Wind turbine designs. What are the differences between multi-bladed propellers and small-bladed ones Program – design and verification aerodynamic calculations of a wind generator – file technical report.doc

WIND ENGINE
a device that converts wind energy into rotational energy. The main working part of a wind turbine is a rotating unit - a wheel driven by the wind and rigidly connected to a shaft, the rotation of which drives equipment that performs useful work. The shaft can be installed horizontally or vertically. Wind turbines are usually used to generate energy consumed periodically: when pumping water into a tank, grinding grain, in temporary, emergency and local power supply networks.
Historical reference. Although surface winds do not always blow, change their direction and their strength is not constant, the wind turbine is one of the oldest machines for obtaining energy from natural sources. Due to the questionable reliability of ancient written accounts of wind turbines, it is not entirely clear when and where such machines first appeared. But, judging by some records, they already existed before the 7th century. AD It is known that they were used in Persia in the 10th century, and in Western Europe the first devices of this type appeared at the end of the 12th century. During the 16th century. The tented type of Dutch windmill was finally formed. No significant changes in their design were observed until the beginning of the 20th century, when, as a result of research, the shapes and coatings of mill wings were significantly improved. Since low-speed machines are cumbersome, in the second half of the 20th century. began to build high-speed wind turbines, i.e. those whose wind wheels can make a large number of revolutions per minute with a high efficiency of wind energy utilization.
Modern types of wind turbines. Currently, three main types of wind turbines are used - drum, wing (screw type) and rotor (with an S-shaped repeller profile).
Drum and vane. Although the drum-type wind wheel has the lowest wind energy utilization rate compared to other modern repellers, it is the most widely used. Many farms use it to pump water if for some reason there is no mains electricity. A typical shape of such a wheel with sheet metal blades is shown in Fig. 1. Drum and vane type wind wheels rotate on a horizontal shaft, so they must be turned into the wind to get the best performance. To do this, they are given a rudder - a blade located in a vertical plane, which ensures that the wind wheel turns into the wind. The diameter of the wheel of the world's largest vane-type wind turbine is 53 m, the maximum width of its blade is 4.9 m. The wind wheel is directly connected to an electric generator with a power of 1000 kW, which develops at a wind speed of at least 48 km/h. Its blades are adjusted in such a way that the rotation speed of the wind wheel remains constant and equal to 30 rpm in the wind speed range from 24 to 112 km/h. Due to the fact that the winds blow quite often in the area where such wind turbines are located, the wind turbine typically produces 50% of the maximum power and powers the public electrical grid. Vane wind turbines are widely used in remote rural areas to provide electricity to farms, including charging the batteries of radio communication systems. They are also used in onboard propulsion systems of aircraft and guided missiles.

S-shaped rotor. An S-shaped rotor mounted on a vertical shaft (Fig. 2) is good because a wind turbine with such a repeller does not need to be brought into the wind. Although the torque on its shaft varies from minimum to one-third of maximum value in half a turn, it does not depend on the direction of the wind. When a smooth circular cylinder rotates under the influence of wind, a force perpendicular to the direction of the wind acts on the body of the cylinder. This phenomenon is called the Magnus effect, after the German physicist who studied it (1852). In 1920-1930, A. Flettner used rotating cylinders (Flettner rotors) and S-shaped rotors instead of bladed wind wheels, and also as propulsors of a ship that made the transition from Europe to America and back.

Collier's Encyclopedia. - Open Society. 2000 .

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The power of the flow, or as it is also called second energy, is proportional to the cube of the wind speed. What does it mean - if the wind speed increases, say, twice, then the energy of the air flow will increase by 2 3 times, namely 2 3 = 2x2x2 = 8 times.

The power developed by the wind engine will vary in proportion to the square of the diameter of the wind wheel. What does it mean when the diameter of the wind wheel is doubled - we get a fourfold increase in power at the same wind speed.

However, not all the energy flowing through a wind wheel can be converted into useful work. Some of the energy will be lost when overcoming the resistance of the wind wheel to the air flow, as well as other losses. Also, a fairly large part of the air energy will be contained in the flow that has already passed through the wind wheel. The theory of vane wind turbines proves:

• The speed of the wind flow behind the wind wheel is not zero;
• The best mode of operation of a wind turbine is one in which the flow speed behind the wind wheel will be equal to 2/3 of the initial flow speed that will flow onto the wind wheel.

Energy utilization factor

This is a number that shows how much of the air flow power will be usefully used by the wind wheel. This coefficient is usually denoted by the Greek letter χ (xi). Its value depends on a number of factors, such as the type of wind motor, the quality of manufacture and the shape of its blades and other factors. For high-speed wind turbines that have streamlined aerodynamic wings, the coefficient χ is approximately 0.42 to 0.46. This means that machines of this type can convert about 42%-46% of the wind flow passing through the installation into useful mechanical work. For low-speed vehicles, this coefficient is about 0.27 - 0.33. The theoretical maximum value of χ for ideal vane wind turbines is approximately 0.593. Vane installations have become quite widespread, and they began to be produced en masse by industry. They are divided into two groups:

• High-speed – number of blades up to 4;

Low-speed - from 4 to 24 blades;

High-speed and low-speed wind turbines

Speed ​​is one of the advantages, as it makes it easier to transfer wind energy to such high-speed devices as an electric generator. Moreover, they are lighter and have a higher wind speed utilization factor than low-speed ones, as mentioned above.

However, in addition to their advantages, they also have a serious drawback, such as several times less torque with a stationary wind wheel and with the same wheel diameters and wind speed than low-speed installations. Below are two aerodynamic characteristics:

Where the dotted line shows an 18-bladed wind wheel, and the solid line shows a 3-bladed one. The horizontal axis shows the number of modules Z of the wind wheel or its speed. This value is determined by the ratio of the speed ωхR of the blade tip to the wind speed V.

From the characteristics of the wind engine we can conclude that each wind speed can have only a single number of revolutions at which the maximum χ can be obtained. In addition, in the presence of the same wind speed, a low-speed device will have a torque several times greater than a high-speed one, and accordingly it will begin to operate at a wind speed lower than the high-speed one. This is quite a significant factor, as it increases the number of operating hours of the wind turbine.

Vane wind turbines

The principle of their operation is based on the aerodynamic forces that arise on the blades of the wind wheel when an air flow hits them. In order to increase power, the wings are given streamlined, aerodynamic profiles, and the wedging angles are made variable along the blade (the closer to the shaft, the larger the angles, and the smaller at the end). The diagram is shown below:

There are three main parts of this mechanism - the blade, the swing, with the help of which the wheel is attached to the hub. Wedging angle φ is the angle between the plane of rotation of the wheel and the blade. Angle of attack α is the angle of wind impact on the blade elements.

When the wind wheel was braked, the directions of the flow flowing onto the blade and the direction of the wind coincided (along arrow V). But since the wheel has a certain rotation speed, then, accordingly, each of the elements of the blade will have a certain speed ωxR, which will increase with distance from the wheel axis. Therefore, the flow blowing over the blade at a certain speed will consist of the speed ωxR and V. This speed is called the relative flow speed and is designated W.

Since only at certain angles of attack there is the best operating mode for a vane wind turbine, the wedging angles φ have to be made variable along the entire length of the blade. The power of a wind engine, like any other, is determined by the product of the angular velocity ω and its torque M: P = Mxω. We can conclude that with a decrease in the number of blades, the moment M will also decrease, but the number of revolutions ω will increase. That is why the power P = Mxω will remain almost constant and will weakly depend on the number of windmill blades.

Other types of wind turbines

As you know, in addition to winged ones, there are also drum, carousel and rotary wind engines. In carousel and rotary types the axis of rotation is vertical, and in drum types it is horizontal. Perhaps the main difference between winged wind turbines and drum and rotary wind turbines is that all the blades of the winged wind turbines operate simultaneously, while the drum and rotary wind turbines operate only that part of the blades, the movement of which will coincide with the direction of the wind.

To reduce the resistance of the blades that go towards the wind, they are either made curved or covered with a screen. The torque when using this type of engine occurs due to different pressures in the blades.

Since rotary, carousel and drum types of wind engines have rather low efficiency (χ for these types does not exceed 0.18), and are also quite bulky and low-speed, in practice they have not received widespread use.

The growth of energy production through the use of non-renewable natural resources is limited by the threshold beyond which there is complete production of raw materials. Alternative energy, including wind power generation, will reduce the load on the environment.

The movement of any mass, including air, generates energy. A wind turbine converts the kinetic energy of the air flow into mechanical energy. This device is the basis of wind energy, an alternative direction in the use of natural resources.

Efficiency

It is quite simple to evaluate the energy efficiency of a unit of a certain type and design and compare it with the performance of similar engines. It is necessary to determine the wind energy utilization factor (WEF). It is calculated as the ratio of the power received at the wind turbine shaft to the power of the wind flow acting on the surface of the wind wheel.

The wind energy utilization rate for various installations ranges from 5 to 40%. The assessment will be incomplete without taking into account the costs of design and construction of the facility, the quantity and cost of generated electricity. In alternative energy, the payback period for wind turbine costs is an important factor, but it is also necessary to take into account the resulting environmental effect.

Classification

Wind turbines are divided into two classes based on the principles of using the generated energy:
linear;
cyclical.

Linear type

A linear or mobile wind turbine converts the energy of air flow into mechanical energy of movement. This could be a sail or a wing. From an engineering point of view, this is not a wind turbine, but a propulsion device.

Cyclic type

In cyclic engines the housing itself is stationary. The air flow rotates, making cyclic movements, its working parts. Mechanical rotational energy is most suitable for generating electricity, a universal form of energy. Cyclic wind engines include wind wheels. Wind wheels, from ancient windmills to modern wind power plants, differ in design solutions and in the complete use of air flow power. The devices are divided into high-speed and low-speed, as well as according to the horizontal or vertical direction of the rotor axis of rotation.

Horizontal

Wind turbines with a horizontal axis of rotation are called vane engines. Several blades (wings) and a flywheel are attached to the rotor shaft. The shaft itself is located horizontally. The main elements of the device: wind wheel, head, tail and tower. The wind wheel is mounted in a head rotating around a vertical axis, in which the engine shaft is mounted, and transmission mechanisms are located. The tail plays the role of a weather vane, turning the head with the wind wheel against the direction of the wind flow.

At high speeds of air flow (15 m/s and above), the use of high-speed horizontal wind turbines is rational. Two and three blade units from leading manufacturers provide KIEV of 30%. A self-made wind turbine has an air flow utilization rate of up to 20%. The efficiency of the device depends on careful calculation and quality of manufacturing of the blades.

Vane wind turbines and wind turbines provide high shaft rotation speed, which allows power to be transferred directly to the generator shaft. A significant disadvantage is that in weak winds such wind turbines will not work at all. There are starting problems when moving from calm to increased wind.

Low-speed horizontal engines have a larger number of blades. The significant area of ​​interaction with the air flow makes them more effective in weak winds. But the installations have significant windage, which requires taking measures to protect them from gusts of wind. The best KIEV indicator is 15%. Such installations are not used on an industrial scale.

Vertical carousel type

In such devices, blades are installed on the vertical axis of the wheel (rotor) to receive the air flow. The housing and the damper system ensure that the wind flow hits one half of the wind wheel, and the resulting resulting moment of application of forces ensures rotation of the rotor.

Compared to vane units, a rotary wind turbine generates more torque. As the air flow speed increases, it reaches operating mode faster (in terms of traction force) and stabilizes in terms of rotation speed. But such units are slow-moving. To convert shaft rotation into electrical energy, a special generator (multipole) capable of operating at low speeds is required. Generators of this type are not very common. The use of gearbox systems is limited by low efficiency.

A carousel wind turbine is easier to operate. The design itself provides automatic control of the rotor speed and allows you to monitor the direction of the wind.

Vertical: orthogonal

For large-scale energy production, orthogonal wind turbines and wind turbines are the most promising. The range of use of such units, in terms of wind speed, is from 5 to 16 m/s. The power they generate has been increased to 50 thousand kW. The profile of an orthogonal blade is similar to that of airplane wings. In order for the wing to start working, you need to apply a flow of air to it, as during the takeoff run of an airplane. The wind turbine also needs to be spun up first, expending energy. After this condition is met, the installation switches to generator mode.

conclusions

Wind energy is one of the most promising renewable energy sources. Experience from the industrial use of wind turbines and wind turbines shows that efficiency depends on the placement of wind generators in places with favorable air flows. The use of modern materials in the design of units, the use of new schemes for generating and storing electricity will further improve the reliability and energy efficiency of wind turbines.

The wind wheel wings are the most important part of a wind turbine. The power and speed of the wind generator depend on the shape of their blades.

In this brochure we will not dwell on the calculation of new wings due to the complexity of this task, but will use ready-made wings that have a certain shape and are characterized by a high efficiency of wind energy utilization and high speed. We only need to solve the question of how to determine the dimensions of new wings for the desired power, based on the dimensions of known wings while maintaining their original characteristics.

For low-power windmills, we will accept a high-speed two-blade wind wheel with the following characteristic known from practice:

Wind energy utilization coefficient……………………………0.35

The speed of a wind wheel should be understood as the ratio of the peripheral speed of the tip of the blade to the wind speed

Taking the same speed equal to 7 for wind wheels of different diameters, we will get different speed of the wind wheels at the same wind speed. The wind wheel with the smallest diameter will develop the highest speed. In general, the revolutions of wind wheels with equal speeds will relate to each other in inverse proportion to their diameters, i.e.

This means a wind wheel with a diameter D 1 will make revolutions per minute as many times as much as the diameter of this wind wheel D 1 is less than the diameter D 2 of another wind wheel. For example, if a wind wheel with a diameter of 1.5 m makes 714 rpm, then a wind wheel with a diameter of 3 m will make 357 rpm, i.e., half as much, although their speed is the same.

For the convenience of calculating the sizes of wind wheel blades of different diameters, but with the same speed, in the table. Figure 4 shows the dimensions of a two-blade wind wheel with a diameter of 1 m. At the top of the table there is a drawing of a blade with letter designations of its dimensions, and under the picture in the table the digital values ​​of these dimensions are given.

On the left, 4 columns show the dimensions of the blade to the left figure; on the right, in 10 columns, the dimensions of five profiles of this blade are given. How to set profile dimensions is shown in the table figure on the right.

In order to comply with the accepted characteristics of a wind wheel with a change in its diameter, it is necessary to change all the dimensions of these blades in the same ratio in which we change the diameter of the wind wheel. In this case, we will maintain geometric similarity, without which it would be impossible to use this method of recalculation.

Since the wind wheel with the dimensions given in table. 4, has a diameter of 1 m, then the ratio of the diameter of the other wind wheel to unity will be equal to D, i.e.

Therefore, in order to obtain the dimensions of a wind wheel blade with a different diameter, each dimension given in the table is necessary. 4, multiply by the value of this diameter. Only the wedge angles of each blade section and their number should remain unchanged. For example, for a wind wheel with a diameter of 1.2 m, each size of the table is required. 4 multiplied by 1.2, we get:

To enlarge the table, click on it with the mouse

To obtain the finished shape of the blade, it is necessary in size, p

calculated in table. 5, draw points for five blade profiles on a sheet of paper and trace the contours along the points using a pattern, as shown in Fig. 13. The profiles of each section are drawn in full size so that templates can be cut out from them when manufacturing the blade.

For a generator with a power of 1 kW, you need a wind wheel with a diameter of 3.5 m. To obtain the dimensions of the blade of this wind wheel, you need those given in table. 4 multiply the dimensions of a wind wheel with a diameter of 1 m by 3.5 and make a table, and then draw the blade profiles that will be required during manufacture.

The power and speed of two-blade wind wheels with the characteristics given above are given in Table. 6.

This table must be used when choosing the diameter of a wind wheel of a given power and determining the gear ratio if the generator speed turns out to be greater than the wind wheel speed it develops at a wind speed of 8 m/sec.

For example, when using an automobile-type GBF generator with a power of 60 W at 900 rpm for a wind-electric unit, a wind wheel with D==1.2 m and a power of 0.169 hp is suitable. With. at 895 rpm (see the first two lines of Table 6).

In this case, the wind wheel can be mounted on the generator shaft. The result is the simplest and most convenient wind-electric unit to operate.

If we were planning to build a wind-electric unit with a power of 400 W, then it would be necessary to adopt a wind wheel diameter of 3 m, which at a wind speed of 8 m/sec develops 1,060 hp. With. or 1.060 X 0.736 = 0.78 kW. Taking the generator efficiency equal to 0.5, we obtain:

The wind wheel develops 357 rpm at a wind speed of 8 m/sec, and the generator with a power of 390 watts requires 1,000 rpm. Therefore, in this case, a gearbox is required that increases the speed in the transmission from the wind wheel to the generator. The gearbox must increase the speed in relation.

The value 2.8 is called the gear ratio. Using this ratio, the number of gear teeth of the gearbox is determined. For example, if we assume that the gear mounted on the generator shaft has 16 teeth, then the drive gear sitting on the wind wheel shaft should have

High-speed wind wheels suffer from a very significant drawback, which is that they do not start well, therefore, they can only start working at high wind speeds.

Many novice wind engineers think that the greater the number of blades on a wind wheel, the more power it will develop. This idea is wrong. Two wind wheels, small-bladed and multi-bladed, with equally well-built blades and the same diameters of the swept surface will develop the same power. This is explained by the fact that since they are equally well executed, then their wind energy utilization rates will be equal, that is, they will transfer the same amount of energy to the working machine. The amounts of incoming wind energy to both wind wheels are equal, since their swept surfaces are equal. As for the revolutions, the fewer the blades, the greater the speed, if they have the same width on both wind wheels; in other words, the smaller the total surface of the blades forming the swept surface, the greater the number of revolutions.

How to determine the dimensions of the wings of a homemade windmill (wind generator) for a given power

Calculation of wind generator blades

On the optimal angle of attack of a propeller windmill

In methods for calculating wind turbines, there is a recommendation to set the angle of attack at which the maximum aerodynamic quality of the blade is achieved. Those. It is proposed to construct a tangent to the polar from the origin of coordinates, and take the coordinates of the tangent point as the initial ones for calculating the windmill. Most likely, what is meant is an analogy with aviation, where as the ratio of lift to drag increases, the duration of the aircraft's gliding increases. Or it is suggested to use blades with maximum lift. A wind turbine operates according to different laws.

Rice. 1 Aerodynamic forces in a wind turbine

Figure 1 shows a diagram of the effect of aerodynamic forces on the blade. The wind speed when approaching the windmill slows down by a certain amount a, which according to Zhukovsky (Betz) theory is equal to 2/3, and according to Sabinin’s theory it is 0.586. The circumferential movement of the blades gives an additional component of speed, which can be found if we consider the blades to be stationary and the air to be moving in the opposite direction to the rotation. These two components are added according to the triangle rule and give the total vector of the flow on the plane of the wind wheel. The speed angle ψ is determined by the ratio a / Z and does not depend on the wind speed:

Here and below, all calculations are carried out for the tip of the blade. For other sections, it is necessary to replace Z everywhere in the formulas with the expression Zr / R, where Z is the speed determined as the ratio of the wind speed to the speed of the blade tip; R – radius of the windmill; r – radius of the selected section.

The speed angle ψ is the sum of the angle of attack α and the blade installation angle β. The angle of attack is determined by the characteristics of the blade, therefore, given the speed of the windmill, it is possible to make the task of calculating the blades unambiguous.

The flow flowing onto the blade causes two forces: the drag force X, directed towards the flow, and the lift force Y, perpendicular to it.

C X , C Y – drag and lift coefficients;

ρ – air density;

S – area of ​​the blade element;

V embankment – the magnitude of the incursion vector, which in turn is equal to:

The last term in parentheses is very small, and in high-speed windmills the incoming speed is almost equal to the peripheral speed of the blade.

The circumferential force is obtained as the difference between the projection of the lift force and the projection of the drag on the plane of rotation.

The expression in the last brackets can be called the aerodynamic circumferential force coefficient, or briefly the circumferential coefficient

The power of a windmill is the product of the peripheral force and the peripheral speed.

This formula does not give the power of the windmill, but the power of the element of the blade located at the tip. The power of a windmill is calculated by integrating over the radius, but the purpose of the article is different.)

Let's consider the polar of the blade in Fig. 2.

Rice. 2 Finding the circumferential force coefficient.

Let us draw a tangent OA to the polar. And let’s build the speed line OZ, which is given by the equation

Those. the velocity straight line forms the velocity angle ψ with the Cy axis, discussed earlier.

OB is equal to the magnitude of the lift at point A. Therefore:

Angle ABD is equal to angle ψ, and hypotenuse AB is the drag coefficient at point A. Therefore, leg BD is equal to:

The segment DE is the difference of two segments

The result is the same expression as in the windmill power formula. All other components in the power formula are given, so the power is determined by this segment or, in other words, the distance from the OZ speed line to the operating point. It is clear from the graph that the coefficient Ccr is maximum at the point of contact of the speed line Z’ to the polar, and not at the point of maximum aerodynamic quality. Therefore, having set the speed and built a high-speed line, you can clearly analyze the operation of the windmill.

TsAGI profile R -ll-12

In Fig. Figure 3 shows the TsAGI P-ll-12 profile, superimposed for comparison on the CLARK – Y profile popular in wind turbines. The polarity of the TsAGI P-ll-12 profile for extension 5 is shown in Fig. 4

Rice. 3 TsAGI profiles R-ll-12 and CLARK – Y

The polar on the left is shown in its usual form with different scales along the coordinate axes. On the right polar, drawn on the same scale, the same constructions are made. The high-speed straight line at Z = 2 gives the maximum circumferential coefficient at an angle of attack of 16°. The point of maximum lift-to-drag ratio is reached at an angle of attack of 2 degrees. At this point, the circumferential coefficient is approximately three times less than at the optimum point. Of course, in a windmill you can choose a working angle of attack of 2 degrees. The power of a wind turbine depends on wind energy. Therefore, the circumferential coefficient, which has decreased three times, will need to be compensated by increasing the chord of the blade by three times. (An idealized case is considered) Squared, the volume of the blade will increase by 9 times. As the area increases, friction losses increase. KIEV is falling. The elongation of the blade decreases and its inductive resistance increases. At the point of maximum aerodynamic quality, the windmill is better coordinated in terms of the degree of air braking in the plane of the windmill and the magnitude of the circumferential force. Coordination increases KIEV. Therefore, the calculation must be carried out taking into account all factors. Here, only the value of the circumferential coefficient and the width of the blade, which directly depends on it, are considered.

Fig. 4 TsAGI profile polars R-ll-12

With increasing speed, the optimum point (at the minimum blade width) approaches the point of maximum aerodynamic quality. With a speed of 6 and an angle of attack of 8°, the gain in the circumferential coefficient, and therefore in the width of the blades compared to 2°, is 1.5 times. But from the analysis of the polars it follows that at high speed values ​​it makes sense to choose an operating point lower along the polar. If there is insufficient load or no load in emergency mode, the windmill picks up speed and goes into overdrive. The speed angle decreases, and since the installation angle in unregulated wind turbines remains constant, the angle of attack decreases. The operating point shifts downwards, and the speed line approaches the polarity. At some speed, the circumferential coefficient will become zero. The onset of this moment (limit value Z) during separation depends on the initial position of the operating point. The lower the starting point is chosen, the lower the spread speed the windmill will gain. But this statement must be tested in practice.

When constructing the high-speed straight line Z = 6, it is clearly seen that the polar in the range of angles of attack from 3 to 12 degrees runs almost parallel to the high-speed straight line. This explains the fact that the use of various theories and concepts for calculating wind turbines has virtually no effect on the operation of the designed high-speed wind turbine.

The sections of the blades located closer to the axis move more slowly than the outer sections, so their speed straight lines lie lower. The internal sections have an optimum point, i.e. The maximum value of the circumferential coefficient lies at high angles of attack, therefore the installation angle and twist of the blades, which are technically complex, are reduced.

As a result of constructing speed lines, a family of optimal points for different speeds is obtained. Which of these points is the most optimal? What speed should you prefer? In the formula for windmill power, speed Z is included in the third power, and the circumferential coefficient is included in the first. Therefore, by multiplying the circumferential coefficients by the corresponding speed cubes, we obtain a series of maxima from which the maximum can be selected. Maximum-maximum lies approximately in the region of half the lift-to-drag ratio, at high speed

Here K is the maximum Cy/Cx ratio. For the profile under consideration, the maximum occurs at an angle of attack of 2 degrees and is equal to 24.

This blade has a lift-to-drag ratio of 24, therefore, the maximum-maximum will be around Z = 10. This estimate is approximate in order to understand the order of magnitude.

It is impossible to construct the circumferential coefficient using the left graph in Fig. 4. There are different scales along the axes, right angles are distorted and lengths are distorted. From the right graph it can be determined that

at Z = 2 the product Z3Cab is equal to:

Those. at a speed of Z = 10, the width of the blades at the tip decreases by 2.3 times compared to a fairly high-speed propeller Z = 6.

Let me draw your attention once again to the fact that the maximum-maximum point gives the minimum width of the blades, and not the maximum power. Power is determined by the wind. And power is also determined by losses, i.e. KIEV wind turbines, which are not considered here.

Program – Design and verification aerodynamic calculations of a wind generator – file TECHNICAL REPORT.doc

TECHNICAL REPORT.doc

Calculation of the aerodynamic characteristics of a wind generator blade and determination of its geometric parameters.

B – number of blades

The report presents the results of calculations of the aerodynamic characteristics of the wind wheel blade and the wind turbine as a whole. The geometric characteristics of the blade are presented.

^ 1. Initial data for calculation.

Estimated wind speed V=12 m/s.

From the experience of creating wind generators of this class, the value of the relative speed is within 6...8. The wind energy utilization factor (or power factor Cp) for existing wind generators is in the range of 0.43...0.47. The speed of the blade tip is in the range of up to 80…100 m/s. This limitation is due to aerodynamic noise and erosive wear of the blade. As an aerodynamic profile of wind generator blade sections, we can use the NACA 44100 series profile, which is currently widely used. The use of laminar profiles makes it possible to obtain higher performance, but subject to high manufacturing precision, the absence of contamination of the blade surface, the absence of structural vibrations and turbulence of the wind flow. Failure to comply with the above conditions reduces the performance of wind generators with laminar blade profiles by 25...30%.

Relative speed =7.

^ Table 1. Coordinates of the NACA 44100 profile.

Where: – new relative profile thickness.

Relative speed (speed) =7.

Figure 2. Wind wheel power and revolutions depending on wind speed (=7).

As can be seen from the calculation results, the designed wind wheel satisfies the requirements of the initial data and the practice of creating wind turbines of this class.

The blade geometry is constructed as follows. The direction of rotation of the rotor is counterclockwise when viewed in the direction of the wind. The installation angles of the sections are indicated from the plane of rotation. A positive value is against the wind direction (Figure 3).

The resulting blade geometry data is presented in Table 2

In electronic form, the data for constructing the geometry of the blade is presented in the files:

VG100.scr – script file (or script file) for the program

VG100.dwg is a blade model built in AutoCAD (Figure 4) based on data from the VG100.scr file.

VG100.CATPart – blade model built in CATIA (Figure 5)

Figure 4. Frame model of the blade.

1. Patrick J. Moriarty, AeroDyn Theory Manual , National Renewable Energy Laboratory, December 2005 NREL/EL-500-36881.

2. John Wiley & Sons, Wind Energy Explained – Theory, Design And Application,

3. E. M. Fateev, Wind engines and wind turbines, OGIZ-SELKHOZGIZ, M., 1948.

4. H. Pigot, Calculation of wind turbine blades, 2000.

5. G. Glauert, Fundamentals of the theory of wings and propellers, State Scientific and Technical Institute, 1931.

6. E. Makarov, Engineering calculations in Mathcad 14, PETER, 2007

TECHNICAL REPORT - Program - Design and verification aerodynamic calculations of a wind generator - TECHNICAL

Very often people are mistaken that multi-blade propellers are for weak winds, and three or two blades are for strong winds. And many believe that for weak winds a multi-blade propeller is more effective, because there are many blades, this makes the thrust higher, more wind is covered by the blades, the torque is higher, and therefore the power, but this is not so. Due to the larger number of blades, the starting torque is higher, so if the generator has strong magnetic sticking, then something has to be done to increase the starting torque, and usually this is adding blades.

Let's first imagine one blade and the physical factors acting on it. The blade has a twist, angles relative to the wind flow, and the wind leaning on it forces the blade to move under pressure (squeeze forward along the axis of rotation). But the blade, moving in its plane, overcomes the frontal resistance of the dense air flow. This flow slows down the blade, preventing it from gaining more speed, and the higher the speed, the higher the aerodynamic drag.

If there are more than one, two or three, or 12 blades, then the aerodynamic drag of all blades does not remain equal to one, it adds up, the losses add up to the total and the propeller speed drops. A lot of energy is wasted just spinning. Plus, the passing blades greatly disturb the flow by twisting it, from this the blades behind receive even more drag and again the power taken from the wind is wasted and the speed drops. It is on revolutions that a lot of power taken from the wind is spent.

Also, when there is a whole forest of blades in a circle, it becomes more difficult for the wind to fall through the propeller. The wind wheel delays the flow of wind, an air “cap” is formed in front of the propeller, and new portions of wind encountering this “cap” are scattered to the sides. You know how the wind bends around obstacles, so the propeller is like a solid shield for the wind.

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But many will think that the more blades, the more energy can be taken from the wind per unit of time, but this is also not true, what is important here is not the number of blades, but the speed and speed of the propeller. For example, 6 blades, say, at 60 rpm will make one revolution, passing a cube of wind and taking away a certain portion of energy from it, and 3 blades will make two revolutions in the same time, and take away the same amount of energy. If you increase the speed further, more energy will be taken away. It doesn’t matter how many blades there are, one or ten, since one blade rotating ten times faster will take the same amount of energy as ten slowly rotating blades.

Speed ​​of the wind wheel.

Also, the faster the blade moves, the more the angle of attack of the wind on the blade changes. Let's imagine that you are sitting in a car and snow is hitting your side window, but when you start driving, the snow will already be hitting the windshield, and when you pick up speed, the snow will already be hitting directly on the windshield, although when you stop the snow will fall from the side again. Likewise, when the blade picks up speed, the wind will hit it at a different angle. Therefore, the tip of the blade is made only 2-5 degrees, since when it accelerates it will reach the optimal angle of attack of the wind and will take away the maximum possible energy. In the middle of the blade, the speed is two times less, therefore the angle is twice as large, 8-12 degrees, and at the root it is even greater, because there the speed is several times less.

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For high-speed small-bladed propellers, the angles are made smaller. For example, for three-blade propellers the usual speed is about Z5, that is, the propeller has maximum power when rotating at a speed five times higher than the wind speed. In this case, the tip of the blade has about 4 degrees, the middle 12 degrees, and the root about 24 degrees. If there are six blades, then the speed is two times lower, which means the angles are twice as large. Well, the thinner the blade and the smaller its area, the faster it is, and the lower its aerodynamic drag, therefore, three blades, if they are wide, will have low speed, and six or twelve thin, narrow blades will have higher speed.

As a result, for example, a three-blade and a six-blade propeller will have equal power in low winds, because three blades with Z5 speed will make twice as many revolutions as six blades with Z2.5 speed in the same time, which means they will take the same amount of energy from the wind. But in a stronger wind, a six-bladed propeller will lose greatly to a three-bladed one, since three blades have less aerodynamic drag and will be able to gain higher speeds, and therefore work with more wind per unit of time, because the faster the blade moves, the more power it will take from the wind .

The only plus is that the more blades, the better the starting torque, and if the generator has magnetic sticking, then the multi-bladed propeller will start earlier, but the torque and power will be higher for small-bladed propellers.

Yes, and torque, as the high-speed propeller picks up speed, the angles of the blade will become optimal for the wind actually flowing onto the blade, and we know that the real angle changes depending on the speed of the blade itself and the torque will be higher, since there is less energy loss on the drag of the blades.

Also, multi-bladed propellers are heavier, which means they work like a flywheel. If the wheel has gained momentum, then the propeller itself stores energy and is more difficult to stop abruptly, but even when the wind blows stronger, this flywheel must still be spun, so multi-bladed propellers react less well to changes in wind strength, and short-term gusts of wind may not even be noticed. And light propellers can provide energy even from a short gust of wind. This is clearly visible on the ammeter when you observe the current strength. The six-bladed one works more softly, there are no large current surges. But the three-bladed one handles every gust and the needle quickly runs back and forth, but this is energy that ultimately accumulates in the battery, and the difference in recoil can be very significant, especially in gusty winds and if the mast is installed low where the wind flow is turbulent.

Another factor is the speed, a multi-blade propeller means a low-speed one, which means the generator is the same, which means there are more generators, more magnets, more winding wires, more iron weight, and as a result the price is much higher. And the generator is usually the most expensive part of a wind generator. And revolutions have the most important role, because the higher the propeller speed at the same wind speed, the generator will produce more power, and then if there are not enough revolutions, then either the generator is larger and more powerful, or a multiplier can be invented.

But everywhere there are their own, but, of course, the cheapest and most efficient single-blade propellers, but they need to be made very accurately and balanced, everything must be calculated, the aerodynamics of the blade must be ideal, otherwise vibrations and beating of the propeller, and then a windmill that will fall apart, are guaranteed. In principle, this is why almost no one even produces factory-made single-blade windmills. Three-bladed propellers turned out to be more optimal; they are not so high-speed, so some imbalance of the propeller is not a problem, but the speeds are also high, which means the generator is cheaper.

But still, high-speed blades require correct aerodynamics, otherwise all efficiency can drop significantly. Therefore, at home it is often easier, although more expensive, to make a crude, large, ineffective, but easy-to-manufacture windmill, improve it without any calculations, redo it, and redo it again, and finally, either gain knowledge and bring everything to fruition, or give it up and say that all this is bullshit, I bought it from the Chinese and don’t worry, you still can’t make it better than at the factory, you’ll just waste your money.