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WINDMILLING OF TURBOFAN ENGINE

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42 Leonardo Times JUNE 2014 A turbofan is a modied version of a turbojet engine. Both share the same basic components, but the turbofan has an additional turbine to turn a fan located at the front. This is called a “two-spool” engine. Some of the air from this fan en- ters the engine core for combustion while around 90% of it goes through a duct called the bypass duct. When the fuel supply to the combustion chamber is cut o, then the engine is said to run in the windmilling condition, i.e., the spool rota- tion is only due to the ram pressure ratio. It is an extreme operating condition and is usually avoided. During windmilling, the compressors and turbines work far away from their design points with poor eciencies and the fan operates in a very dierent way as if it was self-windmilling. The objective of this project is to create a model to predict the mass ow rate and the speed of the two spools at the wind- milling condition. This requires the coupling of the 3-D component simulations with the simu- lation tools. The development of these models relies on the use of characteristic elds, which are formulated using the map tting tools or the articial map pa- rameter (β) and focus on the issue of ex- trapolation of these elds, particularly at the fan stages. Testing on the test bench then validates these extrapolation tech- niques. From these models, we study the performance compared to the operation in windmilling in terms of the mass ow rate, dilution and stability range of the combustion chamber. A parametric study will evaluate the area of convergence of the models on the ight envelope of the engine. Later, an analysis will be made on the inuence of the secondary nozzle section and/or on estimating losses in the bearings of windmilling. EXPERIMENTAL SETUP The development is carried on an engine with a high bypass ratio, unmixed ow geared turbofan by Price induction. The engine is used for a two- to four seater private light aircraft for a maximum take- oweight between 1550 and 2550 kg. Modern materials such as composites and light alloys are used to achieve an opti- mized weight. The diameter of the fan is less than fourteen inches and consists of fourteen blades and forty for the Outlet Guide Vanes, which is driven by a single stage low-pressure turbine. The engine core consists of a centrifugal compressor and a single stage high pressure turbine. The engine is provided with instruments to measure the steady temperature and pressure at dierent locations. Tests are carried out at the test facility with minor changes in order to simulate the windmilling conditions at the ground. During operation a exible air tight tube is used to connect the engine with the fan duct in order to reduce vibrations from the fan that would aect the engine. SIMULATION SOFTWARE The simulation uses software called PROOSIS propulsion object oriented sim- ulation software developed by Empresari- os Agrupados for the engine cycle analysis and the CFD code for accurate 3-D com- Calculation of Performance Characteristics of a Turbofan Engine under Windmilling. The turbofan is a type of air breathing jet engine that nds wide use in aircraft propulsion. During the normal operation of a turbofan engine installed in aircraft, the combustor is supplied with fuel, ow to the combustor is cut oand the engine runs under so called Windmilling conditions being driven only by the ram pressure ratio by producing drag. In-depth analysis is done to study the performance characteristics at this state. TEXT Aditya Ramanathan, MSc Student Aerospace Engineering, ISAE Toulouse WINDMILLING OF TURBOFAN ENGINE
Transcript
Page 1: WINDMILLING OF TURBOFAN ENGINE

42 Leonardo Times JUNE 2014

A turbofan is a modified version of a turbojet engine. Both share the same

basic components, but the turbofan has an additional turbine to turn a fan located at the front. This is called a “two-spool” engine. Some of the air from this fan en-ters the engine core for combustion while around 90% of it goes through a duct called the bypass duct. When the fuel supply to the combustion chamber is cut off, then the engine is said to run in the windmilling condition, i.e., the spool rota-tion is only due to the ram pressure ratio. It is an extreme operating condition and is usually avoided. During windmilling, the compressors and turbines work far away from their design points with poor efficiencies and the fan operates in a very different way as if it was self-windmilling. The objective of this project is to create a model to predict the mass flow rate and the speed of the two spools at the wind-milling condition.This requires the coupling of the 3-D component simulations with the simu-lation tools. The development of these

models relies on the use of characteristic fields, which are formulated using the map fitting tools or the artificial map pa-rameter (β) and focus on the issue of ex-trapolation of these fields, particularly at the fan stages. Testing on the test bench then validates these extrapolation tech-niques. From these models, we study the performance compared to the operation in windmilling in terms of the mass flow rate, dilution and stability range of the combustion chamber. A parametric study will evaluate the area of convergence of the models on the flight envelope of the engine. Later, an analysis will be made on the influence of the secondary nozzle section and/or on estimating losses in the bearings of windmilling.

EXPERIMENTAL SETUPThe development is carried on an engine with a high bypass ratio, unmixed flow geared turbofan by Price induction. The engine is used for a two- to four seater private light aircraft for a maximum take-off weight between 1550 and 2550 kg.

Modern materials such as composites and light alloys are used to achieve an opti-mized weight. The diameter of the fan is less than fourteen inches and consists of fourteen blades and forty for the Outlet Guide Vanes, which is driven by a single stage low-pressure turbine. The engine core consists of a centrifugal compressor and a single stage high pressure turbine. The engine is provided with instruments to measure the steady temperature and pressure at different locations.Tests are carried out at the test facility with minor changes in order to simulate the windmilling conditions at the ground. During operation a flexible air tight tube is used to connect the engine with the fan duct in order to reduce vibrations from the fan that would affect the engine.

SIMULATION SOFTWAREThe simulation uses software called PROOSIS propulsion object oriented sim-ulation software developed by Empresari-os Agrupados for the engine cycle analysis and the CFD code for accurate 3-D com-

Calculation of Performance Characteristics of a Turbofan Engine under Windmilling.

The turbofan is a type of air breathing jet engine that finds wide use in aircraft propulsion. During the normal operation of a turbofan engine installed in aircraft, the combustor is supplied with fuel, flow to the combustor is cut off and the engine runs under so called Windmilling conditions being driven only by the ram pressure ratio by producing drag. In-depth analysis is done to study the performance characteristics at this state.

TEXT Aditya Ramanathan, MSc Student Aerospace Engineering, ISAE Toulouse

WINDMILLING OF TURBOFAN ENGINE

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JUNE 2014 Leonardo Times 43

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ponent simulations. It also models both continuous and discrete model systems. It impacts diff erent confi gurations and preliminary dimensioning of equipment, mono-point and multi-point design, parametric studies, sensitivity analyses, customer deck generation, optimization studies, multi-fl uid models, maps han-dling, etc. The entire model is created by linking the diff erent component models in a graphical user friendly interface. There are simple averaging techniques available to handle the 3D-0D-component data ex-change though the boundary conditions of the whole engine model remain the same. While the boundary conditions of the 3-D simulations are automatically fed by the PROOSIS to the CFD software. The fi gure shows the schematic view of the DGEN 380 model in PROOSIS.

PROOSIS primarily performs the simula-tions based on the thermodynamic gas turbine cycles using averaged variables to describe the fl ow properties. The dif-ferent component characteristics are fed through the maps obtained from either the CFD simulation or the test results. The modeling is determined by the thermody-namic and the fl uid functions. The calcu-lation of the performance at the required operating point is done by solving a set of linear mass and power balance equations for all the components using Newton-Raphson method.

COMPONENT MFT MAPS The Map Fitting Tool is a representation based on similarity parameters. These

tools are very accurate and result in much smoother maps. The similarity parameters used are scalars of the effi ciency, mass fl ow and the rotational speed calculated separately for each of the rotating com-ponents. With the help of the steady ex-periment, PROOSIS was able to simulate a large envelop of conditions up to low pressure ratios using the MFT maps in order to run the steady state calculations. The graph in the fi gure shows steady state windmilling i.e. when the fuel is cut off and the pressure ratio drops and reaches a state where Π<1 but when the pressure ratio increases with higher mass fl ow rate the engine operates under normal condi-tion.The CFD simulations are fi rst run for the fan blade alone and large separations were seen near the tip. Adding the Outlet Guide Vane blades and using the mixing technique for the fan and OGV interface had little eff ect on the fl ow across the fan.The turbine produced little work: roughly of the order of 10-15% of the design work. The high pressure compressor operates at a very low value of the pressure ratio and the fan operates with a pressure ratio of less than one with a very high increase in the bypass ratio. All the results sug-gested that the fan is the most important component to simulate with CFD. The CFD simulations were achieved for a rota-tional speed of 20% of the design speed. The fan and OGV behaved properly at the hub whereas separations appear at the top, especially for the OGV. The hub por-tion of OGV appears to be occupied with high Mach numbers whereas the shroud

portion is not much aff ected compared to the hub part. Further, the lower part of the fan blade compresses a little while the up-per part expands the fl ow with an overall aerodynamic load of zero.

CONCLUSION FUTURE WORKThe present work has been dealt with the behavior of the fan stage of a high bypass ratio turbofan engine-out conditions by reproducing windmilling operation in a ground level test bed. The results dem-onstrate the challenges that arise in char-acterizing the fl ow due to the extremely low temperature and pressure variations. Work is in progress to complete the da-tabase with unsteady measurements to characterize the turbulent and unsteady components of the separated fl ow and provide a reference validation test case. Further work is constantly going on in im-proving the PROOSIS model. The extrapo-lation model is also being studied and improved to meet the new challenges. A detailed study of the MFT map methodol-ogy is also being studied for a step by step calculation procedure.

References

[1] W. Braig, H. Schulte et C. Riegler, « Comparative analysis of the windmilling performance of turbojet and turbofan engines »,[2] N. García Rosa, J. Pilet, J.-L. Lecordix, R. Barènes et G. Lavergne, « Experimen-tal analysis of the fl ow through the fan stage of a high-bypass turbofan in wind-milling conditions

Figure 3. Stagnation pressure at the rotor exit

Figure 1. Mach number on Fan (left) and OGV (right) of hub (top) and shroud (bottom) Figure 2. Steady state windmilling

Figure 4. PROOSIS Model

Windmilling of turbofan.indd 43 03-Jul-14 23:24:10


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