A dimensionless number used in momentum transfer in general and flow through beds of solids calculations in particular. Proportional to (Inertial force) / (Viscous force).

Bl = (V rho) / (mu (1-epsilon) s)

Bl = G / (mu (1-epsilon) s)

epsilon: void fractrion, G: mass velocity, mu: viscosity, rho: density, s: particle area/particle volume, V: velocity

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