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Numerical simulation of cavitation phenomena inside fuel injector nozzles

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  1. Tez No: 403376
  2. Yazar: BARIŞ BİÇER
  3. Danışmanlar: Prof. AKIRA SOU
  4. Tez Türü: Doktora
  5. Konular: Deniz Bilimleri, Gemi Mühendisliği, Marine Science, Marine Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 2015
  8. Dil: İngilizce
  9. Üniversite: Kobe University
  10. Enstitü: Yurtdışı Enstitü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 150

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Özet (Çeviri)

The occurrence of cavitation phenomena inside a nozzle of a fuel injector for diesel engines is directly connected with local pressure drop. Understanding of cavitation flow in a nozzle of a fuel injector has major importance, since it plays a significant role in the fuel spray atomization, which strongly affects diesel engine performance and emissions. The main goal of this dissertation is to establish an effective combination of numerical cavitation models, which can accurately simulate the complex recirculation flow, the cloud cavitation shedding and the re-entrant jet inside fuel injector nozzles. For this purpose, both an in house code and the free computational fluid dynamics (CFD) package OpenFOAM are used. Numerical results are validated quantitatively through the comparison with experimental results of turbulent cavitating flows in a one-side rectangular nozzle. The images of cavitation are captured by a high-speed camera and the turbulent velocity is measured by a Laser Doppler Velocimetry (LDV). The presented work is divided into three major parts: 1. The first objective of the presented thesis is to assess the applicability of the existing bubble dynamics models, i.e., the Rayleigh-Plesset (RP) equation and simplified RP equation, which is called Rayleigh (R) equation, to the prediction of the growth and collapse of cavitation bubbles in Diesel fuel injector. Then, a Modified Rayleigh (MR) equation, based on the critical pressure PC, is proposed to overcome the drawbacks of the existing models. The agreement between calculated and measured bubble radii confirmed the validity of the RP equation. Numerical calculations are performed under various conditions, such as a water injection at low injection pressure and a diesel fuel injection at high injection pressure. The proposed MR equation is confirmed to give a good estimation of the growth and collapse rates of cavitation bubbles under various pressure conditions. 2. Next, the applicability of the various combinations of the models on the turbulent flow and cavitation to the numerical simulations of the transient cavitating flows in a nozzle is examined. (a) The first combination consists of the RNG k􀀀“ model and barotropic cavitation model or Kunz's cavitation model, which are not based on the bubble dynamics models. OpenFOAM is used for the numerical calculations. As a result, it is confirmed that the combination of Homogeneous Equilibrium Model (HEM), a Barotropic (Baro) equation and a RANS turbulence model, RNG k 􀀀 ”model, underestimates cavitation length and cannot reproduce transient cavitation behaviour, which plays a dominant role in atomization of injected liquid jet and spray. While the combination of Kunz's cavitation model and RNG k 􀀀“ model is able to predict the recirculation flow and the cloud shedding well by tuning the model's empirical constants. (b) Second, the combination in a house code based on Lagrangian Bubble Tracking Method (BTM), RP equation and Large Eddy Simulation (LES) is examined. This combination is confirmed to give a good prediction for the cavitation length, thickness as well as cavitation cloud shedding. However, it requires a fine grid and a long CPU time, and is applicable only to incipient cavitation. (c) The final combination of Volume-of-Fluids (VOF), RNG k􀀀”model and Mass Transfer Model (MTM), whose source terms are given by R or MR equations, is tested using OpenFOAM. It is found that the recirculation flow, the cloud shedding and the re-entrant are well simulated by the combination with MR equation, whereas cavitation length and thickness are overestimated with R equation. 3. Finally, the two-equation RANS turbulence models, such as k 􀀀 ! SST and RNG k 􀀀“ models, with various meshes of different cell sizes and the one equation eddy viscosity model under the framework of Large Eddy Simulation (LES) with a fine mesh are investigated to simulate the turbulent flow in an one-side rectangular nozzle. The results conclude that RNG k 􀀀 ”model with MR equation gives a good prediction for the cavitation length and thickness in a nozzle with the fine mesh of less than 50 m in the minimum mesh size xmin. The cavitation cloud shedding is well reproduced by RNG k 􀀀 " using the mesh with the minimum mesh size xmin=50 m. The k􀀀! SST model with MR equation predicts well the cavitation length and thickness in a nozzle with the finer mesh with less than 25 m in xmin. Also, the cavitation cloud shedding is well simulated with the k 􀀀 ! SST model and MR equation using fine mesh size xmin=25 m. The recirculation flow and the vortex shedding accompanied by cavitation cloud until the exit of the nozzle are well simulated with the combination of MR/LES models using the fine grid with minimum mesh size xmin of 4.4 m compared to RANS models. The study concludes that the MR equation together with appropriate turbulence model and a fine mesh can simulate not only the complex cavitating recirculation flow, cloud cavitation shedding and re-entrant jet flow but also cavitation thickness, length as well as mean and turbulence velocities quantitatively, and can be used to explore cavitation phenomena inside fuel injector nozzles.

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