PP210 Turbine Designs

What is an engine spool?

"Spool" refers to the unit of rotating components inside of a jet engine, so named because the rotating shaft with compressor and turbine blades on each end resembles an empty thread spool.

List the seven sections of a gas turbine engine.

1. An air inlet, 2. Compressor section, 3. Diffuser 4. Combustion section, 5. Turbine section, 6. Exhaust section, 7. Accessory section.

List four current applications for bleed air

1. Cabin pressurization, heating, and cooling; 2. Deicing and anti-icing equipment; 3. Pneumatic starting of engines; and 4. Auxiliary drive units (ADU).

What are the basic elements of the combustion section?

1. Casing 2. Perforated inner liner 3. Fuel injection system 4. Some means for initial ignition 5. Fuel drainage system to drain off unburned fuel after engine shutdown

What are the two principal types of compressors currently being used in gas turbine engine aircraft?

1. Centrifugal Flow 2. Axial Flow

List four advantages of the axial flow compressor?

1. High peak efficiency 2. Small frontal area forgiven airflow 3. Straight-through flow, allowing high ram efficiency 4. Increased pressure rises due to increased number of stages with negligible losses.

What are the basic elements of the accessory section?

1. The accessory case, which has machined mounting pads for the engine-driven accessories, and 2. The gear train, which is housed within the accessory case.

What determines the amount of air passing through the turbine engine?

1. The compressor speed (rpm) 2. The forward speed of the aircraft 3. The density of the ambient (surrounding) air

The typical turboprop engine can be broken down into assemblies as follows:

1. The power section assembly—contains the usual major components of a gas turbine engine (i.e., compressor, combustion chamber, turbine, and exhaust sections). 2. The reduction gear or gearbox assembly—contains those sections unique to turboprop configurations. 3. The torque meter assembly—transmits the torque from the engine to the gearbox of the reduction section. 4. The accessory drive housing assembly—mounted on the bottom of the compressor air inlet housing. It includes the necessary gear trains for driving all power section driven accessories at their proper rpm in relation to engine rpm.

Name and describe the three types of combustion chambers for the turbine engines.

1. the can/multiple can type combustion chamber - These combustion chamber sustains with their own air supply duct, fuel nozzle and flame tube within their casing. 2. the can-annular combustion chamber - The can-annular type combustion chamber is much simpler in design. It has a set of flame tubes installed around the combustor section. All the flame tubes have a common outer casing and inner casing. 3. the annular combustion chamber - The annular combustion chamber is most commonly used on modern gas turbine engines. An annular combustion chamber consists of only one annular flame tube. An inner and an outer combustion chamber casing provides the support for such flame tube. The combustion chamber casings are part of the load carrying engine structure.

What is the speed range of a turboprop aircraft?

300 to 400 mph speed range

How is the diffuser in a centrifugal compressor constructed?

A compressor outlet is bolted to each of the outlet ports. These air outlets are constructed in the form of ducts and are elbow known by a variety of names, such as air outlet ducts, outlet elbows, or combustion chamber inlet ducts.

What is the secondary function for the compressor section?

A secondary function of the compressor is to supply bleed air for various purposes in the engine and aircraft

What is the secondary function of the accessory section?

A secondary function of the compressor is to supply bleed air for various purposes in the engine and aircraft.

Why is it sometimes necessary to have more than one stage in a turbine engine?

A single turbine wheel often cannot absorb enough power from the exhaust gases to drive the components dependent on the turbine for rotative power; thus, it is necessary to add additional turbine stages.

How is a turbine stage constructed?

A turbine stage consists of a row of stationary vanes or nozzles, followed by a row of rotating blades

How does the air get from the diffuser to the combustion chamber with a centrifugal compressor?

Centrifugal impeller diffuser pipes and straitening vanes maximize compressor discharge air pressure and redirect airflow at 90⁰ to combustion chamber

Is the turbine engine nozzle diaphragm convergent or divergent and what it's purpose?

Convergent, the vanes in the nozzle diaphragm form a series of convergent ducts which accelerate the air as it leaves the combustors and direct it into the turbine blades at the angle that gives the turbine its maximum efficiency.

What is the function of the exhaust system on a turbojet engine?

Direct the flow of hot gases rearward in such a manner as to prevent turbulence and, at the same time, impart a high final or exit velocity to the gases.

Describe the exhaust section of a turbojet engine, each component and it's purpose.

Exhaust cone: collects the exhaust gases discharged from the turbine section and gradually converts them into a solid flow of gases Tailpipe: portion of the exhaust system of a gas turbine engine through which the gases leave Exhaust nozzle: produce thrust

What are the two basic types of turboprop engines?

Fixed and free turbine

How are most turbine blades constructed?

Forged or cast

What are the two different exhaust nozzle designs used with the turbofan engine?

In the convergent-divergent, or C-D nozzle, the convergent section is designed to handle the gases while they remain subsonic, and to deliver the gases to the throat of the nozzle just as they attain sonic velocity. The divergent section handles the gases, further increasing their velocity, after they emerge from the throat and become supersonic. As the gas flows from the throat of the nozzle, it becomes supersonic (Mach 1 and above) and then passes into the divergent section of the nozzle.

What is the primary function for the compressor section?

Its primary function is to supply air in sufficient quantity to satisfy the requirements of the combustion burners. Specifically, to fulfill its purpose, the compressor must increase the pressure of the mass of air received from the air inlet duct, and then discharge it to the burners in the quantity and at the pressures required.

How many stages can an axial flow compressor have?

Most engines utilize up to 16 stages and more.

What is the primary function of the combustion section?

Provide the means for proper mixing of the fuel and air to assure good combustion, • Burn this mixture efficiently,• Cool the hot combustion products to a temperature that the turbine inlet guide vanes/blades can withstand under operating conditions, and • Deliver the hot gases to the turbine section.

How are the gears & supporting bearing lubricated in the accessory gear box?

The accessory case is also provided with adequate tubing or cored passages for spraying lubricating oil on the gear train and supporting bearings.

What supports the accessory drives?

The accessory drives are supported by ball bearings assembled in the mounting pad bores of the accessory case.

What determines the bypass ratio of a turbine engine?

The amount of air that is bypassed around the core of the engine determines the bypass ratio

What is the most common method for retaining turbine rotor blades?

The blades are retained in their respective grooves by a variety of methods, the more common of which are peening, welding, lock tabs, and riveting

What does the centrifugal flow compressor consist of?

The centrifugal-flow compressor achieves its purpose by picking up the entering air and accelerating it outwardly by centrifugal action.

What is the diffuser, it's purpose and where is it located in the gas path?

The diffuser is the divergent section of the engine after the compressor and before the combustion section. It has the all-important function of reducing high-velocity compressor discharge air to increased pressure at a slower velocity. This prepares the air for entry into the flame burning area of the combustion section at a lower velocity so that the flame of combustion can burn continuously. If the air passed through the flame area at a high velocity, it could extinguish the flame.

What is the difference between the fixed turbine and the free turbine?

The fixed turbine has a mechanical connection from the gas generator (gas-turbine engine) to the reduction gear box and propeller. The free turbine has only an air link from gas generator to the power turbines.

What is the purpose of the stator vanes in an axial flow compressor?

The function of the stator vanes is to receive air from the air inlet duct or from each preceding stage and increase the pressure of the air and deliver it to the next stage at the correct velocity and pressure.

What drives the gears in the accessory gear box?

The gear train is driven by the engine high-pressure compressor through an accessory drive shaft (tower shaft) gear coupling, which splines with a gear box gear and the high-pressure compressor.

Where is the combustion section located in the gas path and what it's purpose?

The location of the combustion section is directly between the compressor and the turbine sections. The combustion section houses the combustion process, which raises the temperature of the air passing through the engine. This process releases energy contained in the air/ fuel mixture.

What is the main difference between the variable inlet guide vanes & variable stator vanes?

The main difference between variable inlet guide vane (VIGV) and a variable stator vane (VSV) is their position with regard to the rotor blades. VIGV are in front of the rotor blades, and VSV are behind the rotor blades.

What is the most satisfactory method utilized for attaching the turbine rotor blades?

The most satisfactory method utilizes the fir-tree design

What is the primary function of the accessory section?

The primary function is to provide space for the gear train is driven by the engine high-pressure compressor through an accessory drive shaft (tower shaft) gear coupling, which splines with a gear box gear and the high-pressure compressor.

What is the principal difference between the two types of impellers?

The principal differences between the two types of impellers are size and ducting arrangement. The double-entry type has a smaller diameter but is usually operated at a higher rotational speed to assure sufficient airflow. The single-entry impeller permits convenient ducting directly to the impeller eye (inducer vanes) as opposed to the more complicated ducting necessary to reach the rear side of the double-entry type.

What is the purpose of the rotor blades in an axial flow compressor?

The rotor, turning at high speed, takes in air at the compressor inlet and impels it through a series of stages. From inlet to exit, the air flows along an axial path and is compressed at a ratio of approximately 1.25:1 per stage

What is the sole purpose of the gas generator?

The sole purpose of the gas generator turbine is to absorb approximately 60 to 70 percent of the total pressure energy from the exhaust gases.

What is a turbine rotor referred to without blades?

The turbine disk is referred to as such without blades

Essentially what two elements does the turbine section consist of?

The turbine inlet guide vanes and turbine disk

Where is the highest heat to metal contact in a turbine engine?

The turbine inlet nozzle vanes are located directly aft of the combustion chambers and immediately forward of the turbine wheel. This is the highest or hottest temperature that comes in contact with metal components in the engine.

How is the turbine shaft attached to the compressor rotor hub?

The turbine shaft must have some means for attachment to the compressor rotor hub. This is usually accomplished by a spline cut on the forward end of the shaft.

The turbine section transforms kinetic energy of the exhaust gases into what type of energy?

The turbine transforms a portion of the kinetic (velocity) energy of the exhaust gases into mechanical energy to drive the gas generator compressor and accessories

How many igniters would a turbofan engine have?

There are usually two igniters mounted on the boss provided on each of the chamber housings. The igniters must be long enough to protrude from the housing into the combustion chamber.

What is the purpose for blade profiling?

These profiles prevent serious damage to the blade or housing should the blades contact the compressor housing.

Why are turbine blades shrouded?

This improves efficiency and vibration characteristics and permits lighter stage weights.

What is the general principle of the fan engine?

To convert more of the fuel energy into pressure

When a turbine rotor has blades installed, what is it referred to as?

Turbine Disk

Which turbine engine has more than one rotor shaft?

Turbofan Engine

Which turbine engine is used by most of the airlines?

Turbofan Engine

What type of turbine engine uses an inlet screen, and what is the screed used for?

Turboprops and turboshafts can use an inlet screen to help filter out ice or debris from entering the engine

What are the two types of fuel nozzles?

Two types of fuel nozzle currently being used in the various types of combustion chambers are the simplex nozzle and the duplex nozzle.

What are the two main elements of the axial flow compressor?

a rotor and a stator

What is the turbine engine inlet type dictated by?

by the type of gas turbine engine

Where are the flame propagation tubes located and what is there purpose?

flame propagation tubes, which is a necessary part of the can-type combustion chambers must have some way to spread combustion during the initial starting operation. This is accomplished by interconnecting all the chambers. As the flame is started by the spark igniter plugs in two of the lower chambers, it passes through the tubes and ignites the combustible mixture in the adjacent chamber and continues until all the chambers are burning.

What is the impeller usually made from?

forged aluminum alloy, heat treated, machined, and smoothed for minimum flow restriction and turbulence.

What is the major problem of the turbojet?

fuel consumption and noise

What does the inlet design contribute materially to?

increasing the ratio of compressor discharge pressure to duct inlet pressure.

What is used to straighten the air flow entering the turbine engine?

inlet guide vanes are used to straighten the air flow, then they also have anti-icing air flowing through them. The inlet also contains some sound-reducing materials that absorb the fan noise and make the engine quieter.

What are turboshaft engines used for?

is a gas turbine engine made to transfer horsepower to a shaft that turns a helicopter transmission or is an onboard auxiliary power unit (APU).

Why are plenum chambers used for the double in the double entry compressor engines?

necessary for a double-entry compressor because the air must enter the engine at almost right angles to the engine axis. Therefore, in order to give a positive flow, the air must surround the engine compressor at a positive pressure before entering the compressor.

How do we determine the compressor ratio?

outlet pressure divided by the inlet pressure

What are the significant features of the gas turbine engine?

separate sections are devoted to each function, and all functions are performed simultaneously without interruption.

What is the purpose for the second set of stator vanes at the outlet of the compressor?

the purpose of the stator vanes at the outlet end of the compressor is to straighten the airflow and eliminate turbulence

Which is first in the gas pass of an axial flow compressor, the rotor or stator and why?

the rotor because it increases the absolute velocity of the fluid and then the stator converts this into pressure rise.

What are the two types of compressor rotors?

the single rotor/spool and the dual rotor/spool, sometimes referred to as solid spool and split spool (two spool, dual spool).

The turbine assembly consist of what two basic elements?

turbine inlet guide vanes and turbine disk.

What is the difference between the turboprop and the turbofan engine?

turboprop is a torque producing and turbofan is a thrust producing engine.

List four disadvantages of the axial flow compressor?

• Good efficiencies over only narrow rotational speed range, • Difficulty of manufacture and high cost, • Relatively high weight, and • High starting power requirements (partially overcome by split compressors).

List five advantages of the centrifugal flow compressor:

• High pressure rise per stage, • Efficiency over wide rotational speed range, • Simplicity of manufacture and low cost, • Low weight, and • Low starting power requirements.

List two disadvantages of the centrifugal flow compressor?

• Its large frontal area for a given airflow and • Losses in turns between stages.