 ## Multiphase Flows

• FAQ
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Section 8.5.3.2 of the CFX Modeling Guide, entitled “Particle Shape Factors”, explains the significance of the Cross Sectional and Surface Area Factors. The Guide mentions that CFX assumes all particles to be spherical by default, and continues this assumption while calculating a particle’s diameter from dividing its mass by its density. The Cross Sectional Area Factor may be included to modify the assumed spherical cross-sectional area if the particles are actually non-spherical. This factor is multiplied by the calculated cross-sectional area when assuming spherical particles, and affects the calculated drag force. To elaborate, let a cuboid particle’s dimensions be length (L) x width (W) x depth (D), where L>W>D. Then, let the following definitions be: •Cross-sectional area, A = W*D •Volume, V = L*W*D •Equivalent spherical radius, Rv = (3/4*V/pi)^(1/3) •Cross Sectional Area Factor, CSAF = A/(pi*Rv^2) CSAF is then multiplied by the cross-sectional area of the spherical particle equivalent of the cuboid particle. The Surface Area Factor (SAF) is analogous to the CSAF in that it adjusts the assumed spherical particle shape to enable non-spherical particles. However, this factor is defined as the ratio of the surface area of the non-spherical particle to the surface area of the spherical particle with the equivalent diameter (non-spherical divided by spherical). The SAF affects mass and heat transfer correlations and thus must be figured out based on the surface geometry of the non-spherical particle compared to the surface of a spherical particle of equivalent mass. 