Browsing by Author "Lin, Yushu"

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    Droplet Drag Modeling on Spray Conditions
    Lin, Yushu (Virginia Tech, 2024-03-04)
    Numerical approaches have been conducted to investigate the effect of droplet deformation and internal circulation on droplet dynamics. Although droplet drag is a classical area of study, there are still theoretical gaps in understanding the motion of large droplets. In applications such as spray combustion, droplets of various sizes are generated and move with the flow. Large droplets tend to deform in the flow, and they have complex interactions with the flow because of this deformation. To better model spray, the physical understanding of droplets needs to be improved. Under spray conditions, droplets are subjected to a high-temperature-and-pressure environment, and the coupling between liquid and gas is enhanced. Therefore the deformation and internal circulation will affect the droplet drag coefficient more significantly than they would under atmospheric conditions. To study the mechanism of how droplet shape and internal circulation influence droplet dynamics, we have used direct numerical simulation (DNS) to simulate a droplet falling at its terminal velocity in high-pressure air. An in-house code developed for interface-capturing DNS of multiphase flows is employed for the simulation. The drag coefficient is calculated, and the results are consistent with the existing literature for slightly deformed droplets. The results show that the drag coefficient is directly related to the droplet deformation and droplet internal circulation. This paper also develops an analytical theory to account for the effect of the Weber number and fluid properties on droplet deformation.
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    A gravity update scheme using PID controller for droplet traveling at terminal velocity in air flow
    Lin, Yushu; Palmore, John A., Jr. (2021-05-26)
    To improve the combustion efficiency in gas turbines, it will be important to find out positions of droplets and the evaporated fuel vapors in the combustion chamber. In our study, we investigate the drag coefficient of droplets, because drag force will affect droplet dynamics. In practice, simple drag correlations are commonly used in DNS, which are single-parameter dependent on Reynolds number. However, if droplets have complex interactions with the flow, more parameters should be taken into consideration. For example, in gas turbine combustion, it is natural to think that deformation, evaporation and internal circulation of droplets have impact on drag coefficient as well. Therefore, drag coefficient will have a dependence on related parameters (effective diameter, Weber number, density ratio, etc.). To reveal the dependence of drag coefficient on probable parameters, we perform simulations of an evaporating droplet traveling at its terminal velocity in high-temperature air by using the numerical framework developed by our research group. This framework uses an interface-capturing DNS for vaporizing multiphase flows. Since the drag force is dynamically changing, an algorithm which updates the gravity to balance the drag force is developed. We incorporate PID controller in our scheme to mimic the difference between gravity and drag force in a more robust manner compared to our previous work [Setiya and Palmore, ESSCI, 2020]. The resultant gravity is then used to characterize the behavior of drag coefficient.
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    The study of droplet internal circulation and its interaction with droplet deformation
    Lin, Yushu; Palmore, John A., Jr. (2023-11-19)
    The study of liquid droplet is important for applications like spray-painting, fire suppression, and spray combustion. Droplet morphology has a great impact in these applications, for example, in spray conditions, droplets of various sizes are generated from jet atomization, and the large droplets have strong deformation. The highly deformed droplets have very different characteristics compared to spherical droplets, but many studies on droplet dynamics are based on the spherical droplet assumption. To develop a more accurate modeling of liquid droplet in jet simulations, we use numerical approaches to investigate the mechanism of droplet deformation. Weber number, which measures the balance of surface tension and inertia, is a key non-dimensional group that quantifies droplet deformation. However, droplets with same Weber number do not always have an identical shape. For example, our previous work[Lin and Palmore, 2022] demonstrated that internal circulation also influences droplet shape. Therefore, a deeper understanding in droplet internal circulation is needed. In this work, we will explore a wider range of droplet parameters relevant to a wide array of applications for droplets to study the interaction between droplet internal circulation and deformation.