Effects of low temperature heat release on the aerodynamics of a flat piston rapid compression machine: Impact on velocity and temperature fields

The study of auto-ignition under temperature stratification is of great interest. Indeed, further understanding of the thermo-kinetic interactions and its influence on the combustion propagation regime is needed. In a previous work [1], experiments in a flat piston Rapid Compression Machine (RCM) demonstrated that the apparent propagation of reaction fronts is highly influenced by the typical temperature stratification observed at inert conditions. Nevertheless, the influence of low temperature heat release (LTHR) on the internal aerodynamics and temperature of the RCM is not well understood. In the present study, we first address the LTHR-flow interaction then address the LTHR-temperature interaction. We performed 2D-PIV experiments at 10 kHz for inert and reactive lean isooctane mixtures. We averaged spatially the acceleration to present the time evolution during the cool flame period. We found that the normalized acceleration has a decreasing trend in both inert and reactive tests. No significant effect of the cool flame was observed on the trend. We performed temperature measurements using thin wire (7.6 µm) type K thermocouples at inert and reactive n-hexane mixtures (same test conditions of Fig. 7 in [1]). The temperature evolution of the hot (adiabatically compressed) and the colder gases were recorded when cool flame occurs. The corrected gas temperature showed good agreement with the theoretical adiabatic core temperature as well as previous measurements with toluene LIF. In the tested case, we found that the cool flame induces an equal temperature rise of approximately 110 K in both the adiabatically compressed and the colder vortex gases. These results confirm quantitatively that LTHR does not significantly affect the mixing of the temperature stratification of our flat piston RCM. In the studied test conditions, the temperature stratification is conserved globally despite the LTHR.