Research On Aerodynamic Characteristics Of Combined Vertical Axis Wind Turbine With Lift Drag Combined Starting Structure

Wind energy is a kind of renewable energy which is the most widely used,most successful commercial operation,most complete industrial chain and most promising.The technology of wind turbines will gradually develop toward both directions,that is,the large size and proportion in grid-connected power generation,and the miniaturization and intelligentization in off-grid power generation.When it comes to the form of wind energy utilization,large-scale grid-connected wind power becomes the absolute mainstream,while distributed,discrete,micro-grid,multi-energy complementary and other forms of wind energy utilization receive more and more attention.In China,the role of small-scale wind power is very important and it can be an effective supplement to large-scale wind power.In the field of rural energy utilization,small-scale wind power has become one of the important measures to realize rural electrification.As the second type of wind turbine,vertical axis wind turbine?VAWT?has been increasingly applied,which contributes to the continuous emergence of various new types.Straight blade VAWT?SB-VAWT?is a typical representative;however,along with its development and application,there comes some problems,which resist the development and promotion.Among them,the problems of start performance,aerodynamic output,and structural safety and stability are more prominent.Aiming at these problems,a new lift drag combined starting structure?LDCS?was designed to improve the starting and aerodynamic characteristics of VAWT.In this paper,a variety of research methods are used to deeply analyze the aerodynamic characteristics and improvement mechanism of the SB-VAWT installed the lift drag combined starting structure,and explore the structure design and structural mechanical analysis of the combined VAWT with LDCS?LDCS-CVAWT?.The main research contents are as follows:1.Aiming at the shortcoming of simple Drag structure,a new LDCS is proposed,and working principle and aerodynamic characteristics are studied,and the flow mechanism of LDCS is revealed.The aerodynamic principles of the composite blade of LDCS are analyzed by the theory of double multi-flow tube model with four actuator disks,and disciplines of aerodynamic characteristics change with tip speed ratio and static starting performance with azimuth angle were explored through numerical simulation.The results show that the rotating moment coefficient appears two peaks and one valley value during a rotating cycle and the positive range of torque coefficient is more than 90%of the rotating cycle.The maximum torque coefficient value is around 0.12,where the minimum torque coefficient value is around-0.06.The power coefficient curve shows a trend of rising first and then falling,the peak value of which appears at the tip speed ratio of 0.6,and the maximum working point is around 0.8 tip speed ratio.Under wind speed of 4 m/s and the rotating speed of 100 r/min,the maximum moment coefficient was near the?=60°,which value is 0.12;minimum torque coefficient appears around the?=120°,which value is 0.06.Moment coefficient varies very obviously with the azimuth waving.The flow field characteristics of LDCS were tested by PIV method,and the results showed that the aerodynamic forces of the blades in LDCS were produced by the cooperation of lift small blade and drag blade.2.Comparing the aerodynamic characteristics of LDCS and traditional drag structure?DS?,the performance improvement of LDCS and its causes were analyzed.The aerodynamic characteristics of LDCS and DS were obtained by numerical simulation.The results showed that:1)Single LDCS blade static moment coefficient varies trend as the azimuth change is consistent with single drag blade static moment coefficient.Except?=120°and?=300°,static moment coefficient of single drag blade are always less than that of single LDCS blade.2)Referring to static moment coefficient in a rotating cycle of the LDCS and DS,except the range of 100°<?<140°,the former were higher than the latter.The average static moment coefficients of the two structures were 0.0365 and 0.0338 respectively,and the improvement rate of the former is 8%higher than that of the latter.3)The rotating moment coefficient of the LDCS were significantly higher than that of the DS within the ranges of 40°<?<110°and160°<?<180°.LDCS rotating moment coefficient peak value is 0.11,valley value is-0.052,and DS rotating moment coefficient peak value is 0.076,valley value is-0.04.Compared with the latter,the maximum improvement rate of the rotating torque coefficient is 89%,and the minimum reduction rate is only 30%.In conclusion,the aerodynamic characteristics of LDCS are better than DS.The reason carried by PIV test of flow field characteristics was that the lift small blade in LDCS increased the aerodynamic force and improved the flow quality.3.Compared with SB-VAWT and DS-CVAWT respectively,the aerodynamic characteristics of LDCS-CVAWT with NACA7715 airfoil and NACA0018 airfoil were studied,and the aerodynamic characteristics advantage of LDCS-CVAWT and its flow mechanism are analyzed.The results showed that:1)Under the wind speed of 10 m/s and?=0.8,the comparing result of the moment coefficient of NACA7715 airfoil CVAWT installing different structure,LDCS or DS,is,when?=80°,the former moment coefficient increase 37%than the latter;in?=20°,torque coefficient of the former increase 250%than the latter.The static torque coefficients of NACA7715 airfoil with the three different structures are as follows:LDCS-CVAWT static moment coefficient was 11.2%higher than that of DS-CVAWT and294%higher than that of SB-VAWT.2)When U=6 m/s,NACA0018 CVAWT attached LDCS can achieve a working tip speed ratio of over 1.8,while DS-CVAWT only reaches 1.4.With the increase of wind speed,the maximum power coefficient of LDCS-CVAWT gradually increases,but the working tip speed ratio decreases,and the power coefficient will drop rapidly from the peak,indicating that NACA0018 airfoil LDCS-CVAWT has better aerodynamic characteristics in the lower wind speed area.NACA0018 airfoil LDCS-CVAWT average static moment coefficient at U=10 m/s is 0.196,which is higher than it at U=6 m/s,0.12.It indicates that the static torque coefficient of LDCS-CVAWT tends to grow with the increase of wind speed,which is benefit to the start of wind turbine.The PIV test of flow field characteristics showed that when the flow entered the LDCS-CVAWT causing the increased fluid quantity,and increased the blade thrust force,and generated lift force around the lift small blade.4.The influence of the blades number on the static start performance of LDCS-CVAWT was studied.The structural parameters of triple-blade LDCS-CVAWT were optimized by orthogonal experimental design method,and the aerodynamic characteristics of LDCS-CVAWT were studied by wind tunnel test.The results show that:1)when the wind speed is 6 m/s,triple-blade LDCS static moment coefficient presents a trend of high single-peak and low trough values,and the peak moment coefficient reaches about 0.45.Double-blade and quadruple-blade LDCS static moment coefficient presents a double-peak and single-trough trend,and the difference between the force peak and trough values is not big.The peak value of static moment coefficient of double-blade LDCS is around 0.25,which of quadruple-blade is around 0.1,and both of them are far less than the peak value of triple-blade LDCS.The average moment coefficient of double-blade is 0.13,triple-blade was 0.22,and quadruple-blade was 0.09.It is proved that the average static moment coefficient of triple-blade LDCS is the highest.Its static start performance is best at low wind speed.2)LDCS-CVAWT parameters are defined as e₂,L₁,?₁,?₁,?₂.The power coefficient of LDCS-CVAWT with optimized parameters grows with the increase of wind speed;the maximum value achieves 0.231 with 6 m/s,0.244 with 8 m/s,and 0.256with 10 m/s.The static moment coefficient curve value under three kinds of wind speed increases with the wind speed rising,and the static moment coefficient reaches the maximum value of 0.472 when the azimuth is 80o,minimum value is 0.15 when the azimuth Angle is 0°,are all positive number,obtained good static start performance.5.A triple-blade LDCS-CVAWT prototype with rated power of 3 kW was designed and produced.The structural mechanical characteristics of the CVAWT were analyzed,and the prototype was preliminarily tested in the field.The results show that:when the rated wind speed U=10 m/s,the maximum stress of LDCS-CVAWT is 237.83 MPa and the maximum displacement is 37.572 mm,All of them did not exceed the allowable tensile stress of the material,320 MPa,meeting the structural strength requirements.The lowest natural frequency of the complete machine is 2.9318 Hz,and the critical speed of the first order is 175.9 r/min,while the working speed range of the wind turbine is 5-100 r/min,far less than the critical speed of the first order,and resonance will not appear when the wind turbine is working.To sum up,this study of LDCS,aiming at improve aerodynamic characteristics of SB-VAWT,has carried on the theoretical analysis,numerical simulation,the aerodynamic characteristics wind tunnel test,PIV visualization wind field experiment,and field test.The aerodynamic characteristics and performance improvement mechanism of the structure and its wind turbine are obtained,which provides a theoretical and experimental basis for the development of new type SB-VAWT.Through the study of the structure design,the analysis of structural mechanical characteristics,and the wind turbine prototype field test of the CVAWT,it provides reference for the design and production of the wind turbine and its practical application.This study is of certain significance for the promotion and application of small and medium-sized VAWT in agricultural and pastoral areas,remote areas and low-wind areas of China.