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Forces acting on a typical aerofoil section of axial flow fan blade 

Forces acting on a typical aerofoil section of axial flow fan blade 

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Purpose: This study focuses on one of the key design aspects of mine ventilation fans, i.e. the selection of an appropriate aerofoil blade profile for the fan blades in order to enhance the energy efficiency of axial flow mine ventilation fans, using CFD simulations. Methods: Computational simulations were performed on six selected typical aerofoil...

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... where C l is the coefficient of lift, C d is the coefficient of drag, L is the lift force, D is the drag force, ρ is the density of air, V is the velocity of undisturbed airflow and A is the blade reference area. The aerodynamic lifting force is a vital component and must be much greater than the drag component. Since lift contributes to the head generated by the fan and the drag causes loss due to skin friction in the wake behind the vane, the profile offering higher L/D ratio is considered more efficient (Misra, 2002). Maximum lift to drag provides a combi- natory measure of the performance of the aerofoil and, therefore, the C l / C d curve can be likened to the efficiency charac- teristic of a fan. In the case of mine ventilation fans, an aerofoil profile generating high lift coefficient and offering high lift to drag ratio is needed for minimizing losses, pro- ducing high head and improving the efficiency of the fan provided it fulfils the other constraints like economy, change in mine resistance over time and other factors. These re- quirements are better fulfilled by the non-symmetrical aerofoils and are, therefore, commonly preferred for fan blades than symmetrical ones. The lift and profile drag of the aerofoil shaped blades, when move through air, vary with the structure of the aerofoil and variations in the angle of attack ( α ). The angle of attack is the angle between the velocity vector and the chord line of the aerofoil ( Figure 2). As the angle of attack increases, the coefficient of lift increases in a near-linear manner. However, at an angle of attack usually between 12 and 18°, breakaway of the boundary layer occurs on the upper surface. This causes a sudden loss of lift and an increase in drag, known as stall condition. In this condition, the formation and propagation of turbulent vortices causes the fan to vibrate excessively and to produce additional low frequency noise (McPherson, 1983). The aerodynamic design of axial flow fans is a very com- plex process and mainly consists of designing two- dimensional blade sections at various radii. Over the past few decades, lots of effort has been put in to aerodynamic improvements of mine fans. Advances in aerodynamic design, which incorporate aerofoil section blading with lower aspect ratios, higher solidities and higher stagger angles have led to an increase in static-pressure rise. In addition to aerodynamic improvements, the use of improved materials and advanced mechanical design techniques also had a significant contribu- tion. In conventional aluminium alloy bladed mine fans, their cross section barely matches a suitable aerofoil geometry that can develop sufficient lift and minimal drag forces. Moreo- ver, the roughness of the blade surface considerably increases the losses. As a result, they may not reach the desired efficiency and cause a loss of energy. Eckert (1953) found that unmachined cast-iron blades give an efficiency ~10% lower than machined blades. Hence, careful smoothening of the blade surfaces is essential to obtain better fan efficiency. In this study, the CFD package ANSYS Fluent 6.3.26 is used to perform the numerical simulation of airflow around the selected aerofoil sections. Fluent solvers are based on the finite volume method in which the domain is discretized into a finite set of control volumes (or cells). It solves conserva- tion equations for mass and momentum to determine the pressure distribution and therefore fluid dynamic forces acting on the wing as a function of time. Six aerofoil sections, viz. EPPLER 420, EPPLER 544, EPPLER 855, FX 74 CL5 140, NACA 747A315 and NACA 64(3)-418 have been chosen based on an extensive literature review for 2-D simulation to select a suitable aifoil section for the blades of axial flow mine ventilation fans. Most of these aerofoils are used in aeroplanes, wind turbines, high velocity rotors, sail- planes and rotorcrafts etc. The aerofoil geometries of the chosen aerofoils have been created with the coordinates obtained from the airfoil coordinates database ( database.html) and shown in Figures 3. Gambit 2.4.6, the pre-processor of Fluent 6.3.26 is used to create the discretized domain or mesh with a far-field and near-field area and exported to Fluent for analysis. Computational domain or the far-field distance, which is about 15 times the size of that of the chord length surrounding the centrally located aerofoil, is created for each profile. The domain boundary is set as a pressure-far-field and the surface of the aerofoil is set as wall. The meshing is done based on two-dimensional structured C-grid topology in x-y direction giving rise to average quadrilateral cells of about 160000 for all of the aerofoils. Figure 4 shows the meshed flow domain surrounding an aerofoil. The resolution of the mesh close to the aerofoil region is made greater for better computational accuracy. The height of the first cell adjacent to the surface of – 5 + the aerofoils is set to 10 , corresponding to a maximum y of approximately 0.2. The aerofoils with meshed flow domain are solved with Fluent solver for determining the lift and drag coefficients giving various input parameters, such as airflow direction, flow 3 velocity (15 m/s), air density (1.225 kg/m ), angle of attack, boundary conditions etc. The free air stream temperature is considered as 300 K, which is same as the environment temperature and at this given temperature, the density and viscosi- ty of the air is ρ = 1.225 kg/m 3 and μ = 1.7894 × 10 – 5 kg/ms respectively. The flow is assumed to be incompressible. At this stage it is important to mention that the accuracy of CFD-based lift and drag prediction depends on the geometry representation, mesh size, flow solver, convergence level, transition prediction and turbulence model (Oskam & Sloo, 1998). For the numerical modelling of fluid-structure interac- tion problems, the choice of turbulence model is important. In this study the most widely used k - ε turbulence model is chosen for simulation due to its simplicity (Kieffer et al., 2006; Hoo, Do, & Pan, 2005). This model, proposed by Launder and Spalding (1974), includes standard renormaliza- tion-group (RNG) and realizable models. The Reynolds number for the simulations is considered as Re = 3 × 10 6 . The angles of attack (AoA) are varied from 0 to 21  at 3  intervals for all the aerofoils. Additionally, the coefficients of lift ( C l ) and drag ( C d ) ...
Context 2
... ventilation fans run 24 hours a day and throughout the year for maintaining a comfortable working environment for the miners working below ground. Mostly, conventional aluminium alloy bladed fans are used for underground mine ventilation, which are neither properly designed nor appro- priately selected to suit the desired condition. As a result, the consumption of electricity during their operation constitutes the largest component of the operating cost for mine ventilation and accounts for one third of a typical underground mine's entire electrical power cost (Vergne, 2003). Belle (2008) has reported that the consumption of electricity by the main ventilation fan of a major mining company alone can be in the region of 120 MW. Therefore, ventilation is undoubt- edly the most significant cost component of any underground mine and a major part of this is made up by the main mine fans. The electric power consumption profile in Polish mines, reported by Krzystanek and Wasilewski (1995), revealed that the mine fans together with dewatering pumps and compres- sors that operate continuously consume over 40% of the total electrical energy consumption of the mines and out of which 14% is consumed by the main fans itself. In this energy crunch world, the mining industry is facing the challenges of increasing energy costs. Therefore, investigations are ongoing and more focus is being given to reducing energy consumption in the ventilation systems. Enhanc- ing energy efficiency is one of the alternatives for minimizing energy consumption in any system. In mine ventilation, energy consumption can be reduced by improving efficiency with the suitable design of ventilation fans. The design and selection of the ventilation fans are of foremost importance with regards to operation and energy consumption. Furthermore, improvement in overall ventilation system efficiency can be achieved by reducing wasteful air power and enhancing fan efficiency. It is obvious that the power consumption will be less if the fan efficiency is more and vice versa (Sharma, 2002). The power consumption in fans depends to a large extent on the various losses involved, viz. impeller friction loss, entry loss, shock loss, clearance loss and diffuser loss, and the high losses in fans which are attributed to poor designs (Eck, 1973). Sen (1997) observed that the excess power consumption in axial flow main mine ventilation fan occurs because of the unrefined design approach, careless selection and incorrect installation of the fan. According to Sen (1997), a combination of aerodynamic, mechanical, electrical, struc- tural and operational factors are involved in any overall opti- mization exercise, i.e. attainment of high fan efficiency or minimum power consumption. According to Hustrulid and Bullock (2001), the main reasons for very poor efficiency of old design colliery fans are due to poor aerodynamic design, improper fan selection and poor aerodynamics at the fan site, which are the root causes that yield static efficiencies below 50%. Therefore, enhancing energy efficiency in mine fans by minimizing various losses is an important facet of energy saving and it is mainly governed by proper design of the ventilation fans. The design of fan blades constitutes the most significant feature of the fans. Belle (2008) has reported that a 10% increase in main fan efficiency with a 10% reduction in electricity consumption may result in a saving of 10.81 MW of electricity per annum; this would have a net present benefit of US$ 16.08 million over 10 years. Several other approach- es, viz. use of fan blades made of composite material such as FRP (fibreglass reinforced plastic), application of variable speed drives and the ventilation-on-demand (VOD) concept have also been tried in order to reduce energy consumption in ventilation fans (Mishra, 2004; Panigrahi, Mishra, Divaker, & Sibal, 2009). The shape of the blades, which are of aerofoil sections in the case of axial flow fans, plays an important role in the performance of the fans and fan efficiency is greatly dependent on the profile of the blades. Therefore, there is a scope for improving energy efficiency by reducing losses and enhancing the efficiency of fans by using aerodynamically designed fan blades made of suitable material. From this point of view, the design of mine fan blade profiles with suitable aerofoil section is very much desired. Nowadays, computational fluid dynamics (CFD) is widely used in the field of aerodynamics for design and analysis of aerospace vehicles. Simulation of airflow around the aerofoil sections using CFD has been studied by several researchers (Kieffer, Moujaes & Armbya, 2006; Eleni, Athanasios, & Dionissios, 2012; Rajakumar & Ravindran, 2012; Bai, Sun, Lin, Kennedy, & Williams, 2012). Rumsey and Ying (2002) reported CFD capability in predicting surface pressures, skin friction, lift, and drag with reasonably good accuracy at angles of attack below stall in high-lift flow fields. Keeping this in mind and with a broader aim of reducing energy consumption in mine ventilation fans, numerical simulations of the aerodynamic effects of different angles of attack on the selected aerofoil sections have been carried out in this study in a turbulent Reynolds number flow using the k - ε turbulence model. The aerodynamic characteristics of the aerofoil sections viz. lift and drag coefficients at various angles of attack are determined for selecting a suitable profile for the axial flow mine ventilation fan blades giving the highest lift to drag ratio. A fan is simply a machine which develops the pressure necessary to produce the required airflow rate and overcome flow resistance of the system by means of a rotating impeller using centrifugal or propeller action, or both. The axial flow fans are commonly used in mine ventilation in lieu of centrifugal fans due to high efficiency, compactness, non-overlo- ading characteristics, development of adequate pressure, etc. (Misra, 2002). The axial flow fan in its simplest form as diagrammatically shown in Figure 1 incorporates a rotor, which consists of a hub fitted with aerofoil section blades in a radial direction. The blades or vanes which constitute the main component of axial flow fan are the surfaces that work by means of dynamic reaction on the air and develop positive air pressure during their rotation due to the development of lift force. The forces acting on a typical aerofoil section of an axial flow fan blade are shown in Figure 2. The lifting force acts at right angles to the air stream and the dragging force acts in the same direction of the air stream and is responsible for losses due to skin friction. The efficiency of axial flow fans is greatly dependent on the profile of the blade, and the aerodynamic characteristics of the fan blades are strongly affected by the shape of the blade cross section. The cross section of fan blades is of a streamlined asymmetrical shape, called the blade’s aerod y- namic profile and is decisive when it comes to blade performance. Even minor alterations in the shape of the profile can greatly alter the power curve and noise level. Therefore, it is essential to choose an appropriate shape with great care, in order to obtain maximum aerodynamic efficiency. An aerodynamic profile with optimum twist, taper and higher lift- drag ratio can provide total efficiency as high as 85 – 92%. The axial flow fan blades are of aerofoil sections and the idea behind using aerofoil blades is to maintain the proper stream- lining of air to reduce losses caused due to form drag as well as from strength considerations (Misra, 2002). The blade performance characteristics may be predicted from the aerodynamic characteristics such as lift and drag coefficients of the chosen aerofoil section and given by the following ...

Citations

... In [30], a study on the selection of an aerofoil blade profile has been conducted. Simulations were performed on six typical aerofoil sections at different angles of attack, ranging from 0 to 21. and at Reynolds number Re = 3 × 10 6 , and various aerodynamic parameters. ...
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... Panigrahi and Mishra studied an appropriate aerofoil blade profile using computational fluid dynamics (CFD) for the fan blades to improve the efficiency of axial flow mine ventilation fans by varying angles of attack and various aerody-namic parameters. The study revealed that an appropriate aerofoil blade profile will increase energy efficiency of mine ventilation fans [14]. Zhao et al. investigated modification of fan geometry with the refined numerical and experimental approaches to reduce noise of outdoor unit. ...
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