Aeration-duct spacing in tropical maize silos: a factorial ventilation experiment linking airflow short-circuiting to stored-grain moisture-drift bounds

Aeration-duct spacing in tropical maize silos is conventionally sized from bulk airflow rate, yet the spacing that actually controls moisture migration is the spacing at which duct-to-duct airflow short-circuiting begins to dominate the through-grain path. the present analysis treats short-circuiting as a design constraint rather than a commissioning nuisance and derives spacing bounds from it. A factorial ventilation experiment was specified over four duct spacings and three airflow rates, with the inter-duct mixing length defined as the distance over which the wall-adjacent and core airstreams equalise. The experiment was evaluated by a replication-based power analysis of the before-after-control-impact estimator under a declared variance structure (between-cluster SD = 6.0, residual SD = 4.5, intraclass correlation = 0.64), with 2000 replications per design cell. Power rose with effect size at every spacing, and the minimum detectable effect at 80% power fell as the number of instrumented clusters increased, reaching 5 percentage points at 54 clusters. The intraclass correlation of 0.64 implied that cluster-level replication, not within-silo sampling density, carried the design. The contribution is a spacing rule in which the duct pitch is bounded above by the short-circuit mixing length and below by the plenum pressure budget, converting a ventilation-rate specification into a moisture-drift specification.