Surface Evaporation Mass Flux and Bulk Superheat

1 Surface Evaporation Mass Flux and Bulk Superheat 

To gain an understanding of the relationship between evaporation mass flux and bulk superheat, microthermometer studies were first made on LIN, LCH4 and LNG [8-11]. 

The boil-off vessel was an 80 mm inner diameter, double walled, vacuum insulated dewar surrounded by a second liquid bath 120 mm inner diameter. The boil-off from the inner vessel could be varied via a uniform heat-flux electrical heater mounted in the vacuum space around the inner wall. The micro-thermometers consisted of 25 pm diameter copper/constantan thermocouple junctions mounted horizontally in differential or absolute configurations (see Fig. 4.2). 

For the differential configuration, the two junctions were separated vertically by a distance of 100 mm, so that temperatures in the region of the surface could be measured relative to that of the bulk liquid. For the absolute configuration, the reference junction was the ice-point while a single calibration point was made against a Platinum thermometer in rapidly boiling LIN. The pool depth was kept between 200 and 250 mm. 

The results are summarised in Fig. 4.2 as a log-log plot of average mass-flux through the surface in g/m s against bulk superheat AT (Tb-To). With the uniform heater used, it proved possible to generate superheats of 3.0 K in LIN and LNG, before any nucleate boiling occurred. 

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