It is seen from the above relations that while the maximum propulsive efficiency of the conventional system approaches 1 when Uj/U∞→1, the corresponding value of the wake-filling propulsion system can reach 2 when Uw/U∞→0. The fact that ηp can be greater than 1 is well known to the naval industry, and the term propulsive coefficient is sometimes used for ηp.
Smith [27] showed that reduction of the wake distortion behind a body through ingestion of the viscous wake into an engine reduces the necessary propulsive power. Using actuator disk theory, Smith demonstrated that it is possible to achieve propulsive efficiencies greater than 1, the theoretical maximum for propulsion without wake ingestion. In the same light, if the engine ingests the boundary layer over the surface of a wing, the same benefit can be realized by re-energizing the low momentum fluid and expelling it out at the trailing edge to fill in the wake. Even without considering boundary-layer ingestion, Ko [28] estimated that for a blended wing body aircraft with embedded propulsion, the propulsive efficiency could be increased from about 80% to 90% using the effect of wake filling. Finally, Kummer and Dang [20] employed CFD to demonstrate that an embedded propulsion system based on the cross-flow fan can reach propulsive efficiency greater than 1.
In addition to increased propulsive efficiency, embedded propulsion can potentially provide reduced noise and increased safety, since the propulsor is now buried within the structure of the aircraft (e.g. no exposed blading). Also, by eliminating the engine pylon/nacelle support structure, the aircraft parasitic drag can be reduced by 18–20%, thus improving cruise efficiency and range. Design of embedded propulsion systems using conventional propulsors presents many challenges. First, by embedding the engines within the wing structure, the fan size becomes restricted, and conventional axial propellers and turbofan engines incur performance penalties as their sizes are reduced. Also, embedded propulsors will inherently ingest non-uniform boundary-layer flows, which tend to reduce engine performance further [29]. With cross-flow fans, however, these problems are less severe as their performance is less affected by the size constraint imposed by the wing thickness, and their performance has been shown through CFD to be less sensitive to inlet flow distortion. In particular, Kummer [30] showed that, using the in-line CFF housing geometry he developed, the exhaust velocity profile was a weak function of the shapes of the inlet velocity profile, which included uniform and boundary-layer-like profiles.
The integration of the cross-flow fan into an airfoil has also been shown to be capable of providing circulation control [21] and [22]. In particular, increased flow ingestion into the fan by increasing its rotational speed or redirecting the exhaust flow via flaps (jet-flap effect) can result in increased lift coefficient. Sectional lift coefficient on the order of 7 has been shown possible with this powered-lift device, resulting in short takeoff and landing (STOL) capability and low in-flight aircraft stall speed without the use of additional high lift devices such as slotted flaps or leading edge slats. The combination of circulation control and differential thrust, accomplished through fan speed and external exhaust nozzle shape, may eliminate the need for other flight control surfaces.
A well-known disadvantage of the cross-flow fan is its low efficiency. However, it can compete with other propulsion technologies at the system level. The embedded cross-flow fan yields lower drag relative to the conventional pylon/nacelle support structure, and as mentioned earlier, its performance is relatively insensitive to wake ingestion, making it suitable for applications that require thick wings (e.g. flying wing platform). Even at low angle-of-attack, the wake created by a thick airfoil can be quite large for thick wings, producing high levels of pressure drag. This renders thick wings sections impractical for most aircraft applications as the drag penalty outweighs any benefits gained in lift or interior volume.
This article seeks to review the basic aerodynamics of cross-flow fans and their application in aircraft propulsion, and is organized as follows. Section 2 presents the basic fluid mechanics and energy transfer processes of the fan with the aid of a simplified one-dimensional model. Performance data of cross-flow fans specifically developed for aviation applications are reviewed in Section 3. In Section 4, prediction methods based on unsteady-flow CFD methods and simplified steady-flow methods are described, along with comparisons of prediction results to test data. The unsteady-flow CFD results are also used to illustrate the detailed flow field of the cross-flow fan, emphasizing the specific flow regions. In Section 5, we review several aircraft concepts that employ the cross-flow fan as a propulsor and/or flow control device along with test data and computational results. Finally, concluding remarks are given in Section 6.
2. Fundamental aerodynamics of cross-flow fans
2.1. Three flow regions
In this section, we review the basic aerodynamics and energy transfer processes in cross-flow fans. We show that the flow within the impeller can be classified into three distinct regions and a mean streamline analysis is used to describe the through-flow and loss characteristics of the fan. Computational and experimental data are presented along the way to aid in the physical description of the behavior of this unique turbomachine.
The flow field of the cross-flow fan is predominantly two-dimensional (2D), moving perpendicular to the impeller axis. Flow enters the blade row in the radially inward direction on the upstream side, passing through the interior of the impeller, and then passes radially outward through the blading a second time. The flow is characterized by the formation of an eccentric vortex that runs parallel to the rotor axis with rotation in the same direction. Fig. 8 shows an example flow field prediction based on a URANS analysis. In the figure the path-lines in the region exterior to the impeller are referenced to the stationary frame, while those on the interior are referenced to the rotating frame. Fan rotation is counter-clockwise.