Thermal and Aerodynamic Characterization of a Creviced and Squared Piston Rapid Compression Machine: Extending the Validity of the Adiabatic Core Hypothesis, Generating Controlled Temperature Gradients

Rapid compression machines (RCM) are well-known tools to study the autoignition phenomenon under engine-relevant conditions. Covering a wide range of pressure and temperature at the top dead center (TDC), it can be employed with different types of mixtures and thermal stratification. Creating a homogeneous hot core region after compression in the combustion chamber is one of the challenges to overcome for RCM studies. The objective of the present work is to characterize from aerodynamic and thermal points of view a new configuration in the optical RCM of Pprime Institute. The latter aims at ensuring a wider adiabatic core region in terms of time and space through the installation of a creviced piston, specifically adapted to the square cross-section cylinder of this particular RCM. For this purpose, the internal flow has been qualified using high-frequency Particle Image Velocimetry with different laser sheet locations. Temperature variation during and after compression is measured at several positions with respect to the cylinder head, using thermocouples with wire diameter of 7.6 µm. It is observed that the piston cavity is able to collect the boundary layer created during compression and maintain a wide region at low velocity after the top dead center. Furthermore, it is demonstrated that different temperature gradient values can be generated and quantified within the adiabatic core region through differential heating of the chamber. This feature is promising for future works devoted to the analysis of combustion regimes. More generally, the thin wire thermocouples are shown to be accurate and reliable sensors to measure temperature in severe and transient pressure and temperature conditions specific to RCM internal flows.