Investigation On Aerodynamics And Speed Control Methods Of Fixed-pitch Variable-speed Horizontal Axis Wind Turbines

Under the background of worldwide energy crisis and environmental pollution and with the policy of Chinese government’s Strategy of Sustainable Development,the utilization of renewable energy,especially wind energy has become the research focus both at home and abroad.To resolve the worldwide problem of wind power integration,the large-scale non-grid-connected wind power system rises.In this system the fixed-pitch horizontal axis wind turbine is combined with variable-speed generators.This combination not only continues the advantages of fixed-pitch wind turbine including straightforward structure,running reliability,easy manufacture and maintenance,but also improves the energy conversion efficiency by regulating the rotating speed of the turbine rotor.Therefore,it is of great significance to carry out the research on the aerodynamic performance and corresponding speed control model on fixed-pitch variable-speed wind turbine.With the method of BEM theory,CFD and experimental measurements adopted in the dissertation,wind turbine performances and detailed flow field structures in the wake and around the blade were studied on three fixed-pitch variable-speed horizontal axis wind turbines,a NREL Phase VI rotor,a SVIAB WG500 rotor and its scaled model rotor.And thus the major flow structures of these wind turbines were obtained.Based on the performances from CFD,a speed control model and a blade design method for fixed-pitch variable-speed wind turbine were developed.Also the advanced technique in aircraft and aircraft engine design was applied to control the flow field of the wind turbine.This dissertation mainly consisted of the following four parts:In part one,a computing platform was set up using BEM theory with the correction of Glauert,hub-tip loss,and stall delay,and was realized in the FPVS-HAWT-Perf code.The torque,power and detailed flow structures were acquired by adopting CFD method and the influences of stall delay and Reynolds number on wind turbine were simulated.The BEM results were in accordance with that in the experiments,with an average error rate around 4%;after the correction of loss and stall delay,the results were still consistent with the experiments’figures even when the wind speed was increased to around 14m/s.By using?-Re_?transition model,the flow field of the scaled small wind turbine was simulated at low Reynolds numbers.The results showed that the Reynolds number had a great impact on airfoil when the attack angle was small.As the attack angle getting larger,the influence decreased.Especially when the attack angle was larger than 30 degrees,an obvious flow separation at the trailing edge of the airfoil appeared.Therefore at lager attack angle,the Reynolds number exerted little impact.For the small-scale wind turbine,there was a critical Reynolds number,below which the maximum power coefficient would decrease rapidly.These findings were of great significance in guiding the conversion between the big wind turbine and small-scale turbine model in wind tunnel experienment.An experiment platform for a scaled wind turbine model was built in the 2^ndpart.The wind turbine aerodynamic performances were obtained in the wind tunnel.The average flow field behind the wind turbine were measured by L-type 5 hole probe driven by a displacement mechanism.The results showed the rotating direction was to the contrary of wind turbine blade both at high and low rotating speed.At 700RPM,the radial veloc ity reduced both along the spanwise and at the downstream.At 1500RPM,the radial velocity witnessed an obvious reduction at near turbine location but no reduction at far downstream location along spanwise.And the axial velocity was lower than that at 700RPM.That is to say,the axial velocity behind the wind turbine reduced with the increase of wind turbine power.With the use of PIV technology,the velocity vector of wind turbine flow field was acquired.The absolute velocity direction behind the rotor matched the results from 5 hole probe measurements.From the comparison of relative velocity vector on five planes,separation areas were found under low rotating speed condition.And the distribution of vorticity revealed an obvious flow loss at blade trail and tip.The instantaneous vorticity on meridian plane showed how the tip and hub vortices were generated and spread to the downstream.These two vortices were not dissipated immediately.The vortex was generated at the blade trailing edge when the blade swept the meridian plan.Due to its insufficient strength,the vortex was dissipated at a near distance.With a combination of flow direction and vortex developing,the three-dimensional vortex structure was profiled.In the 3^rdpart a speed control model FPVS-HAWT-SCM for fixed-pitch variable-speed wind turbine was put forward by applying the aerodynamic characteristics derived from CFD results in this dissertation.And based on BEM theory,a design method of fixed-pitch variable-speed wind turbine was developed,which was successfully applied in designing a wind turbine with 50KW.And under full wind speed condition,the rotating speed fluctuation rate of this wind turbine was limited within20%,actually being 15.63%,fully meeting the design requirements.In last part two typical flow control schemes for fixed-pitch variable-speed wind turbine were explored.By using numerical method,the influence of air jet at blade tip was investigated.And the corresponding rotating speed control strategies were established for a series of cases with the FPVS-HAWT-SCM platform.The results illustrated that within a wide wind speed range,the rotating speed in case 1 and case 5 were more stable and proper for variable-speed control.Thus these attempts provided a new idea for the design and modification of fixed-pitch variable-speed wind turbine.