Boiling heterogeneous nucleate

Increasing the temperature or lowering the pressure on a superheated liquid will increase the probability of nucleation. Also, the presence of solid surfaces enhances the probability because it is often easier to form a critical-sized embryo at a solid-liquid interface than in the bulk of the liquid. Nucleation in the bulk is referred to as homogeneous nucleation whereas if the critical-sized embryo forms at a solid-liquid (or liquid-liquid) interface, it is termed heterogeneous nucleation. Normal boiling processes wherein heat transfer occurs through the container wall to the liquid always occur by heterogeneous nucleation. 

R. Cole, Homogeneous and Heterogeneous Nucleation in Boiling Phenomena, Vol. 1, S. Van Stolen, and R. Cole, Eds., McGraw-Hill, New York, 1979, Chapter 3. 

The nucleation temperature, which exceeds the boiling point of the species, is the temperature at which bubbles spontaneously appear in the liquid. Bubble nucleation is a rate process, and its description on the basis of a nucleation temperature is a simplification. Homogeneous nucleation temperatures are substantially above the boiling point heterogeneous nucleation—aided, for example, by impurities like dust—may occur at somewhat lower temperatures that nevertheless still exceed the boiling point. 

Nucleate Boiling from Wall into Bulk Liquid 4.4.1 Heterogeneous Nucleate Pool-Boiling... 

The mechanism of heterogeneous nucleate pool-boiling on a submerged heated wall is well documented [1]. Because vapour bubbles have an increased internal vapour pressure (proportional to the ratio of surface tension/bubble diameter), they have an increased saturation temperature which must be exceeded for the bubble to grow. The increase in wall temperature needed to create vapour bubbles in the first place can be reduced by nucleation centres in, or on, the surface of the wall. 

Heterogeneous nucleate boiling on plain heated surfaces can be significantly enhanced today by treating the surfaces so as to create a wide variety and a high density of nucleation sites, for example, by the application of porous coatings [2]. 

For LCH4, heterogeneous nucleate boiling on plain surfaces occurs between heat fluxes of a minimum of about 10 kW/m, rising to a maximum or critical heat flux of about 500 kW/m, with wall superheats from about 0.5 K to a maximum of about 20.0 K, respectively, the actual values depending on the particular surface and its immediately previous, thermal history. 

For heterogeneous nucleate boiling in a liquid at its saturation temperature To, streams of vapour bubbles rise to the surface and break through to become the boil-off vapour mass flow. 

In most, if not all, storage situations, the heat flow through the insulation and tank walls into the liquid is a much more gentle process with a heat flux of typically less than 100 W/m for LNG. This level of heat flux is some two orders of magnitude less than the minimum of 10,000 W/m required for heterogeneous nucleate boiling. It can only be released from the liquid by surface evaporation, with no boiling at all. 

A model of explosive boiling making use of the idea of intensive homogeneous nucleation allows us not only to give a qualitative explanation to effects observed in two-phase nonequilibrium flows but also to make trustworthy quantitative evaluations (for example of critical flow rates throu short channels) which cannot be obtained with the aid of traditional schemes of heterogeneous media mechanics. The field of applicability of the model is outlined quite definitely. This model is a usefxil addition to all other models of fluid mechanics. 

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