Heterogeneous Nucleate Pool-Boiling

4 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 extended vertical surfaces, like those in evaporators or the walls of large tanks, the integrated heat transfer into the liquid over a large vertical distance depends on several factors. These include whether the heat transfer takes place at constant wall temperature (the evaporator condition, which is very difficult to simulate experimentally) or constant wall heat-flux (the usual laboratory experimental simulation), and whether the bulk liquid temperature is constant with depth, or not. 

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