Experimental and theoretical investigation of soot onset threshold, chemical speciation and flame properties of ethylbenzene-ethanol blends.

摘要

This work investigates five different one-dimensional, laminar, atmospheric pressure premixed ethanol/ethylbenzene flames (0, 25, 50, 75 and 90% ethanol by weight) at their soot onset threshold (&phis;critical). Liquid ethanol/ethylbenzene mixtures were pre-vaporized in nitrogen, blended with an oxygen-nitrogen mixture and, upon ignition, burned in premixed one-dimensional flames at atmospheric pressure. The flames of the investigation were controlled so that each was at its visual soot onset threshold, and all had similar temperature profiles. The effect of mixture proportions in ethanol/ethylbenzene blends was investigated to see how it impacted the critical equivalence ratio (&phis; critical), chemical speciation and flame properties. Modeling of the flames was conducted to investigate species profiles, important reaction pathways and flame flow characteristics. An optical investigation on flame height and thickness was also conducted to see how the mixture proportions affected these parameters. The results showed that the critical equivalence ratio (&phis; critical) increased in a parabolic fashion as ethanol was increased in concentration. This occurred because as ethanol replaced ethylbenzene, more carbon was removed from polycyclic aromatic hydrocarbons (PAH) and soot formation pathways. As ethanol was increased in proportion, the carbon to oxygen ratio decreased and at the same time more carbon monoxide was produced in the flames, both of these factors removed carbon from the system. Conducting a reaction flux analysis revealed benzyl as a major contributor to the formation of benzene (through toluene formation) and naphthalene. Benzyl concentrations dropped significantly as ethylbenzene was replaced with ethanol and thus greatly reduced the production of key soot precursors. The flow modeling revealed a fairly laminar flame, with nice parallel streamlines except at the periphery of the flame. At the periphery the difference in velocity of the sheath flow caused the streamlines to diverge. The velocity profile takes on the shape of a flattened parabola with significantly differing velocities only in or around the sheath flow. Thus, the results were in line with the uniformity assumptions made in all premixed flame studies. The flame height and thickness investigation revealed that the 90% ethanol flame was both thicker and rose higher off the burner surface than the neat ethylbenzene flame.