Thin film heat transfer, boiling

In high heat flux (heat transfer rate per unit area) boilers, such as power water tube (WT) boilers, the continued and more rapid convection of a steam bubble-water mixture away from the source of heat (bubbly flow), results in a gradual thinning of the water film at the heat-transfer surface. A point is eventually reached at which most of the flow is principally steam (but still contains entrained water droplets) and surface evaporation occurs. Flow patterns include intermediate flow (churn flow), annular flow, and mist flow (droplet flow). These various steam flow patterns are forms of convective boiling. 

Thin-film and surface renewal evaporators are mostly applied to materials with medium to high viscosity, or to high-boiling contaminated mixtures. A typical thin-film evaporator employs a unique rotor with an array of discrete, plowlike blades attached to the rotor core. The blades transport the viscous concentrate or melt through the evaporator while simultaneously forming films to facilitate heat and mass transfer. 

It was supposed, that for a high vapor velocity and a thin liquid film the influence of gravity is small and the correlation for up flow was used. Total boiling suppression occurs when mass quality more than 0.3 for a film thickness less than 60 pm. That value is close to the bubble departure diameter observed for flow boiling in a film. When the film thickness is smaller than the critical one, the forced convection occurs with a small heat transfer coefficient. The crisis of the heat transfer was observed for a complete liquid evaporation on a heated wall. While the mass quality less than 0.3, we have the cell or slug flow mode, so boiling is not suppressed. 

In confined space, the heat transfer enhancement is caused by evaporation a thin liquid film in near comer area (see Fig. 12) and dry spot formation on the channel wall. The dry spot formation in this area can explain low dependence of heat transfer coefficient on wall superheat [21]. For this case heat flux in vicinity of liquid-solid-vapor contact line has higher level due to evaporation in ultra thin film area [20]. In that way high level of heat transfer in vicinity of contact line is responsible for the heat transfer enhancement during boiling in mini-channels. The possibility of dry spot formation on the wall for water boiling in narrow annular channel was observed in [35] also. At wall superheat over 4.5 K the drying-out of liquid is responsible for decrease of heat transfer when the size of dry area becomes very large. 

The fact that boiling heat transfer at the microscale is governed by the evaporation, hydrodynamics, capillary effects of the thin liquid film, has led to the introduction of some relevant dimensionless numbers. These numbers are used by many authors to analyse the behaviour of boiling heat transfer at the microscale (see Table 2). 

The heat transfer mechanisms that are active in boiling in micro-channels can be summarized as follows (i) in bubbly flow, nucleate boiling and liquid convection would appear to be dominant, (ii) in slug flow, the thin film evaporation of the liquid film trapped between the bubble and the wall and convection to the liquid and vapor slugs between two successive bubbles are the most important heat transfer mechanisms, also in terms of their relative residence times, (iii) in annular flow, laminar or turbulent convective evaporation across the liquid film should be dominant, and (iv) in mist flow, vapor phase heat transfer with droplet impingement will be the primary mode of heat transfer. For those interested, a large number of two-phase videos for micro-channel flows from numerous laboratories can be seen in the e-book of Thome [22]. 

The use of structured surfaces to enhance thin-film evaporation has also been considered recently. Here, in contrast to the flooded-pool experiments noted above, the liquid to be vaporized is sprayed or dripped onto heated horizontal tubes to form a thin film. If the available temperature difference is modest, structured surfaces can be used to promote boiling in the film, thus improving the overall heat transfer coefficient. Chyu et al. [43] found that sintered surfaces yielded nucleate boiling curves similar to those obtained in pool boiling. T-shaped fins did not exhibit low AT boiling however, a threefold convective enhancement was obtained as a result of the increased surface area. 

Close-clearance scrapers for viscous liquids are included in the review by Uhl [253]. An application of scraped-surface heat transfer to air flows is reported by Hagge and Junkhan [256] a tenfold improvement in heat transfer coefficient was reported for laminar flow over a flat plate. Scrapers were also suggested for creating thin evaporating films. Lustenader et al. [257] outline the technique, and Tleimat [258] presents performance data. The heat transfer coefficients are much higher than those observed for pool evaporation (without nucleate boiling). 

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