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Computer Simulation Solidifies Advanced Design for Gas Turbine Engines

By Chris Connor, Project Engineer, Allison Engine Company, Indianapolis, Indiana
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Computer simulation helped Allison engineers improve the efficiency of a new gas turbine engine by helping to optimize and validate the transition duct design between the compressor, combustor, and turbine. Scroll transitions, shaped like a tuba, were evaluated for industrial gas turbine engines in the 1970s but abandoned because of the difficulty in achieving good temperature, mass and velocity distribution within the complex scroll geometry. Allison revived this concept for its new generation of low NOx power generation turbines and used the latest simulation technology to optimize critical flow parameters. A preprocessor was used to generate a 3D multiple block, structured grid of the complex scroll geometry and computational fluid dynamics (CFD) software modeled flow around the exterior and through the interior of the scroll.

Allison Engine Company designs, manufactures, markets and supports gas turbine engines and components for aviation, marine and industrial applications, with over 140,000 engines produced. Allison's current product line includes: (1) T56/501D large turboprops for aircraft such as the C-130H Hercules, P-3 Orion and E-2 Hawkeye (2) 501K and 570 turboshaft engines for industrial and marine applications (3) Model 250 turboshaft and turboprop engines for a wide range of civil and military aircraft (4) T406 turboshaft for the tiltrotor V-22 Osprey (5) T800 turboshaft for the U.S. Army RAH-66 helicopter (6) the AE 2100 turboprop for the Lockheed C-130J military transport and for high speed regional aircraft such as the SAAB 2000 and IPTN N250 (7) the AE 3007 turbofan for medium to large business jets and jet-powered regional aircraft such as the Embraer RJ135 and RJ145.

In 1995, the U.S. Department of Energy signed an $82.5 million agreement with Allison to develop a new family of turbine engines for electrical power generation that will provide reduced emissions and lower operating costs as well as increased fuel flexibility. Allison is infusing into the Advanced Turbine System (ATS) program many of the technologies that have been developed by the company under a wide array of military and commercial turbine engine programs. Hazel O'Leary, Energy Secretary when the contract was awarded, said: "Implementation of the ATS program will keep U.S. manufacturers on the cutting edge of turbine technology for power generation applications and enforce the nation's economic competitiveness significantly beyond what would otherwise be achieved by industry alone."

A critical factor in the design of ATS industrial engine, and virtually every other industrial gas turbine engine, is the transition duct from the combustor to the turbine. Unless the combustor is positioned within the same diameter and orientation as the turbine itself, it is necessary to redirect the flow of hot gases from the combustor into the turbine inlet. The flow pattern and distribution of these gases as they enter the turbine is a crucial factor in the engine's performance. The temperature, flow and pressure of the gases should be distributed as uniformly as possible across the exit plane of the transition duct where it enters the turbine to achieve maximum power from a given quantity of combustion gas. Another major issue in transition duct design is controlling the flow pattern of the hot gases in order to prevent overheating and premature failure of the transition hardware.

Many existing power generation engines in the industrial class use a bifurcated transition duct shaped like the letter "U". The combustor is connected to the transition at the bottom of the "U" and the two arms of the "U" form a ring connecting to the turbine. The problem with this approach is that the flow angle or the direction of the swirl in the two flow channels is inherently in the opposite direction. This reduces the efficiency of the turbine. Another problem with the bifurcated transition duct approach is that the hot gases coming out of the combustor by necessity impinge on the bifurcated transition duct on the center of the wall opposite the inlet, which can adversely affect the life of the component.

The primary alternative to the bifurcated transition duct is a scroll shape, which looks like a tuba or a snail shell. The inlet of the tuba or head of the snail shell is where hot gases from the combustor enter the device. Gases flow through the helical body of the scroll and are redirected into the turbine. A key advantage of the scroll is that the device inherently provides a constant flow angle at the turbine inlet. Engineers also have the ability to fine-tune the helical transition duct to provide excellent flow, pressure and temperature distributions. But engine manufacturers that have tried to develop scroll transitions ducts have in most cases given up because of difficulties in developing the intricate geometric details of the scroll. To date, this type of transition duct has only been successfully used on very small engines where the geometrical issues are less involved.

Turbine System

Allison engineers felt that the latest generation of computer simulation tools gave them an excellent opportunity to revisit the scroll design on the new ATS. One of the major problems faced by the earlier generation of scroll designers was the time and expense required to build a scroll prototype and the difficulty of making more than just a few experimental measurements. In recent years, CFD simulations have developed to the point that they are able to provide fluid velocity, flow direction, pressure and temperature values throughout the solution domain for time-dependent problems with complex geometries and boundary conditions. By simulating the problem on the computer, the engineer can easily change the geometry of the model and the boundary conditions, such as inlet velocity, flow rate, etc. and view the effect on engine performance.

Turbine System

Allison engineers developed their own FORTRAN code that defined basic geometrical parameters for the scroll. Using this program they developed a concept design in which hot gas from the combustor flows through the scroll to the turbine while cold gas from the compressor flows around the exterior surface of the scroll to the combustor. Before they could even begin to evaluate this concept, however, Allison engineers faced a major challenge. The exterior flowpath around the scroll posed great difficulty for grid generation. The scroll, as it sits in the casing, transitions from the large cross-section of the combustor exit to the smaller cross-section where the scroll wraps back on itself, forming an annulus to meet with the turbine. This transition creates a considerable problem in the important step of subdividing the problem domain into a grid or mesh of cells. The density or number of cells in the grid is very important because a finer mesh increases accuracy but also substantially increases the run time of the analysis. The problem was that a mesh that was fine enough to provide accuracy in the larger diameter area near the combustor would have so many cells in the smaller diameter area that the analysis would take too long to run.

Allison engineers overcame this problem by using a preprocessor called Gridgen from Pointwise, Inc., Fort Worth, Texas ( http://www.pointwise.com), that allowed them to break the scroll's geometry into contiguous sub-domains called blocks. They generated grids on the scroll and casing surfaces, creating faces for each block, letting the software tool fill the volume grids within the blocks. Using separate blocks for regions where the casing volume varied, in combination with arbitrary interface boundaries in the flow solver, the engineers made it possible to maintain the mesh at a nearly uniform density throughout the casing. In this way, they were able to achieve the accuracy that they needed in the large diameter area without burdening the smaller diameter areas with an excess of elements that would slow down solution times.

The starting point for the grid was the set of curves and surfaces describing the scroll and casing contours, which were imported into Gridgen using the IGES neutral and Network file formats. Allison engineers applied grid curves defining the geometry in the areas where they wanted to separate the geometry into blocks. They then defined 2D grids, called domains, on each surface. In cases where the grids did not adhere precisely to the surface, the engineers used a Gridgen feature that automatically projects a grid onto the geometry to ensure design accuracy. Once the domains were in place, the blocks were created by meshing their volumes. After viewing the resulting mesh, engineers redistributed cells in a few cases in order to insure sufficient accuracy in areas where they expected high gradients.

Turbine System

The irregularity of the scroll geometry meant that the initial grid had areas of negative volume that would have made it impossible to analyze. With a conventional grid generator, Allison engineers would have been forced to modify the grid cell by cell to improve its quality, a process that would have taken weeks. Fortunately, Gridgen provides an elliptic smoother that allowed engineers to improve the quality of the mesh by automatically applying elliptic partial differential equation methods. Engineers applied smoothness, clustering and orthogonality controls to improve the mesh. With each iteration of mesh improvement, the software provided a color-coded graphical display of negative and skewed volume cells. In only about an hour, they had refined the grid, producing an excellent quality mesh ready for computational analysis.

The first analysis of the scroll interior showed that the design produced by the in-house FORTRAN program was quite good in terms of flow velocity, direction of flow, mass distribution and pressure distribution. The analysis of the exterior flowfield highlighted some unevenness that was fixed by making adjustments to the geometry. Gridgen streamlines this process by regenerating the volume grid whenever changes are made to the geometry or grid that would affect that block. The CFD results were used as boundary conditions for a heat transfer analysis that defined local cooling flux requirements for the scroll transition. The resulting design is clearly superior to the conventional approach and has been approved for use on the ATS.