Peak Heat Flux and Temperature Difference in Nucleate Boiling of Liquefied Gases
Two-phase phenomena are encountered frequently in cryogenic systems. Since the thermal diffusivity and kinematic viscosity of the liquids under consideration (and their vapors) are low, the critical value of the Rayleigh or Reynolds number is easily exceeded. Consequently, turbulent conditions prevail and a complicated flow pattern results. A special type of two-phase flow is involved in nucleate boiling which is characterized by the appearance of the vapor phase in the form of single bubbles. It may be caused by heat currents leaking through the outer boundaries of the system (if a conventional type of insulation is used), or by a heat input required for the operation of the system. A thermal boundary layer of superheated liquid is built up near the confining or heat emitting solid surfaces from which the bubbles grow. Since the thermal conductivity of the vapor is much lower than that of the liquid, heat flows to the growing bubbles via the conducting liquid in between the bubbles. Further details on the mechanisms involved have been given in the literature.
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