Ahmet Yozgatligil

BURNING AND SOOTING BEHAVIOR OF ETHANOL DROPLET COMBUSTION UNDER MICROGRAVITY CONDITIONS

In an effort to gain a better understanding of the burning and sooting behavior of ethanol, isolated droplet combustion experiments were performed in the 2.2-s drop tower at NASA Glenn Research Center. The measurement of the burning rate, soot standoff ratio, and soot volume fraction are described in which initial droplet diameter, oxygen concentration, and ambient pressure were varied. The experiments reveal that while ethanol droplets burn in 1-atm air without soot formation, luminous radiation from soot particles is observed at higher pressures, with increased sooting at higher oxygen volume fraction. Increases in the oxygen concentration at elevated pressures results in a nonmonotonic behavior in the measured soot volume fraction. These experiments provide the first quantitative measurements of the soot volume fraction for ethanol droplet burning under microgravity conditions.

An experimental study on the effects of blockage ratio and ventilation velocity on the heat release rate of tunnel fires

It is very important to accurately predict the fire-induced air velocity, temperature, and smoke concentrations in tunnel fires to design efficient fire protection systems. In this study a scaled model of a tunnel was constructed based on Froude number scaling and wood sticks with different configurations which were burned in a controlled environment. Model vehicles having a square base were built according to the wood crib theory. The impact of varying longitudinal ventilation velocity and the cross-sectional area of the burning substances on the heat release rate and temperature distribution in the tunnel were measured.

New Observations of Isolated Ethanol Droplet Flames in Microgravity Conditions

Spherically symmetric ethanol droplet combustion experiments were performed to investigate the influence of initial droplet diameter, ambient pressure and inert substitution on the burning and sooting behaviors of ethanol droplet flames. Experiments were performed using the 2.2 sec reduced-gravity droptower facilities at the NASA Glenn Research Center. Noting the importance of transport characteristics of heat and species and their attendant effects on flame temperature and residence time on the sooting mechanism of diffusion flames, parameter adjustments were made to vary the sooting over a wide range of conditions. In these experiments, the residence times for fuel vapor transport were varied using changes in initial droplet diameters (from 1.6 mm to 2.2 mm) and ambient pressure (from 0.10 MPa to 0.24 MPa) and inert substitutions (He, Ar, and N2). The flame temperatures and flame standoff ratios were varied using different inert substitutions. For each experiment, the soot volume fraction, droplet burning rate, sootshell and flame dynamics, flame temperatures, and flame radiative emission were measured. These measurements enabled calculation of the fuel vapor transport residence times (from droplet surface to the flame front) which provides a measure of the duration for pyrolysis reactions, soot nucleation, and soot growth. The experimental measurements demonstrated that ethanol droplets burning in Ar inert environments produced the highest soot volume fraction, followed by N2 inert environments, and He inert environments, which produced the lowest soot volume fraction. For the various inert environments, the flame temperature distribution and the flame standoff ratio were only weakly affected by changes in both initial droplet diameters and ambient pressures. However, significant increases in soot volume fraction were observed as the initial droplet diameter and ambient pressure were increased. The coupled analysis of the flame temperature and the residence time for fuel vapor transport provided good correlation with the observed variations in sooting in microgravity droplet flames.

Effect of Ventilation and Geometrical Parameters of the Burning Object on the Heat Release Rate in Tunnel Fires

A 1/13 scaled model tunnel is constructed in order to investigate the blockage effect (the ratio of the model cross sectional area to the tunnel cross sectional area) on the heat release rate inside the tunnel with different ventilation velocities. Scaling is done based on Froude number. In the experiments, wood sticks assembled in different geometrical configurations are burned with various longitudinal ventilation velocities inside the model tunnel. Combustion gases are sampled to calculate the heat release rate. A statistical model is developed using the analysis of variance (ANOVA) method. According to the statistical model, 79.8% of the variation in heat release rate is attributed to changes in blockage ratio, 10.6% to changes in thickness, and 4.5% to changes in velocity.

Sooting Behavior of Ethanol Droplet Combustion at Elevated Pressures Under Microgravity Conditions

Liquid ethanol is widely used in practical fuels as a means to extend petroleum-derived resources or as a fuel additive to reduce emissions of carbon monoxide from spark ignition engines. Recent research has also suggested that ethanol and other oxygenates could be added to diesel fuel to reduce particulate emissions. In this cursory study, the combustion of small ethanol droplets in microgravity environments was observed to investigate diffusion flame characteristics at higher ambient pressures and at various oxygen indices, all with nitrogen as the diluent species. At the NASA Glenn Research Center 2.2-second drop tower, free ethanol droplets were ignited in the Droplet Combustion Experiment (DCE) apparatus, and backlit and flame view data were collected to evaluate flame position and burning rate. Profuse sooting was noted above 3 atm ambient pressure. In experiments performed at the Japan Microgravity Center 10-second (JAMIC) drop shaft with Sooting Effects in Droplet Combustion (SEDC) apparatus, the first data that displayed a spherical sootshell for ethanol droplet combustion was obtained. Because of the strong sensitivity of soot formation to small changes in an easily accessible range of pressures, ethanol appears to be a simple liquid fuel suitable for fundamental studies of soot formation effects on spherical diffusion flames. The results impact discussions regarding the mechanism of particulate reduction by ethanol addition to fuels in high-pressure practical combustors.

Nanostructure of Soot Collected from Ethanol Droplet Flames in Microgravity

The nanostructure of soot particles collected from spherically symmetric ethanol droplet flames were analyzed using a high resolution transmission electron microscopy (HRTEM). Nanostructure properties, including fringe length and curvature of carbon lamellae, were measured for soot particles collected in various inert environments. The sampling experiments were performed in the reduced gravity environments produced in the NASA 2.2 sec Droptower at the Glenn Research Center in Cleveland, Ohio. Microgravity droplet combustion experiments provide unique opportunities to vary the residence times over a large range and to independently vary the temperature. In this study, the time-temperature histories experienced by the soot particles were adjusted by means of inert substitutions (argon vs. helium) and variations in the initial droplet diameters (ranging from 1.6 to 2.2 mm). The variations in the initial droplet diameter were found to affect only the residence time necessary for soot inception and growth, whereas inert substitutions modified both residence time and temperature. The measurements of soot nanostructure properties indicated that the higher temperatures produced in the argon inert environment produced more graphitic nanostructures, while the lower temperatures produced in the helium inert environment produced more amorphous nanostructures at the inner core of the soot primary particle. The variations in the initial droplet diameter produced distinct soot nanostructures on the periphery of the soot particle. The higher residence times experienced for the largest initial droplet diameter experiments produced longer carbon lamellae with negligible curvature, while the lower residence times for the smallest initial droplet diameter experiments produced shorter carbon lamellae with higher degrees of curvature. These experimental results provide important foundational understanding of the influence of residence time and temperature on the soot nanostructure that has not been studied previously.

Ethanol Droplet Combustion At Elevated Pressure and Enhanced Oxygen Concentrations

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:

Experimental Investigation on the Mass Loss Rates of Thin-Layered n-Heptane Pool Fires in Longitudinally Ventilated Reduced Scale Tunnel

ABSTRACT Thin-layered n-heptane pool fires are burned with varied pool depths under longitudinal ventilation velocities ranging between 0.5–2.5 m/s in a reduced scale tunnel model. The combined effects of ventilation, pool size, and depth are investigated on the heat release rate, temperature distribution, and mass loss rate of fire. The gas temperature distribution and heat release rate results indicate that the critical ventilation velocity is achieved around 1 m/s in the scaled model, corresponding to 3.6 m/s in the real scale tunnel. It is observed that the gas temperature downstream of the fire increases at 2.5 m/s ventilation due to an enhancing effect of oxygen supply to the fire and increased flame deflection towards the leeward side of the pan. Results show that maximum heat release rate and total heat release normalized by fuel amount tend to occur at critical ventilation velocity. The measured mass loss rates show a considerable increasing trend with pool depth.

Combustion Characteristics of Biomass Ash and Lignite Blend Under Oxy-Fuel Conditions

In this study an attempt was done to profoundly explore the pyrolysis and combustion behaviors and emission characteristics of lignite samples in O2/N2 and O2/CO2 (oxy-fuel conditions) ambients. A special focus was allocated to the effects of three inorganic materials, potassium (K), calcium (Ca) and iron (Fe) on combustion characteristics of Turkish lignite using non-isothermal Thermo-gravimetric Analysis (TGA) technique combined with Fourier Transform Infrared (FTIR) spectroscopy and the effects of ambient gases and various oxygen mole fractions were considered. Eventually the co-processing combustion tests of lignite and the ash contents of different biomass fuels were investigated and the possible way of using biomass as a potential source of inexpensive catalysts in combustion processes were discussed. Co-processing combustion tests of lignite and biomass ash contents indicated that the hazelnut shell and walnut shell ash contents were significantly effective in increasing the char reactivity of lignite due to high concentration of potassium based oxides during combustion tests carried out in both air and 30% O2 in CO2 ambients. Furthermore the catalytic reactivity of wheat straw and cattle manure ash contents were observed in the second region of combustion regarding volatile matter release and combustion in both air and 30% O2 in CO2 ambients. These results are thought to be due to high concentrations of Alkali and Alkaline earth metals existed in the impregnated lignite samples with wheat straw and cattle manure ash contents and especially Na-based oxides in the cattle manure form. Finally in the case of lignite sample impregnated with saw dust ash content, it was observed that the impregnated lignite was significantly more reactive in devolatalization process in 30% O2 in CO2 ambients. These results revealed that the ash contents of walnut and hazelnut shell biomass fuels can be used as a potential source of inexpensive K-based catalysts in the co-processing of coal and biomass ash. Furthermore high concentrations of Alkali and Alkaline earth metals existed in the ash contents of biomass fuels like wheat straw, cattle manure and saw dust can make them suitable sources of inexpensive catalysts and develop a step forward in economic aspects of catalytic coal combustion.© 2013 ASME

MEASUREMENT OF SOOT SURFACE GROWTH KINETICS

Surface growth is one of the major steps in the soot formation processes where most of the total mass is formed. Freshly generated soot from an ethylene diffusion flame was sampled and mixed with ethylene in a flow tube at elevated temperatures and using nitrogen as the carrier gas. The changes in the size of the soot particles after hydrocarbon addition were measured using a differential mobility analyzer. These measurements showed a significant mass increase of soot particles as a result of the hydrocarbon addition where the total number density remained the same. Soot surface growth kinetics were obtained from the changes in the size, surface area and volume distributions. The activation energy (Ea) for ethylene was obtained. Pure pyrolysis experiments for ethylene were also performed. The surface growth process yield was found to be higher than the yield of the pyrolysis process.