A thermodynamic model of a rapid compression expansion machine applied for the estimation of the flame speed of air/hydrogen mixtures

Abstract

This work presents a thermodynamic inverse model to extract the main flame propagation characteristics of an ultra-lean air/hydrogen mixture burning in a rapid compression/expansion machine (RCEM). Differently from combustion vessels, RCEMs allow fundamental combustion studies under thermodynamic conditions of temperature and pressure relevant to internal combustion engine (ICE) operations.The presented thermodynamic model takes into account all the main phenomena occurring in an RCEM, such as the wall heat transfer, and mass, species, and energy exchange with the crevice volumes. Moreover, combining the inverse model with a simplified geometrical description of the flame front, the flame propagation velocities are derived from experimental data of pressure and piston position.The model is applied to investigate the combustion process of a hydrogen/air mixture with λ between 2.1 and 2.6 and thermodynamic states relevant to ICE operations (initial pressure and temperature of around 30 bar and 720 K, respectively). The methodology is validated against direct measurements of flame front velocities realized by optical access in the combustion chamber. The model and measurements underline that the flame speed related to the unburned gas progressively decreases as λ increases, passing from 4.4 m/s at λ = 2.1 down to 2.5 m/s at λ = 2.6. The model/experiment RMSE on the absolute flame velocity is of 0.6 m/s over twelve test points, supporting the consistency of the presented thermodynamic model.