Development of a rapid compression controlled-expansion machine for chemical ignition studies.

摘要

The ability to accurately model fuel combustion processes is essential to the development of transportation, power generation, and manufacturing technology. Models describing the kinetics of chemical oxidation are readily available and highly refined for a wide range of test fuels. However, these models still suffer from high levels of uncertainty under engine-relevant conditions, largely due to a lack of consistency between published validation data.;An experimental testing apparatus, known as the Rapid Compression Controlled-Expansion Machine (RCCEM) has been designed and fabricated to conduct chemical kinetic studies. The RCCEM features a pneumatically-driven, custom-designed cam, which governs the volumetric compression and expansion of the combustion chamber. This machine has been designed to test various compression ratios, compressed pressures, and compressed temperatures. Central to the operation of the RCCEM, the cam assembly is modular with the ability to incorporate different cams with unique compression and expansion profiles. This capability is intended to control heat loss rates in experiments via volumetric expansion, and as a result, increase understanding of its influence on the interpretation of validation data. Performance characterization of the RCCEM, using iso-octane and hexane, has shown that the machine is capable of testing a wide range of conditions with exceptional repeatability. Ignition delay times for iso-octane are reported for compressed temperatures of 630-700 K.;Additionally, two computational fluid dynamics (CFD) studies have been conducted to investigate the role of non-uniform boundary temperatures as a potential cause of discrepancies among data in the literature. The effect of these boundary conditions on ignition delay time predictions and compressed-gas temperature field development has been investigated for heated RCM experiments that use either creviced or flat pistons. Three unique boundary temperature cases for non-reactive simulations showed that a large temperature gradient forms over the crown of the piston due to heterogeneities present in the initial temperature fields. Subsequently, five boundary temperature cases were investigated for reactive simulations and demonstrated the effect of these non-uniformities on ignition delay time predictions. Through this work, it was determined that the flat piston is susceptible to these non-uniform conditions causing discrepancies in ignition delay times, whereas the creviced piston data was only minimally influenced.