A “blower” tunnel is driven by an impeller at entry, usually a true (centrifugal) blower which is almost always of the backward-airfoil or squirrel-cage type rather than the old-fashioned radial-blade variety seen, for example, in car water pumps and domestic hair dryers. The airfoil blades of a centrifugal blower run at nominally the same angle of attack all along the span, and the reduction in pressure rise as the blades stall is gradual, without much deterioration in outlet flow steadiness and uniformity. It must be said that the outlet flow frm a centrifugal blower is disturbed at the best of times, but not much worse than the likely condition of the flow at exit from the main Aroma Diffuser of a closed-circuit tunnel.