8 research outputs found
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A numerical study of fluid-solid interaction in screw compressors
Efforts are continually being made to produce screw compressors with smaller clearances in order to reduce internal leakage. However, since the compression process induces large pressure differences across the rotors and temperature rise, they deform. A reliable method of estimating the interaction between fluid flow parameters and rotor deflection is thus needed in order to minimise clearances while avoiding contact between the rotors and the casing. A 3-D mathematical procedure is presented here to generate a numerical grid comprising both solid and fluid domains. This can be used to calculate the fluid flow and compressor structural deformation simultaneously by means of a suitable commercial numerical solver. Simulation results demonstrate the effects of change in working clearances, caused by rotor deformation, on compressor performance
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CFD Integrated Design of Screw Compressors
Positive displacement screw machines are used in variety of applications such as compressors, expanders, blowers, vacuum pumps, liquid and multiphase pumps. To improve their appearance, efficiency and robustness they are designed with the aid of analytical tools, based often on one-dimensional flow models solved by numerical methods that are confirmed by experiment. Continuing demand for further improvements has led to the need for improved assessment of fluid flow losses in the inlet and outlet openings and how these are affected by the shape of the ports, the deformation of machine components due to the effects of pressure and temperature gradients and their effect on performance, the behaviour of multiphase flows and many other effects. These require more advanced analytical procedures, based on three dimensional numerical flow analysis and fluid-structure interaction. The way to estimate these phenomena is to use CFD analysis and to integrate the results with three dimensional CAD systems. As computers become cheaper and faster and advances are made in numerical methods, such techniques are becoming available for everyday use by design engineers. This paper describes how CFD is merged with other design software by means of an integral management system to obtain interactive control of the entire design process of screw compressors. The methods described are of considerable scope and can be applied, not only to screw compressors but also to any other type of twin rotor rotary machines with parallel axes, such as gear pumps, multiphase pumps, vacuum pumps and roots blowers
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Extending the Role of Computational Fluid Dynamics in Screw Machines
Previous publications show that computational fluid dynamics (CFD) can be readily used for the flow prediction and analysis of screw compressors. Several case studies are presented in this article to show the scope and applicability of such methods. These include solid–fluid interaction in screw compressors, prediction of flow generated noise in screw machines, cavitation modelling in gear pumps, and flow in multiphase pumps for oil and gas industry. Numerical grids for all these cases were generated by the authors using an in-house grid generator, while the CFD calculations were performed with a variety of commercially available CFD codes.
In order to validate the accuracy of the CFD calculations, an extended test programme was carried out using laser Doppler velocimetry to measure the mean and fluctuating velocity distribution in screw compressor flow domains. The measurement results are then compared with the CFD simulations. The results confirm the viability of the developed techniques.
It is shown in this publication that the flexibility of the developed method creates further opportunities for a broader use of CFD for analysis of twin screw machines in a range of new applications
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Grid Generation for Screw Compressors with Variable Geometry Rotors
An algebraic grid generation algorithm is presented in this paper which enables the per- formance of twin screw compressors with variable rotor geometry to be predicted, by means of Computational Fluid Dynamics (CFD). It is based on a method, previously developed by the authors, for compressors with standard uniform pitch rotors and con- stant cross-section profile, which has now been extended to include rotors with variable pitch and/or variable profile. By its use with the commercial CFD solver ANSYS CFX, it has been possible to obtain performance predictions for three variants of an oil-free 3/5 screw compressor, namely uniform helical rotors, variable pitch rotors and variable profile rotors. The variable pitch and variable profile rotors achieve steeper internal pres- sure rise and a larger discharge area for the same pressure ratio. Variable pitch rotors also showed lower leakage rates due to reduced sealing line length in the high pressure domain. This grid generating procedure advances the ability to evaluate both existing and novel compressor configurations
Performance Evaluation and CFD Simulation of Multiphase Twin-Screw Pumps
Twin-screw pumps are economical alternatives to the conventional multiphase system and are increasingly used in the oil and gas industry due to their versatility in transferring the multiphase mixture with varying Gas Void Fraction (GVF). Present work focuses on the experimental and numerical analysis of twin-screw pumps for different operating conditions. Experimental evaluation aims to understand steady state and transient behavior of twin-screw pumps. Detailed steady state evaluation helped form better understanding of twin-screw pumps under different operating conditions. A comparative study of twin-screw pumps and compressors contradicted the common belief that compressor efficiency is better than the efficiency of twin-screw pumps. Transient analysis at high GVF helped incorporate necessary changes in the design of sealflush recirculation loop to improve the efficiency of the pump. The effect of viscosity of the sealflush fluid at high GVF on pump performance was studied. Volumetric efficiency was found to be decreased with increase in viscosity.
Flow visualization was aimed to characterize phase distribution along cavities and clearances at low to high GVF. Dynamic pressure variation was studied along the axis of the screw which helped correlate the GVF, velocity and pressure distribution.
Complicated fluid flow behavior due to enclosed fluid pockets and interconnecting clearances makes it difficult to numerically simulate the pump. Hence design optimization and performance prediction incorporates only analytical approach and experimental evaluation. Current work represents an attempt to numerically simulate a multiphase twin-screw pump as a whole. Single phase 3D CFD simulation was performed for different pressure rise. The pressure and velocity profile agreed well with previous studies. Results are validated using an analytical approach as well as experimental data. A two-phase CFD simulation was performed for 50% GVF. An Eulerian approach was employed to evaluate multiphase flow behavior. Pressure, velocity, temperature and GVF distributions were successfully predicted using CFD simulation. Bubble size was found to be most dominant parameter, significantly affecting phase separation and leakage flow rate. Better phase separation was realized with increased bubble size, which resulted in decrease in leakage flow rate. CFD results agreed well with experimental data for the bubble size higher than 0.08 mm