CFD Simulations as a Support of Experimental Research in a Rapid Compression Expansion Machine Facility

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The main aim of this study to reproduce methane combustion experiment conducted in a rapid compression-expansion machine using AVL FIRETM software in order to shed more light on the in-cylinder processes. The piston movement profile, initial and boundary conditions as well as the geometry of the combustion chamber with a prechamber were the same as in the experiment. Authors by means of numerical simulations attempted to reproduce pressure profile from the experiment. As the first step, dead volume was tuned to match pressures for a non-combustion (air-only) case. Obtained pressure profile in air compression simulations was slightly wider (prolonged occurrence of high pressure) than in the experiment, what at this stage was assumed to have negligible significance. The next step after adjusting dead volume included combustion simulations. In the real test facility, the process of filling the combustion chamber with air-fuel mixture takes 15 s. In order to shorten computational time first combustion simulations were started after the chamber is already filled assuming uniform mixture. These simulations resulted in more than two times higher maximum pressure than recorded in experiments. It was concluded that turbulence decays quickly after filling process, what was also confirmed by next combustion simulations preceded by the filling process. Then the maximum pressure was significantly decreased but still it was higher than in the experiments. Based on the obtained results it was assumed that the discrepancy noticed in air cases is further increased when combustion is included. Moreover, the obtained results indicated that pre-combustion turbulence level is very low and suggested that either piston profile movement is not correct or there is high-pressure leak in the test facility.

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