Experimental investigation of spray cooling on micro-, nano- and hybrid-structured surfaces

The heat transfer during spray cooling was studied experimentally using deionized water to investigate the spray characteristics and the differences between spray cooling on a smooth silicon surface and micro-, nano- and hybrid micro/nano-structured surfaces. The spray cooling experiments show that the heat transfer rates were better for the nano-structured surface, followed by the smooth surface coated with the SiO2 film and the pure silicon surface since the contact angle was smallest on the nano-structured surface and increased on the latter two. The droplet parameter results show that most droplets were 40–60 μm in size. The heat transfer coefficient increased and the wall temperature decreased on the 25G × 25S surface coated with the SiO2 film compared with the 50G × 50S surface coated with the SiO2 film as the heat transfer moved into the partial dryout region due to the SiO2 film’s stronger hydrophilicity so the heated area was more fully utilized, while the CHF was larger for the 50G × 50S surface. Coating the micro-structured surfaces with carbon nano-tube (CNT) films having characteristic sizes smaller than the droplet size was more effective than on the surfaces with larger characteristic sizes. The CHF was largest on the 25G × 25S surface coated with 4 carbon nano-tube films with a 75.3% increase over the smooth surface. The wall temperature increase and the temperature fluctuations were small in the boiling regime as the power increases for the enhanced surfaces.