$17.$ Pneumatic conveying of solids is influenced by the conveying $\_\_\_\_\_\_\_\_$ and the $\_\_\_\_\_\_\_\_$ characteristics of the particles.

$18.$ In trickle bed reactors, the distribution of the $\_\_\_\_\_\_\_\_$ phase is critical to avoiding channeling and $\_\_\_\_\_\_\_\_.$

$19.$ The drift flux model accounts for the relative velocity between phases, known as $\_\_\_\_\_\_\_\_$ velocity, and the spatial distribution termed the $\_\_\_\_\_\_\_\_$ fraction.

$20.$ The application of the drift flux model to gas$-$liquid flows in mini channels helps in understanding the $\_\_\_\_\_\_\_\_$ behavior and $\_\_\_\_\_\_\_\_$ characteristics.