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Figure 1: Components of a centrifugal fan An external motor belt driven inline centrifugal fan discharging inline to the exterior of a building through a duct. Unlike non-inline/non-concentric impeller casing design with a cutoff blade above, the concentrically symetric cylinder casing and impeller geometry of inline type redirects the outflow around so that it is parallel to the inflow of gases.
In these rotary implements, the affinity laws apply both to centrifugal and axial flows. The laws are derived using the Buckingham π theorem. The affinity laws are useful as they allow the prediction of the head discharge characteristic of a pump or fan from a known characteristic measured at a different speed or impeller diameter.
The centrifugal design uses the centrifugal force generated by a rotating disk, with blades mounted at right angles to the disk, to impart movement to the air or gas and increase its pressure. The assembly of the hub, disk and blades is known as the fan wheel, and often includes other components with aerodynamic or structural functions.
A fan may have two maps, one for the bypass (i.e. outer) section and one for the inner section which typically has longer, flatter, speed lines. Military turbofans tend to have a much higher design fan pressure ratio than civil engines. Consequently, the final (mixed) nozzle is choked at all flight speeds, over most of the throttle range.
The fan is designed to produce a pressure difference, and hence force, to cause a flow through the fan. Factors which determine the performance of the fan include the number and shape of the blades. Fans have many applications including in wind tunnels and cooling towers. Design parameters include power, flow rate, pressure rise and efficiency. [1]
In an airfoil, the mean line curvature is designed to change the flow direction, the vane thickness is for strength and the streamlined shape is to delay the onset of boundary layer separation. Taking all the design factors of an airfoil, the resulting forces of lift and drag can be expressed in terms of lift and drag coefficient.
The simplest inlet to a centrifugal compressor is typically a simple pipe. Depending upon its use/application inlets can be very complex. They may include other components such as an inlet throttle valve, a shrouded port, an annular duct (see Figure 1.1), a bifurcated duct, stationary guide vanes/airfoils used to straight or swirl flow (see Figure 1.1), movable guide vanes (used to vary pre ...
A cross-flow fan is a centrifugal fan in which the air flows straight through the fan instead of at a right angle. The rotor of a cross-flow fan is covered to create a pressure differential. A cross-flow fan has two walls outside the impeller and a thick vortex wall inside. The radial gap decreases in the direction of the impeller rotation.