Three Dimensional Numerical Analysis of Screw Compressor Performance

Modern manufacturing methods enable screw compressors to be constructed to very close tolerances where full 3-D numerical calculation of the heat and fluid flow through them is then required to obtain the maximum possible improvements in their design. An independent stand-alone interface program has been developed by the authors in order to generate a numerical grid for this purpose. The interface employs a procedure to produce rotor profiles and an analytical transfinite interpolation method with adaptive meshing to obtain a fully structured 3-D numerical mesh, which is directly transferable to a CFD code. This was required to overcome problems associated with moving, stretching and sliding rotor domains and with robust calculations in domains with significantly different geometry ranges. Some changes had to be made within the solver functions both to enable calculations and to make them faster. These include a means to maintain constant pressures at the inlet and outlet ports and consideration of two-phase flow resulting from oil injection in the working chamber. Modifications implemented to the CFD procedure improved solutions in complex domains with strong pressure gradients. The pre-processor code and calculating method have been tested on a commercial CFD solver to obtain flow simulations and integral parameter calculations. The results of calculations on an oil injected screw compressor are presented in this paper and compared with experimental results

Grid generation for the solution of partial differential equations

A general survey of grid generators is presented with a concern for understanding why grids are necessary, how they are applied, and how they are generated. After an examination of the need for meshes, the overall applications setting is established with a categorization of the various connectivity patterns. This is split between structured grids and unstructured meshes. Altogether, the categorization establishes the foundation upon which grid generation techniques are developed. The two primary categories are algebraic techniques and partial differential equation techniques. These are each split into basic parts, and accordingly are individually examined in some detail. In the process, the interrelations between the various parts are accented. From the established background in the primary techniques, consideration is shifted to the topic of interactive grid generation and then to adaptive meshes. The setting for adaptivity is established with a suitable means to monitor severe solution behavior. Adaptive grids are considered first and are followed by adaptive triangular meshes. Then the consideration shifts to the temporal coupling between grid generators and PDE-solvers. To conclude, a reflection upon the discussion, herein, is given

Three dimensional numerical analysis for flow prediction in positive displacement screw machines

A substantial proportion of all industrial compressors now produced are of the twin-screw type. These are rotary positive displacement machines, which operate at high efficiency over a wide range of speeds and pressure differences. Currently their performance is estimated by assuming simplified one-dimensional flow through passages with dimensions that are invariant with temperature and pressure. As manufacturing accuracy increases, clearances can be reduced and compressors thereby made smaller and more efficient. However to obtain full advantageo f this at the design stage,it must be possible to estimate accurately internal fluid flow patterns, pressure and temperature distribution and their effects on the working process. An interface has therefore been developed in order to generate a 3-D numerical grid for this purpose. This employs a procedure to produce rotor profiles and an analytical transfinite interpolation method with adaptive meshing to obtain a fully structured 3-D numerical mesh, which is directly transferable to a Computational fluid dynamics (CFD) code. Robust calculations can then be performed while allowing for moving, stretching and sliding between the rotors with large variations in the chamber shape and proportions. Changes in the solver functions have improved convergence and increased the speed of solution. One of these is to include a means to maintain constant pressure within the inlet and outlet ports throughout the calculating procedure. Also, the CFD procedure has been modified to enable fast calculations to be made with real working fluids and to estimate two-phase flow effects due to phase change and oil injection in the working chamber. The interface, pre-processing code and calculating procedures have been used with a commercial CFD solver to estimate the performance of three different compressor applications. The predicted results for one of these were compared with those obtained from the author's measurement in an experimental test rig and good agreement was obtained

CFD Analysis of Oil Flooded Twin Screw Compressors

Modelling of screw compressors using Computational Fluid Dynamics (CFD) offers better insight into the working chamber of twin screw machines when compared with chamber models. As shown by authors in earlier publications, CFD models predict performance of dry gas and refrigeration compressors fairly accurately. However numerical flow models used for modelling of oil flooded twin screw compressors are still at the development stage. This is mainly due to the lack of understanding of the flow complexity and the techniques used for solving coupled equations that represent interactions between the gas and the oil in such machines. This paper presents the modelling approach used for calculation of the performance of an oil flooded screw compressor. It requires a structured numerical mesh which can represent all moving parts of the compressor in a single numerical domain. Such mesh is generated by SCORGTM using novel boundary distribution technique called casing-to-rotor conformal boundary mapping. A test oil injected twin screw compressor with rotor configuration 4/5 and 127 mm main rotor diameter was measured in the compressor rig of the Centre for Compressor Technology at City University London. Measurements of the chamber pressure history and integral parameters of the compressor such as mass flow rate of gas and oil, indicated power and temperatures are used for the comparison with CFD results. The analysis showed a close match in the prediction of the mass flow rates of gas. The indicated power obtained by CFD predictions matched well with the measured shaft power. The model provided an exceptional visualization of the interaction of gas and oil inside the compression chamber. The mixing of the phases, distribution of oil, heat transfer between gas and oil and also effects on sealing due to high oil concentration in leakage gaps were well captured

Adaptive Grid Solution Procedure for Elliptic Flows.

This thesis deals with the formulation of a computationally efficient adaptive grid system for two-dimensional elliptic flow and heat transfer problems. The formulation is in a curvilinear coordinate system so that flow in irregular geometries can be easily handled. An equal order pressure-velocity scheme is formulated in this thesis to solve the flow equations. An adaptive grid solution procedure is developed in which the grid is automatically refined in regions of high errors and consecutive calculations are performed between the coarse grid and adapted grid regions in the same spirit as that of a Multi-Grid method. In orthogonal coordinate systems, checkerboard pressure and velocity fields are avoided by using staggered grids. In curvilinear coordinates however, the geometric complications associated with staggered grids are overwhelming and therefore a non-staggered grid arrangement is desirable. To this end, an equal order pressure-velocity interpolation scheme is developed in this thesis. This scheme is termed as the SIMPLEM algorithm and is shown to have good convergence characteristics, and to suppress checkerboard pressure and velocity fields. The adaptive grid technique developed flags the important regions in the calculation domain from an initial coarse grid calculation. Then, adaptation is performed by generating a nonuniform mesh in the flagged region using Poisson\u27s equations in which the nonhomogeneous terms are chosen so that a denser clustering of grid points is obtained where needed most in the flagged region. Coarse grid calculations in the whole domain, and fine grid calculations in the flagged region are consecutively performed until convergence, with correction terms from the fine grid solution added to the coarse grid equations in the flagged region in every cycle of calculation. Thus, the solution in the non-refined regions improves due to the influence of the correction terms added to the coarse grid equations. The effectiveness of the method is demonstrated by solving a variety of test problems and comparing the results with those obtained on a uniform or fixed grid. The adaptive grid solutions are shown to be more accurate than the fixed uniform grid solutions for the same level of computational effort

Evaluation and generic application scenarios for curved hexahedral adaptive mesh refinement

In (dynamic) adaptive mesh refinement (AMR) an input mesh is refined or coarsened to the need of the numerical application. This refinement happens with no respect to the originally meshed domain and is therefore limited to the geometrical accuracy of the original input mesh. We presented a novel approach to equip this input mesh with additional geometry information, to allow refinement and high-order cells based on the geometry of the original domain. We already showed a limited implementation of this algorithm. Now we evaluate this prototype with a numerical application and we prove its influence on the accuracy of certain numerical results. To be as practical as possible, we implement the ability to import meshes generated by Gmsh and equip them with the needed geometry information. Furthermore, we improve the mapping algorithm, which maps the geometry information of the boundary of a cell into the cell's volume. With these preliminary steps done, we use out new approach in a simulation of the advection of a concentration along the boundary of a sphere shell and past the boundary of a rotating cylinder. We evaluate the accuracy of our approach in comparison to the conventional refinement of cells to answer our research question: How does the performance and accuracy of the hexahedral curved domain AMR algorithm compare to linear AMR when solving the advection equation with the linear finite volume method? To answer this question, we show the influence of curved AMR on our simulation results and see, that it is even able to outperform far finer linear meshes in terms of accuracy. We also see that the current implementation of this approach is too slow for practical usage. We can therefore prove the benefits of curved AMR in certain, geometry-related application scenarios and show possible improvements to make it more feasible and practical in the future

Integrated modeling and analysis methodologies for architecture-level vehicle design.

In order to satisfy customer expectations, a ground vehicle must be designed to meet a broad range of performance requirements. A satisfactory vehicle design process implements a set of requirements reflecting necessary, but perhaps not sufficient conditions for assuring success in a highly competitive market. An optimal architecture-level vehicle design configuration is one of the most important of these requirements. A basic layout that is efficient and flexible permits significant reductions in the time needed to complete the product development cycle, with commensurate reductions in cost. Unfortunately, architecture-level design is the most abstract phase of the design process. The high-level concepts that characterize these designs do not lend themselves to traditional analyses normally used to characterize, assess, and optimize designs later in the development cycle. This research addresses the need for architecture-level design abstractions that can be used to support ground vehicle development. The work begins with a rigorous description of hierarchical function-based abstractions representing not the physical configuration of the elements of a vehicle, but their function within the design space. The hierarchical nature of the abstractions lends itself to object orientation - convenient for software implementation purposes - as well as description of components, assemblies, feature groupings based on non-structural interactions, and eventually, full vehicles. Unlike the traditional early-design abstractions, the completeness of our function-based hierarchical abstractions, including their interactions, allows their use as a starting point for the derivation of analysis models. The scope of the research in this dissertation includes development of meshing algorithms for abstract structural models, a rigid-body analysis engine, and a fatigue analysis module. It is expected that the results obtained in this study will move systematic design and analysis to the earliest phases of the vehicle development process, leading to more highly optimized architectures, and eventually, better ground vehicles. This work shows that architecture level abstractions in many cases are better suited for life cycle support than geometric CAD models. Finally, substituting modeling, simulation, and optimization for intuition and guesswork will do much to mitigate the risk inherent in large projects by minimizing the possibility of incorporating irrevocably compromised architecture elements into a vehicle design that no amount of detail-level reengineering can undo

An analytical approach to grid sensitivity analysis for NACA four-digit wing sections

Sensitivity analysis in computational fluid dynamics with emphasis on grids and surface parameterization is described. An interactive algebraic grid-generation technique is employed to generate C-type grids around NACA four-digit wing sections. An analytical procedure is developed for calculating grid sensitivity with respect to design parameters of a wing section. A comparison of the sensitivity with that obtained using a finite difference approach is made. Grid sensitivity with respect to grid parameters, such as grid-stretching coefficients, are also investigated. Using the resultant grid sensitivity, aerodynamic sensitivity is obtained using the compressible two-dimensional thin-layer Navier-Stokes equations