الفهرس 
Atomization BackgroundOnly 14 pages are availabe for public view
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Abstract
Through centuries, man was captivated with flying. This was finally provided by inventing gas turbine engine. But soon was obstructed with the limited fossil fuels and associated environmental issues. So replacement of fossil fuel is vital to insure steady fuel supply. For this purpose, jatropha biofuel stands as one of the promising sustainable alternatives, but with obstacle of high viscosity, which inhibits the atomization process of the fuel. The effervescent atomizer, a type of internal-mixing twin-fluid atomizer, has been showed to work well with biofuels and high viscosity liquids, in terms of lower droplets size at relatively low injection pressure. The basic idea of effervescent atomizer is to load the bulk liquid with bubbles of atomizing gas, using an aerator, to form two phase bubbly mixture upstream of the final discharge orifice. Like the trapped air bubbles in an opened water tap, the bubbles explode just at the exit of the atomizer which shatters the liquid into droplets. Therefore, the atomization is augmented.
The main objectives of the current study is to investigate the feasibility of replacement of conventional aviation Jet-A1 fuel with Jatropha biofuel as a renewable alternative fuel. For this purpose, numerical simulations and experimental measurements were performed. The numerical simulations modelled the internal two-phase flow and external spray emerged from effervescent atomizer. In the internal flow modelling, three dimensional simulations using the volume of fluid interface tracking model were performed. For turbulence simulation, the realizable k-ɛ model was selected following the previous studies.
Eulerian-Lagrangian technique was adopted for external spray modelling in a two dimensional frame work. The 4th order Runge-Kutta method was used to solve the discrete phase trajectories. For simplicity, the standard k-ɛ model was selected for turbulence modelling of the continuous phase.
For all models, the finite volume approach was used to discretize the corresponding governing equations. Further, the SIMPLE algorithm was applied for pressure-velocity coupling. Validation with experimental work was performed and the current results were compared well. The present results showed that the gas to liquid mass ratio (GLR) is one of the major contributory factor affecting the atomizer performance. The internal two phase flow was identified as slug flow in the discharge passage at low GLR (.08%). The flow evolved to slug-annular flow at GLR of 0.5%. At high GLR (0.8%) the annular flow was distinguished. The mixing between phases was augmented with increasing GLR. Finally the liquid film thickness at the atomizer outlet was calculated and compared with the conventional aviation Jet-A1 fuel. The results showed that the liquid film thickness remains unchanged at low GLRs (less than 0.15%), though the higher biofuel viscosity has order of four. But, for higher GLRs (above 0.3%), the liquid film thickness slightly changed with maximum deviation less than 10%. Further the deviation between jatropha and Jet-A1 liquid film thickness decreases at higher GLR of 0.8%. The results unveil the superiority of effervescent atomizer with handling jatropha biofuel in terms of the internal flow.
The results of spray modelling show that the Sauter Mean Diameter (SMD) increases with spray tips. Further, the jet momentum and penetration length are augmented.
In the experimental measurements, the visualization technique was applied to characterize both internal and external flow in a transparent effervescent atomizer model. The measurements ensured that the GLR is one of the major contributory factors in effervescent atomization. The internal flow photography showed the flow development from bubbly, slug and annular flow with aeration level increases.
The atomization process was highly augmented with increasing aeration level at low GLRs (less than 14%). Increasing GLR further has no appreciable effect on atomization. Instead the spray cone angle was noticed to decrease at high aeration levels (above 17%).