多葉片Savonius垂直軸風(fēng)力機(jī)的設(shè)計(jì)與數(shù)值分析

1、華中科技大學(xué)碩士學(xué)位論文多葉片Savonius垂直軸風(fēng)力機(jī)的設(shè)計(jì)與數(shù)值分析姓名:仁飄零申請(qǐng)學(xué)位級(jí)別:碩士專業(yè):流體機(jī)械及工程指導(dǎo)教師:王軍2011-01-06華 中 科 技 大 學(xué) 碩 士 學(xué) 位 論 文摘要垂直軸風(fēng)力機(jī)分為兩大類:DARRIEUS(升力型,簡(jiǎn)稱H型和SAVONIUS(阻力型, 簡(jiǎn)稱S型。 兩葉片的升力型風(fēng)力機(jī)啟動(dòng)轉(zhuǎn)矩低但效率較高,阻力型風(fēng)力機(jī)具備高的啟動(dòng)轉(zhuǎn)矩但效率很低?，F(xiàn)在流行設(shè)計(jì)混合型風(fēng)力機(jī)(將S型和H型風(fēng)力機(jī)的設(shè)計(jì)思想綜合在同一模型上,它能夠利用升力與阻力。許多學(xué)者利用低成本的辦法來(lái)單獨(dú)提高啟動(dòng)轉(zhuǎn)矩和效率。最常見的措施是改變?nèi)~片數(shù)以及優(yōu)化葉型。SAVONIUS風(fēng)力機(jī)總成本

2、低,制造和維修簡(jiǎn)單,雖然效率較低,但它適用于風(fēng)速低的居民區(qū)。本文研究的目標(biāo)就是設(shè)計(jì)一臺(tái)高效低成本的阻力型垂直軸風(fēng)力機(jī),并選用六個(gè)葉片的SAVONIUS型式。SAVONIUS風(fēng)力機(jī)的轉(zhuǎn)子在風(fēng)速很低的情況下開始向外輸出功率,在風(fēng)速很高的情況下依然能夠持續(xù)輸出。因?yàn)槔蔑L(fēng)速的范圍很廣,雖然效率低,但是這種風(fēng)力機(jī)還是很受歡迎。由于SAVONIUS風(fēng)力機(jī)基本上能夠一直輸出功率,它的年發(fā)電量遠(yuǎn)遠(yuǎn)高于同尺寸的水平軸風(fēng)力機(jī)。原裝SAVONIUS的輸出功率較低有兩個(gè)原因:(1風(fēng)打到葉片凸面上引起負(fù)轉(zhuǎn)矩;(2隨著轉(zhuǎn)子的旋轉(zhuǎn),其轉(zhuǎn)矩由高到低變化幅度很大。兩葉片SAVONIUS風(fēng)力機(jī)的最大缺點(diǎn)是:隨著轉(zhuǎn)子的旋轉(zhuǎn),轉(zhuǎn)矩

3、會(huì)下降到為接近零的程度。在某一些角度下,轉(zhuǎn)子無(wú)法啟動(dòng),而多葉片能夠確保轉(zhuǎn)矩的穩(wěn)定性。本文所介紹的提高效率的方法為:(1增加葉片數(shù)目和優(yōu)化設(shè)計(jì)(2不讓來(lái)流風(fēng)打到葉片的凸面上,即可以用一種“盾”或?qū)~把葉片的凸面蓋住。目前的阻力型風(fēng)力機(jī),尚未看到含“盾”或擋風(fēng)板的型式,所以本文以這種含“盾”的風(fēng)力機(jī)為研究對(duì)象,開展風(fēng)力機(jī)的CAD設(shè)計(jì)、CFD分析和樣機(jī)制造等工作。二維CFD計(jì)算的結(jié)果顯示,這種新型的風(fēng)力機(jī)比常規(guī)垂直軸風(fēng)力機(jī)效率高出約20%。 從樣機(jī)的制造角度來(lái)看,含導(dǎo)風(fēng)結(jié)構(gòu)的風(fēng)力機(jī)有弊病,即進(jìn)口須保持對(duì)風(fēng)以及穩(wěn)定性,減少摩擦;要達(dá)到這種要求將帶來(lái)設(shè)計(jì)和運(yùn)行的復(fù)雜化。另外,單獨(dú)采用轉(zhuǎn)子的風(fēng)力機(jī)型式的效

4、率并不低,而且簡(jiǎn)單方便,可以接受任何角度的風(fēng)源而不需要對(duì)風(fēng)的條件。因此,開展轉(zhuǎn)子的優(yōu)化設(shè)計(jì)研究工作非常有必要。本研究設(shè)計(jì)出了一種效率高且成本低的垂直軸風(fēng)力機(jī)。關(guān)鍵詞:SAVONIUS風(fēng)力機(jī);CFD;垂直軸風(fēng)力機(jī);VAWT華 中 科 技 大 學(xué) 碩 士 學(xué) 位 論 文AbstractAmong the vertical axis wind turbines, we find two main families: the Darrieus (H or V rotor and the Savonius (S rotor which work respectively on lift and drag

5、. The first family has a low starting torque and a high efficiency and the second has a high starting torque and a low efficiency. The trend is to find a cheap way to increase the starting torque and the efficiency of these turbines. Mostly, designers either increase the number of blades or work on

6、their geometry. Some designs include both rotors on the same axis to form a mixed lift-drag turbine. Although Savonius turbines have a lower aerodynamic efficiency, they have a high starting torque. Consequently, they start producing power under lower winds. Moreover, they continue producing power e

7、ven under very strong winds. As a result, Savonius turbines make a very good use of the wind resource available because they have a broader working interval; their capacity to transform wind into usable energy is higher than horizontal axis rotors of the same swept area. Besides, Savonius rotors ove

8、rall costs remain low; these rotors are also easy to manufacture and to operate. Despite their low efficiency, they are suitable turbines for populated areas where the wind speed is at minimum.The efficiency of Savonius turbines is low because of the “negative drag” exerted on the convex part of the

9、 blades, this drag results in a negative torque lowering the efficiency. The best way to improve the Savonius rotor efficiency is:(1Preventing the wind from striking the convex parts: it can be done by hiding theconvex part of the blades behind a shield or a vane.(2Increasing the number of blades an

10、d finding the related optimum geometry.The aim of the present study is to design a cheap high efficient vertical axis drag wind turbine. The concept chosen is a six-bladed Savonius rotor.Among all the drag turbines available on the market, no model with wind shield has been found. Therefore, the typ

11、e of turbine designed in the present study is considered new.The full turbine has been designed by CAD and manufactured; its aerodynamic behavior tested by CFD. The CAD design was modified according to the CFD computations results. The present thesis mainly shows the results of the two-dimensional C

12、FD computations and also some observations made during operation of the prototype. In theory this new design is a success because the power coefficient improves significantly. However, in application it is not perfect yet because it also brings some drawbacks: orientation of the shield towards the w

13、ind, friction and stability. The principal issue of adding external devices is that: if the inlet is not maintained at its optimum position, the efficiency would drop immediately. The rotor alone (without shielding is a simple but effective configuration, it can start at very low wind speeds with wi

14、nd blowing from any direction. As a reason, the future work should be focused on improving the design of the rotor (geometric parameters of the blades, number of blades, dimension ratios.This study is a success because it opens the way to a new type of cheap and effective wind turbines.Key words: CF

15、D, Vertical Axis Wind Turbine, Savonius獨(dú)創(chuàng)性聲明本人聲明所呈交的學(xué)位論文是我個(gè)人在導(dǎo)師指導(dǎo)下進(jìn)行的研究工作及取得的研究成果。盡我所知,除文中已經(jīng)標(biāo)明引用的內(nèi)容外,本論文不包含任何其他個(gè)人或集體已經(jīng)發(fā)表或撰寫過的研究成果。對(duì)本文的研究做出貢獻(xiàn)的個(gè)人和集體,均已在文中以明確方式標(biāo)明。本人完全意識(shí)到本聲明的法律結(jié)果由本人承擔(dān)。學(xué)位論文作者簽名:日期:年月日學(xué)位論文版權(quán)使用授權(quán)書本學(xué)位論文作者完全了解學(xué)校有關(guān)保留、使用學(xué)位論文的規(guī)定,即:學(xué)校有權(quán)保留并向國(guó)家有關(guān)部門或機(jī)構(gòu)送交論文的復(fù)印件和電子版,允許論文被查閱和借閱。本人授權(quán)華中科技大學(xué)可以將本學(xué)位論文的全部或

16、部分內(nèi)容編入有關(guān)數(shù)據(jù)庫(kù)進(jìn)行檢索,可以采用影印、縮印或掃描等復(fù)制手段保存和匯編本學(xué)位論文。保密,在_年解密后適用本授權(quán)書。本論文屬于不保密。(請(qǐng)?jiān)谝陨戏娇騼?nèi)打“”學(xué)位論文作者簽名:指導(dǎo)教師簽名:日期:年月日日期:年月日華 中 科 技 大 學(xué) 碩 士 學(xué) 位 論 文1.Introduction1.1 Role of Wind PowerKofi Annan said 53 “Energy is essential for development. Yet two billion people currently go without, condemning them to remain in th

17、e poverty trap. We need to make clean energy supplies accessible and affordable. We need to increase the use of renewable energy sources and improve energy efficiency”.Fifty six per cent of the worlds rural population does not have access to electricity and electricity production accounts for thirty

18、 eight per cent of worldwide CO2 emissions.Wind Energy could be part of the solution as it provides 100% clean and renewable energy. Wind turbines could provide electricity for people who have no access to the electrical grid. People living in developing countries with low industrialization level an

19、d low budgets need wind turbines that are: cheap, robust, simple, easy to manufacture and needing low maintenance. Such wind turbines could also be installed in developed countries for reducing the non-renewable energy consumption. Wind turbines could be installed in urban areas: if each building ha

20、d its own wind turbine, it would diminish the amount of energy required from power plants.The amount of electric current that the turbine can produce each year depends on the wind resource of the site. Therefore, the choice of the turbine depends first on the wind power potential of the site. The en

21、ergy which the turbine takes from the wind varies with the cube of the wind speed meaning that small variations in wind speed induce great variations in the power output. It is well known that wind is extremely variable in both space and time.The present thesis focuses on urban wind turbines because

22、 they produce energy right where it is needed. In urban environment, average wind speeds are very low. Therefore, efficient turbines have to be able to produce a significant amount of energy at low wind speeds. Figure 1 Wind Power華 中 科 技 大 學(xué) 碩 士 學(xué) 位 論 文 Figure 2 Basic diagram showing how wind is tur

23、ned into electricity using V AWTs1.2 Comparing Wind TurbinesThe important characteristics when comparing wind turbines are:Coefficient of Power CpTorque coefficient CmOverall costsNoise and visual pollutionYearly produced electric energy (most importantAccording to Betz, the maximum energy that an i

24、deal wind turbine can absorb is 16/27 of the total kinetic energy of the incoming wind. Therefore, the maximum efficiency of an ideal turbine would be 0.593 which means that only less than 60% of the incoming wind energy can be used for wind turbines installations.華 中 科 技 大 學(xué) 碩 士 學(xué) 位 論 文 Figure 3 Co

25、efficients of power of the main different types of turbines 11.2.1 Vertical Axis Wind Turbines (V AWTThe most evident particularity of V AWTs is that they dont need to be oriented into the wind because they can catch wind from any direction. V AWTs can be installed on top of buildings roofs, on the

26、ground or on a self-supporting tower. Installing the turbine on the ground or on roofs facilitates the mounting and the maintenance because the eventual drive train and electric generator are on the ground.The mobile parts in V AWTs are less than in HAWTs (Horizontal Axis Wind Turbines. Fatigue life

27、 is also longer for V AWTs than HAWTs due to lower stress and rotating speed; which makes them more reliable. V AWTs have low tip speed ratios which reduces to the minimum their noise pollution. On the other hand, the major inconvenient of installing a turbine near the ground is that the rotor is lo

28、cated in the lowest part of the atmospheric boundary layer where the wind speed is very low and the turbulence very high. Turbulence induces both wind speed and direction variations. As a reason,V AWTs can also be mounted on self-supporting towers to benefit from higher wind speeds. Close to the gro

29、und, due to surface friction and heat exchanges between the air and the surface (convection, the wind speed decreases and the wind direction changes. In urban areas, the surface roughness takes its highest value since there are a lot of obstacles to the wind flow. The turbulence intensity of the win

30、d flow depends on the surface roughness and turbulence intensity decreases with height. The surface roughness takes its lowest value in desert areas and above the sea, where the largest wind turbines are installed (onshore and offshore wind farms. Figure 4 Axial wind velocity respectively: linear, e

31、xponential and turbulent 32華 中 科 技 大 學(xué) 碩 士 學(xué) 位 論 文 Figure 5 Respectively: Windside products (Helical Savonius, MUCE products (Multi-blade Darrieus, GUAL INDUSTRIE products (Multi-blade Savonius with stators and standard Savonius1.2.2 Standard Savonius Wind Turbine, drag typeThe Savonius rotor starts

32、 producing electrical energy at very low wind speeds and can still continue producing electricity at very high wind speeds. This is the reason why this type of turbine is very popular despite is low coefficient of power. Savonius turbines have a very good annual electricity production rate compared

33、to HAWTs of the same swept area and this because Savonius rotors are producing power almost all the time.Weak pointsNegative torque for a range of azimuth anglesLow rotation speed (generators work better with higher speedsLow power coefficient (CpStrong pointsEasy to realize and to operateLow costsC

34、an be realized with recuperation material such as oil barrelsLow maintenanceCan start producing electricity at very low wind speeds1.2.3 Standard Darrieus Wind Turbine, lift type Figure 6 Darrieus rotorThe standard Darrieus wind turbine has such a low starting torque that it cannot self-start. The r

35、otation of the turbine has to be initiated by an auxiliary mean. Commonly, they are the following: using the generator has a motor by injecting current in order to produce the torque necessary to start the turbine;華 中 科 技 大 學(xué) 碩 士 學(xué) 位 論 文installing a pitch control system to vary the angle of attack o

36、f the airfoils along with the wind speed thus the turbine can self-start at lower wind speeds; installing a Savonius rotor on the shaft of the Darrieus rotor (see hybrid wind turbine systems.Weak points-Low starting torque-Need of relatively high wind speeds-The surface roughness of the airfoils and

37、 the type of airfoils chosen has a strong impact on the efficiency (Reynolds number influence on the boundary layer-Manufacturing airfoils needs high precision machining: higher costs-Presence of a control system (pitch controlStrong points-High aerodynamic efficiency-Can be built in big scalesMulti

38、-bladed Darrieus Wind Turbines Figure 7 Shenzhen Chengyuan Huanbao productAdding blades to the Darrieus rotor leads to the same conclusions as in HAWTs: multi-bladed machines are used for low speed and high torque operations; they can self-start at low wind speeds because they have higher torque but

39、 they have a lower rotational velocity at which optimum performance is reached. Two-bladed Darrieus wind turbines are used for low torque and high speed operations; they have a better aerodynamic efficiency than multi-bladed Darrieus but they start only at higher wind speeds.In urban areas where the

40、 wind speed is low, multi-bladed Darrieus have the upper hand because they start producing power at lower wind speeds. They are also self-starting suppressing any related control device. Adding more blades from the aerodynamic point of view is not good because each added blade will add another wake

41、then more turbulence is added to the air flow resulting in an increase in drag and a decrease in lift on each blade. The interactions between the blades and the wakes lower the efficiency of the turbine. The number of blades should be carefully determined.華 中 科 技 大 學(xué) 碩 士 學(xué) 位 論 文 Figure 8 Flow field

42、around a three-bladed Darrieus 16 Figure 9 HAWTs respectively water pumping application (high torque low speed, electricity production(low torque high speed1.2.4 Three-bladed Horizontal Axis Wind Turbine (HAWT Figure 10 Large scale Horizontal Axis Wind Turbines華 中 科 技 大 學(xué) 碩 士 學(xué) 位 論 文Weak pointsNeed

43、of high wind speeds (average wind speed of the site should be at least 8m/sNeed a system to make the rotor facing the incoming wind (yaw control as the wind direction changes, yawing the rotor also induces gyroscopic loadsCosts are still too high, long period before the return on investmentDrive tra

44、in and generator are inside the nacelle which complicates maintenance (see pictures aboveFatigue problemsStructural stability problemsAeroelastic instability problemsNoise problems due to the high tip speed ratioThe bigger the turbine is, the more complicate is the installation and transportationPeo

45、ple dont live in areas where the wind has an average speed of 8 m/s. Therefore this kind of turbine is not appropriate for urban areas where the average wind speed is low. In addition; the noise generated by the turbine and the visual pollution could both be problems. The major drawback of HAWTs is

46、that the rotor constantly needs to be oriented into the wind (yaw control. When wind direction is changing randomly HAWTs rotors cannot catch effectively the energy from the incoming wind.Strong pointsHigh aerodynamic efficiencyCan be realized in huge scales (see picture above since the power produc

47、ed is directly proportional to the swept area.Big scale HAWTs axis of rotation is at a high distance from the ground where the wind is stronger and less turbulent (Mega-Watt size turbines are more than 100m high.1.3 Optimized Types of the Savonius Wind TurbineIn populated areas: the wind speed is mo

48、st of the time very low, because since the dawn of time people settle down mostly in low winds areas. The wind direction is also highly variable and the turbulence higher. For those reasons, Savonius types of wind turbines are more appropriate. The major drawback of Savonius rotors is the low coeffi

49、cient of power; many researchers have looked for simple and cheap methods to enhance the efficiency of the classical Savonius wind turbines.In the conventional Savonius rotor: the starting torque can be high, low, null or even slightly negative for several angles. Giving a helical form to the rotor

50、can decrease this variation of static torque and thus give a constant torque along 360 degrees; improving the rotors starting ability. The helical rotor has a lower coefficient of power than the conventional Savonius rotor 2. On the other hand, the helical rotor presents an advantage in terms of fat

51、igue life: the maximum power coefficient occurs at a lower tip speed ratio. For a given wind speed, a helical rotor would rotate at a slower angular velocity than a conventional rotor which diminishes vibrations and loads. However lower rotating speeds yield lower performances from the generator.華 中

52、 科 技 大 學(xué) 碩 士 學(xué) 位 論 文 Figure 11 Conventional Savonius rotor, Coefficient of torque when starting the turbine according to theposition of the rotor 31.3.1 Helical SavoniusThe helical form of the blade ensures that there is always a concave surface to catch the wind at the optimum angle (90 degrees and

53、 create the optimum drag. Figure 12 Windside products helical Savonius rotor / two stages conventional Savonius rotor 4 / three-stage Savonius turbine made with oil barrels for pumping water in Argentina To solve the problem of negative starting torque, it is also possible to superpose conventional

54、Savonius rotors with an angle shift. They are all on the same axis but their angular position is different. Two stages can be shifted of 90 degrees for example. Adding stages ensures that there is always a surface of optimum torque for the wind to blow in. Unfortunately, adding stages increases the

55、inertia of the turbine; for that reason, adding too many stages would decrease the maximum coefficient of power 2 1.3.2 The Lebost Wind Turbine 5 Figure 13 The Lebost Wind Turbine installed on the roof of a building (1975-1981 / Schema of theLebost Wind Turbine for the wind tunnel tests 5華 中 科 技 大 學(xué)

56、 碩 士 學(xué) 位 論 文 The LWT is a four-bladed Savonius type rotor inside a flow-focusing housing. It is a kind of diffuser augmented wind turbine. According to the results of Hoffert et al. 5, the efficiency of the LWT can equal the Betz limit of 16/27. Figure 14 Comparison of coefficients of power between

57、the Lebost Wind Turbine and other classicalturbines 51.3.3 Multi-bladed Savonius rotorsAccording to Shikha et al. 6, the conventional Savonius rotor does not work efficiently for a few reasons. One of the reasons is that: there is a negative torque result of the wind exerting pressure on the convex

58、part see figure 15 (red circle. Figure 15 6 Flow field around conventional Savonius rotor / diffuser augmented Savonius rotor respectively position 1 and 2 / diffuser augmented Savonius six-bladed rotor This negative torque could be suppressed if all the incoming flow was converged on the concave part. This could be done by adding a convergent nozzle. The convergent nozzle has another advantage: it increases the wind speed (according to 6. The energy of the wind varies with the cube of the wind speed, increasing the wind