Modeling Reciprocating Compressors Using A Cartesian Cut-Cell Method With Automatic Mesh Generation

Computational Fluid Dynamics (CFD) models can offer great insight into flow phenomena and complex fluid-structure interactions present in reciprocating compressors. This is often achieved, however, at large computational cost and considerable user setup time. In this study, a Cartesian cut-cell finite-volume method is applied to model a small displacement refrigeration compressor. The cut-cell method has the key feature of representing discrete cell volumes exactly without requiring the computational grid to coincide with the bounding geometry. Additionally, the grid is dynamically generated at each time step based on the instantaneous boundary positions and is automatically refined based on gradients of local flow variables. These two features make this method ideal for modeling the deformation of the valves, the motion of the piston, and the complex geometries of the suction and discharge mufflers. The model is validated against experimental data. The sources of numerical error in the model are assessed, including the spatial and temporal discretization error and the model treatment for valve opening, closure, and contact. Lastly, several automated grid generation strategies are presented to establish guidelines for balancing cost and accuracy. The model formulation highlights the ease of incorporating complex and moving geometries characteristic of reciprocating compressors into a CFD model at a reasonable cost

Investigations of Automatic Meshing in Modeling a Dry Twin Screw Compressor

In order to design screw compressors for optimal performance, it is crucial to understand the complex fluid flow processes within them. Computational fluid dynamics (CFD) is one approach for doing so. Considerable progress has been made over the last several years in both commercial and academic solution packages for this application; however, due to the complex moving geometries of the screw rotors and the tight clearances between the moving parts, a major challenge that remains is the generation of numerical grids that are increasingly efficient, accurate, robust, and easily created. In this study, an alternate methodology for this problem is presented. The grid is created automatically at every time step based on the instantaneous geometry using a Cartesian cut-cell based method which preserves exactly the changing control volume shapes. Automatic mesh refinement is employed to adaptively increase mesh resolution where the flow variables have large gradients in order to resolve the large-scale flow structures. To address the problem of efficiently modeling the flows in the small clearance gaps, an empirical model is applied so that the cells within the gaps can remain relatively coarse. This removes a major bottleneck from the computational cost and allows more mesh resolution to be applied in accurately capturing the physics of the port flows. The effect of the thermal expansion on the gap sizes is accounted for by considering the heat transfer from the fluid to the solid walls and then periodically solving the solid to steady state using cycle-averaged heat transfer coefficients; the clearances therefore vary throughout the length of the rotors. The model is validated against experimental measurements of the internal pressure, mass flow rate, temperature, and power for two operating conditions. A global grid convergence study demonstrates the spatial and temporal convergence of the numerical model, and establishes necessary computational costs for varying levels of accuracy. It is shown that for the tested configurations, numerically accurate results are achieved with a total turn-around time that is low enough for practical use in engineering applications

Computational Fluid Dynamics Study on Transonic Axial Compressors using Cartesian Cut-Cell Based Method with Adaptive Mesh Refinement and Boundary Layer Mesh

Axial compressors are used extensively in the energy, power, and transportation industries. Computational Fluid Dynamics (CFD) has been widely used in research and development of dynamic compressors. CFD modeling for such designs often presents great challenges in terms of meshing and computational cost due to moving parts with complex shapes, tiny gaps, and a large range of length scales and time scales to resolve. In this work, a Cartesian cut-cell based method with adaptive mesh refinement (AMR) is used to study Rotor 67, a transonic axial compressor design from NASA. The adopted method is demonstrated to be easily implemented and copmutationally efficient through a mesh convergence study, largely due to the advantage of an autonomously generated Cartesian cut-cell grid and AMR. Additionally, a boundary layer mesh can be used in conjunction with the Cartesian cut-cell mesh in order to resolve the near-wall flow more efficiently. Both the frozen-rotor approach with a single non-inertial reference frame (SRF) and a moving-rotor approach in a single inertial reference frame are used for the computation of the global pressure ratio and the isentropic efficiency as well as the local flow velocity, pressure, and temperature. Results show great grid convergence and good agreement with previously published experimental data for multiple operating conditions in terms of both global and local flow quantities

Modeling A Reciprocating Compressor Using A Two-Way Coupled Fluid And Solid Solver With Automatic Grid Generation And Adaptive Mesh Refinement

Computational fluid dynamics has been increasingly used in the design and analysis of reciprocating compressors over the last several years. One of the major challenges in the use of such tools is the creation of the numerical grid on which the modeled equations are solved. Since these compressors typically consist of many interconnected and moving parts, manual creation of the grid can be labor-intensive. Furthermore, it is necessary that the choice of grid yields a sufficiently resolved solution, so that the numerical error is significantly less than the modeling error. In this work, a small displacement refrigeration compressor is modeled using a numerical grid created with an automatic meshing approach. The grid is then automatically adapted to the flow based on the local flow field variables at each time step. This cut-cell based grid matches the supplied fluid volume exactly and permits general motion of all bounding surfaces. An explicit two-way coupled approach is used to account for the fluid-structure interaction between the deforming reed valves and the flow. The fluid is solved using a finite-volume approach, whereas the solid is solved using a finite-element model. The model is validated in comparison to measured mass flow rate, pressure, temperature, and valve lift for two different operation conditions and two different working fluids, namely R-404a and R-449a. The numerical accuracy of the calculations is demonstrated through an automated grid convergence study, and the effect of the grid and time-step resolution on the pressure pulsations and valve lift is shown. While computations on a relatively coarse grid yield power, mass flow rate, and pressure oscillation frequency comparable to measurements, a finer mesh is required inside the cylinder and in the discharge muffler to predict adequately the amplitude of the pressure fluctuations

Modeling A Scroll Compressor Using A Cartesian Cut-Cell Based CFD Methodology With Automatic Adaptive Meshing

The recent requirements on the scroll compressors used in air conditioning systems are focused on the reduction of the noise and the improvement of the efficiency. To achieve this, the complex fluid flow phenomenon taking place inside of the compressor must be better understood. Two modeling approaches for investigating this problem are one-dimensional multi-physics modeling and three-dimensional computational fluid dynamics (CFD) modeling. The one-dimensional multi-physics based models are available to perform these calculations with low computational cost, but the influence of detailed geometrical effects on the fluid flow behavior is not taken into account. This paper deals with the development and application of a three-dimensional CFD model for a scroll compressor. To deal with the challenge of the complicated moving geometries of the orbiting scroll and the deforming reed valves, an automated meshing strategy is employed to dynamically calculate the working chamber volumes based on the instantaneous geometry positions. Thereby no user meshing is required, and the resulting Cartesian cut-cell based mesh has the desirable numerical properties of orthogonality and low numerical diffusion. The motion of the discharge reed valve is determined using a fluid-structure-interaction model considering the valve as a one-dimensional deforming cantilever beam and considering the valve geometry and variable cross sections. The comparisons between the simulation and experimental results, e.g. indicator diagram, discharge valve motion and deformation, and pressure pulsation indicate that a good correlation is achieved, while the computational time stays acceptably low. The spatial and temporal convergence of the numerical method is demonstrated, particularly for the computation of the internal pressure and discharge pressure pulsation. The results show that the developed simulation model can be used to improve and to optimize the compressor design process by reducing the demand on prototype testing and to improve the understanding of the internal flow in the system

CFD Modeling And Performance Evaluation Of A Centrifugal Fan Using A Cut-Cell Method With Automatic Mesh Generation And Adaptive Mesh Refinement

Computational Fluid Dynamics (CFD) is a convenient and powerful tool for modeling and evaluating the performance of centrifugal compressors, fans, and pumps. Typically, the use of CFD requires significant labor in mesh generation and refinement. Moreover, for rotating transient simulations, a grid-to-grid interface is usually needed to incorporate the rotating impeller. In this work, a finite-volume-based Cartesian cut-cell method is employed to model a centrifugal fan. Both rotating transient simulations and steady steady multiple reference frame (MRF) simulations are analyzed. The cut-cell-based method automatically generates the Cartesian mesh on-the-fly based on the current location of the rotating boundaries, without requiring any grid-to-grid interface between the rotating part and the stationary part for the rotating transient cases. The grid is also dynamically refined based on the velocity field and y-plus values at the walls. These features greatly save the pre-processing time of the modeling and make it easy for performing global grid convergence studies. The flow is validated against experiment data for both point velocity measurements, and global measurements of mass flow rate and pressure rise. Key simulation parameters and model constants that can affect the global output and local flow details are discussed

Scapula fractures: interobserver reliability of classification and treatment

OBJECTIVES:There is substantial variation in the classification and the management of scapula fractures. The first purpose of this study was to analyze the interobserver reliability of the OTA/AO and the New International Classification of scapula fractures. The second purpose was to assess the proportion of agreement among orthopaedic surgeons on operative or nonoperative treatment. DESIGN:: Web-based reliability study SETTING:: Independent orthopaedic surgeons from several countries were invited to classify scapular fractures in an online survey. PARTICIPANTS:One-hundred and three orthopaedic surgeons evaluated 35 movies of 3DCT-reconstruction of selected scapular fractures, representing a full spectrum of fracture patterns. MAIN OUTCOME MEASUREMENTS:Fleiss' kappa (κ) was used to assess the reliability of agreement between the surgeons. RESULTS:: The overall agreement on the OTA/AO Classification was moderate for the types (A, B, and C, κ = 0.54) with a 71% proportion of rater agreement (PA) as well as for the nine groups (A1 to C3, κ = 0.47) with a 57% PA. For the New International Classification, the agreement about the intra-articular extension of the fracture (Fossa (F), κ = 0.79) was substantial, the agreement about a fractured body (Body (B), κ = 0.57) or process was moderate (Process (P), κ = 0.53), however PAs were more than 81%. The agreement on the treatment recommendation was moderate (κ = 0.57) with a 73% PA. CONCLUSIONS:The New International Classification was more reliable. Body and process fractures generated more disagreement than intra-articular fractures and need further clear definitions