An innovative methodology for the analysis of microparticle deposits in transonic and subsonic blades for the assessment of compressor degradation

Solid particle ingestion is one of the principal degradation mechanisms in the compressor section of heavy-duty gas turbines. Usually, foulants in the ppm range not captured by the air filtration system cause deposits on blading and result in a severe performance drop of the compressor. It is of great interest to the manufacturer and industry to determine which areas of the compressor airfoils are affected by these contaminants as a function of the location of the power unit. The aim of this work is the estimation of the actual deposits on the blade surface in terms of location and quantity. Particle trajectory simulations use a stochastic Lagrangian tracking method which solves the equations of motion separately from the continuous phase. Then, a transonic rotor and subsonic rotor are considered as a case study for the numerical investigation. The compressor rotor numerical model and the discrete phase treatment have been validated against the experimental and numerical data available in literature. The size of the particles, their concentrations and the filtration efficiency are specified in order to perform a realistic quantitative analysis of the fouling phenomena in an axial compressor. This study combines the impact/adhesion characteristic of the particles obtained through a Computational Fluid Dynamics (CFD) numerical simulation and the real size distribution of the contaminants in the air swallowed by the compressor. The kinematic characteristics (velocity and angle) of the impact of micrometric and sub-micrometric particles with the blade surface of an axial transonic and subsonic rotor are shown. The blade zones affected by particle impact are extensively analyzed. This work has the goal of combining the kinematic characteristics of particle impact on the blade with fouling phenomenon through the use of a quantity called ‘sticking probability’ adopted from literature. The analysis shows that particular fluid-dynamic phenomena such as separation, shock waves and tip leakage vortex strongly influence pattern deposition. The combination of the smaller particles (0.15 ― 0.25) μm and the larger ones (1.00 ― 1.50) μm determines the highest amounts of deposits on the leading edge of the compressor airfoil. The blade zones affected by deposits are clearly reported by using easy-to-use contaminant maps realized on the blade surface in terms of contaminant mass per unit of time. From these analyses, some guidelines for proper installation and management of the power plant (in terms of filtration systems and washing strategies) can be drawn

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

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

An Interdisciplinary Approach to Study the Fouling Phenomenon

Abstract Solid particle ingestion is one of the principal degradation mechanisms in the compressor section of heavy-duty gas turbines. Foulants in the ppm range which are not captured by the air filtration system usually cause deposits on blading,which results in a severe drop in the performance of the compressor.Through the interdisciplinary approach proposed in this paper, it is possible to determine the evolution of the fouling phenomenon through the integration of several studies in different research fields: (i) numerical simulation, (ii) power plant characteristicsand (iii) particle-adhesion characteristics.This paper shows the possibility of linking the numerical results related to the impact/adhesion characteristic of the particles with the actual air contamination data and operating condition of the power units. In fact, the size of the particles, their concentrations and the filtration efficiency represent the major contributors to performing a realistic quantitative analysis of the fouling phenomena in an axial compressor.The integration of these research fields could represent a valuable support for the investigation of the relationship between compressor airfoil design and fouling rate

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

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

CFD Approaches Applied To A Single-Screw Expander

Organic Rankine Cycle (ORC) systems rely on the expander performance to generate power output in an efficient manner. Especially in the low power range (below 100 kWe), positive displacement (PD) expanders (e.g. scroll, twin-screw, reciprocating, vane, spool, etc.) result to be cost-effective. However, commercially available PD expanders are still limited and, in many cases, the existing PD compressors are operated in reversed mode by introducing design modifications to sealing, bearings, port sizes, lubrication requirements to increase both their performance and reliability. Computational fluid dynamics (CFD) as a design and analysis tool of positive displacement machine has been proven to be viable. Challenges arise when CFD is applied to PD machines due to the dynamics of the expansion (or compression) process, presence of internal leakages and heat transfer mechanisms, as well as deforming working chambers. Different grid generation methods and solution schemes have been successfully implemented to scroll, twin-screw and reciprocating machines (Rane et al. 2012, Rane et al. 2013). The limitation of such methodologies to be applied directly to complex multi-rotor machines has been highlighted by Rane et al. (Rane at al. 2012). In this paper, a single-screw expander is used as benchmark to evaluate different grid generation methodologies (dynamic remeshing and Chimera strategy overlapping grid) and commercial software, in terms of computational resources required, accuracy of the results and limitations. The calculations have been performed with air to reduce the complexity of the problem.     REFERENCES Rane S., Kovacevic A., Kethidi M., “CFD Modeling in Screw Compressors with complex multi rotor configurationsâ€(2012), Int. Compressor Engineering Conference at Purdue Univ. Paper 2141. Rane S., Kovacevic A., Stosic N., Kethidi M., “Grid deformation strategies for CFD analysis of screw compressorsâ€, Int. J. Refrigeration, 36(2013), 1883-1893