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Computational Study of Under-Expanded Jets, Mixture Formation and Combustion in Direct-Injection Spark-Ignition Hydrogen Engines

Hamzehloo, A; (2016) Computational Study of Under-Expanded Jets, Mixture Formation and Combustion in Direct-Injection Spark-Ignition Hydrogen Engines. Doctoral thesis , UCL (University College London). Green open access

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Abstract

The present PhD thesis studied under-expanded gaseous fuel jets, in-cylinder flow, in-cylinder mixture formation and spark ignition combustion with emphasis on their applications in advanced direct injection spark ignition gaseous-fuelled internal combustion engines particularly with hydrogen fuelling. Open-source and commercial computational fluid dynamics frameworks based on OpenFOAM® and STAR-CCM+® were developed, validated and employed. A fourth-order accurate Runge-Kutta solver specifically for modelling shock-containing compressible flows including underexpanded jets was developed using OpenFOAM® C++ libraries. An adaptive mesh refinement technique was created for STAR-CCM+® and employed to study highly turbulent under-expanded jets. Reynolds-Averaged Navier-Stokes and Large Eddy Simulation techniques were employed to investigate sonic and mixing characteristics and also three-dimensional structures of various underexpanded jets including air, nitrogen, methane and hydrogen. Various nozzle profiles, ambient thermodynamics, injection pressures and nozzle pressure ratios were also examined. Additionally, a new methodology was developed within the STAR-CCM+® framework using its JAVA™ scripting capability in order to simulate the in-cylinder flow, mixture formation and combustion in advanced hydrogen engines with complex geometries. It was noticed that the compressible in-nozzle flow in high pressure gaseous injection from a millimetre-size nozzle exhibited a significant transient behaviour, finished by the formation of an under-expanded jet with a nozzle exit Mach number higher than unity. Existence of a strong transient vortex ring in hydrogen jets (unlike those of nitrogen and methane jets) was found to be a contributing factor to the flow instabilities in the vicinity of the intercepting shock that promoted airhydrogen mixing before the Mach reflection. It was noticed that increasing the injection pressure did not necessarily increase the penetration length of under-expanded gaseous fuel jets significantly and there would be an optimum injection pressure which could provide desirable mixing characteristics while maintaining manufacturing and safety-related costs at a reasonable level. It was confirmed that the azimuthal star-shape (petal) structure is a natural characteristic of under-expanded jets which through a vortex merging process promotes mixing at the boundary of gaseous jets. It was noticed that a cylindrical nozzle with lower length to diameter ratio would result in a relatively higher jet penetration. It was found that with a conical nozzle profile, if an under-expanded jet formed, a higher penetration length was obtained compared to that of a simple orifice nozzle. It was shown that flow around the intake valves and its associated vortical structures had significant effect on the formation and evolution of the in-cylinder flow and its tumbling motion. It was found that higher level of turbulent kinetic energy could be obtained by retarding end of injection timing. It was concluded that a double pulse injection strategy could provide desirable level of mixture richness, fuel stratification and turbulence around the spark location. Finally, by applying a detailed kinetic combustion model it was found that the impact of the fuel stratification was less dominant on the hydrogen flame propagation characteristics compared to that imposed by the velocity field.

Type: Thesis (Doctoral)
Title: Computational Study of Under-Expanded Jets, Mixture Formation and Combustion in Direct-Injection Spark-Ignition Hydrogen Engines
Open access status: An open access version is available from UCL Discovery
Language: English
UCL classification: UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science
URI: http://discovery.ucl.ac.uk/id/eprint/1474463
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