Non-Ideal Compressible-Fluid Dynamics (NICFD) is the branch of fluid mechanics devoted to the study of compressible flows whose behaviour deviates from the one predicted by the ideal-gas model. Non-ideal behaviour is typically observed in dense-gas flows, namely fluids made of complex molecules operating in thermodynamic conditions close to the liquid-vapour saturation curve. One of the most intriguing phenomena predicted by the NICFD theory is the non-monotone variation of the Mach number along isentropic expansions in converging-diverging nozzles. So far, this peculiar behaviour has never been observed experimentally. In the present thesis, an experiment is devised for the observation of the nonmonotone behaviour of the Mach number in a de Laval nozzle operating with MM (hexamethyldisiloxane, C6H18OSi2) in the TROVA (Test Rig for Organic VApours) wind tunnel of Politecnico di Milano. From the NICFD theory and state-of-the-art thermodynamic models, it is found that the reservoir thermodynamic states to be attained in order to observe the non-monotone trend of the Mach number along the expansion are spread over a narrow range, close to the thermal stability limit of MM. In order to correctly reproduce the desired reservoir thermodynamic states and to observe the non-monotone Mach trend, tests require a particularly accurate setup. To this purpose, a dynamic simulation model capable of treating the discontinuous mode of operation of the TROVA is formulated and implemented. The simulation tool predicts the trend of the flow variables during the time evolution of the experiment and, therefore, it can be exploited to compare different experimental setup. The dynamic simulation tool and the nozzle design procedure are assessed against experimental data from the TROVA wind tunnel. Experiments are performed to characterize the expansion in a supersonic nozzle designed ad hoc in the present work to operate with MM in less demanding process conditions compared to the ones required to observe the non-monotone Mach behaviour, though still in the NICFD regime. The final design of the nozzle for the observation of non-monotone Mach behaviour is performed. For both nozzles, CFD simulations are carried out to compute the flow field in design and selected off-design conditions.
La fluidodinamica comprimibile non ideale è la branca della meccanica dei fluidi volta allo studio di correnti comprimibili il cui comportamento devia da quello predetto dal modello del gas ideale. Il comportamento non ideale viene osservato tipicamente in correnti di gas densi, cioè fluidi composti da molecole complesse che operano in condizioni termodinamiche vicine alla curva di saturazione. Uno dei fenomeni più interessanti predetti dalla teoria NICFD è l’andamento non monotono del numero di Mach nelle espansioni isentropiche in ugelli convergenti-divergenti. Questo particolare fenomeno non è mai stato osservato perimentalmente ad oggi. In questo lavoro di tesi è stato concepito un esperimento per l’osservazione sperimentale dell’andamento non monotono del numero di Mach in un ugello de Laval,che operi con il fluido MM (hexamethyldisilossano, C6H18OSi2) nella galleria del vento supersonica del politecnico di Milano, denominata TROVA (Test Rig for Organic VApors). Dalla teoria NICFD e dai più sofisticati modelli termodinamici, si evince che le condizioni totali che devono essere raggiunte per permettere un comportamento non monotono del numero di Mach si trovano in una regione del piano termodinamico ristretta, in prossimità del limite di stabilità termica del fluido. Per riprodurre sperimentalmente tali condizioni, è necessario un accurato setup dell’esperimento. A questo proposito, è stato formulato ed implementato un modello per la simulazione dinamica del TROVA. Il software di simulazione sviluppato consente di predire l’andamento delle variabili fluidodinamiche durante l’esperimento e, perciò, può essere usato per la pianificazione degli esperimenti. Il software di simulazione e la procedura di design degli ugelli sono verificati attraverso il confronto con i dati sperimentali del TROVA. Gli esperimenti sono effettuati su un ugello progettato ad hoc in questa tesi per operare con MM in condizioni meno critiche di quelle richieste dall’osservazione del comportamento non monotono del Mach, benchè non ideali. In questo modo è stato possibile progettare un ugello che mostrasse un andamento non monotono del numero di Mach durante ’espansione. Per entrambi gli ugelli, lo studio del campo di moto è effettuato attraverso simulazioni CFD, in condizioni di design e off-design.
Non-ideal nozzle design and simulation for an organic vapour wind tunnel
BOTTIN, MATTEO
2016/2017
Abstract
Non-Ideal Compressible-Fluid Dynamics (NICFD) is the branch of fluid mechanics devoted to the study of compressible flows whose behaviour deviates from the one predicted by the ideal-gas model. Non-ideal behaviour is typically observed in dense-gas flows, namely fluids made of complex molecules operating in thermodynamic conditions close to the liquid-vapour saturation curve. One of the most intriguing phenomena predicted by the NICFD theory is the non-monotone variation of the Mach number along isentropic expansions in converging-diverging nozzles. So far, this peculiar behaviour has never been observed experimentally. In the present thesis, an experiment is devised for the observation of the nonmonotone behaviour of the Mach number in a de Laval nozzle operating with MM (hexamethyldisiloxane, C6H18OSi2) in the TROVA (Test Rig for Organic VApours) wind tunnel of Politecnico di Milano. From the NICFD theory and state-of-the-art thermodynamic models, it is found that the reservoir thermodynamic states to be attained in order to observe the non-monotone trend of the Mach number along the expansion are spread over a narrow range, close to the thermal stability limit of MM. In order to correctly reproduce the desired reservoir thermodynamic states and to observe the non-monotone Mach trend, tests require a particularly accurate setup. To this purpose, a dynamic simulation model capable of treating the discontinuous mode of operation of the TROVA is formulated and implemented. The simulation tool predicts the trend of the flow variables during the time evolution of the experiment and, therefore, it can be exploited to compare different experimental setup. The dynamic simulation tool and the nozzle design procedure are assessed against experimental data from the TROVA wind tunnel. Experiments are performed to characterize the expansion in a supersonic nozzle designed ad hoc in the present work to operate with MM in less demanding process conditions compared to the ones required to observe the non-monotone Mach behaviour, though still in the NICFD regime. The final design of the nozzle for the observation of non-monotone Mach behaviour is performed. For both nozzles, CFD simulations are carried out to compute the flow field in design and selected off-design conditions.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/139435