Integrated Design and Analysis of Turbofan Engines

[:en]High bypass ratio (BPR) fans are of heightened interest in the area of civil air vehicle propulsion. It increases the air inhaling and improves both the thrust and the propulsive efficiency. The specific fuel consumption is also reduced in today’s turbofan engines.

The inlet fan designs and optimizations are very important as the fan can be subjected to different inlet conditions. As a matter of fact, a modern high bypass fan system provides over 85% of the engine’s net thrust. Hence, a well-designed bypass fan system is crucial for the overall propulsion characteristics of a turbofan engine. A tool which can perform both inverse tasks and direct tasks on bypass fan system is a necessity for turbofan design.

Figure 1 - Turbofan
Figure 1 Meridional Section of the Turbofan Engine
AxSTREAM ® Streamline Solver

The AxSTREAM® streamline solver is a throughflow solver, the specificity of the outcome one should expect from this solver is up the meridional flow field. Hence, when we develop the model, we shall take Acarer and Özkol’s work [2016] as a reference example.

The current AxSTREAM® streamline solver is only available for duct flow with single inlet and single outlet. Hence, the flow splitting effect is not available in the existing solver. Moreover, the splitter and the stagnation effect on the lip of it cannot be captured by current solver either. However, if one can take the philosophy used in Acarer and Özkol’s work and divide the bypass fan system into three zones – fan rotor, bypass and core – which share a common interface at the fan rotor exit. Each zone is a single-inlet-single-outlet configuration. Then it would be feasible to use the AxSTREAM streamline solver to simulate each zone individually.

Figure 2 - Divided Method of Bypass Fan Systen
Figure 2 – Divided Method of Bypass Fan System
Two Critical Calculations

It is very important to determine how the flow at the fan exit will separate into two stream. In terms of modelling, the radius at which the streamline diverges into two, designated as point of fan flow divider (PFD), needs to be obtained so that the radial distributions of quantities at the fan exit can be separated into two sets accordingly, and the resultant two sets of radial distributions are to be fed to the bypass inlet and the core inlet at their inlet boundary conditions. The bypass ratio (BPR) is used were BPR is denoted as χ, and defined as:

χ = m-bypass/ m_core

It is a systematic parameter that greatly influence the overall propulsion system (fan + core engine). Hence, it is usually determined from propulsion characteristics based on required propulsion requirements, such as thrust, propulsive efficiency etc.

In Design mode, the BPR is given as an initial data and mass flow rate is determined for bypass and core. In Analysis mode, the PFDs are given as an initial data so BR and mass flow rates for each stream are determined.

Point of Fan Flow Divider (PFD):
  • – Radius where streamline diverges into two
  • – Radial distributions of quantities at the fan exit separated into two boundary condition sets:
    • – Feeds into bypass inlet
    • – Feeds into core inlet
Baseline Turbofan Engine TF-34

As a baseline engine layout, look at the well-known General Electric turbofan aircraft engine TF-34.

The turbofan stage consists of the rotor and the inseparable stator. The separation of the flow is carried out downstream of the stage.

Figure 3 TF-34 Original Engine Layout
Figure 3 TF-34 Original Engine Layout
Figure 4 Engine Cycle Data
Figure 4 Engine Cycle Data

Parameters for redesign are taken from Figure 4 (highlighted in red).

  • The goal of the re-design: reduce the axial length of the engine which leads to a reduction in engine weight, increased rigidity of shafts and housings.
  • Proposed changes: the rotor is moved along the axis to the position of the stator. The stator is divided into two parts: the Outlet Guide Vane (OGV) is located in the bypass-channel and Inlet Guide Vane is located in the core-channel. The main diameter of the nozzle is reduced, while the outlet area of the nozzle remains the original one.
  • Output: the new flow paths for the fan, bypass and core. Determine the point of fan flow divider (PFD) for a given bypass ratio (BR). Creating a performance map for bypass and core.
Figure 5 Modification of Turbofan Layout
Figure 5 Modification of Turbofan Layout
Figure 6
Figure 6 Turbofan System in AxSTREAM® Projects

 

Boundary Conditions and Calculation Procedure

Boundary Conditions

Boundary Conditions Transfer Schematic

Parameters are averaged in height and transferred from one project to another (from Rotor to OGV + Nozzle and IGV + Frame). The point of separation of two streams is calculated using the radial equilibrium equation.

Figure 7 Transfer Schematic of Parameters
Figure 7 Transfer Schematic of Parameters

For interconnection between the projects, AxSTREAM ION™ software was used which allows data to be transferred between projects.

Design and Analysis Modules in ION Project
  • – Design module is designed to calculate the configuration of the turbofan, determine the PFD depending on the given BR, and find integral parameters such as pressure ratio and efficiency.
  • – Analysis module is designed to calculate the existing configuration of the turbofan, determine the BPR depending on the given PFD, find integral parameters such as pressure ratio and efficiency for:
    • – Inlet Fan: Inlet Bypass;
    • – Inlet Fan: Inlet Core;
    • – Inlet Fan: Outlet Bypass;
    • – Inlet Fan: Outlet Core.
Figure 8 ION Scheme of Design Module
Figure 8 ION Scheme of Design Module
Figure 9 ION Scheme of Analysis Module
Figure 9 ION Scheme of Analysis Module
Figure 10 ION Scheme for Performance Map Creation
Figure 10 ION Scheme for Performance Map Creation

 

Description of Block Scheme
  • – Input initial data – block of initial parameter for setting the fan operation mode. A split point will be found for this mode of operation.
  • – Fan – block of Fan Streamline calculation;
  • – Determination of radius PFD – block of radius finding for flow separation;
  • – Determination of radius PFD – block of finding geometric position of PFD
  • – Interpolation of parameters – block of parameters interpolation for transferred of parameters to Bypass and Core;
  • – Bypass – block of Bypass Streamline calculation;
  • – Core – block of Core Streamline calculation;
  • – Determination of the integral parameters – block of calculation of pressure total ratio and efficiency (From Inlet Fan to Outlet Fan, From Inlet Fan to Outlet Bypass and From Inlet Fan to Outlet Core).
  • – Map – block of Performance map calculation;
Results
Figure 11 Total Pressure Ratio
Figure 11 Total Pressure Ratio
Figure 12 Adiabatic Efficiency
Figure 12 Adiabatic Efficiency
Summary

On the basis of the work done, such software products as AxSTREAM® and ION were used, which will allow for gas-dynamic calculation of the fan and also allowing for the depletion of several projects into a single working scheme which allowed for the calculation of the entire flow path (it allows you to calculate the bypass of a gas turbine engine). A characteristic of the fan was also obtained which allows judging its area of ​​applicability and the gas-dynamic efficiency.

The flow path of the gas turbine engine was also modified. The axial length was reduced, which in turn leads to a decrease in engine mass and increased rigidity of shafts and housings. The stator was shifted to the Bypass channel and to the Core channel. Also, the bypass channel was slightly modified to keep the outlet area.

The main challenge was to find the flow split point. To solve this problem, the Acarer and Özkol’s method was applied.

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