Positive Displacement Machines Research Articles

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. SCORGTM (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.

The Influence of Rotor Geometry on Power Transfer Between Rotors in Gerotor-Type Screw Compressors

Abstract The configuration of a twin-screw positive displacement machine is proposed, consisting of an internally geared outer rotor meshing with an externally geared inner rotor. Helical rotors with constant profile and pitch are used with parallel rotor axes and stationary end plates incorporating inlet and discharge ports to achieve internal compression or expansion. The focus of this paper is to understand the effect of rotor geometry on two key performance indicators; the swept volume of the machine and the proportion of input power transferred between the inner and outer rotors. This requires a detailed analysis of the limitations on rotor profile generation, the formation of working chambers, and the forces exerted on the rotors. The choice of the rotor for power transfer to or from the machine is shown to be an important consideration, and helical rotors are found to enable lower power transfer between rotors during operation when compared with straight-cut rotors, but with reduced swept volume for the same machine size. For particular applications, this compromise is characterized through multi-objective optimization of the rotor profile and wrap angle in order to identify appropriate configurations for the proposed machine.

Numerical analysis and correlations for radial and tangential leakage of gas in scroll compressors

Scroll compressors are positive displacement machines of orbital motion that compress a gas by means of two interfitting spiral-shaped scroll members. Gas leakage is an important source of inefficiency in scroll compressors and it therefore needs to be estimated. Currently, correlations for leakage required in simulation models are based on simplified flow conditions. Thus, a high degree of uncertainty is associated with leakage predictions for the wide range of operating conditions found in actual applications. This paper reports a numerical analysis of radial and tangential leakage in scroll compressors considering different gases, operating conditions and geometries expressed as dimensionless parameters. Numerical models were developed to analyze the aforementioned phenomena in detail, including the geometry of the wraps. Based on the predicted values, correlations for the radial and tangential leakage of gas particularly suitable for comprehensive simulation models were developed and validated through comparison with experimental data and other models available in the literature.

PDSim: Demonstrating the capabilities of an open-source simulation framework for positive displacement compressors and expanders

In this paper, the first open-source generalized simulation framework for positive displacement machines (PDSim) reported in literature is employed to model different compressor and expander types. In particular, it is shown how PDSim handles specific numerical and modeling aspects common to positive displacement machines such as stiffness of differential equations, discontinuous volume curves, leakage flow models, in-chamber and shell heat transfer models, mechanical and frictional losses. A control volume approach based on a set of differential equations is used to predict the performance of different positive displacement machines under steady-periodic operations. Reciprocating, scroll, and two rotary compressors as well as a single-screw expander have been considered as examples. The source code of PDSim with examples are provided as an electronic annex.

PDSim: A general quasi-steady modeling approach for positive displacement compressors and expanders

A novel generalized framework is presented that can be used to simulate the quasi-steady-state performance of a wide range of positive displacement compressors and expanders (scroll, piston, screw, rotary, spool, etc.). The complete simulation algorithm is described, and an emphasis is placed on the numerical methods required to obtain robust behavior of the simulation. This formulation has been implemented into an open-source software package entitled PDSim written in the Python language. PDSim is the first open-source generalized compressor and expander simulation package and the complete source code is included in the Supplemental material. A piston expander is used as an example of the utilization of this framework. The framework has been applied to several positive displacement machines in the companion paper in order to demonstrate the flexibility of the approach (Ziviani et al., 2019).

Сравнительный анализ массогабаритных показателей бескрейцкопфной и крейцкопфной поршневых гибридных энергетических машин объемного действия

Currently, one of the main ways to improve the efficiency of piston compressors and volumetric pumps is to combine them into a single unit, called a piston hybrid energy machine. Recently, a new class of crossheadless piston hybrid energy positive displacement machines has appeared. In this work, a comparison of mass-dimensional indicators of experimental crosshead and crossheadless hybrid energy machines is performed. The analysis of the results shows that the crossheadless machine has significantly smaller masses of moving parts performing reciprocating motion (almost 3 times) and a smaller maximal overall size (almost 1.5 times) in comparison to the crosshead machine. The specific mass-dimensional indicators of the two machines are comparable.

Thermodynamic Foundations of Working Processes in Positive-Displacement Machines Designed for Compression of Two-Phase, Two-Component Working Fluids

A mathematical model of the working processes in positive-displacement machines designed to compress two-phase, two-component working fluids is developed on the basis of the general laws of conservation of energy, mass, volume and motion. A mathematical model of the working processes of a hybrid piston power machine where there is a layer of liquid above the piston in the compressor section is given. Two open thermodynamic systems (gas and droplet liquid) in the presence of deformation and thermal interactions are considered.

Transient flow analysis in a Roots blower: Experimental and numerical investigations

Abstract It is widely acknowledged that rotary positive displacement machines exhibit highly unsteady flow fields that affect their performance. The presence of the operational clearances impacts this unsteady flow field and further affects the performance. However, the exact nature of these unsteady flow mechanisms remains largely unknown that necessitates both detailed experimental investigations and computational modelling. Thus, the present study employs both optical visualization and unsteady Reynolds-Averaged Navier Stokes (URANS) computational modelling methods while focussing on investigating the transient flow field inside a Roots blower, a general type of the rotary positive displacement machine. Straight lobes in a Roots blower provide convenient optical access to experimentally analyse internal flow and compare it with the predictions obtained by standard computational models. In the first part of this paper, this study covers the low-speed experimental investigations using i) High-Speed Camera (HC), ii) the continuous High-Speed Particle Image Velocimetry (CPIV) and, iii) the instantaneous PIV (IPIV) obtained with a double pulse laser and a double shutter camera. Relative merits from these techniques are discussed with respect to the Roots blower unsteady flow mechanisms. In addition, computational analyses are performed using a combination of in-house and commercial modelling methods and the results are compared against the experiments. The results confirm the existence of highly three-dimensional and unsteady flow field where certain distinct flow mechanisms originating from the operational clearances impact the performance of the Roots blower. The study also highlights challenges of the experimental and computational methods used for evaluation of positive displacement machines that impact the accuracy of results.

Foreword

It is my great pleasure to welcome you to the 11th International Conference on Compressors and their Systems, in City, University of London from 9th - 11th September 2019. The conference is organised in collaboration with our esteemed partners; Institution of Mechanical Engineers (IMechE), Institute of Refrigeration (IOR) and International Institute of Refrigeration (IIR) and on behalf of the Organising Committee I would like to thank them for supporting and promoting this event.

Model predictive control of a free piston compressor/expander with an integrated linear motor/alternator

Linear positive displacement machines are becoming increasingly more attractive for applications that are normally known as unconquerable niches of rotary and scroll machines. Free-piston machines are characterized by the absence of a crank mechanism, since there is a direct transformation of electrical energy into the piston movement. From the point of view of manufacturing, these machines benefit from a higher robustness and reliability because of less mechanical components involved and reduced frictional losses associate with a conventional crank mechanism.However, the major challenge in replacing the rotary machines by linear ones is a lower efficiency at lower speeds which is unavoidable because of the nature of linear motion: continuous operation means a reciprocating movement within a stroke length with significantly long periods of acceleration and deceleration when the speed is far from its optimal value. However, the advantage of free-piston machines is the fact that the motion profile is freely configurable within physical constraints, which provides a possibility to optimize the speed given the efficiency map of particular linear motor.While the methods and results of the efficiency assessment for rotary machines are widely available, there is a lack of these analyses for linear machines. The current study provides in-depth analyses of a double-coil iron core linear motor also acting as a generator.

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

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.

Investigation of a Thermo-Fluidic Exchange Pump in Trilateral Flash and Organic Rankine Cycles

It is well known that large amounts of energy loss occurs at low temperature states in a wide range of industrial processes., The recovery and reuse of this energy is at the forefront of increasing the overall efficiencies of industrial systems. The aim of this paper is to investigate the effectiveness of using a Thermo-Fluidic Exchange (TFE) pump at low temperature conditions in both a Saturated-Vapour Organic Rankine Cycle (SORC) and a Trilateral Flash Cycle (TFC).For some low temperature applications, TFCs have been shown to achieve higher net power output than conventional SORCs, due to their ability to extract more heat from the source fluid. This is the subject of current research as a result of advancements made in the design of positive displacement machines for operation as two-phase expanders. Conventional turbines cannot be used for TFCs as they must operate in the vapour phase.One drawback of the TFC is the higher working fluid mass flow rate required. Depending on the scale of the system, this can potentially cause difficulties with pump selection. A TFE pump uses heat input to the system to increase the pressure and temperature of the working fluid, rather than the work input in a standard mechanical pump. This paper compares the net power output achievable using both mechanical and TFE pumps with SORC and TFC systems. The results suggest that the TFE pump could be a viable option for TFC systems.

Validation of FlowVision CFD on ICCS2015 Test Case: Application of Gap ModelTM and SGGRTM for Leakage Flow Prediction in a Dry Screw Compressor

Clearances and leakages through them play a vital role in determining the efficiency of positive displacement machines, especially of screw compressors. It is of interest to accurately predict the overall machine behavior including the leakage flows, even during the early design phases.CFD (Computational Fluid Dynamics) methodologies can potentially address this problem; however, the computational efficiency is limited by the well-known issue: dimensionality. This issue arises due to the fact that rotors are sized in tens of centimeters whereas the clearances are in the order of (sub) microns.Here we present a reduced order model called the Gap Model, embedded in FlowVision CFD software package to overcome this problem. Our approach is based on Poiseuille flow assumption and intends to evaluate fluid flows within very thin channels without resolving these regions with traditional fine mesh discretization. In such cases when two wall boundary conditions, such as rotor-to-rotor or rotor-to-chamber in a screw compressor, are close to each other less than mesh resolved dimensions, only one row of computational cells are constructed between the walls and an extra source term is added to the momentum equation, describing the frictional force produced by clearance walls.The presented model is applied to ICCS2015 Test Case Screw Compressor developed by Centre for Compressor Technology of City, University of London. CFD predictions using this leakage model is in agreement with experimental results within 6.3 percent maximum error. The calculation times are reduced down to less than a day for obtaining stable and converged results.

Grid Generation for CFD Analysis and Design of a Variety of Twin Screw Machines

A detailed study of the fluid flow and thermodynamic processes in positive displacement machines requires 3D CFD modeling in order to capture their real geometry, including leakage gaps. However, limitations in the conventional computational grids, used in commercial software packages, exclude their use for classical twin screw machines. The screw compressor rotor grid generator (SCORG) is a customized grid generation tool developed to overcome these limitations. This paper shows how it can be further extended to include non-conventional rotor designs, such as those with variable lead or profile variation and even internally geared machines with conical rotors. Other arrangements possible with this improvement include multiple gate rotors to increase volumetric displacement or dual lead, high wrap angle rotors for very high-pressure differences and vacuum applications. A case study of a water-injected twin screw compressor is included to demonstrate its use for both detailed flow analysis and design.

Pressure Pulsation and Cavitation Phenomena in a Micro-ORC System

Micro-ORC systems are usually equipped with positive displacement machines such as expanders and pumps. The pumping system has to guarantee the mass flow rate and allows a pressure rise from the condensation to the evaporation pressure values. In addition, the pumping system supplies the organic fluid, characterized by pressure and temperature very close to the saturation. In this work, a CFD approach is developed to analyze from a novel point of view the behavior of the pumping system of a regenerative lab-scale micro-ORC system. In fact, starting from the liquid receiver, the entire flow path, up to the inlet section of the evaporator, has been numerically simulated (including the Coriolis flow meter installed between the receiver and the gear pump). A fluid dynamic analysis has been carried out by means of a transient simulation with a mesh morphing strategy in order to analyze the transient phenomena and the effects of pump operation. The analysis has shown how the accuracy of the mass flow rate measurement could be affected by the pump operation being installed in the same circuit branch. In addition, the results have shown how the cavitation phenomenon affects the pump and the ORC system operation compared to control system actions.

Comparison of different ORC typologies for heavy-duty trucks by means of a thermo-economic optimization

This study focuses on the design phase of ORC systems recovering the heat wasted from two of the sources available on a Heavy-Duty Truck (HDT): the exhaust and recirculated gases. From these heat sources and their combinations, 5 possible architectures are considered. The main components (i.e. the heat exchangers, the pump and the expander) of the WHR systems are investigated and modeled. Plate type heat exchangers are considered for both the hot and cold sides of the system. Regarding the expansion devices, 5 positive displacement machine technologies, the scroll, screw, piston, vane and roots expanders, are considered and modeled while, among the turbo-expanders, the radial-inflow turbine is taken into consideration. A semi-empirical model is proposed to simulate a volumetric pump. The models of components are first confronted with experimental data. The validated models are then used as references for the design of the new components, which is achieved following similitude rules. This ultimately leads to 30 typologies that will be used with 6 of the various investigated working fluids. In order to identify the most promising system(s), a 3-step optimization tool is developed. First, the most suitable conditions are identified for the design of the ORC systems using a simplified model of an expansion machine. In the second step, the design phase, using more detailed models for the expanders and a proposed economic model for the overall system, a thermo-economic optimization is performed. In the third step, the output power for each of the obtained system models is maximized, optimizing the evaporating pressure and the superheating degree for various off-design conditions. The average power weighted using the frequency distribution of the gas operating conditions is computed and used to compare the 180 systems. Finally, because power is not the only criterion to select the most suitable system topology, additional criteria are taken into consideration.

Simulation-based investigation of spontaneous condensation in steam-driven screw expanders

Condensation and its effects on turbomachinery operation are well understood and have been widely investigated. However, only little scientific work on condensation in positive displacement machines has been published. Although, depending on machine type, high expansion rates and, as a consequence, significant supersaturation can be achieved for working fluids with a negative saturation vapour curve. In this paper, the effects of spontaneous steam condensation in screw expanders are discussed. Classical nucleation theory is used for the thermodynamic simulation of operational behaviour. The study shows at which point during the expansion phase spontaneous condensation can be expected and typical nucleation rates are determined. The impact of released latent heat during expansion on chamber states is depicted. Furthermore, a comparison of purely metastable expansion with equilibrium expansion is provided in order to show the full-range discrepancies. Additionally, the influence of internal leakage and throttling effects during the inlet phase on the course of spontaneous condensation and droplet growth is analysed. Typical operating parameters are widely varied in simulation so as to identify the impact of steam parameters and expander parameters on the condensation process.

An Attempt to Evaluate the Cycloidal Rotor Fan Performance

In this work, the cycloidal rotor fan (CRF) performance was estimated by means of a numerical method based on Unsteady Reynolds Averaged Navier-Stokes equations (URANS). The fan with a cycloidal rotor belongs to the positive displacement machines of the rotary type. The numerical algorithm for simulation of the flow in the cycloidal rotor as well as postprocessing of the CFD results was prepared using Ansys CFX CEL. The methodology for the assessment of the CRF performance was proposed and verified. It was found out that the CRF performance strongly depends on the shape of the profile of the applied rotor blade. The NACA 0012 and CLARK Y profiles were tested for the same CRF structure and flow conditions. Also, the crucial importance for the CRF performance has the range of the blade pitch angle change.

Scaling Criteria for Axial Piston Machines Based on Thermo-Elastohydrodynamic Effects in the Tribological Interfaces

In lieu of reliable scaling rules, hydraulic pump and motor manufacturers pay a high monetary and temporal price for attempting to expand their production lines by scaling their existing units to other sizes. The challenge is that the lubricating interfaces, which are the key elements in determining the performance of a positive displacement machine, are not easily scalable. This article includes an analysis of the size-dependence of these units with regard to the significant physical phenomena describing the behavior of their three most critical lubricating interfaces. These phenomena include the non-isothermal elastohydrodynamic effects in the fluid domain, and the heat transfer and thermal elastic deflection in the solid domain. The performance change due to size variation is found to be unavoidable and explained through fundamental physics. The results are demonstrated using a numerical fluid–structure–thermal interaction model over a wide range of unit sizes. Based on the findings, a guide to scaling swashplate-type axial piston machines such as to uphold their efficiency is proposed.