Due to the bulk liquid flowing over the heating surface, the interaction of the flow regime and the heat transfer mechanism make the flow boiling process become more complex.
Two arrangements are normally considered for flow boiling inside tubes; vertical tubes and horizonal tubes. The boiling characteristics of both orientations are significantly
CHAPTER 3 Heat Transfer in Thin Film Evaporators - Literature Review 23 different, so that different correlations are used for each of them. An extensive review of flow boiling inside tubes was given by Collier ( 198 l a).
There are two important types of heat transfer mechanisms in flow boiling, nucleate boiling and convective interface boiling. However, the proportion of these two mechanisms varies over the length of the heating tubes.
In most (but not all) systems of practical interest, the onset of nucleate boiling is achieved at or just beyond the point where the bulk flow reaches the saturated liquid condition (boiling temperature) (Carey, 1 992).
Thome (1990) pointed out that the wall superheat required to initiate boiling in flow boiling is not the same as that for nucleate pool boiling because the thermal boundary layers created by the respective processes are quite different.
Many correlations have been proposed to estimate the wall superheat required for the onset of nucleate boiling inside tubes (Bergles and Rohsenow, 1 964; Sato and Matsullura, 1 96 1 ; and Frost and Dzakowic, 1967). A widely quoted expression is that of Davis and Anderson ( 1966):
where: !1T = cr
Tsar
qJ
B h fg P v k (3-2)1::.. T B is the temperature difference between the wall and saturation temperature (K),
cr is the surface tension (N/m),
Tsat is the saturation temperature (K), q is the heat flux(W/m2),
htg is the latent heat of vaporization (J/leg), Pv is the vapour density (kg/m3),
The physical properties are evaluated at the saturation conditions corresponding to the local pressure.
3.3 Heat transfer through a thin liquid mm
In general, the heat transfer mechanisms through a thin liquid film could be conduction, convection or nucleate boiling, the proportions of each one are mainly dependent on the heating rate and the liquid flow pattern. The thermophysical properties of the liquid, namely thermal conductivity, viscosity, density, specific heat, latent heat, surface tension and boiling point elevation, and the conditions of the heating surface all may also have an effect on the heat transfer process. As a result of the heat transfer through the liquid film, part of liquid is evaporated. The vapour velocity over the vapour-liquid interface may also affect the liquid fIlm flow pattern and consequently the heat transfer.
Four possible mechanisms for liquid film evaporation are discussed in the literature (Dengler and Addoms, 1956; Chun and Seban, 1 97 1 ; Angeletti and Moresi 1983; and Thome, 1990). These include:
( 1 ) Convective interface boiling, where heat transfer through the liquid film is by conduction and convection, the evaporation occurs only at the vapour-liquid interface of the film. The whole process is mainly governed by liquid "flow conditions and thermophysical properties.
(2) Low rate nucleate boiling, where bubbles form and break but do not affect heat transfer greatly. Observations show that in this case bubbles move with the fIlm without reaching the surface.
(3) High rate nucleate boiling, where the presence of the bubbles significantly affects liquid flow pattern and where the bubbles follow one another in channels across the film. The mixing of the liquid entails a significant improvement in heat transfer.
CHAPTER 3 Heat Transfer in Thin Film Evaporators - Literature Review 25
(4) Vapour film evaporation, where vapour film is evolved from bubbles produced at the tube surface.
The heating rate for a liquid film on the heating surface could be expressed as the heat flux, which is the heat transferred per specific area, or the wall superheat, which is the temperature difference between the wall and the liquid-vapour interface, similar to the wall superheat used in the pool boiling curve. From the pool boiling curve for water, it can be seen that, at atmospheric pressure, natural convection occurs when wall superheat is less than 5°C and isolated bubble nucleate boiling occurs when wall superheat is in the range 5 to 10°e. When a water film trickles down a tube under gravity, the critical degree of wall superheat required for vapour bubbles to be formed on the surface is up to 7°C, but if the heating rate is not very high, e.g. if heat flux is less than 30 kW/m2, only convective interface boiling applies on the water film (Billet, 1 989). Chun and Seban ( 1 971) showed in their experimental results that a wall superheat of 3. 7°C was required for nucleation in water at atmospheric pressure. According to Blass ( 1979), for water and aqueous solutions, transition from interface boiling to nucleate boiling in liquid films occurs at heat fluxes in the range of 40 to 60 kW/m2, and at temperature differences in the range of 6 to 7.SoC. In his study of the effect of surface roughness on the boiling phenomena, Bell (1982) found that the ordinary engineering surfaces require surface superheats of the order of at least 3 to 5°C to initiate nucleation of a stable boiling phase.
Dengler and Addoms ( 1956) pointed out that in all cases an increase in liquid velocity past a surface was shown to raise the temperature difference required to initiate nucleate boiling at that surface. In other words it means that at a constant temperature difference the heat flux contribution by nucleate boiling decreases with increasing velocity until the boiling ceases entirely. It may be explained as that the greater velocity tends to prevent bubble formation at the wall (O'Connor and Russell, 1978). This effect of liquid velocity on nucleation, incidentally, can be observed by anyone stirring a pot of boiling water.
The wall superheat required for nucleate boiling also increases when the boiling temperature is reduced (MUller-Steinhagen, 1 989). Thome ( 1990) argued that the formation of the vapour core in a tube may lead to a complete suppression of bubble growth in the liquid film at the tube wall.
In falling film evaporators, liquid distributed on the top of the tubes flows down on the inner surface as a thin film under the action of gravity and later accelerated by the vapour velocity. Several heat-transfer mechanisms may coexist depending on local conditions (O'Connor and Russell, 1978). Kroll and McCutchan ( 1 968) found the interface evaporation plays the major role in the heat transfer at lower heat flux. But the low rate nucleate evaporating mechanism is regarded by Sinek and Young ( 1960) as the most probable mechanism in the falling film evaporators.
Billet ( 1 989) thought that the flow pattern in the evaporator decidedly affects heat transfer on the liquid side of the heater, and that the amount of vapour in the liquid is a crucial factor. He also pointed out that the calculation of the heat transfer coefficient in evaporators is subjected to various physical laws that depend on whether boiling is nucleate or convective.
Housova ( 1970) presented experimental data for falling film evaporation of water in a 3 metre long tube at the evaporation temperature range of 50 to 74°C. The data indicates that the change to nucleate boiling takes place above a wall superheat of 1 0°C.
Chen ( 1 992) found that the overall heat transfer coefficient in a single tube falling film evaporator decreased with increasing of the temperature difference between the steam condensing and the liquid evaporating temperatures. There was a change in the rate of decrease of heat transfer coefficient with increases in the temperature differences between the steam condensing and the liquid evaporating temperature at about 8K, which was attributed to the changes of heat-transfer mechanisms in the thin liquid film from convective interface boiling to nuc1ear boiling.
CHAPTER 3 Heat Transfer in Thin Film Evaporators - Literature Review 27
Stephan ( 1992) stated that nucleate boiling is generally avoided In falling film
evaporators because liquid is dragged along by the vapour bubbles resulting in dry spots forming on the wall, which can reduce the heat transfer rate and promote the fouling of the heating surface. However, Bouman et al. (1993) found that nucleate boiling starts at temperature difference, between the liquid and wall temperature, of about 0.5°C for milk and about 5°C for water. It also found that the transition from convective boiling to nucleate boiling takes place at much lower heat fluxes with skim milk than with water (van Stralen and Cole, 1979). This can partly be explained by the difference in surface tension, which for milk is lower (Walstra and Jenness, 1 984), but may also be due to the presence of milk fat globules which may act as nuclei. Therefore, based on above discussions, Mackereth ( 1995) postulated that, with water, falling film evaporators are likely to operate solely under the convection boiling regime, while on milk, nucleate boiling may dominate.