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An experimental and computational study of flow in the Squish Jet combustion chamber Lappas, Petros

Abstract

Fast controlled burning of the fuel-air charge inside combustion chambers of spark ignition engines typically leads to reductions in specific fuel consumption and exhaust emissions. These advantages are especially useful in engines running on natural gas—a fuel with a relatively low flame speed. 'Squish-Jet' combustion chambers have a unique geometry that forms jets of gas that converge radially inwards as TDC (Top Dead Centre) is approached. The turbulence generated by the jets has been shown to increase the burn rate and the positive results warrant numerical optimisation of Squish-Jet combustion chambers. In this study a prerequisite to the optimisation was carried out, namely, testing the validity of numerical flow predictions inside Squish-Jet combustion chambers. The validity study also led to a greater understanding of the flow processes inside these complex combustion chambers. The CFD (Computational Fluid Dynamics) code chosen for the numerical work was KIVA-3V. Ensemble mean velocities and turbulence intensities that were measured in three different combustion chambers exhibiting squish flow were compared with corresponding values from KIVA-3V. One of the combustion chambers was a plain Bowl-in-Piston type while the remaining two were Squish-Jet chambers (named 'Squish-Jet 1' chamber and 'Squish-Jet 2' chamber). While each Squish-Jet chamber had a 10 mm wide fence surrounding the piston bowl rim, Squish-Jet 1 chamber had a taller fence with wider squish grooves than Squish-Jet 2 chamber. Measurements were made by PIV (Particle Image Velocimetry) and LDV (Laser Doppler Velocimetry) and in order to closely reproduce the initial and boundary conditions set in KIVA-3V, the UBCRICM (University of British Columbia Rapid Intake and Compression Machine) was used. PIV enabled the in-cylinder flow to be visualised in planes parallel to the cylinder head, while LDV produced single-point velocity data that were used to more accurately determine rms velocity fluctuations. The microscopic particles used for PIV and LDV were seeded into each combustion chamber by a novel system developed to suit the momentary flow in the UBCRICM. The combustion chambers did not contain fuel and they were transparent to allow optical access for the laser measurements. A compression ratio of 9.1:1, a squish clearance of 2 mm and a crank speed of 800 rev/min were used for all tests. The numerical and experimental results indicated that Squish-Jet chambers tend to generate more turbulence than plain Bowl-in-Piston chambers, even though the former may have smaller squish velocities. This affirms the potential for Squish-Jet chambers to accelerate combustion; thus improving thermal efficiency and emissions in reciprocating internal combustion engines. Although the measured and predicted rms velocity fluctuations were all below 1.7 m/s, the LDV measurements made at the jet opening of one of the Squish-Jet chambers revealed some flaws in the k-e turbulence model used in KIVA-3V. The assumption of turbulence isotropy in KIVA-3V was violated there, as the rms velocity fluctuations in the radial direction were higher than in the tangential direction especially for crank angles prior to 10 degrees BTDC (Before Top Dead Centre). In addition, the turbulent kinetic energy was generally overestimated while the turbulent viscosity may have been underestimated. Consequently, it is likely that KIVA-3V overpredicted the dissipation rate of the turbulent kinetic energy. The general mean flow trends (with respect to time and space) measured in important regions of each squish chamber were well predicted by KIVA-3V. Nevertheless, weaker predictions were made at the (only measurable) squish groove outlet for each Squish-Jet chamber due to a suspected local PIV error. Aside from this, the mean radial velocity was overpredicted by at most 17%. The symmetrical inward migration of squish flow that was observed by PIV was present in KIVA-3V output. KIVA-3V also forecast the abatement of the squish velocity observed at TDC. The greater turbulence intensity observed just inside the piston bowls near TDC was also predicted. These flow predictions are valuable for chamber shape optimisation because they can be used to develop trends governing a chamber's propensity to produce turbulence and to transport it toward the ignition point.

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