Contra-rotating Fan Research Articles

Development of Carbon Composite Blades within the Context of the Experimental Validation of a CFD-Based Design Tool for Contra-Rotating, Electric Fan Engines

Electric propulsion systems have emerged as a disruptive technological approach towards achieving sustainable and climate-neutral aviation. To expand the operational envelope of such propulsion units in terms of altitude and velocity, an enclosing duct and counter-rotating rotors to enhance efficiency can be utilized. In this study, an iterative CFD-based design tool developed for these novel propulsion systems is utilized to design a reference engine, having a classic rotor–stator configuration. Being the key component of this propulsor, a manufacturing process for composite blades is presented. This effort aims to make state-of-the-art technology accessible to smaller research projects, promoting the widespread adoption of electric propulsion technology in the aviation sector. By experimental investigations of the blade elongation both in tensile tests and engine operation, measuring the tip clearance with a high-speed camera, this process could be validated to facilitate the transferability of research. Finally, the performance of the manufactured engine is measured by a five-hole miniature probe, not only in design point but also in off-design operation. The results indicate that a substantial reduction in discrepancies between initial specifications, subsequent CFD simulations, and experimental investigations compared to conventional design tools relying on empirical formulations can be achieved due to the CFD-based approach. This allows the CFD-based tool to be validated for designing scalable contra-rotating fan engines.

Effect of inlet distortion on internal flow and performance of a contra-rotating fan operating with single-stage impeller

To establish a theoretical foundation for the rational selection of contra-rotating fans operating with a single-stage impeller under inlet distortion. Unsteady numerical simulations were conducted using the SST k-ω turbulence model to analyze the airflow fields, respectively, for the single-stage operation of front impeller (SSOFI) and the single-stage operation of rear impeller (SSORI) of the fan. The internal flow characteristics with single-stage operation under both distorted inlet condition (DIC) and uniform inlet condition (UIC) were studied. Furthermore, a comparison of the performance parameters of the fan, including total pressure and total pressure efficiency, was conducted. Focusing on the SSOFI with satisfactory operational performance of the contra-rotating fan, this study aims to reveal the propagation of inlet distortion inside the fan and reveal the influence mechanisms of inlet distortions on the performance of the fan. The results indicate that the impact of inlet distortion on the flow field is primarily concentrated in the upstream area of the front-stage impeller for SSOFI. However, the flow disturbance from the exit of the front-stage impeller to the diffuser is relatively minimal. For SSORI, the inlet distortion exacerbates the adverse effect of the front stage impeller as the front guide vane on the flow inside the rear stage impeller. The entropy loss in the two-stage impellers is significantly higher than that under the uniform inlet condition, and the turbulence level in the diffuser is intensified. While the efficiency of the fan is significantly influenced by the inlet condition and operating stages, their impact on the total pressure is small. Specifically, under uniform inlet conditions, the total pressure efficiency for SSOFI exceeds that of SSORI by 4.9 %, whereas the difference rises to 11 % under inlet distortion condition.

Impact of solidity and speed ratio on the performance of a contra-rotating fan

In this study, the effects of solidity and rotor speed ratio on the performance of a contra-rotating fan are investigated numerically. Simulation results have been validated by experimental tests. The test stand construction and the measurements have been performed according to ISO-5801 standard. In order to investigate the effect of blade solidity, simulations have been conducted with constant blade count of the front rotor and four configurations of the rear rotor (having 6, 8, 12 and 16 blades), and also constant blade count of the rear rotor and three configurations of the front rotor (having 8, 12, and 16 blades). Results suggest that there is an optimum solidity for the rear rotor, which gives maximum pressure rise and efficiency. Furthermore, the effects of the rotor speed ratios have been investigated in this study. It is shown that the speed of the front rotor is more effective on the fan performance, as compared to the rear rotor. Results reveal that the performance drop caused by a 25% reduction in the rear rotor blade count, can be compensated by 5% increase of the front rotor rotational speed. Therefore, a suitable choice of the speed ratio can result in light-weight and high-performance contra-rotating stages.

Numerical Study on Local Entropy Production Mechanism of a Contra-Rotating Fan.

Contra-rotating fans (CRFs) have garnered significant attention due to their higher power-to-weight ratio compared to traditional fans; however, limited focus has been given to the localization and development of local aerodynamic losses. Furthermore, there is a need for further research on the impact of load distribution along the radius on local entropy production. Therefore, this study aims to investigate a contra-rotating fan as the research subject. An optimal design for load distribution along the radius is achieved by constructing a surrogate model in combination with a genetic algorithm. The effectiveness of this design has been verified through experimentation using a specific test device. In this study, a local entropy production rate (EPR) model adapted to the shear stress transport-detached eddy simulation (SST-DES) technique is constructed to evaluate the loss distribution of the contra-rotating fan. This paper primarily focuses on comparing and analyzing the blade profile and overall performance of the CRFs before and after optimization. The EPR contribution of each interval along the radius is compared to the corresponding blade channel to identify the approximate range of high-EPR regions. Furthermore, an investigation is conducted to examine the distribution of EPR along the streamwise direction in these high-EPR regions. After that, by comparing the development of the flow structure near a stall before and after optimization, combined with the analysis of the EPR contours, the EPR mechanism of this CRF is revealed.

Detailed Performance Studies on a Small Electric-Powered Contra-Rotating Ducted Fan Engine

Electrical propulsion has been identified as one of the key fields of future research within the aerospace sector. This paper aims to contribute to the ongoing development of small electric-powered ducted fan engines with a thrust in the range of a few hundred newtons. A special emphasis is placed on introducing contra-rotating fans to such engines. Recently, such an innovative contra-rotating engine has been developed and put into operation on a novel test bench. This study intends to take a closer look at the performance of the engine at design speed ratio by means of experimental and numerical investigations. Global performance parameters are analyzed to validate parallel numerical simulations. Results indicate that both experiment and simulation are in good agreement. Moreover, detailed flow studies of the spanwise distribution of relevant parameters are conducted, and results are discussed. From these results, the stall behavior of the stage is identified. Finally, a comprehensive performance map is established with the help of a number of numerical simulations, which reveal a good off-design performance behavior of the engine.

Effect of the Radial Velocity Distribution on the Loss Generation of a Contra-Rotating Fan in a Ventilation System.

Quantification of the loss generation of ducted contra-rotating fan (CRF) blades is difficult to achieve, since there are no guide vanes between rotors. A blade design program was established to investigate the relationship between radial velocity distribution and incurred loss. Numerical and experimental techniques were used to confirm the optimal configuration's overall performance. The relationship between loss and velocity distribution under the impact of spanwise load distribution was confirmed by the entropy contour from various perspectives. The appropriate radial velocity distribution can improve the operating efficiency of a CRF by reducing the entropy around the annulus under design and near-stall conditions. This regularity could provide some strategies in the design of contra-rotating blades.

Flow effects of microperforated-panel casing treatments in a contra-rotating fan

The flow effects of microperforated-panel (MPP) casing treatments installed over the front rotor of a small contra-rotating (CR) fan are numerically studied, with emphasis put on the reshaped front-rotor tip leakage flow, the blade response to rotor-rotor interaction, and the unsteadiness of static pressure on blade surfaces. It is found that the secondary flow through the sub-millimeter holes of the MPP hinders the formation of the front-rotor tip leakage flow and reduces its circumferential stretching, resulting in an improved inflow to the rear rotor. Consequently, the unsteady characteristics of static pressure and lift force on the rear rotor are generally attenuated. An enlarged back cavity can enhance such attenuation, in which case a 14.5% decrease in pressure unsteadiness is observed. The first, second, and third harmonics of the unsteady lift force experience reductions of 15.6%, 37.4%, and 60.9%, respectively. However, the unsteady characteristics on the front rotor dominated by potential interaction are not significantly influenced by the MPP. Furthermore, analysis of blade-to-blade phase differences shows that, whether the CR fan is treated with the MPP or not, the phase differences are not precisely repeated but vary randomly and that the higher the order of the force harmonic, the more likely the phase relationship is disturbed by the treatments. The varying nature of the phase relationship would render the acoustic energy of an induced interaction tone leaked into shaft tones and broadband noise. Radiation efficiency analysis of force harmonics demonstrates that the front rotor is not an efficient contributor to interaction noise compared with the rear rotor, which radiates more interaction tones with much higher efficiency. The current work is expected to be useful not only for devising effective MPP casing treatments to control small CR fan noise but also for understanding their underlying flow effects on rotor-rotor interaction.

Identification and classification of operating flow regimes and prediction of stall in a contra-rotating axial fan using machine learning

AbstractPrediction of stall before it occurs, or detection of stall is crucial for smooth and lasting operation of fans and compressors. In order to predict the stall, it is necessary to distinguish the operational and stall regions based on certain parameters. Also, it is important to observe the variation of those parameters as the fan transitions towards stall. Experiments were performed on a contra-rotating fan setup under clean inflow conditions, and unsteady pressure data were recorded using seven high-response sensors circumferentially arranged on the casing, near the first rotor leading edge. Windowed Fourier analysis was performed on the pressure data, to identify different regions, as the fan transits from the operational to stall region. Four statistical parameters were identified to characterise the pressure data and reduce the number of data points. K-means clustering was used on these four parameters to algorithmically mark different regions of operation. Results obtained from both the analyses are in agreement with each other, and three distinct regions have been identified. Between the no-activity and stall regions, there is a transition region that spans for a short duration of time characterised by intermittent variation of abstract parameters and excitations of Fourier frequencies. The results were validated with five datasets obtained from similar experiments at different times. All five experiments showed similar trends. Neural Network models were trained on the clustered data to predict the operating region of the machine. These models can be used to develop control systems that can prevent the stalling of the machine.

The Effect of Load Control on the Performance of Contra-Rotating Fans

In recent years, few studies focused on adjusting the load distribution of contra-rotating fan (CRF) blades. To improve the overall performance of CRFs, we used a design code to build 32 sets of CRFs to determine the effects of three factors—the front and rear rotor load matching, the load distribution of each rotor and the axial distance between the rotors—on the total pressure rise and efficiency of CRFs using numerical calculations. The relationship between the CRF blades load and velocity components was theoretically analyzed using blade element analysis and the forward problem method. According to the performance curve, it can be concluded that the rear rotor (RR) is the key factor that determines the performance of CRFs. Through analyzing Mach number contours from different perspectives, the relationship between velocity and adjustment load was verified. Furthermore, the flow field characteristics for three specific CRFs were explored at the stall points, design points and choke points to reveal their flow mechanisms. This study provides a reference for the CRF blade design method.

Experimental and numerical studies on small contra-rotating electrical ducted fan engines

Electrical propulsion has been identified as one of the key fields of future research within the aerospace sector. The Institute of Aeronautical Engineering at the Universität der Bundeswehr München aims to contribute to the ongoing development of small-sized electrical ducted fan engines with a thrust in the range of 100 N. A special emphasis is placed on electrically powered contra-rotating fan stages. When compared to a conventional rotor–stator stage, contra-rotating fan stages allow for a more compact design, considering a given pressure ratio, or an increased pressure ratio at a constant fan diameter. Since numerous new aircraft concepts are presently being developed, a high demand for compact and powerful electrically driven engines arises. Electrically driven contra-rotating fan engines provide a high potential in terms of compactness, emissions and efficiency. Using electric motors offers the ability to overcome common issues, such as design and integration of a contra-rotating stage into a gas turbine. An innovative new engine design featuring such a contra-rotating stage is developed and tested at one of the Institute’s test benches for electrical propulsion. Key components are two brushless motors powering the fan stage, one for each rotor. Various operation points are investigated experimentally during an extensive test campaign. Experimental results are compared to results of numerical simulations computed by ANSYS CFX. Results indicate a good agreement between experiment and simulation. The engine is running very smooth throughout all tested operation points. Yet, intensive heating up of the electric motors and high-temperature zone are found to be an issue at higher rotation speeds.

Analyzing Interaction Noise from a Small Contra-Rotating Fan Enclosed with Lined and Unlined Casings

AbstractThe interaction noise that radiates from ducted contra-rotating (CR) fans are theoretically characterized using Green’s analytical functions for lined and unlined circular ducts containing ...

Windmilling Characteristics of a Contra-Rotating Fan

Abstract In the present experimental study, a low aspect ratio, low hub-tip ratio contra-rotating axial fan is investigated to understand its performance under windmilling conditions. Two configurations are tested; in the first configuration (event A), the front rotor of the contra-rotating fan is powered and the rear rotor is allowed to windmill; in the second configuration (event B), the rear rotor of the contra-rotating fan is powered and the front rotor is allowed to windmill. The spanwise distribution of the loading coefficient and the flow angles at different streamwise positions reveal the details of the flow development across the rotors. Though the average total pressure drops across the windmilling rotor for both the events, a small spanwise region behaves as a fan or a stirrer. Thus, a “neutral radius” on the windmilling rotor is identified for both events A and B. The neutral radius appears close to the tip for event A and it appears close to the hub for event B. Thus, the span regions close to the tip behave as a fan for event A and the span regions close to the hub behave as a stirrer for event B. It is also observed that the neutral radius shifts to a lower span location as the flow coefficient reduces. Thus, the flow coefficient is a significant parameter that decides the position of the neutral radius. Further, the unsteady pressure measurements recorded at the casing capture the fundamental phenomena during the stall inception. The paper thus relates the similarities and unveils the contrasting features of the windmilling events A and B. In summary, this paper discusses the performance, flow physics, and stall inception characteristics of a contra-rotating axial fan under windmilling conditions.

Coupling analysis of contra-rotating fan interstage pressure pulsation and blade vibration based on wavelet reconstruction.

In recent years, the flow characteristics research of the interstage region in counter-rotating axial fans in terms of fluid dynamics has attracted much attention. Especially, the dynamic relationship between interstage pressure pulsation and blade vibration in counter-rotating axial fans has not yet been clarified. This paper performs the signal processing method of wavelet decomposition and reconstruction in time-frequency analysis process. Under different flow conditions, weak-coupling numerical simulation program is employed to analyze the fluid-structure coupling interaction between interstage pressure pulsations and blade vibrations in counter-rotating axial fans. The results indicate that the fluid-structure coupling interaction field in the interstage of counter-rotating axial fans mainly excites the first-order vibration of the second-stage blade. At the same time, the consistency between the pulsation frequency and the vibration frequency of the airflow reflects the good coupling property. Two stage blades cut the airflow to cause field changes and airflow pulsation, and then, airflow pulsation causes blades deformation and produces vibrations of the same frequency at the blade. The deformation of the blades, in turn, causes the flow field changes. Rotating stall, vortex movement and breakdown produced low-frequency airflow pulsation and vortex vibration of the blade.

Influence of distorted inflows on the performance of a contra-rotating fan

AbstractA contra-rotating fan offers several aerodynamic advantages that make it a potential candidate for future aircraft engine configurations. Stall in a contra-rotating axial fan is interesting since instabilities could arise from either or both of the rotors. In this experimental study, a contra-rotating axial fan is analysed under clean or distorted inflow conditions to understand its performance and stall inception characteristics. The steady and unsteady measurements identified the relative contribution of each rotor towards the performance of the stage. The tip of rotor-1 is identified to be the most critical region of the contra-rotating fan. The contribution of rotor-2 to the overall loading of the stage is observed to be relatively less than rotor-1. The penalty due to distortion in the stage pressure rise is mostly felt by rotor-1, while rotor-2 also shows a reduction in performance for distorted inflows. Rotor-2 stalls at a high flow coefficient marking the initiation of partial stall of the stage, and the stall of the whole stage occurs once rotor-1 stalls. A fluid phenomenon that is attached to the blade surface marks the stall of rotor-1, and this fluid phenomenon initially rotates at a speed close to the speed of rotation of the blade. As the stage moves towards the fully developed stall, this fluid phenomenon sheds from the blade surface. The fluid phenomenon thus propagates at a speed much lower than the rotational speed of the blade during fully developed stall.

Effect of Circumferential Casing Treatment on Low-Speed Contra-Rotating Fans

Effect of Circumferential Casing Treatment on Low-Speed Contra-Rotating Fans

A theory for rotating stall in contra-rotating fans

This paper reports on a theory of rotating stall in contra-rotating fans and compressors. The theory is developed from Moore’s theory. A second-order hysteresis is used in the current study for the pressure rise of the counter-rotating rows. This enables the model to predict the transient behavior of the stall cell. Comparing the experimental results with the theory shows that the modified model can predict the speed of the stall cells fairly accurately. Results show that the rotor speed ratio plays a critical role in the stall cell speed and its direction of rotation. Furthermore, the developed model makes it possible to study the effects of stagger angle and number of stall cells. The conditions under which pure rotating stall can occur in contra-rotating fans are also discussed in this paper. It is shown that the stall cells merge to form a single cell before a stable fully-developed rotating stall is established.

Noise attenuation and performance study of a small-sized contra-rotating fan with microperforated casing treatments

This study investigates the benefits of noise control for a small-sized contra-rotating (CR) fan using microperforated-panel (MPP) casing treatments over the rotor. Several configurations of this type of casing treatment were proposed and then simulated with the finite element method (FEM) to determine their acoustic performance. In general, the FEM simulations indicated that the configurations of placing the treatment over the front rotor are more acoustically effective than those over the rear rotor. Experimentally, it was found that each axial row of holes of the MPP casing behaves like a stator vane, interacting with the fan rotor and thus generating a noisy interaction tone. However, such a tone can be sufficiently suppressed by optimizing the row numbers at the design stage of the MPP casing to increase the interaction mode and frequency. Experimental results further showed that for the treatment over the front rotor, the fan performance was not significantly influenced, especially in the flow rate region above stall point, while for the treatment over the rear rotor, a pressure loss of no more than 10 Pa was perceived, particularly in the flow rate region around the fan design point. Acoustically, the former was able to reduce overall noise throughout the flow rate region, with a maximum noise reduction of 3.5 dBA around the design point, whereas the latter increased overall noise slightly. Moreover, an enlarged volume lowering cavity stiffness for the treatment over the front rotor was found to improve the reduction of overall noise by 0.7 dBA (from 2.8 dBA to 3.5 dBA) as well as to attenuate rotor-alone tones effectively, of which the front treatment with the regular cavity failed in noise reduction. In contrast, no substantial enhancements of noise reduction were noticed for the rear treatment with an enlarged cavity. It is anticipated that the current investigation can provide a reference for controlling CR fan noise with MPP casing treatments.

Stall inception in a contra-rotating fan under radially distorted inflows

The fan of a gas turbine engine is often subject to distortion due to non-uniformities in the incoming flow which could eventually result in an early stall of the fan. Contra-rotating fans have potential aerodynamic advantages and could replace the conventional fan configurations of gas turbine engines. Understanding the phenomenon prior to stall is important to characterise the behaviour of the stage at low mass flow rates. This paper investigates the phenomena leading to stall in a low-aspect ratio contra-rotating fan stage operating at different speed combinations under radially distorted inflows. Unsteady casing static pressure measurements obtained at a location upstream of rotor-1 leading edge are analysed using wavelet and Fourier analysis techniques. The results primarily concluded that a higher speed of rotation of rotor-2 could possibly suppress the pre-stall disturbances. The fluid structures leading to stall are initially associated with the blade passing frequency (BPF). These frequencies stretch to lower frequencies during the onset of fully developed stall and fully developed stall finally occurs at low frequencies.

Aerodynamic performance assessment of a ducted fan UAV for VTOL applications

The aerodynamic performance of a recent developed ducted fan Unmanned Aerial Vehicle (UAV) was experimentally assessed by means of force, pressure and PIV measurements. Experiments were conducted on the UAV for both hover and forward flight configurations. Particular attention was given to the in-ground-effect which is important for the take-off and landing modes of the UAV during flight. Variations included rotational speed of the contra-rotating fan, incident angle, forward flight speed and the exit-to-ground distance were examined, allowing a comprehensive understanding of the overall aerodynamic characteristics of the ducted fan UAV. Wind tunnel measurement has successfully defined an initial operation region of the ducted fan UAV. The inlet velocity distortion at the inlet of the UAV during forward flight can significantly influence the inlet circumferential pressure distribution and therefore in principle affect the overall aerodynamic performance. The presence of the ground during take-off and landing can enhance the total lift generation with slightly sacrificing propulsive efficiency. In addition, when the UAV flies in proximity of the ground the back pressure at exit will be increased and thus alleviates the lift production by the duct lip. The outcomes can provide essential information in design and optimization of ducted fan type flight vehicles.

Quasi-unsteady Flow Field Computation and Performance Prediction of Contra-Rotating Fan

Discussion of numerical flow field prediction with two rotor-stator interface models is presented in the paper. The mixing plane and harmonic models are applied to contra rotating fan. Both models confirmed good performance and high power density leading to compact configuration of the contra rotating fan compared to centrifugal fan. The harmonic model provides realistic wake and potential effect propagation through the rotor-rotor interface. In addition, it provides user with static pressure frequency and amplitude inputs for aerodynamic noise assessment.