A "fan" is an axial or centrifugal impeller (usually, an axial impeller is called a fan and a centrifugal one a "blower", and the rotating machinery of a high-speed tunnel is called a "compressor", because the pressure rise is a large fraction of the absolute pressure and therefore a multi-stage axial machine like a jet-engine compressor is used).
Most closed-circuit tunnels are driven by axial-flow fans, which produce a static pressure rise (with no appreciable change in axial velocity or dynamic pressure unless the pressure rise is comparable with the absolute pressure) at one point in the circuit, to compensate for the total-pressure losses in the rest of the circuit.
The design of axial fans for tunnels covers a wide range. Lightly-loaded fans, which usually have a high ratio of tip speed to axial velocity ("fine pitch") and a correspondingly high relative velocity at the blades, produce the required pressure rise with a fairly small blade area and look very much like aircraft propellers. However, fans with high tip speeds cause a great deal of vibration if the approaching stream is not uniform over the cross-section, and a tip speed of more than 150 or 200 m/s in air implies a relative Mach number approaching that at which shock waves occur, again resulting in noise and vibration. In modern low-speed tunnels, therefore, the fan tip speed is kept as low as possible, not more than two or three times the local axial velocity, and the blade arrangements more closely resemble one stage of an axial-flow compressor, with a stator row in front of the rotor.
Because it is necessary to return to uniform, non-swirling flow in a circular or polygonal section downstream of the fan, the diameter of the central nacelle ("boss", "hub") is usually a smaller fraction of the fan diameter than in multi-stage compressor practice, and rarely exceeds 0.5 to 0.6 of the fan diameter. As a result the space between adjacent blades, measured around the circumference, varies considerably from root to tip. The space/chord ratio typifies the fan loading, and it also determines the allowances to be made for the effect of one fan blade upon the adjacent ones. Near the tip the interference is small and propeller design rules can be used, but near the root the space/chord ratio is smaller and turbomachinery design rules apply. Unfortunately there is often a region near mid-radius where neither set of rules is accurate and some kind of interpolation is needed. It is usual to keep the cross-sectional area of the air stream nearly constant over the length of the nacelle, so the outer casing has to bulge outwards. The structural alternatives are a fiberglass moulding or, more crudely, an expanding cone, a cylindrical section and a contracting cone, in that order.
In closed-circuit tunnels, axial fans are usually mounted downstream of the second corner, where the cross-sectional area is two or more times that of the test section. This reduces the optimum tip speed of the fan, leading to less noise and vibration. There is also the practical advantage that objects blown out of the test section have some chance of being caught or fragmented by the corner vanes before reaching the fan blades.
Axial fans have a much more limited operating range (in terms of the pressure-rise coefficient, defined as the ratio of pressure rise through the fan to the dynamic pressure of the flow) than centrifugal blowers. If the pressure coefficient is required to be too high, the fan blades will stall, usually beginning at the root where the blade angle to the plane of rotation is largest, so that the flow direction may be downstream near the blade tips and upstream near the roots. If the pressure coefficient is too low, then a negative-angle stall near the tips will occur. Axial-flow fans, usually with the electric motor mounted in the central boss, are commercially available but high-performance tunnels normally have custom-built fans.
A good detailed account of axial-flow fan design, nominally for mine ventilation fans but applicable to wind tunnels as well, is given by R.A. Wallis, Axial Flow Fans and Ducts, Wiley-Interscience (1983).
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 diffuser of a closed-circuit tunnel.
Centrifugal blowers are almost always bought "off the shelf" as low-pressure
commercial blowers intended for ventilation systems, so the design process
reduces to searching the manufacturer's catalog for a unit that produces the
required total-pressure rise at the required volume flow rate. These blowers are
usually made of sheet metal and should be distinguished from high-pressure
centrifugals with cast metal cases.