Mechanical design of bypass casing for Aero Gas Turbine

K. S. Shivakumar Aradhya, M. Rudra Goud, S. K. Patel, N. Leela and K. Ramachandra

GTRE, Bangalore 560093, India

 

ABSTRACT

 

Bypass casing, situated between the intermediate and load casing houses the core engine of the gas turbine and forms the skin of the engine body. This paper presents the details of the design and development of bypass casing for a military aircraft engine with a low bypass ratio. The different phases of design and development engineering design, design analysis and production are presented in detail.  The engineering design consists of two major steps: aerodynamic design and mechanical design. The different aspects associated with these are discussed in brief.  The design analysis includes stress, vibration and instability analysis and lifing details. Forming route was selected for the production of the casing.  The details of forming, machining and final inspections are presented in brief.

 

INTRODUCTION

 

Bypass casing is a cylindrical shell shaped structure situated between the intermediate frame and load ring in a gas turbine. It houses the core engine of the gas turbine, which comprises of High Pressure Compressor unit (HPC), Combustion chamber and High Pressure Turbine (HPT), and, forms the skin of the engine body. This paper presents details of the design and development of bypass casing for a light combat military aircraft engine with a low bypass ratio. The design and development route for the casing is depicted in the flow chart shown in Fig.1. It comprises of three major steps:

 

                     I.      Engineering Design

                   II.      Design Analysis and

                  III.      Production

 

 

 

I ENGINEERING DESIGN

 

The engineering design consists of two stages:

1.    Aero-dynamic Design and 

2.      Mechanical Design

 

Following sections present brief details of the two:

 

1.      Aerodynamic Design:

 

The aerodynamic analysis and design decide the major dimensions of the bypass casing namely the mean diameter and the axial length. The mean diameter of the casing depends on the bypass ratio chosen and subsequent mass flow through it. The axial length is decided by the length of the core engine.  The bypass ratio in the present design is chosen as 16.0 % which provides adequate mass flow to meet the required aerodynamic performance of the engine cycle.

 

2. Mechanical Design:

 

The mechanical design is mainly based on strength and stiffness requirements of the casing under different operating/flight conditions. The design also makes provision for the accommodation of different units of the core engine, namely High Pressure Compressor, Combustion chamber and High Pressure Turbine. 

 

The titanium alloy Ti-64 is used as the casing material in the present design.   With this material a thickness of 1.2 mm was arrived at, at the end of mechanical design.

 

II DESIGN ANALYSIS

 

The design analysis consists of the following stages:

 

1.      Heat transfer analysis

2.    Stress analysis

3.    Vibration studies

4.    Instability analysis

5.       Fatigue-fracture analysis and lifing

 

Finite element technique was extensively used in the design analysis. The front-end commercial software MSC-NASTRAN [1] along with the pre-processor MSC-XL [2] was used.

 

1.      Heat transfer analysis:

 

The temperature distribution in the casing was obtained through a rigorous heat transfer analysis. The finite element model used in the analysis is shown in Fig. 2.

 

 

 

 

Following boundary conditions were used in the analysis:

 

a.       On the interior side the temperature of the hot compressed air was considered.

b.      On the exterior ambient temperature & pressure conditions were used.

 

2.      Stress Analysis:

 

A detailed stress analysis was carried out to evaluate the static strength of the casing under critical operating/flight conditions considered over different mission cycles. The working stresses as well as stress distributions were checked for safety on the basis of “maximum shear” and “von Mises stress theory.”  The following loads were considered in the analysis:

 

Air pressure distribution along the length of the casing (The average differential pressure across the thickness of casing is 0.3167 MPa which is obtained through a rigorous CFD analysis). 

 

Thermal loads due to temperature gradients in the casing (The temperature distribution obtained in the heat transfer analysis is used to generate the thermal loads).

 

Inertial loads:

 

The table below gives the range of g-values considered under different load & flight conditions for different mission cycles.

 

Table 1: Ranges of g-values under different load & flight conditions

 

Range of gx

Range of gy

Range of gz

Backward

Forward

Left

Right

Down

Up

-3.82

2.00

-3.42

3.42

-4.77

8.5

 

A three-dimensional finite element model of the by-pass casing is shown in Fig. 3a. The FE mesh was generated using 4-noded shell elements with No. of elements = 1448, No. of nodes = 1516 & Total No. of degrees of freedom = 8296. Details of boundary conditions used in the analysis are shown in Fig. 3b.

 

 

Material Properties:

 

In the present design the casing is made of the titanium alloy Ti-64. The chemical composition of the material is shown in Table 2 below:

 

Table 2: Chemical Composition of Ti-64 (%tage)

 

 

Al

V

Fe

H2

Balance

5.5-6.75

3.5-4.5

0.30 max

0.0125

Titanium

 

Mechanical properties of Ti-64 at the average operating temperature of 260C are shown in the following table.

 

Table 3: Mechanical Properties of Ti-64 at 260C

 

           Property

           Value

Young’s Modulus (E)

        98.28 Gpa

Poisons Ratio ()

          0.33

Density ()

     4420.00 kg/m3

CTE ()

          9.50 x 10-6/ºC

6

0.2 % Proof Stress (yt)

       558.85 Mpa

Tensile Strength (su)

       669.28 Mpa

Elongation

         17.00 %

Reduction in Area

        38.50 %
 
Results and Discussion:

 

The results of the analysis for the most severe load case are presented in the following sections:

 

(1) Displacements: The peak values of displacements and their locations are presented in Table 4 below:

 

Fig. 4 depicts the resultant displacement distribution for the maximum load case.

 

 

The peak values of displacements are well within allowable limits.

 

Table 4: Displacement results

 

Displacement

Value

(mm)

Location

x-coor

y-coor

z-coor

(X-disp)max

0.6355

1103.0

353.6

198.70

(Y-disp)max

1.1330

1030.0

397.5

-80.75

(Z-disp)max

1.2570

1030.0

30.62

404.40

(R-disp)max

1.3800

1030.0

30.62

404.20

 

(2) Stresses: Figures 5 and 6 represent the principal shear stress(t23) and von Mises stress respectively. The corresponding peak stress values are shown in Table 5 below:

 

 

 

 

 

Table 5:  Peak stress values for critical load condition

         

Stress

Value

(MPa)

Location

x-coor

y-coor

z-coor

[t23]max

134.70

1030.0

30.62

404.2

[sv]max

233.40

1030.0

30.62

404.4

 

Table 5 shows that the stresses are well within design limits and hence the design is safe.

 

3.       Buckling analysis:

 

An instability analysis was carried out to evaluate the buckling load (crippling load). Fixed-free boundary condition was used in the analysis. The normal mode displacement pattern obtained at the end of the analysis is shown in Fig. 7.  The buckling load was found to be 1,12,000 N, which is well within the axial g-load acting on the casing.            

 

 

 

4.      Vibration Studies:

 

An eigen value analysis was carried out to evaluate the natural frequencies and mode shapes of the casing. While applying the boundary conditions the intermediate frame side of the casing   was fixed and the load ring side was radially constrained. Accelerated subspace iteration scheme was used in the FEM computation. Table 6 gives the values of the first ten natural frequencies.  The mode shape corresponding to the natural frequency 36.93 Hz is shown in Fig. 8.

 

Table 6: Natural frequencies of the casing

 

Mode.

No

Frequency

(Hz)

1

36.93

2

96.58

3

105.62

4

121.07

5

125.93

6

126.07

7

128.74

8

133.78

9

134.85

10

143.53

 

 

The natural frequencies are much below the forcing frequencies envisaged during engine operation under different flight conditions.

 

5.      Fatigue-fracture analysis and lifing:

 

Since the peak values of stresses and displacements are well within elastic limits (considering all severe load and flight conditions) the casing does not enter into low-cycle fatigue regime. The high cycle fatigue life was estimated considering the stress and load histories in a typical mission cycle, which lasts for about 60 minutes. The life was found to be more then 2x106 cycles, which meets the MIL standard requirement.

 

The casing is not creep critical since its operating tem-perature is below 300°C and stress magnitudes are low.

 

III  PRODUCTION

 

Forming and machining route was selected in the manufacture of the casing. The casing consists of two parts (Figures 9 & 10).  A tapering section of length 737.00mm on the left and a cylindrical section of length 377.00 mm on the right. The tapering portion of the casing was made of two halves (150° and 210°sectors each) and bolted along  the  common edges. The cylindrical section is formed from a single sheet and welded at the common edge. The tapering and cylindrical sections are bolted together (Fig. 11).

 

CONCLUSIONS

 

This paper presents the details of the design and development of bypass casing for a military aircraft engine with a low bypass ratio. The different phases of design and development -- engineering design, design analysis and production, and, the problems faced at different stages of design and the solutions arrived at are presented.

 

 

 

 

The present design has led to a bypass casing with an approximate length 1.1 m, and an average wall thick-ness of 1.2 mm. This design meets the required strength and life requirements and further provides additional stiffness to hold the core engine intact without allowing it to deform/buckle even under most severe operating/flight conditions.

 

Weight optimization was a major concern in the design, which resulted in a search for newer material option for bypass casing. Polymide matrix composite is found to be one of the promising materials for the casing design.

 

An initial design carried out with PMR-15 is found to give a weight reduction of 30%. A detailed design program is planned to replace the titanium alloy Ti-64 with PMR-15 in the next phase of development.

 

ACKNOWLEDGEMENT

 

The authors thank Shri. V. Sundararajan, Director, Gas Turbine Research Estd., for having given permission to publish this paper.

 

References

 

1)      MSC/NASTRAN Users' Manual, Vers.   67, 1991, MacNeal-Schwendler   Corp., 815, Colorado   Boulevard, Los Angeles, CA  90041-1777, USA

2)      MSC/XL Users' Manual, Vers. 2.0, 1991, MacNeal-Schwendler Corp., 815, Colorado Boulevard, Los Angeles, CA 90041-1777, USA

3)      American Military Standard, MIL-E- 005007E(AS)

 

                                                                                                  

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