Boiling Mechanisms

In considering the effect of mass transfer on the boiling of a multicomponent mixture, both the boiling mechanism and the driving force for transport must be examined (17—20). Moreover, the process is strongly influenced by the effects of convective flow on the boundary layer. In Reference 20 both effects have been taken into consideration to obtain a general correlation based on mechanistic reasoning that fits all available data within 15%. 

The boiling mechanism can conveniently be divided into macroscopic and microscopic mechanisms. The macroscopic mechanism is associated with the heat transfer affected by the bulk movement of the vapor and Hquid. The microscopic mechanism is that involved in the nucleation, growth, and departure of gas bubbles from the vaporization site. Both of these mechanistic steps are affected by mass transfer. 

In the macroscopic heat-transfer term of equation 9, the first group in brackets represents the usual Dittus-Boelter equation for heat-transfer coefficients. The second bracket is the ratio of frictional pressure drop per unit length for two-phase flow to that for Hquid phase alone. The Prandd-number function is an empirical correction term. The final bracket is the ratio of the binary macroscopic heat-transfer coefficient to the heat-transfer coefficient that would be calculated for a pure fluid with properties identical to those of the fluid mixture. This term is built on the postulate that mass transfer does not affect the boiling mechanism itself but does affect the driving force. 

Essentially, except for once-through boilers, steam generation primarily involves two-phase nucleate boiling and convective boiling mechanisms (see Section 1.1). Any deposition at the heat transfer surfaces may disturb the thermal gradient resulting from the initial conduction of heat from the metal surface to the adjacent layer of slower and more laminar flow, inner-wall water and on to the higher velocity and more turbulent flow bulk water. 

Landau LD, Lifshitz EM (1959) Fluid mechanics, 2nd edn. Pergamon, London Landerman CS (1994) Micro-channel flow boiling mechanisms leading to Burnout. J Heat Transfer Electron Syst ASME HTD-292 124-136 Levich VG (1962) Physicochemical hydrodynamics. Prentice HaU, London Morijama K, Inoue A (1992) The thermohydraulic characteristics of two-phase flow in extremely narrow channels (the frictional pressure drop and heat transfer of boiling two-phase flow, analytical model). Heat Transfer Jpn Res 21 838-856... 

To give a qualitative description of various boiling mechanisms and facilitate the empirical correlation of data, it is necessary to employ dimensional analysis. 

Commonly used nondimensional groups. The commonly used nondimen-sional groups in boiling heat transfer and two-phase flow are summarized as follows. Some are used more frequently than others, but all represent the boiling mechanisms in some fashion. 

These results are surprising in that a reduction in U is normally obtained with a decrease in boiling temperature. On this basis U3 is high, which may indicate a change in boiling mechanism although A T3 is reasonable. Even more important is the very low value of U in effect 4. This must surely indicate that part of the area is inoperative, possibly due to the deposition of crystals from the highly concentrated liquor. 

The transition from nucleate boiling to forced-convection heat transfer is gradual. As fluid velocity increases, nucleation becomes more difficult, and therefore this boiling mechanism makes up a decreasing part of the total heat transfer. The transition will occur at higher velocities as pressure increases, and because the velocity at a constant quality decreases with pressure, a higher quality will also be required to suppress nucleation. 

Fig. 8.1 Schematic representation of the devolatilization process. The hatched area represents the polymer melt being devolatilized, which is almost always subject to laminar flow. The bubbles shown are created by the boiling mechanism and by entrapped vapors dragged into the flowing/ circulating melt by moving surfaces.

The objective is to reduce volatiles to below 50-100-ppm levels. In most devolatilization equipment, the solution is exposed to a vacuum, the level of which sets the thermodynamic upper limit of separation. The vacuum is generally high enough to superheat the solution and foam it. Foaming is essentially a boiling mechanism. In this case, the mechanism involves a series of steps creation of a vapor phase by nucleation, bubble growth, bubble coalescence and breakup, and bubble rupture. At a very low concentration of volatiles, foaming may not take place, and removal of volatiles would proceed via a diffusion-controlled mechanism to a liquid-vapor macroscopic interface enhanced by laminar flow-induced repeated surface renewals, which can also cause entrapment of vapor bubbles. 

The foaming-boiling mechanism described previously may be characteristic not only to polymeric melts, but it may also be the inherent boiling mechanism of viscoelastic liquids in general. 

Clearly, this mechanism is more complex than ordinary boiling mechanisms, and any theoretical formulation of devolatilization must take into account this complexity. An initial attempt to formulate semiquantitative elements of this mechanism was made by Albalak et al. (41). They proposed that once a nucleus of a macrobubble is created and the bubble begins to grow, the stretched inner surface of the bubble enhances the rate of nucleation just beneath the soft surface, thus generating new blisters, as shown schematically in Fig. 8.20. 

Boiling Mechanisms Vaporization of liquids may result from various mechanisms of heat transfer, singly or combinations thereof. For example, vaporization may occur as a result of heat absorbed, by radiation and convection, at the surface of a pool of liquid or as a result of heat absorbed by natural convection from a hot wall beneath the disengaging surface, in which case the vaporization takes place when the superheated liquid reaches the pool surface. Vaporization also occurs from falling films (the reverse of condensation) or from the flashing of liquids superheated by forced convection under pressure. 

Boiling Mechanisms Vaporization of liquids may result from various mechanisms of heat transfer, singly or combinations thereof... 

For an extended review of experimental work on mini and microchannels, the reader is refered to the Thome (2004) and Kandlikar (2002) papers. This brief review covers a representative selection of heat transfer studies in minichannels and its aim is to illustrate the tendencies observed in the presented data. Recently Kandlikar (2004) developed a new general correlation adapted to minichannels which gives very good results for low qualities but fails to take dry-out into account, as noted by the author in question. Lately Thome et al. (2004) and Dupont et al. (2004) proposed a semi-empirical three zone model which is the only published work to predict the unique trends observed in minichannels. In this model the dominant boiling mechanism is the evaporation of the liquid film pressed under confined bubbles. 

For i)h = 0.77 mm q is always proportional to - 7sat and a is independent of lii. Since Co is greater than 0.5, microscale boiling should prevail and according to the three zone model of Thome et al. (2004) film evaporation would be the boiling mechanism occurring in this tube. 

To explain why the boiling number seems to govern the transition between heat flux increasing a and vapour quality increasing a, the following interpretation is proposed, based on macroscale boiling mechanisms. From the Rohsenow (1952) and Kew and Cornwell (1997) analysis, an inertial characteristic time "Ccv for the liquid layer and a characteristic time Xb for bubbles leaving the wall can be defined. Then, from the Kutaleladze (1981) and Rohsenow (1952) analysis it can be shown that the ratio of these two characteristic times can be written ... 

The effect of confinement on the heat transfer coefficient before dry-out was found to be an increase of 74% when the hydraulic diameter decreased from 2 to 0.77 mm. The effect of confinement on dry-out was found to be a decrease in the critical quality from 0.3-0.4 to 0.1-0.2 for the same reduction of the hydraulic diameter. Heat flux dependent boiling prevailed in the 2 mm hydraulic diameter tube while quality dependent boiling prevailed in the 0.77 hydraulic diameter tube because of the difference in boiling and confinement numbers. The transition from one regime to another occurred for Bo - (1 - x) si 2.2-10 regardless of the heat and mass velocity. Moreover it was found that dry-out could even be the dominant boiling mechanism at low qualities. The results obtained with the 2 mm hydraulic diameter tube were in total agreement with Huo et al. (2004) s work. Finally frictional pressure losses seem to dominate up to mass velocities of 469 kg/m s. 

The character of the bubbles (i.e. shape and size) is also likely to be affected by the presence of the deposit. For instance the so-called wick boiling mechanism mentioned earlier, is likely to play an important role in the heat transfer process. The evaporation may be considered to take place at the bottom of the steam chimney or on the walls of that channel. If the steam chimneys are absent as might be the case with small pore size or without interconnecting chaimels, heat transfer is only possible by conduction. It could also be possible to consider that the liquid film was directly on the heating surface. Mass transfer rather than heat transfer might also form the basis of a mathematical model. 

With kettle reboilers, the liquid level at the coltunn base is set by the reboiler liquid level plus the head for overcoming reboiler circuit friction. The boiling mechanism is pool boiling with some convective effects. Kettle reboilers normally operate with high ( s 80 percent) fractional vaporization and are therefore prone to fouling. 

Figure 4.8-1. Boiling mechanisms for water at atmospheric pressure, heat flux us.

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