Dynamics and heat transfer of a hollow droplet impact on a wetted solid surface Dashu Li a , Xili Duan b,⇑ , Zhiwei Zheng c , Yusen Liu d a CNOOC Research Institute, Beijing 100028, China b Memorial University of Newfoundland, St. John’s, NL A1B 3X5, Canada c China Agricultural University, Beijing 100083, China d University of Shanghai for Science and Technology, Shanghai 200093 China article info Article history: Received 27 October 2017 Received in revised form 5 February 2018 Accepted 5 February 2018 Keywords: Hollow droplet impact Dynamics Heat transfer Coupled level set and volume of fluid (CLSVOF) abstract A numerical model based on the coupled level set and volume of fraction (CLSVOF) method is developed to investigate dynamics of a hollow droplet impacting on a wetted solid surface and the associated heat transfer process. Numerical results show different impact behaviors of the hollow droplet as compared to a continuous dense droplet. One of the distinctive flow features of the hollow droplet is a central counter jet, which is not present in continuous dense droplets. Analysis of pressure distribution indicates that the central counter jet is mainly due to the pressure gradient inside the hollow droplet during the impact and spreading. Simulation results also show that the impact velocity of the hollow droplet determines its spread factor, dimensionless spreading edge-jet height and central counter-jet height, all with positive relations. The complicated transient heat transfer between the impacting droplet and the surface was analyzed. The average heat flux increases with higher impact velocity. Overall heat transfer during hollow droplet impact is found to be lower than that during continuous dense droplet impact. These results pro- vide better understanding of hollow droplet impingement and heat transfer on wetted surfaces. Ó 2018 Elsevier Ltd. All rights reserved. 1. Introduction Droplet impact on a solid surface is a phenomenon widely appearing in both nature and industry. Examples include raining and icing on surfaces, dynamics of liquid fuels in combustion sys- tems, ink-jet printing, and material casting [1,2]. Understanding droplet dynamics and associated heat transfer in the impingement process helps better design of these systems for higher efficiency and reliability. Great research progress has been made in this area. Depending on different impacting targets, droplet impact can be categorized into three types: (1) dry-surface impact, (2) wetted- surface impact and (c) liquid-pool impact. The underlying mecha- nisms of these processes are fundamentally different. Droplet impact on a wetted surface involves far more complex droplet- liquid-solid interactions than the other two types of impacts [3,4]. Many experimental studies have been conducted to understand dynamics of liquid droplet impacting on dry or wetted surfaces. Rioboo et al. [5] observed six different conditions of droplet impact: deposition, prompt splash, corona splash, receding break-up, partial rebound, and complete rebound and later sum- marized four stages of a droplet impact process, i.e., kinematic phase, spreading phase, relaxation phase and equilibrium phase [6]. Based on different impact conditions, liquid properties, and surface wettability, to experimentally analyze droplet dynamics, high speed camera is commonly used to capture detailed images during droplet impact and spreading process [7–10], while the dro- plet impact speed can be controlled by the height of droplet gener- ation system. Guo et al. [8] applied high speed imaging to study the flow phenomenon such as spray, splash and especially the interest- ing bell-like spray that appears after a droplet impacts on the liq- uid film. They investigated the relationships between the flow phenomenon and diameter and speed of the droplet, as well as the effects of viscosity, surface tension and thickness of the film. Liang [9] conducted a review on mass and momentum interactions during droplet impact on a liquid film and described different behaviors of droplet impact including spread, ejecta sheet, crown sheet and splash. Recently Pan et al. [10] used a high speed camera to study water droplet dynamics on stainless surfaces with differ- ent wettability conditions. With a statistical design of experiments method, they investigated the effects of surface temperature, impact speed, droplet temperature and surface wettability. It was found that the impact speed and surface wettability are significant https://doi.org/10.1016/j.ijheatmasstransfer.2018.02.017 0017-9310/Ó 2018 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: xduan@mun.ca (X. Duan). International Journal of Heat and Mass Transfer 122 (2018) 1014–1023 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt