Darsini Kathirgamanathan - Gas Turbine Performance Engineer, OPRA Turbines International B.V.
Thijs Bouten - Principal Combustion Engineer, OPRA Turbines International B.V.
OPRA Turbines develops, manufactures, markets and maintains generator sets using the OP16 series of gas turbines, which is rated at 1.85 MWe. Depending on the application and requirements, different combustion systems can be mounted on the OP16 gas turbine. The gas turbine generator set comes in a containerized package (shown in Figure 1) that includes the OP16 gas turbine, fuel systems, generator, control system, air intake and ventilation system.
Figure 1: OPRA’s OP16 gas turbine generator set
CFD simulations are an integral part of OPRA’s design process. Simulations are used to analyze and optimize flow within various areas, including combustion, turbomachinery and gas dispersion analysis. OPRA has been exploring the use of the open source software package OpenFOAM as a complement to commercial CFD software packages. A good mesh is essential for performing a CFD analysis and OpenFOAM benefits from hex-dominant meshes. Various open source meshing tools are available for OpenFOAM, which work generally fine for simple geometries. However, these meshing tools are hard to use for real (industrial) geometries because of limited geometry import, non-intuitive operation and limited documentation.
"AutoMesh™ tools are OPRA's preferred solution because of the full -hex and hex-hybrid meshes, automatic meshing and direct export to OpenFOAM."
NUMECA's AutoMesh™ has been implemented in OPRA’s design and CFD methodology, as shown schematically in Figure 2. These meshing tools are a preferred solution for OPRA because of the full-hex and hex-hybrid meshes, automatic meshing and direct export to OpenFOAM. HEXPRESS™/Hybrid is used for combustion and gas dispersion analysis and AutoGrid5™ for turbomachinery.
Figure 2: CFD simulation process at OPRA
The following case study shows the use of HEXPRESS™/Hybrid mesh for CFD simulation.
OPRA has developed a marine version of its OP16 gen-set for use onboard ships. As part of this project, a gas dispersion analysis of the OP16 marine package has been performed. The OP16 marine gen-set will be operating on a range of fuels with significantly varying gas composition and energy density. Enclosures, including gas systems, should be ensured of explosion protection in case of gas leakages as the enclosure ventilation air and the gas mixture may form LEL (Low Explosion Limit) volume clouds which may ignite in presence of an ignition source. To ensure a non-hazardous atmosphere inside the enclosure, gas detectors need to be placed at the right locations to identify the eventual leakages. As part of this enclosure gas dispersion analysis, a CFD study has been carried out to investigate the ventilation flow behavior inside the package as well as the behavior of the LEL volume clouds due to gas leakages.
Figure 3: Simplified OP16 marine gen-set enclosure
The gas leakages that are considered in the CFD study are very small leakages that can occur at the flanges, fittings or small cracks in the pipeline. To perform the gas dispersion analysis, possible leakage locations in the gas systems are identified and small leakages are simulated at these locations. The CFD simulation is carried out in two steps: firstly, the flow simulation is carried out where the leaked gas flow will mix with the air at specified boundary conditions. Second simulation is carried out to capture the LEL volume clouds. The latter simulation is performed based on the passive scalar approach.
"A good mesh is one of the fundamentals to run the CFD analysis successfully. A complicated geometry is easily meshed by HEXPRESS™/Hybrid and delivers a very fine mesh. As many features are automated, it saves the user a significant amount of time to generate the mesh. Furthermore, any small changes made to the geometry in CAD can be easily modified in HEXPRESS™/Hybrid."
A good mesh is one of the fundamentals to run the CFD analysis successfully. Figure 3 shows the simplified OP16 marine gen-set enclosure geometry that was meshed. The HEXPRESS™/Hybrid meshing tool is used to mesh this marine package. The package has a complicated geometry including thin, small surfaces, small holes, and narrow pipe lines. A complicated geometry like this is easily meshed by HEXPRESS™/Hybrid and it delivers a very fine mesh. As many features are automated in HEXPRESS™/Hybrid, it saves the user a significant amount of time to generate the mesh. Furthermore, any small changes made to the geometry in CAD can be easily modified in HEXPRESS™/Hybrid. This feature helped us to quickly generate the mesh for different leakage sizes. Figure 4 shows the mesh created by HEXPRESS™/Hybrid for one of the gas leakages.
Figure 4: OP16 marine gen-set package mesh created by HEXPRESS™/Hybrid
Figure 5 shows the LEL volume clouds of one of the leakages obtained from the CFD analysis. The LEL volume clouds shown in red represent the 100% LEL volume clouds and the volume clouds shown in green represent the volume clouds that will be detected by the gas detector set point. Based on the volume clouds required for the gas detector set point (shown in green), the locations of the gas detectors are defined. In case of any gas leakages in the enclosure, the gas detectors placed in these locations ensure that the leakages are detected.
Figure 5: LEL volume clouds obtained from CFD simulations
NUMECA's AutoMesh™ significantly reduced our meshing time and improved the mesh quality resulting in better CFD simulations.