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A Presentation on Variable Cycle Engines

By: Praveen Pratap Singh M.Tech- Aircraft Propulsion 15MT1AP006, IIAEM

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Contents

1.Abstract2. Introduction3.VCE Configuration and working. 4. Working cycle of VCE and work calculation 5. Effect of VABI’s on performance parameters 6. Research, Development and Future 7. Revolutionary Turbine Accelerator : Another

Air breathing VCE8. Conclusion 9. References

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Abstract Aircraft performance lies in the complexity of engine design. The need for

supersonic passenger aircraft has led to a compromise between turbofan and turbojet engine.

Turbofan is optimized for subsonic thrust with better fuel efficiency and less noise but performs poor at high speeds. On the other hand turbojet could serve purpose for achieving supersonic speeds but at the cost of higher efficiency and jet noise.

Hence a concept of hybrid engine known as Variable Cycle Engine(VCE) has been formulated to combine best traits of both turbofan and turbojet engine so as to achieve supersonic speed at less specific fuel consumption and noise.

Variable Cycle Engine is a next generation supersonic transport engine that is designed to operate efficiently under mixed flight conditions such as subsonic, transonic and supersonic which attains high specific thrust at supersonic cruise.

In this presentation the objective and features of VCE with examples of already existing engines, variation in parameters with variable geometry are discussed and results are interpreted. Thus, the main aim being to project some light on variable cycle engines, their characteristics and the present study being made and research carried out on the same.

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Gas Turbine Engines High speed air breathing engines used in aircrafts. The forward thrust is produced by accelerating hot gases in backward

direction. In a typical gas turbine engine, this is accomplished as follows.

Entry of high speed air in diffuser (a

diverging section)

Compression of high speed air in compressor (a rotating part)

Burning of fuel air mixture in combustion chamber

Expansion of hot gases in turbine (a

rotating part)

Further expansion of gases up to ambient

pressure of air in nozzle (a converging section) and release of gases at

high speed

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Gas Turbine Engines Gas turbine engines are classified as follows : Turbojet (Conventional ) Ramjet (No rotating part) Turboprop (an additional propeller at front) Turbofan ( Fan instead of propeller) Turboshaft (An additional rotor produces shaft power (Torque)which is

further converted into thrust) Schematic (Ref. 11) of a conventional GT Engine is shown below

Air flow

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Turbojet and Turbofan Engines Turbojet Thrust through core engine only

Low bypass engine (BR=1:1) Operates at supersonic speeds High specific thrust due to high jet

velocity High SFC High operating noise due to high jet

velocity Used in military/fighter aircrafts Less take off thrust Low propulsive efficiency

Turbofan Typically, 80% thrust through air

bypassed by fan, 20% through core engine

High bypass engine (BR= 2-5:1) Operates at subsonic speeds High specific thrust at low speeds. Low SFC Low operating noise

Used in civil/passenger aircrafts High takeoff thrust High propulsive efficiency

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Idea of Supersonic Passenger Aircraft Concorde (in figure, Ref. 12)was developed

jointly by UK and France in 1976 as one of the commercial supersonic aircraft.

It incorporated a turbojet engines and achieved speed of Mach no 2.4

Operation of Concorde was stopped in 2001 due to many reasons.

One of technical reasons being enormous noise produced by high speed jet during take off .

Specific fuel Consumption was very high 22000 litres/hours

Concorde needed to fly at longer distances to be economically viable.

For high speed and optimisation of flight, concord used double delta shaped wings and variable engine intake ramp system controlled by digital computers.

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Variable Cycle Engines The need for supersonic passenger

aircrafts has led to the concept of variable cycle engines .

Most of the aircrafts today use Turbofan Engines.

Turbofan engines are optimised for subsonic thrust with fuel efficiency and produce less noise but they perform poorly at high speed .

Using Turbojet engines for passenger aircrafts could serve the purpose of achieving supersonic speed.

But this speed is achieved at terrible fuel efficiency and high jet noise.

Hence, a concept of a hybrid engine known as variable cycle engine was derived which aims to combine best traits of turbojet and turbofan engine so as to achieve supersonic speed at less specific fuel consumption and less noise.

Variable Cycle Engine

Passenger Aircraft

TurbofanTurbojet

Supersonic

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Variable Cycle Engines Quest for Variable Cycle engines began in late 1980s/early 1990s. GE’s YF120 developed in late1980s was one such engine which was used in

Lockheed YF22 (powered with two YF120) had achieved flight of up to Mach 1.58

These engines are being developed so as to have minimum 25% of fuel saving. Schematic of YF120 is shown in figure. (Ref. 4)

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Objective of VCEs

High specific thrust at low specific fuel consumption at high speed

Low noise (of Range that of turbofan )

Ability to alter mass flow rate

Maintain Overall pressure ratio

High propulsive efficiency

High mechanical efficiency of shaft

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Mass flow rate and Thrust

where, ρ- density of fluid (Kg/s) A- Area ( ) C- Velocity of fluid (m/s) where, m- mass flow rate of air and fuel mixture

(Kg/s) Cj- velocity of jet Ca- Velocity of air (speed of aircraft) Aj- Jet exit area Pj- Pressure at jet exit Pa- Ambient air pressure

From mass flow rate equation, change in area or velocity will change mass flow rate also, and hence a variable area geometry is required.

In thrust equation, if gases are expanded completely up to ambient air pressure, second term of equation will be zero.

Again it can be seen from equation that thrust is directly proportional to jet velocity , increasing or decreasing jet velocity will increase or decrease thrust.

If second term exists, changing jet exit area will change the value of thrust.

Also, thrust is proportional to mass flow rate, and hence changing mass flow rate will change thrust.

Both thrust and mass flow rate are dependent on jet velocity, an important parameter to be considered in VCEs

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Jet Noise and Propulsive Efficiency Noise level of an aircraft is measured

by acoustic power (Ref.2) in subsonic turbulent jet

Kinetic power in jet is proportional to third power of jet velocity hence a “Acoustic Efficiency Parameter “may be defined as

where ,Mj is ratio of Cj and ambient speed of sound .

Therefore to reduce broadband noise of an aircraft exhaust, jet speed needs to be reduced.

Also, reduction in jet speed yields better propulsive efficiency.

It is defined as thrust power to the

sum of thrust power and change in K.E of jet.

It is evident from the equation that lesser the value of jet velocity, greater will be the propulsive efficiency

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SFC and Overall Efficiency

where m(dot)f is mass flow rate of fuel in Kg/hr and F is thrust in N.

where Q is calorific value of fuel

Hence it can be seen that Low SFC = High Efficiency High SFC = Low Efficiency

For low SFC, Thrust produced should be high enough while fuel flow rate should be low.

Thrust is dependent on mass flow rate and jet velocity, hence increasing jet velocity or mass flow rate or jet velocity will increase thrust.

This can be achieved by incorporating variable geometry units in the engine.

Hence evolved the concept the variable cycle engine

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VCE ConfigurationA variable cycle air breathing GT engine includes A Fan and a high pressure compressor A High pressure turbine that drives a pressure compressor A low pressure turbine that drives a fan A combustor and a afterburner Variable Geometry components such as 1. Variable Area Bypass Injector (VABI) 2. Variable Inlet Guide vane 3. Variable Area Nozzle 4. Variable Nozzle Area Turbine There are two VABI’s used in the engine. The first VABI is a selector valve downstream of fan. The second VABI is valve which is located between core driven fan stage and

inlet to HP compressor.

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Working Principle of VCE The schematic of VCE working principle and connection between engine parts are

shown in figure (Ref.5)

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Working Principle of VCE There are two kinds of working mode about the variable cycle engine, turbojet and

turbofan. When engine working in subsonic cruise low power state, the mode conversion valve

open, make more air into the deputy external duct, at the same time the former mixer is wide opened. Open the after mixer, increasing the bypass ratio; reduce fuel consumption, the engine working at the model of turbofan this time.

When engine working in the state of supersonic cruise, acceleration and climbing, the former mixer is smaller, selector valve closed, most of the gas forced into the core engine, produce high thrust, the engine working at the model of turbojet this time

One of the first attempts to achieve this operation was the Variable Pumping Compressor shown in Fig (Ref.3)

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Variable area bypass injector It is used to vary the relative proportions of flow in a mixer. Effectively, it acts like a

variable area mixer. The key function of the rear VABI is to enable control of the fan operating line

independently of the core gas generator system It does this by matching static pressures of the streams entering the mixer by varying

the Mach number in the bypass stream to attain the static pressure balance for mixing the flows.

VABI settings may range from 0.5 to 1.5. In any condition other than the nominal position, a pressure drop must occur across the VABI in order to match static pressures resulting in increased mixing losses associated with the velocity mismatch of the two streams.

VABI position diagram is shown (Ref. 3)

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Variable area nozzle At constant speed a nozzle downstream of the compressor is closed. This increases the pressure ratio in the compressor because the compressor must

now increase the density of the gas to push it through the smaller flow area. In such a way the working line of a compressor can be controlled. For an engine with an afterburner a variable area nozzle becomes a necessity. Due to the increase in temperature in the afterburner, the pressure increases to the

point where compressor stall would occur if the nozzle area where not adjusted.

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Variable Inlet Guide Vane The aim of using VIGVs in a VCE is to

enable transition from one cycle to another Variable inlet guide vanes operate by

varying the inlet axial velocity. As can be seen in the vector diagrams in

Figure (Ref.3), closing the vanes produces a smaller value of axial velocity and thus a smaller mass flow.

Similarly, opening the vanes away from the nominal position produces a larger axial velocity and thus higher mass flow.

By varying the axial velocity through changing the IGV angle, one can effectively vary the mass flow through the component (as, = , where c is axial 𝑚̇ 𝜌𝑐𝐴velocity)

In addition to enabling cycle transition, VIGVs offer better performance at off design, surge margin control, and increasing the overall compressor range of operation.

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Variable Nozzle Area Turbine Variable area turbines (also VABI 3) primarily attempt to “change the speed-speed

relationship of the high and low rotors in order to enable optimal engine efficiency over the entire flight envelope”,

Thus allowing OPR to be controlled as turbine inlet temperature varies to match the fan power demand.

They also allow independent control of high and low rotor speeds and provide increased cycle matching capability.

This variable geometry occurs at the inlet of the turbine and can be attained two ways.

1.The first method involves re-staggering the stator blades and acts in a similar fashion to VIGVs.

2. The second method is mechanically simpler and involves introducing an obstruction into the flow or by introducing secondary airflow

Use of variable geometry in an HP turbine is seen as a large technology risk but use in an LP turbine can lead to modest alteration of bypass ratio, TSFC, and specific thrust at subsonic speeds.

The specific improvement depends largely on the cycle engine design

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Total Pressure Recovery Through Inlet The intake is provided with a variable geometry mechanism such that the throat is

wide open at low subsonic speeds and closes progressively towards the supersonic regime.

The mismatch between intake and compressor is avoided during the supersonic regime by deviating through the bypass duct and the variable geometry bypass nozzle (BN), the excess of mass low captured.

Hence, it is assumed that the intake delivers the exact mass flow requested by the air compressor

The total pressure recovery through the intake TPR is the ratio of outlet to inlet stagnation pressures. For air behaving as perfect gas and provided that the flow is adiabatic, the total pressure recovery is related to the intake kinetic efficiency ηk by: (Ref. 8)

TPR =(1 + 0.5(γ −1)(1−ηk) Ma∞ ^2)^−γ/(γ−1)

where Ma ∞ is the free stream Mach number and γ the ratio of specific heats. The kinetic efficiency is the ratio of kinetic energy of the outlet flow (if expanded to

ambient pressure) to the free stream kinetic energy ηk = v2out/v2 in

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Working cycle of VCE Working cycles for adaptive engines of VCE types are typical cycles of a double

flow engine, where the working area is split between bypass channels, depending on the aircraft flight conditions and operation range of the engine

On the contrary, the working cycle of the ‘bypass’ engine is a typical cycle of a single-flow engine with variable area of the corresponding thermal cycle. The curve shape depends on the amount of air that is bled from downstream of the compressor and fed downstream the turbine.

Real cycle of a turbojet adaptive engine(left) and Real cycle of a turbojet engine of the ‘bypass’ type (right) are shown in fig. (Ref 6)

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Work calculation The work during a full cycle of adaptive engines can be determined for individual

channels with use of the following formulas:

For an adaptive engine that incorporates a mixer of streams, the relationship between the effective work of the external cycle on one side and the ηM mixer efficiency and φM ratio of velocities of air stream to be mixed can be expressed in the following form:

Whilst the corresponding relationship for the engine with an afterburner in the external channel is the following

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Work calculation The work of the full cycle executed by the engine of the ‘bypass’ type can be

expressed in the form that exemplifies the difference between the work during a cycle executed by a turbojet engine without bleeding facilities and the working cycle of the ‘bypass’ engine, i.e.

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VCE Rear VABI and Nozzle Area Effect on Flow and Bypass Ratio Figure (Ref.4) shows how the two front VABI’s

affect the engine bypass ratio W16/W21. While VABI 1 (downstream of the fan) is open,

both the nozzle area (varied in the range from nominal to +10%) and VABI 3 have a certain influence on bypass ratio.

If VABI 1 is closed then the bypass ratio is nearly constant.

The numbers describing the VABI 3 position are in the range from 0.5 to 1.5.

Values lower than 1 indicate that the bypass mixer area A163 is smaller than its nominal value.

VABI 3 = 0.5 mean that the bypass mixer area is reduced to 50% of the design point value.

VABI 3 values bigger than 1 mean that the core mixer area A63 is reduced.

A 50% reduction of A63 is described with VABI 3 = 1.5

The sum of A163 and A63 is always equal to the total mixer area A64.

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VCE Rear VABI and Nozzle Area Effect on SFC and Thrust Figure(Ref.4) shows the SFC for the

various geometry positions. These results are somewhat surprising since the lowest SFC is achieved with the lowest bypass ratio and the highest SFC goes with the highest bypass ratio. This result is affected by the engine rating limiters:

If VABI 1 is open and VABI 2 closed (this VABI setting makes the engine a conventional turbofan), then the engine is limited by the HP spool speed – T4 is much lower than in the other cases.

If VABI 1 is closed and VABI 2 open, then the active limiter is the burner exit temperature and the HP spool speed is much lower than in the other cases

If both VABI 1 and VABI 2 are open then the engine operates with big nozzle and bypass mixer areas at the spool speed limiter, otherwise at the temperature limit.

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VCE Rear VABI and Nozzle Area Effect on Flow and Thrust

One claimed advantage of the VCE is that it can vary thrust without changing mass flow which allows for optimum flow conditions at the aircraft intake during supersonic flight.

One can adapt the mass flow in such a way that the intake operates at the most favourable pressure recovery and without or reduced spillage drag.

From Figure , one can read at constant mass flow (∼60 kg/s) a thrust range of more than 15 to 20%. Low thrust is connected with big bypass mixer and nozzle areas, high thrust with small areas.

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Research, Development & Future Research is being carried out extensively on these types of engines. GE Aviation has

been developing a revolutionary engine that aims to combine the best traits of turbojet and turbofan engines, delivering supersonic speed capability and fuel efficiency in one package.

The new engines are being developed under the USAF ADVENT project, which is seeking 25 percent fuel saving which will in turn lead to an increase in mission capability.

The variable cycle technology used in the YF120 would be extended to not only turn the engine into a Turbojet but also into a Ramjet. In that mode all airflow would bypass the core and be diverted into the afterburner-like "hyper burner" where it would be combusted like a ramjet.

In context of the development of advanced launchers, the Revolutionary Turbine Accelerator (RTA) was developed by NASA and General Electric (GE) in order to allow space access and hypersonic commercial transport.

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RTA: An Another ABVCE The RTA (Ref.7) is a turbine based combined cycle (TBCC) and variable cycle engine

(of GE’s YF120) with a ramjet running on kerosene. Ground tests with a prototype, RTA-1, were performed in 2006

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RTA: An Another ABVCE The RTA has following improvements over the YF120:1. New fan and fan frame,2. New core driven fan stage,3. New rear VABI 4. New hyperburner and slave exhaust ,5. New fuel and thermal management system

the RTA has a core driven fan stage (CDFS) fit with a separation between the tip and hub of the fan.

Using such a fan in combination with variable inlet guiding vanes (VIGV) makes it possible to change the flow path behind the fan separately.

The new fan and frame are necessary considering the RTA reaches a higher bypass ratio (BPR).

Furthermore, the RTA has a larger rear VABI system which reduces mixing losses by allowing a smoother transition of the flow.

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RTA: An Another ABVCE The RTA was designed with the following trajectory

(Ref.7)or space access in mind .The trajectory is divided into the following zones:

1. SLS=M=1.6 :From take-off until Mach 1.6, the RTA performs like a single-bypass augmented turbofan engine, i.e. the first VABI is fully closed and most of the air delivered to the hyperburner originates from the core stream. At take-off the engine delivers a thrust of more than 150 kN.

2. 2 < M = 3 : At Mach 2, the engine begins transition to ramjet mode, with the fuel flow to the core decreasing while the fuel flow to the hyperburner increases. Within this range, the front VABI also reaches its maximum opening position.

3. 3 < M = 3.5 : Between Mach 3 and Mach 3.5 both the rotor speed and the turbine temperature decrease. At Mach 3.5, the core engine is completely wind milling to ensure a quick engine starting, to drive the accessory engine parts and to reduce the mechanical loads on the rotating components exposed to the highest inlet temperatures.

4. M > 3.5 : Beyond Mach 3.5, the engine is in full ramjet mode and at Mach 4 the launch vehicle separates.

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Conclusion1. Variable cycle engine is a very complex power plant, involving fluid mechanics,

structural mechanics, heat transfer, combustion control and many other subjects.2. Complexity in design with variable geometry is a big challenge to deal with.3. Cost of manufacturing and maintenance are both very high. 4. VCE’s are in development phase and research is being carried out to make it cost

effective. 5. Several optimization techniques are being incorporated for modelling engine design

so as to achieve desired objectives.6. Mathematical modelling plays an important role in this research. 7. While there is no doubt that with advent of Variable Cycle Engines supersonic speed

in commercial aircrafts would be achieved however it should have benefits over conventional subsonic commercial aircrafts.

8. Success in VCE technology will even make space access to human easier.

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References 1. HIH Saravanamuttoo, H. Cohen, GFC Rogers. “Gas Turbine Theory , 5th edition (Pearson, 2006)”. 2. Saeed Farukhi. “Aircraft Propulsion, 2nd edition” The University of Kansas, USA. (Wiley 2014). 3. Sean T Ford, “Aerothermodynamic Cycle Design and Optimization method for aircraft engines”,

PhD Thesis. Georgia Institute of Technology , Dec. 2014. 4. Kurzke, Joachim. "The Mission Defines the Cycle: Turbojet, Turbofan and Variable Cycle Engines

for High Speed Propulsion." N.p., n.d. Web. 20` Dec. 2012.5. Zhongzheng You. “Component-level Modelling Technology and Optimization for Variable Cycle

Engine.” American Journal of Mathematics and Statistics, 2013. 6. Mirosáaw Kowalski. “Adaptive Jet Engines Work Analysis And Control”. Journal of KONES

Powertrain and Transport, Vol. 18, No. 2 2011.7. Peter Vyvey1 and Walter Bosschaerts, Victor Fernandez Villace, Guillermo Paniagua. “Study of an

Airbreathing Variable Cycle Engine”. 47th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, 2011.

8. Victor Fernandez-Villace andGuillermoPaniagua. “Numerical Model of a Variable-Combined-Cycle Engine for Dual Subsonic and Supersonic Cruise”. Energies 2013. www.mdpi.com/journal/energies

9. www.wikipedia.org 10.www.nasa.gov 11.https://www.ohio.edu/mechanical/thermo/Intro/Chapt.1_6/gasturbine/turbojet_schema.gif12.http://t.wallpaperweb.org/wallpaper/aircraft/1600x1200/British_Airways_Concord.jpg

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