Development and Design of Energy Efficient Oil-Flooded Screw Compressors

It is estimated that about 17% of the world's generated power is used for compression. Thus all, even minor improvement of the efficiency of compressors will substantially reduce CO2 emission. This paper presents development of family of energy efficient oil-flooded screw compressors for Kirloskar Pneumatic Company Ltd. The developmental techniques adopted to improve efficiency such as introduction of superior 'N' rotor profile, rotor clearance management, performance calculation using 3D CCM (Computational Continuum Mechanics), direct parametric interface to CAD (Computer Aided Design), which contains bearing selection for complete 3D solid modelling. Also, contemporary prototyping and experimental investigation is supported by the fully computerised data acquisition and processing. The cumulative improvement of all these elements of the design process resulted in a very efficient machine which guarantees the competitive position of Kirloskar Pneumatic Company Limited in the screw compressor market

3D CFD analysis of an oil injected twin screw expander

Small scale Organic Rankine Cycle (ORC) systems have a big potential for waste heat recovery in the market. Due to the smaller volume flows inside these systems, non-conventional expansion technologies such as screw expanders become more interesting. Recent economic studies have shown the important role of screw machines in such cycles. However, in order to get a better understanding of the expansion behaviour in an ORC, appropriate simulation models of screw expanders are necessary. The flow inside an oil-injected twin screw expander is modeled in detail with 3D CFD (Computational Fluid Dynamics) calculations. These simulations are challenging because of the deforming domain and the narrow gaps between the screws or between a screw and the casing. The deforming mesh motion is handled by an in-house code which generates a block-structured grid with the help of the solutions of the Laplace problem. The oil-phase was modeled with an Eulerian multiphase model and the working fluid is treated compressible. The performance of the screw expander is strongly affected by the oil-injection which provides lubrication and a better sealing of the gaps. Therefore, the different types of leakages inside the screw expander are studied and monitored. As the result of the simulations, knowledge about the flow process and the losses inside the oil-injected screw expander is built up

Numerical investigation of cavitation in twin-screw pumps

In order to investigate the flow characteristics and the formation process of cavitation in twin-screw pumps, three-dimensional 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 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 has been analysed and discussed. For analysis of cavitation in clearances, a 2D numerical model which includes radial and inter-lobe clearances was used. The relationship between volumetric efficiency and cavitation intensity was developed by variation of boundary conditions

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

Review of Mathematical Models in Performance Calculation of Screw Compressors

The mathematical modelling of screw compressor processes and its implementation in their design began about 30 years ago with the publication of several pioneering papers on this topic, mainly at Purdue Compressor Conferences. This led to the gradual introduction of computer aided design, which, in turn, resulted in huge improvements in these machines, especially in oil-flooded air compressors, where the market is very competitive. A review of progress in such methods is presented in this paper together with their application in successful compressor designs. As a result of their introduction, even small details are now considered significant in efforts to improve performance and reduce costs. Despite this, there are still possibilities to introduce new methods and procedures for improved rotor profiles, design optimisation for each specified duty and specialized compressor design, all of which can lead to a better product and new areas of application. A review of methods and procedures which lead to modern screw compressor practice is presented in this paper. This paper is intended to give a cross section through activities being done in mathematical modelling of screw compressor process through last five decades. It is expected to serve as a basis for further contributions in the area and as a challenge to the forthcoming generations of scientists and engineers to concentrate their efforts in finding future and more extended approaches and submit their contributions

Combining Boundary-Conforming Finite Element Meshes on Moving Domains Using a Sliding Mesh Approach

For most finite element simulations, boundary-conforming meshes have significant advantages in terms of accuracy or efficiency. This is particularly true for complex domains. However, with increased complexity of the domain, generating a boundary-conforming mesh becomes more difficult and time consuming. One might therefore decide to resort to an approach where individual boundary-conforming meshes are pieced together in a modular fashion to form a larger domain. This paper presents a stabilized finite element formulation for fluid and temperature equations on sliding meshes. It couples the solution fields of multiple subdomains whose boundaries slide along each other on common interfaces. Thus, the method allows to use highly tuned boundary-conforming meshes for each subdomain that are only coupled at the overlapping boundary interfaces. In contrast to standard overlapping or fictitious domain methods the coupling is broken down to few interfaces with reduced geometric dimension. The formulation consists of the following key ingredients: the coupling of the solution fields on the overlapping surfaces is imposed weakly using a stabilized version of Nitsche's method. It ensures mass and energy conservation at the common interfaces. Additionally, we allow to impose weak Dirichlet boundary conditions at the non-overlapping parts of the interfaces. We present a detailed numerical study for the resulting stabilized formulation. It shows optimal convergence behavior for both Newtonian and generalized Newtonian material models. Simulations of flow of plastic melt inside single-screw as well as twin-screw extruders demonstrate the applicability of the method to complex and relevant industrial applications

Expander selection for an on board ORC energy recovery system

This paper deals with the comparison between volumetric expanders (screw, scroll and rotary vane) and an Inlet Forward Radial (IFR) micro turbine for the exploitation of an on board Organic Rankine Cycle (ORC) energy recovery system. The sensible heat recovered from a common bus engine (typically 8000cc) feeds the energy recovery system that can generate sufficient extra power to sustain the air-conditioning system and part of the auxiliaries. The concept is suitable for all kind of thermally propelled vehicles, but the application considered here is specific for an urban bus. The ORC cycle performance is calculated by a Process Simulator (CAMEL Pro) and the results are discussed. A preliminary design of the considered expanders is proposed using ad-hoc made models implemented in MATLAB; the technical constraints inherent to each machine are listed in order to perform the optimal choice of the expander based on efficiency, reliability and power density. Last step will be the selection of the expander that suites the specific technical and design requests. The final choice relapsed on the screw motor, for it is the best compromise in terms of efficiency, lubrication and reliability

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

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