Real-fluid behavior in rapid compression machines: Does it matter?

Rapid compression machines (RCMs) have been extensively used to quantify fuel autoignition chemistry and validate chemical kinetic models. Historically, the analyses of experimental and modeling RCM autoignition data have been conducted based on the adiabatic core hypothesis where real-fluid behavior has been completely overlooked, though this might be significant at common RCM test conditions. This work presents a first-of-its-kind study that addresses two significant questions for autoignition studies within RCMs in the fundamental combustion community: (i) can real-fluid behavior in RCMs affect interpretation and analysis of RCM experimental data? and (ii) can real-fluid behavior in RCMs affect RCM autoignition modeling and the validation of chemical kinetic models? To this end, theories for real-fluid isentropic change are newly proposed and derived based on high-order Virial EoS, and are further incorporated into an effective-volume real-fluid autoignition modeling framework newly developed for RCMs. With detailed analyses, the strong real-fluid behavior in common RCM tests is confirmed, which can greatly influence the interpretation of RCM autoignition experiments, particularly the determination of end-of-compression temperature and evolution of the adiabatic core in the reaction chamber. Furthermore, real-fluid RCM modeling results reveal that considerable error can be introduced into simulating RCM autoignition experiments when assuming ideal-gas behavior, leading to contradictory validation results of chemical kinetic models. Therefore, we recommend the community adopt frameworks with real-fluid behavior fully accounted for to analyze and simulate past and future RCM experiments, so as to avoid misinterpretation of RCM autoignition experiments and eliminate the errors that can be introduced into the simulation results with the existing RCM modeling frameworks.