Low Pressure Axial Flow Fans Research Articles

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.

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.

Multi-Objective Optimization for the Radial Bending and Twisting Law of Axial Fan Blades

The performance of low-pressure axial flow fans is directly affected by the three-dimensional bending and twisting of the blades. A new blade design method is adopted in this work, where the radial distribution of blade angle and blade bending angle is composed of standard-form rational quadratic Bézier curves. Dendrite Net is then trained to predict the pneumatic performance of the fan. A non dominated sorting genetic algorithm is employed to solve the global optimization problem of the total pressure coefficient and efficiency. The simulation results show that the optimal blade load distribution along the radial direction becomes uniform, and the suction surface separation vortex and passage vortex are restrained. On the other hand, the tip leakage vortex is enhanced and moves toward the blade leading edge. According to the experimental results, the maximum efficiency increases by 5.44%, and the maximum total pressure coefficient increases by 2.47% after optimization.

An optimization on the stacking line of low-pressure axial-flow fan using the surrogate-assistant optimization method

An optimization on the stacking line of low-pressure axial-flow fan using the surrogate-assistant optimization method

Experimental investigation of tip-leakage flow in an axial flow fan at various flow rates

We investigate the evolution of tip-leakage vortex (TLV) in a low-pressure axial flow fan using digital particle image velocimetry. The blade rotational speed is fixed at 1000 rpm, and the Reynolds number is 547000 based on the blade tip radius and tip velocity. The evolution of TLV in downstream is highly influenced by the incoming flow rate. The TLV breakup occurs for the peak efficiency and stall conditions and sometimes for a higher flow rate, but does not occur for the highest flow rate considered. Thus, the migration speed of the TLV for the peak efficiency condition is faster due to the TLV breakup than those for higher flow rates. The scatter plot of the TLV center location indicates that the TLV wanders around its mean location and the region swept by the wandering motion increases as the TLV migrates downstream. High turbulent kinetic energy exists at the phase-averaged TLV center and its upstream location, respectively, owing to the wandering motion and the interaction between the TLV and main axial flow.

Experimental evaluation of energy efficiency and velocity fields on a low-pressure axial flow fan (desktop type)

Energy efficiency labeling programs are worldwide applied to provide energy savings (electrical, mechanical, thermal, etc.) in machines and equipment for residential and industrial use, and also a potential reduction in greenhouse gas emissions. Actual research and development indicate that low-pressure axial flow fans represent a significant amount of energy consumption (electricity) especially in developing countries with warm climates, as in Brazil. In this work, bench tests were performed at university facilities, taking into account Brazilian standards for energy efficiency (Brazilian Labeling Program) for residential fans. Equipment under test is a free blow fan (desktop type, residential use), three-bladed with 500 mm diameter and angular speed ranging from ~ 500 up to 1500 s−1. Experimental analysis and measurement employed hot-wire anemometry, tachometer, wattmeter, chronometer, and data acquisition system. The results are for performance evaluation of turbomachinery characteristic curves. It includes airflow (m3 s−1), angular speed (s−1), the power supplied/received (W), efficacy ηENCE,n [(m3 s−1 W) m], and non-dimensional parameters: efficiency η (%) and design flow rate, as well as velocity field, among others. Main contributions indicate that (a) velocity and airflow in turbulent and fully developed conditions are consistent with energy efficiency and efficacy literature comparison, (b) power received by the fluid flow increases ten times more than supplied electrical energy (17/1.8), in the test range, (c) maximum energy conversion efficiency is ηmax ~ 33% at ~ 1300 RPM and nominal flow rate φ ~ 0.165, (d) efficacy is lower than 0.0030 {[(m3 s−1) W)] m}, classified in “D” level, and (e) expected energy savings ~ 30% by reaching “A” efficacy levels.

Correlation between Aerodynamic Noise and Velocity Fluctuation of Tip Leakage Flow of Axial Flow Fan

In the present paper the attention is focused on correlation between fan noise and velocity fluctuations of tip leakage vortex around rotor blade of a low pressure axial flow fan at the maximum pressure operating point. We measured time fluctuating velocity near the rotor tip around the rotor blades by using a hot-wire sensor from a relative flame of refer- ence fixed to the rotor blades. As the results, it is clear that the velocity fluctuation due to tip leakage vortex has weak periodicity and the hump portion appeared in its spectrum. If the flow rate was lower than the design condition, the tip leakage flow became to attach to the following blade and the sound pressure level at frequency of velocity fluctuation of this flow was increased. The correlation measurements between the velocity fluctuation of tip leakage flow and the aerodynamic noise were made using a rotating hot-wire sensor near the rotor tip in the rotating frame. The correlation between the velocity fluctuation due to tip leakage flow and acoustic pressure were increased due to generation of weak acoustic resonance at the maximum pressure operating point.

Correlation between Aerodynamic Noise and Velocity Fluctuation of Tip Leakage Flow of Axial Flow Fan

In the present paper the attention is focused on correlation between fan noise and velocity fluctuations of tip leakage vortex around rotor blade of a low pressure axial flow fan at the maximum pressure operating point. We measured time fluctuating velocity near the rotor tip around the rotor blades by using a hot-wire sensor from a relative flame of refer- ence fixed to the rotor blades. As the results, it is clear that the velocity fluctuation due to tip leakage vortex has weak periodicity and the hump portion appeared in its spectrum. If the flow rate was lower than the design condition, the tip leakage flow became to attach to the following blade and the sound pressure level at frequency of velocity fluctuation of this flow was increased. The correlation measurements between the velocity fluctuation of tip leakage flow and the aerodynamic noise were made using a rotating hot-wire sensor near the rotor tip in the rotating frame. The correlation between the velocity fluctuation due to tip leakage flow and acoustic pressure were increased due to generation of weak acoustic resonance at the maximum pressure operating point.

Numerical Prediction on Noise from Axial Flow Fan Based on Modified Blade Force Model

This paper presents a modification to the existing blade force model of Wu et al. [5] to predict noise radiated from a low-pressure axial flow fan. The aerodynamic theory for the fan impeller is introduced and the module analysis is conducted to calculate the flow parameters more accurately, the lift coefficient is modified by taking into account the specific shape of the blade, and an empirical parameter is also replaced by the CFD simulation based on FLUENT software. The numerical prediction on the noise radiated from the fan is performed by using the modified model, and the predicted results are more closely to the experimental results than those before modification.

Experimental research on aerodynamic performance and exit flow field of low pressure axial flow fan with circumferential skewed blades

In this article, the low pressure axial flow fan with circumferential skewed rotor blade, including the radial blade, the forward-skewed blade and the backward-skewed blade, was studied with experimental methods. The aerodynamic performance of the rotors was measured. At the design condition at outlet of the rotors, detailed flow measurements were performed with a five-hole probe and a Hot-Wire Anemometer (HWA). The results show that compared to the radial rotor, the forward-skewed rotor demonstrates a wider Stable Operating Range (SOR), is able to reduce the total pressure loss in the hub region and make main loading of blade accumulating in the mid-span region. There is a wider wake in the upper mid-span region of the forward-skewed rotor. Compared to the radial rotor, in the backward-skewed rotor there is higher total pressure loss near the hub and shroud regions and lower loss in the mid-span region. In addition, the velocity deficit in the wake is lower at mid-span of the backward-skewed rotor than the forward-skewed rotor.

Large Eddy Simulation of Acoustical Sources in a Low Pressure Axial-Flow Fan Encountering Highly Turbulent Inflow

A large eddy simulation (LES) was applied to predict the unsteady flow in a low-speed axial-flow fan assembly subjected to a highly “turbulent” inflow that is generated by a turbulence grid placed upstream of the impeller. The dynamic Smagorinsky model (DSM) was used as the subgrid scale (SGS) model. A streamwise-upwind finite element method (FEM) with second-order accuracy in both time and space was applied as the discretization method together with a multi-frame of reference dynamic overset grid in order to take into account the effects of the blade-wake interactions. Based on a simple algebraic acoustical model for axial flow fans, the radiated sound power was also predicted by using the computed fluctuations in the blade force. The predicted turbulence intensity and its length scale downstream of the turbulence grid quantitatively agree with the experimental data measured by a hot-wire anemometry. The response of the blade to the inflow turbulence is also well predicted by the present LES in terms of the surface pressure fluctuations near the leading edge of the blade and the resulting sound power level. However, as soon as the effects of the turbulent boundary layer on the blades become important, the prediction tends to become inaccurate.

Large Eddy Simulation of Acoustical Sources in a Low Pressure Axial-Flow Fan due to Highly Turbulent Inflow Conditions

Large Eddy Simulation of Acoustical Sources in a Low Pressure Axial-Flow Fan due to Highly Turbulent Inflow Conditions

Phenomenon of labyrinth weal with Low static pressure difference and large clearance

The low pressure axial flow fans with an outer ring, used for cooling automobile radiators, have a significantly large tip clearance between the ring tip and the fan shroud. It has been found that the turbulent reverse flow, or leakage flow, which occurs at the tip clearance, greatly affects the fan performance and noise level. Therefore, in order to improve the fan performance and noise level it is important to decrease the effect of leakage at the tip clearance. The authors investigated the performance of the straight-through type of labyrinth seal which operates in an extremely low static pressure difference with a large clearance. It was hoped that by sealing this clearance with the labyrinth seal the performance would be improved. It was verified that the labyrinth seal satisfied almost the same performance as that predicted by the previous theory. This theory was established by experimental studies in the condition of quite high static pressure difference when the labyrinth is stationary. However, it was later discovered that the leakage rate decreased significantly even though there was far lower ring speed in comparison to past research results where the ring rotated. This phenomenon is conspicuous in a lower differential pressure. However, the cause of this phenomenon has not been determined.

Axial Flow Fan Noise Generated by the Interaction between Tip Leakage Flow and an Adjacent Rotor Blade

To clarify the mechanism of noise generation from a low pressure axial flow fan, time fluctuating velocity was measured around the rotor blades, especially near the rotor tip, by using a hot-wire sensor from a relative flame of reference fixed to the rotor blades. Two sensors were used to obtain the cross-correlation between velocity fluctuations at the reference point and other locations. The results are summarized as follows. If the flow rate was lower than the design condition and the tip leakage flow became to attach to the following blade, the sound pressure level was increased. From the cross-correlation of velocity fluctuations the leakage flow flew downstream spirally, in which a discrete frequency velocity component with low frequency was predominant. It is possibly considered to be the cause of the noise generation of low frequency band.

107 リング付低圧軸流ファン用直通形ラビリンスシールの特性

The low-pressure axial flow fans with an outer ring, used for cooling automobile radiators, have a significantly large tip clearance between the ring tip and the fan shroud. It has been known that the turbulence of reverse flow, or leakage flow, which occurs at the tip clearance, greatly affects the performance and noise level. In order to improve the performance and noise level, the authors studied about the labyrinth seal which was used at the clearance. As a result, we discovered that it was possible to predict the performance by conventional theory when the labyrinth was stationary, and that the leakage rate decreased significantly even though the ring speed was much less than that of past research when the ring was rotated.

Performance of Straight-Through Type Labyrinth Seal for a Low-Pressure Axial Flow Fan with an Outer Ring

The low-pressure axial flow fans with an outer ring, used for cooling automobile radiators, have a significantly large tip clearance between the ring tip and the fan shroud. It has been found that the turbulent reverse flow, which occurs at the tip clearance, greatly affects the fan performance and noise level. Therefore, in order to improve the fan performance and noise level it is important to decrease the effect of leakage at the tip clearance. The authors investigated the performance of the straight-through type of labyrinth seal which operates in an extremely low static pressure difference with a large clearance. It was hoped that by sealing this clearance with the labyrinth seal the performance would be improved. It was verified that the labyrinth seal satisfied almost the same performance as that predicted by the previous theory. This theory was established by experimental studies in the condition of quite high static pressure difference when the labyrinth is stationary. However, it was later discovered that the leakage rate decreased significantly even though there was far lower ring speed in comparison to past research results where the ring rotated. This phenomenon is conspicuous in a lower differential pressure. However, the cause of this phenomenon has not been determined.

Internal Flow of Straight-Through Type Labyrinth Seal for a Low-Pressure Axial Flow Fan with an Outer Ring

Internal Flow of Straight-Through Type Labyrinth Seal for a Low-Pressure Axial Flow Fan with an Outer Ring

105 軸流ファン構造の斜流ファンの設計における可視化手法の応用

Low pressure axial flow fans without stator blades or guide vanes are generally used in order to achieve light weight, low cost and a simple structure, and to conserve space when mounted for automobile cooling system. However, since flow resistances in front and back of the fan are very strong, air tends to flow diagonally at the fan outlet side. The fan does not operate as an axial flow fan. In this report, we describe work conducted on a motor fan, mainly from the standpoint that analysis and improved understanding of actual air flow conditions lead to improved design. As for the measurement of air flow at the outlet and inlet side of the fan, the five hole-pitot tubes were used. And the reverse flow area was visualized to adhere tracer particles which are in the air flow to the pressure surface of the blades. As a result, we could predict the air flow among fan blades and portion and shape of the reverse flow area. These data were useful for designing the mixed flow fan which has the structure of an axial flow fan.

Flow characteristics of the sound pressure level and its prediction for a low pressure axial flow fan.

The relation between the noise level of an axial flow fan and the flow around the fan rotor was investigated over a wide range of fan flow rates. The magnitude of the wake width in the region where the velocity of air relative to the blade is large controls the noise level. The change in the noise level with fan flow rate can be estimated by a working formula proposed by the present authors for predicting the noise level of a fan working at its design point.