Numerical study on flow-boiling heat transfer in square channels partially centrally filled with gradient porous media

Boiling heat transfer is widely used in industry and daily life and can be enhanced by porous media structure. Previously HPM have been shown to significantly improve heat transfer performance, but how the porosity gradient affects boiling heat transfer remains unknown. In this paper, the subcooled flow boiling in vertical square channels partially filled with gradient porous media was numerically investigated by using a newly developed solver named GPMPhaseChangeFoam embedded in the open-source platform OpenFOAM. The results indicate that in positive GPM inserted channels, when z <= 0.16 m Nu(z) increases with increasing H-p*, and when z >= 0.16 m Nu(z) increases with increasing H-p* from 0.2 to 0.8 but deceases with H-p* rises from 0.8 to 1.0, and the effect of H-p* on Nu(z) is more pronounced in the entrance region over the exit region. In negative GPM inserted channels, Nu(z) increases with increasing H-p* throughout the channel length in positive GPM inserted channels, and the effect of H-p* on Nu(z) is less pronounced in the entrance region than the exit region. Flow resistance increases with H-p*, but changes little with Delta epsilon, and Nu(fd) increases with H-p* in both positive and negative GPM inserted channels. However, Nu(fd) decreases with an increase in triangle epsilon from 0 to 0.3 in positive GPM inserted channels, from -0.3 to 0 in negative GPM inserted channels, and triangle epsilon = -0.3 at H-p* = 1.0 is 13.4 % higher than that of triangle epsilon = 0.3. Furthermore, the negative GPM inserted channel behaves higher flow boiling heat transfer performance than the HPM and positive GPM.