Sprays: Dropsize Distribution

In order to optimize combustion processes it is essential to study the fuel/air-mixing process and the spatial and temporal fuel distribution (spray evaporation) in the combustion chamber. Imaging techniques require a significantly lower measurement times compared to point measurements such as Phase-Doppler anemometry (PDA). Furthermore, detection of several snapshots allows the obeservation of transient spray processes. It is our goal to develop imaging techniques which can be applied in engine-like environments. This implies the usage of conventional fuels, engine-like temperatures and pressures and an identical shape of the combustion chamber in our experiments. For an optical measurement technique these are challenging conditions, i.e. due to absorption phenomena in dense sprays.

Figure 1: Experimental setup of high-pressure high-temperature chamber at DaimlerChrysler, Stuttgart.

Fig. 3 shows simultaneous LIF and Mie images. The heatable high-pressure chamber can simulate various operating conditions of a gasoline engine. A temporal sequence of simultaneous images yields information about spray development and spray evaporation.

Figure 2: Schematic setup for simultaneous measurement of LIF and Mie signals. Two ICCD cameras and a Nd:YAG laser (532 nm) are used, LIF detection at 580 nm ± 30 nm, Mie detection at 532 nm.

This simultaneous detection of fluorescence and elastic scattering yields information on the mean droplet size. The ratio of LIF and Mie intensity is proportional to the Sauter Mean Diameter (SMD) assuming that no evaporation takes place. The Sauter Mean Diameter is an average diameter used for the characterization of sprays. A droplet of one SMD has the same volume and surface area ratio as the sum of all volumes and surfaces in the investigated spray.

Figure 3: Simultaneous measurement of LIF and Mie in a Common Rail Diesel injection. The LIF signal is only produced by liquid fuel. ASOI = After start of injection.

Evaporation of the fuel leads to accumulation of the low-vapor-pressure tracer in the liquid and allows the evaporation history of the spray to be traced. The LIF signal keeps constant since the number of tracer molecules in the drop keeps constant, too. Smaller, already evaporating drops show a decreasing Mie signal while the LIF signal keeps constant. Therefore, the LIF/Mie signal ration is increasing.

see also: Mie/LIF-Dropsizing

 

References:

[1] I. Düwel, H.-W. Ge, H. Kronemayer, R. W. Dibble, E. Gutheil, C. Schulz, and J. Wolfrum, "Experimental and numerical characterization of a turbulent spray flame," Proc. Combust. Inst. 31, in press (2006).
[2] I. Düwel, J. Schorr, J. Wolfrum, and C. Schulz, "Laser-induced fluorescence of tracers dissolved in evaporating droplets," Appl. Phys. B 78, 127-131 (2003).
[3] I. Düwel, J. Schorr, P. Peuser, P. Zeller, J. Wolfrum, and C. Schulz, "Spray diagnostics using an all solid-state Nd:YAlO3 laser and fluorescence tracers in commercial gasoline and Diesel fuels," Appl. Phys. B 79, 249-254 (2004).