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

An open-source framework for the numerical analysis of constant and variable thickness scroll compressors

Le simulazioni numeriche rappresentano uno degli strumenti più rilevanti per la valutazione delle performance di compressori ed espansori di tipo scroll. I modelli monodimensionali sono una soluzione importante per ottenere informazioni veloci ed affidabili sul funzionamento generale di queste macchine, mentre le analisi CFD forniscono dati più accurati sul loro comportamento transitorio, in particolare durante i processi di aspirazione e scarica. Sfortunatamente, la disponibilità di strumenti open-source per condurre analisi di basso ordine su macchine volumetriche è ancora limitata. In questa tesi, l'autore ha realizzato sia analisi monodimensionali che simulazioni di fluidodinamica computazionale su macchine di tipo scroll. Il software scelto per le simulazioni a basso ordine è PDSim, un codice open-source rilasciato di recente e utilizzato per le analisi quasistatiche di macchine volumetriche. In alternativa, il ben noto software OpenFOAM è stato adottato per realizzare analisi tridimensionali. Contribuendo all'espansione delle possibilità di modellazione open-source di macchine scroll, sono stati realizzate due serie di librerie software per OpenFOAM. La prima, CoolFOAM, è un wrapper che connette OpenFOAM con la libreria termofisica CoolProp. La seconda, scrollFOAM, estende le possibilità del software permettendo di realizzare simulazioni di scroll totalmente tridimensionali. Tutti gli strumenti numerici utilizzati in questa tesi sono stati validati utilizzando dati sperimentali e modelli recuperati in letteratura. In particolare, i risultati di PDSim e OpenFOAM sono stati confrontati tra di loro e con una serie di misurazioni effettuate su un compressore scroll di origine industriale. Le librerie sviluppate sono state impiegate nell'ambito di diverse analisi, tra cui uno studio degli effetti di gas reale sulle simulazioni CFD di un espansore scroll e un'analisi multi-componente di una porzione di un ciclo ORC. Dal punto di vista del compressore, è stato studiato il comportamento della macchina in condizioni fuori progetto. In aggiunta, è stata effettuata un'analisi di sensibilità sull'effetto dei trafilamenti all'interno della macchina. In seguito, sia scrollFOAM che PDSim sono stati adattati per supportare una nuova formulazione geometrica per geometrie scroll a spessore variable. L'adozione di spirali a spessore variabile è teoricamente associata con diversi vantaggi, tra cui una riduzione dei costi e un aumento dei rendimenti. Infine, PDSim e OpenFOAM sono stati accoppiati per formare una catena open-source per la progettazione e ottimizzazione di scroll a spessore costante e variabile. Questa catena è stata testata realizzando un'ottimizzazione di tipo kriging centrata su un compressore a spessore variabile operante in condizioni di under-compression. Partendo da un riferimento iniziale a spessore costante, la geometria ottimizzata ha dimostrato di raggiungere prestazioni comparabili associate a una consistente riduzione dei costi di produzione.Numerical analyses are one of the most important tools to evaluate the performance of scroll compressors and expanders. 0-D models are an important solution to obtain fast and reliable information on the general performance of these machines, while CFD simulations give more accurate data on the transient behaviour of the compressors, especially during the suction and discharge process. Unfortunately, the availability of open-source tools for low-order and CFD simulations of positive displacement machines is still limited. In this thesis, the author has performed both 0-D and CFD analyses of scroll machines. The numerical tool selected for low-order simulations is a recently published open-source tool for the quasi-static analysis of positive displacement machines, PDSim. On the other hand, the well known OpenFOAM software has been adopted for 3-D numerical analyses. Contributing to the expansion of the open-source modelling capabilities of scrolls, two sets of libraries for OpenFOAM have been developed. The first, CoolFOAM, is a wrapper that couples OpenFOAM with the thermophysical library CoolProp. The second, scrollFOAM, extends the software capabilities allowing to perform full 3-D simulations of scrolls. All the numerical tools employed in this thesis have been validated against experimental references and test cases retrieved in the literature. In particular, the results of PDSim and OpenFOAM simulations have been compared to each other and to a set of measurements taken from an industrial scroll compressor. The libraries have been employed for different analyses, including the investigation of real gas effects on CFD simulations of a scroll expander and a multi-component numerical investigation of a portion of an ORC. From the compressor point of view, the behaviour in off-design conditions and a sensitivity analysis of gap widths have been objects of additional studies. Lastly, both scrollFOAM and PDSim have been adapted to support a new geometrical formulation for variable thickness scroll geometries. The adoption of variable thickness wraps is theoretically associated with several advantages, including costs reduction and increased efficiencies. Finally, PDSim and OpenFOAM have been coupled to form an open-source tool-chain for the design and optimization of constant and variable thickness scrolls. This tool-chain has been tested by performing a kriging optimization process of a variable thickness compressor working in under-compression. Starting from a constant thickness reference, the optimized geometry reached comparable performance with a consistent reduction of the manufacturing costs

Structured Mesh Generation and Numerical Analysis of a Scroll Expander in an Open-Source Environment

The spread of the organic rankine cycle applications has driven researchers and companies to focus on the improvement of their performance. In small to medium-sized plants, the expander is the component that has typically attracted the most attention. One of the most used types of machine in this scenario is the scroll. Among the other methods, numerical analyses have been increasingly exploited for the investigation of the machine’s behaviour. Nonetheless, there are major challenges for the successful application of computational fluid dynamics (CFD) to scrolls. Specifically, the dynamic mesh treatment required to capture the movement of working chambers and the nature of the expanding fluids require special care. In this work, a mesh generator for scroll machines is presented. Given few inputs, the software described provides the mesh and the nodal positions required for the evolution of the motion in a predefined mesh motion approach. The mesh generator is developed ad hoc for the coupling with the open-source CFD suite OpenFOAM. A full analysis is then carried out on a reverse-engineered commercial machine, including the refrigerant properties calculations via CoolProp. It is demonstrated that the proposed methodology allows for a fast simulation and achieves a good agreement with respect to former analyses

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

Low-Noise Airfoils for Turbomachinery Applications: Two Examples of Optimization

Automotive fans, small wind turbines, and manned and unmanned aerial vehicles (MAVs/UAVs) are just a few examples in which noise generated by the flow’s interaction with aerodynamic surfaces is a major concern. The current work shows the potential of a new airfoil shape to minimize noise generation, maintaining a high lift-to-drag ratio in a prescribed Reynolds regime. This investigation uses a multifidelity approach: a low-fidelity semiempirical model is exploited to evaluate the sound pressure level (SPL). Fast evaluation of a low-cost function enables the computation of a large range of possible profiles, and accuracy is added to the low-fidelity response surface with high-fidelity CFD data. The constraint of maintaining a predefined range of the lift coefficient and lift-to-drag ratio ensures the possibility of using this profile in usual design procedures

CFD-based optimization of scroll compressor design and uncertainty quantification of the performance under geometrical variations

Positive displacement (PD) machines are widely used in several applications such as in vapor and power generation systems. Nevertheless, their design is still based on standard approaches mainly driven by thermodynamic analysis and theoretical correlations. Geometrical details influence on the machine performance is often neglected. Because of this, PD machines hydraulic efficiency has not really been improved. The present work shows an innovative design strategy aimed at maximizing the machine efficiency by topology optimization. The geometry of a scroll compressor has been parametrized. The parameters were optimized with a Particle Swarm Optimization (PSO) based procedure integrated with Computational Fluid Dynamics (CFD) to achieve the maximum efficiency. In order to better understand the influence of these parameters on machine performances, a Design of Experiment (DOE) approach was also used. Afterward an Uncertainty Quantification framework is applied to the compressor to identify the reliability of the optimal design subject to geometrical variations. Among all the investigated parameters, the most important seem to be the high-pressure port shape and size. Performance are highly affected also by the number of coils which defines the built-it compression ratio

A strategy for the robust forecasting of gas turbine health subjected to fouling

Fouling represents a major problem for Gas Turbines (GTs) in both heavy-duty and aero-propulsion applications. Solid particles entering the engine can stick to the internal surfaces and form deposits. Components' lifetime and performance can dramatically vary as a consequence of this phenomenon. These effects impact the whole engine in terms of residual life, operating stability, and maintenance costs. In the High-Pressure Turbine (HPT), in particular, the high temperatures soft the particles and promote their adhesion, especially in the short term. Unfortunately, predicting the GT response to this detrimental issue is still an open problem for scientists. Furthermore, the stochastic variations of the components operating conditions increase the uncertainty of the forecasting results. In this work, a strategy to predict the effects of turbine fouling on the whole engine is proposed. A stationary Gas Path Analysis (GPA) has been performed for this scope to predict the GT health parameters. Their alteration as a consequence of fouling has been evaluated by scaling the turbine map. The scaling factor has been found by performing Computational Fluid Dynamic (CFD) simulations of a HPT nozzle with particle injection. Being its operating conditions strongly uncertain, a stochastic analysis has been conducted. The uncertainty sources considered are the circumferential hot core location and the turbulence level at the inlet. The study enables to build of confidence intervals on the GT health parameters predictions and represents a step forward towards a robust forecasting tool