Convection evaporative

Under purely convective evaporation from gas phase heating, the liquid surface temperature... 

When continuous liquid jets are employed, the liquid film covering the surface is continuous and the heat is removed mainly by convection. Evaporation from the thin film may occur at high heat fluxes or low flow rates. 

Evaporation of moisture within a solid and gas transport out of the solid by diffusion and/or convection. Evaporation can occur within a solid if it is boiling or porous. Subsequently vapor must move out of the sample. 

Heat transfer in nucleate boiling and convective evaporation... 

The region of saturated boiling is followed by that of convective evaporation. With the increasing vapour content the heat transfer from the wall to the fluid improves. The thermal resistance of the boundary layer decreases in comparison to the thermal resistances in nucleate boiling. Likewise, the wall temperature drops, cf. Fig. 4.53, so that only a few or no bubbles are formed at the wall. The heat transfer is predominantly or exclusively determined by evaporation at the phase boundary between the liquid at the wall and the vapour in the core flow. 

Of the many different methods for calculating heat transfer, that from Chen [4.92] will be discussed here, as it was established with a model that is plausible in physical terms. It also has the advantage that it is not only valid for convective evaporation but also for saturated boiling. Similarly to saturated boiling, it is assumed that the heat transfer coefficient is a combination of two parts which are... 

For convective evaporation in the annular space between two tubes heated from only one side, an equivalent diameter, de, has to be introduced. It is given by... 

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]. 

Kandlikar and Balasubramanian [24] extended the correlation proposed by Kandlikar for conventional tubes, where the local two-phase heat transfer coefficient was determined according to the value of the dominant mechanism between nucleate boiling (nb) and convective evaporation (cv) ... 

Correlations for Pure Convection. It is usually assumed that if the heat transfer coefficient is independent of heat flux, the mode of heat transfer is pure forced convection (see the preceding section on mechanisms). Earlier correlations for forced convective evaporative heat transfer had the form... 

General Empirical Correlations. The correlations described above were either fluid specific or related to correlations for forced convection evaporation and/or pool nucleate boiling, respectively. An alternative approach is to develop correlations based only on the data for forced convective boiling. The most widely used correlation of this form is that of Shah [265], who correlated data for convective flow boiling in both vertical and horizontal pipes in the form... 

At high enough qualities and mass fluxes, however, it would be expected that the nucleate boiling would be suppressed and the heat transfer would be by forced convection, analogous to that for the evaporation for pure fluids. Shock [282] considered heat and mass transfer in annular flow evaporation of ethanol water mixtures in a vertical tube. He obtained numerical solutions of the turbulent transport equations and carried out calculations with mass transfer resistance calculated in both phases and with mass transfer resistance omitted in one or both phases. The results for interfacial concentration as a function of distance are illustrated in Fig. 15.112. These results show that the liquid phase mass transfer resistance is likely to be small and that the main resistance is in the vapor phase. A similar conclusion was reached in recent work by Zhang et al. [283] these latter authors show that mass transfer effects would not have a large effect on forced convective evaporation, particularly if account is taken of the enhancement of the gas mass transfer coefficient as a result of interfacial waves. 

S. M. MacBain and A. E. Bergles, Heat Transfer and Pressure Drop Characteristics of Forced Convection Evaporation in Deep Spirally Fluted Tubing, in Convective Flow Boiling, J. C. Chen ed., pp. 143-148, Taylor Francis, Washington, DC, 1996. 

In terms of process technology the most easily realized evaporators are the natural-convection evaporator with heating elements and the thin-film evaporator with rotating wiper blades. When the exhaust vapors of a wiped thin-film evaporator are withdrawn at the bottom, for small heights, a falling-film evaporator can be simulated (Figure 2.3.1-6). Exceptions are cases, especially with evaporators, in which material-, temperature-, and residence-time-dependent changes in the product are ex-... 

Over phase 4, the temperature changes are obtained by evaluating the sum of convective, evaporative, and radiative heat losses. 

The design of natural convection evaporators is difficult because a complex interrelationship between the liquid circulation rate due to density differences and heat transfer coefficients exists. The circulation flow rate depends on the amount of evaporated liquid and is not controlled by an external device as in forced circula-... 

Fig. 7.4-2 Possible temperature profiles of the liquid in the tubes of a natural convection evaporator...

J. Bellan, K. Harstad Analysis of the convective evaporation of non-dilute clusters of drops, Int. J. Heat Mass TransfCT, 30, 125-136 (1987). 

Directing Convection Marangoni Convection/ Evaporation/Self- assembly Coffee rings, polygonal network structures, fingering instabilities, cracks, chevron patterns, etc. Nanometer to micrometer Marangoni convection and stick-slip motion can determine the final patterns observed [132-134]... 

Thus, clothing should ensure appropriate heat and mass transfers between the human body and its environment in order to maintain die physiological thermal balance of the wearer. To obtain this thermal balance, heat loss needs to balance heat production (metabolic rate). Fig. 19.1 shows that for body heat loss, several pathways are available, ie, external work, conduction, radiation, convection, evaporation, and... 

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