Abstract
Pool boiling of cryogenic nitrogen and oxygen is important, as both substances represent more than 99 % of all particles in the atmosphere. Boiling of cryogens is commonly utilized in gas processing, air separation, cooling, or superconducting systems. The so-called chilldown is a characteristic process when handling cryogens, which occurs when a surface at ambient temperature gets into contact with a cold cryogenic liquid. The chilldown is accompanied by transient film boiling. Investigating critical heat fluxes and film boiling of cryogens is thus more important compared with non-cryogenic liquids. This contribution deals with heat transfer during pool boiling of cryogenic nitrogen, oxygen, and nitrogen–oxygen mixtures. Knowledge of heat transfer coefficients and of critical heat fluxes is crucial for qualified design of cryogenic apparatuses and for improved handling of cryogens. In this contribution, pool boiling performance of pure nitrogen, oxygen, and their mixtures is investigated and suitable predictive correlations for the heat transfer coefficient and critical heat flux are analyzed. It was found that variations of thermophysical properties with composition are quite weak and sufficiently linear for nitrogen–oxygen mixtures. The pure-fluid correlation of McNelly is shown to be suitable for HTC estimation during boiling of pure nitrogen and oxygen. The mixture correlation of Thome was found to be suitable for nitrogen–oxygen mixtures.
