Bio-butanol fuel atomization and combustion processes.

摘要

Three bio-butanol fuel atomization and combustion technologies were considered, namely butanol electrostatic sprays, butanol non-premixed flames, and butanol kinetic modeling in view of the emergence of methods of production of this fuel from biological sources.;Butanol electrospray (e-spray) phenomenology was investigated through high-speed visualization and compared with the corresponding electrosprays of ethanol, heptane and butanol-containing mixtures. Electrospray structure was probed using Phase Doppler Anemometry and both droplet size and velocity measurements were obtained for sprays of butanol and butanol-containing fuel blends. These results indicated an unstable and polydisperse electrospray behavior for most conditions. Several factors were identified as responsible for this unstable behavior and were investigated experimentally. These included: e-spray menisci oscillations, instabilities initiating the droplet break-up, secondary droplet break-up because of high Weber numbers and finally stability of butanol electrical conductivity with applied voltage. Stable butanol electrosprays were achieved within a narrow region of low flow rates and a non-dimensional analysis was performed in order to develop an empirical expression correlating the dimensionless average diameter, flow rate and applied voltage.;Butanol non-premixed flames were studied in a counter-flow burner configuration. Major combustion species were measured using line Raman imaging and K-type thermocouples were used in order to perform temperature scans across the flame. Also, extinction strain rates were measured as a function of overall stoichiometry. Butanol flames were compared with flames of methane (which is not oxygenated) as well as ethanol which is a currently widely employed biofuel and with butanol-methane mixture flames. It was shown that butanol flames could sustain higher strain rates at extinction than ethanol flames but significantly smaller than methane flames. For the strongly diluted flames under consideration, it was shown that temperature followed a very closely linear relation with nitrogen concentration. For the same nitrogen concentration, butanol exhibited lower temperatures at the same overall stoichiometry and heat release, because of the higher average molecular weight of the fuel stream. In addition, the possibility of estimating the scalar dissipation rate at the stoichiometric surface chi stoich, was investigated through a measurement of the mixing layer thickness. It was proven that approximating the mixing layer thickness through the gradient of the mixture fraction at the stoichiometric surface, offered a good estimation of chistoich.;Butanol kinetic modeling was studied in a zero-dimensional piston-cylinder assembly and a MATLAB code was used in order to solve the energy conservation and species equations. Pressure and temperature results were provided as a function of time, along with mol fractions of major species and combustion intermediates. The same calculation was performed for ethanol and n-heptane. It was shown that butanol peak pressures and temperatures were lower than the corresponding values for the other two fuels and that its autoignition occurred after a longer time interval.