Düwel I. Spray combustion diagnostics with tracer-based laser-induced fluorescence imaging

Inaugural-Dissertation zur Erlangung der Doktorwürde der Naturwissenschaftlich-Mathematischen Gesamtfakultät der Ruprecht-Karls-Universität Heidelberg, 2007. 149 p.Abstract
To understand and optimize the combustion processes in new types of internal combustion engines, like gasoline direct-injection engines, non-intrusive laser-based diagnostics methods have proven to be versatile and important tools to measure combustion characteristics. Most engine developers already rely on optical methods in the development of new engines. To guarantee accurate and reliable results of optical measurements it is necessary to carefully evaluate and characterize these techniques under realistic conditions.
In this work a variety of laser-based diagnostics techniques, mostly based on the tracer-LIF principle, have been evaluated for their applicability and limitations and have been applied to different spray systems – reaching from single droplets, over model sprays to fuel sprays in a firing test engine.
The tracer-laser-induced-fluorescence (LIF) method is based on doping the fuel with a fluorescent substance whose fluorescence characteristics are used to measure various physical properties of the spray, such as temperature, droplet size, or droplet velocity. From these properties information about heat and mass-transfer mechanisms in the spray can be gained. However, the characteristics of the used tracers must be known in detail in order to guarantee useful and comparable results. Therefore, in this work various tracers were characterized in terms of their evaporation and temperature-dependent properties. The tracers Atto 680, Rhodamine 800 and Rhodamine B have further been tested for their applicability to realistic fuels.
High-repetition-laser systems and high-resolution CCD or CMOS cameras allow the detailed analysis of spray formation, evaporation and combustion by tracer LIF with high temporal and spatial resolution. The so-called laser-light-sheet method provides two-dimensional information from the area of interest. The application of the before characterized tracers to a model spray burner provided new insights into these processes by comparison with numerical spray simulations implementing detailed chemistry through a spray flamelet model. To study the evaporation of a single droplet, an electrodynamic levitator (Paul-trap), that traps charged droplets in high-frequency electric field, was designed and setup. The droplet was heated by a pulsed CO2-laser. Comparison with an new droplet evaporation model, that implements these conditions, showed promising results.
In addition, in a gasoline direct-injection test engine the interaction of spray and spark plug, the influence of the spray on the spark and the effect of fluctuations in the spray angle during injection were analyzed and provided insight into the reasons of misfires and partial burns. Further, laser-induced exciplex fluorescence yielded information about the spatial distribution of the ignitable fuel/air mixture for different gasoline injectors at varying conditions.