Abstract
A numerical study is presented to examine the behavior of a single liquid droplet initially passing through air or steam, followed by impingement onto a static or vibrating surface. The fluid dynamic equations are solved using the Volume of Fluid method, which includes both viscous and surface tension effects, and the possibility of droplet evaporation when the impact surface is hot. Initially, dynamic behavior is examined for isothermal impingement of a droplet moving through air, first without and then with boundary vibration. Isothermal simulations are used to establish how droplet rebound conditions and the time interval between initial contact to detachment vary with droplet diameter for droplet impingement onto a stationary boundary. Heat transfer is then assessed for a liquid droplet initially at saturation temperature passing through steam, followed by contact with a hot vibrating boundary, in which droplet evaporation commences. The paper shows that, for droplet impingement onto a static boundary, the minimum impact velocity for rebound reduces linearly with droplet diameter, whereas the time interval between initial contact and detachment appears to increase linearly with droplet diameter. With the introduction of a vibrating surface, the minimum relative impact velocity for isothermal rebound is found to be higher than the minimum impact velocity for static boundary droplet rebound. For impingement onto a hot surface, in which droplet evaporation commences, it is shown that large-amplitude surface vibration reduces heat transfer, whereas low-amplitude high-frequency vibration appears to increase heat transfer.
Highlights
Single-phase liquid spray cooling is attractive for its high heat removal capabilities.Spray evaporative cooling is potentially even more attractive owing to its very high heat removal potential
Getting an understanding of droplet rebound when impinging onto a hot vibrating boundary is of practical importance for spray evaporative cooling
The largest reduction in overall heat transfer could be seen to occur at an amplitude of 10 mm and frequency of 10 Hz, which was a reduction of 3.32% below the stationary case
Summary
Single-phase liquid spray cooling is attractive for its high heat removal capabilities. Another successful dynamic contact angle model is from Blake and Coninck (2002) [11] Their simpler model combined the molecular kinetic theory of wetting, the out-of-balance surface-tension force, and the Frenkel–Eyring activated-rate theory of transport in liquids [12]. Chen et al (2016) [14] numerically simulated droplet impingement dynamics and evaporation using the VOF method deploying the Blake and Coninck contact-angle calculation method [11]. They validated their predictions by comparison with experimentally measured droplet impingement data obtained by Dong et al (2006) [15], showing good agreement. The objectives were to assess the computational feasibility of simulating droplet impingement in terms of accuracy and efficiency
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