The collection efficiency by inertial impaction in fibrous filters increases with which parameter(s)? Identify the most comprehensive choice.

Introduction:
Inertial impaction is dominant for relatively large particles and higher velocities where particle inertia prevents perfect streamline following. Recognizing what increases impaction helps optimize filters, cyclones, and sampling inlets for coarse particle capture.

Given Data / Assumptions:

• Flow around fibers in the appropriate Reynolds number range.
• Particles have sufficient inertia to deviate from streamlines as parameters change.
• Filter geometry is held constant while velocity and particle size vary.

Concept / Approach:
Impaction correlates with Stokes number, Stk ∝ d_p^2 * U (for given density and characteristic length). Thus, increasing particle diameter (d_p) and/or air velocity (U) raises Stk, increasing the probability that particles cross streamlines and strike the collector. Interception (geometry-driven) and diffusion (Brownian motion-driven) have different parametric dependencies; here the key factors are size and velocity enhancing inertia.

Step-by-Step Solution:

Verification / Alternative check:
Filtration correlations and cyclone cut-size equations show improved coarse particle capture with higher velocity and larger particle size, consistent with increasing Stk.

Why Other Options Are Wrong:

• (a) or (b) alone are incomplete; both factors matter.
• (d) duplicates (a) without including velocity.

Common Pitfalls:
Over-speeding filters may increase pressure drop and energy use; balance capture efficiency with operational constraints.

Final Answer:
both (a) and (b)