Laser Diagnostic Techniques with Ultra-High Repetition Rate for Studies in Combustion Environments
Abstract: When conducting laser based diagnostics in combustion environments it is often desirable to obtain temporally resolved information. This can be due to several factors such as combustion taking place in a turbulent flow field, flame propagation from a spark plug in an initially quiescent combustible mixture, or rapid, multi-point fuel consumption in a homogeneous charge as a result of compression ignition in an engine cycle. A multi-YAG laser cluster and a high-speed framing camera capable of recording sequences of up to eight image frames, and having a framing rate up to the megahertz range were originally set up for these types of studies. Within the framework of this thesis, further developments of this high-speed diagnostic system aiming at extending the wavelength palette and thus the range of detectable species, was carried out. In addition, the system was used for measurements with ultra-high repetition rates for the detection of different flame species in a variety of combustion devices. The high-speed laser system was redesigned for the generation of laser radiation at 355 nm, in addition to the original 532 nm and 266 nm, and a successful feasibility test for high-speed formaldehyde planar laser-induced fluorescence (PLIF) was carried out for the new design. Moreover, a novel multi-dye laser cluster has been set up. By pumping each of the four dye lasers individually using the Nd:YAG lasers in the multi-YAG cluster, tunable laser radiation with an ultra-high repetition rate can be produced, without the drawback of either losses in laser pulse energy or significant deterioration of the beam intensity profile often occurring when a single dye laser is pumped at ultra-high repetition rates. The multi-YAG and multi-dye laser clusters were used for high-speed visualization of the OH radical by means of planar laser-induced fluorescence in a low-swirl methane/air flame for tracking flame front movements over time while simultaneously measuring the flow-velocity field. Simultaneous high-speed OH visualization and imaging of the temperature field was also performed. The work carried out was a first step in the development of a detailed Large Eddy Simulation validation database for turbulent, premixed methane/air flames. High-speed OH PLIF using a single dye laser was employed in several other studies of the reaction zone, including an investigation of the ignition properties of hot jets in explosive environments, a study of combustion processes in a pulse combustor, and an investigation of the governing processes leading to electrical signals in an ion-current sensor. The last of these also included high-speed fuel tracer LIF. An alternative technique for flame studies involving measurement of the chemiluminescence from OH and CH in order to determine the equivalence ratio was investigated in terms of spatial and temporal resolution. The capability of the technique for resolving flame fronts was compared to reference measurements of OH PLIF. The tests showed that the spatial resolution in the depth direction suffered from line-of-sight detection, which significantly reduced the resolution. As the sensor was designed for monitoring spatial and temporal inhomogenities in mixtures within industrial gas turbine combustors, the temporal and spatial scales in such a combustor were evaluated using the high-speed laser diagnostic system for time-resolved visualization of OH. Also, fuel tracer PLIF was performed in order to visualize the fuel distribution in the combustor. The multi-YAG laser cluster was used in several studies of combustion processes in a homogeneous charge compression ignition (HCCI) engine, involving both high-speed fuel tracer PLIF and formaldehyde PLIF, with the aim of studying different types of ignition control. Acetone was used as a fuel tracer in investigating the effects of combustion chamber geometry on combustion. In studies of spark-assisted HCCI operation, the engine was run on a fuel mixture containing n-heptane, which produces formaldehyde early in the cool-flame region. Formaldehyde can thus be used as a fuel marker, eliminating the need of an added fuel tracer in this situation. Furthermore, three-dimensional imaging of formaldehyde in a laboratory flame as well as of Jet-A vapour in a slow non-reacting flow was demonstrated. This was achieved by rapidly scanning the laser sheet across a measurement volume spatially separating the eight laser pulses. A stack of closely spaced PLIF images was acquired by the framing camera, which could be used to re-create the three-dimensional shape of the investigated species by means of interpolation between the sheets.
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