Laser-Induced Phosphor Thermometry - Feasibility and Precision in Combustion Applications

Sammanfattning: Temperature is one of the most fundamental parameters to be measured in many research disciplines. In combustion science, a thorough knowledge of temperature is essential for improving and optimizing combustion processes. Increasingly strict environmental legislation, greater demands for energy, and efforts to reduce dependence upon fossil fuels are forcing the combustion industry to obtain more adequate knowledge of combustion processes generally. Laser Induced Phosphorescence (LIP) can serve as a tool for measuring temperature. It utilizes the temperature-dependent properties of the phosphorescence emitted by inorganic luminescent materials referred to as thermographic phosphors, after these have been illuminated by laser radiation. Either point- or two-dimensional measurements are usually performed by the use of either the temporal or the spectral temperature-dependent properties involved. The technique has many advantages. When used wisely it can be close to non-intrusive, offers remote sensing capabilities, and allows a high degree of temporal resolution, accuracy and precision to be obtained. The phosphor is usually applied to a surface either as a point or covering some area. The phosphorescence is detected using either a point light detector, such as a photomultiplier tube (PMT), or an image detector, such as a CCD camera. By seeding phosphor particles in free flow it is also possible to perform temperature measurements in gaseous media. In the present thesis, temperature measurements using laser-induced phosphorescence will be described and certain limitations of it will be addressed. Since the phosphor is applied to a surface as a very thin layer, the measurements performed are often considered non-intrusive. However, in situations in which the temperature changes very rapidly, such as in a combustion engine, and large temperature gradients are present for short periods of time, a relevant question to ask is whether the temperature of the phosphor agrees with the temperature it is intended to measure. One can also ask whether the phosphor layer acts as an insulator, making the measurements indeed intrusive. These are questions investigated in the thesis. At the same time, the technique is quite susceptible to small systematic errors if measurements of high precision and accuracy are to be performed. The technique also requires highly stable detectors in order for correct temperature values to be obtained. In the thesis, the characteristics of different detectors, both for point- and for 2D measurement, are investigated in terms of non-linear features and saturation effects. Also the precision of the technique, and how this is related to the spatial resolution achieved, are investigated. In addition, the suitability of the technique for measurements in gaseous media is investigated in terms of the laser heating of the phosphor particles and the relaxation time required for thermal equilibrium to be achieved.