Development of Multi-Dimensional Laser Techniques for In-situ Combustion Diagnostics

Sammanfattning: Multi-dimensional, laser-based measurements of quantities important for obtaining a better understanding of combustion processes, particularly of temperature and of species distributions, were performed using laser-induced fluorescence (LIF). The main purpose of the temperature experiments was to investigate the potential of employing a two-line atomic fluorescence technique (TLAF), using indium as the tracer species, in sooting environments such as in diesel engines. The initial studies were performed in a laboratory flame in which the amount of soot was varied for investigating whether the technique suffers from strong background radiation due to heated soot particles, absorption of laser wavelengths or interference caused by poly-aromatic hydrocarbons (PAH). The results indicate In-TLAF thermometry to perform well over a large range of -values. Indium is an attractive tracer candidate since both its excitation and detection wavelengths are in the visible range (410 nm and 451 nm), where absorption by hydrocarbons and other native combustion species was found to be negligible. Because of the oscillator strength of indium being high, the laser energy required is very low. This is an advantage since lasers of high power give rise either to laser-induced incandescence (LII) from soot particles or to strong LIF from PAH. The broad temperature sensitivity range, some 700–3000 K, of the technique when indium atoms are employed allows measurements to be performed in most practical combustion environments. In fuel-rich flames the seeding efficiency also increases, since the loss of active species due to oxidation there plays only a subordinate role. This is an advantage, since it permits sufficient signal-to-noise ratios to be achieved with use of a lower seeding concentration minimising the effects of the seeded species on the combustion chemistry. This makes In-TLAF highly attractive for thermometry applications in fuel-rich and turbulent combustion processes. In-TLAF may well prove to be a good candidate for future measurements in diesel engines, where temperature information is of crucial importance for further development of engine design. Engine measurements were performed using a high-speed laser and camera system. The laser cluster and the framing camera employed are able to capture eight images within 50 , making studies of time and spatially resolved combustion events possible. Concentration measurements were performed in engines of three types; a spark-ignition (SI) engine, a gasoline-direct-injection (GDI) engine and a homogeneous-charge- compression-ignition (HCCI) engine. The high-speed system was used so as to be able to perform true single-cycle-resolved measurements in the engines, all the data being recorded in a single engine cycle without any averaging effects caused by cycle-to-cycle variations. Studies of the sort are impossible to perform using a system that captures only single-shot images from subsequent cycles. The detection of single-cycle-resolved images of fuel and OH distributions was performed initially in a lab-top SI-engine with the aim of investigating the applicability of the technique. The same technique was then applied to a GDI engine at the Volvo Car Corporation. In that engine, fuel transport from the start of injection to the time of ignition was studied, along with flame propagation using both OH and fuel-tracer PLIF. The onset and development of combustion was also studied in the HCCI engine located at the Division of Combustion Engines in Lund, using fuel-tracer PLIF. The main goal there being to capture the appearance and growth of auto-ignition kernels for investigating whether any flame propagation occurs in a combustion process of this type. Cycle-to-cycle variations were also studied by comparing different cycles. To investigate whether a fuel island in a two-dimensional image is isolated or simply looks like an island due to wrinkling effects in and out the laser sheet, an experiment was performed in which 3D data was recorded. The topology of the fuel distribution can also be studied by obtaining measurements in three dimensions. From such experimental data, spatial gradients in all three directions can be calculated, providing information of interesting to modellers.