Browse Publications Technical Papers 2019-24-0131
2019-09-09

Large Eddy Simulations and Tracer-LIF Diagnostics of Wall Film Dynamics in an Optically Accessible GDI Research Engine 2019-24-0131

Large Eddy Simulations (LES) and tracer-based Laser Induced Fluorescence (LIF) measurements have been performed to study the dynamics of fuel wall-films on the piston top of an optically accessible, four-valve pent-roof GDI research engine for a total of eight operating conditions. Starting from a reference point, the systematic variations include changes in engine speed (600; 1,200 and 2,000 RPM) and load (WOT and 500 mbar intake pressure); concerning the fuel path the Start Of Injection (SOI=360°, 390° and 420° CA after gas exchange TDC) as well as the injection pressure (10, 20 and 35 MPa) have been varied. For each condition, 40 experimental images were acquired phase-locked at 10° CA intervals after SOI, providing valuable insights with respect to the wall film dynamics in terms of spatial extent, thickness and temperature. The simulation framework was developed as follows: first, the spray model was calibrated using spray morphology evolution data of the same injector, characterized in a constant volume spray chamber by means of high-speed shadow imaging. In a second step, the wall impingement and film models were calibrated using the reference condition. The model is subsequently validated by means of data from the remaining operating points using unchanged model constants, showing very good capabilities in terms of capturing all trends observed experimentally while good quantitative agreement is reported concerning the transient films’ spatial extents and thicknesses. The insights obtained experimentally, in conjunction with the proposed modelling framework, constitute an important basis towards improved understanding of wall film dynamics and near-wall fuel-air mixture stratification in GDI engines, which are considered to play a major role in soot formation processes during combustion.

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