Study of Multiphase Flow at the Suction of Screw Compressor

Screw compressors are commonly used for industrial and commercial gas processing and refrigeration. These machines are known to be able to admit mixtures of gasses and liquids to a certain concentration. In oil injected compressors, oil is mostly injected in the working domain to seal, cool and lubricate. But would the injection of atomized oil or other liquid in the suction of the compressor be useful for better control of the discharge temperature and reduction in energy consumption, is still to be determined. Similarly, liquid neutral to the process may be injected in an oil free compressor suction to help controlling discharge temperature. It can be erosive and corrosive to the compressor rotors. Therefore mapping a two phase suction flow of a screw compressor may help in understanding the means to improve compressors efficiency and reliability. This paper is the initial phase of PhD program to determine the multiphase flow characteristic at suction of twin screw compressors by means of experimental techniques. Review of most common and up to date measurement techniques in field of multiphase flow was carried out to determine their suitability and feasibility. Also Modelling of single and multiphase flow at the suction domain of a twin screw compressor were performed in order to have a better understanding of flow distribution. The research is performed on an oil free screw compressor with “N” rotor profiles of 128 mm and configuration of 3/5 lobes with L/D of 1.6 and 93 mm centre distance. A simplified CFD model of only suction domain which reduces computational time was compared with the CFD model of the entire compressor and it was found that it predicts most of flow features with same accuracy. The experimental study which will be used to validate the CFD model has been presented

Numerical investigation of flow characteristics in twin-screw pump under cavitating conditions

In order to investigate the flow characteristics and the formation process of cavitation in twin-screw pumps, three-dimensional CFD (Computational Fluid Dynamics) numerical analysis has been carried out. A conformal structured moving mesh generated by an in-house code SCORG was applied for the rotor domain. The VOF (Volume of Fluid) Method has been adopted for dealing with the liquid-gas two-phase flow, while the bubble dynamics was handled by a homogenous cavitation model. By changing the rotation speed and discharge pressure, the intensity, distribution area and variation of cavitation at different rotor angle were obtained. The effects of rotation speed and discharge pressure on cavitation characteristics have been analysed. Calculation results with cavitation model are compared with the results without cavitation and the experimentally obtained values. The influence of cavitation on the performance of a screw pump in terms of the mass flow rate, pressure distribution, rotor torque and the shaft power have been analysed and discussed

Highly Deforming Computational Meshes for CFD Analysis of Twin-Screw Positive Displacement Machines

Commercial flow solvers can be used to obtain flow solutions in applications with deforming domains, but, in general, are not suitable for screw machine flow calculations. This is due to the large magnitude of deformation of the domain and the geometrical complexity of helical rotors. In this chapter, the governing equations for deforming domains and three methods of obtaining mesh movement, commonly used by FVM solvers, have been analysed. A comparative study of customised methods of grid generation for screw machines, using algebraic and differential approaches, is shown to help in the selection of techniques that can improve grid quality, robustness and speed of grid generation. The analysis of an oil-injected twin-screw compressor is included as a test case to demonstrate the application of SCORG, a deforming grid generator, as a means of predicting performance

CFD Analysis of A Twin Screw Compressor with Oil Injection

In this paper, a full 3D transient Computational Fluid Dynamics (CFD) model of a twin screw compressor with oil injection is described in detail. Volume of Fluid (VOF) two phase flow model is used for gas and liquid phases. The rotor meshes are generated by SCORGTM grid generator and read into the CFD solver at each time step. The simulation runs efficiently and results are obtained in about 12 hours on 24 CPU cores. The CFD prediction of air mass flow rate and indicated power has a good agreement with the experimental data measured at City University of London. The compression pocket is tracked using a customized postprocessing technique and the pressurevolume diagram within it is also obtained and analyzed. Simulations demonstrate that the approaches used in this paper are robust and fast, and can be readily applied to industrial compressor systems for rapid design iterations and improvements

CFD Analysis of an ORC Vane Expander using OpenFOAM Solver

Recent studies on the use of 3D Computational Fluid Dynamics (CFD) for the analysis and design of sliding vane machines has proved beneficial for the detailed evaluation and optimisation of the vane expanders for a given working fluid and operating condition. The authors have earlier developed a customised rotor grid generator for integration with commercial CFD solvers and validated it for use in typical small-scale ORC expanders for waste heat recovery. In this paper, this customised grid generation is extended to an open source CFD solver OpenFOAM, by using a connectivity methodology originally developed for roots blower and twin-screw machines. The control of the rotor grid deformation is through a user code integrated within the flow solver. A case study of the reference ORC expander operating with R245fa was used for validation. The available experimental data for three operating conditions are compared with the results calculated with ANSYS CFX and OpenFOAM-v1912 solvers. During the filling and expansion process, the internal pressure traces are accurately captured by both the solvers and the difference is within 0.05 bar with measurements. However, between the outlet port closure and inlet port opening process the pressure and temperature prediction with OpenFOAM solver is considerably different from the ANSYS CFX solver. It was observed that the OpenFOAM solver is resulting into a non-physical low temperature zone upstream to the tangency region of the rotor and the stator that goes below 80℃. Overall, CFD solution obtained with the commercial solver ANSYS CFX is much more stable and robust than the open source OpenFOAM solver. The generic nature of the deforming grid generation used with an open source CFD solver presented in the paper allows broadening of the utilisation of CFD modelling tools for the design of vane machines

Grid generation methodology and CFD simulations in sliding vane compressors and expanders

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 computational grids suitable for fast and reliable numerical analysis. The paper addresses this challenge presenting the development of an analytical grid generation for vane machines that is based on the user defined nodal displacement. In particular, 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 performed 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. In this way, the moving and deforming fluid domain of the sliding vane machine is discretized and the conservation of intrinsic quantities in ensured by maintaining the cell connectivity and structure. For validation of generated grids, a mid-size air compressor and a small-scale expander for Organic Rankine Cycle applications have been investigated in this paper. Remarks on implementation of the mesh motion algorithm, stability and robustness experienced with the ANSYS CFX solver as well as the obtained flow results are presente

Influence of approaches in CFD Solvers on Performance Prediction in Screw Compressors

Computational Fluid Dynamics (CFD) offers insight into screw compressor designs beyond the capabilities of other conventional methods. It allows evaluation of local flow patterns which influence performance but are difficult or impossible to investigate experimentally. Implementation of CFD in these machines is challenging due to the physics of the flow, the properties of the working fluids and the complexity of flow passages which change size and position. This is additionally challenged by a lack of methodologies available to generate the meshes required for the full three dimensional transient simulations. Commercially available CFD solvers need to fully interact with customized grid generators to enable resolution of grid deformation during a flow solution. However, the factors that influence flow predictions are not only related to grids but also to the approach which CFD solvers use to calculate distribution of parameters such as pressure, velocities, temperatures, etc. In this paper, two approaches most commonly used in commercial CFD software are compared and analysed. The first is a segregated cell-centre based solver and the second is a coupled vertex-centre based solver. Both are pressure based finite volume solvers. Customized grid generation software is used for meshing of moving rotors and flow domains around the rotors in an oil free air screw compressor with ‘N’ rotor profile of 3/5 lobe combination. The deforming rotor grid is maintained as identical in both solvers. The performance predictions obtained by calculations with these two CFD models are compared with measurements obtained on the test compressor in the City University London test rig. The comparison includes pressure in the compressor chamber, mass flow rate, indicated power and the volumetric efficiency. The study reveals differences between the results obtained by two different solvers and the experimental results. Analysis presented in this paper provides a good basis for further consideration of differencing schemes and other characteristics and settings for different CFD solvers in order to achieve accurate predictions of flows in positive displacement machines

Numerical CFD simulations on a small-scale ORC expander using a customized grid generation methodology

Positive displacement machines are the most suitable devices for small-scale waste heat to power conversion units based on an Organic Rankine Cycle (ORC) due to their capabilities of handling small mass flow rates and high pressure ratios. Among the technologies, sliding vane machines provide unique features such as low operating revolution speed, geometrical flexibility and uncomplicated manufacturing. Nonetheless, research and product development in this field have been constrained by the lack of interfaces between deforming and moving fluid domains that characterize sliding vane devices and the design tools at the state of the art. This research work tackles this challenge and presents the development of an analytical grid generation for sliding vane machines that is based on user defined nodal displacement. Through this approach, the numerical methodology discretizes the fluid domains enclosed between the cells and ensures conservation of intrinsic quantities by maintaining the cell connectivity and structure. Transient 3D single phase simulations on a small scale ORC expander were further set up in the ANSYS CFX solver and provided insights on the main flow field as well as in the leakage paths between rotor blade tips and casing. The numerical results were eventually validated with reference to an experimental dataset related to a waste heat to power conversion application in compressed air systems where the sliding vane ORC expander worked with R236fa, at a pressure ratio of 2.65 and at 1551 RPM

Numerical investigation of oil injection in a Roots blower operated as expander

The adoption of positive displacement machines as expanders in Organic Rankine Cycles (ORCs) is increasingly common, especially in the low to medium power range. At the same time, these devices often serve as compressor in Vapor-Compression Refrigeration Systems. In both cases, the application of Computation Fluid Dynamics (CFD) to optimize such machines has become an integrated tool in the design process. As a consequence, several challenges associated with the numerical simulation have to be taken into account. For example, the modeling of the gap represents a challenge for the stability of the numerical analysis. The dynamic of the process, combined with deformations of the clearances and of the working chamber has to be considered with extra care. To raise the efficiency of the machine, oil is typically injected. Its numerical modeling imply an extra challenge in the simulation of the actual operation of the machine. The present work is mainly focused on the multi-phase nature of the flow, with a particular analysis of the lubricant oil injected. In this work, a two-lobe Roots blower operated as expander has been simulated with the open-source software OpenFOAM-v1812, using the SCORG-V5.2.2. This analysis highlights the areas that are affected the most by the oil presence in order to highlight the sealing effect it provides