Experimental study of blade thickness effects on the overall and local performances of a Controlled Vortex Designed axial-flow fan

The purpose of this work is to study the effects of blade thickness on the performances of an axial-flow fan. Two fans that differ only in the thickness of their blades were studied. The first fan was designed to be part of the cooling system of an automotive vehicle power unit and has very thin blades. The second fan has much thicker blades compatible with the rotomoulding conception process. The overall performances of the fans were measured in a test bench designed according to the ISO-5801 standard. The curve of aerodynamics characteristics (pressure head versus flow-rate) is slightly steeper for the fan with thick blades, and the nominal point is shifted towards lower flow-rates. The efficiency of the thick blades fan is lower than the efficiency of the fan with thin blades but remains high on a wider flow-rate range. The mean velocity fields downstream of the rotors are very similar at nominal points with less centrifugation for the thick blades fan. Moreover, the thick blades fan maintains an axial exit-flow on a wider range of flow-rates. The main differences concern local properties of the flow: phase-averaged velocities and wall pressure fluctuations strongly differ at the nominal flow-rates. The total level of fluctuations is lower for the thick blades fan that for the thin blades fan and the spectral decomposition of the wall fluctuations and velocity signals reveal more harmonics for the thick blades fan, with less correlation between the different signals. For this kind of turbomachinery, the use of thick blades could lead to a good compromise between aerodynamic and acoustic performances, on a wider operating range.     

An engineering approach to blade designs for low to medium pressure rise rotor-only axial fans

Low to medium pressure rise axial fan equipment of the arbitrary vortex flow rotor-only type is widely used in industrial and commercial applications, with many of the installations and rotor designs being far from optimum. Complex computational methods exist for analyzing flows in, for example, high-speed axial flow compressors with multistage blade rows; however, the designers and manufacturers of low-speed, general-purpose axial flow fan equipment have been reluctant to embrace this technology. A simpler yet reliable design technique is presented that allows this category of ducted axial fan rotors, in the presence of swirl-free inlet flow, to be designed to achieve a specified duty with sufficient accuracy for engineering purposes. Practical blade design recommendations and limits, similar to those that exist for free vortex flow axial rotors, have been established for the arbitrary vortex flow rotor-only case.

The technique employs a straightforward engineering approach to arbitrary vortex flow axial fan rotor design, and the equation set can be solved by using relatively simple numerical methods. Estimates of pressure rise and shaft power characteristics for a proposed fan/rotor design can be computed and the design loop iterated until an acceptable set of blade parameters is identified. It is also possible to analyze the performance of an existing axial fan installation as a prelude to the design of a more efficient and effective replacement rotor.

Experimental data used in validating the design and analysis techniques are also presented. These data include comprehensive Cobra pressure probe surveys of local flow parameters downstream of three different low boss ratio, low solidity, arbitrary vortex flow rotors (all with circular arc camber line type blades) as well as fan performance characteristics for one of the experimental rotors configured as a direct-exhaust fan unit. Installation-dependent factors such as direct-exhaust losses and tip clearance effects are also examined. The analytical technique is shown to provide acceptable estimates of fan/rotor pressure rise performance and shaft power characteristics over a moderately wide range of blade angles and operating conditions.     

An actuator-disc model for the prediction of abrupt stall in an axial compressor rotor

The representation of loss in a cascade by the appearance of blockage has been extended to deal with blade rows by the use of a source distribution to represent this blockage, and in the case of the actuator disc approximation, the presence of sources is confined to an axi-symmetric dìstribution over the actuator disc. It is found that if a typical dependence of loss (and consequently diffusion ratio) upon incidence for each section of an axial compressor rotor is represented in this manner, the influence of blockage on the axial velocity distribution may be found using the potential equation combined with the usual actuator disc approximation. Study of the behaviour of the controlling ordinary differential equation for the axial velocity ahead of the disc reveals that as the flow is reduced, the equation contains a singularity within the range of radius and a meaningful solution does not exist. This result is interpreted as the limit to continuous operation and reasonable agreement between this predicted limit and the appearance of abrupt stall (experimentally) is found.     

A study of the flow characteristics in micro-abrasive jets

An experimental study of particle velocities in micro-abrasive jets by using the particle image velocimetry (PIV) technique is presented. It has been found that the particle jet flow has a nearly linear expansion downstream. The particle velocities increase with air pressure, and the increasing rate increases with nozzle diameter within the range considered. The instantaneous velocity profile of the particle flow field in terms of the particle velocity distribution along the axial and radial directions of the jets is discussed. For the axial profile in the jet centerline downstream, there exists an extended acceleration stage, a transition stage, and a deceleration stage. For the radial velocity profiles, a relatively flat shape is observed at a jet cross-section near the nozzle exit. Mathematical models for the particle velocities in the air jet are then developed. It is shown that the results from the models agree well with experimental data in both the variation trend and magnitude.     

Internal flow numerical simulation of a cross‐flow fan in refrigerant operating condition using user‐defined functions

In the paper, a cross‐flow fan in refrigerant operating condition is systematically simulated using user‐defined functions. Three‐dimensional simulations are acquired with Navier–Stokes equations coupled with k–ε turbulence model, and internal flow characteristics of an indoor split‐type air conditioner are obtained, which is mainly composed of cross‐flow fan and heat exchanger. It has systematically been simulated in the isothermal flow condition that the performance of cross‐flow fan may be reduced easily with dry or humid air, and in the refrigerant operating condition in which user‐defined functions are applied to the humid air, considered as a mixture of dry air and vapor. A density‐modulated function is adopted to deal with the condensation of the vapor at the heat‐transfer region approximately. The results show flow mechanism of the two gas‐phase flow, including phase‐vary process. The distribution of the parameters is not uniform at the inlet of the machine, the intensity and position of pressure and velocity vary along the axial direction of the fan, the distribution of vapor volume fraction and turbulent intensity in heat‐transfer region is obtained, and the external characteristic data of the indoor machine are obtained and analyzed. Compared with the experimental data, the calculated characteristic curves and designed parameters are on target. © British Crown Copyright 2010/MOD. Reproduced with permission. Published by John Wiley & Sons, Ltd.     

Numerical analysis of turbulent flow in a rotating U-bend with roughened walls

A numerical analysis has been performed for a developing turbulent flow in a rotating U-bend of strong curvature with rib-roughened walls using an anisotropic turbulent model. In this calculation, an algebraic Reynolds stress model is used to precisely predict Reynolds stresses, and a boundary-fitted coordinate system is introduced as a method of coordinate transformation to set the exact boundary conditions along the complicated shape of U-bend with rib-roughened walls. Calculated results for mean velocity and Reynolds stresses are compared to the experimental data in order to validate the proposed numerical method and the algebraic Reynolds stress model. Although agreement is certainly not perfect in all details, the present method can predict characteristic velocity profiles and reproduce the separated flow generated near the outer wall, which is located just downstream of the curved duct. The Reynolds stresses predicted by the proposed turbulent model agree well with the experimental data, except in regions of flow separation.     

A plasma-fluid model for EHD flow in DBD actuators and experimental validation

A numerical method to simulate plasma induced electrohydrodynamic flow is proposed in this study. The numerical model consists of three components. Firstly, a potential module to simulate temporal potential and electric field generated in the ionized fluid. Secondly, a plasma module to simulate plasma development and charge particle densities. Finally, a fluid module to simulate the flow affected by the body forces induced by the movement of the charged particles. Fluid flow is modeled using modified predictor-corrector strategy as proposed in the marker and cell method. The velocity field was corrected to achieve incompressible flow by calculating pressure correction factors, considered in all cells. Numerical convergence and time sensitivity analysis were carried to confirm grid independence and determine an efficient time step for simulations. Numerical computations are validated by comparing with experimental results of discharge currents and current densities. They were found to be in very good agreement thus providing an extensive validation. Furthermore, quiescent flow over a dielectric barrier discharge actuator is simulated in the this study, using the proposed plasma-fluid model, to model flow evolution and resolve temporal flow features for detailed analysis. The streamline and vorticity plots were analyzed and compared with experimental results, and flow results were found to be in-line with the experiments.     

Numerical simulation and experimental validation of gas–solid flow in the riser of a dense fluidized bed reactor

Gas–solid flow in the riser of a dense fluidized bed using Geldart B particles (sand), at high gas velocity (7.6–15.5 m/s) and with comparatively high solid flux (140–333.8 kg/m² s), was investigated experimentally and simulated by computational fluid dynamics (CFD), both two- and three-dimensional and using the Gidaspow, O’Brien-Syamlal, Koch-Hill-Ladd and EMMS drag models. The results predicted by EMMS drag model showed the best agreement with experimental results. Calculated axial solids hold-up profiles, in particular, are well consistent with experimental data. The flow structure in the riser was well represented by the CFD results, which also indicated the cause of cluster formation. Complex hydrodynamical behaviors of particle cluster were observed. The relative motion between gas and solid phases and axial heterogeneity in the three subzones of the riser were also investigated, and were found to be consistent with predicted flow structure. The model could well depict the difference between the two exit configurations used, viz., semi-bend smooth exit and T-shaped abrupt exit. The numerical results indicate that the proposed EMMS method gives better agreement with the experimental results as compared with the Gidaspow, O’Brien-Syamlal, Koch-Hill-Ladd models. As a result, the proposed drag force model can be used as an efficient approach for the dense gas–solid two-phase flow.     

Modeling of lifting‐device aerodynamics using the actuator surface concept

The actuator surface (AS) concept and its implementation within a differential, Navier–Stokes control volume finite‐element method (CVFEM) are presented in this article. Inspired by vortex and actuator disk methods, the AS concept consists of using porous surfaces carrying velocity and pressure discontinuities to model the action of lifting surfaces on the flow. The underlying principles and mathematics associated with AS are first reviewed, as well as their implementation in a CVFEM. Results are presented for idealized 2D cases with analytical solutions, as well as for the 3D cases of a finite wing and an experimental wind turbine. In the case of the finite wing, wake induction is well handled by the model with accurate predictions of induced angles and drag when compared with the Prandtl lifting line model. Comparisons with volume force approaches, often used to model the action of propellers or wind turbine blades in a simplified analysis, show that the AS concept has some interesting advantages in terms of accuracy and respect of flow physics. This new approach is easy and rapid to embed in most computational fluid dynamics (CFD) methods. It is applicable to a wide range of problems involving thin lifting devices like finite wings, propellers, helicopter or wind turbine blades. Copyright © 2009 John Wiley & Sons, Ltd.     

Experimental study of the three-dimensional flow field in cross-flow fans

High-resolution PIV measurements of the flow field inside cross-flow fans have been performed in planes normal and parallel to the fan axis, both outside and inside the impeller. The well known difficulties in obtaining the optical access inside the impeller have been overcome by allowing the internal flow planes to be illuminated by the laser light sheet or shot by the CCD camera through the moving blade vanes. Measurements have been performed in two cross-flow fans having the same two-module impeller but casing geometries based on very different design concepts. PIV data in planes normal to the rotor axis show a strong correlation between vorticity distribution and turbulent shear stresses inside the eccentric vortex of each fan. Furthermore, they provide useful elements to explain the very different performance of the two fans evidenced by their characteristic curves. Measurements in planes parallel to the impeller axis show that wide three-dimensional recirculation structures develop near the casing end walls at the discharge of the fans. These mean flow structures are responsible for the backflow into the end portions of the impeller of part of the discharged fluid, which is then transported axially by the eccentric vortex towards the rotor central disc before being discharged once again outside the impeller. In the case of cross-flow fans including few rotor modules, the existence of significant axial velocity components inside the eccentric vortex can alter substantially the flow picture, common in the current literature, resulting from 2-D numerical models or measurements performed in a single transverse plane of the fan.     

A comprehensive analysis of the viscous incompressible flow in quasi-three-dimensional aerofoil cascades

A rigorous model of the fully elliptic flow over the blade-to-blade stream surface in an annular aerofoil cascade is developed. The model accuracy stems from its precise simulation of the meridional hub-to-casing flow effects, including those of the shear stress components that are created by the spanwise velocity gradients. These stresses are unprecedentedly introduced in the flow-governing equations in the form of source terms and are modelled as such. The final set of flow-governing equations are solved using the Galerkin weighted residual method coupled with a biquadratic finite element of the Lagrangian type. The flow solution is verified against the numerical results of a fully three-dimensional flow model and a set of experimental data, both concerning a low-aspect-ratio stator of an axial flow turbine under a low Reynolds number and subsonic flow operation mode. The numerical results in this case show well predicted aerofoil loading and pitch-averaged exit flow conditions. Also evident is a substantial capability of the analysis in modelling such critical regions as the wake subdomain. It is further proven that the new terms in the governing equations enhance the quality of the numerical predictions in this class of flow problems.     

Flow driven by a stamped metal cooling fan – Numerical model and validation

This paper presents a numerical fluid flow model for the stamped metal cooling fans popularly employed in electric motors. An experimental system is constructed to measure the performance of the cooling fan. The agreements between model prediction and experimental data are reasonably good. Parametric studies with the numerical model indicate that the viscous heating in the fluid and the variation of the air density have negligible effects on the fan performance. The blade edge thickness affects the flow driving capability of the fan. With various pressure differentials, three flow regimes are recognized. The first is the axial component dominated flow. In the second regime, the flow has a forward axial flow and a backward leakage flow. The third one is the leakage flow dominated regime, when the pressure differential across the fan is large.     

On the numerical study of bubbly flow created by ventilated cavity in vertical pipe

The dispersion of bubbles into a down-liquid flow in a vertical pipe is investigated. At low flow rates, the intended design of a swarm of discrete bubbles is achieved. At high flow rates, a ventilated cavity is nonetheless formed, which is attached close to the gas sparger. Behind this ventilated cavity, three different flow regimes characterize the complex bubbly flow field downstream of the down-liquid flow: vortex region with high void fraction, transitional region and pipe flow region. In this study, a numerical model that solved the entire development of the gas–liquid flow including the extended single-phase liquid region upstream to the wall-jet and recirculating-vortex zones in order to allow a more realistic determination of the boundary conditions of the down-liquid flow was adopted. Coupling with the Eulerian–Eulerian two-fluid model to solve the respective gas and liquid phases, a population balance model was also applied to predict the bubble size distribution in the wake right below the cavity base as well as further downstream in the transitional and fully-developed pipe flow regions. The numerical model was evaluated by comparing the numerical results against the data derived from theoretical, numerical and experimental approaches. Prediction of the Sauter mean bubble diameter distributions by the population balance approach at different axial locations confirmed the dominance of breakage due to the high turbulent intensity below the ventilated cavity which led to the generation of small gas bubbles at high void fraction. Further downstream, the coalescence effect dominated leading to merging of the small bubbles to form bigger bubbles.     

Computational and experimental investigation of flow over a transient pitching hydrofoil

The present study is developed within the framework of marine structure design operating in transient regimes. It deals with an experimental and numerical investigation of the time–space distribution of the wall-pressure field on a NACA66 hydrofoil undergoing a transient up-and-down pitching motion from 0° to 15° at four pitching velocities and a Reynolds number Re = 0.75 × 10⁶. The experimental investigation is performed using an array of wall-pressure transducers located on the suction side and by means of time–frequency analysis and Empirical Modal Decomposition method. The numerical study is conducted for the same flow conditions. It is based on a 2D RANS code including mesh reconstruction and an ALE formulation in order to take into account the foil rotation and the tunnel walls. Due to the moderate Reynolds number, a laminar to turbulent transition model was also activated. For the operating flow conditions of the study, experimental and numerical flow analysis revealed that the flow experiences complex boundary layer events as leading-edge laminar separation bubble, laminar to turbulent transition, trailing-edge separation and flow detachment at stall. Although the flow is relatively complex, the calculated wall pressure shows a quite good agreement with the experiment provided that the mesh resolution and the temporal discretization are carefully selected depending on the pitching velocity. It is particularly shown that the general trend of the wall pressure (low frequency) is rather well predicted for the four pitching velocities with for instance a net inflection of the wall pressure when transition occurs. The inflection zone is reduced as the pitching velocity increases and tends to disappear for the highest pitching velocity. Conversely, high frequency wall-pressure fluctuations observed experimentally are not captured by the RANS model. Based on the good agreement with experiment, the model is then used to investigate the effects of the pitching velocity on boundary layer events and on hydrodynamic loadings. It is shown that increasing the pitching velocity tends to delay the laminar-to-turbulence transition and even to suppress it for the highest pitching velocity during the pitch-up motion. It induces also an increase of the stall angle (compared to quasi-static one) and an increase of the hysteresis effect during pitch-down motion resulting to a significant increase of the hydrodynamic loading.     

Application of the Dorodnitsyn finite element method to swirling boundary layer flow

The Dorodnitsyn finite element method for turbulent boundary layer flow with surface mass transfer is extended to include axisymmetric swirling internal boundary layer flow. Turbulence effects are represented by the two-layer eddy viscosity model of Cebeci and Smith¹ with extensions to allow for the effect of swirl. The method is applied to duct entry flow and a 10 degree included-angle conical diffuser, and produces results in close agreement with experimental measurements with only 11 grid points across the boundary layer. The introduction of swirl (w_e/u_e = 0.4) is found to have little effect on the axial skin friction in either a slightly favourable or adverse pressure gradient, but does cause an increase in the displacement area for an adverse pressure gradient. Surface mass transfer (blowing or suction) causes a substantial reduction (blowing) in axial skin friction and an increase in the displacement area. Both suction and the adverse pressure gradient have little influence on the circumferential velocity and shear stress components. Consequently in an adverse pressure gradient the flow direction adjacent to the wall is expected to approach the circumferential direction at some downstream location.     

Exact calculation of the flow of a staggered tandem cascade with moving second blade row

Summary  The plane flow around a tandem cascade of flat plates is calculated by means of conformal mapping. The blades of the two rows are perpendicular to each other. The first row is stationary, the second row moves with constant velocity. The conformal mapping will be constructed by a “mapping flow”. The blades of one row are stream lines and those of the other row are potential lines of the flow. By conformal mapping, the physical flow around the tandem cascade of the physical ζ-plane is converted into a flow between infinitely long straight walls in the z-plane, each wall corresponding to one of the blades. The conditions far upstream and far downstream of the cascade are represented by source-vortices. In the z-plane, the boundary conditions may be easily fulfilled by reflection and repetition of the source-vortices, and the flow may be calculated by well-known methods. The physical flow searched for is then obtained by inverse mapping. Received 24 July 2000; accepted for publication 6 December 2000     

Quasi-three-dimensional calculation of flow in blade passages of turbines

The flow in turbomachines is currently calculated either on the basis of a single successive solution of an axisymmetric problem (see, for example, [1-A]) and the problem of flow past cascades of blades in a layer of variable thickness [1, 5], or by solution of a quasi-three-dimensional problem [6–8], or on the basis of three-dimensional models of the motion [9–11]. In this paper, we derive equations of a three-dimensional model of the flow of an ideal incompressible fluid for an arbitrary curvilinear system of coordinates based on averaging the equations of motion in the Gromek–Lamb form in the azimuthal direction; the pulsation terms are taken into account in the equations of the quasi-three-dimensional motion. An algorithm for numerical solution of the problem is described. The results of calculations are given and compared with experimental data for flows in the blade passages of an axial pump and a rotating-blade turbine. The obtained results are analyzed.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 69–76, March–April, 1991.I thank A. I. Kuzin and A. V. Gol'din for supplying the results of the experimental investigations.     

Turbulent flow in a tube containing an offset rod

A numerical finite volume prediction method for arbitrary-shaped passages has been applied to the case of fully developed axial turbulent flow past a rod eccentrically placed in a circular tube. The numerical method was based on an orthogonal curvilinear mesh and employed an algebraic stress transport model to calculate the full three-dimensional velocity field directly from the governing partial differential equations. This study is one of a series of applications of this prediction method to a range of different non-circular passages that have been made in order to establish the capabilities and usefulness of this type of procedure. The present eccentric rod case was the subject of a comprehensive experimental investigation by Kacker¹ which has enabled a detailed comparison to be made between the present predictions and the measurements. This comparison included local distributions of axial velocity, wall shear stress and secondary velocities; and although found to be satisfactory overall, some differences in detail revealed possible shortcomings in the measurement of secondary flow. This, together with other previously reported cases, indicates, that, although the present method cannot be expected to replace experiment in providing turbulent passage flow data, it has an important role to play in interpreting and supplementing experiments.     

Determination of real-time predictors of the wind turbine wake meandering

The present work proposes an experimental methodology to characterize the unsteady properties of a wind turbine wake, called meandering, and particularly its ability to follow the large-scale motions induced by large turbulent eddies contained in the approach flow. The measurements were made in an atmospheric boundary layer wind tunnel. The wind turbine model is based on the actuator disc concept. One part of the work has been dedicated to the development of a methodology for horizontal wake tracking by mean of a transverse hot wire rake, whose dynamic response is adequate for spectral analysis. Spectral coherence analysis shows that the horizontal position of the wake correlates well with the upstream transverse velocity, especially for wavelength larger than three times the diameter of the disc but less so for smaller scales. Therefore, it is concluded that the wake is actually a rather passive tracer of the large surrounding turbulent structures. The influence of the rotor size and downstream distance on the wake meandering is studied. The fluctuations of the lateral force and the yawing torque affecting the wind turbine model are also measured and correlated with the wake meandering. Two approach flow configurations are then tested: an undisturbed incoming flow (modelled atmospheric boundary layer) and a disturbed incoming flow, with a wind turbine model located upstream. Results showed that the meandering process is amplified by the presence of the upstream wake. It is shown that the coherence between the lateral force fluctuations and the horizontal wake position is significant up to length scales larger than twice the wind turbine model diameter. This leads to the conclusion that the lateral force is a better candidate than the upstream transverse velocity to predict in real time the meandering process, for either undisturbed (wake free) or disturbed incoming atmospheric flows.     

NUMERICAL MODELLING AND PARTICLE IMAGE VELOCIMETRY MEASUREMENT OF THE LAMINAR FLOW FIELD INDUCED BY AN ENCLOSED ROTATING DISC

The fluid flow field within an enclosed cylindrical chamber with a rotating flat disc was calculated using a finite volume computational fluid dynamics (CFD) model and compared with particle image velocimetry (PIV) measurements. Two particular laminar cases near the Transitional flow regime were investigated: Reynolds number Re=2.5×1 ⁴, chamber aspect ratio G (h/R_d)=0.2 and Re=4.2×10⁴, G (h/R_d)=0.217. This enabled direct comparison with the numerical and experimental results reported by other researchers. The computational details and some major factors that affect the computed accuracy and convergence speed are also discussed in detail. PIV results containing some 4300 velocity vector points in each of seven planes for each case were obtained from the flow field parallel to the rotating disc. It was found that PIV results could be obtained in planes within the boundary layers as well as the core flow by careful use of a thin laser illumination sheet and correct choice of laser pulse separation. There was close agreement between numerical results, the present PIV measurements and other reported experimental and numerical results.