Modern Manufacturing - Assignment Example

The article is with reference to the design and fabrication of blade for wind turbine. The blade is the rotary part of the turbine which is either mounted or integral part of the turbine. This part of the turbine converts the kinetic energy of air into mechanical energy through rotary motion, this motion then excites the generator and electrical energy if produced. The performance of the entire machine can be improved through design optimization of the blades; the blades are exposed to drag which is responsible for the power and frictional loss. The design of blade is critical exercise, and shall include desirable factors to cater for the power and frictional losses. The integrity of the wind turbine is protected with appropriate design of the blades with safe margin. The blades are the most exposed part of the machine, exposed to air, rust, moisture and friction; therefore during designing such factors have to be incorporated to protect the integrity of blades. The blades are considered as "balanced integration of economic, aerodynamic, structural dynamic, noise, and aesthetic considerations"; and their size and shape determines the power generation through wind turbine. The design of the blade is based upon the material, blade number, airfoils, chord and twist distributions. Each of these factors determines the efficiency of the rotary machine; the material of the blade shall govern strength, airfoil geometry shall desired high lift-to-grad ratio and/or blade stiffness. The design of the horizontal wind has been categorised into upwind and downwind configurations; and various additional characteristics i.e. performance enhancers have been included such as diffusers and concentrators. Aerodynamics The modified design of the blades have been introduced which is the three-bladed rotor; this is "industry-accepted configuration". The advantages of such design are related to the modest noise parameters; beside such design has also translated into lower blade fatigue. The selection of the blade number is based upon the stiffness of the blades, aerodynamic efficiency, and tower-shadow impulsive noise. The three-bladed rotor is preferred due to "rotor-noise and aesthetic considerations". It is important to realise that for specific rotor diameter, the three-bladed rotor will have "two-thirds the blade loading of a two-bladed rotor and one-third that of a one-bladed rotor". The three-bladed rotor has "lower impulsive noise resulting from blade loading for either a downwind or upwind tower shadow". In case of two-bladed and one-bladed rotor, the lower aerodynamic efficiency is controlled through "lower solidity and higher tip speeds for a given diameter or power relative to three-bladed rotors". As per general rule, the increase in rotor noise is subject to the higher tip speed; where the "noise is proportional to the fifth power of tip speed". From technical perspective, the balancing of the rotor is critical exercise and small magnitude of imbalance can lead to excessive vibration on the machine which shall result in the fatal damage. The three-blade rotor are much balanced than two-bladed rotor, "the 120-degree spacing between the blades, rotor dynamics are more benign than for the 180- and 360-degree spacing associated with two- and one-bladed rotor systems, respectively". It has been proven that the three-bladed rotor is reliable, and requires minimum operating and maintenance cost, as compared with the cost incurred by two-bladed and one-bladed rotors. Airfoils Airfoils are the critical component of the rotor, the airfoil itself is the blade. The thickness and geometry of blades determines the performance characteristics of the rotor. The airfoil is designed such that laminar flow is developed across the airfoil and low drag is generated. Airfoils developed for high Reynolds numbers experience "significant laminar separation bubbles", such occurrence can lead to substantial damage to the airfoil, and can lead to roughness. The formation of the bubbles is not desirable, because it will "lead to large variations in airfoil performance as a function of roughness". The airfoils for the wind-turbine "lack adequate thickness for the blade-root region, this shall cater for the structural requirement of high flap stiffness for tower clearance and efficient material use to accommodate high root-bending moments". During the design phase, it is important to verify the minimal sensitivity of wind turbine airfoils, the minimum sensitivity shall be based upon maximum lift coefficient to roughness effects. Several issues relevant to the design of airfoils have been recorded, "excessive peak power was another significant problem for fixed-pitch, stall-regulated turbines that resulted in high drive-train loads and generator failures". The solution towards such problems was resolved through design modifications, "the low-maximum-lift tip airfoils were designed for passively controlling peak power while actually increasing overall performance". Blade Geometry The modification of the blade geometry has resulted in the minimum cost of energy; this was made possible through demonstration. The blades were exposed against the peak powers, and several critical factors including mean wind speed, maximum root-bending moment were calculated. It is common in high load application machines that "optimization normally results in a blade with less solidity than if it were optimized for maximum annual energy". Materials The material commonly preferred for the fabrication of the blades for wind turbine is "steel, aluminium, and composite materials such as wood, fibreglass, and carbon fibres". The physical properties of the material i.e. strength and stiffness are important, and the selection of the material is based upon the suitable values of these factors. The losses related to the technical design can be minimised through inclusion of the light-weight materials in the product, the light weight blades are therefore popular "to minimize inertial and gyroscopic loads, which contribute to blade fatigue". The blades manufactured from steel and aluminium has low fatigue life, this is due to their heavy weight. In industries most of the blades are manufactured from fibreglass. The positive characteristics of such blades include "highest strength-to-weight ratio and stiffness", however such blades are used on need basis due to its "high cost, and strain incompatibility". The composite materials include common resin systems comprising of polyester, vinyl-ester, and epoxy. Polyester and vinyl-ester are preferred due to their lower cost, however many industries have switched to epoxy, "to achieve better material properties". The benefits of epoxy include alleviation of shrinkage, non-brittle characteristics, and improved fatigue characteristics. Manufacturing Methods Hand Lay-Up This process is considered to be old and popular method. The cost of material and labour is identical for this manufacturing method. As per the process, the multiple fibreglass fabric layers are placed upon each another and coated with resin. The procedure fails to "achieve an optimum glass-to-resin ratio and reproducible blade weights". The air pollution is the major concern relevant to this process, "undesirable working conditions resulting from the styrene out-gasing as the polyester resin cures". Filament Winding The machines required for the fabrication includes Kaman 40-kW and WTS4-MW. These machines used "filament-wound 9.6-m and 38-m rotor blades, respectively". The advantage of this procedure is the minimum labour expenses, and the technique develops strong blades. The procedure is considered to be "automated process whereby continuous strands of glass fibre pass through a resin bath and are then wound at an angle around a mandrel". The mandrel is used for two distinct purposes i.e. production of internal spar or the external blade shape. Through this procedure concave blade surfaces are developed, which is possible through airfoil camber or twist. The filament winding requires "lightweight, non-extractable mandrel shells of the blade external geometry". The concern relevant to this technique is that the surface finish of the object is not desirable, and this lacking severely influences the airfoil performance characteristics. It is therefore generally believed that "filament winding is best suited for interior tubular blade spars that are later moulded into the blade". Pultrusion The procedure is considered to be cheap relative to others. Through this procedure "compromised aerodynamic and structural efficiencies" have been achieved, however the process is applicable for the fabrication of large blades only. The blades manufactured through this technique "do not lend themselves to nonlinear twist and tapered chord distributions"; thus the aerodynamic efficiency is lowered by 12percent. The drop in efficiency can be reduced through "bonding on an inboard, twisted cuff and a tapered tip". The structural reliability of the blade is affected by the secondary bonds. The blades manufactured under this procedure have greater flexibility; therefore these blades are regarded unsuitable for "large, upwind commercial machines for which tower clearance is important". It is important to realise that increased flap stiffness and structural efficiency are possible through inclusion of external doublers in the root region; this is the part where the bending moment is concentrated. Resin-Transfer Moulding The manufacturing process consumes minimum labour and resin cost. The manufacturing process is based upon application of resin-transfer moulding. The description of the procedure is as follows; the fibreglass layers are kept in dry mould, and covered with membrane; the intent of this membrane is the sealing of mould. During the process, catalyzed resin is "introduced between the mould and the membrane under pressure, vacuum, or a combination of both". Through this process, "more consistent, higher quality parts having 70 percent fibre content by weight can be achieved". The manufacturing process is suitable for large wind turbine blades. Fig 1. Description of blade materials. Fig 2. Description of shapes of blades. Fig 3. Airfoil designs References 1. Kalpakjian & Schmid. Manufacturing Engineering and Technology. Addison-Wesley Publication. 2000. 2. Bruce Gregg et al. Modern Materials and Manufacturing Processes. US Imports & Phipes Publication. 1997. 3. Kalpakjian. Manufacturing Processes for Engineering Materials. Addison-Wesley Publication. 1997. 4. Mikell P Groover. Fundamentals of Modern Manufacturing. Wiley Publication. 1996. 5. GE Thyer. Computer Numerical Control of Machines. Butterworth-Heinemann. 1991. Read More