Experimental investigation of the intermittent spray heat transfer characteristics of nanofluids in single- and two-phase states

Traditional air-cooled cooling systems used in engineering vehicles consume significant energy and do not align well with future development trends. To address this problem, a single- and two-phase intermittent spray cooling technique for nanofluids was proposed in this study. The effects of the duty cycle and spray frequency on the heat transfer performance and surface temperature uniformity of intermittent spray were determined by experiments, and the thermal behaviour of intermittent spray with different mass fractions of Al2O3 nanofluids was compared in detail. The results revealed that when the spray frequency is constant, the heat transfer coefficient increases with an increase in duty cycle, but the cooling efficiency decreases. The spray frequency mainly affects the average heat transfer surface temperature and its uniformity. At 2.0 Hz, nanofluids with a mass fraction of 0.7 % exhibited the best intermittent spray performance, with an average heat transfer coefficient of 431.8 W/(m2 & sdot;K). However, a continuous increase in the mass fraction of the nanofluids inhibited the heat transfer performance. The impact of droplets on the heat transfer surface, the liquid film heat transfer process, and the heat transfer characteristics of the nanofluid intermittent spray in the single- and two-phase states were further studied. The results revealed that the heat transfer of intermittent spray mainly depends on the properties and flow rate of the coolant in a single-phase state. Although the heat transfer performance of intermittent spray is evidently enhanced in the two-phase state, it still does not exceed that of continuous spray. These findings highlight the flexibility of nanofluid intermittent spray cooling in enhancing heat transfer control and can inform the development of energy-saving and intelligent cooling systems for engineering vehicles.