Layer Stratification in Single Component Cryogenic Liquid with Boil-off at 1 bar

7 Layer Stratification in Single Component Cryogenic Liquid with Boil-off at 1 bar 

From Chap. 3, it can be seen how liquid convective circulation can lead to density stratification in single component liquids during boil-off at 1 bar. In the absence of any continuous mechanical mixing of the single component liquid, the intrinsic mechanisms of supeiheated wall boundary layer flow, incomplete evaporation at the surface, and central downward jet of less superheated liquid, can lead to stratification into two layers. Each layer will be at a more or less uniform but different temperature, with a hotter, less dense layer on top of a colder, more dense layer. 

Since the temperature difference across the Uquid-liquid interface between the two layers is small, of the order of 0.1-1.0 K, the ntixing effect of thermally driven molecular diffusion in the absence of any convective motion is relatively small. The associated density difference across the Uquid-liquid interface acts so as to suppress local convective mixing and the stratification is therefore extremely stable. 

However, the heating of the top layer by the waU boundary layer flow continues, leading to a continuing rise in temperature, or an increase in thickness accompanied by a downward migration of the layer interface, or a combination of the two. 

When the density difference between the two layers becomes large enough, of the order of 1.0 % (corresponding to an approximate AT of 2.5 K for LNG (from Table 5.1), the supeiheated wall boundary layer flow in the denser, lower layer suddenly has insufficient buoyancy and inertia to penetrate the liquid-Uquid layer interface. The boundary layer flow is trapped in the lower layer and no evaporation can take place to release its superheat Instead, the wall boundary layer turns over at the interface and its motion and superheat is locked into flie lower liquid causing it to heat up instead. When this happens, the boil-off rate will fall as the first indication 

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