1. Introduction
The cross-flow fan consists of a drum-like rotor with forward curved blades, encased within housing walls as shown in Fig. 1. The inlet and outlet have rectangular cross-sections, and the key advantage of the fan in most applications is its ability to extend lengthwise, producing a uniformly distributed inflow and outflow. Therefore, the impeller length-to-diameter ratios are often large. Geometric design of the fan housing (or casing) tends to be quite varied and in some cases difficult to classify and parameterize. However, the primary features can be identified as the rear wall, vortex wall (or stabilizer) and end-walls. Fig. 2 from Eck [1] gives an idea of the broad range of possible housing designs that can be used. Most housings are intended for approximately 90° flow turning from inlet to outlet, however, the fan is capable of 180° turning (casing 6) and in-line flow (casing 7). The in-line design is of particular interest for aircraft applications.