Unit - 4

Steam nozzles

Q1) What is steam nozzles?

A1)

Steam nozzles:

In steam turbines, the general transformation of warmth electricity of steam into mechanical paintings takes location in stages:-

Available steam electricity into kinetic electricity Kinetic electricity into mechanical paintings.

The first level is performed with the gadgets referred to as steam nozzles.

A steam nozzle is a duct or passage of easily various go sectional region which converts warmth electricity of steam into kinetic electricity.

The form of the nozzle is designed such that it's going to carry out this conversion of electricity with minimal loss. The go segment of the nozzles can be circular, rectangular, elliptical or square

Applications of Nozzles

1) Steam and Gas turbines,

2) Jet engines,

3) For propulsion of rocket motors,

4) Flow measurements,

5) In injectors for pumping water,

6) In ejectors for casting off air from condensers.

The nozzles are simply placed simply earlier than the steam turbines.

A steam nozzle can be described as a passage of various cross-phase, thru which warmth electricity of steam is transformed to kinetic electricity. Steam Nozzle most important feature is to provide steam jet with excessive speed to power steam generators. The electricity conversion is introduced approximately with inside the following ways:

1. The excessive-strain, excessive-temperature steam first expands with inside the nozzles emanates as an excessive-speed fluid circulation.

2. The excessive-speed steam popping out of the nozzles impinges at the blades installed on a wheel. The fluid circulation suffers a lack of momentum at the same time as flowing beyond the blades this is absorbed through the rotating wheel entailing the manufacturing of torque.

3. The shifting blades circulate due to the impulse of steam (because of the alternate of momentum) and additionally due to the growth and acceleration of the steam relative to them. In different phrases additionally they act because the nozzles.

A steam nozzle is a duct or passage of easily various cross-sectional region which converts warmth electricity of steam into kinetic electricity. The form of the nozzle is designed such that it'll carry out this conversion of electricity with minimal loss.

Working of Steam Nozzle:

During the primary a part of the nozzle, the steam will increase its speed. And then the later a part of the nozzle, wherein the steam derives greater in quantity than in speed.

1. Since the mass of steam that is surpassed thru any phase of the nozzle remains’ constant.

2. The version of steam strain with inside the nozzle relies upon the speed, particular quantity, and dryness fraction of steam.

Uses:

The foremost use of a steam nozzle in steam generators is to provide a jet of steam with an excessive speed.

The smallest phase of the nozzle is referred to as the throat.

Convergent Nozzle:

A usual convergent nozzle is proven in fig. In a convergent nozzle, the cross-sectional region decreases constantly from its front to exit. It is utilized in a case in which the backpressure is same to or extra than the important strain ratio.

Divergent Nozzle:

The cross-sectional region of the divergent nozzle will increase constantly from its front to exit. It is utilized in a case, in which the returned strain is much less than the important strain ratio.

Convergent-Divergent Nozzle:

In this case, the cross-sectional region first decreases from its front to the throat after which will increase from throat to exit. It’s far extensively used in lots of forms of steam generators.

Q2) Explain adiabatic expansion in nozzles

A2)

Adiabatic expansion in nozzles

A specifically built test suggests the close to best adiabatic enlargement of an excellent electron fuelling ensuing in a polytrophic index more than 1.4, drawing close the adiabatic price of 5 / 3, while casting off electric powered fields from the gadget, whilst the polytrophic index near cohesion is determined while the electrons are trapped via way of means of the electrical fields.

The measurements had been made on collision less electrons in an argon plasma increasing in a magnetic nozzle.

The collision lengths of all electron collision tactics are more than the dimensions period of the enlargement, which means the gadget can't be in thermodynamic equilibrium, but thermodynamic standards may be used, with caution, in explaining the results.

In particular, a Lorentz force, created via way of means of in homogeneitieswith inside the radial plasma density, does paintings at the increasing magnetic field, decreasing the inner power of the electron fuelling that behaves as an adiabatically increasing perfect fuelling.

• A turbine is a rotary regular kingdom regular waft device whose cause is the manufacturing of shaft strength on the price of the strain of the operating fluid.
• Two well-known training of generators are steam and fuelling generators relying at the operating substance used.
• Usually, adjustments in capability power are negligible, as is the inlet kinetic power.
• Often the go out kinetic power is neglected (if in a problem, the waft velocities are specified, the kinetic power time period need to be included).
• Normally, the technique with inside the turbine is adiabatic and the paintings output reduces to lower in enthalpy from the inlet to go out kingdom
• A nozzle is a spout at the quilt of a hose or pipe used to manipulate the motion of a fluid like water or air.
• A convergent nozzle is a nozzle that begins off evolved large and receives smaller-a lower in cross-sectional area.
• As a fluid enters the smaller cross-section, it has to hurry up because of the conservation of mass.
• To keep a steady quantity of fluid transferring via the restrained part of the nozzle, the fluid should circulate quicker.
• The power to make this fluid accelerate has to return back from somewhere.
• Some power is with inside the random movement of molecules, which we take a look at as strain.
• The power on this random movement is transformed into quicker ahead movement, referred to as circulation flow.
• This alternate makes the strain drop.

Q3) What is maximum discharge?

A3)

Maximum discharge

A discharge is a measure of the quantity of any fluid flow over unit time. The quantity may be either volume or mass. Thus the water discharge of a tap (faucet) can be measured with a measuring jug and a stopwatch. Here the discharge might be 1 liter per 15 seconds, equivalent to 67 ml/second or 4 liters/minute. This is an average measure.

For measuring the discharge of a river we need a different method and the most common is the 'area-velocity' method. The area is the cross sectional area across a river and the average velocity across that section needs to be measured for a unit time, commonly a minute. Measurement of cross sectional area and average velocity, although simple in concept, are frequently non-trivial to determine.

The units that are typically used to express discharge in streams or rivers include m3/s (cubic meters per second), ft3/s (cubic feet per second or cfs) and/or acre-feet per day.

A commonly applied methodology for measuring, and estimating, the discharge of a river is based on a simplified form of the continuity equation. The equation implies that for any incompressible fluid, such as liquid water, the discharge (Q) is equal to the product of the stream's cross-sectional area (A) and its mean velocity(), and is written as:

Q = A

{\displaystyle Q=A\,{\bar {u}}}Where

{\displaystyle Q}Q Is the discharge ([L3T−1]; m3/s or ft3/s)

{\displaystyle A}A Is the cross-sectional area of the portion of the channel occupied by the flow ([L2]; m2 or ft2)

{\displaystyle {\bar {u}}} Is the average flow velocity ([LT−1]; m/s or ft/s)

For example, the average discharge of the Rhine river in Europe is 2,200 cubic meters per second (78,000 cu ft./s) or 190,000,000 cubic meters (150,000 adrift) per day.

Because of the difficulties of measurement, a stream gauge is often used at a fixed location on the stream or river.

Q4) Write a short note on critical pressure ratio

A4)

Critical strain ratio is the ratio of strain at which the device offers most mass glide charge and it can't be improved in addition with the aid of using adjusting the device strain. The vital strain ratio of any fluid relies upon at the polytrophic index (n) of that fluid. This most mass glide charge circumstance is reached whilst the suit variety at minimal cross-segment will become same to

1. The vital strain calculated at downstream is likewise referred to as choking strain. It manner that if the strain at downstream reduced under the vital strain then choking will occur.

By the definition, the critical pressure ratio is given by,

Critical pressure ratio = PcPo

Where Pc = Pressure at the minimum cross-section

Po = Inlet pressure

Q5) Explain various terminology used in nozzle?

A5)

EFFECTS OF NOZZLE FRICTION

The effects of nozzle friction are

1. Reduction in enthalpy drop
2. Reheating of steam i.e improving the quality of vapour at the exit
3. Reduction in exit velocity
4. Increase in specific volume
5. Decrease in mass flow rate

NOZZLE EFFICIENCY

Consider a convergent-divergent nozzle, the (h-S) diagram with friction is as shown in fig above. It is assumed that the friction losses in convergent portion are negligible and the whole loss occurs in the divergent portion.

It represents the pressure at inlet to the nozzle, pt is the throat pressure which is equal to critical pressure and pe is the exit pressure. The nozzle efficiency can be given as Nozzle efficiency, η=h1−he/h1−he

NOZZLE OR VELOCITY COEFFICIENT, CC:

It is defined as the ratio of actual exit velocity to the ideal exit velocity. Accordingly,

Significance of critical pressure ratio:

The significance of the critical pressure ratio is given below

1. The critical pressure ratio helps to attain maximum mass flow rate through the nozzle.
2. It also helps us to avoid choking of nozzle.

Q6) Write a calculation of throat and exit areas

A6)

The reason of this applet is to simulate the operation of a converging-diverging nozzle, possibly the maximum vital and fundamental piece of engineering hardware related to propulsion and the excessive pace go with the drift of gases.

This tool turned into invented via way of means of Carl de Laval towards the quilt of the l9th century and is as a consequence frequently called the 'de Laval' nozzle.

This applet is meant to assist college students of compressible aerodynamics visualize the go with the drift via this kind of nozzle at a number of conditions.

Technical Background The regular configuration for a converging diverging (CD) nozzle is proven with inside the figure.

Gas flows via the nozzle from a place of excessive stress (typically called the chamber) to one in all low stress (called the ambient or tank).

The chamber is typically huge sufficient in order that any go with the drift velocities right here are negligible.

The stress right here is denoted via way of means of the image pc.

Gas flows from the chamber into the converging part of the nozzle, beyond the throat, via the diverging component after which exhausts into the ambient as a jet. The stress of the ambient is called the 'again stress' and given the image pb.

Where,

A∗ is the area of the throat (m2)

A is the area of the nozzle exit (m2)

M is the Mach number (No unit)

γ is the specific heat ratio (No unit)

The area-Mach number relation gives the ratio of the local area to throat area as a function of Mach number.

The area-Mach number relation is valid for isentropic flows (i.e. no shocks allowed) and calorically perfect gases.

To get a fundamental sense for the conduct of the nozzle believe appearing the easy test proven in parent 2. Here we use a converging diverging nozzle to attach air cylinders.

Cylinder A consists of air at excessive strain, and takes the vicinity of the chamber.

The CD nozzle exhausts this air into cylinder B, which takes the vicinity of the tank. Imagine you're controlling the strain in cylinder B, and measuring the ensuing mass waft charge via the nozzle.

You might also additionally anticipate that the decrease you're making the strain in B the extra mass waft you will get via the nozzle.

This is true, however handiest as much as a factor. If you decrease the again strain sufficient you return back to an area wherein the waft charge abruptly stops growing all collectively and it would not count how a whole lot decrease you're making the again strain (even in case you make it a vacuum) you cannot get any extra mass waft out of the We say that the nozzle has become 'choked'.

You may want to postpone this conduct via way of means of making the nozzle throat bigger (e.g. Gray line) however ultimately the equal component could happen. The nozzle becomes choked even in case you removed the throat altogether and simply had a converging nozzle.

The motive for this conduct has to do with the manner the flows behave at Mach 1, i.e. while the waft velocity reaches the rate of sound. In a consistent inner waft (like a nozzle) the Mach quantity can handiest attain 1 at a minimal with inside the cross-sectional place.

When the nozzle is not choked, the waft via it's far absolutely subsonic and, in case you decrease the again strain a little, the waft is going quicker and the waft charge increases.

As you decrease the again strain similarly the waft velocity on the throat ultimately reaches the rate of sound (Mach 1).

Any similarly decreasing of the again strain cannot boost up the waft via the nozzle any extra, due to the fact that could entail transferring the factor wherein M=1 far from the throat wherein the place is a minimal, and so the waft receives stuck.

The waft sample downstream of the nozzle (with inside the diverging segment and jet) can nonetheless extrude in case you decrease the again strain similarly, however the mass waft charge is now constant due to the fact the waft with inside the throat (and for that count with inside the whole converging segment) is now constant

Q7) Explain in detail supersaturated flow

A7)

When the release of to start with dry and saturated steam increasing via a nozzle, it's far measured to be 1 to three% extra than theoretically calculated discharge. This is precisely the opposite of what's expected.

Expectations:

Due to the impact of friction the real discharge need to be approximately three to 5% much less than what's theoretically calculated.

But in Reality, the measured cost end up 1 to three% extra than the theoretical one. In the case of preliminary superheated steam, the boom is simply enough to atone for the losses because of friction, so on this case, the measured discharge is extra or much less is in settlement with the theoretical one.

The boom in measured discharge to the theoretically calculated discharge is because of the time lag in condensation of steam and for this reason the steam stays with inside the dry country rather than wet; this phenomenon is called “Super saturation”.

Sometimes, the steam on this situation is stated to be in a metastable country. This impact changed into investigated via way of means of Wilson and Martin. There a few reasons worried in super saturation:

The condensation manner usually begins off evolved round any tiny nuclei.

Generally, tiny dirt debris are continually gift with inside the commercially generated steam in enough quantities.

In the absence of such overseas debris, condensation is behind schedule and the temperature of steam maintains to fall, that is referred to as super saturation.

2. In the herbal manner of condensation, the random kinetic power of molecules falls right all the way down to a degree that isn't always enough to conquer those appealing forces of molecules and a few decrease transferring molecules come collectively and shape small water droplets.

A sure time c language is important for the gathering of those molecules to shape droplets.

The glide of steam via a brief convergent a part of nozzle is with a excessive pace and it's far extraordinarily speedy which can be pretty insufficient for the condensation of steam and the condensation is behind schedule, the glide is supersaturated.

3. The molecules of a completely small water droplet will cross on evaporating right into an area this is saturated and area will, therefore, end up superheated with water vapor

Q8) Write an effect of Supersaturated glide in nozzle

A8)

The impact of supersaturated is to lessen the warmth drop barely for the duration of the growth and therefore a corresponding discount in very last pace.

The very last dryness fraction and the entropy additionally elevated and the measured discharge is extra than that theoretically calculated.

Consequently, the condensation of steam is delayed for a bit while.

This phenomenon is referred to as super saturation, and the steam that exists within side the moist area without containing any liquid is thought as supersaturated steam

The locus of factors wherein condensation will take vicinity now no longer thinking about of the preliminary temperature and strain on the nozzle front is known as the Wilson line lies among four and five percentage moisture curves in the saturation area at the h-s diagram for steam, and is regularly approximated with the aid of using the4 percentage moisture line.

The super saturation phenomenon is proven at the h-schart. The waft among theoretical saturated line and real saturated line is called metastable waft or exceptional saturated waft

Q9) Write a short note no Wilson line.

A9)

Wilson Line

However, it's far appropriate to lessen the steam wetness as a ways as feasible with the moisture keeping apart gadget in the steam generator. The Westinghouse four-loop 3425 MW (thermal) layout, which has been tailored for the UK's first PWR station at Sizewell, offers steam to the turbine whose complete load situations are nominally: Pressure = 66. 6 bar; Temperature = 282°C Maximum wetness component = 0.25%

This enlargement effects in steam on the later levels with a moisture content material of 25%.

An excessive moisture content material is insupportable because it effects in blade erosion and level performance losses. The undesirability of blade erosion changed into mentioned in advance on this chapter.

As steam is accelerated via the turbine it begins off evolved to condense within side the moist area alongside the 'Wilson line' which corresponds to a wetness component of 3-4% (see Fig 1.50).

Above the Wilson line, the steam behaves as dry steam; beneath the line, wetness turns into obvious within side the turbine enlargement. Water droplets entrained within side the steam go with the drift best boost up to a small share of the dry steam pace as they by skip via the constant blades.

The resultant mist has a pace which does now no longer fit the inlet pace triangle of the transferring blade (Section 1.2.2 of this chapter), neither in importance nor direction. As the mist progresses via the turbine, the droplets migrate towards the blade guidelines via way of means of distinctive feature of centripetal inertia. The resultant impact of the moist steam is a mechanical drag similar to a 1% cylinder loss for each 1% wetness.

The results of an excessive moisture content material are unattractive from the viewpoints of turbine performance and operational economics. The cycle defined in advance blanketed a moisture separator reheated (MSR), which takes in part accelerated steam, eliminates the moisture and reheats to a superheat temperature. The desire of reheat or crossover strain is stricken by numerous parameters for a device rotating at 3000 r/min; those are:

The most wetness within side the LP turbine exhaust (10-12%). The most wetness within side the HP turbine exhaust (14-15%). The temperature to which reheat steam may be raised, confined via way of means of the most steam temperature to be had on the turbine prevent valve (TSV). Overall cycle performance.

Plant capital costs.

The wetness within side the HP turbine may be a bit better than that for the LP turbine because the blades are shorter and the end velocities are substantially much less. Considering those parameters at the Mollie chart (Fig 1.51) offers a 'window' inside which the crossover strain can vary.

The resultant LP circumstance line could be very much like that of a traditional LP cylinder on a fossil-fired or AGR plant. The choice to mirror the LP cylinder is attractive, because it offers financial savings in phrases of layout and manufacture and gives a utility, which includes the CEGB, the choice to rationalize its countrywide spares holding

Q10) What is steam turbine?

A10)

Steam turbines

A steam turbine is a gadget that extracts thermal power from pressurized steam and makes use of it to do mechanical paintings on a rotating output shaft. Its cutting-edge manifestation changed into invented through Charles Parsons in 1884.

Fabrication of a cutting-edge steam turbine entails superior metalwork to shape high-grade metal alloys into precision components the use of technology that first have become to be had with inside the twentieth century; endured advances in sturdiness and performance of steam mills stays principal to the power economics of the twenty first century.

The steam turbine is a shape of warmth engine that derives lots of its development in thermodynamic performance from using more than one ranges with inside the enlargement of the steam, which leads to a better method to the appropriate reversible enlargement process.

Because the turbine generates rotary motion, it's far mainly suitable for use to power an electrical generator—approximately 85% of all energy era with inside the United States with inside the yr. 2014 changed into through use of steam mills.

A steam turbine related to an electric powered generator is referred to as a rapid generator.

As of 2021, amongst the biggest steam mills with inside the global is the Arabella steam mills synthetic through GE primarily based totally on a unique layout through Alstom.

An Arabella turbine is 7 m in diameter, weighs 4000 lots and spins at 1500 rpm.

In a standard nuclear installation, every other 4000 lots of helping metal shape is required, in addition to a thousand lots of pumps, valves, and pipes.

Technical issues consist of rotor imbalance, vibration, bearing wear, and choppy enlargement (diverse styles of thermal shock).

In big installations, even the sturdiest turbine is able to shaking itself aside while operated out of trim.

A main venture dealing with turbine layout became lowering the creep skilled via way of means of the blades.

Because of the excessive temperatures and excessive stresses of operation, steam turbine substances grow to be broken via those mechanisms.

As temperatures are extended with a purpose to enhance turbine performance, creep turns into significant.

To restriction creep, thermal coatings and super alloys with solid-answer strengthening and grain boundary strengthening are utilized in blade designs.

Protective coatings are used to lessen the thermal harm and to restriction oxidation.

These coatings are frequently stabilized zirconium dioxide-primarily based totally ceramics.

Using a thermal shielding coating limits the temperature publicity of the nickel super alloy. This reduces the creep mechanisms skilled with inside the blade.

The nickel-primarily based totally blades are alloyed with aluminum and titanium to enhance power and creep resistance.

The microstructure of those alloys consists of various areas of composition.

A uniform dispersion of the gamma-top phase – a mixture of nickel, aluminum, and titanium – promotes the power and creep resistance of the blade because of the microstructure.

Refractory factors together with rhenium and ruthenium may be introduced to the alloy to enhance creep power.

The addition of those factors reduces the diffusion of the gamma top phase, accordingly keeping the fatigue resistance, power, and creep resistance.

Q11) What is working principle of steam turbine?

A11)

Working principle of steam turbines

Working precept of steam turbine relies upon at the dynamic motion of steam. A high-speed steam is coming from the nozzles and it moves the rotating blades which can be outfitted on a disc established on a shaft.

This high-speed steam produces dynamic stress at the blades wherein blades and shaft each begin to rotate with inside the equal direction.

Basically, in a steam turbine stress electricity of steam extracts after which it transformed into kinetic electricity through permitting the steam to go with the drift thru thaw nozzles.

The conversion of kinetic electricity does mechanical paintings to the rotor blades and the rotor is attached to a steam turbine generator which acts as a mediator.

Turbine generator collects mechanical electricity from the rotor and converts into electric electricity.

Since the development of steam turbine is simple, its vibration is a great deal much less than the alternative engine for equal rotating speed.

Though one-of-a-kind sorts of governing gadget are used to enhance turbine speed.

The working precept of this tool is primarily based totally at the dynamic motion of the steam. The expanded stress steam which comes out from the nozzles hits the revolving blades which might be close-suited to the disc that is located at the shaft.

In general, the steam turbine isolates the power of the stem after which transforms it into the kinetic power which then flows via the nozzles.

So, the rework of kinetic power plays mechanical movement to the rotor blades and this rotor has a reference to the steam turbine generator and this plays because the intermediary.

Because the development of a tool is so streamlined, it generates minimum noise whilst in comparison to different styles of rotating devices. In maximum of the turbines, the revolving blade velocity is linear to that of the steam velocity flowing throughout the blade.

When the vapor receives elevated with inside the single-segment itself from that boiler pressure to the exhausted pressure, then the vapor pace is extraordinarily expanded.

Whereas the principal turbine that is utilized in nuclear flora wherein the steam enlargement fee is sort of 6 MPa to 0.0008 MPa having a fee of velocity at 3000 revolutions in step with 50 Hz of frequency and 1800 revolutions at 60 Hz frequency.

Q12) Give the classification of impulse turbine.

A12)

i. Simple Impulse Turbine (De Laval Turbine):

An easy impulse turbine or single-degree impulse turbine is appropriate for low-stress steam.

The stress stays steady at some point of at the same time as flowing over the transferring blades. In this case, the excessive-stress steam is to begin with multiplied in a nozzle or a hard and fast of nozzles that is located through the facet of rotor leaving a few hole of approximately 5-6 mm.

Due to stress drop, an excessive speed is acquired at the opening of the nozzle.

The steam at this excessive kinetic power flows over the blade channel after which it's far discharged from the turbine.

Ii. Compounded Steam Turbines:

Due to development in technological ability, the fashion is to generate steam at excessive stress and temperature, as excessive as 100-a hundred and fifty bar stress and approximately 550°C great heat.

For acquiring most thermal efficiency, the full stress drop from boiler to condenser stress have to be absolutely transformed into kinetic power.

If the whole stress drop from excessive to low stress takes location best in a single set of nozzles, then the turbine rotor rotates at a completely excessive pace so as of approximately 20,000 rpm.

Such an excessive revolution of turbine rotor isn't always beneficial for realistic purposes. It poses some of technical issues which includes structural failure because of excessive centrifugal strain developed, boom in vibration, immoderate noise produced, and overheating of bearings.

Q13) Explain Pressure-Compounded Turbines (Rameau)and Pressure-and-Velocity-Compounded Turbines:

A13)

Pressure-Compounded Turbines (Rameau):

Pressure-compounded impulse turbine is one wherein some of easy impulse generators are organized in collection and located on a not unusual place shaft.

In this case, a row of constant nozzles is located on the access of every row of transferring blades, i.e., this contains extrude rows of constant nozzles and transferring row of blades in collection.

The overall stress drop from excessive stress to exhaust stress is break up into collection of smaller stress drop to take location in specific tiers in collection.

Each set combining one row of constant and one row of transferring blade is referred to as one degree.

High-stress steam is multiplied with inside the first row of constant nozzles of the primary degree with a small stress drop.

Due to small stress drop, much less kinetic power is to be had on the go out of first row of constant nozzles.

The steam with small kinetic power enters to the primary row of transferring blades in which it undergoes extrude of momentum and the kinetic power is absorbed.

Pressure-and-Velocity-Compounded Turbines:

Such generators are the aggregate of stress and speed compounding. If the stress variety may be very large, this association may be very a whole lot appropriate. As we know, a -row speed-compounded turbine is greater green than a three-degree speed-compounded turbine.

But the development of -row speed-compounded turbine possesses a few problems because it will increase the speed in line with blade. Hence, the full stress drop of steam from boiler stress to exhaust stress might break up up in and 3 steps, as carried out in stress compounding, and the kinetic power won in every step is absorbed absolutely in transferring wheels earlier than the subsequent stress drop occurs.