Nucleation saturation

The formation of a liquid phase from the vapour at any pressure below saturation cannot occur in the absence of a solid surface which serves to nucleate the process. Within a pore, the adsorbed film acts as a nucleus upon which condensation can take place when the relative pressure reaches the figure given by the Kelvin equation. In the converse process of evaporation, the problem of nucleation does not arise the liquid phase is already present and evaporation can occur spontaneously from the meniscus as soon as the pressure is low enough. It is because the processes of condensation and evaporation do not necessarily take place as exact reverses of each other that hysteresis can arise. 

Crystallization. Acidified aluminum sulfate solutions can be supercooled 10 °C or more below the saturation point. However, once nucleation begins, the crystallization rate is rapid and the supersaturated solution sets up. The onset of nucleation in a gentiy stirred supersaturated solution is marked by the appearance of silky, curling streamers of microscopic nuclei resulting from orientation effects of hydraulic currents on the thin, platelike crystals. Without agitation, nucleation in an acidified solution, in glass tubes, can yield extended crystalline membranes of such thinness to exhibit colors resulting from optical interference. 

Nucleation is initiated by secondary mechanisms involving the seed crystals or low super-saturation and high surface area of seed crystals eliminate or minimize nucleation seed crystals grow... 

The evolution of gases, such as in dre example given above of dre formation of CO(g) in dre U airsfer of sulphur between carbon-saturated iron and a silicate slag, requires dre nucleation of bubbles before dre gas can be eliminated from the melt. The possibility of homogeneous nucleation seems unlikely, and the more probable source of gas bubbles would either be at the container ceramic walls, or on detached solid particles of the containing material which are... 

Nucleation is the growth of clusters of molecules that become a thermodynamically stable nucleus. This process is dependent on the vapor pressure of the condensable species. The molecular clusters undergo growth when the saturation ratio, S, is greater than 1, where saturation ratio is defined as the actual pressure of the gas divided by its equilibrium vapor pressure. S > 1 is referred to as a supersaturated condition (14). 

Figures 6.30 and 6.31 present the same information for saturated hydrocarbons. In Figure 6.30, the saturated liquid state is on the lower part of the curve and in Figure 6.31 it is on the upper part of the curve. Below T y, the line width changes, indicating that the liquid probably does not flash below that level. Note that a line has been drawn only to show the relationship between the points a curve reflecting an actual event would be smooth. Note that a liquid has much more energy per unit of volume than a vapor, especially carbon dioxide. Note It is likely that carbon dioxide can flash explosively at a temperature below the superheat limit temperature. This may result from the fact that carbon dioxide crystallizes at ambient pressure and thus provides the required number of nucleation sites to permit explosive vaporization.

With continued heating, the local saturation temperature is reached and the steam bubbles move into the larger, bulk-water nucleate boiling region. Because the resulting steam bubble-water mixture close to the heated metal surface has a lower density than cooler water farther away from the heated surface, the steam bubble-water mixture rises. 

During the subcooled nucleate flow boiling of a liquid in a channel the bulk temperature of the liquid at ONB, 7b, is less than the saturation temperature, and at a given value of heat flux the difference ATsub.oNB = 7s - 7b depends on L/d. The experimental parameters are presented in Table 6.2. 

The bubble nucleation occurs at the location where the wall temperature exceeds the saturation temperature. 

The detail experimental study of flow boiling heat transfer in two-phase heat sinks was performed by Qu and Mudawar (2003b). It was shown that the saturated flow boiling heat transfer coefficient in a micro-channel heat sink is a strong function of mass velocity and depends only weakly on the heat flux. This result, as well as the results by Lee and Lee (2001b), indicates that the dominant mechanism for water micro-channel heat sinks is forced convective boiling but not nucleate boiling. 

For qualitative analysis of the conditions at which the boiling incipience was studied experimentally the parameter D = ATsub.oNB / s may be used. Depending on the value of D, the channels can be subdivided into two groups D < 1 and D < .. When D < D = 0.125—0.25) the onset of nucleate boiling occurred at a bulk temperature significantly less than saturation. When D nucleate boiling occurred at values of the bulk temperature close to saturation. 

Liu Z, Winterton RHS (1991) A general correlation for saturated and subcooled flow boiling in tubes and annuli, based on a nucleate pool boiling equation. Int J Heat Mass Transfer 34 2759-2766... 

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