Enhanced convection and boiling heat transfer with ethanol additive in a closed-loop water spray cooling system

Deionized water and 2-8 vol.% ethanol-water mixtures at 2 vol.% intervals are used as working fluids to experimentally study the heat transfer characteristics of closed-loop spray cooling. Two types of solid-cone pressure swirl nozzles (TG-0.3 and TG-0.5) were adopted. The surface heat flux and heat transfer coefficient are calculated based on the measured temperatures by the inserted thermocouples. The temperature and flow rate of the liquid film are also measured to partition the contribution of convection and boiling heat transfer modes. The results demonstrated that at the same spray pressure of 0.3 MPa, a higher total mass flow rate using the TG-0.5 nozzle (∼6.5 g/s) induces superior spray cooling heat transfer coefficients than the TG-0.3 nozzle (∼4.7 g/s). Adding low-concentration ethanol significantly improves the heat transfer rate compared to pure water. Furthermore, this enhancement exhibits a non-monotonic relationship with ethanol content, peaking at 4 vol.% ethanol for both nozzles. The liquid film mass flow rate decreases at higher surface temperatures since more liquid undergoes phase change. Meanwhile, adding ethanol leads to a notable increase in liquid film mass flow rate relative to pure water, while further increases in ethanol content result in a slight reduction. The liquid film temperature increases from subcooled to near saturated with the increased surface temperature. It also decreases with ethanol volume fraction for both nozzles, while the temperature difference between the ethanol-water mixtures and pure water is related to the nozzle type. Although the convective heat flux declines with ethanol concentration, all ethanol-water mixtures exhibit higher values than pure water. Ethanol also significantly enhances boiling heat flux, and this enhancement is particularly noticeable during the later stage of nucleate boiling with the TG-0.3 nozzle. Besides, the highest boiling heat flux values are also achieved by the 4 vol.% ethanol-water mixture.