Reducing the noise level of a turbojet twin-circuit engine with a thrust class of 7 tons using the chevron nozzle design of the external circuit

The development of aircraft engines of a high degree of double-circuit is faced with the tasks of noise reduction. The necessary reduction is a target condition for the operation of dual-circuit turbojet engines (turbojet engines) at take-off modes on runways and airport terrain to ensure health safety systems and environmental factors. The analysis of noise sources is based on a detailed study of the nodal systems of compressors, turbines, combustion chambers, external and internal turbofan nozzles. [4]

When designing a device and a system for reducing the noise of a turbofan engine, the constancy of the calculated gas dynamic and dimensional characteristics is taken into account. An example of changing the dimensions of the nozzle is the installation of a silencer, which leads to an increase in the internal resistance of the channels and a drop in thrust. Taking into account the specified characteristics, the author proposed the introduction of the chevron geometry of the external nozzle on the designed turbojet thrust class 7ts with a degree of double-circuit 9. A feature of the chevron geometry is the reduction of the noise of the loss of total pressure due to the difference in velocities, from which a vortex flow is formed, increasing the range of mixing flows.[6]

To confirm the task, the author used modeling tools in the ANSYS CFX software package, which allow solving problems requiring qualitative research in the field of aircraft engine gas dynamics.[1]

The final calculated data necessary for the task used values such as thrust at take-off (R = 7 tons), the nozzle speed of the inner contour (C1 = 302.76 m/s), the nozzle speed of the outer contour (C2 = 232.47 m/s), the diameter of the inner nozzle (D1 = 666 mm), Diameter at the cut of the external nozzle (D1 = 1400 mm) 

At the design stage of turbojet nozzles, an important part of the simulation is the central body, for the construction of which the thickness (f = 1.26 mm) and the position relative to the axes of the engine and nozzles (l = 16.25) are taken into account. The thickness value is necessary for the direction of the flow jet, which is accompanied by a narrowing of the flow part. [7]

The geometry of the nozzles and the central body was constructed in 2D and 3D formulation in the NX CAD system [1]. The chevron «petals» on the outer nozzle are modeled symmetrically to the radius. [6]

The choice of a chevron conical curve along the nozzle section by the author is based on the studied range of patents and aircraft engines [2,3,4,5]:

1) GE 1997 chevron «Chevron exhaust nozzle for gas turbine» (US6360528B1, 1997), USA. NASA.

2) «Nozzle for turbofan aircraft engines» (GB2289921A, 1994), Britain. British Aircraft Engine Company.

3) GE 2006 GEnx engine with chevrons.

4) Rolls-Royce with RR Trent 1000

5) CFM (GE-SNECMA) 2013 engine with chevron LEAP

6) PD-14

An important condition for setting a 3D model is the absence of a pylon and struts of the external circuit of the engine. Design and optimization techniques allow the construction of an incomplete path of the external contour in the take-off and cruising modes of the projected aircraft engine with subsonic design mode. [6,7,8]

The construction of a cylindrical shape around the model and the operation «subtraction» of the entire computational domain were performed. This action gives the actual flow area. However, it is taken into account that the form is solid-state. [1]

The original graphic prototype is exported to the Parasolid format, for subsequent import into the Ansys Meshing software tool, which allows you to build a grid with the final value of the elements: 83444621. The simulation of the undisturbed part of the stream by the Opening boundary condition in the CFX Pre processor was performed. The Entrainment option is activated, that is, free entry and exit through the conditional wall of the calculation model. The boundary conditions are set in the preprocessor. Switching to the CFX Solver tool makes it possible to make the necessary calculation. A forced stop of the calculation is sufficient for a numerical value of 800 iterations. [1]

Along the longitudinal section of the nozzles, a contour of the parameters of velocity, total pressure, and static pressure is displayed. It is important to take into account that the central body is accelerating the flow due to the fact that in the formulation of the problem the nozzle is narrowing. There is a zone of reduced total pressure. In real conditions, the flow during the flow in the narrowing nozzle has a maximum speed after the cut, where further acceleration occurs. After the critical section of the nozzle, further acceleration of the flow occurs.

With the velocity distribution on the inner nozzle slice, the velocity coefficient is approximately 0.99.

The velocity values in the «petals» zone decrease in the range from 302.76 m/s to 197 m/s. In this area, there is a deceleration of the flow, which affects the noise reduction according to the Lighthill formula for sound power, depending on the static density and flow velocity.[4]