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Modelling of cavitation in diesel injector nozzles
- E. GIANNADAKIS, M. GAVAISES, C. ARCOUMANIS
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- Journal:
- Journal of Fluid Mechanics / Volume 616 / 10 December 2008
- Published online by Cambridge University Press:
- 10 December 2008, pp. 153-193
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- Article
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A computational fluid dynamics cavitation model based on the Eulerian–Lagrangian approach and suitable for hole-type diesel injector nozzles is presented and discussed. The model accounts for a number of primary physical processes pertinent to cavitation bubbles, which are integrated into the stochastic framework of the model. Its predictive capability has been assessed through comparison of the calculated onset and development of cavitation inside diesel nozzle holes against experimental data obtained in real-size and enlarged models of single- and multi-hole nozzles. For the real-size nozzle geometry, high-speed cavitation images obtained under realistic injection pressures are compared against model predictions, whereas for the large-scale nozzle, validation data include images from a charge-coupled device (CCD) camera, computed tomography (CT) measurements of the liquid volume fraction and laser Doppler velocimetry (LDV) measurements of the liquid mean and root mean square (r.m.s.) velocities at different cavitation numbers (CN) and two needle lifts, corresponding to different cavitation regimes inside the injection hole. Overall, and on the basis of this validation exercise, it can be argued that cavitation modelling has reached a stage of maturity, where it can usefully identify many of the cavitation structures present in internal nozzle flows and their dependence on nozzle design and flow conditions.
Vortex flow and cavitation in diesel injector nozzles
- A. ANDRIOTIS, M. GAVAISES, C. ARCOUMANIS
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- Journal:
- Journal of Fluid Mechanics / Volume 610 / 10 September 2008
- Published online by Cambridge University Press:
- 08 August 2008, pp. 195-215
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Flow visualization as well as three-dimensional cavitating flow simulations have been employed for characterizing the formation of cavitation inside transparent replicas of fuel injector valves used in low-speed two-stroke diesel engines. The designs tested have incorporated five-hole nozzles with cylindrical as well as tapered holes operating at different fixed needle lift positions. High-speed images have revealed the formation of an unsteady vapour structure upstream of the injection holes inside the nozzle volume, which is referred to as ‘string-cavitation’. Computation of the flow distribution and combination with three-dimensional reconstruction of the location of the strings inside the nozzle volume has revealed that strings are found at the core of recirculation zones; they originate either from pre-existing cavitation sites forming at sharp corners inside the nozzle where the pressure falls below the vapour pressure of the flowing liquid, or even from suction of outside air downstream of the hole exit. Processing of the acquired images has allowed estimation of the mean location and probability of appearance of the cavitating strings in the three-dimensional space as a function of needle lift, cavitation and Reynolds number. The frequency of appearance of the strings has been correlated with the Strouhal number of the vortices developing inside the sac volume; the latter has been found to be a function of needle lift and hole shape. The presence of strings has significantly affected the flow conditions at the nozzle exit, influencing the injected spray. The cavitation structures formed inside the injection holes are significantly altered by the presence of cavitation strings and are jointly responsible for up to 10% variation in the instantaneous fuel injection quantity. Extrapolation using model predictions for real-size injectors operating at realistic injection pressures indicates that cavitation strings are expected to appear within the time scales of typical injection events, implying significant hole-to-hole and cycle-to-cycle variations during the corresponding spray development.
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