Emergency cooling of superheated surfaces by nanofluids additives in stop-and non-stop modes of heat load rise

Early [Technical Physics Letters, 2016, Vol. 42, P. 677–681. — https://doi.org/10.1134/S106378501607004X] we have shown the possibility of emergency cooling of an overheated Ni/Cr surface using additives of aluminosilicate nanofluids (AlSi-NF) at the time of developed film boiling of water (crisis), provided that the increase in thermal load is stopped and its fixation at the level of Q ≈ 1.0 MW/m, exceeding the critical heat flux (CHF) of water (0.7 MW/m). However, in real operating conditions of cooling systems (especially for nuclear reactors), emergency situations sometimes arise in which it is very problematic to immediately turn off the heat load supply or maintain it at a certain predetermined level. In this regard, in this work, on a fully automated stand, the peculiarities of eliminating the water boiling crisis and emergency cooling of the overheated surface of the mini-reactor heater by injecting a portion of hot AlSi-NF in conditions of film boiling of water and a steady increase in heat load were studied, and the results were compared with previously obtained in the mode of stopping the rise of the thermal load. The test was carried out on an aqueous AlSi-NF nanofluid obtained on the basis of a natural mixture of aluminosilicates montmorillonite + palygorskite (Ukraine). The boiling-overheating-cooling curves, as well as the time dependences of the heat transfer coefficient and the heating surface temperature, were recorded in automatic real-time mode. Emergency cooling of the overheated surface (from 600 to 125 °C) after the introduction of a portion of hot AlSi-NF occurred in a matter of minutes due to a sharp increase in the heat transfer coefficient α up to 55,000 W/(mK). Such a phenomenon of a sharp intensification of heat transfer and a 3-fold increase in the specific heat flux (q) during boiling of AlSi-NF compared to the base liquid (water) is explained by the deposition on the heating surface of a gel-like layer of nanoparticles with high hydrophilicity and mobility, which can sharply increase nucleate boiling and convection. Regardless of the mode of supplying the heat load, the principal possibility of overcoming the boiling crisis and emergency cooling of the superheated surface with the addition of AlSi-NF nanofluid has been established, for a time sufficient to eliminate the accident. Bibl. 27, Fig. 4, Tab. 1.