Spray cooling has been shown to be the best cooling technique for applications that require efficient high heat flux removal. Technically, this environment-friendly process assists heat transfer from hot surfaces by spraying water droplets on a surface, extending the water droplets, and evaporating water. As of today, spray cooling has attracted widespread attention due to its advantages in high heat flux removal, such as: less flow rate demand, high heat dissipation capacity, low superheat degree, no temperature overshoot and no contact thermal resistance with the heating surface. Attempts to control the wetting property and realize rapid extension of water droplets on a surface have been demonstrated in recent studies with much effort being channeled towards surface design.
Although recent publications have reported surface designs that could improve water extension and consequently a large possibility of increasing the apparent water drying rate, they have limited design capability, depending on the surface materials that make it challenging to apply a surface design onto existing surface of a typical devise.
In fact, a review of existing literature reveals that most studies have used as-synthesized and as-received particles. Worse off, studies focusing on the structural design of coating particles and their relationships with the water drying properties in the layer of the designed particles are quite few. To address this, a team of researchers from the Yokohama National University in Japan: Dr. Motoyuki Iijima, Masahiro Hayakawa and Dr. Junichi Tatami, in collaboration with Koichiro Nakamura and Shinichi Nagashima at the Lion Corporation designed a new nanoscale structured composite particles to enhance the water extension on the core particle surface and amongst the composite particles in the powder layer, respectively. They aspired to realize their design through a high-shear mechanical composing process. Their work is currently published in the research journal, Chemical Engineering Science.
In their approach, the team adopted composite particles consisting of spherical porous SiO2 particles (assembly of nanoparticles) with nano-scaled surface roughness as the core particles and SiO2 nanoparticles as the depositing particles. The effects of the composite particle nanoparticle content on the morphology of the composite particles and their relationships with the water droplet extending and drying properties in the powder layer of the designed composite particles were systematically investigated based on FE-SEM observations, characterization of N2 adsorption/desorption isotherms, and 1H spin-spin relaxation rate measurements of composite particles.
Based on the FE-SEM observations, the authors were able to show that the pulverized nanoparticles could be successfully attached to porous core particles without the core particles collapsing. Additionally, the reduction of the specific surface area and increase of the particle specific external surface area after the compositing process strongly supported the fact of nanoparticle attachment.
In summary, nanoscale structured composite particles consisting of spherical porous SiO2 particles with nano-scaled surface roughness as the core particle and SiO2 nanoparticles as the depositing particles were successfully designed through a high-shear mechanical composition process. In an interview with Advances in Engineering, Dr. Motoyuki Iijima further emphasized one of their observations stating that 15 wt% loading of nanoparticles successfully increased apparent water drying rate.
M. Iijima, M. Hayakawa, J. Tatami, K. Nakamura, S. Nagashima. Design of nanoscale structured composite particles through mechanical process for fabricating a powder layer with rapid drying properties. Chemical Engineering Science volume 203 (2019) page 113–121.