The Simulation of an Axial Flow Fan Performance Curve at Low Flow Rates

2011 ◽  
Author(s):  
S. J. van der Spuy ◽  
F. N. le Roux ◽  
T. W. von Backstro¨m ◽  
D. G. Kro¨ger
Keyword(s):  
Static Pressure ◽  
Axial Flow ◽  
Low Flow ◽  
Operating Point ◽  
Pressure Curve ◽  
Flow Rates ◽  
3 Dimensional ◽  
Air Cooled Condensers ◽  
Actuator Disc ◽  
Experimental Values

Simplified fan models are often used to simulate the effect of axial flow fans in large arrays of air-cooled condensers. The actuator disc method and its shortcomings are discussed and illustrated using computational fluid dynamics. A full 3-dimensional analysis of the fan is also performed at a single operating point to evaluate the flow conditions around the blade. This analysis confirms the results of the actuator disc method at the specific operating point. An adaptation of the actuator disc method, to improve its ability to model fan operation at low flow rates, is proposed. The effectiveness of the adaptation is evaluated by comparing the fan static pressure curve obtained from experimental results to the fan static pressure curve obtained from the simulations. The comparison shows that an analysis using the adapted actuator disc method produces results that correlate well with experimental values.

Prediction of Rotor Dynamic Destabilizing Forces in Axial Flow Compressors

1992 ◽  
Vol 114 (4) ◽  
pp. 621-625 ◽  
Author(s):  
J. Colding-Jorgensen
Keyword(s):  
Axial Compressor ◽  
Axial Flow ◽  
Present Theory ◽  
Tip Clearance ◽  
Low Flow ◽  
Normal Operation ◽  
Flow Rates ◽  
Backward Whirl ◽  
Actuator Disc ◽  
Rotor Dynamic

It has been shown by Thomas (1958) and Alford (1965), that axial flow turbo-machinery is subject to rotor dynamic destabilizing gas forces produced by the circumferential variation of blade-tip clearance when the rotor is whirling. However, the magnitude and direction of these forces have yet to be clarified. For example, it is still uncertain, under which circumstances the rotor whirl direction will be forward, and when it will be backward, with respect to the rotation. In the present paper, a simple analysis of the perturbed flow in an axial compressor stage with whirling rotor is presented, based on the actuator disc analysis of Horlock and Greitzer (1983), and the gas force on the rotor is calculated on this basis. It appears that in the normal operation range of an axial compressor, the whirl direction is predicted to be forward always. Backward whirl is predicted to take place only at very low flow rates, well below the normally expected stall limit. Experimentally, forces were indeed found in direction of backward whirl for low flow rates, and in direction of forward whirl for high flow rates, in the results reported by Vance and Laudadio (1984), as analyzed by Ehrich (1989). While this experimental evidence supports the present theory qualitatively, a direct comparison of the measured and predicted destabilizing force has yet to be carried out.

scholarly journals Investigation of the Flow Field in the Vicinity of an Axial Flow Fan During Low Flow Rates

2014 ◽  
Author(s):  
Francois G. Louw ◽  
Theodor W. von Backström ◽  
Sybrand J. van der Spuy
Keyword(s):  
Flow Field ◽  
Numerical Simulations ◽  
Flow Rate ◽  
Axial Flow ◽  
Operating Conditions ◽  
Design Flow ◽  
Low Flow ◽  
Axial Flow Fan ◽  
Free Vortex ◽  
Flow Rates

Large axial flow fans are used in forced draft air cooled heat exchangers (ACHEs). Previous studies have shown that adverse operating conditions cause certain sectors of the fan, or the fan as a whole to operate at very low flow rates, thereby reducing the cooling effectiveness of the ACHE. The present study is directed towards the experimental and numerical analyses of the flow in the vicinity of an axial flow fan during low flow rates. This is done to obtain the global flow structure up and downstream of the fan. A near-free-vortex fan, designed for specific application in ACHEs, is used for the investigation. Experimental fan testing was conducted in a British Standard 848, type A fan test facility, to obtain the fan characteristic. Both steady-state and time-dependent numerical simulations were performed, depending on the operating condition of the fan, using the Realizable k-ε turbulence model. Good agreement is found between the numerically and experimentally obtained fan characteristic data. Using data from the numerical simulations, the time and circumferentially averaged flow field is presented. At the design flow rate the downstream fan jet mainly moves in the axial and tangential direction, as expected for a free-vortex design criteria, with a small amount of radial flow that can be observed. As the flow rate through the fan is decreased, it is evident that the down-stream fan jet gradually shifts more diagonally outwards, and the region where reverse flow occur between the fan jet and the fan rotational axis increases. At very low flow rates the flow close to the tip reverses through the fan, producing a small recirculation zone as well as swirl at certain locations upstream of the fan.

A Comparison of Actuator Disc Models for Axial Flow Fans in Large Air-Cooled Heat Exchangers

2016 ◽  
Author(s):  
Michael B. Wilkinson ◽  
Francois G. Louw ◽  
Sybrand J. van der Spuy ◽  
Theodor W. von Backström
Keyword(s):  
Heat Exchangers ◽  
Three Dimensional ◽  
Axial Flow ◽  
Velocity Profiles ◽  
Low Flow ◽  
Disk Model ◽  
Flow Rates ◽  
Fan Performance ◽  
Actuator Disk ◽  
Performance Results

The performance of large mechanical draft air-cooled heat exchangers is directly related to fan performance which is influenced by atmospheric wind conditions, as well as the plant layout. It is often necessary to numerically model the entire system, including fans, under a variety of operating conditions. Full three-dimensional, numerical models of axial flow fans are computationally expensive to solve. Simplified models that accurately predict fan performance at a lesser expense are therefore required. One such simplified model is the actuator disk model (ADM). This model approximates the fan as a disk where the forces generated by the blades are calculated and translated into momentum sources. This model has been proven to give good results near and above the design flow rate of a fan, but not at low flow rates. In order to address this problem two modifications were proposed, namely the extended actuator disk model (EADM) and the reverse engineered empirical actuator disk model (REEADM). The three models are presented and evaluated in this paper using ANSYS FLUENT. The models are simulated at different flow rates representing an axial flow fan test facility. The resulting performance results and velocity fields are compared to each other and to previously simulated three dimensional numerical results, indicating the accuracy of each method. The results show that the REEADM gives the best correlation with experimental performance results at design conditions (ϕ = 0.168) while the EADM gives the best correlation at low flow rates. A comparison of the velocity profiles shows that none of the three models predict the radial velocity distribution at low flow rates correctly, however the correlation improves at flow rates above ϕ = 0.105. In general the upstream velocity profiles, where reversed flow occurs through the fan, are poorly predicted at low flow rates. At the flow rates above ϕ = 0.137 the correlation between the velocity profiles for the simplified modes and the three dimensional results is good.

High Pressure Fan Design for Biogas Plants

2009 ◽  
Author(s):  
Philipp Epple ◽  
Mihai Miclea ◽  
Harald Schmidt ◽  
Antonio Delgado ◽  
Hans Russwurm
Keyword(s):  
High Pressure ◽  
High Pressures ◽  
Thermal Power ◽  
Flow Analysis ◽  
Single Stage ◽  
Low Flow ◽  
Operating Point ◽  
Flow Rates ◽  
New Class ◽  
Biogas Plants

High pressure fans for thermal power generation stations, especially biogas plants, usually operate in a spiral casing at high pressures of about p = 12.000–15.000 Pa and low flow rates of around Q = 100–600 m3/s. The motor drive has a constant speed of 3.000 l/min. This corresponds to specific speeds of nq = 3–6 min−1, which is already beyond the conventional range of single stage radial machines. Nowadays these fans for biogas plants usually operate at higher flow rates than specified or are multiple stage radial fans. Therefore a new class of radial impellers has been developed. These single stage impellers have a unique high pressure at a low flow rate operating point. In this work several impellers of this new class have been designed and validated with a commercial Navier-Stokes solver (ANSYS CFX). The design process is described in detail. It is based on a new extended analytical and numerical design method. It is shown that the prescribed unusual operating point can be achieved with single stage radial impellers. An in detail flow analysis is given showing the fundamental flow physics of these impellers.

Lift and Drag Characteristics of an Air-Cooled Heat Exchanger Axial Flow Fan

2015 ◽  
Vol 137 (8) ◽  
Author(s):  
Francois G. Louw ◽  
Theodore W. von Backström ◽  
Sybrand J. van der Spuy
Keyword(s):  
Heat Exchanger ◽  
Lift Coefficient ◽  
Axial Flow ◽  
Design Flow ◽  
Low Flow ◽  
Axial Fan ◽  
Flow Rates ◽  
Blade Element Theory ◽  
Lift And Drag ◽  
Air Cooled Heat Exchanger

Actuator-disk models (ADMs) use blade element theory to numerically simulate the flow field induced by axial fans. These models give a fair approximation at near design flow rates, but are of poor accuracy at low flow rates. Therefore, the lift/drag (LD) characteristics of two-dimensional (2D) sections along the span of an air-cooled heat exchanger (ACHE) axial fan are numerically investigated, with the future prospect of improving ADMs at these flow conditions. It is found that the blade sectional LD characteristics are similar in shape, but offset from the 2D LD characteristics of the reference airfoil (NASA LS 413 profile) at small angles of attack (αatt<5deg). A deviation between these characteristics is observed at higher angles of attack. The blade sectional lift coefficients for αatt>5deg always remain lower compared to the maximum lift coefficient of the reference airfoil. Conversely, the blade sectional drag coefficients are always higher compared to that of the reference airfoil for αatt>5deg.

Effect of Convergent Angle on Flow Characteristics of Y-Shaped Diffusers Using CFD

2014 ◽  
Vol 592-594 ◽  
pp. 1909-1913
Author(s):  
R. Thundil Karuppa Raj ◽  
M.P. Dhyan Shankar
Keyword(s):  
Static Pressure ◽  
Flow Characteristics ◽  
Cross Flow ◽  
Axial Flow ◽  
Momentum Equation ◽  
Convergence Criterion ◽  
Second Phase ◽  
Hexahedral Mesh ◽  
3 Dimensional ◽  
Two Phases

Diffusing ducts are used in fluid flow systems, mainly in aeroplane engine inlets to decelerate the flow and to correspondingly increase the static pressure. The main problem in achieving a high pressure recovery is the flow separation which results in non-uniform distribution and excessive losses. The present work is aimed to study the flow characteristics in Y-shaped diffusing ducts. The Y-shaped diffuser has rectangular inlets and the outlet is circular with a certain settling length for the flow to be stabilized. The diffuser is modeled in CATIA V5 and further discretized using ICEMCFD12.1. Hexahedral mesh is generated for all diffuser cases, which have been used to capture the hydrodynamic boundary layers. ANSYS CFX 12.1 based on finite volume technique, using k-ε turbulence model is adopted for predicting the flow. The flow field through the 3-dimensional domain is captured by solving the appropriate governing equations namely, the continuity equation and the momentum equation. The convergence criterion is set to 10E-06 for mass and momentum. The whole investigation is done in two phases: in the first phase a commercial CFD code is validated for the results obtained for an S-shaped diffuser and in the second phase the same idea is then extended for the analysis of Y-shaped diffuser. The coefficient of static pressure, cross flow and axial flow velocity distributions are calculated based on the mass averaged quantities for the Y-shaped diffusers (30o and 40o).

scholarly journals Design and Performance Analysis of Blades Based on the Equal–Variable Circulation Method

2021 ◽  
Vol 9 ◽  
Author(s):  
D. Liang ◽  
C. Song ◽  
S. Liang ◽  
S. Wang ◽  
Y. Li ◽  
...  
Keyword(s):  
Design Method ◽  
Axial Flow ◽  
Aerodynamic Performance ◽  
Low Flow ◽  
Improve Performance ◽  
Flow Rates ◽  
Circulation Method ◽  
Blade Tip ◽  
New Type ◽  
And Performance

With the aim of improving the aerodynamic performance of axial turbomachinery, a new type of blade is designed using the equal–variable circulation method. Taking an axial flow fan as the research object, this article describes the development of a new type of turbomachinery by changing the design method and producing a blade with forward sweep. The aerodynamic performance of the fan is simulated and compared with the experimental data. The numerical results show that the equal circulation design method improves the aerodynamic performance of the blade roots, while the variable circulation design method enhances the aerodynamic performance of the blade tips. By adopting the equal–variable circulation design method, the total pressure of the experimental fan is increased by about 4%, while the efficiency remains unchanged. Forward-swept blades with an equal–variable circulation design also improve performance over the conventional blades by changing the center-of-gravity stacking line. At low flow rates, the efficiency of the experimental fan can be increased by 7.5%, and the working range of the flow is expanded. Under high flow rates, the restriction of the blade tip on the airflow is decreased and the fluidity is slightly reduced.

The Effect of Hub Configuration on the Performance of an Air-Cooled Steam Condenser Fan

2020 ◽  
Author(s):  
Z. Meiring ◽  
S. J. van der Spuy ◽  
C. J. Meyer
Keyword(s):  
Flow Rate ◽  
Static Pressure ◽  
Axial Flow ◽  
Pressure Rise ◽  
Volumetric Flow Rate ◽  
Superior Performance ◽  
Design Point ◽  
Static Efficiency ◽  
Air Cooled Condensers ◽  
The Impact

Abstract Axial flow fans used in air-cooled condensers are typically analysed with smooth rounded hubs as they offer superior performance when compared to other hub configurations. However, such a hub configuration is impractical and may increase the manufacturing and installation costs of air-cooled condensers. As such, it is desirable to use a simpler, yet effective, hub configuration in order to reduce the installation cost. This paper assesses the impact that a simpler hub configuration may have on the performance of an axial flow fan. This is done through a comparison of three hub configurations: a cylindrical hub with a flat nose, a cylindrical hub with a hemispherical nose, and a disk hub, installed on the B2a-fan. Computational fluid dynamics modelling, utilising OpenFOAM, is used to simulate each hub configuration. It is found that the impact on performance due to hub configuration is dependent on the volumetric flow rate through the fan. A thin disk hub exhibits superior performance at low flow rates, resulting in a 8.4% improvement in total-to-static pressure rise and a 5.7% point improvement in total-to-static efficiency. As volumetric flow rate increases, the effectiveness of the disk hub configuration reduces while the hemispherical and flat nosed cylindrical hub configurations result in similar performance metrics at the design point flow rate. At above design point flow rate, the flat nosed cylindrical hub configuration shows an improvement in performance over the hemispherical nose cylindrical hub configuration, with a 9.5% increase in total-to-static pressure rise and a 5.1% point improvement in total-to-static efficiency.

Effect of Circumferential Static Pressure Non-Uniformity Caused by a Volute on Flow in High Pressure Ratio Centrifugal Compressor With Vaneless and Vaned Diffuser

2013 ◽  
Author(s):  
Hideaki Tamaki ◽  
Masaru Unno ◽  
Xinqian Zheng ◽  
Yangjun Zhang
Keyword(s):  
Centrifugal Compressor ◽  
Gas Turbines ◽  
Static Pressure ◽  
Incidence Angle ◽  
Low Flow ◽  
Vaneless Diffuser ◽  
Vaned Diffuser ◽  
Flow Rates ◽  
Operating Range ◽  
Surge Line

Centrifugal compressors are deemed to have a wide operating range. Recirculation devices, which comprise a bleed slot, an upstream slot and an annular cavity connecting them, are often used particularly compressors for turbochargers. They remove low energy fluid at the inducer and improve the incidence angle of the impeller leading edge, i.e. the blade loading of the inducer, at low flow rates due to the recirculation flow supplied to the compressor inlet. The impeller of a centrifugal compressor is often housed in a volute. Since the geometry of the volute is not axisymmetric, the impeller might be surrounded by an asymmetric flow field, hence there is the potential to enlarge the compressor operating range and improve efficiency using a recirculation device with an asymmetrically-distributed bleed slot, referred to here as a non-axisymmetric recirculation device. The authors [1] applied non-axisymmetric recirculation devices to a compressor with a vaneless diffuser. The results showed the effectiveness of a non-axisymmetric recirculation device with a bleed slot partially channeled in the circumferential direction. They also showed that the surge line of the compressor characteristics, which is the line connecting the operational points of the smallest flow rates on all peripheral Mach numbers, was significantly affected by the change in the circumferential position of the bleed slot relative to the volute tongue. The tested compressor was originally designed to feature a vaned diffuser [2]. Enhancing the compressor operating range is the key for marine use turbochargers, integrally geared compressors, multistage compressors and gas turbines as well as automotive turbochargers. These compressors normally use vaned diffusers. In this study the authors tried to apply non-axisymmetric recirculation devices developed in their previous study [1] to compressors with a vaned diffuser. Moreover different circumferential positions of the bleed slot relative to the volute tongue were tested as well as the vanelss diffuser case. The change in the surge line of the compressor characteristics was much smaller compared to the compressor with the vaneless diffuser. The circumferential static pressure distributions in the compressors in combination with vaneless and vaned diffusers were measured to determine the above reason as well as conducting unsteady calculations with a simplified outlet boundary condition. These measurements and calculations showed that the impeller with the vaned diffuser was surrounded by a less distorted static pressure field than that with the vaneless diffuser. These results implied that the vaned diffuser depresses the spread of the circumferential static pressure non-uniformity effect caused by a volute to the impeller.

A Numerical Model for the Flow Within the Tower of a Tornado-Type Wind Energy System

1981 ◽  
Vol 103 (4) ◽  
pp. 299-305 ◽  
Author(s):  
S. S. Ayad
Keyword(s):  
Numerical Model ◽  
Pressure Drop ◽  
Wind Energy ◽  
Boundary Conditions ◽  
Axial Flow ◽  
Energy System ◽  
Optimum Ratio ◽  
Flow Rates ◽  
Wind Energy System ◽  
Experimental Values

The two-equation k–ε turbulence model is used to predict numerically the flow within the tower of a tornado-type wind energy system. Calculations are carried out for a tower in a uniform flow. Both cases of closed-bottom tower and simulated turbine flow with a variety of turbine-to-tower diameter ratios and turbine flow rates are considered. Calculated values of pressure for closed-bottom tower are compared with experimental values. Considerable reduction of the pressure drop is found due to the interaction of the nonrotating axial flow from the turbine with the vortex within the tower. Values of the maximum power that can be extracted by the turbine are estimated for a given set of tower flow boundary conditions. The optimum ratio of turbine-to-tower diameters was found to be around 0.4.

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