Surface Evaporation, the Only Way of Dissipating Superheat and Thermal Overfill

As mentioned in Chap. 3, for depfli/diameter ratios less than 0.5, such as in large cylindrical LPG and LNG tanks, flie boundary layer flow across the base may be broken by the creation of vertical fliermals spaced horizontally at intervals approximating to the liquid depth according to Rayleigh s instability criteria for natural convection. In aU cases, the heat in-flow is carried by boundary layer flows, and thermals, to the liquid surface. 

6 Surface Evaporation, the Only Way of Dissipating Superheat and Thermal Overfill 

After the superheated boundary layer flows and thermals reach the surface, some, or aU, of the excess heat is absorbed by the process of surface evaporation, hr fact, the wall boundary layer flow turns over into a horizontal flow and moves radially inwards just below the liquid/vapour interface. During this inward radial motion, surface evaporation takes place as described in Sects. 4.6.1. 6.5 below (see also Fig. 4.1). 

When the inward radial motion reaches the centre (of a cylindrical vessel or tank), it turns over to become a strong downward jet carrying the excess heat, not released by surface evaporation, into the core of the liquid where secondary convection produces mixing and superheating. 

The highest fluid temperature is undoubtedly at the wall-Uquid-vapour interface, where the evaporative mass flux will be larger than at the centre of flie liquid pool. The mechanism of surface evaporation during inward radial flow is extraordinarily complicated and sensitive, while the rate of evaporating mass flow through the surface is controlled by several local factors. Let us develop the picture of this mechanism as it was discovered at Southampton. 

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