Generation of Turbulence in a RCEM towards Engine Relevant Conditions for Premixed Combustion Based on CFD and PIV Investigations

The interaction of turbulent premixed methane combustion with the surrounding flow field can be studied using optically accessible test rigs such as a rapid compression expansion machine (RCEM). The high flexibility offered by such a test rig allows its operation at various thermochemical conditions at ignition. However, limitations inherent to such test rigs due to the absence of an intake stroke do not allow turbulence production as found in IC-engines. Hence, means to introduce turbulence need to be implemented and the relevant turbulence quantities have to be identified in order to enable comparability with engine relevant conditions. A dedicated high-pressure direct injection of air at the beginning of the compression phase is considered as a measure to generate adjustable turbulence intensities at spark timing and during the early flame propagation. Based on former engine measurements and corresponding CFD simulations, the regime of relevant operating conditions in terms of velocity and length scale ratios in the Borghi diagram was derived for the RCEM. The main goal of this study is A) to characterize experimentally the flow field evolution for the optically accessible part of the domain in the spark plug vicinity to assess if the target conditions can be reached, and, B) support interpretation of the experimental data by means of CFD calculations which provide insights w.r.t. the flow field evolution also in the non-observable regions. To this end, the underexpanded jet of the angled single-orifice air injector used for the secondary air injection was experimentally and numerically assessed first in a constant volume setup and thereafter under transient conditions (moving piston). Schlieren imaging of the Mach disc location and time-resolved Particle Image Velocimetry (PIV) measurements around the spark plug were conducted to deduce the characteristics of the injection and the resulting tumbling air motion respectively. The numerical findings suggest that the underexpanded jet first impinges on the piston and then interacts also with the cylinder liner leading to the formation of a tumbling motion which is subsequently compressed due to piston motion. Reasonable agreement with the experiment is reported at the end of this process chain for the observable part of the domain. It has been shown that the use of an underexpanded secondary air injection in the RCEM allows for reproduction of the desired velocity and length scale ratios representative of the engine under consideration.