Experimental and Computational Investigation of Cross-Flow Fan Propulsion for Lightweight VTOL Aircraft

Progress in Aerospace Sciences, 2009

Cross-flow fans offer unique opportunities for distributed propulsion and flow control due to their potential for spanwise integration in aircraft wings. The fan may be fully or partially embedded within the wing using a variety of possible configurations. Its inlet may be used to ingest the boundary-layer flow, and its high-energy exhaust flow may be injected into the wake at the wing trailing edge for drag reduction or vectored thrust. Cross-flow fans are high-pressure coefficient machines, so they can be diametrically compact. However, their efficiency is fundamentally limited by unavoidable recirculation flows within the impeller at all flight speeds, and by additional compressibility losses at high speeds. This article reviews the fundamental aerodynamics and flow regions of cross-flow fans using a simple mean-line analysis to examine the basic energy transfer and loss processes. Experimental data for fans intended for aircraft application are then reviewed and compared to calculations using unsteady Navier-Stokes methods, showing the state-of-the art in flow field and performance prediction capability. Alternative prediction methods where blade action is modeled in terms of body-force or vortex elements are discussed, including challenges in handling arbitrary non-uniform, unsteady blade flows for various design configurations. The article concludes with a review of cross-flow fan propulsion and flow control concepts that have been investigated by various researchers, and with discussions on future challenges in their application.

Journal of Thermal Science

This present paper describes three dimensional computational analysis of complex internal flow in a cross flow fan. A commercial computational fluid dynamics (CFD) software code CFX was used for the computation. RNG k-e two equation turbulence model was used to simulate the model with unstructured mesh. Sliding mesh interface was used at the interface between the rotating and stationary domains to capture the unsteady interactions. An accurate assessment of the present investigation is made by comparing various parameters with the available experimental data. Three impeller geometries with different blade angles and radius ratio are used in the present study. Maximum energy transfer through the impeller takes place in the region where the flow follows the blade curvature. Radial velocity is not uniform through blade channels. Some blades work in turbine mode at very low flow coefficients. Static pressure is always negative in and around the impeller region.

Energies

Cross-flow fans (CFFs) have become increasingly popular in recent years. This is due to their use in several domains such as air conditioning and aircraft propulsion. They also show their utility in the ventilation system of hybrid electric cars. Their high efficiency and performance significantly rely on the design parameters. Up to now, there is no general approach that predicts the CFFs’ performance. This work describes a new methodology that helps deduce the performance of CFFs in turbomachinery, using both analytical modeling and experimental data. Two different loss models are detailed and compared to determine the performance–pressure curves of this type of fan. The efficiency evaluation is achieved by realizing a multidisciplinary study, computational fluid dynamics (CFD) simulations, and an optimization algorithm combined to explore the internal flow field and obtain additional information about the eccentric vortex, to finally obtain the ultimate formulation of the Eck/Lai...

International Journal of Fluid Mechanics Research, 2012

This present paper describes three dimensional computational analysis of complex internal flow in a cross flow fan, (CFF) with a special emphasis on the performance improvement by varying impeller and casing geometries, inlet and tongue clearance gaps. RNG k-ε two-equation turbulence model is used to simulate the model with unstructured mesh. Sliding mesh interface is used at the interface between the rotating and stationary domains to capture the unsteady interactions. Three impeller geometries, seven casing configurations and three inlet and tongue clearances are varied in the present study. The efficiency of the fan has improved from 59 % to 71 % and that of casing from 37 % to 61 %. Maximum energy transfer through the impeller takes place in the region where the flow follows the blade curvature.

Journal of Energy Systems, 2017

Axial flow fans are broadly applied in numerous industrial applications because of their simplicity, compactness and moderately low cost, such us propulsion machines and cooling systems. Computational fluid dynamics techniques are commonly applied to investigate flow phenomena through the axial fan and the rotor dynamic performance. In the present work, a computational model of an axial fan is presented in the current study. Numerical simulations of a single stage axial fan on variable conditions have been performed to obtain the detailed flow field of the centrifugal fan. The investigation of the current work is focused on the rotor–stator configuration and the modeling of aerodynamic behavior of the blade rows. The precise prediction of axial force and efficiency has essential implication for the optimized operation of axial fan and the choice of thrust bearing. Furthermore, it can act as guide for the geometrical and structural axial fan design and the study of axial force predic...

Rotor-only axial fans feature rotors designed according to different vortex criteria. Nowadays the literature does not exhaustively clarify when a specific swirl distribution has to be used and which are the advantages/drawbacks in terms of fan performance and efficiency. A review of the experimental performance of rotor-only axial fans designed with different vortex criteria is summarized here in Φ − Ψ and σ − δ (specific speed-specific diameter) graphs to identify the best operating conditions of each design. Four rotor-only axial fans (two free-vortex, a constant-swirl and a rigid-body swirl one) are tested on an ISO-5801-A rig. For two of them, flow velocities at rotor exit are measured with a 5-hole probe. The result is an experimentally based map around the Cordier curve for rotor-only axial fans. Indications on the best Φ − Ψ range for fans designed using different vortex criteria are provided and explained. The effects of increasing the tip clearance on the rotor performance at design duty are investigated as well.

Journal of Aircraft, 2012

Ducted-fan-based vertical and/or short takeoff and landing uninhabited aerial vehicles are frequently encountered in aeronautical applications. In edgewise flight, the performance of these vehicles is, in general, poor because of the increasingly distorted inlet flow as the flight speed is increased. The present experimental study uses a planar particle image velocimeter system to investigate the near duct aerodynamic performance in hover and edgewise flight conditions. High-resolution particle image velocimetry measurements provide reliable and highresolution aerodynamic data forming a validation basis for further analytical and computational design studies. A radial equilibrium-based fan aerodynamic model is also integrated into a three-dimensional Reynold-averaged-Navier-Stokes-based computational system. Particle image velocimetry measurements and computational predictions of the mean flow near the fan inlet plane are in very good agreement at hover conditions. The aerodynamic modifications due to fan inlet flow distortion in an edgewise flight regime are clearly displayed in particle image velocimetry results. A comparison of the current particle image velocimetry measurements and the accelerated Reynold-averaged-Navier-Stokes predictions supported by the simple radial equilibrium-based rotor model indicates that the current rotor model can be highly effective and time efficient in the design cycle of future vertical and/or short takeoff and landing uninhabited aerial vehicle systems based on ducted fans.

Experimental Thermal and Fluid Science, 2004

Cross-flow fan performance is strongly influenced by the geometry of the casing, as the latter, in turn, affects the position and the strength of the eccentric vortex that characterizes the operation of this category of fans. The paper presents a systematic experimental investigation of the flow field within the impeller at different throttling conditions and for different geometries of the fan casing. Both pressures and velocities are measured using a three-dimensional five-hole probe that is inserted in the flow. This study helps determine the relationship between the design parameters of the casing and the flow field pattern, and it is part of an extensive work, by the same research group, aimed at establishing a general theory on cross-flow fan operation and at outlining the design guidelines for this particular type of turbomachines.

Polish Journal of Environmental Studies, 2014

This paper presents a CFD study on flow characteristics in the centrifugal fan in nominal and off-design conditions. Numerical calculations were carried out using ANSYS CFX package [10]. The numerical model was verified on the grounds of experimental tests using the standard methods to determine the performance curve of centrifugal fans. The following paper also presents the pressure distributions in select cross-sections of a machine, relative velocity, and static pressure profiles inside the rotor blade channels for the full range of flow characteristics.

Based on the requirement of energy consumption level and weight and dimension restriction, compact axial machines are highly demanded in many industrial fields. The counter-rotating axial-flow fans could be a promising way to achieve these requirements. Because of the reduction of rotational speed and a better homogenization of the flow downstream of the rear rotor, these machines may have very good aerodynamic performances. However, they are rarely used in subsonic applications, mainly due to poor knowledge of the aerodynamics in the mixing area between the two rotors, where very complex structures are produced by the interaction of highly unsteady flows. The purpose of the present work is to compare the global performances (static pressure rise and static efficiency) and the wall pressure fluctuations downstream of the first rotor for three different stages operating at the same point: a single subsonic axial-flow fan, a conventional rotor-stator stage and a counter-rotating system that have been designed with inhouse tools. The counter-rotating system allows large savings of energy with respect to the other two systems, for lower rotation rates and by adjusting the distance between the two rotors, a solution with comparable wall pressure fluctuations levels for the three systems is found.