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Nahme-Griffith number, Nakashima, CY Nanoparticles, for heat transfer augmentation, Naphthalene: Napthenes: National practice, in mechanical design, guide to, Natural convection: Natural draft cooling towers: Natural frequency of tube vibration in heat exchangers, Navier-Stokes equation, Neon: Neopentane: Net free area, in double-pipe heat exchangers, Netherlands, guide to national mechanical design practice, Networks, of heat exchangers, pinch analysis method for design of, Neumann boundary conditions, finite difference method, Nickel, thermal and mechanical properties Nickel alloys, Nickel steels, Niessen, R, Nitric oxide: Nitriles: Nitrobenzene: Nitro derivatives: Nitroethane: Nitrogen: Nitrogen dioxide: Nitrogen peroxide: Nitromethane: m-Nitrotoluene: Nitrous oxide Noise: Nonadecane: Nonadecene: Nonane: Nonene: Nonanol: Nonaqueous fluids, critical heat flux in, Non-circular microchannels: Noncondensables: Nondestructive testing, of heat exchangers Nongray media, interaction phenomena with, Nonmetallic materials: Non-Newtonian flow: Nonparticipating media, radiation interaction in, Nonuniform heat flux, critical heat flux with, Non-wetting surfaces, in condensation augmentation,
Condensation Enhancement
North, C, No-tubes-in-window shells, calculation of heat transfer and pressure drop in, Nozzles: Nowell, D G, Nucleate boiling: Nuclear industry, fouling problems in, Nucleation: Nucleation sites: Nuclei, formation in supersaturated vapor, Number of transfer units (NTU): Numerical methods: Nusselt: Nusselt-Graetz problem, in laminar heat transfer in ducts, Nusselt number:
O P Q R S T U V W X Y Z
N Non-wetting surfaces, in condensation augmentation,
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Condensation Enhancement

DOI 10.1615/hedhme.a.000189

2.6 CONDENSATION
2.6.6 Condensation Enhancement

Ralph L. Webb

A. Introduction

Condensation will occur on a surface whose temperature is below the vapor saturation temperature. The condensed liquid formed on the surface will exist either as a wetted film or in droplets. The condensate forms as droplets on the surface, if the condensate does not wet the surface. Although dropwise condensation yields a very high heat transfer coefficient, it cannot be permanently sustained. Dropwise condensation (see Section 188) may be promoted by liquid additives or surface coatings that inhibit surface wetting. As the surface slowly oxidizes, the surface will eventually become wetted, and the process will revert to filmwise condensation (see Section 185). Hence, filmwise condensation is currently the more important process.

This section is concerned with enhancement of condensation. Geometries include plates and tubes (horizontal and vertical). Condensation may occur either inside or outside the tube. The condensation coefficient will be increased by surface or body forces, which act on the condensate film and reduce its thickness. Without special "enhancement" effects the film thickness on a stationary surface is influenced by gravity and interfacial shear stresses. Depending on the surface orientation, interfacial shear forces may aid or impede the gravity force.

The technology of enhancement of film condensation involves the following basic phenomena: (1) Additional surface forces, such as surface tension, to locally thin the film, (2) Additional body forces, such as electric fields or centrifugal force to pull the condensate off the surface, (3) Surface roughness tomix the condensate film. The effectiveness of these possible methods depends on the magnitude and direction of the imposed force, relative to the existing interfacial shear and gravity forces. The surface orientation and vapor velocity have a significant effect on the importance of the interfacial shear and gravity forces, respectively. Because the surface orientation and the number of forces that may act on the condensate film will affect the condensation coefficient, it is appropriate to segregate the discussion of enhancement into sub-sections, which depend on the surface orientation, vapor velocity, and the imposed enhancement techniques.

Because interfacial shear force may significantly alter the condensation coefficient, we will first address enhancement "without vapor shear" effects. Then, the survey will be concluded by geometries for which significant vapor shear effects exist. Vapor shear effects are important for both condensation inside tubes and on tube bundles.

1.4, = 4.742 and = 0.0. Again, this empirical correlation does not account for probable surface tension drainage effects or fin efficiency. However, it is probably the most general of those presented. The correlation predicted 71% of the data points within ± 30%.

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