Boiling phenomena nucleation

When does a liquid boil Clearly, boiling at constant pressure—say, atmospheric pressure—begins when we increase the temperature of a liquid or solution and the vapor pressure reaches a pressure of one atmosphere. Alternatively, the pressure over a liquid or solution at constant temperature must be reduced until it reaches the vapor pressure at that temperature (e.g., vacuum distillation). Yet it is well known that liquids can be superheated (and vapors supersaturated) without the occurrence of phase transfer. In fact, liquids must always be superheated to some degree for nucleation to begin and for boiling to start. That is, the temperature must be raised above the value at which the equilibrium vapor pressure equals the surrounding pressure over the liquid, or the pressure must be reduced below the vapor pressure value. As defined earlier, these differences are called the degree of superheat. When the liquid is superheated, it is metastable and will reach equilibrium only when it breaks up into two phases. 

The resulting expression, Eq. 8.6-2, has a weak temperature-dependent preexponential term, and a temperature sensitive exponential term. The latter contains the surface tension to the third power and the superheat to the second power. With increasing temperature the surface tension drops, and superheat increases, giving rise to an increase of orders of magnitude in J over a very narrow temperature range. 

The computed kinetic limit of superheat of /i-butane, for example, is 378.3 K and the experimentally measured3 value is 376.9 K. With ordinary liquids, the kinetic limit of superheat approaches the critical temperature (7k S/7 crit = 0.89). However, under ordinary conditions, when the liquid is in contact with solid surfaces, it boils far below the kinetic limit of superheat. Thus, the boiling point of u-butane, for example, is 272.5 K. Similarly, the theoretical kinetic superheat of water is 300°C, while the ordinary boiling point of water is 100°C. 

When the vapor phase is generated at a solid interface rather than in the bulk of the liquid, the process is known as heterogeneous nucleation. Heterogeneous nucleation theories on smooth surfaces yield similar expression to Eq. 8.6-1 for J, with modified groupings A and B that account for the contribution of geometry and energy of the solid surface (22). 

Finally, it is worth mentioning that the cavitation phenomenon observed in low viscosity liquids is also caused by (explosive) boiling induced by sudden reduction of pressure, such as that occurring in regions behind moving surfaces, such as impellers, or as the result of flow acceleration (Bernoulli effect) (23). 

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The maximum heat flux achievable with nucleate boiling is known as the critical heat flux. In a system where the surface temperature is not self-limiting, such as a nuclear reactor fuel element, operation above the critical flux will result in a rapid increase in the surface temperature, and in the extreme situation the surface will melt. This phenomenon is known as burn-out . The heating media used for process plant are normally self-limiting for example, with steam the surface temperature can never exceed the saturation temperature. Care must be taken in the design of electrically heated vaporisers to ensure that the critical flux can never be exceeded. 

Because the bubble population increases with heat flux, a point of peak flux may be reached in nucleate boiling where the outgoing bubbles jam the path of the incoming liquid. This phenomenon can be analyzed by the criterion of a Hemholtz instability (Zuber, 1958) and thus serves to predict the incipience of the boiling crisis (to be discussed in Sec. 2.4.4). Another hydrodynamic aspect of the boiling crisis, the incipience of stable film boiling, may be analyzed from the criterion for a Taylor instability (Zuber, 1961). 

The rate of heat transfer from the tubes to the fluid depends primarily on turbulence and the magnitude of the heat flux itself. Turbulence is a function of mass velocity of the fluid and tube roughness. Turbulence has been achieved by designing for high mass v ocities, which ensure that nucleate boiling takes place at the inside surface of the tube. If sufficient turbulence is not provided, departure from nucleate boiling (DNB) occurs. DNB is the production of a film of steam on the tube surface that impedes heat transfer and results in tube overheating and possible failure. This phenomenon is illustrated in Fig. 27-40. 

The main difficulty is to establish the dependence of the heat transfer coefficient on vapour quality in relation to different mechanisms controlling flow boiling. Some correlations do not take into account the two mechanisms. Others account for convective and nucleate boiling. To the present author s knowledge, none take into account the influence of channel size. The aim here is to summarise recent work on flow boiling, to describe an experiment on the phenomenon in minichannels and to compare the results with classical correlations. 

Boiling Limitation. At very high radial heat fluxes, nucleate boiling may occur in the wick-ing structure and bubbles may become trapped in the wick, blocking the liquid return and resulting in evaporator dryout. This phenomenon, referred to as the boiling limit, differs from the other limitations previously discussed in that it depends on the evaporator heat flux as opposed to the axial heat flux [7],... 

Effect of Pressure. Results illustrating the effect of pressure on nucleate pool boiling are shown in Fig. 15.34 the superheat required for a given heat flux decreases with increasing pressure as shown. Results for subatmospheric conditions are presented by Schroder et al. [74] the trends shown in Fig. 15.34 are continued (increasing superheat for reducing pressure), but at the lowest pressure studied (0.1 bar), a new phenomenon is observed. Boiling starts with the creation... 

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