Effects of CuO mass concentrations on water-based CuO nanofluid heat pipe for thermal energy extraction: inhibition of coal spontaneous combustion

To inhibit coal spontaneous combustion (CSC) effectively, the heat pipe (HP) technology has been employed for extracting thermal energy. Nanofluids are a new type of working fluid with special properties that accelerate HPs heat transfer efficiency. Experimental tests for heat transfer were conducted at various CuO mass concentrations (0, 5, 10, 15, and 20 mass%) on water-based CuO nanofluid HP to further investigate the influence of the distribution of the coal temperature and heat extraction. Results indicated that the coal pile to vertically distribute temperature in the top area was the highest, that in the middle area was lower, and that in the bottom area was the lowest. The high-temperature area of the coal pile was concentrated at a distance < 150 mm from the heat source. The cooling efficiency and value of the HP on the coal pile exhibited the following order: 5 mass% > 10 mass% > 15 mass% > 0 mass% > 20 mass%. In addition, due to the increase in the CuO mass concentration, the cooling efficiency had an initial increase first followed by subsequent decrease. An optimal concentration was predicted at approximately 8.5–9 mass%. When the optimal concentration was 8.71 mass% CuO, the maximum heat transfer radius was 341 mm. Moreover, the temperature of the condensing part of the HP gradually drops and moves towards the cold end. Therefore, in the HP, the area near the bottom was the optimal location for heat extraction and utilisation. Thus, the scientific applications of the HP in the field of CSC should be considered substantially crucial.