Limbach Research Group

Microscopic Studies of Plasma-Particle and Plasma-Droplet Interactions

Investigator: Atulya Kumar

With growing pollution and environmental concerns many engine and power plant manufactures are actively developing combustion engines to become more efficient and operate using leaner fuel mixtures. Leaner fuel would allow for more thermal efficiency and significantly reduce NOx emissions [1]. To achieve this, a fair amount of challenges like slower ignition, combustion instability and propensity to misfire have to be addressed. Laser based ignition systems have been shown to have a huge potential for addressing issues related to lean-fuel combustion [2-3]. As lasers can precisely deliver energy at an optimal location in the combustion chamber and at a given time. This can minimize the heat loss to the engine block by igniting fuel-oxidizer mixture away

Figure 1: Three regimes of laser-droplet interactions: a) direct droplet irradiation, b) plasma formation and subsumption of droplet, c) shock wave deformation/atomization.

from the walls making flame kernel development faster. Despite all the capabilities listed, we see that laser ignition events are probabilistic [3-4]. This begs the fundamental question whether a direct irradiation of a micro droplet or an off-particle plasma interaction result in a laser ignition event.

The proposed research focuses on quantifying the fundamental, microscopic interactions between regions of localized heat release with individual levitated fuel droplets under conditions relevant to a wide range of combustion applications. The goal will be developing a thorough understanding of the physical processes associated with single droplets and aerosol particles through controlled experiments. In order to obtain

Figure 2: Copper particle trapped in the EDB exhibiting Mie scattering

controllable, localized, precisely timed and repeatable ignition events, a laser spark will be used to directly irradiate individual droplets or create a nearby plasma hotspot via dielectric breakdown of the surrounding gas. With the help of an electrodynamic balance (EDB) [5] and micrometer positioning capability of the laser focal point, three main types of interactions will be observed in the proposed study are shown in figure 1.

For this purpose, an electrodynamic balance has been constructed that is used to trap charged particulates /droplets at the null point of the balance. With its ability to constrain the particle in space, makes it the ideal tool to be coupled with the Q-switched laser giving the flexibility to position the laser focal point relative to the particle. The EDB employs two different types of electric fields to constrain a single particle in space. An AC voltage which creates a time averaged restoring force for the particle to the null point of the balance and a DC voltage to counter act the weight of the charged particle. Figure 2 shows a 30um copper particle trapped in the EDB. The particles we have used or intend to are Ti, Cu, Al, water and fuels (particle size ranging from 20-60um).

Figure 3: Schematic of the Experimental Apparatus

A stereo-imaging optical arrangement has been implemented (figure 3) to observe the test section using a single camera sensor coupled to a long-distance microscope. Currently, we use laser induced breakdown spectroscopy (LIBS) diagnostics to study the interaction of the spark plasma with solid particles which will be directly relevant to droplets as well. This allows us to establish the variation of plasma parameters like the plasma temperature and electron density for each type of interaction. Several recent studies have shown that the location [6-7] and viewing angle [8-9] of particles subsumed

Figure 4: A zoomed set of stereo images of titanium dust particle(Left) and Emission lines obtained (Right).

by a laser produced plasma significantly influences the observed LIBS signal intensity. The controllable positioning and observation of droplet-plasma interactions proposed in this study will directly contribute to a better understanding of aerosol LIBS.

The preliminary data collected are shown in figures 4 and 5. The variations of LIBS signal intensity and relative distance will be investigated while capturing the stereo images of the event itself.

Figure 5: Preliminary results on spectral signal variation due to particle position relative to the point of breakdown.

We have added a highspeed imaging capability and will be incorporating a LIF diagnostic techniques to further our understanding of the various fundamental processes taking place during the plasma -particle interaction soon. THe two videos below demonstrate this capability by showing a TiO2 particle subjected to (1) a direct laser induced breakdown and (2) heating by a laser. Finally, with the implementation of the 3D reconstruction of the stereo-images we can precisely obtain the position the particle with respect to the plasma plume. With this, we can calculate the plasma parameters like electron temperature and electron density and understand its variation with breakdown distance. This will give us insight on the most optimal location for laser breakdown for a successful ignition event.

 

 

 

References:

  1. Richardson, S., McMillian, M. H., Woodruff, S. D., and McIntyre, D., “Misfire, Knock and NO x Mapping of a Laser Spark Ignited Single Cylinder Lean Burn Natural Gas Engine,” SAE transactions, 2004, pp. 858–865.
  2. McMillian, M. H., Woodruff, S. D., Richardson, S. W., and McIntyre, D. L., “Laser spark ignition: laser development and engine testing,” ASME Paper No. ICEF2004-917, 2004.
  3. Dale, J. D., Smy, P. R., and Clements, R. M., “Laser Ignited Internal Combustion Engine – An Experimental Study,” 1978 Automotive Engineering Congress and Exposition, SAE International, Feb 1978.
  4. El-Rabii, H., Zähringer, K., Rolon, J. C., and Lacas, F., “Laser ignition in a lean premixed pre-vaporized injector,” Combustion Science and Technology, Vol. 176, No. 9, 2004, pp. 1391–1417.
  5. Davis, E. J., Buehler, M. F., and Ward, T. L., “The double-ring electrodynamic balance for microparticle characterization,” Review of Scientific Instruments, Vol. 61, No. 4, Apr 1990, pp. 1281.
  6. David W. Hahn and Nicolo Omenetto. Laser-induced breakdown spectroscopy (LIBS), part I: Review of basic diagnostics and plasma-particle interactions: Still-challenging issues within the analytical plasma community. Applied Spectroscopy, 64(12), 2010.
  7. David W. Hahn and Nicolo Omenetto. Laser-induced breakdown spectroscopy (LIBS), part II: Review of instrumental and methodological approaches to material analysis and applications to different fields. Applied Spectroscopy, 66(4):347-419, 2012.
  8. G A Lithgow and S G Buckley. Effects of Focal Volume on Uncertainty in Single-Aerosol Laser-Induced Breakdown Spectroscopy Measurements. Spectroscopy, (February), 2005.
  9. Erin S. Simpson, Gregg A. Lithgow, and Steven G. Buckley. Three-dimensional distribution of signal from single monodisperse aerosol particles in a laser induced plasma: Initial measurements. Spectrochimica Acta – Part B Atomic Spectroscopy, 62(12):1460{1465, 2007.
  10. Limbach, Christopher & Robinson, Ryan & Adams, Dylan & Wilbanks, Megan & Yalin, Azer. (2016). Toward a Microscopic Study of Laser Interactions with Levitated Liquid Fuel Droplets. 10.2514/6.2016-3382.