Sham Rane

Algebraic generation of single domain computational grid for twin screw machines. Part I. Implementation

New algebraic algorithm for deforming grid generation of twin screw machines developed.Regularized distribution achieved by control function and a reference blocking structure.Independent grid refinement in interlobe region achieved for better accuracy in the leakage gaps.Non-conformal interface between two rotor domains eliminated, thereby producing a single domain.New grid adaptable with multiple CFD flow solvers and capable of solving multiphase models. Special attention is required for generation of computational grids in highly deforming working chambers of twin screw machines for 3D CFD calculations. Two approaches for customised grid generation are practically available. The first is an algebraic grid generation and the second is a differential decomposition method. This paper reports on new developments in the algebraic approach that has the advantages associated with both algebraic and differential methods. Two control functions are introduced for regularisation of the initial algebraic distribution. One is based on an analytical control function in transformed coordinate system while the other uses background blocking structure in order to guide the initial algebraic distribution towards a single computational mesh. This paper presents implementation and grid characteristics of these new functions. Developed grids have been tested and results from flow calculations on a dry air compressor have been validated in part II of the paper [29].It was possible to achieve two distinct characteristics desirable in a twin screw rotor domain mesh. Firstly, it is possible to independently control grid refinement in the interlobe region thereby providing better accuracy in representation of the leakage gaps. Secondly and most importantly, it is possible now to eliminate the non-conformal interface between the two rotor domains thereby producing a single domain structured grid for the rotors, while still maintaining the fully hexahedral cell topology. An improvement in the global orthogonality of the cells was achieved. Despite of a decrement in the Face warp quality, aspect ratio of cells retained similar scale.

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 connectivity 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 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. 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, stability and robustness experienced with the solvers when working with highly deforming grids and the obtained flow results.

Identification and Quantification of Start Up Process in Oil Flooded Screw Compressors

Oil injected screw compressors, used for air compression, refrigeration and air conditioning, are usually installed in simple packages which often do not include an oil pump to supply oil to the bearings and the working chamber, since the oil is pressurised, together with the gas being compressed, in the working chamber. Consequently, normal lubrication will only start when the back pressure reaches the chamber pressure,. Due to the lack of lubrication in this start up period, the rotors will be in direct contact with insufficient or no oil film between them, while the pressure in the compression chamber will increase causing the temperature to rise. The period of un-lubricated operation then depends on the size of the oil system and the length of the discharge piping. Also, during this time, due to the lack of oil in the working chamber, the leakage flow will be higher than normal. This will increase the overall temperature in the compression chamber. It is expected that some surface damage may occur on the rotors. When the compressor operates intermittently, with frequent starts and –stops, mode, this may result in rapid wear and decrease in the compressor performance. This paper addresses such issues through experimental investigation and simulation and defines the scope of work required to understand the oil flooded compressor process in transient operation mode. This will allow prediction of wear in such compressors and give some insight to the modifications required to increase the compressor reliability.Copyright

Numerical modelling of twin-screw pumps based on computational fluid dynamics

Increasing demands for high-performance screw pumps in oil and gas as well as other applications require deep understanding of the fluid flow field inside the machine. Important effects on the performance such as dynamic losses, influence of the leakage gaps and presence and extent of cavitation are difficult to observe by experiments. However, it is possible to study such effects using well-validated computational fluid dynamics models. The novel-structured numerical mesh consisting of a single-computational domain for moving screw pump rotors was developed to allow three-dimensional computational fluid dynamics simulation of such machine possible. Based on finite volume method, the instantaneous mass flow rates, rotor torque, local pressure field, velocity field and other performance indicators including the indicated power were predicted. A calculation model for the bearing friction losses was introduced to account for mechanical losses. The geometry of the inlet and outlet passages and piping system are taken into consideration to evaluate their influences on the pressure distribution and shaft power. The paper also shows the influence of rotor clearances on the pump performance. The computational fluid dynamics model was validated by comparing the numerical results with the measured performance obtained in the experimental test rig through the comprehensive experiment performed for a set of discharge pressures and rotational speeds. Validation includes comparison of mass flow rates, shaft power and efficiency under variety of speeds and discharge pressure. It has been found that the predicted results match well with the measurements. The results also showed that the radial clearances have larger influence on the mass flow rate than the interlobe clearance. The correct design of the flow passages within the screw pump plays significant role in minimizing required power consumption. The analysis presented in this paper contributes to better understanding of the working process inside the screw pump and offers a good reference to improve design and optimize such machines in terms of clearance selection, shape of the ports, piping system, etc. In future, this model will be used for analysis of cavitating flows and determining performance of other multiphase screw pumps.

CFD grid generation and analysis of screw compressor with variable geometry rotors

This paper presents development of an algebraic grid generation algorithm applicable to Finite Volume Method for Computational Fluid Dynamics (CFD) calculation of variable pitch twin screw machines. It is based on the principles developed for the uniform pitch rotors with constant cross-section profile. The same algorithm could be also used for rotors with variable profile geometry. Performance predictions are obtained by ANSYS CFX for an oil-free 4/5 lobed twin screw compressor with variable pitch rotors and uniform ‘N’ profile. A comparison with the performance of a compressor of the same rotor size and wrap angle, but with the uniform pitch rotors showed that the variable pitch rotors give better compression characteristics. This is achieved by reduced throttling losses, reduced length of the sealing line towards the high pressure end and a larger discharge area for the same pressure ratio.

Rotor profile design and numerical analysis of 2–3 type multiphase twin-screw pumps

Increasing demands for high-performance handling of fluids in oil and gas as well as other applications require improvements of efficiency and reliability of screw pumps. Rotor profile plays the key role in the performance of such machines. This paper analyses difference in performance of 2–3 lobe combination of twin-screw pumps with different rotor profiles. A-type profile formed of involute–cycloid curves and D-type formed of cycloid curves are typical representatives for 2–3 type screw pumps. The investigation is performed by use of a full 3-D computational fluid dynamics analysis based on a single-domain structured moving mesh obtained by novel grid generation procedure. The real-time mass flow rate, rotor torque, pressure distribution and velocity field were obtained from 3D computational fluid dynamics calculations. The performance curves were produced for variable rotation speeds and variable discharge pressures. The computational fluid dynamics model was validated by comparing the simulation results of the A-type pump with the experimental data. In order to get the performance characteristics of D-type profile, two rotors with D-type profile were designed. The first has the same displacement volume as A-type while the second has the same lead and rotor length as A-type but different displacement volume. The comparison of results obtained with two rotor profiles gave an insight on the advantages and disadvantages of each of them.

Numerical Analysis of Screw Compressor Rotor and Casing Deformations

Performance and reliability of screw compressors is highly dependent on their operational clearances. Compressor structural parts including rotors and the casing are affected both by pressure and temperature of the working fluid to which they are exposed. The standard approach when simulating performance is to neglect these deformations and assume rigid compressor elements. In this paper a numerical solution which combines the solution of fluid field from Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA) of solid elements is used to calculate deformations of the compressor elements. The temperature field obtained from CFD is extracted and applied to the surface of the solid parts where it was averaged in time and served as boundary conditions for solid body calculations. The FEM analysis performed in ANSYS showed encouraging results which can be used for analysis of changes in compressor clearances.

Screw Rotor Profiles of Variable Lead Vacuum and Multiphase Machines and Their Calculation Models

Twin screw machines are mechanically simple and reliable, having only two moving parts, while their manufacturing cost is low and, as a result of optimization of their rotor proportions and profiles, their efficiency is high. Consequently, they are now used widely both as multiphase and vacuum pumps. This study gives the results of an investigation of how their performance is affected by changing the rotor lead, i.e. the axial step along the rotor wrap, from a constant value to a variable one. A number of different configurations have been considered and it is shown that, for machines with a large wrap angle, a variable lead results in greater displacement and improved efficiency.

Numerical and Experimental Investigation of Pressure Losses at Suction of a Twin Screw Compressor

Rotary twin screw machines are used in the wide range of industrial applications and are capable of handling single and multiphase fluids as compressors, expanders and pumps. Concentration of liquid in the inlet flow can influence the performance of the machine significantly. Characteristics of the multiphase flow at the suction of a screw compressor depend on the local flow velocities and concentration. Local flow velocity measurements inside the screw compressors are difficult to obtain. However other flow properties such as local pressures are easier to attain. It is therefore useful to carry out experiments with local pressure variations in the suction which can be used to validate the 3D numerical Computational Fluid Dynamic (CFD) models that could help in studying the single and multiphase flow behaviour in screw compressors. This paper presents experimental efforts to measure the local pressure losses inside the suction plenum of the screw compressor. Pressure variations are measured at 23 locations in the suction port at various operating conditions and compared with 3D CFD model. The grid generator SCORGTM was used for generating numerical mesh of rotors. The flow calculations were carried out using commercial 3D solver ANSYS CFX. It was found that the local pressure changes predicted by the CFD model are in the good agreement with measured pressures. This validated the use of CFD for modelling of the single phase flows in suction of screw machines.

Prediction of Heat Transfer and Visualisation of Temperature Field in Screw Compressors

The heat transfer within a screw compressor is not considered to affect its performance significantly, because thermal energy dissipation represents less than 1% of the compressor power input. However, it can influence the machine reliability because heat transfer, resulting from the compression process, creates a non-uniform three dimensional temperature field leading to local distortions which may be larger than the clearances between the machine parts. This phenomenon is widely known and special control procedures are required to allow for start-up and shut down as well as for steady running operation without seizing. However, these are normally derived only from test data and may result in larger internal clearances than are really necessary, thereby reducing the optimum performance.This paper describes a method for calculating the heat transferred during screw compressor processes, more precisely and hence how to obtain the temperature fields within the screw machine parts, which are needed to establish the size of the necessary clearances to maintain safe operating conditions.Copyright