Comparison between flat and creviced pistons in a controlled trajectory rapid compression and expansion machine (Ct-rcem)

In this work, we present a comparison of the thermo-chemical evolution of the fuel-air mixture when compressed with a flat piston v/s a creviced piston, in a controlled trajectory rapid compression and expansion machine (CT-RCEM). With the unique capability to precisely control the piston trajectory inside the compression chamber, the CT-RCEM provides a much wider operating range and extreme flexibility of operation in addition to ability to investigate the dynamic relationship between the thermodynamic path of compression and the chemical kinetics of the resulting autoignition. However, for reliably conducting such thermo-chemical investigations, it is essential to ensure spacial homogeneity of temperature field inside the combustion chamber during the compression process, for which creviced pistons have been used in the conventional RCM investigation. Here, we numerically investigate the effectiveness of a creviced piston for ensuring such homogeneity in the CT-RCEM for various piston trajectories. First, the compression of pure air using a creviced piston for piston trajectories and compression ratios in the CT-RCEM is simulated to illustrate the suppression of roll-up vortex as compared to the compression with a flat piston. Next, for a lean fuel-air mixture, two-stage auto-ignition of dimethyl ether (DME) is simulated for the compression with a flat and a creviced piston, for same piston trajectories. The simulation results have been validated against the experimental data from the CT-RCEM. Subsequently, using the temperature, pressure, and mole fraction of chemical species obtained from the numerical model, chemical kinetic investigations are performed using the Global reaction Pathway Selection (GPS) algorithm and compared between the flat and creviced pistons.