Voidage

Voidage, also called porosity, is the fraction of a material’s volume that is not occupied by solid matter and is available to contain fluid, typically a gas or liquid. It is defined as ε = V_voids / V_total, where V_voids is the volume of empty space and V_total is the total volume of the medium. The solid fraction is then 1 − ε. In porous and granular media, ε depends on packing structure, particle size distribution, and the degree of compaction. For a simple packed bed of spheres, typical voidage values lie in the mid-range of about 0.3 to 0.5, but can vary with packing geometry and sorting.

In practical terms, voidage controls how easily fluids can flow through a material and how much of the volume is available for phase transport. In gas–solid systems such as packed or fluidized beds, ε influences pressure drop, gas residence time, heat and mass transfer rates, and reaction kinetics. In a fluidized bed, gas flow causes bed expansion, increasing voidage from its packed value to a higher operating level; the minimum fluidization voidage is a key parameter for design and operation, and the relationship between gas velocity, voidage, and pressure drop is often described by correlations based on the Ergun equation.

Voidage also appears in modeling diffusion and transport within porous media. Effective diffusivity and mass-transfer coefficients depend on ε and the tortuosity of the pore network. Measurements of voidage can be made by geometric methods (knowing solid mass, density, and bed dimensions), tracer or gas-holdup techniques, image-based methods, or porosimetry, depending on the system and scale.

Overall, voidage is a fundamental descriptor of how much space inside a material is available for fluid flow and transport.