation are required. In this region, the gas-phase temperature at the flame is limited by the interphase heat transfer. In the second inner region (the reaction length scale), the chemical reaction occurs in a very thin zone where the highest gas-phase temperature is found. The results showed that superadiabatic effects are reduced for leaner mixtures, smaller pore sizes and smaller fuel Lewis numbers. The results also show that there is a minimum superadiabatic temperature for the flame propagation to be possible, which corresponds to the lean flammability limit for the premixed combustion in porous inert media. A parameter that universalizes the leading-order flame properties is identified and discussed.

On the Effect of Pressure Oscillations on Droplet Autoignition

The paper discusses the possible interaction between combustion instabilities and induction times of droplets (and sprays) to autoignition. It is shown that acoustic pressure/temperature oscillations significantly affect the induction times of n-heptane droplets. This may play an additional role in low frequency dynamics and might be the main driver of high frequency dynamics. Experiments on single droplets in an acoustic field were used to validate numerical simulations on the autoignition of large n-heptane droplets. The simulations were then extended towards technical droplet sizes and a gas turbine typical pressure range of 17 bar. It was found that the acoustic-scale changes of the pressure and temperature result in significant changes of the ignition delay. Applying numerical calculations to micro-sized droplets enabled to study the thermo-acoustic effects under conditions approximating real gas-turbines. The findings reveal the importance of thermo-acoustic effects on ignition processes in the instability-driving mechanisms of combustion and indicate that “acoustics-ignition”-interactions must be taken into account for low-frequency as well as for high-frequency dynamics; this in addition to the flow and mixture perturbations which are well known to drive combustion instabilities in gas-turbines.Copyright

Ignition and Extinction of Monopropellant Droplet with Lewis Number Different from Unity

ConclusionIn this analysis we have studied the Lewis number effects and the kinetics of the reaction in the ignition, extinction and combustion of the monopropellant droplets. We have limited the analysis for the condition of ignition in the cases that, when it occurs, there is an exterior region, in chemical equilibrium, and an interior region, in frozen condition, separated by a thin reaction zone.To describe the process of vaporization with combustion, for large activation energies, we have formulated the problem to find the value of the Damköhler number that corresponds to the value λ for the vaporization rate, when the heat flux at the surface of the droplet is known.To obtain the results we needed to calculate for the three regions the first terms of the expansion in power of the inverse of Zeldovich number,E/RT∞. In the expression that relates the vaporization rate and the Damköhler number, the flame temperature, θf, in first approximation, and the correctionH1f, due to the leakage of fuel by the flame appear.In the text we have mentioned the effect of the Lewis number, the physi-chemical parameters and the global reaction order on the process of ignition, extinction and combustion of monopropellent droplets.