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大型風(fēng)輪葉片設(shè)計技術(shù)的現(xiàn)狀與發(fā)展趨勢
摘要：介紹目前風(fēng)電葉片的外形設(shè)計、結(jié)構(gòu)設(shè)計和材料方面的技術(shù)，并分析葉片在翼型、結(jié)構(gòu)設(shè)計和材料方面的發(fā)展趨勢和新的設(shè)計理念。
1前言
能源是人類社會發(fā)展和經(jīng)濟增長的原動力。目前以化石燃料為主的能源結(jié)構(gòu)，不僅資源難以支撐，而且對環(huán)境帶來嚴(yán)重題目，特別是溫室氣體排放造玉成球天氣變化將帶來一系列生態(tài)和環(huán)境題目。解決這一困難的出路在于開發(fā)清潔的可再生能源。目前在可再生能源中，除水電以外，風(fēng)電最具有貿(mào)易開發(fā)條件。風(fēng)能作為環(huán)境友好型的可再生能源，它的開發(fā)和利用不僅可以緩解世界能源危機，而且還具有常規(guī)化石能源不可相比的上風(fēng)，如可持續(xù)開發(fā)，不存在資源枯竭題目，不排放二氧化碳等溫室氣體和其他有害物質(zhì)等。地球上風(fēng)能資源非常豐富，占有關(guān)調(diào)查結(jié)果顯示，全球的風(fēng)能儲量約為2．74×10MW，其中可經(jīng)濟開發(fā)利用的風(fēng)能為2×10MW，比可開發(fā)利用的水電總量還要大1O倍。隨著常規(guī)化石能源的枯竭和生態(tài)環(huán)境的惡化，以風(fēng)電為代表的可再生能源的開發(fā)和利用受到各國政府的重視，經(jīng)過最近二十多年的發(fā)展，尤其是近幾年，風(fēng)電產(chǎn)業(yè)日益成為一個迅速增長的新興產(chǎn)業(yè)。在過往十年中，全球風(fēng)能產(chǎn)業(yè)以每年30％左右的速度快速增長，且這種趨勢還會持續(xù)下往。截止2006年底，全球風(fēng)電總裝機容量已超過74GW。
全球風(fēng)電產(chǎn)業(yè)的迅猛發(fā)展帶動了風(fēng)電機組及其上游產(chǎn)業(yè)鏈的快速發(fā)展，其中葉片是風(fēng)電機組的關(guān)鍵部件之一，其性能好壞直接影響風(fēng)電機組的風(fēng)能利用效率和機組所受載荷，在很大程度上決定了機組的整體性能和風(fēng)電開發(fā)利用的經(jīng)濟性。同時，葉片也是風(fēng)機的核心部件，其本錢約為風(fēng)電機組總本錢的20％[1]。因此，世界各大主要風(fēng)機制造商都非常重視葉片的設(shè)計和生產(chǎn)，并盡可能保持獨立的設(shè)計和生產(chǎn)能力。
2風(fēng)輪葉片設(shè)計
風(fēng)輪葉片的優(yōu)化設(shè)計要滿足一定的設(shè)計目標(biāo)，其中有些甚至是相互矛盾的¨：年輸出功率最大化；最大功率限制輸出；振動最小化和避免出現(xiàn)共振；材料消耗最小化；保證葉片結(jié)構(gòu)局部和整體穩(wěn)定性；葉片結(jié)構(gòu)滿足適當(dāng)?shù)膹姸纫蠛蛣偠纫蟆?
葉片設(shè)計可分為氣動設(shè)計和結(jié)構(gòu)設(shè)計這兩個大的階段，其中氣動設(shè)計要求滿足前兩條目標(biāo)，結(jié)構(gòu)設(shè)計要求滿足后四條目標(biāo)。通常這兩個階段不是獨立進(jìn)行的，而是一個迭代的過程，葉片厚度必須足夠以保證能夠容納腹板，進(jìn)步葉片剛度。
2.1外形設(shè)計
葉片氣動設(shè)計主要是外形優(yōu)化設(shè)計，這是葉片設(shè)計中至關(guān)重要的一步。外形優(yōu)化設(shè)計中葉片翼型設(shè)計的優(yōu)劣直接決定風(fēng)機的發(fā)電效率，在風(fēng)機運轉(zhuǎn)條件下，活動的雷諾數(shù)比較低，葉片通常在低速、高升力系數(shù)狀態(tài)下運行，葉片之間活動干擾造成活動非常復(fù)雜。針對葉片外形的復(fù)雜活動狀態(tài)以及葉片由葉型在不同方位的分布構(gòu)成，葉片葉型的設(shè)計變得非常重要。目前葉片葉型的設(shè)計技術(shù)通常采用航空上先進(jìn)的飛機機翼翼型設(shè)計方法設(shè)計葉片葉型的外形。先進(jìn)的CFD技術(shù)已廣泛應(yīng)用于不同類型氣動外形的設(shè)計，對于低雷諾數(shù)、高升力系數(shù)狀態(tài)下風(fēng)機運行條件，采用考慮粘性的N—S控制方程分析葉片葉型的流場是非常必要的。
在過往的1O多年中，水平軸風(fēng)機葉片翼型通常選擇NACA系列的航空翼型，比如NACA44XX，NA—CA23XX，NACA63XX及NASALS(1)等。這些翼型對前緣粗糙度非常敏感，一旦前緣由于污染變得粗糙，會導(dǎo)致翼型性能大幅度下降，年輸出功率損失最高達(dá)30％[3]。在熟悉到航空翼型不太適合于風(fēng)機葉片后，80年代中期后，風(fēng)電發(fā)達(dá)國家開始對葉片專用翼型進(jìn)行研究，并成功開發(fā)出風(fēng)電葉片專用翼型系列，比如美國Seri和NREL系列、丹麥RISO—A系列、瑞典FFA—W系列和荷蘭DU系列。這些翼型各有上風(fēng)，Seri系列對翼型表面粗糙度敏感性低；RISO—A系列在接近失速時具有良好的失速性能且對前緣粗糙度敏感性低；FFA—W系列具有良好的后失速性能。丹麥LM公司已在大型風(fēng)機葉片上采用瑞典FFA—W翼型，風(fēng)機專用翼型將會在風(fēng)機葉片設(shè)計中廣泛應(yīng)用。表1為對NREL翼型系列性能進(jìn)步的估算。
目前葉片外形的設(shè)計理論有好幾種，都是在機翼氣動理論基礎(chǔ)上發(fā)展起來的。第一種外形設(shè)計理論是按照貝茨理論得到的簡化設(shè)計方法，該方法是假設(shè)風(fēng)力機是按照貝茨公式的最佳條件運行的，完全沒有考慮渦流損失等，設(shè)計出來的風(fēng)輪效率不超過40％。后來一些著名的氣動學(xué)家相繼建立了各自的葉片氣動理論。Schmitz理論考慮了葉片周向渦流損失，設(shè)計結(jié)果相對正確一些。Glauert理論考慮了風(fēng)輪后渦流活動，但忽略了葉片翼型阻力和葉稍損失的影響，對葉片外形影響較小，對風(fēng)輪效率影響卻較大。Wilson在Glauert理論基礎(chǔ)上作了改進(jìn)，研究了葉稍損失和升阻比對葉片最佳性能的影響，并且研究了風(fēng)輪在非設(shè)計工況下的性能，是目前最常用的設(shè)計理論。
2.2結(jié)構(gòu)設(shè)計
目前大型葉片的結(jié)構(gòu)都為蒙皮主梁形式，如圖1所示為典型的葉片構(gòu)造形式_4j。蒙皮主要由雙軸復(fù)合材料層增強，提供氣動外形并承擔(dān)大部分剪切載荷。后緣空腔較寬，采用夾芯結(jié)構(gòu)，進(jìn)步其抗失穩(wěn)能力，這與夾芯結(jié)構(gòu)大量在汽車上應(yīng)用類似。主梁主要為單向復(fù)合材料層增強，是葉片的主要承載結(jié)構(gòu)。腹板為夾芯結(jié)構(gòu)，對主梁起到支撐作用。
葉片結(jié)構(gòu)設(shè)計應(yīng)依據(jù)相關(guān)設(shè)計規(guī)范。目前葉片結(jié)構(gòu)設(shè)計規(guī)范主要建立在IEC國際標(biāo)準(zhǔn)和德國GL標(biāo)準(zhǔn)基礎(chǔ)上，要求結(jié)構(gòu)滿足靜力強度、疲憊強度和葉尖撓度要求。復(fù)合材料葉片各展層是交錯展放的，實際初步設(shè)計時，將所有雙軸布視為一層，所有單軸布視為一層，這樣做對結(jié)構(gòu)強度和性能影響不大_4J。葉片結(jié)構(gòu)展層是分段設(shè)計，各段厚度都不一致，應(yīng)對厚度進(jìn)行連續(xù)化處理，終極設(shè)計的各展層厚度還應(yīng)為各單層厚度的整數(shù)倍。
結(jié)構(gòu)展層校核對葉片結(jié)構(gòu)設(shè)計來說也必不可少。前在校核方面，大多用通用貿(mào)易有限元軟件，比ANSYS、NASTRAN、ABAQUS等。對葉片進(jìn)行校核時，考慮單層的極限強度、自振頻率和葉尖撓度J，分析模型有殼模型和梁模型等，并且能夠做到這兩種模型的相互轉(zhuǎn)換，如圖2，3所示。與其他葉片結(jié)構(gòu)相比，目前大型葉片的中空夾芯結(jié)構(gòu)具有很高的抗屈曲失穩(wěn)能力，較高的自振頻率，這樣設(shè)計出來的葉片相對較輕。有限元法可用于設(shè)計，但更多用于模擬分析而不是設(shè)計，設(shè)計與模擬必須交叉進(jìn)行，在每一步設(shè)計完成后，必須更新分析模型，重新得到展層中的應(yīng)力和應(yīng)變數(shù)據(jù)，再返回設(shè)計，更改展層方案，再分析應(yīng)力和變形等，直到滿足設(shè)計標(biāo)準(zhǔn)為止，如圖4所示。由于復(fù)合材料正交各向異性的特殊性，葉片各展層內(nèi)的應(yīng)力并不連續(xù)，而應(yīng)變則相對連續(xù)，所以葉片結(jié)構(gòu)校核的失效準(zhǔn)則有時候完全采用應(yīng)變失效準(zhǔn)則。
2.3材料選擇
葉片發(fā)展初期，由于葉片較小，有木葉片、布蒙皮葉片、鋼梁玻璃纖維蒙皮葉片、鋁合金葉片等等，隨著葉片向大型化方向發(fā)展，復(fù)合材料逐漸取代其他材料幾乎成為大型葉片的唯一可選材料。復(fù)合材料具有其它單一材料無法相比的上風(fēng)之一就是其可設(shè)計性，通過調(diào)整單層的方向，可以獲得該方向上所需要的強度和剛度。更重要的是可利用材料的各向異性，使結(jié)構(gòu)不同變形形式之問發(fā)生耦合。比如由于彎扭耦合，使得結(jié)構(gòu)在只受到彎矩作用時發(fā)生扭轉(zhuǎn)。在過往，葉片橫截面耦合效應(yīng)是一個讓設(shè)計職員頭疼的困難，設(shè)計工程想方想法消除耦合現(xiàn)象。但在航空領(lǐng)域人們開始利用復(fù)合材料的彎扭耦合，拉剪耦合效應(yīng)，進(jìn)步機翼的性能J。在葉片上，引進(jìn)彎扭耦合設(shè)計概念，控制葉片的氣彈變形，這就是氣彈剪裁。通過氣彈剪裁，降低葉片的疲憊載荷，并優(yōu)化功率輸出。
玻璃纖維增強塑料(玻璃鋼)是現(xiàn)代風(fēng)機葉片最普遍采用的復(fù)合材料，玻璃鋼以其低廉的價格，優(yōu)良的性能占據(jù)著大型風(fēng)機葉片材料的統(tǒng)治地位。但隨著葉片逐漸變大，風(fēng)輪直徑已突破120m，最長的葉片已做到61．5m，葉片自重達(dá)18t。這對材料的強度和剛度提出了更加苛刻的要求。全玻璃鋼葉片已無法滿足葉片大型化，輕量化的要求。碳纖維或其它高強纖維隨之被應(yīng)用到葉片局部區(qū)域，如NEGMiconNM82．40m長葉片，LM61．5m長葉片都在高應(yīng)力區(qū)使用了碳纖維。由于葉片增大，剛度逐漸變得重要，已成為新一代MW級葉片設(shè)計的關(guān)鍵。碳纖維的使用使葉片剛度得到很大進(jìn)步，自重卻沒有增加。Vestas為V903．0MW機型配套的44m系列葉片主梁上使用了碳纖維，葉片自重只有6t，與V802MW，39m葉片自重一樣。美國和歐洲的研究報告指出，含有碳纖維的承載玻璃纖維層壓板對于MW級葉片是一個非常有效的選擇替換品。在E．c．公司資助的研究計劃¨。。中指出，直徑為120m風(fēng)輪葉片部分使用碳纖維可有效減少總體自重達(dá)38％，設(shè)計本錢減少14％。但碳纖維價格昂貴，極大地限制其在風(fēng)機葉片上的使用?，F(xiàn)今碳纖維產(chǎn)業(yè)仍以發(fā)展輕質(zhì)、良好結(jié)構(gòu)和熱性質(zhì)佳等附加值大的航空應(yīng)用材料為主。但很多研究員卻大膽預(yù)言碳纖維的應(yīng)用將會逐步增加。風(fēng)能的本錢效益將取決于碳纖維的使用方式，未來若要大量取代玻璃纖維，必須低價才具有競爭力。
3風(fēng)輪葉片發(fā)展趨勢
3.1葉片造型的發(fā)展
前面提到風(fēng)機葉片專用翼型已成系列，但還存在很大改進(jìn)空問。采用柔性葉片也是一個發(fā)展方向，利用新型材料進(jìn)行設(shè)計，從而改進(jìn)空氣動力和葉片受力狀態(tài)，增加可靠性和對風(fēng)能捕捉量。在開發(fā)新的葉片外形上也進(jìn)行大量嘗試，Enercon公司對33m葉片進(jìn)行空氣動力實驗，經(jīng)過精確的測定，葉片的實際氣動效率為56％，比按照Betz計算的最大氣動效率低約3—4％。為此，該公司對大型葉片外形型面和結(jié)構(gòu)都進(jìn)行了必要的改進(jìn)，包括為抑制天生擾流和旋渦在葉片端部安裝“小翼”，如圖5所示；為改善和進(jìn)步渦輪發(fā)電機主艙四周的捕風(fēng)能力，對葉片根莖進(jìn)行重新改進(jìn)，縮小葉片的外形截面，增加葉徑長度；對葉片頂部和根部之間的型面進(jìn)行優(yōu)化設(shè)計。在此基礎(chǔ)上，Enercon公司開發(fā)出旋轉(zhuǎn)直徑71131的2MW風(fēng)力發(fā)電機組，改進(jìn)后葉片根部的捕風(fēng)能力得以進(jìn)步。Enercon公司在4．5MW風(fēng)力發(fā)電機設(shè)計中繼續(xù)采用此項技術(shù)，旋轉(zhuǎn)直徑為112m的葉片端部仍安裝有傾斜“小翼”，使得葉片單片的運行噪音小于3個葉片(旋轉(zhuǎn)直徑為66m)運行時產(chǎn)生的噪音。
3.2葉片材料的進(jìn)展
風(fēng)機機組正朝著大型化發(fā)展，葉片長度越來越長，捕捉的風(fēng)能越來越多。風(fēng)場經(jīng)營者和能源公司都看好大葉片，因此Enercon公司的6MW機組應(yīng)運而生，GE公司的7MW機組研發(fā)緊鑼密鼓，而英國正在研制IOMW的巨型風(fēng)力機¨。如此大功率風(fēng)機配套的葉片將是超規(guī)模的。目前普遍采用的玻纖增強聚脂樹脂、玻纖增強環(huán)氧樹脂將無法滿足要求。所以必須開發(fā)更為先進(jìn)的材料，具備輕質(zhì)、高強以及剛性好的性能。
碳纖維的使用已成必然，但一般以碳／?；祀s的形式出現(xiàn)。3TEX開發(fā)了一種三維混雜結(jié)構(gòu)，如圖6所示。這種結(jié)構(gòu)具備高強度、高剛度特性，同時該結(jié)構(gòu)能使樹脂灌注速度加快，縮短工作時間。且這種結(jié)構(gòu)較厚，減少了展層層數(shù)，節(jié)約勞動力，降低了生產(chǎn)本錢。實際結(jié)果表明，使用這種混雜纖維形式比全玻璃鋼葉片減輕質(zhì)量約為10％左右。
在未來的十幾年里，有大量的葉片將會退役，退役后葉片的處理將是我們所面臨的一個非常棘手的題目。目前使用的復(fù)合材料葉片屬于熱固性復(fù)合材料，很難自然降解。廢棄物處理一般采用填埋或者燃燒等方法處理，基木上不再重新利用，易對環(huán)境造成影響，為此，人們開始積極研究開發(fā)“綠色葉片”一熱塑性復(fù)合材料葉片l121。愛爾蘭Gaoth風(fēng)能公司與日木三菱重工及美國Cyclics公司正在探討如何共同研制低本錢熱塑性復(fù)合材料葉片。根占有關(guān)資料介紹，與環(huán)氧樹脂／玻璃纖維復(fù)合材料大型葉片相比較，若采用熱塑性復(fù)合材料葉片，每臺大型風(fēng)力發(fā)電機所用的葉片重量可降低10％，抗沖擊性能大幅度進(jìn)步，制造本錢至少降低l／4，制造周期至少降低l／3，而且可完全回收和再利用。安全快捷地制造“綠色”的復(fù)合材料葉片正期待著復(fù)合材料葉片制造商往實現(xiàn)，Gaoth公司及其合作伙伴就是實現(xiàn)這一目標(biāo)的先驅(qū)。
3.3葉片設(shè)計新的研發(fā)理念
現(xiàn)在大型葉片的結(jié)構(gòu)基本為蒙皮加主梁的形式，主梁為預(yù)先成型，然后粘接到葉片蒙皮。國外有設(shè)計公司提出葉片整體成型概念，意在打破蒙皮主梁的結(jié)構(gòu)形式。
丹麥LM公司提出了“FutureBlade”的概念，且已在其54m和61．5m巨型葉片上使用了這種設(shè)計概念。LM公司研發(fā)部經(jīng)理FrankV．Nielsen以為未來葉片設(shè)計的關(guān)鍵已從效率最大化轉(zhuǎn)移到能量本錢(COE)最優(yōu)化，葉片將會更加細(xì)長，這種設(shè)計技術(shù)將會降低葉片載荷，葉片質(zhì)量分布更加優(yōu)化，材料本錢將會降低，產(chǎn)品質(zhì)量將更加得到保證。
2008年三月，美國Knight&Carver的風(fēng)電葉片公司成功開發(fā)了一種新型葉片STARBlade¨。這種具有創(chuàng)新性的葉片不同于當(dāng)前使用的盡大部分葉片，是專門針對低風(fēng)速區(qū)域設(shè)計的。這種葉片葉尖采用“柔性”設(shè)計理念進(jìn)行設(shè)計，在外形上與傳統(tǒng)葉片后緣線性變化不同，逐漸向后緣彎曲，降低了葉片風(fēng)壓和風(fēng)機的驅(qū)動扭矩，并最大限度捕捉所有可用風(fēng)速范圍內(nèi)的風(fēng)能，包括邊沿的低風(fēng)速區(qū)域，比傳統(tǒng)的葉片捕風(fēng)能力進(jìn)步了5～10％。第一片該種葉片已經(jīng)進(jìn)行了靜力測試，年內(nèi)還將生產(chǎn)第二片。
國內(nèi)中材科技風(fēng)電葉片股份有限公司研制的1．5MWsinoma40．2m葉片已經(jīng)成功下線，并在今年7月份通過了靜力測試。該葉片采用新的“柔性、預(yù)彎”設(shè)計技術(shù)，針對國內(nèi)風(fēng)況設(shè)計，葉尖部分向上風(fēng)向彎曲，葉片細(xì)長，柔性好，其整機載荷低于同類37．5m1．5MW葉片。
4結(jié)語
風(fēng)電將在全球范圍繼續(xù)高速發(fā)展，國內(nèi)、國外風(fēng)電市場巨大，中國的目標(biāo)是累計裝機容量在2010年達(dá)到500萬kW，2020達(dá)到年3000萬kW，這個目標(biāo)將會提前實現(xiàn)，國內(nèi)葉片市場將供不應(yīng)求。按目前國內(nèi)引進(jìn)技術(shù)比較普遍的1．5MW葉片來計算，2006—2010年，需要葉片數(shù)為7000片左右，而2010—2020年之間，所需葉片數(shù)將為50000片，國內(nèi)葉片市場巨大。
葉片設(shè)計技術(shù)的發(fā)展將會為我們提供更加高效，低本錢，高可靠性的葉片，國內(nèi)葉片設(shè)計技術(shù)相對落后，目前MW級別上，葉片設(shè)計技術(shù)基本依靠進(jìn)口，但該局面有看在未來的幾年內(nèi)逐步得到改觀，完全依靠國內(nèi)氣力設(shè)計的葉片不久的將來會在國內(nèi)風(fēng)電場上空運轉(zhuǎn)。
附錄B
Large turbine blade design technology of the current situation and trend of development
Abstract: the article introduces the current wind power blade shape design, structure design and material technology, and analyze the blade in the aspect of airfoil, structure design and materials and the development trend of the new design concept.
1 introduction
Energy is the impulsion of human social development and economic growth. Is given priority to with fossil fuel energy structure, resources not only difficult to support, but also to the environment have serious topic, especially greenhouse gas emissions made right ball weather change will bring a series of ecological and environmental questions. To solve the difficult way out lies in the development of clean renewable energy. At present in renewable energy, in addition to the water and electricity, wind power is most trade development condition. Wind as environmentally friendly renewable energy, its development and utilization can not only ease the world energy crisis, but also has the conventional fossil energy can not be compared to the upper hand, such as sustainable development, there is no resource depletion topic, do not emit carbon dioxide and other greenhouse gases and other harmful substances, etc. Earth wind energy resources are very abundant, accounts for the investigation, according to the results of the global wind energy reserves is about 2.74 x 10 mw, which can be economic development and utilization of wind energy for 2 x 10 mw, than can be larger amount of hydropower development and utilization of 1 o. As the conventional fossil energy depletion and the deterioration of ecological environment, represented by wind power development and utilization of renewable energy to the attention of the governments, after recent more than 20 years of development, especially in recent years, the wind power industry has increasingly become a fast-growing emerging industry. In the past ten years, the global wind energy industry rapid growth at the rate of about 30% a year, and this trend will continue. By the end of 2006, the global wind power total installed capacity of more than 74 gw.
The rapid development of global wind power industry to drive the rapid development of the wind turbines and the upstream industry chain, the blade is one of the key components of wind turbines, its performance is good or not directly affect the wind energy utilization efficiency of wind turbines and unit load, to a great extent, determines the overall performance of the unit and the efficiency of wind power development and utilization. At the same time, the leaf is the core component of fan, its capital is about 20% of the total capital wind turbines [1]. As a result, the world's large main fan manufacturer attaches great importance to the design and production of blade, and keep the independent design and production capacity as soon as possible.
2 turbine blade design
Turbine blade optimization design to meet certain design goals, some of them even conflicting ¨ : year maximum output power; Maximum power limit output; Vibration to minimize and avoid resonance; Material consumption minimization; Ensure the blade structure of local and global stability; Leaf blade structure satisfies the requirement of appropriate strength and stiffness requirements.
Blade design can be divided into pneumatic design and structure design of the two big stage, including pneumatic design requirements to meet the first two goals, structure design requirements to meet after four goals. Usually these two phases is not independent, but an iterative process, the blade thickness must be in place to ensure that can accommodate enough web, progress blade stiffness.
2.1 the appearance design
Blade aerodynamic design mainly shape optimization design, this is a crucial step in the design of blade. Shape optimization design of vane airfoil design quality directly decided to efficiency of the fan, under the condition of wind turbine, activity of the low Reynolds number, leaf blade usually run in the condition of low speed, high lift coefficient, blade between disturbance caused activity is very complicated. For complex activity state of blade shape and leaf by leaf type in different orientation, the distribution of leaf blade design becomes very important. At present the design technology of the leaf blade usually adopt advanced aircraft on aviation machine carefully design method to design the shape of leaf blade. Advanced CFD technology has been widely used in different types of aerodynamic shape design, for low Reynolds number and high lift coefficient condition fan operation conditions, controlled by considering viscous N - S equation of the flow field analysis of leaf blade is very necessary.
For many years in the past 1 o, the horizontal axial fan blade airfoil usually choose the committee series air airfoil, such as NACA44XX, NA - CA23XX, NACA63XX and NASALS (1), etc. The airfoil is sensitive to the leading edge roughness, once the leading edge because of pollution become rough, to cause a decline in airfoil performance greatly, annual output of up to 30% power loss [3]. In familiar to the air after the airfoil is not suitable for fan blade, after the mid - 80 - s, wind power in the developed countries began to study of special blade airfoil, and successfully developed A special wind vane airfoil series, such as the United States begawan and NREL series, Denmark RISO - A series, Sweden FFA - DU series W series and the Netherlands. These airfoils have the upper hand, begawan series of airfoil surface roughness sensitivity was low; RISO - the series A stall near the end of the good stall performance and low sensitivity to the leading edge roughness; FFA - W series has a good after stall performance. LM company in Denmark has been used on large fan blade airfoils FFA in Sweden - W, fan dedicated airfoil will be widely used in the design of fan blade. Table 1 for estimating of the NREL airfoil series performance improvement.
At present there are several kinds of blade shape design theory, is developed based on airfoil aerodynamic theory. The first design theory is according to the simplified design method of Bates theory, the method is to assume that the wind machine is run in accordance with the formula of optimum condition Bates, no considering the eddy current loss, etc., designed a turbine efficiency is not more than 40%. Later, some famous pneumatic scientists have established respective blade aerodynamic theory. Schmitz theory considering the vane axial eddy current loss, relatively correct some design results. Glauert theory after considering the rotor eddy current activity, but ignores the blade airfoil drag and leaves a little loss, the influence of the leaf shape is affected, the impact on the efficiency of wind turbines is larger. Wilson in Glauert improved on the basis of theory, to study the leaf slightly loss and lift-to-drag ratio on the blade for best performance, and studied the turbine under the off-design performance, is by far the most commonly used design theory.
2.2 structure design
At present large blade structure for skin girder form, as shown in figure 1 for typical _4j leaf structure form. Skin is mainly composed of biaxial composites layer increased, providing aerodynamic shape and bear most of the shear load. With wider rear cavity, USES the sandwich structure, advances its ability to resist buckling, the sandwich structure similar to a large number of applications in the car. Main girder of unidirectional composite material layer, is the main bearing structure of the blade. Web for sandwich structure, on long-term support.
Blade structure design should be based on the related design specifications. The blade structure design code based on GL standards based on IEC international standards and Germany, requires structure satisfies the requirement of static strength, fatigue strength and tip deflection. And the exhibition floor is interlaced with the blade, preliminary design, put all the biaxial cloth as a layer, all uniaxial cloth as a layer, do not _4J influence on structure strength and performance. Blade structure layer is segmented design exhibition, the thickness of the paragraphs are not consistent, should make continuous processing for the thickness, the ultimate design of each exhibition layer thickness should be also the integer times of single layer thickness.
Structure show layer check for blade structure design is essential. Before checking ways, mostly using general finite element software to trade, such as ANSYS, NASTRAN, ABAQUS. When the blade was checking, considering the limit of single layer strength, natural frequency and tip deflection J, analytical model and shell model and beam model, and the ability to do this two kinds of model transformation, as shown in figure 2, 3. Compared with other blade structure, large blade hollow sandwich structure has a high ability to resist buckling instability, high natural frequency of vibration, the designed blade relatively light. Finite element method can be used in the design, but more is used for simulating analysis rather than design, design and simulation must be crossed, after completion of each step in the design, must update the analysis model, to get the stress and strain of the exhibition floor, return to design again, change the exhibition floor plan, stress and deformation analysis, and so on, until meet the design standards, as shown in figure 4. Due to the particularity of orthotropic composite materials, the stress of each exhibition layer within the blade is not straight, the strain is relatively continuous, so check the failure criterion of blade structure sometimes completely using strain failure criteria.
2.3 material selection
Leaf development initial period, due to the small blades, a konoha, cloth skin leaves, steel fibre glass skin blade, aluminum alloy blades, etc., with the blade in the direction of large-scale development, composite material gradually replace other materials almost become the only optional large blade material. Cannot be compared with other single composite material is one of the upper hand of its design, by adjusting the direction of the single layer, can obtain the direction on the required strength and rigidity. More important is the anisotropy of available materials, make the structure deformation of different forms of ask a coupling. Such as bending torsional coupling, making the structure shift when only by bending moments. In the past, the cross section blade coupling effect is a design staff have a headache, want to design engineering ideas to eliminate the coupling phenomenon. But people began to use composite material in aviation crankle coupling, shear coupling effect, the performance of the progressive wing J. On the blade, the introduction of crankle coupling design concept, control gas elastic deformation of the blade, which is gas spring clipping. Through an air spring clipping, lower blade fatigue load, and optimize the power output.
Glass fiber reinforced plastic (FRP) is the most widely used in modern fan blades composite materials, glass fiber reinforced plastic with its low price, excellent performance occupies a large fan blade material dominance. However, as the leaves grow bigger, now rotor diameter of 120 m, the longest blade has to be 61.5 m, leaf weight of up to 18 t. The strength and rigidity of materials more stringent requirements are put forward. All GRP blades have been unable to meet the blade large, lightweight requirements. Carbon fiber or other high strength fiber is then applied to the blade local area, such as NEGMiconNM82.40 m long blade, LM61.5 m long blade are used carbon fiber in high stress area. Increases due to the blade, stiffness becomes increasingly important, has become a new generation of MW level the key to the blade design. The use of carbon fiber have a big progress, blade stiffness weight did not increase. Vestas V903.0 MW model matching the 44 m series main girder with the carbon fiber blades, leaf weight of 6 t, and V802MW, 39 m leaf weight. Study in the United States and Europe, bearing glass fiber laminate containing carbon fiber for MW stage blade replacement is a very effective choice. ¨. The E.c. company funded research plan. Part, according to a diameter of 120 m turbine blade using carbon fiber can effectively reduce the overall weight of was 38%, and the design of capital by 14%. But the carbon fiber is expensive, greatly limiting its use in the fan blades. Current lightweight carbon fiber industry is still in development and good structure and thermal properties, such as value-added large aviation applied materials. But many researchers boldly predicted that the application of carbon fiber will gradually increase. Wind way of capital benefit will depend on the use of carbon fiber in the future if you want to replace the glass fiber in great quantities, must be low price is competitive.
The development trend of 3 rotor blades
3.1 the development of the blade shape
Previously mentioned special fan blade airfoil has become a series, but also has great improvement empty asked. Using the flexible blade is also a development direction, using new materials to design, to improve the aerodynamic and blade stress, increase the reliability and the amount of wind energy capture. In the development of new blade shape is also a large number of attempts to Enercon company to 33 m blade aerodynamic experiments, through the accurate determination of blade's actual aerodynamic efficiency is 56%, lower than the maximum aerodynamic efficiency calculated on the basis of the Betz about 3-4%. To this end, the company of vane shape large surface and structure are the necessary improvements, including to curb naturally disturbed flow and vortex at the end of blade installation \"wing\", as shown in figure 5. To improve and progress the chasing after the wind around the turbine generator main tank capacity, improve of blade root, cross section shape of narrow leaves, increase leaf length to di