High-speed imaging diagnostics for the study of transport and reaction processes

Advances in solid-state laser and CMOS technology provided high-repetition rate lasers and high­frame rate cameras that enabled the development of imaging techniques for measurements that resolve important turbulent time scales. We have developed high-speed particle image velocimetry (PIV) techniques using infrared, visible, and ultraviolet lasers, each providing unique features that make them the preferred choice for particular measurement conditions. Ultraviolet high­repetition rate lasers were crucial in enabling planar laser induced fluorescence (PLIF) measurements of scalar distributions.
Flow structures, turbulence characteristics, scalar mixing, and their evolution are critically important in technical processes such as internal combustion engines. The interaction of fuel sprays with in-cylinder flow controls the formation of combustible mixtures. Flow around the spark plug affects the interaction of the spark plasma with the fuel cloud with significant impact on the rate of misfires. Simultaneous high-speed PIV and PLIF measurements were performed to quantify the combined role of turbulence and mixture properties on misfires in a spray-guided spark ignition engine.
Furthermore, high-resolution measurements were performed of the flow in the boundary layer at the cylinder head of a running engine with micro-PIV and particle tracking velocimetry (PTV). The role of the instantaneous flow structure in determining local heat transfer rates becomes evident from observations of small-­scale (several hundred micrometers) vortices that penetrate the boundary layer. Comparison of the measured flow structure to the commonly used law of the wall highlights the shortcoming of this model for engine applications. An outlook on future needs and opportunities for boundary layer measurements closed the presentation.