Basics of evaporation
From Thermal-FluidsPedia
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Evaporation processes generally occur from liquid films, drops, and jets. Films may flow on a heated or adiabatic surface as a result of gravity or vapor shear. Drops may evaporate from a heated substrate, or they may be suspended in a gas mixture or immiscible fluid. Jets may be cylindrical in shape or elongated (ribbon-like).
In film evaporators, a liquid film is evaporated in order to: (1) cool the surface on which it flows, (2) cool the liquid itself, or (3) increase the concentration of some component(s) in the liquid. Heat may be transferred to the film surface by conduction or convection. The design of film evaporators is most frequently constrained by the extent of the liquid superheat, since nucleate boiling must be prevented. Nucleate boiling can lead to product deterioration because elevated temperatures increase chemical reaction rates. Also, the accumulation of deposits on walls (fouling) as a result of nucleate boiling hinders heat transfer. To avoid nucleate boiling, liquid superheating should not exceed 3 to 40 K, depending on the liquid. This chapter deals with evaporation with no nucleate boiling.
In addition to the capability for low liquid superheating, film evaporators may be designed to achieve short residence time (that is, contact time between the liquid and the evaporator wall), which further reduces the tendency for chemical reactions to occur. The tendency for fouling is also reduced in more rapidly flowing films. These features are ideal for heat-sensitive fluids such as those in food processing and polymer devolatilization (Alhusseini et al., 1998).
A thin liquid film in an evaporator can be produced by several arrangements, two of which are falling and climbing films (See Fig. 1). In falling film evaporators, the liquid is evenly distributed around the opening of a vertical channel – usually a tube – and falls under gravitational force as its surface evaporates to a vapor or gas mixture. If the vapor is drawn to the bottom, the flow is cocurrent as shown in Fig. 1(a) and the shear stress of the vapor on the liquid film increases its speed downward. When the vapor is drawn to the top, the flow is countercurrent as shown in Fig. 1(b). A climbing film evaporator Fig. 1(c) is designed so that liquid is fed into the bottom, and vapor shear pushes the liquid film along the wall to the top. In all of these cases, the lack of a hydrostatic pressure drop eliminates the corresponding temperature drop along the evaporator length, thereby increasing the uniformity in temperature. A more complicated example, shown in Fig. 1 (d), is a two-phase closed thermosyphon where evaporation takes place in the evaporator section and liquid and vapor flow in opposite directions.
The residence time of a liquid in a falling film evaporator may be increased with a countercurrent configuration. If this configuration is chosen, however, the likelihood of entrainment, flooding, or liquid blocking should be considered. Unlike the cocurrent configuration, countercurrent flow is more prone to entrainment, which occurs when liquid is carried upward by the vapor. Liquid blocking – when liquid bridges the channel and blocks the vapor path – can also be more likely in a countercurrent configuration.
In addition to the film evaporation described above, evaporations from liquid droplets attached to a heated wall or surrounded by hot gas can also find their application in irrigation of crops, firefighting, and combustion. Figure 2 shows evaporation from a liquid droplet attached to a heated wall; its application can be found in surface spray cooling, where liquid droplets are sprayed onto a hot surface. Heat is conducted through the droplet to the interface, where an abrupt temperature drop takes place due to evaporation. The mass fraction of the vapor component in the gas mixture, ωv, is greatest next to the interface, and by diffusion the mass fraction decreases to its bulk level with increasing distance from the wall. The size of the liquid drop and the temperature distribution at two different times (t2 > t1) are shown in Fig. 2. As time goes on, the liquid droplet becomes smaller (as indicated by the dashed-line), while the temperature at the heated wall and interface remain unchanged.
When a liquid droplet is surrounded by a hot gas mixture, evaporation takes place on the surface of the droplet, as shown in Figure 3. Such processes are used in bulk cooling as well as spray fuel injection in diesel engines. The time required for such a droplet to completely evaporate as a member of a dispersed phase will determine the overall heat transfer (cooling effect). The partial pressure, pv, and the mass fraction, ωv, of the vapor in the gas mixture are the greatest adjacent to the surface of the droplet, and diffusion causes the partial pressure and mass fraction to decrease with increasing distance from the interface. The temperature profile in the drop is dominated by transient conduction, but convection takes place outside the drop. Since the evaporating liquid is directly supplied by the liquid droplet, evaporation on the drop surface is dominated by heat transfer, not by mass diffusion.
References
Alhusseini, K.A., Tuzla, K., and Chen, J.C., 1998, “Falling Film Evaporation of Single Component Liquids,” ASME Journal of Heat Transfer, Vol. 41, pp. 1623-1632.
Faghri, A., and Zhang, Y., 2006, Transport Phenomena in Multiphase Systems, Burlington, MA.
Faghri, A., Zhang, Y., and Howell, J. R., 2010, Advanced Heat and Mass Transfer, Global Digital Press, Columbia, MO.