CFD analysis on swirl angle effect in gas turbine combustion chamber

Swirl vanes are used for imparting swirling motion to the flow. These swirl vanes with its vanes induce the air circulation in order to create a turbulence at the primary zone of the combustion chamber. A doublet swirl vanes influence air flow in both axial and radial direction. The CFD analysis has been performed in a doublet swirl vanes under a subsonic speed. The objective of this paper is to increase the vortices around the swirl vanes by changing the area, angle of swirl vanes and increasing the swirl number. Further, the primary zone length and dimension has been observed to find the extend of vortex core and turbulence intensity growth. Several parametric studies have been made to find the best simulation results among various turbulence models like k-ε and SST Reynolds stress model. Major parametric comparisons like pressure, velocity, and TKE changes have been studied. Finally, through the analysis results, it has been evident that by changing the swirl vane angle and swirl number the recirculation zone's length and vortices has been increased and improves the mixing characteristic


Introduction.
The jet engine combustion chamber is the part of a jet propulsion system which mainly determines the efficiency and reliability of operation of the engine. The enhanced aerodynamic mixing process taking place inside the combustion chamber due to swirler are complex. The construction of the combustion chamber of a jet propulsion engine is an extremely difficult task due to the nature of smaller diameter and in length. The better progress of solution to this problem requires an understanding of the mixing processes taking place in a combustion chamber. The combustion chamber has to work within the conventional system of a jet propulsion system. This superimposes upon the combustion chamber design efficiency and upon the system of the process of combustion in it, a succession of superior mixing characteristics. As to achieve this, a combustion chamber should be designed with a minimum dimension in a such a way to achieve greater efficiency in mixing the products with compressed air and finely atomized fuel. The swirler used in the combustion chamber, prior to primary zone enhances the turbulence which eventually results in good combustion rate. In order to revamp with minimum dimensions with good mixing characteristic, the concept of doublet swirler with angle effect is studied and analysed in this paper with using CFD at various turbulence models.

Literature survey
There are several authors who have studied about the swirl vanes in the combustion chamber. But the multiple inlet swirler or the Doublet has been discussed by Prashant Singh et .al. 21 . He mainly focused on the study of multiple inlet swirler in which the flow is divided into multiple streams. Hence the performance can be improved efficiently. Multiple inlet swirlers results in complete combustion thus maintaining emissions and pollution. Further, Multiple inlet swirlers divides the flow into two streams one stream is guided through the conventional path while the other stream is guided through multi tangential vanes. By referring various concepts related to multiple inlet swirler, he found that for same inlet mass flow, the swirl number can be varied in fixed swirl configuration. A. Gusseabet .al. 1 performed the reactive flow analysis of gas turbine combustion system in his paper. Also, the comparison between 3D / 2D RSM and SST Model is done. The results obtained shows that 3D URANS RSM model provides more confined vortex cone, high velocity, overestimates the negative circumferential velocity by approximately 5% and also captures the intensity of flow better than the 3D URANS SST model. The main difference between 3D URANS RSM and SST k-ω is the circumferential velocity profile. Khairul Fikri Tamrinaet .al. 10 explained that the swirl flows are generally turbulent in nature. CFD analysis is carried in the 2D axisymmetric channel. The results produced in both second order and third order equations are almost similar. Both two equation k-ε model and five equation Reynolds Stress model are used. The physical model used consists of burner geometry divided into two parts -Swirl generator and Combustion chamber. Swirl generator produces two coaxial jetsaxial and annular swirling jet. The main difference between k-ε model and RSM model differs in the ability to predict turbulence level. P.Muthukumar et.al. 21 studied the flow through axial swirler. It is important that the design of swirler is required to enhance the mixing characteristic and to minimize the NOx emissions. In this swirl vanes of four different angles 30 0 , 40 0 , 50 0 and 60 0 were analyzed and the results were compared. The swirl vanes were analyzed in two different turbulence models of Reynolds Stress Model (RSM) and k-ε. Straight vanes are easy to manufacture but Curved vanes produce a better aerodynamic performance. The analysis results show that as Reynolds number increases axial velocity increases, turbulence intensity increases and inlet Kinetic energy also increases.

Methodology
The basic geometry design of the Doublet swirler has been designed using tool CATIA. Further, the geometry is meshed using the tool ANSYS-ICEM CFD and Aerodynamic analyze is carried out in the simulation software called ANSYS-CFX. Finally, the results are extracted from post-processing tool.

Geometry details
The doublet swirler consists of a central hub of diameter 40 mm with a total length of 104 mm. A total of eight axial vanes surrounds this hub inscribed in a circle of diameter 102 mm. The axial vanes start at a distance of 80 mm from one end of the hub. Each axial vane starts from the origin and extends to a length of 51 mm and thickness of 4 mm. The depth of each axial vane is 20 mm. These axial vanes are covered by a cylindrical tube of outer diameter of 104 mm and thickness of 1mm. This outer cylindrical tube extends to a length of 80 mm above the axial vanes a circular layer of outer diameter 160 mm and inner diameter 102 mm is placed whose depth is about 2 mm. The tangential vanes of the doublet swirler are placed above this circular layer. The number of tangential vanes present is also eight. Each tangential vane is inclined to an angle of about 75 0 . The length of a tangential vane is about 45 mm and thickness is 4 mm. The depth of each tangential vane is about 20 mm above the tangential vanes a circular layer is placed with the same dimensions as the previous layer. Similarly, the Doublet is twisted with an angle of 54°. The twisted model is also designed with same parts and same dimensions. But both the axial vanes and tangential vanes are twisted by inclining the trailing edge of the vanes to an angle of about 54 0 .

Grid Details
ANSYS Meshing is a discretization tool. Grid independence study has been carried out and meshing has been made to the doublet with nodes and element as 460662 and 2614729 for 90°, 461169 and 2616519 for 54° respectively. The mesh quality parameter skewness and orthogonality are kept at optimal level as per standards.

Turbulence Models
The four different turbulence models have been analyzed for the Standard Doublet and the Twisted Doublet. They are the k-ε standard, k-ε RNG, BSL Reynolds stress and SSG Reynolds stress. The k-ε model is studied to predict free-shear layer flows with considerably small pressure gradients.

Boundary Conditions
The analysis of both the models is carried out in CFX solver. Boundary conditions for inlet, outlet and wall is set as follows: Inlet is set with an average combustion chamber velocity of 60m/s. Wall is operated as a No slip wall. The outlet is adjusted to Average static pressure and zero Pascal pressure. Double precision solvers are used with High order resolution.

Results and discussion
The doublet swirler with standardized and twisted configurations is designed and simulated in ANSYS, post-processing has been made in order to compare the best performance among the two swirler design. The significant factor of a swirl vane is Turbulence that is analyzed and compared to four different turbulent models. These models show the Formation of the Recirculation zone around the combustion chamber. It is evident that the BSL RSM Turbulence model produces the highest number of turbulent recirculation zone. Such streamlines are simulated with standardized and twisted doublet. Further, the turbulent kinetic energy, velocity, pressure parameters are correlated and the results are sketched in the CFX graph in order to know the finest turbulent model.        Here, 7 asymmetric recirculation zones are formed and that is observe in figure 9 at length about 50 mm, 60 mm, 85 mm, 120 mm, 150 mm, 280 mm and 400 mm from the swirler.

Conclusion
The evident result obtained from the CFD Analysis on a different angle of swirl vane with various Turbulence models. That, the result predicted by BSL RSM Turbulence model agrees best with the other turbulence models with the comparison of parameter velocity, pressure, and TKE. Adding to that, the value of pressure drop is greater in angle 54 0 model compared to angle 90 0 model. The velocity value of angle 54 0 modelhas a large velocity drop than angle 90 0 model. The highest value of TKE for both angle 54 0 and angle 90 0 model is almost same. Ultimately the angle 54 0 swirler model is more efficient in mixing characteristic and better than angle 90 0 model.