Numerical simulation of motion characteristics and heat transfer enhancement of free particles in boiling heat transfer

• VOF-DEM coupling model is established for boiling with free particles. • Three modes of bubble and particle motion are observed. • Both temperature and fluctuation range of heating surface are reduced. • The heat transfer coefficient with free particles is increased by 35.67%. Nanofluids enhance boiling heat transfer mainly by changing fluid properties and depositing porous coatings on heating surfaces. Increasing the size of particles, a porous layer can be formed by the accumulation of particles on heating surface to promote nucleate boiling, and the particles also can be carried into the fluid to enhance convective heat transfer. To find the appropriate particle size, the VOF-DEM coupling model is established for boiling heat transfer with free particles. The effect of the same volume fraction of free particles with different sizes of 30 μm, 50 μm, 70 μm, and 90 μm are compared. The motion of free particles with different sizes in boiling heat transfer can be divided into three modes, including particles carried inside bubbles (Mode I), particles moving under the entrainment of the bubble wake (Mode II), and bubbles leaving from the side of particle aggregates (Mode III). The boiling heat transfer performance with free particles is improved, and both the average temperature and the fluctuation range of the heating surface are reduced. The boiling heat transfer is improved through the formed porous layer on heating surface and convective heat transfer by free particles. The particle size of 70 μm shows the greatest improvement within the selected size range, and its boiling heat transfer coefficient is the highest at different heat fluxes. At a heat flux of 170.55 kW/m 2 , the boiling heat transfer coefficient reaches 13.54 kW/(m 2 ·K), which is 35.67% higher than that of a polished surface.