| dc.description.abstract | Understanding the flow behaviour of liquid jets discharging from orifices is essential for several applications such as fuel spraying, ink-jet printing, aerosol devices in pharmaceutical industries, micro-machined fuel injectors, and 3-D printing. Also, the atomization process of liquid jet discharging from an orifice is pivotal for the design and development of fuel injectors employed in aircraft gas turbines, rocket engines, and automotive engines. Academic interest in the problem of liquid jet breakup and atomization, whether experimental or theoretical, are confined to circular orifices and there is very little information available on the effect of non-circular shape of orifice on the fluid dynamic behaviour of liquid jets. Several studies are devoted to elucidate the role of orifice geometrical parameters such as orifice diameter, orifice length, the shape of orifice inlet, orifice inner wall surface roughness, and the shape of the orifice cross-section on the atomization of discharging liquid jets. It is of practical interest to understand the fluid dynamics of liquid jets discharging from non-circular orifices, compared to an equivalent circular orifice.
The interfacial oscillation characteristics of liquid jets discharging from orifices is essential to describe their breakup mechanism and subsequent spray formation. This thesis presents the results of a systematic experimental study on the characteristics of liquid jets discharging from orifices of non-circular shapes, namely ellipse, square, and triangle. The study is carried out in two stages: the first is to characterise the capillary force governed jet oscillations of liquid jets discharging from orifices of non-circular shapes, by the interface oscillation and axis-switching phenomenon in the case of liquid jets issued from all noncircular orifices and also to understand the effect of non-circular orifice shape on the breakup behaviour of liquid jets. The interfacial oscillation behavior of liquid jets discharging from elliptical, triangular, and square shaped non-circular orifices with an equivalent diameter of 5mm. The measurements of wavelength and amplitude of jet oscillation are extracted from digital images of the liquid jets captured via still photography technique at different flow conditions using three different liquids (water, water-glycerol mixture, and Jet A-1 fuel). The measured wavelength of jet oscillation, irrespective of the shape geometry of the tested non-circular orifices, increases linearly with increase in liquid jet velocity and scales with the liquid jet Weber number. The experimental measurements of wave length recorded at different flow conditions exhibit good agreement with the theoretical predictions for liquid jet undulating in its cross section. For a given velocity of the jet, the liquid jet of Jet A-1 displays higher value of wave length and undergoes a larger increase along the axis of the jet compared to the jets of other two liquids. Further the trends on the variation of maximum and minimum amplitudes of the first wave segment of liquid jets discharging from the elliptical and square orifices with liquid jet velocity are also discussed in detail.
In the second phase, studies on the liquid jets and spray formation from non-circular orifices of equivalent diameter of 2.5 mm and 1 mm are studied for all the three experimental liquids. The thesis presents the results on the features of spray formation of water jets discharging from elliptical, triangular, and square orifices of equivalent diameter in the range 0.96 mm to 1.06 mm. The experiments are carried out in a spray test facility with pressure drop across the orifice as high as 4.2 MPa. The spray from the non-circular orifices are visualized using backlight and front light techniques and the images of spray at different flow conditions are captured to deduce details on jet breakup, spray spread at different axial locations and spray morphology. The measurements of spray drop size, expressed as Sauter
Mean Diameter, (SMD) are obtained using laser diffraction instrument Spraytec. The effect of orifice shape on SMD is studied for all the three jetting liquids and the measured SMD agrees very well with the available theoretical models. The orifice shape influences the spray spread significantly and it is well observed in the water-glycerol mixture jets that the spray width of triangular orifice jets is larger than that of the square orifice, maybe due to the enhanced surface atomization in the triangular jets. The analysis of SMD variation for Jet A-1 liquid from the non-circular orifices reveals that the triangular orifice exhibits better atomization characteristics compared to that of the elliptical and square orifices. The enhanced atomization of Jet A-1 jets from the triangular orifice may be attributed to the
generation of most unstable ligaments from the orifice. Further work is needed to ascertain this observation. | en_US |
