How Gas Turbine Work

A Little Background
There are many different kinds of turbines:

• You have probably heard of a steam turbine. Most power plants use coal, natural gas, oil or a nuclear reactor to create steam. The steam runs through a huge and very carefully designed multi-stage turbine to spin an output shaft that drives the plant's generator.
• Hydroelectric dams use water turbines in the same way to generate power. The turbines used in a hydroelectric plant look completely different from a steam turbine because water is so much denser (and slower moving) than steam, but it is the same principle.
• Wind turbines, also known as wind mills, use the wind as their motive force. A wind turbine looks nothing like a steam turbine or a water turbine because wind is slow moving and very light, but again the principle is the same.

A gas turbine is an extension on the same concept. In a gas turbine a pressurized gas spins the turbine. In all modern gas turbine engines the engine produces its own pressurized gas, and it does this by burning something like propane, natural gas, kerosene or jet fuel. The heat that comes from burning the fuel expands air, and the high-speed rush of this hot air spins the turbine.

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Advantages and Disadvantages of Gas Turbine Engines
So why does the M-1 tank use a 1,500 horsepower gas turbine engine instead of a diesel engine? It turns out that there are two big advantages:

1. Gas turbine engines have a great power-to-weight ratio compared to reciprocating engines.  That is, the amount of power you get out of the engine compared to the weight of the engine itself is very good.
2. Gas turbine engines are also smaller than their reciprocating counterparts of the same power.

The main disadvantage of gas turbines is that, compared to a reciprocating engine of the same size, they are expensive. Because they spin at such high speeds and because of the high operating temperatures, designing and manufacturing gas turbines is a tough problem from both the engineering and materials standpoint. Gas turbines also tend to use more fuel when they are idling and they prefer a constant load rather than a fluctuating load. That makes gas turbines great for things like trans-continental jet aircraft and power plants, but explains why you don't have one under the hood of your car.

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How Gas Turbine Engines Work
Gas turbine engines are, theoretically, extremely simple. They have 3 parts:

• A compressor to compress the incoming air to high pressure.
• A combustion area to burn the fuel and produce high pressure, high velocity gas.
• A turbine to extract the energy from the high pressure, high velocity gas flowing from the combustion chamber.

The following figure shows the general layout of an axial-flow gas turbine - the sort of engine you would find driving the rotor of a helicopter, for example: In this engine air is sucked in from the right by the compressor. The compressor is basically a cone-shaped cylinder with small fan blades attached in rows (8 rows of blades are represented here). Assuming the light blue represents air at normal air pressure, then as the air is forced through the compression stage its pressure and velocity rise significantly. In some engines the pressure of the air can rise by a factor of 30. The high-pressure air produced by the compressor is shown in dark blue.
This high-pressure air then enters the combustion area, where a ring of fuel injectors injects a steady stream of fuel. The fuel is generally kerosene, jet fuel, propane, or natural gas. If you think about how easy it is to blow a candle out, then you can see the design problem in the combustion area - entering this area is high-pressure air moving at hundreds of miles per hour. You want to keep a flame burning continuously in that environment. The piece that solves this problem is called a "flame holder", or sometimes a "can". The can is a hollow, perforated piece of heavy metal (shown here is half of the can in cross-section):
The injectors are at the right. Compressed air enters through the perforations. Exhaust gases exit at the left. You can see in the previous figure that a second set of cylinders wraps around the inside and the outside of this perforated can, guiding the compressed intake air into the perforations.
At the left of the engine is the turbine section. In this figure there are two sets of turbines. The first set directly drives the compressor. The turbines, the shaft and the compressor all turn as a single unit:
At the far left is a final turbine stage, shown here with a single set of vanes. It drives the output shaft. This final turbine stage and the output shaft are a completely stand-alone, freewheeling unit. They spin freely without any connection to the rest of the engine. And that is the amazing part about a gas turbine engine - there is enough energy in the hot gases blowing through the blades of that final output turbine to generate 1,500 horsepower and drive a 63 ton M-1 Tank! A gas turbine engine really is that simple.
In the case of the turbine used in a tank or a power plant, there really is nothing to do with the exhaust gases but vent them through an exhaust pipe, as shown. Sometimes the exhaust will run through some sort of heat exchanger either to extract the heat for some other purpose or to preheat air before it enters the combustion chamber.
The discussion here is obviously simplified a bit. For example, we have not discussed the areas of bearings, oiling systems, internal support structures of the engine, stator vanes and so on. All of these areas become major engineering problems because of the tremendous temperatures, pressures and spin rates inside the engine. But the basic principles described here govern all gas turbine engines and help you to understand the basic layout and operation of the engine.
Other variations
Large jetliners use what are known as turbofan engines, which are nothing more than gas turbines combined with a large fan at the front of the engine. Here's the basic (highly simplified) layout of a turbofan engine:
 
You can see that the core of a turbofan is a normal gas turbine engine like the one described in the previous section. The difference is that the final turbine stage drives a shaft that makes it's way back to the front of the engine to power the fan (shown in red in this picture). This multiple concentric shaft approach, by the way, is extremely common in gas turbines. In many larger turbofans, in fact, there may be two completely separate compression stages driven by separate turbines, along with the fan turbine as shown above. All three shafts ride within one another concentrically.
The purpose of the fan is to dramatically increase the amount of air moving through the engine, and therefore increase the engine's thrust. When you look into the engine of a commercial jet at the airport, what you see is this fan at the front of the engine. It is huge (on the order of 10 feet in diameter on big jets), so it can move a lot of air. The air that the fan moves is called "bypass air" (shown in purple above) because it bypasses the turbine portion of the engine and moves straight through to the back of the nacelle at high speed to provide thrust.
A turboprop engine is similar to a turbofan, but instead of a fan there is a conventional propeller at the front of the engine. The output shaft connects to a gearbox to reduce the speed, and the output of the gearbox turns the propeller.
by Marshall Brain

WHAT IS WORK

Work exists only in transitional form. Work is equal to the product of the force used to move something and the distance that it is moved. If there is no movement there is no work done.

W = F  D
       W    Work
       F    Force
       D    Distance

In a steam engine the pressure of the steam acting upon a piston does work. The force is the product of that pressure and the area of the piston.

F = p  A
       p    pressure
       A    Area pressure is acting upon

The work done by the piston is equal to the force times the distance the piston moves.

W = p A d
       d    Distance the piston moves (Stroke)

Since A d is the displacement volume v of the piston. Work can be expressed in terms of the displacement.

W = p v
        v    Volume displaced by piston.

GAS-TURBINE THEORY

A simple gas turbine is comprised of three main sections a compressor, a combustor, and a power turbine. The gas-turbine operates on the principle of the Brayton cycle, where compressed air is mixed with fuel, and burned under constant pressure conditions. The resulting hot gas is allowed to expand through a turbine to perform work. In a 33% efficient gas-turbine approximately two / thirds of this work is spent compressing the air, the rest is available for other work ie.( mechanical drive, electrical generation)

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However there are variations...

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One variation of this basic cycle is the addition of a regenerator. A gas-turbine with a regenerator (heat exchanger) recaptures some of the energy in the exhaust gas, pre-heating the air entering the combustor. This cycle is typically used on low pressure ratio turbines.

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  Turbines using this cycle are: Solar Centaur / 3500 horsepower class
up to the General Electric Frame 5

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Gas-turbines with high pressure ratios can use an intercooler to cool the air between stages of compression, allowing you to burn more fuel and generate more power. Remember, the limiting factor on fuel input is the temperature of the hot gas created, because of the metallurgy of the first stage nozzle and turbine blades. With the advances in materials technology this physical limit is always climbing.<

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 One turbine using this cycle is: General Electric LM1600 / Marine version

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A gas-turbine employing reheat.

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An Intercooled & Recuperated Turbine

 

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GAS-TURBINE HISTORY

The history of the gas turbine begins with a quest for jet propulsion.

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The earliest example of jet propulsion can be traced as far back as 150 BC to an Egyptian named Hero. Hero invented a toy that rotated on top of a boiling pot due to the reaction effect of hot air or steam exiting several nozzles arranged radially around a wheel. He called this invention an aeolipile.

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In 1232 the Chinese used rockets to frighten enemy soldiers.

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Around 1500 A.D. Leonardo da Vinci drew a sketch of a device that rotated due to the effect of hot gasses flowing up a chimney. The device was intended to be used to rotate meat being roasted. In 1629 another Italian named Giovanni Branca actually developed a device that used jets of steam to rotate a turbine that in turn was used to operate machinery. This was the first practical application of a steam turbine.

Ferdinand Verbiest a Jesuit in China built a model carriage that used a steam jet for power in 1678.

The first patent for a turbine engine was granted in 1791 to an Englishman named John Barber. It incorporated many of the same elements of a modern gas turbine but used a reciprocating compressor. There are many more early examples of turbine engines designed by various inventors, but none were considered to be true gas turbines because they incorporated steam at some point in the process.


In 1872 a man by the name of Stolze designed the first true gas turbine. His engine incorporated a multistage turbine section and a multi stage axial flow compressor. He tested working models in the early 1900's.
Charles Curtis the inventor of the Curtis steam engine filed the first patent application in the U.S. for a gas turbine engine. His patent was granted in 1914 but not without some controversy.
The General Electric company started their gas turbine division in 1903. An engineer named Stanford Moss lead most of the projects. His most outstanding development was the General Electric turbosupercharger during world war 1. ( Although credit for the concept is given to Rateau of France.) It used hot exhaust gasses from a reciprocating engine to drive a turbine wheel that in turn drove a centrifugal compressor used for supercharging. The evolutionary process of turbosupercharger design and construction made it possible to construct the first reliable gas turbine engines.
Sir Frank Whittle of Great Britain patented a design for a jet aircraft engine in 1930.He first proposed using the gas turbine engine for propulsion in 1928 while a student at the Royal Air Force College in Cramwell, England. In 1941 an engine designed by Whittle was the first successful turbojet airplane flown in Great Britain.
Concurrently with Whittle's development efforts, Hans von Ohain and Max Hahn, two students at Gottingen in Germany developed and patented their own engine design in 1936 these ideas were adapted by The Ernst Heinkel Aircraft company. The German Heinkel aircraft company is credited with the first flight of a gas turbine powered jet propelled aircraft on August 27th 1939.The HE178 was the first jet airplane to fly.

The Heinkel HeS-3b developed 1100 lbs. of thrust and flew over 400 mph, later came the ME262, a 500 mph fighter, more than 1600 of these were built by the end of WWII. These engines were more advaced than the British planes and had such features as blade cooling and a variable area exhaust nozzles.

In 1941Frank Whittle began flight tests of a turbojet engine of his own design in England. Eventually The General Electric company manufactured engines in the U.S. based on Whittle's design.

...... information courtesy Ron Munson

PROPERTIES OF STEAM AND WATER

 INTRODUCTION

The process by which we convert water into steam and use the steam to turn a propulsion shaft encompasses the generation and expansion phases of the steam cycle.  A study of the properties of water and steam at these critical phases is necessary to understand the steam cycle.  This lesson defines terms associated with these properties and processes, and explains the use of steam tables to calculate the work and efficiency created by steam.

REFERENCES

            (a)       Elements of Applied Thermodynamics, Robert M. Johnson, et al.

            (b)       Principles of Naval Engineering  NAVPERS 10788 series

            (c)            Introduction to Naval Engineering, Edward F. Gritzen.

INFORMATION

 A.              Basic Thermodynamic Terms

 1.       Enthalpy (h), measured in British thermal units per pound (mass), or BTU/lbm, represents the total energy content of steam.  It expresses the internal energy and flow work, or the total potential energy and kinetic energy contained within a substance.  The advantage of enthalpy is that we can express in one term all of the energy in a substance which is due to its pressure and temperature.  Enthalpy  values are used to represent the energy level of steam entering a turbine, a value useful for determining turbine efficiency.  By superheating steam, we can add enthalpy to steam without raising the pressure of the steam.  For example, steam at 620 psig and 850°F can do more work in a turbine than steam that is 620 psig and 650°F.

 2.       Entropy (s), measured in BTU/lbm-°R, represents the unavailability of energy (°R=Rankine temperature scale where 0°R =  absolute zero and 460°R = 0°F).  The second law of thermodynamics states that when heat is transferred from high temperature to low temperature regions, some of the heat will be rejected and not converted into mechanical work.  Entropy is a measure of how much heat must be rejected to a lower temperature receiver at a given pressure and temperature.

 a.        A complex explanation of the mathematical significance of the definition of entropy is unnecessary.  It is a term which attempts to describe the universe’s tendency to evenly distribute all mass and energy throughout space.  Processes which produce entropy are possible and those which destroy entropy are impossible.

 b.       Bodies with a high temperature will, when brought in contact with a body of a lower temperature, always cause heat to transfer from the hot body to the cold body.  This will lower the internal energy of the hot body and raise the internal energy of the cold body.  This is the principle that guides the design and operation of all naval heat exchangers.  For example, a main engine lube oil cooler directs hot lube oil over cool seawater piping, so that the hot lube oil will transfer some of its heat to the cooler seawater.  If left together indefinitely, the property of entropy would cause the heat from the lube oil to be equally distributed between the oil and the water, so that both would have the same temperature.  

 c.        Entropy would not be important except for the fact that the purpose of any engine is to collect, transfer, and use energy.  Thus, in a steam plant for example, it is not possible to add energy to water, boil it and transmit the resulting high energy steam across the relatively cooler engineroom without some of that energy being lost.  Some of this energy will always be lost through system conditions such as ineffective pipe lagging, piping leaks, and dirty or fouled tubes which retard heat transfer.  Operators must constantly attempt to minimize the effects of these conditions to maximize plant efficiency and reduce fuel and water costs.

 3.       A working fluid is a substance which receives, transfers and transmits energy in a thermodynamic system.  In most systems, the working substance is a fluid (liquid, vapor or gas).  In a steam system, water is the working fluid.

 4.       Density (r), measured in lbm/ft, represents the mass of a substance per unit volume, or how tightly packed the molecules are.  The more molecules packed in a given space, the more dense the material.  The density of water in a given location of the boiler is critical to the steam generation process because relatively dense feedwater will naturally push a less dense steam/water mixture through the boiler generating tubes.

 5.       Specific volume (v_SP), measured in ft³/lbm, represents the space occupied per unit mass of a substance.  It is the mathematical inverse of density.  Most engineering equipment is designed for size and strength taking into consideration the specific volume of the intended working fluid.

 6.       Specific weight (g), measured in lbf/ft³, represents the weight of a substance per unit volume.  This is the density of a substance acted upon by gravity.  The pressure of a fluid at the bottom of a storage tank is a direct function of the height of the fluid in the tank and the specific weight of the feedwater.  This resultant pressure is an important shipboard consideration with respect to providing a minimum suction pressure for a pump below the tank to move the fluid through a system.

7.       The state of a working fluid refers to the physical properties it possesses at a particular pressure, temperature and volume.  If each of these are known with respect to a substance, the state of the substance is known.  The substance can be a subcooled, saturated, or superheated solid, liquid, or gas.  Many systems operate the working fluid with very specific temperature/pressure relationships.  Water is subcooled in the condensate and feed phases of the steam cycle to allow it to be pumped, saturated in portions of the generation and feed phases for natural flow or for maintaining proper chemistry, and superheated in the expansion phase to extract maximum work from the steam to turn a propulsion turbine.   

 8.       A thermodynamic process is any process which changes the state of the working fluid.  These processes can be classified by the nature of the state change that takes place. Common types of thermodynamic processes include the following:

 a.        A reversible process is an ideal process where the working fluid returns to its original state by conducting the original process in the reverse direction.  For a process to be reversible, it must be able to occur in precisely the reverse order.  All energy that was transformed or distributed during the original process must be capable of being returned to its exact original form, amount and location.  Reversible processes do not occur in real life.

 b.       An irreversible process is any process which is not reversible.  All real life processes, such as the basic steam cycle, are irreversible.

 c.        An adiabatic process is a state change where there is no transfer of heat to or from the system during the process.  Because heat transfer is relatively slow, any rapidly performed process can approach being adiabatic.  Compression and expansion of working fluids are frequently achieved adiabatically with pumps and turbines.

 d.       An isothermal process is a state change in which no temperature change occurs.  Note that heat transfer can occur without causing a change in temperature of the working fluid.  In the DFT, auxiliary exhaust heats incoming condensate, then condenses to liquid and falls to the bottom of the tank. Throughout this process, the temperature of the auxiliary exhaust remains constant at 246-249°F.

 e.        An isobaric process is a state change in which the pressure of the working fluid is constant throughout the change.  An isobaric state change occurs in the boiler superheater, as the heat of the exiting steam is increased without increasing its associated pressure.

 9.       A thermodynamic cycle is a recurring series of thermodynamic processes through which an effect is produced by the transformation or redistribution of energy.  In other words, it is a series of processes repeated over and over again in the same order.  Thermodynamic cycles contain five basic elements: (1) a working fluid, (2) an engine, (3) a heat source, (4) a heat receiver, and (5) a pump.  All thermodynamic cycles may be classified as being open cycles or closed cycles.

 a.        A closed cycle is one in which the working fluid is reused.  Steam plants and refrigeration cycles are closed cycles.  In a steam plant, the water undergoes a series of processes that change the state of the water. Eventually the water returns to its original state and is ready to begin the cycle again.

 b.       An open cycle is one in which the working fluid is not reused.  Open cycles typically use the atmosphere as a working fluid.  An internal combustion engine represents a typical open cycle.  Air is drawn into the engine, combusted in the cylinders, and exhausted back to the atmosphere.  Fresh air is drawn into the engine to begin the cycle again.

 B.              Heat Addition and Temperature

 1.       When heat is added to a material, one of two things will occur: the material will change temperature or the material will change state.   When a substance is below the temperature at a given pressure required to change state, the addition of sensible heat will raise the temperature of the substance.  Sensible heat applied to a pot of water will raise its temperature until it boils.  Once the substance reaches the necessary temperature at a given pressure to change state, the addition of latent heat causes the substance to change state.  Adding latent heat to the boiling water does not get the water any hotter, but changes the liquid (water) into a gas (steam).

 2.       One can state that a certain amount of heat is required to raise the temperature of a substance 1°F.  This energy is called the specific heat capacity.  The specific heat capacity of a substance depends upon the volume and pressure of the material.  For water, the specific heat capacity is 1 BTU/lbm-°F and remains constant.  This means that if we add 1 BTU of heat to 1 lbm of water, the temperature will rise 1°F.

 C.              Introduction to Steam Tables

 1.       When a teapot of water is placed on a hot burner, sensible heat begins to heat the water.  The energy added to the water raises its internal energy and its temperature.  When the water reaches 212°F, the temperature no longer rises as latent heat begins to change the water from a liquid to a vapor.  The mass inside the teapot is slowly changing from a 100% water / 0% steam mixture into a 0% water / 100% steam mixture.  If we add only half the necessary latent heat, then only half the water will boil into steam.  The result would be a 50% water / 50% steam mixture at 212°F.  If we add all the latent heat necessary, then the water at 212°F changes completely into steam at 212°F.  Continuing to add heat to the 212°F steam results in a temperature increase (superheating), and we are again raising the temperature by adding sensible heat.  Refer to figure 3.2-1 (sensible/latent heat and enthalpy).

 2.       While the properties of water at atmospheric pressure are commonly known, water under different pressures will exhibit different properties.  When water is boiled at pressures higher than atmospheric, the same events occur as described above with two exceptions.  First, the boiling temperature will be higher than 212°F.  Second, less latent heat is required to be added to change the water completely into steam.  If water were to be boiled at a pressure lower than atmospheric pressure, then we would find that the boiling temperature would be less than 212°F and a larger amount of latent heat would be required to change the water completely into steam.  Refer to figure 3.2-2 (temperature vs. latent heat).

 a.        When water is below the boiling point, the addition of heat is seen as sensible heat.  This water is said to be a subcooled liquid.  When enough sensible heat is added so that the temperature of the water approaches saturation temperature but no steam has yet been formed, the water is said to be a saturated liquid.

 b.       As the water is transformed from a saturated liquid to saturated steam, boiling is occurring.  As latent heat is added, the temperature of the water remains the same but the saturated liquid is being changed into a saturated vapor.  During this period the water is referred to as a liquid/vapor mixture.  When enough latent heat is added so that all of the liquid is converted into vapor, the water becomes a saturated vapor.  Note that the saturated vapor is 100% vapor and exists at the same temperature as the saturated liquid.  Above the saturated steam point, vapor exists at a temperature higher than saturation temperature.  This is the superheated vapor region.

 c.        Steam tables are a useful tool for determining the properties of steam and water at various temperatures and pressures.  The steam tables are broken into three tables.

D.              Mollier Diagram

 1.       The Mollier diagram is a small portion of data from the steam tables graphed onto enthalpy-entropy coordinates.  It presents the region that is commonly found in propulsion plant steam systems.  Examine the last section of the steam tables for a representation of a Mollier diagram.

 2.       Locating information off the Mollier diagram is done as follows: The horizontal axis is entropy (s) in BTU/lbm-°R.  The vertical axis is enthalpy (h) in BTU/lbm.  The dark line across the middle of the chart is called a “steam dome” because of its shape.  Above this line, the data is for superheated steam.  Below this line, the data is for a steam-water mixture.  The data directly on the line is for saturated steam.

 3.       To find data in the steam-water mixture region of the chart, enter the chart using the absolute pressure and %-moisture (y).  Once you find the intersection of these two parameters, read off the number directly across from the intersection point for enthalpy.  Read off the number directly below the intersection point for entropy.

 To find data in the superheated region of the chart, enter the chart using the measured temperature and pressure of the steam.  Again, find the intersection point of these two parameters and read off the values for entropy and enthalpy.  Notice that moisture does not plot in the superheat region.  This is because moisture is a parameter which only exists in saturated conditions.