Experimental and numerical study on sensible heat transfer at droplet/wall interactions

Abstract

The present study addresses a detailed experimental and numerical investigation on the impact of water droplets on smooth heated surfaces. High-speed infrared thermography is combined with high-speed imaging to couple the heat transfer and fluid dynamic processes occurring at droplet impact. Droplet spreading (e.g. spreading ratio) and detailed surface temperature fields are then evaluated in time and compared with the numerically predicted results. The numerical reproduction of the phenomena was conducted using an enhanced version of a VOFbased solver of OpenFOAM previously developed, which was further modified to account for conjugate heat transfer between the solid and fluid domains, focusing only on the sensible heat removed during droplet spreading. An excellent agreement is observed between the temporal evolution of the experimentally measured and the numerically predicted spreading factors (differences between the experimental and numerical values were always lower than 3.4%). The numerical and experimental dimensionless surface temperature profiles along the droplet radius were also in good agreement, depicting a maximum difference of 0.19. Deeper analysis coupling fluid dynamics and heat transfer processes was also performed, evidencing a strong correlation between maximum and minimum temperature values and heat transfer coefficients with the vorticity fields in the lamella, which lead to particular mixing processes in the boundary layer region. The correlation between the resulted temperature fields and the droplet dynamics was obtained by assuming a relation between the vorticity and the local heat transfer coefficient, in the first fluid cell i.e. near the liquid-solid interface. The two measured fields revealed that local maxima and minima in the vorticity corresponded to spatially shifted local minima and maxima in the heat transfer coefficient, at all stages of the droplet spreading. This was particularly clear in the rim region, which therefore should be considered in future droplet spreading models. KeywordsDroplet impact, smooth heated surface, high-speed infrared thermography, VOF, vorticity. IntroductionUnderstanding the fluid dynamic and heat transfer mechanisms of droplet impact on heated surfaces is relevant for a wide range of applications, from fire sprinklers to cooling applications. A popular solution for microprocessors cooling is based on spray impingement [1-2]. The elementary representation of a spray composed by arrays of single droplets impacting onto a heated surface is not straightforward in many of the aforementioned applications, but the complexity of the observed phenomena relays on the study of single droplet impacts to understand the basic governing processes. Such approximation is not so far from the real systems for microelectronics cooling, which actually deal with single droplets or with very sparse sprays [1,3]. In many of these applications, liquid phase change is promoted to take advantage of the latent heat of evaporation. However, efficient cooling can be ...

A Study on the Dynamic Collision Behaviors of a Hydrous Ethanol Droplet on a Heated Surface

“…Additionally, small jets are observed at the top of the "pagoda-like" vapor bubbles, which facilitate the ejection of daughter droplets. A similar "pagoda-like" bubble structure has also been mentioned in other studies [39,40], and this kind of bubble structure was considered one of the main reasons for secondary atomization under this regime.…”