Axial Flow Fan Research Articles

Optimized Casing Treatment to Improve Stall Margin Under Circumferential Pressure Distortion in a High-Speed Axial Flow Fan

Abstract An optimized axial slot casing treatment (OPCT) was designed that experimentally improves the stall margin by 7.31% with an efficiency gain of 1.94% at a design speed of 100% in a high-speed axial flow fan. Subsequently, the OPCT was tested under the distorted inflow with a 9% composite distortion index; it can still improve the stall margin by 6.19% with an efficiency gain of 1.63% at the 100% design speed. The unsteady measurement results indicate that the location of the shock wave obviously moves forward toward the leading edge of the blade tip as the rotor blade rotates out of the distorted region and induces the stall in advance. When the OPCT is applied, the strength of the interaction between the shock wave and tip leakage flow can be suppressed, and the location of the shock wave at the distorted region is pushed backward. Thus, the trigger of the stall inception generated in the downstream of the distortion region is postponed. Meanwhile, a new phenomenon was discovered that the OPCT does not alter the instability mode of the fan, and the fan eventually surges, which is consistent with the B parameter value of 2.942. However, under the distorted inflow, the B parameter can be reduced from 2.942 to 0.863, thereby the instability mode of the fan is changed from surge to stall. It can guide the instability prediction when the boundary condition (distortion or casing treatment) of the fan is changed.

Quantifying the Tip Leakage Vortex Wandering in a Low-Speed Axial Flow Fan via URANS Simulation

The tip leakage vortex is a dominant noise and aerodynamic loss source of low-speed axial flow fans. The tip leakage vortex often has an unsteady behavior, like the periodic motion of the vortex core: the so-called vortex wandering. This periodic vortex wandering results in additional noise, aerodynamic loss, and an oscillation in the moment acting on the blading, which causes an unfavorable vibration of the rotor. In the research campaigns focusing on vortex wandering, complicated measurement techniques (PIV) or high-fidelity but computationally costly simulations are commonly used. The present paper aims to introduce a relatively cost-effective unsteady Reynolds-averaged Navier-Stokes simulation-based investigation method of vortex wandering. The capabilities of the proposed technique are presented through a low-speed axial flow fan case study.

Investigation of varying tip clearance gap and operating conditions on the fulfilment of low-speed axial flow fan

Abstract The present study aims to identify the suitable tip clearance and volumetric flow rates for low-speed axial flow fans. The performance of an axial fan depends on various parameters like fan speed, available tip clearance gap, volumetric flow rate, power consumption, and working fluid. Leakage flow occurring at a rotating component such as a blade and solid casing of a ducted axial fan is typically linked to losses and the potential emergence of a rotating stall. The numerical analysis for this study uses the Reynolds Averaged Navier–Stokes equations with the k-omega SST turbulence model to perform the steady-state simulations by varying tip gap and volumetric flow rate. The results proved that the optimum performance was obtained with a tip clearance gap of 1 mm, and maximum fan efficiency was achieved at a volumetric flow rate of 3.9–4.5 m3/s. The novelty of this proposed work is to enhance the efficiency of axial flow fans with circular arc-cambered aerofoils using optimum tip clearance and volumetric flow rate through steady-state simulations. This method can be used in turbomachinery to improve fan performance.

Axial Flow Fan Performance in a Forced Draught Air-Cooled Heat Exchanger for a Supercritical Carbon Dioxide Brayton Cycle

Abstract An axial flow cooling fan has been designed for use in a concentrated solar power plant. The plant is based on a supercritical carbon dioxide (sCO2) Brayton cycle, and uses a forced draft air-cooled heat exchanger (ACHE) for cooling. The fan performance has been investigated using both computational fluid dynamics (CFD) and scaled fan tests. This paper presents a CFD model that integrates the fan with the heat exchanger. The objective is to establish a foundation for similar models and to contribute to the development of efficient ACHE units designed for sCO2 power cycles. The finned-tube bundle is simplified, with a Porous Media Model representing the pressure drop through the bundle. Pressure inlet and -outlet boundary conditions are used, meaning the air flowrate is solved based on the fan and tube bundle interaction. The flowrate predicted by the CFD model is 0.5% higher than the analytical prediction, and 3.6% lower than the design value, demonstrating that the assumptions used in the design procedure are reasonable. The plenum height is also found to affect the flowrate, with shorter plenums resulting in higher flow rates and fan efficiencies, and longer plenums resulting in more uniform cooling air flow.

Innovative configurations for heat sink integrated with piezoelectric fans

Piezoelectric (PE) fans are gradually replacing conventional axial flow fans in the field of heat dissipation for microelectronics due to small size and high efficiency. However, the flow characteristic of PE fans was seldom considered when arranging heat sink fins, which could not utilize the fans’ advantages. Here, the design of fin arrangement based on the transient flow field generated by the vibration of dual PE fans in the channel was calculated and analyzed by CFD simulation and dynamic grid technology. Four zones with different flow field characteristics were found: Zones 1 and 4 which located inlet and outlet of the channel were Parallel flow zone. Zone 2 with vortices corresponds to the area ranges from the waist of blades to middle section of the channel was Multiple vortices flow zone. Zone 3 with high-speed vortices and jet flows corresponds to the area ranges from middle section of the channel to halfway downstream of blades region was Diffuse flow zone. Innovative configurations of heat sinks where fins arranged at the inlet and surrounding blades were designed according to the flow field characteristics: the loss of high-speed airflow to the inlet was prevented effectively and heat dissipation in the downstream heat sinks was dispersed by fins arranging in Parallel flow zone and Multiple vortices flow zone. The development and merging of high-speed vortices were strengthened by the inhomogeneous pin-fins arranging in Diffuse flow zone. The optimal coordination between the velocity field and the temperature gradient field among innovative configurations was found based on the field coordination analysis. Heat transfer performance was enhanced by new designs where the average temperature compared to conventional heat sink with PE fans was reduced by 16.7°. Our work provided a new fins arrangement to achieve uniform temperature distribution and high heat transfer performance for heat sink integrated with PE fans and would benefit the application in thermal management of high power devices integrated with PE fans.

Optimization design method based on parameter reduction and active subspaces: Redistribution of chordwise loading at blade tips in a transonic axial-flow fan

In practical optimization design, an excessive number of design variables have a highly detrimental influence on the efficiency and accuracy of the final design scheme and expose the optimization problem to the curse of dimensionality. Therefore, incorporating only the most essential variables into an optimization design problem facilitates obtaining accurate and cost-efficient solutions. Reported here is an optimization design method based on parameter reduction and active subspaces, and it is used to redistribute the tip load in a transonic fan. Specifically, a coupled design strategy is developed to reduce the number of parameters needed to describe the three-dimensional blade shape, which leads to far fewer design variables being involved in the optimization design. Moreover, active subspaces are used to perform sensitivity analysis and establish low-dimensional surrogate models. After the coupled design, a blade is represented effectively by only three parameters, each of which has a significant influence on the fan performance. Three one-dimensional active subspaces are established for maximum mass flow rate, maximum total pressure ratio, and maximum efficiency, based on which the linear surrogate models are obtained. Next, the chordwise tip blade loading is optimized, after which the rotor efficiency at the design point is increased by 1.1%, while the total pressure ratio remains nearly unchanged. Finally, the flow field is analyzed to understand the mechanism for this performance improvement, and the results show that the optimized blade loading reduces the aerodynamic losses caused by shock-induced flow separation and the interaction between shocks and tip leakage flows.

Design of an innovative flow isolation device (FID) as an air flow barrier to control the temperature of large storage areas in cold room applications

The virus transmission chain of Covid-19 has a resulted in a significant, global impact on social and working life. Covid-19 vaccines require protective refrigerated conditions during delivery and storage. Warehousing and transportation of these vaccines need to be properly controlled to maintain the product quality and efficacy. Logistics companies developed large-scale refrigerated warehouses as storage relay stations. The heat convection between two rooms of the large-scale, low-temperature warehouses with different temperatures for example (5 °C, -15 °C) should be prevented to improve efficiency. In this study, an innovative flow isolation device (FID) was designed in Taipei Tech to block the storage areas, preventing heat convection. This device can maintain a constant low-temperature in warehouses used for vaccine storage. The axial flow fan draws air from the protected zones (5 °C, -15 °C) into the device. A perforated plate stabilizes the flow rate and maintains a uniform flow direction, creating a flow barrier created between two rooms. This flow can significantly block the heat convection induced by two warehouses with different temperatures. The computational fluid dynamics (CFD) software Ansys Fluent is used to simulate the effect of the FID on the prevention of the heat convection between two rooms. Different case studies were simulated at different machine sizes and supply air flow speeds. The simulated flow patterns and velocity fields are used to evaluate the highest isolation efficiency. The findings show that the FID device can effectively maintain the low temperature of the warehouse. Moreover, the FID helps to avoid heat loss and maintain thermal comfort in working areas, reducing the risk of hypothermia.

Benchmarking the Performance of the Actuator-Disk Method for Low-Pressure Axial Flow Fan Simulation

Abstract The actuator-disk method is a cost-effective simulation tool that implicitly represents the rotor of a turbomachine using a blade-element approach combined with two-dimensional (2D) airfoil coefficient input data. Actuator-disk models further rely upon empirical coefficient corrections to modify the 2D input data to better mimic physical blade aerodynamic characteristics. However, the fabrication of high-fidelity, general-purpose corrections remains a formidable challenge, so the limits of actuator-disk model accuracy have never been rigorously tested and remain uncertain. This is especially the case for low-pressure axial flow fan models, given the relative lack of empirical corrections conceived specifically for fan rotor simulation. In this study, benchmark performance limits of the actuator-disk method for axial flow fan analysis are explored. The limits of the modeling approach are interrogated by simulating actuator-disk fan models embedded with accurate physical blade data derived from explicit three-dimensional (3D) fan model computations. It is subsequently shown that even with a near-precise coefficient description, the method remains unreliable. However, using an unconventional actuator-disk model formulation that is based directly on 3D blade force inputs, it is demonstrated that the accuracy of conventional models can be noticeably enhanced if the input coefficient data is artificially manipulated. This nonphysical tuning of the input coefficient data is required to compensate for the simplified flow fields produced by the reduced-order modeling approach. The needed coefficient adjustments, referred to as modeling corrections, are subsequently defined and explained alongside the presentation of new realistic performance targets for future actuator-disk fan model variants.

Influence of air-guided ring structure on performance of axial-flow fan

Influence of air-guided ring structure on performance of axial-flow fan

Blade retrofit design to reduce the noise of an axial flow fan by mitigating the tip leakage vortex

In the context of controlling axial fan aerodynamic noise, one effective approach involves the redesign of the blade. In the present study, a novel blade design was formulated for a specific model of axial flow fan, targeting the mitigation of the tip leakage vortex (TLV) and subsequent reduction of noise levels. The assessed noise-reduction blades have perforations drilled from the leading edge (LE) to the pressure surface (PS) of the blades. By impeding the tip leakage flow (TLF), these perforations effectively weaken the TLV. The three-dimensional non-constant flow field within the channel is computed employing the shear stress transport (SST) turbulence model. The aerodynamic noise prediction, which is based on the Lighthill’s acoustic analogy theory, shows that the designed blades can effectively reduce the aerodynamic noise of the axial fan. Specifically, enhancing the ratio of inlet area to outlet area of the ventilation holes leads to an improved noise reduction effect. Notably, for ratios of 0.44 and 1.06, the designed blades exhibit noise reductions of up to 3.3 dB(A) and 3.9 dB(A), respectively. Meanwhile, the static efficiency of these two fans is reduced by 0.50% and 0.26%, respectively. The study thus provides good theoretical support for the design of axial flow fans in the context of noise reduction.

Numerical Analysis of a Novel Casing Treatment in an Axial Flow Fan

The present paper reports the use of a slotted casing treatment of trapezoidal shape to enhance the stall margin of an axial flow fan. The Taguchi method was used to optimize the rectangular slotted casing treatment based on that which the trapezoidal slots were designed. The unsteady numerical simulations were carried out using the shear-stress-transport turbulence model. The rotor with trapezoidal slots led to the stall margin improvement of 23.4% and mild pressure rise of 0.54%. Without the casing treatment, the tip leakage trajectory was directed from the suction surface of the blade toward the pressure surface of the adjacent blade. After incorporating the trapezoidal slots, the tip leakage trajectory was significantly shifted toward the suction surface of the same blade. This alteration in flow characteristics near the blade-tip region may be attributed to the exchange of momentum due to the simultaneous suction and blowing through the trapezoidal slots. It was also found that the trapezoidal slots improved the axial flow velocity and reduced the diffusion factor in the blade-tip region as compared to the baseline case.

Enhancing Axial Flow Fan Performance in Air-Cooled Condensers: Tip Vortex Manipulators and Comparative Analysis of Numerical Simulation and Experimental Testing

Abstract Direct dry-cooled power plants typically operate numerous large-diameter axial flow fans in air-cooled condensers (ACCs) to facilitate the condensation of steam in the plant's thermodynamic cycle. Enhanced fan efficiency may, at scale, lead to significant parasitic power reductions and subsequent increases in plant power output. The performance of these fans may be increased by adding blade-tip modifications, which act to control blade-tip leakage flow. Previous studies have shown that manipulating the tip leakage vortex (TLV) increases the blade-to-air momentum transfer and stabilizes the air flow structures as it traverses the fan blade. This paper takes a step toward the development of improved tip vortex manipulators for an ACC fan. Three-dimensional computational fluid dynamics (CFD) simulations, and experimental testing of the modified and unmodified fan within an ISO test facility are performed. The results are then utilized to assess the accuracy of the simulation model, visualize the manipulated flow structures at the blade tip, and ultimately quantify the effect of the endplate designs on fan performance. Excellent correlation between numerical and test results for the unmodified fan is observed, and the benefit of blade-tip modification on fan performance is again confirmed experimentally. The simulation model, however, fails to predict improvements in fan performance under modification, and simulated tip leakage flows are investigated for clarification. It is hypothesized that deflection of tip endplates under operation account for differences between simulation and experiment, and that fluid–structure interaction analyses could potentially resolve discrepancies.

Experimental and numerical comparisons among various skewed blades of low speed axial fan rotors

Literature indicates that there is lack of study related to the effects of axially skewed blades on the aerodynamic performance of axial flow fans. In this study rotors of various blade skews, namely, axial, circumferential and axial-circumferential types, are studied and their aerodynamic performances are compared to those of the baseline radial rotors of two types, namely, constant chord and varying chord. The skew in a blade can be considered as combination of sweep and dihedral. The skew change from the circumferential to axial type causes reversal of dihedral induced radial blade force and hence a reversal of meridional streamline curvature which in turn reverses the modulation of the end-wall loads. The inherent forward sweep in the forward skewed blades also modulates hub and tip loads. Thus, in a forward axial skew blade the forward sweep and positive dihedral combine favorably to unload the tip and to upload the hub. In the forward circumferentially skewed blades, forward sweep and the negative dihedral have mutually countering effects on end-wall load modulation. The highlight of this study is that it has demonstrated experimentally the unloading of the tip in the forward axially skewed blades which resulted in reduced losses and better performance including increased stall margin. Reduced tip loading was also verified from the casing pressure sensor measurements which showed weakening of tip vortex. The relative influence of sweep and dihedral on performance was additionally demonstrated in the axially-circumferentially skewed rotor of the constant chord type, with improvements attributable to higher sweep.

Investigation into the predicted performance of a cooling fan for an sCO2 concentrated solar power plant

An axial flow fan is designed for dry cooling of a sCO2 Brayton cycle for a CSP plant, and a scaled model of the fan is manufactured and tested experimentally in an ISO 5801:2017 Type A fan test facility. While the design procedure’s estimate of the total-to-static efficiency at the design flow rate is relatively high, at 74.4%, the experimental fan efficiency was found to be only 50.6%. This paper details an investigation into the causes of fan performance degradation, using both experimental and numerical methods. The investigation finds that the hub configuration, tip clearance size, and scale of the test fan is responsible for the reduced performance. Two alternative hub and casing configurations are identified which improve the fan’s performance significantly.

Fault diagnosis of mine main ventilator based on multi-eigenvalue selection and data fusion

As a strong nonlinear complex system, the mine main ventilator is difficult to select eigenvalues of the ventilator fault. This study proposed a multiple eigenvalue selection method based on ensemble empirical modal decomposition (EEMD). Firstly, an experimental platform of axial flow fan was built to simulate the bearing fault and fan blade fault. Secondly, the vibration and wind pressure data of the above three faults and the normal state were collected, and the vibration signals were decomposed by EEMD. The normalized Root Mean Square Error was used to evaluate the signal separation degree of different faults under the same eigenvalue and to select the eigenvalue. Then, combined with wind pressure information, multi-data information fusion was used to construct the failure indicator vector table. Finally, the BP neural network was trained and tested with a vector table, and the fault diagnosis of the mine main ventilator was achieved. The experiment results showed that the diagnostic accuracy of test results was 98.87% after eigenvalue selection and data fusion. The method has high accuracy and adaptability, and is very suitable for mechanical equipment such as mine ventilator.

Analysis of aerodynamic and aeroacoustic performances of axial flow fans with variable winglet curvature in chordwise direction

This study introduces a novel concept for axial-flow fans with winglets featuring gradually decreasing curvature along the chordwise direction. The objective is to mitigate the formation of leakage vortices in the gap flow region between the fan blade tip and the shroud. Two types of axial-flow fans, denoted as VR-A and VR-B, are designed, each incorporating variable winglets with differing rates of curvature reduction compared to the baseline CR fan with constant winglet curvature. The research encompasses a comprehensive approach involving computational fluid dynamics (CFD) simulations and experimental analyses. Numerical simulations employ incompressible Unsteady Reynolds-averaged Navier-Stokes (URANS) equations to capture intricate flow behaviors, while a virtual fan performance tester is used for flow performance predictions. Furthermore, the Ffowcs Williams & Hawking (FW–H) integral equation is utilized to estimate the radiated aerodynamic noise. Experimental measurements include flow performance assessments and noise evaluations, yielding data on flow rate, efficiency, and sound pressure spectrum. Comparisons between numerical and experimental results validate the numerical methods. Subsequent evaluations highlight the advantages of the VR fans over the CR fan, showing increased flow rates and reduced noise levels. These enhancements are attributed to the weakened leakage vortex in the gap flow region. The findings provide valuable insights for optimizing axial-flow fans partially covered by shrouds, offering improved flow performance and decreased noise radiation.

Further Development of Rotating Beamforming Techniques Using Asynchronous Measurements

When rotating noise sources, such as turbomachinery, are investigated using phased microphone array measurements and beamforming, sidelobes appear on the resulting beamforming maps. Sidelobes can be decreased by increasing the number of microphones. However, if the investigated phenomenon is steady, then there is a cost-effective alternative: performing asynchronous measurements using phased arrays having a limited number of microphones. The single beamforming maps can be combined in order to arrive at results that are superior in resolution and sidelobe levels. This technique has been investigated in the literature, but according to the authors’ best knowledge, has not yet been applied to turbomachinery. This article introduces a means for applying the asynchronous measurement technique and the combination methods for rotating noise sources. The combination methods are demonstrated on two rotating point sources (both in simulations and measurements), and then on an axial flow fan test case. In the case of the two rotating point sources, the achievable improvement in resolution, average-, and maximum sidelobe levels are shown as compared to the single results. In the case of the axial flow fan, it is demonstrated that the combination methods provide more reliable noise source locations and reveal further noise sources.

The Effects of Rotation and Solidity on the Aerodynamic Behavior of Low-Pressure Axial Flow Fans

Abstract Implicit axial flow fan models are commonly used for the numerical analysis of industrial heat exchanger systems. However, these fan models perform poorly within the often complex off-design flow environments characteristic of these systems, as their low-order formulations lose applicability under highly 3D flow conditions. At off-design flow conditions, rotation and blade solidity effects cause physical fan blade behavior to deviate from the anticipated 2D behavior characterized by the implicit models. Physical fan blade behavior at off-design flow conditions, however, remains uncertain and the specific effects of rotation and solidity are not yet widely understood. This study investigates off-design fan blade behavior by presenting explicit 3D computations of two low-pressure axial flow fans. The effects of rotation and blade solidity are then identified through a side-by-side comparison of the computed 3D fan blade data against isolated 2D airfoil data. The rotating fan blade lift characteristics are shown to be distinct from the comparative 2D airfoil trends. At low-flow (off-design) operating conditions, rotation establishes large radial flow movements, and the high blade solidities cause standing vortices to develop within the rotating blade passages. The radial outflow energizes the blades' boundary layers, and the vortices inhibit the flow from separating from the blades' surfaces. Consequently, blade stall is avoided and high lift coefficient values are realized. The presented fan blade lift characteristics are unique relative to literature on other turbomachines of lower blade solidity, highlighting the importance of considering solidity effects in implicit rotor model developments.

Development of a Performance-Based Design Technique for an Axial-Flow Fan Unit Using Airfoil Cascades Based on the Blade Strip Theory

Axial-flow fans are widely used as cooling fans in the outdoor units of split-type air conditioners. The design of an axial-flow fan blade involves stacking several airfoils that can be differently designed for each spanwise section. However, the complex flow field around the fan blade, including circumferential and axial flows, presents challenges when applying the single airfoil theory. This study proposed a systematic performance-based design method for axial-flow fans using a cascade of airfoils based on the blade strip theory. The theory characterized the complex three-dimensional flow field driven by an axial-flow fan in terms of a two-dimensional cascade of airfoil flows. Computational fluid dynamics based on finite volume methods were used to predict the flow field and aerodynamic sound sources of an existing low-pressure axial-flow fan partially covered by a fan shroud, and the results were validated against experimental measurements. Three radial locations in the spanwise region from the hub to the blade tip that have a significant impact on aerodynamic performance were selected, and the two-dimensional flow field on a cylindrical surface with a constant radius was extracted from the three-dimensional flow field to characterize the performance of an axial fan. Then, the airfoils at the targeted span locations were optimized for a higher flow rate and greater efficiency via two-dimensional simulations using the cascades of the airfoil, and the selected optimized airfoils were applied to existing fan blades. The effectiveness of the proposed performance-based design method for low-pressure axial-flow fans was validated by the results, which showed that the redesigned fan blades with cascades of airfoils performed as predicted, increasing the intended higher flow rate by about 1%, improving power consumption by 8%, and lowering the overall sound pressure level by 1.5 dBA.

Filtering Suppression for Large Peak Conducted EMI from High Speed Axial Flow Fan

Filtering Suppression for Large Peak Conducted EMI from High Speed Axial Flow Fan