Energy efficiency is a broad topic that can fit different fields, from transportation to buildings and power systems. In this thesis, subdivided into three main chapters, the attention is posed to novel energy-conversion systems that convert thermal energy to electrical energy, such as organic Rankine cycles (ORCs) and supercritical carbon dioxide (sCO2) power systems. For both technologies, the system efficiency is strongly connected to the achievable efficiency of the turbomachinery components, whose design practices cannot rely on several decades of experimental activities. A straightforward reworking of established design criteria may lead to shortcomings, owing to the unconventional gas dynamics underlying ORC and sCO2 turbomachinery, which considerably differs from the one based on the ideal-gas thermodynamics. With the goal of developing dedicated design rules that can enable high-efficient ORC and sCO2 turbomachinery, the original contribution of this research is twofold: first, to advance the present knowledge on turbomachinery operating in the so-called non-ideal compressible fluid-dynamics (NICFD) regime, and second, to develop a number of computational tools that can be instrumental for the design and the analysis of such complex energy devices. The first chapter discusses the development of an automated shape-optimization technique based on high-fidelity computational fluid-dynamic (CFD) simulations. To tackle the prohibitive computational cost that would require a straight CFD-based optimization, a surrogate approach based on the Kriging interpolation is used. The optimization tool is then applied to a variety of test cases, involving stator and rotor cascades, transonic and supersonic flow regimes, ideal-like and significantly non-ideal flows, and accounting for single and multiple operating conditions. The substantial increase in efficiency for all reported cases indicates that shape-optimization techniques can be effectively used for the design of turbomachinery for which well-established design rules are presently not available. The second chapter analyses the peculiar gas dynamics that onsets when Γ< 1, where Γ is the fundamental derivative of gas dynamics. It is numerically demonstrated that non-ideal phenomena can occur in supersonic turbine flows, observing simultaneously a non-monotonic variation of the Mach number against the pressure along the expansion and non-ideal oblique shocks/rarefaction fans featuring an increase/decrease of the Mach number. Moreover, as the gas-dynamic evolution is found to be highly sensitive to the upstream stagnation state, the deviations in turbine performance given realistic field variabilities in the power cycles are quantified through an uncertainty-quantification framework based on a polynomial-chaos representation of the flow solver. The third chapter deals with non-ideal effects arising in characteristic flows of carbon dioxide in turbomachinery for sCO2 power systems. Based on a mean-line analysis, it is shown that competitive turbomachinery efficiencies can be obtained when accounting for the actual volumetric flow evolutions. As local flow accelerations within the compressor can promote phase transition, two CFD models capable of dealing with compressible non-ideal two-phase flows are developed and validated. Both models assume thermodynamic equilibrium, but one model also introduces an additional simplification in the thermodynamic treatment. Finally, the simulation of a realistic centrifugal impeller operating in near-critical conditions is undertaken, with a particular emphasis on the effect of variable intake thermodynamic conditions on the compressor performance. As a result, an extended similitude theory that explicitly accounts for both non-ideal and two-phase effects is proposed.
Il concetto di efficienza energetica è utilizzato in diversi contesti, dal trasporto agli edifici e alla produzione di energia. In questa tesi, divisa in tre capitoli, l’attenzione è rivolta a innovativi sistemi di conversione dell’energia che convertono l’energia termica in energia elettrica, come i cicli operanti con fluidi organici (ORC) o con anidride carbonica in condizioni supercritiche (sCO2). Per entrambe le tecnologie, l’efficienza del sistema è considerevolmente dipendente dall’efficienza raggiungibile dalle sue turbomacchine, le cui tecniche di progetto non possono basarsi su dati sperimentali perché al momento deficitari. Riadattare tecniche di progetto per macchine convenzionali può comportare prestazioni subottimali se applicate a nuove macchine, in virtù di una gasdinamica non-ideale che largamente differisce da quella basata sul modello di gas ideale. Con l’obiettivo di sviluppare tecniche di progetto dedicate che possano portare al progetto di turbomacchine efficienti nel campo degli ORC e sistemi sCO2, il contributo di questa tesi è duplice: in primo luogo, avanzare nella comprensione dei fenomeni che avvengono nelle turbomacchine che operano in regime di gasdinamica non-ideale e, in secondo luogo, sviluppare tecniche computazionali che possano aiutare il progetto e l’analisi di questi complessi elementi di conversione dell’energia. Nel primo capitolo è sviluppato un algoritmo di ottimizzazione della forma di profili di turbomacchine basato su codici di fluidodinamica computazionale (CFD). Per ridurre il proibitivo costo computazionale che richiederebbe un’ottimizzazione unicamente basata su calcoli CFD, è stato implementato un approccio surrogato basato sull’interpolazione Kriging. L’ottimizzatore è stato testato su diverse applicazioni, tra cui schiere statoriche e rotoriche che operano in regime di flusso transonico e supersonico, correnti ideali e non-ideali, ed includendo nella funzione obiettivo sia singole che multiple condizioni operative. Il significativo incremento di efficienza per tutti i casi studiati è una chiara prova che tecniche di ottimizzazione di forma possono essere utilizzate in maniera efficace nel progetto di turbomacchine per le quali ancora non esistono tecniche di progetto consolidate. Nel secondo capitolo è stata studiata la particolare gasdinamica che si verifica per Γ< 1, dove Γ è la derivata fondamentale della gasdinamica. Si è dimostrato numericamente che fenomeni non-ideali possono verificarsi in correnti di turbine supersoniche, osservando simultaneamente la variazione non monotona del numero di Mach lungo l’espansione e l’aumento/diminuzione di quest’ultimo attraverso onde oblique di compressione/rarefazione. Inoltre, come conseguenza di un’evoluzione della gasdinamica significativamente sensibile a variazioni dello stato di ristagno a monte, le variazioni prestazionali della turbina soggetta a realistiche variabilità nel ciclo di potenza sono state quantificate attraverso una tecnica di quantificazione dell’incertezza basata su una rappresentazione in caos polinomiale del codice computazionale. Il terzo capitolo affronta gli effetti non-ideali che possono presentarsi in correnti di anidride carbonica caratteristiche di turbomacchine per cicli di potenza sCO2. A partire da un’analisi a parametri concentrati, è stato dimostrato che efficienze competitive possono essere ottenute quando si tiene in considerazione la reale volumetria del fluido in condizioni di gas non-ideale. Dato che la corrente è soggetta ad accelerazioni locali all’interno di turbocompressori, che possono dar luogo a flussi bifase, due tecniche CFD sono state sviluppate per simulare correnti comprimibili di miscele bifase non-ideali. Entrambi i modelli assumono equilibrio termodinamico; inoltre, uno dei due modelli introduce un’ulteriore semplificazione nella descrizione termodinamica del flusso. Grazie a questi codici, è stato possibile simulare una girante di compressore centrifugo che opera vicino al punto critico, focalizzando lo studio sulle prestazioni della macchina al variare delle condizioni termodinamiche in ingresso. Come risultato è stata proposta una teoria della similitudine estesa che ha il merito di includere sia effetti non-ideali che flussi bifase.
Turbomachinery operating in the non-ideal compressible fluid-dynamic regime: a step towards next-generation power systems
Romei, Alessandro
2020/2021
Abstract
Energy efficiency is a broad topic that can fit different fields, from transportation to buildings and power systems. In this thesis, subdivided into three main chapters, the attention is posed to novel energy-conversion systems that convert thermal energy to electrical energy, such as organic Rankine cycles (ORCs) and supercritical carbon dioxide (sCO2) power systems. For both technologies, the system efficiency is strongly connected to the achievable efficiency of the turbomachinery components, whose design practices cannot rely on several decades of experimental activities. A straightforward reworking of established design criteria may lead to shortcomings, owing to the unconventional gas dynamics underlying ORC and sCO2 turbomachinery, which considerably differs from the one based on the ideal-gas thermodynamics. With the goal of developing dedicated design rules that can enable high-efficient ORC and sCO2 turbomachinery, the original contribution of this research is twofold: first, to advance the present knowledge on turbomachinery operating in the so-called non-ideal compressible fluid-dynamics (NICFD) regime, and second, to develop a number of computational tools that can be instrumental for the design and the analysis of such complex energy devices. The first chapter discusses the development of an automated shape-optimization technique based on high-fidelity computational fluid-dynamic (CFD) simulations. To tackle the prohibitive computational cost that would require a straight CFD-based optimization, a surrogate approach based on the Kriging interpolation is used. The optimization tool is then applied to a variety of test cases, involving stator and rotor cascades, transonic and supersonic flow regimes, ideal-like and significantly non-ideal flows, and accounting for single and multiple operating conditions. The substantial increase in efficiency for all reported cases indicates that shape-optimization techniques can be effectively used for the design of turbomachinery for which well-established design rules are presently not available. The second chapter analyses the peculiar gas dynamics that onsets when Γ< 1, where Γ is the fundamental derivative of gas dynamics. It is numerically demonstrated that non-ideal phenomena can occur in supersonic turbine flows, observing simultaneously a non-monotonic variation of the Mach number against the pressure along the expansion and non-ideal oblique shocks/rarefaction fans featuring an increase/decrease of the Mach number. Moreover, as the gas-dynamic evolution is found to be highly sensitive to the upstream stagnation state, the deviations in turbine performance given realistic field variabilities in the power cycles are quantified through an uncertainty-quantification framework based on a polynomial-chaos representation of the flow solver. The third chapter deals with non-ideal effects arising in characteristic flows of carbon dioxide in turbomachinery for sCO2 power systems. Based on a mean-line analysis, it is shown that competitive turbomachinery efficiencies can be obtained when accounting for the actual volumetric flow evolutions. As local flow accelerations within the compressor can promote phase transition, two CFD models capable of dealing with compressible non-ideal two-phase flows are developed and validated. Both models assume thermodynamic equilibrium, but one model also introduces an additional simplification in the thermodynamic treatment. Finally, the simulation of a realistic centrifugal impeller operating in near-critical conditions is undertaken, with a particular emphasis on the effect of variable intake thermodynamic conditions on the compressor performance. As a result, an extended similitude theory that explicitly accounts for both non-ideal and two-phase effects is proposed.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/175160