The detrimental impact of surface fouling on the critical heat flux of microstructures in boiling heat transfer

Boiling heat transfer, owing to its higher efficiency than alternative methods, is attracting considerable attention for application in next-generation cooling systems, which use the latent heat of liquid phase change. The most crucial factor in boiling heat transfer is the critical heat flux (CHF), which is the limiting bound of the heat flux in a cooling system. In this study, we investigated the deterioration of the CHF owing to the fouling effect induced by deposited nanoparticle layers on micro/nano composite structures via pool boiling experiments. The micropillar structures (MPS) had a diameter of 4 μm, a gap of 10 μm, and a height of 15 μm. SiO₂ nanoparticles with diameters of 400 nm and various NaCl concentrations were used herein. To increase the thickness of the deposited nanoparticle layers through fouling, boiling experiments were conducted in salt (NaCl) solutions with concentrations between 0.0001 and 0.1 M. The results revealed that as the salt concentration increased, the CHF of the nanofluids-plain (representing the plain surface with SiO₂ nanoparticles deposited) under salt samples increased, and the nanofluids-plain sample under 0.1 M salt exhibited a 40% enhanced CHF relative to the untreated plain surface. By contrast, the CHF of nanofluids-MPS (representing the surface containing MPS and SiO₂ nanoparticles deposited) under salt samples decreased with increasing salt concentration, with a notable 24% decrease in the CHF of this surface under 0.1 M salt concentration. These results were validated by measuring the thicknesses of the deposited nanoparticle layers and surface wicking performances. As the salt concentration increased, the thickness of the deposited nanoparticle layer increased, enhancing nano-wicking and thereby improving the CHF of the nanofluids-plain sample. However, the thickened nanoparticle layer deposited on the nanofluids-MPS sample obstructed the micro-wicking path between the MPS. Under 0.1 M salt concentration, the thick layer blocked liquid flow between micropillars of the nanofluids-MPS sample, sharply decreasing the CHF. In conclusion, these findings emphasize that both salt concentration and structural dimensions should be carefully considered during boiling experiments involving micro/nano composite structures in diluted salt solutions.