An optical investigation into post compression aerodynamics in a dual opposed piston rapid compression machine
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Experimental chemical kineticists use rapid compression machines (RCMs) to verify models of global reactivity of fuels in as an ideal environment as possible, similar to those experienced in internal combustion engines. An RCM compresses a fuel mixture to an elevated temperature and pressure in less than 20 ms and maintains this high energy state until the onset of ignition. The time from the end of compression to the rapid rise in pressure due to ignition is defined as the ignition delay time (IDT) and is measured using dynamic pressure sensors located in the chamber wall. This phenomenon is characteristic of a given fuel at a given condition and is usually plotted as a function of compressed temperature. Due to the difficulties in performing direct, real time temperature measurements, including the “facility effect” of heat loss from the compressing gases to the chamber wall, the “adiabatic core principle” is used to estimate the temperature of the compressed gases at the end of compression prior to the onset of ignition as a function of compressed pressure. The assumptions made to use this numerically convenient tool fail if aerodynamics, introduced during the compression phase, transports cooler boundary layer gases to the central ‘adiabatic’ core which leads to an inhomogeneous compressed-gas temperature field. Creviced pistons are widely used to suppress the formation of roll up vortices that are initiated at the edge of the piston face caused by the motion of the piston relative to the cylinder wall. This investigation introduces a pilot project to observe the aerodynamic behaviour of the gases during and post compression, and to visually evaluate the efficacy of the creviced pistons in suppressing the formation of corner vortices. The observations were carried out in a working, dual opposed piston RCM. A rudimentary 2-D particle imaging velocimetry (PIV) arrangement was developed in-house to evaluate the feasibility of this type of investigation. A 50 mW, 520 nm Osram synchronised pulsed laser diode with planar sheet optics was used as a light source, in conjunction with a Chronos 1.4 off-the-shelf consumer, high speed camera at 1000 frames per second. Cooking grade corn starch was used as tracer particles. A dedicated, optical chamber was developed to mechanically support the Schott Duran Precision bore sight glass to provide optical access to the compressed volume. The volume was sealed using Epotek T7110 high temperature epoxy. A bespoke particle delivery system using nitrogen as the drive and compressed fluid was employed. As direct compressed measurement was not feasible due to difficulties encountered in penetrating the sight glass, 13 bar compressed pressures were estimated through comparison with pressure time histories from the standard reaction chamber under similar conditions. Image post-processing was carried out using Davinci Resolve, to improve contrast and sharpness, to convert to greyscale and to generate discrete image output. Lens induced distortion was improved using Matlab and an engraved calibration grid. The flow fields were evaluated using PIVlab. Despite some shortcomings of the corn starch particles, in terms of flow tracking fidelity, the system as a whole performed as expected, and insight into the performance of two piston geometries were compared. A qualitative comparison of the non-creviced (flat) and creviced piston geometries revealed a clearly discernible and repeatable improvement in using the creviced geometry compared to the non-creviced configuration. The purported plane of symmetry, characteristic of a dual opposed piston RCM and of special interest to investigators in the field of computational fluid dynamics (CFD), was also confirmed and its location as a function of the synchronicity of piston motion and displacement time history was investigated. Root mean square (RMS) comparisons of particle velocity among the two piston types on a characteristic plane of interest, provided a quantifiable method to compare their performance and also permitted direct comparison with a CFD analysis under similar conditions. This investigation is by no means an exhaustive study of creviced piston performance, but it serves as a low-cost validation of the PIV technique as carried out under the challenges posed by the dual opposed-piston configuration and serves as a roadmap for future studies of this kind. Recommendations and lessons learned from this investigation are presented. The success of the methodologies employed in this investigation warrants further effort to study the influence of a wider range of pistons geometries, diluent gases and compressed pressures and temperatures on corner vortex formation. A number of preparatory and ancillary projects carried during the course of this investigation are presented in the Appendices.
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