Visualization of features of atomizing circular liquid sheet using simultaneous Volume Laser-Induced Fluorescence

Abstract

Present work is an experimental investigation of primary atomization of a radially expanding circular liquid sheet produced from a cylindrical vertical water jet issuing out from the convergent nozzle and impinging orthogonally on a horizontally placed cone-disc deflector below the nozzle. The radially expanding thin circular water sheet formed from the peripheral edge of deflector disc. The atomization process of expanding liquid sheet is captured simultaneously for the first time in the top and side views using a high-speed Volume Laser-Induced Fluorescent (VLIF). The experiments were conducted at 6 kHz framing rate for various jet Weber numbers (Wejet) over a range of 996 <Wejet < 7480. The top view VLIF revealed interesting spatiotemporal features of atomizing sheet such as outer edge thickness, liquid threads, ligaments and droplets and formation of perforations punctures (perforations) on the local sites of liquid sheet. Moreover, the evolution of the perforations leading to the breakup into ligaments and droplets was also captured in top view VLIF as well. Results show the liquid sheet outer edge (rim) thickness is unaffected by change in Wejet, whereas the rim velocity is found linearly increases with increase in Wejet. The liquid threads observed at the rim are subjected to end-pinching effect (as Ohnesorge number, Oh ≪ 1) when detached (because of Plateau-Rayleigh instability) from the rim. Ligament size in terms of length (Llig) and thickness (tlig) are measured at two different locations; (i) at rim (~100 mm from nozzle axis) and (ii) at about 130 mm downstream from the rim. Ligaments measured at downstream of the rim were found to be smaller than those measured at the rim. Further downstream, the droplets are formed whose size decreased with increase in jet Weber number (Wejet). Three different sites of origin of droplets namely rim, perforations edges and liquid bridge are identified. The variation of droplet diameter that produced from the rim with the Wejet is found to follow D10~Wejet-1/4 and D32~Wejet-1/5. Various features originating from the perforations are captured in top view VLIF and quantified for the first time using rigorous image processing methods. These features are perforation diameter, perforation’s front and rear edge thicknesses and velocities, liquid bridge, droplets from perforation via ligaments and via formation of liquid bridges. The perforations are formed close to sheet breakup and number of occurrence of perforations increases with increase in Wejet. The rear edge of perforation is seen thicker than the front edge because of larger accumulation of water in the rear edge. Consequently, the rear edge velocity is seen lower than the front edge velocity during the life span of perforations from the origin to their rupture into ligaments and droplets. Liquid bridge formed by merging/coalescing of two perforations is also observed. It is found that liquid bridge thickness increased with increase in Wejet. The droplets originating from the perforations are weak function of Wejet.