Thermal removal from coal fires by a two-phase closed thermosyphon with a CuO nanofluid

Two-phase closed thermosyphons (TPCTs) are effective devices for transferring heat and could be used to reduce the internal heat of coal piles, inhibiting coal spontaneous combustion. Nanofluids have recently been investigated as working media for improving the thermal efficiency of TPCTs by exploiting the high thermal conductivity of nanoparticles. In this study, the heat transfer performance of a TPCT containing CuO–H2O nanofluids of various nanoparticle concentrations was investigated experimentally and through simulations for a coal pile at different operating temperatures (373.15, 413.15, 473.15, and 573.15 K). At 373.15 K, the optimal CuO–H2O concentration was 10 mass%, which was associated with a 34.5% improvement in heat transfer. At 413.15, 473.15, and 573.15 K, the optimal CuO–H2O concentration was 15 mass%, with the corresponding heat transfer improvements being 8.1%, 41.9%, and 40.8%, respectively. At higher operating temperatures, the thermal resistance of the TPCT decreased and the heat transfer performance increased. In particular, at 473.15 and 573.15 K, increasing the nanoparticle concentration improved heat exchange in the working medium, increasing the heat transfer efficiency. These results could be used to further optimize TPCTs and by practitioners to reduce the risk of coal fires.