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Pool Boiling
Published in Neil E. Todreas, Mujid S. Kazimi, Nuclear Systems Volume I, 2021
Neil E. Todreas, Mujid S. Kazimi
Returning to an overview of condensation heat transfer, the condensation phenomenon can be categorized into two types. Homogenous condensation, which takes place within the steam body itself and the heterogeneous condensation, where the steam condenses on colder surfaces. Homogeneous condensation requires significant subcooling of the steam to form an embryo of a droplet, a process which is inhibited by both thermodynamics and slow heat transfer within the steam body. Heterogeneous condensation requires less of a temperature difference since an embryo may be formed as adsorbed liquid at the cold surface, a more thermodynamically favorable situation and the heat is efficiently rejected within the cold surface. Heterogeneous condensation can occur either on a solid surface or by direct contact condensation on a liquid surface. On a solid surface, condensation can occur as filmwise condensation when complete wetting occurs or as dropwise condensation when droplets only partially wet the surface, creating fresh areas for contact with steam. As such, in dropwise condensation the rate of condensation heat transfer is higher.
Additives for Gases and Liquids
Published in Ralph L. Webb, Nae-Hyun Kim, Principles of Enhanced Heat Transfer, 2004
The objective of such additives is to promote dropwise condensation. Typically, the heat transfer coefficient for dropwise condensation is much higher than that for filmwise condensation. Additives that establish the existence of dropwise condensation are called dropwise condensation “promoters.” Griffith [1985] surveys the technology of dropwise condensation and discusses various promoters that have been found for steam (water) condensation. A satisfactory promoter must cause the surface to become hydrophobic to the condensate. This means that the contact angle of the condensate must be increased to approach 90°. Hence, the condensate will not spread on the surface but will exist as discrete droplets. The droplets agglomerate and run off the surface. This is possible for high-surface-tension fluids, such as water. However, low-surface-tension fluids (e.g., organics, alcohols, and refrigerants) have such a low surface tension, that no promoter has been found that will make the surface hydrophobic to the condensate.
Response of a Containment Building to a Reactor LOCA
Published in Robert E. Masterson, Nuclear Reactor Thermal Hydraulics, 2019
In classical heat transfer, one learns that condensation is an extremely effective way to transfer and remove heat. Condensation occurs when the temperature of a vapor is reduced below its saturation temperature TSAT. Most of the time, this is done by bringing the vapor into contact with a solid surface whose wall temperature TWALL is below the saturation temperature TSAT of the vapor. In reactor work, two distinct forms of condensation occur. The first form is called film condensation, and the second form is called dropwise condensation. In film condensation, the condensate wets the surface and forms a liquid film that slides down the surface under the influence of gravity alone. This film acts as a thermal boundary layer that transfers heat from the condensate to the cooler surface. The thickness of the boundary layer increases in the direction of flow as more vapor condenses on the film. In dropwise condensation, the vapor that is condensing forms small droplets instead of a continuous film. The surface eventually becomes covered by these droplets in various sizes and shapes. Most of these droplets are spherical in shape, and their sizes can be described by a statistical probability distribution. Eventually, these droplets become so large that they slide off of the surface if it is vertical, and this may cause a small pool of water to form at the base of the surface. These processes are compared in Figure 34.11. Now let us examine the flow of heat from the vapor to the walls of the containment building when this occurs.
Lattice Boltzmann simulation of wetting gradient accelerating droplets merging and shedding on a circumferential surface
Published in Engineering Applications of Computational Fluid Mechanics, 2022
Lu Chen, Ming Gao, Jia Liang, Dongmin Wang, Liang Hao, Lixin Zhang
In the field of condensation heat transfer, the heat transfer effect of dropwise condensation is much stronger than that of film condensation. In order to maintain the dropwise condensation, it is necessary to remove the condensate in time. The traditional way mostly relies on gravity to remove the droplets, and the wetting gradient surface can provide a new idea for accelerating the drainage process. At present, there are many kinds of research on directional droplet movement on a horizontal surface with a wetting gradient, but there are few studies on the non-horizontal surface. There are many studies on merging double droplets, but most researchers focus on uniformly wetted surfaces. In this paper, a wetting-gradient surface is introduced to study the process of accelerating the merging and shedding of two droplets on the circumferential surface, the influence of radius and radius ratio on the shedding process of two droplets is investigated, and the velocity field of droplets sliding downward and shedding process is analyzed. In the simulation of this paper, applying a wetting gradient on the circumferential surface is a major difficulty encountered in this study. Finally, we divide the circumferential surface into two parts with left and right symmetry, and each part is divided into 12 sections with continuously changing wettability to form a wetting gradient.
Experimental Investigation of Condensation and Freezing Phenomenon on Hydrophilic and Hydrophobic Titanium Nanopillared Glass Surfaces
Published in Heat Transfer Engineering, 2021
Mohammad Rejaul Haque, Chen Zhu, Chuang Qu, Edward C. Kinzel, Amy Rachel Betz
Droplet dynamics changed significantly for the hydrophobic Ti nanopillared glass surface as shown in Figure 8. This portion of the testing was carried out under laboratory condition due to the unavailability of the humidity-controlled chamber. However, due to higher nucleation energy barrier imposed by the hydrophobic coating, a lower nucleation rate of droplet (Video 4) was observed for the coating as shown in Figure 8b. Less coalescence and lower area coverage was observed. Lower average diameter and lower droplet density were observed for the coated surface, as shown in Figure 9. At t= 15 s, spherical shaped droplet nucleated. It has been observed that dropwise condensation exhibits higher heat transfer improvement compared to filmwise condensation [36]. The smaller droplets also exhibit less thermal resistance to heat transfer as depicted in Equation (4)
Numerical analysis of meniscus dynamics in monolayer-wick dropwise condensation
Published in Numerical Heat Transfer, Part A: Applications, 2019
S. Modak, M. Kaviany, S. Hoenig, R. Bonner
Dropwise condensation is manifested by the droplet-size distribution on the surface and the microstructure of the surface. Studies suggest that even though individual droplets experience growth and coalescence, the droplet distribution on a surface can be assumed to be static [1]. Bonner [2] developed droplet distribution correlations using the Le Fevre and Rose framework [3], for water and organic fluids, and examined the effect of the surface inclination with respect to the gravity. The dropwise condensation demonstrates higher heat transfer coefficient compared to filmwise condensation, due to the low thermal resistance of the smaller droplets [4]. Mikic [5] suggests that droplets with diameter > 0.2 mm can be considered as insulators, while the smaller ones can be assumed to be responsible for all the heat transfer though the surface. Laboratory trials have shown that hydrophobic coatings enhance dropwise condensation by reducing the wettability of the surface, however, efforts to develop durable coatings have had limited success [1].