Deforming grid generation for numerical simulations of fluid dynamics in sliding vane rotary machines

The limiting factor for the employment of advanced 3D CFD tools in the analysis and design of rotary vane machines is the unavailability of methods for generation of a computational grid suitable for fast and reliable numerical analysis. The paper addresses this issue through an analytical grid generation based on the user defined nodal displacement which discretizes the moving and deforming fluid domain of the sliding vane machine and ensures conservation of intrinsic quantities by maintaining the cell connec- tivity and structure. Mesh boundaries are defined as parametric curves generated using trigonometrical modelling of the axial cross section of the machine while the distribution of computational nodes is per- formed using algebraic algorithms with transfinite interpolation, post orthogonalisation and smoothing. Algebraic control functions are introduced for distribution of nodes on the rotor and casing boundaries in order to achieve good grid quality in terms of cell size and expansion. For testing of generated grids, single phase simulations of an industrial air rotary vane compressor are solved by use of commercial CFD solvers FLUENT and CFX. This paper presents implementation of the mesh motion algorithm, sta- bility and robustness experienced with the solvers when working with highly deforming grids and the obtained flow results.The work documented in this paper has been funded by the 2015 Scholarship of the Knowledge Centre on Organic Rankine Cy- cle technology ( www.kcorc.org ), the organization formed by the members of the ORC Power Systems committee of the ASME In- ternational Gas Turbine Institute (IGTI)

User defined nodal displacement of numerical mesh for analysis of screw machines in FLUENT

Growing demands to reduce energy consumption are driving researchers towards in-depth analysis of positive displacement machines. Twin screw compressors are amongst the most common types of positive displacement machines. These machines have inherently complex geometry due to intricate rotor profiles used. As the details of the internal flows are difficult to obtain experimentally, Computational Fluid Dynamics (CFD) offers a good alternative for evaluation of internal flow patterns. However, implementation of CFD is challenging due complex deforming geometries. In this paper, a customised grid generator SCORGTM developed by authors is used to generate numerical meshes for commercially available solver ANSYS FLUENT. FLUENT is an unstructured solver which offers flexibility of using both segregated and coupled solution algorithms. Segregated algorithms are generally faster which results in shorter product development time. Interface with FLUENT is implemented by performing User Defined Nodal Displacements (UDND) of grids generated by SCORG in a parallel framework. For this purpose, SCORG and UDND are coupled and extended to work with FLUENT's parallel architecture. The developed code is compiled within the solver. The oil free air screw compressor with 'N' profile rotors and 3/5 lobe combination is modelled for 8000 RPM and 6000 RPM. Finally, the predicted performance values with FLUENT are compared to previously calculated CFX predictions and experimental results. FLUENT requires shorter solution time to obtain same accuracy of CFX

Analysis of oil-injected twin-screw compressor with multiphase flow models

Growing demands for energy are motivating researchers to conduct in-depth analysis of positive displacement machines such as oil-injected screw compressors which are frequently used in industrial applications like refrigeration, oil and gas and air compression. The performance of these machines is strongly dependent on the oil injection. Optimisation of oil has a great energy saving potential by both increasing efficiency and reducing other impacts on the environment. Therefore, a three-dimensional, transient computational fluid dynamics study of oil injection in a twin-screw compressor is conducted in this research. This study explores pseudo single-fluid multiphase (SFM) models of VOF (Volume of Fluid) and a mixture for their capability to predict the performance of the oil-injected twin screw compressor and compare this with the experimental values. SCORG™ (Screw Compressor Rotor Grid Generator) is used to generate numerical grids for unstructured solver Fluent with the special interface developed to facilitate user defined nodal displacement (UDND). The performance predictions with both VOF and mixture models provide accurate values for power consumption and flow rates with low deviation between computational fluid dynamics (CFD) and the experiment at 6000 RPM and 7.0 bar discharge pressure. In addition, the study reflects on differences in predicting oil distribution with VOF, mixture and Eulerian-Eulerian two-fluid models. Overall, this study provides an insight into multiphase flow-modelling techniques available for oil-injected twin-screw compressors comprehensively accounting for the details of oil distribution in the compression chamber and integral compressor performance

Analytical grid generation and numerical assessment of tip leakage flows in sliding vane rotary machines

© 2021 The Author(s). The research presents a new analytical grid generation methodology for computational fluid dynamics studies in positive displacement sliding vane rotary machines based on the user defined nodal displacement approach. This method is more inclusive than state of the art ones since it enables the investigation of a broader range of design configurations, such as single, double and multiple-acting vane machines with non-circular housing, slanted blade and asymmetric blade tip profiles. Node number and radial divisions of blade tip are the parameters that affect most the mesh quality. The method was validated against indicated pressure measurements on a rotary vane expander resulting in a confidence interval within 4.31%. The benchmark analysis showed that the proposed method is as accurate as the manual ANSYS ICEM one but more than 1500 times faster (111s instead of 48h to generate 360 grids). The paper further proposes a novel method to track the leakage flows at the blade tip gaps of vane machines through a post-processing routine in ANSYS CFD-Post based on rotating monitoring planes. The leakage assessment on the vane expander case study showed that a 10 μm gap between blade tip and the 76 mm stator led to a 0.06 unit increase of the expander filling factor.Research Councils UK (RCUK) Centre for Sustainable Energy Use in Food Chains (Grant No. EP/K011820/1); National Science Foundation of China (NSFC, Grant No. 21978227) and the China Scholarship Council (CSC, Grant No. 201906280153)

Analytical grid generation and numerical assessment of tip leakage flows in sliding vane rotary machines

The research presents a new analytical grid generation methodology for computational fluid dynamics studies in positive displacement sliding vane rotary machines based on the user defined nodal displacement approach. This method is more inclusive than state of the art ones since it enables the investigation of a broader range of design configurations, such as single, double and multiple-acting vane machines with non-circular housing, slanted blade and asymmetric blade tip profiles. Node number and radial divisions of blade tip are the parameters that affect most the mesh quality. The method was validated against indicated pressure measurements on a rotary vane expander resulting in a confidence interval within 4.31%. The benchmark analysis showed that the proposed method is as accurate as the manual ANSYS ICEM one but more than 1500 times faster (111s instead of 48h to generate 360 grids). The paper further proposes a novel method to track the leakage flows at the blade tip gaps of vane machines through a post-processing routine in ANSYS CFD-Post based on rotating monitoring planes. The leakage assessment on the vane expander case study showed that a 10 μm gap between blade tip and the 76 mm stator led to a 0.06 unit increase of the expander filling factor

Numerical methodology and CFD simulations of a rotary vane energy recovery device for seawater reverse osmosis desalination systems

Energy recovery devices in Seawater Reverse Osmosis Systems (SWRO) reduce energy consumption and may facilitate the large-scale deployment of desalination systems. In this paper, a Rotary Vane Energy Recovery Device (RVERD) is analysed and optimised by aiming at weakening cavitation and improving the volumetric performance of the machine. An innovative analytical methodology based on user defined nodal displacement is proposed to address the need to discretise the rotating and deforming computational domain of double-acting vane machines. The generated grids are interfaced with the ANSYS FLUENT solver for multi-phase computational fluid dynamics simulations. The flow topology is analysed to reveal the flow and cavitation features especially in the blade tip regions. A port optimisation is then carried out followed by a sensitivity analysis on the design parameters to improve RVERD performance. The results show that delaying the discharge angle at the high-pressure outlet port by 3° and an optimal port to stator length ratio of 70% helped to prevent backflows and eliminate torque peaks. The sensitivity analysis has identified the rotational speed and the blade tip clearance as the two most influential factors affecting cavitation and, in turn, the volumetric efficiency of the machine. With respect to the baseline design configuration, at the optimal rotational speed of 1000 RPM and with a tip clearance gap of 50 μm, the volume-averaged vapour volume fraction in the core decreased from 20.6 × 10−3 to 0.6 × 10−3 while the volumetric efficiency increased from 85.7% to 91.6%. The axial clearance gap of 70 μm contributed to 2.9% of the volumetric losses

Numerical methodology and CFD simulations of a rotary vane energy recovery device for seawater reverse osmosis desalination systems

© 2021 The Authors. Energy recovery devices in Seawater Reverse Osmosis Systems (SWRO) reduce energy consumption and may facilitate the large-scale deployment of desalination systems. In this paper, a Rotary Vane Energy Recovery Device (RVERD) is analysed and optimised by aiming at weakening cavitation and improving the volumetric performance of the machine. An innovative analytical methodology based on user defined nodal displacement is proposed to address the need to discretise the rotating and deforming computational domain of double-acting vane machines. The generated grids are interfaced with the ANSYS FLUENT solver for multi-phase computational fluid dynamics simulations. The flow topology is analysed to reveal the flow and cavitation features especially in the blade tip regions. A port optimisation is then carried out followed by a sensitivity analysis on the design parameters to improve RVERD performance. The results show that delaying the discharge angle at the high-pressure outlet port by 3° and an optimal port to stator length ratio of 70% helped to prevent backflows and eliminate torque peaks. The sensitivity analysis has identified the rotational speed and the blade tip clearance as the two most influential factors affecting cavitation and, in turn, the volumetric efficiency of the machine. With respect to the baseline design configuration, at the optimal rotational speed of 1000 RPM and with a tip clearance gap of 50 μm, the volume-averaged vapour volume fraction in the core decreased from 20.6 × 10−3 to 0.6 × 10−3 while the volumetric efficiency increased from 85.7% to 91.6%. The axial clearance gap of 70 μm contributed to 2.9% of the volumetric losses.Research Councils UK (RCUK) Centre for Sustainable Energy Use in Food Chains (Grant No. EP/K011820/1), the National Science Foundation of China (NSFC, Grant No. 21978227); the China Scholarship Council (CSC, Grant No. 201906280153)

Numerical and Experimental Study of Screw Machines with Large Helix Angle

An effective and efficient CFD simulation is of high importance to accelerate the design activities of twin screw machines. However, mesh generation for screw machines with large helix angles can produce highly skewed numerical cells which can make simulation unreliable. From the established literature, the two main approachesto generate structured deforming mesh for CFD analysis of twin screw machines are algebraic and differential. The purpose of this thesis is to explore grid generation techniques suitable for 3D numerical modelling of rotatory positive displacement machines with large helix angles. Both cut-cell cartesian and body-fitted grid generation methods are investigated for analysis of machines with different profiles and helix angles. The conservation and simulation accuracy of cut-cell cartesian method was first evaluated using a simple piston cylinder example and a hook and claw rotor profile used in vacuum pump which is difficult to set by a body fitted mesh. It was shown that even with coarse initial mesh, such profiles could have been analysed using a cut-cell cartesian mesh, but it required special model to account for leakage flows. Secondly, screw machines with low helix angle are used to examine the capability of both grid generation methods. ANSYS CFX was used for analysis of body fitted mesh produced by SCORG grid generator, while ANSYS Forte was used for evaluation of cut-cell cartesian method. Thirdly, oil-injected twin screw compressor with intermediate helix angle was explored using body-fitted method. In addition, a case study of a twin-screw vacuum pump with higher helix angle of 62 degree was studied using SCORG and FLUENT to further explore the body-fitted method. Lastly, a twin-screw vacuum pump with variable pitch and cusp points was analysed using cut-cell cartesian method. These five case studies demonstrated advantages and disadvantages of body-fitted and cut-cell cartesian methods for low, intermediate, and large helix angles. This thesis then proposes development of an alternative method called a normal grid generation method which is expected to improve mesh quality of a body fitted mesh for screw machine with large helix angles and in turn could improve accuracy of flow calculation

Numerical analysis of oil injection in twin-screw compressors

Compressors are widely used in the manufacturing, process, construction, and energy industries. They consume nearly 20% of the electricity generated worldwide [1]. Nearly 66% of this electricity comes from burning fossil fuel that greatly impacts the environment. Improving compressor efficiency by even a small percentage will considerably reduce electricity and energy consumption. This thesis focuses on improvement in the efficiency of oil-injected compressors. The injection of oil in the working chamber of a screw compressor increases volumetric efficiency and reliability, but it increases power losses. Also, the oil needs to be separated from the gas, which requires additional equipment and energy losses, not to mention the contamination of the environment with the oil carryover. The injected oil has a significant influence on compressor performance and the environment. The distribution of oil injected in a compression chamber is critical for performance and reliability. Accordingly, investigations are carried out using Computational Fluid Dynamics (CFD), based on a Volume of Fluid (VOF) model, to determine the oil distribution. The predictions were compared with test results. This was applied to an industrial compressor with a traditional single oil injection point. Using different nozzle diameters allowed the evaluation of the oil and temperature distribution close to rotor surfaces to be determined. It was found out that the high gas temperatures coincided with the regions of low oil concentration. With increasing the oil flow rate through a single port, the cooling of these hot spots was not achieved beyond a certain point unless the second oil-injection port was introduced on the opposite rotor. The injection through ports on both rotors reduced the compressor chamber temperature by 30°-35°C and the specific power by 1.8%. Furthermore, the adaptive mesh refinement technique in a simplified compression domain was used to simulate the film formation and disintegration. The disintegration has shown various oil phase breakup levels due to interfacial shear, inertial, and centrifugal forces, leading to ligaments, lobes, and droplets. This study shows that computational methods could be exploited for improving compressor energy consumption through oil distribution. One such way is the enhancement of oil distribution to cool high temperature spots. Another is understanding the oil phase breakup due to the forces acting in a compression chamber that can affect the oil droplet sizes and cooling surface area. Techniques used in this research can be applied to improve efficiency for a wide range of screw compressors