Heat transfer under quenching of cylindrical bodies in subcooled liquids

摘要

Quenching of high-temperature bodies in liquids is inherent for important technological processes. Despite a large number of experimental studies, there is yet no common understanding of an existence of the two distinct modes of the process. The physical nature of the intense cooling regime occurred at the surface temperatures thermodynamically excluding direct liquid/solid contact, requires new studies. This problem is very complex, as the process depends on plenty of factors (liquid subcooling, properties of a coolant and a cooled body, the surface characteristics). However, its solution is essential, since incipience of the intensive heat transfer regime of cooling means a sharp decrease of the process duration. This is actual in connection with the development of socalled the tolerant fuel for NPP, where there is a need to justify the behavior of new materials under the conditions of re-flooding of the core in the event of a loss of coolant accident. On this way, new purposeful experimental researches are necessary. The aim of this work is an experimental study of the effect of coating properties on unsteady film boiling. For this, cylindrical copper specimens coated with gold and Cr18Ni25 alloy were made. Cr18Ni25 coatings had different thickness and roughness. To analyze the effect of liquid properties on cooling processes, water and ethanol were used as a coolant. The experiments were carried out in a wide range of subcooling, both at atmospheric and at elevated pressures, the maximum subcooling for ethanol being 150 K. The experimental results revealed a strong influence on the quenching rate of the coolant physical properties and the thermal effusivity of a cooled body that is in agreement with the model developed by the authors earlier. The experimental results showed a strong influence on the process of both the physical properties of the coolant and the set of coating properties (thermal activity, thickness and roughness), which qualitatively agrees with the model developed by the authors earlier. An essential conjugated effect of the thickness and roughness of the low conductance coating is a principally new result, especially for ethanol as a coolant. This allows determining a direction of further theoretical and experimental researches, which have to account for the separate influence of the coating properties, thickness, roughness height and a liquid subcooling on incipience of the intense heat transfer regime.