2.5. In the vapour space above the liquid, there is closed loop, thermo-syphon type vapour recirculation, with a
finite, recirculating, mass flow R(vapour) kg/sec which may well exceed the boil-off mass flow. The recirculating
flow mixes with the upward boundary layer flow adjacent to the neck wall, with a total mass flow of R plus the boil-off
mass flow, while the centre of the vapour column moves downwards with a reducing mass flow. The
recirculation is driven by conducted heat flow absorbed from the neck wall by the boundary layer flow. The A heat
inflow from the central, downward vapour plume appears to provide the residual value for minimum boil-off performance. Further research is needed to understand how to reduce the recirculating vapour mass flow at the
liquid surface, and the associated residual heat inflow.