Development of a laser induced phosphorescence technique for the investigation of evaporating two-phase flows

Abstract

The prospects of utilising the laser induced phosphorescence emission of common ketone tracers in the study of multi-phase (gas-liquid) flows are investigated within the context of this thesis. The quantification of evaporated fuel concentrations in the vicinity of liquid droplets by means of the laser induced fluorescence imaging technique, and the measurement of fuel concentrations in sprays containing sub-pixel sized droplets by means of the laser induced exciplex imaging technique suffer from well-known limitations; the former is plagued by low vapour phase signal intensities and halation, and the latter by liquid-vapour crosstalk and quenching. Therefore, a literature research was initially carried out, focusing on two main topics: the underlying photophysics of the processes involved in the excitation and deexcitation mechanism of the tracers under investigation, and the relevant optical techniques available for carrying out vaporized fuel concentration measurements in two-phase flow environments. Following a description of the experimental apparatus, a series of calibration experiments is presented. The liquid phase phosphorescence properties of acetone and 3-pentanone were investigated in the bulk and in liquid streams, and the phosphorescence emission characteristics of both tracers were quantified for different bath gas compositions. The phosphorescence signal of gaseous acetone was calibrated for the excitation energy and concentration dependencies. Based on the calibration data, a new technique was developed for the purpose of investigating the vapour phase concentration in the vicinity of liquid droplets. The proposed technique utilizes the phosphorescence rather than the fluorescence emission of vapour and liquid acetone, and is compared with the well-established laser induced fluorescence technique (LIF), in both evaporative and non-evaporative monodisperse droplet streams. The obtained results suggest that laser induced phosphorescence (LIP) imaging clearly improves upon laser induced fluorescence imaging, by successfully addressing both the high signal intensity disparity between the two phases and the ensuing halation that plague measurements carried out by deployment of LIF. The fluorescence and phosphorescence emission of acetone and 3-pentanone, the latter considered in order to demonstrate the feasibility of LIP imaging by deployment of other common ketone tracers apart from acetone, were also examined in sprays by means of a high-pressure gasoline direct injection system. Experiments examining the emission from both electronic states and their potential correlation are presented for non-evaporative sprays.; in particular, liquid phase corrections are carried out in LIF images by deployment of their corresponding LIP images and the obtained correlation functions, and the ensuing errors are quantified. Finally, mean and median filters are employed in limiting these errors and assessing the feasibility of the proposed technique. The obtained results are rendered encouraging, with suggestions for improvement focusing on reducing the noise observed in the LIP images by both signal augmentation and enhancement in the efficiency of the collection optics.