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

Paper #:
  • 2017-24-0043

Published:
  • 2017-09-04
Abstract:
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 machine 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 have 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 ignition timing and during the early flame propagation. Based on former engine measurements and corresponding CFD simulations [1], 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 to compare experimental flow field data with CFD simulations in order to validate (1) this approach as a tool to reproduce engine relevant conditions and (2) the numerical treatment of the involved processes. For this purpose, the under expanded jet of the angled single-orifice injector 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 temporally resolved PIV measurements around the spark plug were conducted to deduce the characteristics of the injection and the resulting tumbling air motion respectively. Numerical simulations were in agreement with the experimentally measured flow field. It has been shown that this approach allows the reproduction of the velocity and length scale ratios as found in the engine under consideration. 1. Koch, J., Xu, G., Wright, Y., Boulouchos, K. et al., "Comparison and Sensitivity Analysis of Turbulent Flame Speed Closures in the RANS G-Equation Context for Two Distinct Engines," SAE Int. J. Engines 9(4):2091-2106, 2016, doi:10.4271/2016-01-2236.
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