Project SECRET - Bi Directional System Coupling

Bi-Directional System Coupling for Conjugate Heat Transfer and Variable Leakage Gap CFD Analysis of Twin-Screw Compressors


Oil-free twin-screw compressors are essential in various industrial applications where clean compressed gas is required. Due to the absence of the cooling oil in these machines, thermal deformations are large. Hence, design clearances are generally set at a relatively large value of more than 150 μm. Leakage through these clearances are the primary source of flow loss. It is essential to predict the change of the gap size in operation accurately so that the design clearances can be minimised, allowing reliable operation and maximising compressor efficiency. To achieve this, CFD and Structural solvers were combined. The CFD model uses a single domain deforming grid of the twin-screw rotors generated in SCORG grid generator, together with ANSYS CFX flow solver. The thermal model of the rotors and housing uses ANSYS Structural solver. Two modelling systems were coupled bi-directionally to obtain variation in the radial leakage gap size for calculation of performance in the CFD model. The predicted compressor performance thus obtained was compared with measurements of flow, internal pressure-rise, power, specific power, volumetric and adiabatic efficiency. For the test case, three variations of radial gap size were evaluated, two of them with the uniform gap size of 10 μm and 160 μm and the third one with a variable gap size as predicted by the bi-directionally coupled model. The coupled model predicted this gap size to vary from 24 to 117 μm, thus predicting an improved flow and volumetric efficiency by 8.2%, lower indicated power by 2.5% and a higher adiabatic efficiency by 5.5%, in comparison to the design gap size of 160 μm. These predicted gap sizes could be used to improve the design clearances of the compressor by reducing them from 160 to 120 μm which would result in a better performance during operation.



As shown in Figure 2, the bi-directional system is comprised of five analytical models, which are coupled and managed via the ANSYS Workbench platform. Each model is identified by an alphabetical letter and its elements are numbered as cells. For example, C5 describes the setup cell of the structural analysis model. The compressor design data, the rotor CAD model and the grids required for the CFD model are generated externally by SCORG. The system coupling procedure is an iterative process that starts with an initial CFD solution to get an estimate of the temperature field. In the current work, the initial CFD solution was obtained with an assumed radial leakage gap of 10 μm. This is derived from a combined flow and CHT analysis with uniform leakage gap sizes. However, the component E estimates the leakage gap size, based on the deformation data received from component C during the simulation process. ANSYS CFX solver with a Finite Volume Method (FVM) is used for both models. Once the initial flow field and temperature distribution are known, the housing and rotor surface temperature data are transferred as boundary conditions to the component B which defines the thermal analysis model. The thermal analysis model uses a Finite Element Method (FEM) to solve temperature distribution in the metal components and the results are transferred as body loads to the transient structural analysis model in the component C. Components C and E are connected via the system coupling component D. In the current work, the housing deformation has been accounted for only in components C and E. For simplification, the rotor deformation is not considered. Accordingly, in order to account for rotor deformation, the leakage gap size is set at a lower value of 30 μm in the model and is updated by the housing deformation data as the simulation progresses.

Figure 2

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