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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. [ABSTRACT FROM AUTHOR]

The boiling performance of functionalized hybrid aluminum surfaces was experimentally investigated for water and self-rewetting mixtures of water and 1-butanol. Firstly, microstructured surfaces were produced via chemical etching in hydrochloric acid and the effect of the etching time on the surface morphology was evaluated. An etching time of 5 min was found to result in pitting corrosion and produced weakly hydrophilic microstructured surfaces with many microcavities. Observed cavity-mouth diameters between 3.6 and 32 μm are optimal for efficient nucleation and provided a superior boiling performance. Longer etching times of 10 and 15 min resulted in uniform corrosion and produced superhydrophilic surfaces with a micropeak structure, which lacked microcavities for efficient nucleation. In the second stage, hybrid surfaces combining lower surface energy and a modified surface microstructure were created by hydrophobization of etched aluminum surfaces using a silane agent. Hydrophobized surfaces were found to improve boiling heat transfer and their boiling curves exhibited a significantly lower superheat. Significant heat transfer enhancement was observed for hybrid microcavity surfaces with a low surface energy. These surfaces provided an early transition into nucleate boiling and promoted bubble nucleation. For a hydrophobized microcavity surface, heat transfer coefficients of up to 305 kW m−2 K−1 were recorded and an enhancement of 488% relative to the untreated reference surface was observed. The boiling of self-rewetting fluids on functionalized surfaces was also investigated, but a synergistic effect of developed surfaces and a self-rewetting working fluid was not observed. An improved critical heat flux was only obtained for the untreated surface, while a lower critical heat flux and lower heat transfer coefficients were measured on functionalized surfaces, whose properties were already tailored to promote nucleate boiling. [ABSTRACT FROM AUTHOR]

Heat transfer coefficients were investigated for saturated nucleate pool boiling of binary mixtures of water and glycerin at atmospheric pressure in a wide range of concentrations and heat fluxes. Mixtures with water mass fractions from 100 % to 40 % were boiled on a horizontal flat copper surface at heat fluxes from about 25 up to 270 kW m − 2 . Experiments were carried out by static and dynamic method of measurement. Results of the static method show that the impact of mixture effects on heat transfer coefficient cannot be neglected and ideal heat transfer coefficient has to be corrected for all investigated concentrations and heat fluxes. Experimental data are correlated with the empirical correlation α = 0.59 q 0.714 + 0.130 ω w with mean relative error of 6 % . Taking mixture effects into account, data are also successfully correlated with the combination of Stephan and Abdelsalam (1980) and Schlünder (1982) correlations with mean relative error of about 15 % . Recommended coefficients of Schlünder correlation C 0 = 1 and β L = 2 × 10 − 4 m s − 1 were found to be acceptable for all investigated mixtures. The dynamic method was developed for fast measurement of heat transfer coefficients at continuous change of composition of boiling mixture. The dynamic method was tested for water–glycerin mixtures with water mass fractions from 70 % down to 35 % . Results of the dynamic method were found to be comparable with the static method. For water–glycerin mixtures with higher water mass fractions, precise temperature measurements are needed. [ABSTRACT FROM AUTHOR]