Sensible Heat Transfer during Droplet Cooling:Experimental and Numerical Analysis

This study presents the numerical reproduction of the entire surface temperature field resulting from a water droplet spreading on a heated surface, which is compared with experimental data. High-speed infrared thermography of the back side of the surface and high-speed images of the side view of the impinging droplet were used to infer on the solid surface temperature field and on droplet dynamics. Numerical reproduction of the phenomena was performed using OpenFOAM CFD toolbox. An enhanced volume of fluid (VOF) model was further modified for this purpose. The proposed modifications include the coupling of temperature fields between the fluid and the solid regions, to account for transient heat conduction within the solid. The results evidence an extremely good agreement between the temporal evolution of the measured and simulated spreading factors of the considered droplet impacts. The numerical and experimental dimensionless surface temperature profiles within the solid surface and along the droplet radius, were also in good agreement. Most of the differences were within the experimental measurements uncertainty. The numerical results allowed relating the solid surface temperature profiles with the fluid flow. During spreading, liquid recirculation within the rim, leads to the appearance of different regions of heat transfer that can be correlated with the vorticity field within the droplet

Effect of wettability on nucleate boiling

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.Heat transfer enhancement at liquid-solid interfaces is often achieved by modifying the surface properties. However, deep efforts to describe the actual role of surface modification only started at the 1980’s and much work has left undone since then. The wettability is a key parameter governing heat, mass and momentum transport at liquid-solid interfaces. However it is usually quantified using macroscopic quantities, which cannot be related with the micro-and-nano-scale phenomena occurring at the interface. In this context, the present paper revises the potential and limitations of using macroscopic apparent contact angles to predict the wetting regimes. Then, these angles are used to relate the wetting regimes with bubble dynamics and heat transfer processes occurring at pool boiling. The results show that the macroscopic angles are useful to establish general trends and differentiate bubble dynamics behaviour occurring for opposite wetting regimes. However, milder wetting changes occurring within each regime, caused, for instance, by surface topography are not well captured by the apparent angle, as the surface topography is not scaled to affect this macroscopic angle, although it can clearly influence the bubble formation and departure mechanisms and, consequently the heat transfer coefficients. In line with this, the concept of the micro-scale contact angle, as introduced by Phan et al. [1] is used here together with a geometrical parameter to include the effect of surface topography, to describe the role of the wettability on bubble dynamics. Based on this analysis, a multi-scale approach is proposed to include the role of wettability on correlations predicting the pool boiling heat transfer coefficients.am201

Experimental and Numerical Study on Sensible Heat Transfer at Droplet/Wall Interactions

[EN] 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.The authors are grateful to Fundação para a Ciência e Tecnologia (FCT) for partially financing the research under the framework of the project RECI/EMS-SIS/0147/2012 and for supporting P. Pontes with a research fellowship. A. S. Moita acknowledges FCT for financing her contract through the IF 2015recruitment program (IF 00810-2015) and E. acknowledges FCT for supporting his PhD fellowship (SFRH/BD/88102/2012).Teodori, E.; Pontes, P.; Moita, A.; Moreira, A.; Georgoulas, A.; Marengo, M. (2017). Experimental and numerical study on sensible heat transfer at droplet/wall interactions. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 304-311. https://doi.org/10.4995/ILASS2017.2017.5024OCS30431

Effect of extreme wetting scenarios on pool boiling conditions in a quiescent medium

This study focuses on the detailed description of the heat transfer and bubble dynamics processes occurring for the boiling of water on surfaces with extreme wetting regimes, namely hydrophilicity and superhydrophobicity. The wettability is changed by modifying the surface chemistry and without significant variations in the mean surface roughness. Under these conditions and for the range studied here the effect of the extreme wetting regimes was dominant, thus the influence of surface topography was not addressed. A particular trend is observed for the boiling curve obtained with the superhydrophobic surfaces, as the heat flux increases almost linearly with the superheat, although with a much lower slope than the hydrophilic surfaces. This occurs due to the formation of a large stable vapour film over the entire surface just at around 1 K superheat, because of the almost immediate coalescence of the bubbles generated on the surface. This behaviour agrees with the so-called “quasi-Leidenfrost” regime recently reported in the literature and with a theoretical prediction of the heat flux that is presented in this study. Furthermore, a comprehensive analysis of bubble dynamics, useful for comparison with numerical simulations is given. Such analysis is based on the temporal evolution of the bubble diameter together with bubble contact angle and with the velocity of the contact line. The results suggest that the existing models and correlations can predict the trends of the bubble growth using a modified contact angle value, called the bubble contact angle (or its supplemental value), for the hydrophilic surfaces, even if they cannot accurately predict bubble sizes. Approximating the modified contact angle with the quasi-static contact angle, obtained during surface characterization is practical for a qualitative evaluation, but the results obtained here do not support for its use when estimating the bubble departure diameter. On superhydrophobic surfaces, the effect of the vapour film must be considered, since although this is not the starting point of the boiling process, it represents the actual working conditions when using this kind of surfaces

Evaluation of pool boiling heat transfer over micro-structured surfaces by combining high-speed visualization and PIV measurements

The present work introduces an alternative approach which combines heat transfer measurements, high speed visualization and PIV to infer on the effect of surface micro-structuring in the various heat transfer parcels. The PIV provides particularly interesting information on the bubbles vertical velocity, allowing to infer how the microstructures affect the bulk induced flow. The micro-patterns are composed by arrays of cavities with fixed shape and depth, only varying the distance between cavities, S. The results confirm the importance of enhancing the pool boiling performance, by promoting the increase of the bubble frequency and active nucleation sites density with micro-patterned surfaces. The role of the patterns is different, depending on the properties of the fluids. Hence, for liquids with very high latent heat of evaporation and surface tension, the parcel of heat transfer related to the phase change is very important. Based on the force balance on the departing bubbles, it is evident that in this case the micro-patterns will mainly affect the bubble coalescence, which may lead to steep deterioration of the heat transfer coefficient. On the other hand, for liquids with lower thermal properties (and lower surface tension) the micro-patterns play an important role in increasing and stabilizing the vertical velocity of the bubbles, thus favoring a very efficient boiling performance, with high boiling frequencies and large active nucleation sites density. This role of the micro-patterned surfaces is well captured by the representation proposed here, which relates the heat transfer coefficient with the distance between micro-patterns S, made dimensionless by the characteristic dimension (Lc=(?lv/(?l-?v))1/2). This representation actually shows the best performance of the HFE7000 in comparison to that of ethanol, when the micro-patterned surfaces are used, despite the lower thermal properties of HFE7000. This trend was confirmed experimentally in our measurements

Wettability effect on pool boiling: a review

This chapter aims at reviewing and discussing the effects of wettability on pool boiling heat transfer. Despite the studies carried out in the last half of the 20th century, enhancement of pool boiling heat transfer by surface wettability changes is still a very complex task to achieve and even to understand. Numerous studies have been proposed in the literature to improve heat transfer coefficients (HTCs) and/or to delay the occurrence of the critical heat flux (CHF) in pool boiling by increasing the number of nucleation sites, the nucleation rate and/or the rate of bubble detachment from the surface. However, many of them modify simultaneously both the chemical structure of either the liquid or the solid surface and the surface topography, making it difficult to separate the effects of each, and also failing to determine whether there are cross-effects due to interactions between them. The present review covers the wettability effect on pool boiling, from the fundamentals of nucleation and onset of boiling to bubble dynamics, and finally to their subsequent effects on the HTC and CHF. This exercise is then used to understand a variety of strategies developed to enhance pool boiling heat transfer. Theoretical approaches predicting heat fluxes and heat transfer coefficients are also addressed. Main conclusions on the role of wettability on pool boiling heat transfer are finally summarized and further research topics are suggested