Abstract
In this study, cavitating flows inside a transparent cylindrical nozzle with an inner diameter of 0.9 mm were visualized, and the effect of cavitation on atomization characteristics of emerging sprays was investigated. Different patterns of cavitating flows inside the nozzle were visualized using a high-speed camera. In-house codes were developed to process the captured images to study the droplet size distribution and droplet velocity in different flow regimes. The results show that cavitating flows at the microscale have significant effects on atomization characteristics of the spray. Two working fluids, namely, water and poly(vinyl alcohol) microbubble (PVA MB) suspension, were employed. Accordingly, the injection pressures were detected as 690 kPa, 1035 kPa, and 1725 kPa for cavitation inception, supercavitation, and hydraulic flip flow regimes in the case of water, respectively. The corresponding pressures for the aforementioned patterns for PVA MB suspension were 590 kPa, 760 kPa, and 1070 kPa, respectively. At the microscale, as a result of a higher volume fraction of cavitation bubbles inside the nozzle, there is no large difference between the cavitation numbers corresponding to cavitating and hydraulic flip flows. Although the percentage of droplets with diameters smaller than 200 μm was roughly the same for both cases of water and PVA MB suspension, the Sauter mean diameter was considerably lower in the case of PVA MBs. Moreover, higher droplet velocities were achieved in the case of PVA MBs at lower injection pressures.
Highlights
The cavitation phenomenon results in phase change inside liquid and formation of several bubbles, and it has significant effects on many processes such as atomization and break-up of the liquid jet.1 Understanding the behavior of cavitating flow regimes in the microscale is a challenging task, and the amount of related data in the literature is not as abundant as that corresponding to macro- or miniscale
The results show that cavitating flows at the microscale have significant effects on atomization characteristics of the spray
In the case of poly(vinyl alcohol) microbubble (PVA MB), the injection pressures corresponding to cavitation inception, supercavitation, and hydraulic flip flows are 590 kPa, 760 kPa, and 1070 kPa, respectively, which shows a decrease of 100 kPa, 275 kPa, and 655 kPa for the same flow regimes in the case of water
Summary
The cavitation phenomenon results in phase change inside liquid and formation of several bubbles, and it has significant effects on many processes such as atomization and break-up of the liquid jet. Understanding the behavior of cavitating flow regimes in the microscale is a challenging task, and the amount of related data in the literature is not as abundant as that corresponding to macro- or miniscale. Suh and Lee investigated the impact of cavitation on diesel fuel atomization in 2D transparent acrylic resin nozzles with different length to width ratios They studied the flow inside the nozzle and the spray morphology under different experimental conditions. In order to intensify the cavitating flows of water inside nozzles and to reduce the required injection pressure for cavitation inception, some studies proposed roughened surfaces and droplets or microbubbles in the working fluid. First, cavitating flows inside a micro-nozzle made of transparent quartz are visualized, and the effect of cavitation on emerging spray is investigated in terms of atomization characteristics and velocity of the departed droplets. The dominant role of supercavitation in atomization properties of the emerging spray is discussed in detail
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