Development and application of laser diagnostics - from laboratory devices towards practical combustion engines

Abstract: For many decades, research work on combustion has been focused on improving combustion efficiency and reducing harmful emissions. Laser diagnostics is one of the best ways to investigate the combustion process and emission formation as it is non-intrusive and it has high spatiotemporal resolution. In this thesis work, many laser diagnostics have been developed and employed for combustion research. The laser-based optical methods cover ballistic imaging (BI), multi-scaler laser introduced fluorescence (LIF) imaging, particle imaging velocimetry (PIV), laser Doppler anemometry (LDA), and high speed LIF measurement (up to 140 kHz). Two BI systems were developed and compared, together with ultrafast shadow imaging (USI) for better imaging through a high optical depth (OD) substance, e.g. a spray. In addition, multi-scaler planar laser introduced fluorescence (PLIF) measurements were developed in Lund University Piloted Jet (LUPJ) burners including simultaneous measurement of temperature, CH radicals and OH radicals distribution, and simultaneous measurement of CH2O radicals, CH radicals and OH radicals distribution. Key parameters, such as Damköhler number, Karlovitz number, and Kolmogorov time scale, were calculated and are listed in this thesis based on LDA measurements. Moreover, high speed PLIF measurements were developed in an LUPJ burner including simultaneous OH/CH2O PLIF at 50 kHz, OH PLIF at 100 kHz and CH2O PLIF at 140 kHz, with more than 100 consecutive images for the first ever time. That was achieved by using a burst-mode laser pumped optical parametric oscillator (OPO) system synchronised with high speed cameras and high speed intensifiers. The diagnostics approaches were capable of following the temporal evolution of the reacting flow down to the Kolmogorov scale for better understanding of the transient behaviour of the eddy/flame interaction in highly turbulent premixed flames.The developed laser diagnostics have also been applied in a diesel spray in a high temperature high pressure (HTHP) constant volume vessel, in a pulsed plasma discharge and in practical combustion devices, e.g. internal combustion engines with elevated pressure and temperature (>90 bar and >1000 ºC). The developed 2f-BI system has been successfully employed for investigation of the spray formation region of a diesel spray, i.e. engine combustion network (ECN) Spray A, and the supercritical phenomenon has been observed with cellular structures of the spray for the first time. It’s also the first time that a burst-mode laser system has been applied for high-speed OH PLIF imaging in pulsed plasma discharges at tens of kHz repetition rate. The changing of OH radical distribution during post discharge was captured at 27 kHz, e.g. the deformation of the OH PLIF intensity from toroidal shape to a filled circle was observed. In addition, the decay rate of OH distribution at the outer layer of the plasma column and the increasing rate of that in the plasma column were calculated. The mixing process of gasoline/diesel and air in homogeneous charge compression ignition (HCCI) and partially premixed combustion (PPC) engines was visualised and investigated by 10 Hz fuel-tracer PLIF measurements and, also, the developed high-speed PLIF techniques with the burst-mode laser system. In addition, the mixture formation and evolution of low temperature combustion, i.e. CH2O distribution, together with the auto-ignition, i.e. high temperature combustion, were captured and followed for more than ten crank angle degrees (CADs) in one engine cycle at 36 kHz. To the best of my knowledge, no one has ever achieved this before. The results are also of significant value for computational fluid dynamics of internal combustion engines. An extended conceptual model for gasoline PPC mode with exhaust gas recirculation (EGR)-dilution and single-injection strategy is proposed. Furthermore, cycle-resolved PIV measurement was performed in a light-duty optical diesel engine with single, double and triple injection strategies. Last but not least, the experimental equipment and setups are introduced in this thesis, together with some practical experience and hands-on advice not mentioned in the attached papers.