WIND TURBINE OPERATING POWER CURVE BASIC INFORMATION AND TUTORIALS

What is the operating power curve of wind turbine?

The power curve of a turbine is built by connecting the maximum points of individual characteristic curves at various blade pitch angles, taking into account the maximum capacity of the turbine generator.

A curve specific to a wind turbine that determines the output power versus the wind speed. This curve is used by the wind turbine controller during operation to adjust the blade pitch.

Illustrates a typical characteristic curve of wind turbines based on which a turbine is to adjust its parameters and set points for the operation at various wind speed conditions. It is interesting to understand how this curve has been generated.

The performance (power coefficient and power capture) of a turbine changes with the tip speed ratio and that for a specific rpm of a turbine the tip speed ratio depends on the wind speed. On the other hand, we would like a turbine to always work, as much as possible, with maximum power coefficient, that is, around the peak of its characteristic curve at each wind speed.

If the maximum points of the characteristic curves for each wind speed are connected together, the resulting curve shows the desired points of operation of a turbine at various speeds. This curve, when blended with a cap for the maximum capacity of the turbine generator, defines a curve based on which a turbine is scheduled and controlled. That is the curve introduced in figure 10.3; it is referred to as a wind turbine power curve.

AGT100 COMBUSTOR BASIC INFORMATION AND TUTORIALS

What is Allison AGT100 Combustor?

The main features of this combustor, shown schematically in have been described by Rizk and Mongia. It comprises a prechamber in which the fuel is vaporized and mixed with air, a pilot and ignition chamber, and the main cylindrical chamber.

Variable geometry is employed to control the stoichiometry in the primary zone. The prechamber contains a centerbody that houses both the main fuel injector and a pilot nozzle, which is employed only for lightup and acceleration to engine idle speed.

The main fuel is introduced from a manifold surrounding the prechamber, just downstream of the prechamber axial swirler. Uniform filming of the fuel is achieved by spraying it through eight tangential holes onto the etched surface of the prechamber.

The swirling air assists in the prefilming process. The high temperatures of the inlet air and the prechamber walls combine to promote rapid vaporization of the fuel within the prechamber.

At power modes higher than idle, additional air is admitted into the prechamber through a radial swirler to merge and mix with the air flowing through the axial swirler.

Engine lightup is initiated in a small pilot chamber located on the side of the main combustion chamber. This piloting device also serves as a sustainer source when the combustor is operating at low inlet air temperatures or at conditions that lie outside the normal lean blowout limits.

The swirling vaporized fuel–air mixture flows into the main chamber through a round opening in the center of the dome. At high-power settings, additional air is injected into the main chamber through eight holes that are drilled in a manner designed to impart a swirling motion to the flowing air.

Four simple rectangular dilution holes were chosen to ease fabrication of the ceramic liner. Variable geometry, in the form of sliding bands, is used to vary and control the flow areas of the dilution holes and the radial swirler in the prechamber.

At low-power modes, most of the air flows through the dilution holes. As the fuel flow rate is increased above idle, the variable geometry is moved to increase the airflow through the radial swirler and to reduce, by a corresponding amount, the airflow through the dilution holes.

The use of variable geometry enabled the AGT100 combustor to meet the program goals of 5.0 and 37 g/kg fuel for NOx and CO, respectively. Moreover, the experimental data acquired in the course of this investigation was used by Rizk and Mongia to develop a model for calculating NOx formation in
LPP combustors.

This model takes into account the effects of pressure, residence time, and air distribution between different combustion zones. It also provides useful insight into the contribution of the pilot chamber to the total NOx emissions.

GE (GENERAL ELECTRIC) LM6000 COMBUSTOR BASIC INFORMATION AND TUTORIALS

What is General Electric LM6000 Combustor?

Another important aeroderivative gas turbine is General Electric’s LM6000. This premix combustor employs about twice the volume of the conventional annular combustor it replaces in order to maintain low levels of CO and UHC while greatly reducing the emissions of NOx. Part of the air used in combustion, which at maximum power is around 80% of the total combustor airflow, flows into the combustion zone through three annular rings of premixers.

The two outer rings each have 30 fuel–air premixers, whereas the inner ring has 15. This arrangement of premixers facilitates fuel staging at part-load operation. The total of 75 fuel nozzles is formed by having 15 stems with three premixers on each stem, plus 15 stems with two premixers on each stem.

Each stem incorporates two or three separate fuel circuits for independently fueling the premixers. A short annular liner was selected to minimize the amount of air needed for wall cooling. Only backside cooling is used, so a thermal barrier coating is applied to both the liner and in the dome area to keep the metal temperatures within acceptable limits.

The use of a multipass diffuser also permits further reduction in overall combustor length. Of special importance to the attainment of low emissions is the design of the premixers. The double annular counter rotating swirler (DACRS) was conceived to satisfy the restraints of autoignition and size. The duct diameter is reduced toward the exit in order to create an accelerating flow, thereby preventing flashback.

The conical centerbody located along the centerline of the premixer can be used to supply liquid fuel to an atomizer at its tip, and gas passages for diffusion burning at low-power conditions. The objective with this type of mixing device is to produce a completely homogeneous mixture of fuel and air at the premixer exit.

As the total area of the fuel-injection holes is fixed by the flow rate and the available fuel njection pressure, the design procedure is essentially one of finding the best compromise between the desire for small injection holes to give a large number of fuel-injection points, and the equally important requirement of large injection holes to allow the fuel jets to penetrate across the airstream.

A big advantage of the premixer module concept is that, once developed, it has broad applications to a wide range of combustor sizes and configurations, as discussed above in connection with the ABB-EV burner.

The basic module remains the same regardless of combustor size; only the number and
arrangement varies. Thus, according to Joshi et al,, the DACRS II and DACRS III mixers could be applied to a range of GE engines, including the LM1600, LM2500, and LM6000, because single digit NOx emissions have been attained with both these mixers at test conditions encompassing the operating ranges of these engines.

GAS TURBINE THRUST BEARING DESIGN FAILURE BASIC INFORMATION AND TUTORIALS

Factors Affecting Thrust-Bearing Design

The principal function of a thrust beating is to resist the thrust unbalance developed within the working elements of a turbomachine and to maintain the rotor position within tolerable limits.

After an accurate analysis has been made of the thrust load, the thrust bearing should be sized to support this load in the most efficient method possible. Many tests have proven that thrust bearings are limited in load capacity by the strength of the babbitt surface in the high load and temperature zone of the beating.

In normal steel-backed babbitted tilting-pad thrust bearings, this capacity is limited to between 250 and 500 psi (17 and 35 Bar) average pressure. It is the temperature accumulation at the surface and pad crowning that cause this limit.

The thrust-carrying capacity can be greatly improved by maintaining pad flatness and by removing heat from the loaded zone. By the use of high thermal conductivity backing materials with proper thickness and proper support, the maximum continuous thrust limit can be increased to 1000 psi or more.

This new limit can be used to increase either the factor of safety and improve the surge capacity of a given size bearing or reduce the thrust beating size and consequently the losses generated for a given load.

Since the higher thermal conductivity material (copper or bronze) is a much better beating material than the conventional steel backing, it is possible to reduce the babbitt thickness to .010-.030 of an inch (.254-.762 mm). Embedded thermocouples and RTDs will signal distress in the beating if properly positioned.

Temperature monitoring systems have been found to be more accurate than axial position indicators, which tend to have linearity problems at high temperatures. In a change from steel-backing to copper-backing a different set of temperature limiting criteria should be used..

COMPRESSOR SURGE BASIC INFORMATION AND TUTORIALS

What is Compression Surge?

Compressor surge is a phenomenon of considerable interest; yet it is not fully understood. It is a form of unstable operation and should be avoided. It is a phenomenon that, unfortunately, occurs frequently, sometimes with damaging results. Surge has been traditionally defined as the lower limit of stable operation in a compressor, and it involves the reversal of flow.

This reversal of flow occurs because of some kind of aerodynamic instability within the system. Usually, a part of the compressor is the cause of the aerodynamic instability, although it is possible for the system arrangement to be capable of augmenting this instability.

Compressors usually are operated at a working line, separated by some safety margin from the surge line. Extensive investigations have been conducted on surge.

Poor quantitative universality or aerodynamic loading capacities of different blades and stators, and an inexact knowledge of boundary-layer behavior make the exact prediction of flow in the compressor at the off-design stage
difficult.

A decrease in the mass flow rate, an increase in the rotational speed of the impeller, or both can cause the compressor to surge. Whether surge is caused by a decrease in flow velocity or an increase in rotational speeds, the blades or the stators can stall.

Note that operating at higher efficiency implies operation closer to surge. It should be noted here that total pressure increases occur only in the rotational part of the compressor, the blades.

The surge line slope on multistage compressors can range from a simple single parabolic relationship to a complex curve containing several break-points or even "notches." The complexity of the surge line shape depends on whether or not the flow limiting stage changes with operating speed from one compression stage to another; in particular, very closely matched stage combinations frequently exhibit complex surge lines. In the case of compressors with variable inlet guide vanes, the surge line tends to bend more at higher flows than with units that are speed controlled.

Usually surge is linked with excessive vibration and an audible sound; yet, there have been cases where surge not accompanied by audible sound has caused failures. Usually, operation in surge and, often, near surge is accompanied by several indications, including general and pulsating noise level increases, axial shaft position changes, discharge temperature excursions, compressor differential
pressure fluctuations, and lateral vibration amplitude increases.

Frequently, with high-pressure compressors, operation in the incipient surge range is accompanied by the emergence of a low frequency, asynchronous vibration signal that can reach predominant amplitudes, as well as excitation of various harmonics of blade passing frequencies. Extended operation in surge causes thrust and journal beating failures.

Failures of blades and stators are also experienced due to axial movement of the shaft causing contact of blades and stators. Due to the large flow instabilities experienced, severe aerodynamic stimulation at one of the blade natural response frequencies is caused, leading to blade failure.

WIND MACHINE BUYING GUIDE - WIND MACHINE BASICS

What to Look for When Buying a Wind Machine?

While there are many turbines on the market, careful load and site analysis will narrow the field considerably. Once you have determined your average monthly electrical load and the average wind speed on your site, you can select a wind turbine  that will produce enough electricity to meet your demands.

Manufacturers provide a plethora of technical data on their wind machines that can be used to make comparisons. Unfortunately, most of it is useless. Further complicating matters,

“There can be a big difference in reliability, ruggedness, and life expectancy from one brand to the next,” according to Mike Bergey, president of Bergey Windpower.

So how do you go about selecting a wind machine?
Although wind turbines can be compared using many criteria, there are only a
handful that really matter: (1) swept area, (2) durability, (3) annual energy output,
(4) governing mechanism, (5) shut-down mechanism, and (6) sound.

Swept Area 
Swept area is the area of the circle described by the spinning blades of a turbine. Because the blades of a wind turbine convert wind energy into electrical energy, the swept area is the collector area of the turbine. The greater the swept area, the greater the collector area.

The bigger the swept area, the more energy you’ll be able to capture from the wind. To get the most out of a wind turbine — to produce the most electricity at the lowest cost — select a wind turbine with the greatest swept area.

Swept area allows for easy comparison of different models. Swept area is determined by rotor diameter. The rotor diameter is the distance from one side of the circle created by the spinning blades to a point on the opposite side or about twice the length of the blades.

When comparing wind turbines, then, the rotor diameter is a pretty good measure of how much electricity a turbine will generate. Although other features such as the efficiency of the generator and the design of the blades influence energy production, for most turbines they pale in comparison to the influence of rotor diameter and, hence, swept area.

Manufacturers list the rotor diameter in feet or meters — often both. The greater the blade length, the greater the rotor diameter and the greater the swept area. Most manufacturers also list the swept area of the rotor. Swept area is presented in square feet or square meters — sometimes both.

Annual Energy Output 
 Another, even more useful, measure is the annual energy output (AEO) or annual energy production (AEP) at various wind speeds. The AEO of a given wind turbine is presented as kilowatt-hours of electricity produced at various average wind speeds.

Like the US EPA’s estimated gas mileage for vehicles, AEO gives buyers a convenient way to compare models. As in the estimated gas mileage rating, however, AEOs won’t tell you exactly how much electricity a wind machine will produce  at a site. Performance varies depending on a number of factors such as turbulence and the density of the air.

Durability: Tower Top Weight 
Another extremely important criterion is durability. The most important measure of durability is tower top weight — how much a wind turbine weighs.

Four turbines that produce about the same amount of electricity are for example, the Proven WT2500 (419 pounds), the ARE110 (315 pounds), the Skystream 3.7 (170 pounds) and the Whisper 500 (155 pounds). The weight differences are in some cases substantial.

In our experience, heavyweight wind turbines tend to survive the longest sometimes many years longer than medium or lightweight turbines. Weight is usually reflected in the price. Remember, however, that you get what you pay for.

Producing electricity on a precarious perch 80 to 165 feet above the ground isn’t a job you want to relegate to the lowest bidder, which is invariably the lightest turbine.

Balance of System Cost 
Before you buy a machine, consider the total system cost. You’ll need to purchase a tower and pay for installation, unless, of course, you install the tower yourself. Even then, you’ll need to pay for concrete, rebar and equipment to excavate the foundation and anchors.

You’ll also need to run electrical wire from the turbine to the house and purchase an inverter (although they’re included in most batteryless grid-tie wind turbines). If you’re going off-grid or want battery backup for your gridconnected system, you’ll also need to buy batteries.

All of this will add to the cost. The cost of the turbine itself may range from 10 to 40 percent of the total system cost. Governing Systems

Found in all wind generators worth buying, governing, or overspeed control, systems are designed to prevent a wind generator from burning out or breaking apart in high winds. They do this by slowing down the rotor when the wind reaches a certain speed, known as the governing wind speed. Why is this necessary?

As wind speed increases, the rotor of a wind turbine spins more rapidly. The increase in the revolutions per minute (rpm) increases electrical output. Although electrical output is a desirable goal, if it exceeds the machine’s rated output, the generator could overheat and burn out.

In addition, centrifugal forces in high wind speeds exert incredible forces on wind turbines that can tear them apart if the rotor speed is not governed.

A governing system is essential because it allows the turbine to shed extra energy when the winds are really strong. Not all wind turbines come with governing mechanisms, however. Many of the smallest wind turbines, the micro-turbines, with rated outputs of around 400 watts, for example, have no governing mechanisms.

(These turbines are too small to produce a significant amount of electricity for most applications.) Larger wind turbines, those with swept areas over 38 square feet, however, come with overspeed controls. Two types are commonly found: furling and blade pitch.

GAS TURBINE TEMPERATURE MEASUREMENTS BASIC INFORMATION AND TUTORIALS

Temperature Measurement Techniques of Gas Turbines

Temperature may be measured by any of the following instruments:

1. Mercury-in-glass thermometers

2. Thermocouples

3. Resistance thermometers

4. Thermometer wells

Thermocouples are the preferred type of instruments because of the simplicity in basic design and operation. They can attain a high level of accuracy, are suitable for remote reading, and are robust and relatively inexpensive.

Regardless of the temperature-measuring device to be used, on-site calibration of the entire measurement system is desirable. Usually, a two-point check can be made by employing frozen and boiling water. 

At the very least, all devices can be checked at a common temperature, preferably in the midrange of expected temperatures so that any deviant devices can be discarded. This check is particularly desirable for low-head machines where the temperature rise will be slight.

Test plans frequently are prepared on the assumption that a laboratory thermometer can replace an operating instrument in an existing thermometer well.

While this change may be satisfactory, the prudent tester needs to be aware that because of the propensity of thermowells to break off and perhaps enter the machine or cause a hazardous leak, their design is compromised such that true gas temperature determination is impossible. The compromise may be to make the well short and/or to make it thick-walled. 

In either event the mass of metal exposed to ambient temperature may exceed that exposed to the gas, resulting in significant error if the gas temperature is much different from the ambient temperature. High-pressure systems requiring thick-wall pipe are particularly susceptible to this fault. However, the use of a good heat-transfer fluid can minimize the error. 

The best gas temperature reading is attained by a calibrated fine-wire thermocouple with the junction directly exposed to the gas near the center of the flow. As deviations from this ideal are made, the potential for error is increased.

Inlet and discharge temperatures are the stagnation temperatures at the respective points and should be measured within an accuracy of 1 ~ (0.55 ~ When the velocity of the gas stream is more than 125 fps (36.6 mps), the velocity effect should be included in the temperature measurement with a total temperature probe. 

This probe is a thermocouple with its hot junction provided with a shielded cup. The cup opening points upstream. A trade-off has to be made in a field test situation where the gas is not clean.

GAS TURBINE PERFORMANCE TESTS BASIC INFORMATION AND TUTORIALS

How to conduct gas turbine performance tests.

The performance analysis of the new generation of gas turbines is complex and presents new problems, which have to be addressed. Performance acceptance tests, which are required to be conducted for contractual guarantees, require that the turbine be cleaned before the test.

The average commissioning time for the advanced gas turbine (G Type) units is longer than the F and FA Type units. This is usually due to the increased number of starts and trips during commissioning, due to a lot of fine tuning required for the DLN combustors, cooling systems, and complicated control systems, which increase the number of equivalent engine hours.

It is recommended that contractually the maximum number of equivalent engine hours be limited to about 600-800 hours regardless of the actual equivalent operating hours.

If this is not done then the power output will be corrected to a larger corrected output, reducing the actual power the plant will produce. There have been many cases of 2000 to 6500 equivalent operating hours recorded during commissioning, which in many cases amount to the power and heat rate being corrected by 2 to 5%. This affects the profitability of the plant.

The new units operate at very high turbine firing temperatures. Thus, variation in this firing temperature significantly affects the performance and life of the components in the hot section of the turbine.

The compressor pressure ratio is high which leads to a very narrow operation margin, thus making the turbine very susceptible to compressor fouling. The turbines are also very sensitive to back pressure exerted on them when used in combined cycle or cogeneration duty. The pressure drop through the air filter also results in major deterioration of the performance of the turbine.

If a life cycle analysis were conducted the new costs of a plant are about 7-10% of the life cycle costs. Maintenance costs are approximately 15-20% of the life cycle costs. Operating costs, which essentially consist of energy costs, make up the remainder, between 70-80% of the life cycle costs, of any major power plant.

Thus, performance evaluation of the turbine is one of the most important parameters in the operation of a plant. Total performance monitoring on- or off-line is important for the plant engineers to achieve their goals of:

1. Maintaining high availability of their machinery.
2. Minimizing degradation and maintaining operation near design efficiencies.
3. Diagnosing problems, and avoiding operating in regions, which could lead to serious malfunctions.
4. Extending time between inspections and overhauls.
5. Reducing life cycle costs.

To determine the deterioration in component performance and efficiency, the values must be corrected to a reference plane. These corrected measurements will be referenced to different reference planes depending upon the point, which is being investigated.

Corrected values can further be adjusted to a transposed design value to properly evaluate the deterioration of any given component. Transposed data points are very dependent on the characteristics of the component's performance curves.

To determine the characteristics of these curves, raw data points must be corrected and then plotted against representative nondimensional parameters. It is for this reason that we must evaluate the turbine train while its characteristics have not been altered due to component deterioration. If component data were available from the manufacturer, the task would be greatly reduced.

WIND TURBINE AERODYNAMIC BLADES BASIC INFORMATION AND TUTORIALS

Aerodynamic Blade Wind Turbines

Each blade of a turbine is subject to aerodynamic forces from the wind. The airstream has a relatively fixed direction for the area swept by the blades, at an instant, but since blades are twisted the relative direction of wind with respect to a blade is not the same for various segments of the blade. 

Moreover, whereas the windstream has a constant speed (for a short period under consideration), the speed of air as a result of the blade rotation is smaller for the segments of the blade closer to the hub than the segments closer to the tip of the blade.

Consequently, the relative motion of air with respect to the blade as a result of wind and blade motion varies along the length of a blade. The resulting aerodynamic force, thus, varies in both magnitude and direction along the blade span. The typical force for a segment is shown in figure below (a blade can be assumed to be made of any arbitrary number of segments).

The force on each blade segment consists of two components: one component along the direction of wind (the drag force, almost in the horizontal direction) and one perpendicular to wind (the lift component, in a near vertical plane). These forces are depicted for a two-blade turbine in figure below.

A two blade-turbine is more appropriate to demonstrate the fact that all the components along the wind direction have the same force direction in the two blades, whereas the forces normal to the wind have opposite directions in the two blades, since the blades are symmetric to each other. 

The forces shown correspond to when the blade is feathered and does not catch much energy (the lift components are smaller than the drag components). This is just for the sake of clarity of the figure. This implies, also, the fact that the horizontal force on the blades is greater when a turbine is parked than when it is working.

It is easy to verify that the resultant of the two sets of forces is a push on both blades in the wind direction and a torque about the turbine shaft axis. In other words, all those force components along the wind direction contribute to a backward push on the blades, and do not generate any rotational motion.

However, those force components that are in the opposite direction in a (near) vertical plane are the only ones that generate a torque that makes the turbine rotate.

Note that the just mentioned aerodynamic forces are functions of wind speed, rotational speed, and pitch angle. Therefore, they are not the same for different operating conditions and for when the turbine is parked. 

A blade must be able to withstand these forces in the harshest condition; that is, when these forces are at their highest.

All the forces on a turbine must ultimately be transferred to the ground through the tower.

DIFFERENCE BETWEEN IMPULSE AND REACTION HYDRAULIC TURBINES BASIC INFORMATION

What is the difference between an impulse and reaction turbine?

Impulse Turbine wherein the available hydraulic energy is first converted into kinetic energy by means of an efficient nozzle. The high velocity jet issuing from the nozzle then strikes a series of suitably shaped buckets fixed around the rim of a wheel. The buckets change the direction of jet without changing its pressure.

Reaction Turbine wherein a part of the total available hydraulic energy is transformed into kinetic energy before the water is taken to the turbine runner. A substantial part remains in the form of pressure energy. Subsequently both the velocity and pressure change simultaneously as water glides along the turbine runner. The flow from inlet to outlet of the turbine is under pressure and, therefore, blades of a reaction turbine are closed passages sealed from atmospheric conditions.



Impulse Turbine

1. All the available energy of the fluid is converted into kinetic energy by an efficient nozzle that forms a free jet.

2. The jet is unconfined and at atmospheric pressure throughout the action of water on the runner, and during its subsequent flow to the tail race.

3. Blades are only in action when they are in front of the nozzle.

4. Water may be allowed to enter a part or whole of the wheel circumference.

5. The wheel does not run full and air has free access to the buckets.

6. Casing has no hydraulic function to perform; it only serves to prevent splashing and to guide the water to the tail race.

7. Unit is installed above the tail race.

8. Flow regulation is possible without loss.

9. When water glides over the moving blades, its relative velocity either remains constant or reduces slightly due to friction.

Reaction Turbine
1. Only a portion of the fluid energy is transformed into kinetic energy before the fluid enters the turbine runner.

2. Water enters the runner with an excess pressure, and then both the velocity and pressure change as water passes through the runner.

3. Blades are in action all the time.

4. Water is admitted over the circumference of the wheel.

5. Water completely fills the vane passages throughout the operation of the turbine.

6. Pressure at inlet to the turbine is much higher than the pressure at outlet ; unit has to be sealed from atmospheric conditions and, therefore, casing is absolutely essential.

7. Unit is kept entirely submerged in water below the tail race.

8. Flow regulation is always accompanied by loss.

9. Since there is continuous drop in pressure during flow through the blade passages, the relative velocity does increase.

IGNITION THEORY BASIC INFORMATION AND TUTORIALS

What is ignition theory? How things ignite?

Most ignition theories are based on the idea that the transient ignition source, usually an electric spark, must supply sufficient energy to the combustible mixture to create a volume of hot gas that just satisfies the necessary and sufficient condition for propagation, namely, that the rate of heat generation just exceeds the rate of heat loss.

The work of Lewis et al. did much to clarify and improve knowledge of spark ignition in quiescent mixtures. The first major contribution to ignition theory for flowing mixtures was made by Swett who studied the influence on ignition energy of variations in pressure, velocity, equivalence ratio, and turbulence.

Swett’s theory is based on the ideas that (1) only a portion of the discharge length is important in the ignition process and (2) heat loss by thermal conduction is negligible compared with heat loss by eddy diffusion.

Both of these ideas were fully confirmed in subsequent experiments carried out by Ballal and Lefebvre on ignition in flowing mixtures. Unfortunately, Swett’s treatment of turbulence is very limited and much of his experimental data are suspect for reason.

Gaseous Mixtures
Ballal and Lefebvre analyzed the processes governing the rate of heat generation in an incipient spark kernel and the rate of heat loss by thermal conduction and turbulent diffusion. They conclude that, for the spark kernel to survive and propagate unaided throughout a gaseous mixture, its minimum dimension should always exceed the quenching distance.

Heterogeneous Mixtures
All the evidence obtained in the studies of Subba Rao, Rao, and Lefebvre on the ignition of flowing mixtures of fuel drops and air serves to suggest that passage of the spark creates a kernel in which high gas temperatures are attained, partly from the energy supplied in the spark, but also from the heat liberated by the evaporation and rapid combustion of the smallest fuel drops.

GAS TURBINE COMBUSTOR REQUIREMENTS BASIC INFORMATION AND TUTORIALS

What is a gas turbine combustor?

A gas turbine combustor must satisfy a wide range of requirements whose relative importance varies among engine types. However, the basic requirements of all combustors may be listed as follows:

1. High-combustion efficiency (i.e., the fuel should be completely burned so that all its chemical energy is liberated as heat)

2. Reliable and smooth ignition, both on the ground (especially at very low ambient temperatures) and, in the case of aircraft engines, after a flameout at high altitude

3. Wide stability limits (i.e., the flame should stay alight over wide ranges of pressure and air/fuel ratio)

4. Low pressure loss

5. An outlet temperature distribution (pattern factor) that is tailored to maximize the lives of the turbine blades and nozzle guide vanes

6. Low emissions of smoke and gaseous pollutant species

7. Freedom from pressure pulsations and other manifestations of combustion- induced instability

8. Size and shape compatible with engine envelope

9. Design for minimum cost and ease of manufacturing

10. Maintainability

11. Durability

12. Petroleum, synthetic, and biomass-based multifuel capability.

For aircraft engines, size and weight are important considerations, whereas for industrial engines more emphasis is placed on other items, such as long operating life and multifuel capability. For all types of engines, the requirements of low fuel consumption and low pollutant emissions are paramount.

JUMO 004 (GERMANY) HISTORICAL ENGINE TURBINE MACHINE

This engine is of great historical interest because it was the world’s first mass produced turbojet and one that saw extensive service in World War II. It was among the first engines to employ axial flow turbo machinery and straight through combustors.

Each of the six tubular combustors was supplied with fuel at pressures up to 5.2 MPa (750 psi) from a pressure-swirl atomizer, which sprayed the fuel upstream into the primary combustion zone. The primary air flowed into the liner through six swirl vanes, the amount of air being sufficient to achieve near-stoichiometric combustion at the engine design point.

Mixing between combustion products and dilution air was achieved using an assembly of stub pipes that were welded to a ring at their upstream end and to the outer perimeter of a 10-cm diameter dished baffle at their downstream end. The hot combustion products flowed radially outward through the gaps between the stub pipes to meet and mix with part of the cold secondary air.

The remaining secondary air flowed through the stub pipes, incidentally serving to protect them from burnout because of their immersion in the hot combustion gases, to provide further mixing of hot and cold gases in the recirculation zone created by the presence of the baffle.

PELTON TURBINE REGULATION BASIC INFORMATION AND TUTORIALS

Hydraulic turbines are usually coupled to an electric generator and the generator must run at constant speed to maintain frequency of supply constant. The speed of generator N in rev/min, the frequency of supply (f) in Hertz and number of poles of the generator P are related by the equation:

f = NP/120

The peripheral velocity u of turbine wheel must remain constant as speed is constant. The velocity u and speed N are connected by the formula:

u = pi DN/60

where D is mean diameter of the wheel.

It is also desirable to run turbine at maximum efficiency and therefore speed ratio u/VI must remain same which means the jet velocity must not change as head available H is constant. The only way to adjust the load is to change hydraulic power input given by p= yQH

As y, specific weight of water and H are constant, the only variable factor is Q volume flow rate of water entering the turbine. The flow rate Q is Q = Area of nozzle x velocity of jet

Thus flow rate will change by changing the area of the jet or more closely the diameter of the jet. This is accomplished by a spear valve and deflector plate shown in figure below.

The spear alters the cross-sectioned area ofthe jet. The position of spear is controlled by a servo mechanism that senses the load change. For a sudden loss of load when the turbine is shut down, a deflector plate rises to remove the jet totally from the buckets and to allow time for the spear to move to new position.

As seen in the figure for high load the spear valve has moved out, then increasing the area of the jet, at low loads the spear has moved in, decreasing the area of the jet. Deflector plate in normal position and fully deflected jet are also seen in the figures.

EULER'S HEAD BASIC INFORMATION AND TUTORIALS

EULER'S HEAD TUTORIALS
What is Euler's Head?

If the whole of mechanical power is converted into hydraulic power then total head H would be given by the relation

P=yQH

where Q is flow rate and y sp. weight of the fluid.

This is called Euler's head of the pump. The head available is actually less than Euler's head. If the water enters the impeller without whirl such that V Iw = 0 then Euler's equation is written as

TYPES OF CENTRIFUGAL HYDRAULIC RECOVERY TURBINES (CHRT) BASIC INFORMATION

Two major types of centrifugal hydraulic power recovery turbines are used.

1. Reaction—Single or multistage Francis-type rotor with fixed or variable guide vanes.

2. Impulse—Pelton Wheel, usually specified for relatively high differential pressures.

HPRTs with Francis-type rotors are similar to centrifugal pumps. In fact, a good centrifugal pump can be expected to operate with high efficiency as an HPRT when the direction of flow is reversed.

The Pelton Wheel or impulse runner type HPRT is used in high head applications. The impulse type turbine has a nozzle which directs the high pressure fluid against bowl-shaped buckets on the impulse wheel.

This type of turbines’ performance is dependent upon back pressure, while the reaction type is less dependent upon back pressure.