Ethanol Droplet Combustion At Elevated Pressure and Enhanced Oxygen Concentrations

Abstract

ion reactions produce vinyl radicals from the ethylene that are then converted to acetylene via: C2H3 → C2H2 + H Reaction 2 Reaction 2 is very sensitive to ambient pressure at conditions relevant to the present experiments. Acetylene is generally accepted as a key species contributing to soot formation processes. Increased levels of acetylene promote aromatic hydrocarbon formation either through the C4 mechanism of Frenklach and co-workers or through subsequent formation of C3 species followed by the C3 ring formation mechanism of Miller and Melius. In an effort to increase the likelihood of forming soot in ethanol experiments, ambient pressure and the oxygen concentration were varied in conjunction. Figure 7 displays the backlit laser-backlit view of ethanol droplets burning in various oxygen concentrations in nitrogen at 2.2 atm. At 21 % and 25 % O2 in N2, there is no visible luminosity exhibited in the flame view and the attenuation of the laser beam in the backlit view was lacking. As the oxygen concentration is increased to 30% O2 in N2, the formation of a distinct sootshell and a luminous flame are observed. Another interesting behavior was noted in which the sooting propensity appears to decrease at 40 % O2 in N2 case compared to the 30% O2 in N2 case. Additional experiments and analysis are required to investigate this interesting behavior. From the experiments shown in figure 7, the maximum soot volume fraction, fv,max, was measured using the tomographic inversion technique. These measurements clearly bear out the interpretation from the visual observation – at 21% O2 in N2, there is no measurable soot concentration, while at 30% O2 in N2, the maximum soot volume fraction is approximately 13 ppm. The soot volume fraction was not measured for the 40% O2 in N2 case since the distribution of soot was not uniform. These measurements represent the first soot volume fraction data obtained for ethanol droplet combustion in microgravity environment. These experiments clearly demonstrate the strong dependence of sooting behavior of ethanol droplets on ambient pressure and oxygen concentration. Concluding Remarks This study provided the first detailed measurement of the spherically-symmetric burning and sooting behavior of isolated ethanol droplets burning in enhanced oxygen and high pressure conditions. The burning rate measurements are strongly influenced by ambient oxygen concentrations (21% to 50% O2 in N2) but are independent of pressure in the range studied (1.0 to 2.2 atm in air). Use of enhanced oxygen concentration combined with higher pressures resulted in distinct sootshell formation. Measurement of soot volume fraction indicates that the sooting propensity increases non-monotonically with oxygen concentration. The effective control of the sooting behavior of ethanol from a soot-free flame to a highly sooting flame by using pressure and oxygen concentration is important for its use as one of the primary fuels to investigate the influence of sooting and radiation influence on droplet combustion. Acknowledgments Support from NASA through Grant NCC3-822 is gratefully acknowledged. Opportunity to perform experiments at JAMIC 10 sec. dropshaft, facilitated through the Japan Space Utilization Program (Mr. T. Sakuraya) is greatly appreciated. References Vanderver T.A., Ed. Clean Air Law and Regulation. The Bureau of National Affairs. Washington, D.C., 1992. Poulopoulos,S.G., Samaras,D.P. , Philippopoulos, C.J., Atmospheric Environment 35(2001) 4399 -4406. Nag, P., Litzinger,T.A., Haworth,D.C. Eastern States Meeting of the Combustion Institute (2001) 4 Abu-Qudais, M., Haddad, O., Qudaisat, M., Energy Conversion and Management 41 (2000) 389-399. Kitamura T., Ito T., Senda J., Fujimoto H., JSAE Review 22 (2001) 139-145 6 Miyamoto,N. et al., SAE paper No.980506 (1998). Choi, M.Y., Dryer, F.L., Science Requirements Document for Experiments and Model Development for Investigation of Sooting and Radiation Effects in Microgravity Droplet Combustion, NASA, 2001 Godsave, G.A.E. (1953). Proc. Combust. Inst. 4: 818-830. 9 Okajima, S. and Kumagai, S. Proc. Combust. Inst. 15 (1975) 401-407. 10 Hara, H. and Kumagai, S. Proc. Combust. Inst. 23 (1991) 1605-1610. Lee, A., and Law, C. K., Combust. Sci. Technol. 86: