Temporally gated imaging for investigation of atomizing sprays

Doctoral thesis, 2016

Fuel sprays play an important role in the combustion process of modern compression ignition engines, but their dynamics are not well understood. This thesis focuses on time-gated imaging techniques as a means to study the optically dense spray formation region of these atomizing sprays. Initially, the performance of two ballistic imaging setups previously used in spray investigations was investigated. The investigation showed that the simpler of the two setups (2f) was less affected by changes in the time-gate setup, and was more robust to component misalignment. The 2f setup is therefore often preferable for time-gated spray investigations. Then, the ballistic imaging system was further developed with the use of three synchronized regenerative laser amplifiers. This system allowed three time-gated images with user defined inter-frame spacing to be generated. Based on these three images, two-dimensional acceleration data of larger liquid structures in the spray formation region were obtained. Such acceleration data can be used in the development of predictive computational spray codes. Next, ballistic imaging was used to study fuel sprays at elevated pressures and temperatures. This was the first time ballistic imaging was applied to sprays at engine-relevant conditions. The ambient conditions were varied and changes in the liquid/gas interfaces in the spray formation region were studied. It was shown that especially for single-component fuels the liquid-gas interfaces exhibited a significant change, which mostly correlated with the thermodynamic properties of the fuel. These findings contribute to the ongoing discussion regarding the potential transient supercriticality of fuel sprays in modern direct injection combustion engines. The investigation of the fuel sprays further showed that there is a need for a technique that provides depth-resolved data from the spray formation region. To this end, we developed a new approach termed time-gated sectioning. This setup captures backscattered light from the spray formation region, and was successfully able to generate a depth-resolution of around 0.3 mm. Collectively, the work presented in this thesis advances the use of time-gated imaging methods for spray investigations.