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- Ganippa, Lionel Christopher, 1970
(author)
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Atomisation and Combustion Studies of Diesel Sprays
- 2003
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Doctoral thesis (other academic/artistic)abstract
- This thesis addresses two potential problems facing investigators involved in Diesel engine development: fuel atomisation and soot formation. A particular concern has been cavitation in nozzles and its influence on the spray and combustion, investigated through impingement studies, scaled-up transparent nozzle studies and evaluation of hydro grinding effects in a real-size nozzle. Besides cavitation, air dilution effects on soot formation were studied using Laser Induced Incandescence (LII) and two-colour pyrometry. A method was developed to study the instantaneous fuel jet momentum and nozzle discharge coefficient for a non-stationary injection process. The time-resolved fuel jet momentum and nozzle discharge coefficient provides information about the transient phenomena taking place during the injection period due to cavitation. Scaled-up transparent nozzles were used to observe the flow structure within the nozzle hole and to evaluate its effect on the jet emerging from the nozzle flow. The asymmetric distribution of cavitation discovered within the hole had a strong influence on the spray pattern. Under realistic engine conditions, two nozzles each with a hole along the nozzle axis and the same momentum distribution in time, but different inlet geometries, were chosen to study the cavitation effects on combustion. Even though there were differences in the internal turbulence and nozzle discharge coefficient due to cavitation, no major differences were observed in spray angle, penetration, ignition delay, flame structure, temperature or soot concentration between these two equivalent nozzles. LII measurements revealed that soot formation occurred on the inside boundary of the flame periphery where the temperatures are high and oxygen concentration is depleted. In addition, no soot formation was observed initially in the central core of the spray. During later stages, soot production in the interior of the flame caused the soot concentration to be higher in the central region close to the tip of the flame. In diluted environments, the effective ambient oxygen concentration available for reactions is lower, so the reaction rates and flame temperatures are reduced, which delays the first appearance of soot near the nozzle tip and decreases the overall soot formation. However, due to lack of oxygen, the soot that is formed is not effectively oxidised in the later stages, particularly close to the flame tip where very low temperatures were observed.
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- Ganippa, Lionel Christopher, 1970, et al.
(author)
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Cavitation: a Contributary Factor in the Transition from Symmetric to Asymmetric Jets in Cross-flow Nozzles
- 2004
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In: Experiments in Fluids. ; Vol. 36, s. pp. 627 - 634
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Journal article (peer-reviewed)abstract
- The structure and evolution of cavitation and its influence on jet patterns from two transparent cross-flow nozzles with holes inclined at 90 degrees (nozzle A) and 80 degrees (nozzle B) to the nozzle axis have been investigated using high-speed motion pictures, flash photography and stroboscopic visualization. At the onset, cavitation inception was in the form of travelling bubbles, which were transported along the flow and clearly detached from the wall. As the flow was increased the bubbles grew and merged into a dense group of bubbles (cloud cavitation), partly unsteady and shedding. Further increasing the flow caused the cavitation at the entrance to transform mainly into a glassy appearance and at this stage the cavitation was well inside the hole and the spray appeared symmetric. When the flow was increased beyond this stage, cavitation extended to the exit of the hole, occupying a significant part of the hole on one side, resulting in a jet that atomized on the side where cavitation was most extensive and a non-atomizing jet on the side with less cavitation. The distribution of cavitation in the hole is very sensitive to the nozzle geometry and it substantially influences the spray dispersion.
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- von Berg, E., et al.
(author)
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Coupled simulations of nozzle flow, primary fuel jet breakup, and spray formation
- 2005
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In: Journal of Engineering for Gas Turbines and Power. - : ASME International. - 1528-8919 .- 0742-4795. ; 127:4, s. 897-908
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Journal article (peer-reviewed)abstract
- Presented are two approaches for coupled simulations of the injector flow with spray formation. In the first approach the two-fluid model is used within the injector for the cavitating flow. A primary breakup model is then applied at the nozzle orifice where it is coupled with the standard discrete droplet model. In the second approach the Eulerian multi-fluid model is applied for both the nozzle and spray regions. The developed primary breakup model, used in both approaches, is based on locally resolved properties of the cavitating nozzle flow across the orifice cross section. The model provides the initial droplet size and velocity distribution for the droplet parcels released from the surface of a coherent liquid core. The major feature of the predictions obtained with the model is a remarkable asymmetry of the spray. This asymmetry is in agreement with the recent observations at Chalmers University where they performed experiments using a transparent model scaled-up injector. The described model has been implemented into AVL FIRE computational fluid dynamics code which was used to obtain all the presented results. Copyright
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