Boiling from metal surfaces immersed in liquid nitrogen and liquid hydrogen

A study of bubble nucleation in liquid nitrogen and liquid hydrogen has been made in a 120 mm bubble chamber using a high speed cine camera.The initiation and growth of bubbles from artificially prepared cavities was studied. These cavities, produced in both copper and brass surfaces, had mouth radii ranging from 50 μm to 190 μm with depths varying from 110 μm to 500 μm. The liquids were superheated by either heating the surface containing the cavities or by reducing the pressure of the liquid, or by combining both of these methods.The heat fluxes required for initiation of boiling were greater than those required to sustain nucleation from a given site. The growth rate of the bubbles at constant liquid pressure was dependent on a number of factors. It changes as a bubble grew out of the thermal or superheated layer above the metal surface. After an initial period of more rapid growth the vapour volume increased linearly with time. The growth rate was dependent on the liquid used; being slower in liquid hydrogen than in liquid nitrogen. Formulae were obtained which predicted the observed variations in growth rate and departure volume.No nucleation was observed from the test surfaces for a pressure drop alone; the surfaces had to be heated before nucleation would occur. It is thought that this was due to the existence of a zero contact angle for cryogenic liquids on solid surfaces.