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Computer Simulation Solidifies
Advanced Design for Gas Turbine Engines

Computer simulation helped engineers at Allison Engine Company, Indianapolis, IN, 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.

Allison revived the concept of scroll transitions for evaluating its new generation of low NOx power generation turbines.

The design of the company's Advanced Turbine System revives a tuba-shaped scroll transition duct design evaluated (and abandoned) by industry in the 1970s. At that time, one of the major problems engineers faced was the time and expense required to build a scroll prototype and the difficulty of making more than just a few experimental measurements. However, in recent years, CFD simulations have developed to the point where engineers 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 a 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.

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. However, they faced a major challenge. 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. A problem domain mesh 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.

A preprocessor called Gridgen from Pointwise, Inc., Fort Worth, TX, 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, they achieved 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 resulting design is clearly superior to the conventional approach and has been approved for use on the ATS.

--KC

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