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

reducing pressure valve with real gases an integrated approach for the design

Abstract In the pursuit of an increasing cleaner fuel, methane represents a widely-employed solution for vehicles. The lower emissions, if compared to gasoline or diesel fuel, makes it an attractive opportunity in tackling transport-related pollution. Methane-powered vehicles are indeed often excluded from driving bans, pushing the demand for such kind of car. Methane is usually stored on board in tanks filled with pressure up to 20 MPa. The fuel injection systems for methane feeding usually work at pressure lower than 1 MPa (around 0.7 MPa). This difference demands a pressure-reducing valve to be installed to adjust the pressure and the fuel flow rate as required by the driver. This component and its design in hostile condition is the object of this study. Particularly, in automotive applications, the fluid operates not far from the critical point and therefore the behavior should be modelled with a real gas approach. In such light, it is immediate to note that, by the throttling procedure, the temperature of the gas drops. In addition to the acceleration of the flow, the Joule-Thomson effect related to the non-ideality of the fluid lowers the static temperature of the gas itself during the expansion. If this is combined with particularly cold environmental conditions, the material of the seals may fail entailing gas leakage. In this work, an integrated numerical and experimental study of methane fluid and thermodynamic conditions when passing through the valve orifice is reported. Extreme environmental conditions have been numerically tested, comparing and validating the results with experiments. The numerical simulations have been carried out with the open-source software suite OpenFOAM-v1712. The capability of real gas modelling has been extended by implementing a new thermophysical strategy based on the CoolProp set of libraries

real gas expansion with dynamic mesh in common positive displacement machines

Abstract Fluids processed by the machinery involved in ORC cycles undergo several transformations among which the expansion in positive displacement machines. The fluid path inside this component is very complicated and gaps play a crucial role. Due to the importance of this technical detail, gap design and optimization is a decisive step in achieving an high efficiency both of the expander and the whole cycle. In this work the fluid dynamics of several fluids commonly used in ORC cycles is investigated. Particularly, their behaviour during the expansion through the gap in operation is numerically investigated. The effects of the gap formation and its evolution on the processed fluid is studied thanks to a dynamic mesh approach. A typical application has been considered in this work: the variable gap between the fixed and mobile spirals of a scroll expander is analysed. The relative motion and in turn, the variation of the gaps during the machine operation, implies the use of particular numerical strategies able to well represent these localized geometrical features. On the top of that, the modelling of the processed fluids as a real gas determines an extra effort in the way of representing the actual behavior involved in the positive displacement machine operation. This analysis shows the local fluid dynamic phenomena due to the variable clearances. R134a and its replacements R152a and R1234ze(E), fluids widespread in the ORC cycles, are used in this work. The fluids are investigated under the same conditions and effects like separation and shock wave are highlighted. This analysis allows the comprehension of how local phenomena could affect the overall machine operation and efficiency. Gaps are the responsible of the volumetric efficiency of the machine and, coupled with (i) time-variable geometry modification, (ii) relative velocities and (iii) fluid characteristics characterize the global ORC system performance

CFD Analysis of a Non-Newtonian Fluids Processing Pump

Abstract Pumps are among the most spread machines in industrial facilities. In this work a comparative CFD analysis using different software is presented. The three-dimensional flow in the semi-open impeller and volute of a centrifugal pump is numerically simulated. The main advantage of semi-open impeller centrifugal pump is its efficiency which can be considered constant thanks to the clearance adjustment. In addition this kind of impeller is less likely to clog with solid bodies (important in case of slurry-processing). The open impeller has all the parts visible, so it is easier to inspect for wear and damages. Eventually it is lighter than a shrouded impeller: it can spin faster. The stress due to centrifugal force is indeed a limit for the speed of this machines. On the other hand its main disadvantage if compared to a shrouded pump is its lower efficiency due to the heavier tip leakage. In addition it cannot be employed in case of explosive products: the risk of contact between impeller and volute causing sparks is not negligible. The simulations have been carried out using both open-source and proprietary software: OpenFOAM®, PumpLinx ® and ANSYS-CFX ®. The performance of the machine handling both Newtonian and non-Newtonian fluids are also investigated. The numerical models and the results of the different computational strategies were compared with the experimental data and the accuracy of different software is evaluated in the case of Newtonian model. It is well known that the performance of a centrifugal pump drops processing a viscous fluid. Even so the behavior during the pumping of non-Newtonian fluids has not been investigated so far. The non-Newtonian fluid processed is a shear-thinning fluid (the apparent viscosity decreases with an increase stress). The slurries which are usually processed in the food industries, chemical plants and oil&gas processes show a usual behavior which correspond to this kind of model

WOM: Whole ORC Model

Over the last decade, environmental and economic concerns have pushed the researchers to find new solutions in the track of a more responsible use of energy. Particularly, small scale organic Rankine cycles (ORCs) have been regarded as candidate for a better employment of waste energy. In order to increase the performance of these cycles and to extend their operating range, attention has been drawn on the behavior of the different components both experimentally as well as by means of computational fluid dynamics (CFD) simulations. The numerical approach has been increasingly used in the study of the machines that compose the cycle, avoiding the problems that typically affect the experimental analyses, e.g. compatibility of refrigerant with sealing systems, and allowing for the preliminary test of new machines to be added to the cycle. In this work, the numerical analysis will be focused not only on the single components considered as a stand-alone, but rather extended on their reciprocal interaction and on the system integration of the different machines. A Whole ORC Model (WOM), can thus be built and employed as a virtual test bench. Such a virtual model can be of paramount importance in predicting the behavior of the cycle in off-design conditions or in gathering information about fluid stagnation locations. The analysis can be even extended by coupling the WOM with the external world. Specifically, the grid demand and the heat flux at the evaporator can vary: such change is translated in a variation in the boundary conditions. The response of the cycle to the external variation can be therefore monitored and studied. A full three-dimensional, transient analysis and the framework in which the WOM is developed are presented in this work. The numerical strategies employed are described, with particular attention to the fashion in which the real gas effect of the working fluid and the motion inside the positive displacement machines are treated. The performance variation in response to an external change is reported to show the capability of the virtual test bench in helping both the system conductor as well as the designer

experimental and numerical characterization of an oil free scroll expander

Abstract Micro-ORC systems are characterized by low efficiency values, but at the same time could be used as energy recovery systems in domestic applications for which reliability and low noise level represent the biggest challenges. In this paper, an integrated Reverse Engineering (RE)-Computational Fluid Dynamics (CFD) methodology is applied in order to study the adaptation of a commercial scroll compressor to be used as an expander in a micro-ORC system. The analyses reported in this paper consist of: (i) the acquisition of the 5-kW oil-free scroll expander through a RE procedure and its CAD reconstruction, (ii) the set-up of fully three-dimensional transient simulations with the Chimera strategy using the Siemens PLM software, (iii) the validation of the computational analysis by means of experimental tests and finally, (iv) the analysis of the geometry-flow features like flank and axial gaps, coupled with the analysis of the scroll volumetric efficiency and overall performance. Chimera strategy is able to move the computational grid at each time step in order to accommodate the shape and size changes of the gas pockets. The scroll characterization was carried out using both experimental and numerical tests. Six different rotational velocities in the range of (400 – 2400) rpm with a fixed pressure level (7.5 bar) were tested for validating the numerical model using air as a working fluid. The numerical model was then used to calculate the scroll expander performance operating in an existing ORC system with R134a as working fluid

PROGRESSES IN PARTICLE-LADEN FLOWS SIMULATIONS IN MULTISTAGE TURBOMACHINERY WITH OPENFOAM

Numerical simulations of particle-laden flows have received growing attention in the last decade, due to the broad spectrumof industrial applications in which discrete phases prediction is of interest. Among these, ingestion of particles by turbomachinery is an area that is seeing vivid research and studies. The mostcommon technique to tackle this kind of problem is the EulerianLagrangian method, in which individual particles are trackedinside the domain. At the same time, in multi-stage turbomachinery simulations interfaces are needed to couple the flow solution in adjacent domains in relative motion. In this work, an open-source extension for Lagrangian simulations in multistage rotating machines is presented in the foam-extend environment.Firstly, a thorough discussion of the implementation is presented, with particular emphasis on particle passage through General Grid Interfaces (GGI) and mixing planes. Moreover, a massconservative particle redistribution technique is described, as such a property is requested at interfaces between Multiple Reference Frame (MRF). The peculiarities of the algorithm are then shown on a relevant test-case. Eventually, three turbomachineryapplications are presented, with growing complexity, to show the capabilities of the numerical code in real-life applications. Simulation results in terms of erosion and impacts on aerodynamic surfaces are also presented as examples of possible parameters of interest in particle-laden flow computation

Full 3D numerical analysis of a roots blower with open-source software

In recent years, computational fluid dynamics (CFD) has been applied for the design and analysis of positive displacement machines (both compressors and expanders) for vapor compression and power generation (e.g., ORCs) applications. In particular, twin screw compressors are widely employed in industrial vapor compression systems because of their high efficiency compared to other compressor types. The numerical modeling of the operation of such machines is challenging: the dynamics of the compression (or expansion) process and the deforming working chambers make the simulation process a not-trivial task. The relative motion of the rotors and the variation of the gaps during machine operation are few of the major challenges towards the implementation of reliable CFD models. Furthermore, the elaborated working fluid (i.e. the refrigerant) operates in many cases either close to the critical point or to the saturated-vapor line. Under such conditions, the ideal gas model does not hold and, thus, a compressible real gas solver is required. Among the several numerical techniques that have been developed throughout the years, the custom predefined mesh generation is one of the most used techniques. In such an approach, a set of meshes (one for each time step) is generated in advance before running the CFD simulation. The solver is fed with the mesh for each time step retaining the configuration of the mesh unchanged. In this work, SCORG-V5.2.2 was used to generate the meshes of the deforming domain around rotating parts of the machines. This was interconnected with OpenFOAM-v1606+, which is used to compute the flow field associated with the operation of the twin screw machine. It was demonstrated that the proposed methodology allows for a fast simulation and to achieve a good agreement with experimental test results

Computational Models for the Analysis of positive displacement machines: Real Gas and Dynamic Mesh

Abstract In recent years, computational fluid dynamics (CFD) has been applied for the design and analysis of positive displacement machines (both compressors and expanders) with numerous challenges due to the dynamics of the compression (or expansion) process and deforming working chambers. The relative motion and in turn, the variation of the gaps during machine operation implies several obstacles for the implementation of reliable CFD models. The majority of the studies reported in literature focused on scroll, twin screw and reciprocating machines. The limitation of the developed methodologies to be applied directly to positive displacement machines with more complex meshing such as that of single-screw has been highlighted in literature. In this paper, a single screw expander is studied by means of (i) a moving mesh technique (dynamic mesh in the Key Frame Remeshing approach) and (ii) a real gas model of a R134a (Peng-Robinson model) implemented in OpenFOAM ®. On the top of that, all the possible techniques that come with the software are investigated in their application to single screw. An useful review of the state of the art CFD with open-source software (OpenFOAM-v1606+ and foam-extend4.0) is therefore carried out. The reliability of CFD model represents indeed the first step on which the design process and further optimization will be based

A Smad3 transgenic reporter reveals TGF-beta control of zebrafish spinal cord development

TGF-beta (TGFβ) family mediated Smad signaling is involved in mesoderm and endoderm specification, left-right asymmetry formation and neural tube development. The TGFβ1/2/3 and Activin/Nodal signal transduction cascades culminate with activation of SMAD2 and/or SMAD3 transcription factors and their overactivation are involved in different pathologies with an inflammatory and/or uncontrolled cell proliferation basis, such as cancer and fibrosis. We have developed a transgenic zebrafish reporter line responsive to Smad3 activity. Through chemical, genetic and molecular approaches we have seen that this transgenic line consistently reproduces in vivo Smad3-mediated TGFβ signaling. Reporter fluorescence is activated in phospho-Smad3 positive cells and is responsive to both Smad3 isoforms, Smad3a and 3b. Moreover, Alk4 and Alk5 inhibitors strongly repress the reporter activity. In the CNS, Smad3 reporter activity is particularly high in the subpallium, tegumentum, cerebellar plate, medulla oblongata and the retina proliferative zone. In the spinal cord, the reporter is activated at the ventricular zone, where neuronal progenitor cells are located. Colocalization methods show in vivo that TGFβ signaling is particularly active in neuroD+ precursors. Using neuronal transgenic lines, we observed that TGFβ chemical inhibition leads to a decrease of differentiating cells and an increase of proliferation. Similarly, smad3a and 3b knock-down alter neural differentiation showing that both paralogues play a positive role in neural differentiation. EdU proliferation assay and pH3 staining confirmed that Smad3 is mainly active in post-mitotic, non-proliferating cells. In summary, we demonstrate that the Smad3 reporter line allows us to follow in vivo Smad3 transcriptional activity and that Smad3, by controlling neural differentiation, promotes the progenitor to precursor switch allowing neural progenitors to exit cell cycle and differentiate