Modern aero-engine combustors burn a lean and premixed mixture. These new combustion chamber concepts have been adopted to reduce flame temperature, but they are responsible for the hot flow unsteady character developed after combustion. The turbulent flame so produced induces larger heat release fluctuations which generate Entropy Waves and hot-spots. Furthermore, to enhance the fuel air mixing, combustion air is swirled leading to vorticity disturbances at first stage stator inlet. These instabilities generated during combustion are the cause of some drawbacks. First, vorticity disturbances interact with secondary flows. These, together with hot-spots and Entropy Waves, make more challenging the cooling technique design constraining the designers to take on board blades overheating. Lastly, all these disturbances involve an indirect noise increase. Due to the proximity of airports to residential zones and to make more comfortable passengers flight, the reduction of acoustic emissions is now a major field of research both for industry and research groups. To properly study combustor-turbine interactions, each single disturbance component should be insulated and studied. For this purpose an Entropy Wave Generator (EWG) has been designed and characterized in this Thesis project, so to allow future investigation in the High Pressure Turbine Test Rig of the Laboratorio di Fluidodinamica delle Macchine. The EWG must be able to produce an Entropy Wave that suits with aero-engine constructors requirements. At the beginning of this Thesis work, the design phase is approached and the different manufactured prototypes are illustrated. Afterwards, by means of properly calibrated measuring instruments, the different configurations are tested in the convergent nozzle of LFM Wind Tunnel to simulate EWG insertion in the turbine flux. Thanks to the experience acquired during experimental tests, some decisions taken in the design phase have been revised so to achieve better outcomes. Experiments show that Entropy Wave temperature fluctuation amplitude reduces as the frequency increases and the feeding pressure decreases. EW properties measured in the nozzle at the representative stator inlet position and with the engine-representative frequency of 120 Hz have a minimum temperature of 50°C, a peak-to-peak temperature value of 20°C and a Swirl Number of 0.4. These instabilities generated during combustion are the cause of some drawbacks. First, vorticity disturbances interact with secondary flows. These, together with hot-spots and Entropy Waves, make more challenging the cooling technique design constraining the designers to take on board blades overheating. Lastly, all these disturbances involve an indirect noise increase. Due to the proximity of airports to residential zones and to make more comfortable passengers flight, the reduction of acoustic emissions is now a major field of research both for industry and research groups. To properly study combustor-turbine interactions, each single disturbance component should be insulated and studied. For this purpose an Entropy Wave Generator (EWG) has been designed and characterized in this Thesis project, so to allow future investigation in the High Pressure Turbine Test Rig of Politencico di Milano. The EWG must be able to produce an Entropy Wave that suits with aero-engine constructors requirements. At the beginning of this Thesis work, the design phase is approached and the different manufactured prototypes are illustrated. Afterwards, by means of properly calibrated measuring instruments, the different configurations are tested in the convergent nozzle of LFM Wind Tunnel to simulate EWG insertion in turbine flux. Thanks to the experience acquired during experimental tests, some decisions taken in the design phase have been revised so to achieve better outcomes. Experiments show that Entropy Wave temperature fluctuation amplitude reduces as the frequency increases and the feeding pressure decreases. EW properties measured in the nozzle at the representative stator inlet position and with the engine-representative frequency of 120 Hz have a minimum temperature of 50°C, a peak-to-peak temperature value of 20°C and a Swirl Number of 0.4.
I moderni combustori dei motori aeronautici bruciano una miscela magra e premiscelata per ridurre la temperatura di fiamma. L'utilizzo di tale tecnica comporta una forte instazionarietà del flusso d'aria caldo generato dopo la combustione. Una fiamma turbolenta comporta rilevanti rilasci di calore non stazionari che dunque sono responsabili della generazione di onde entropiche o Entropy Waves e hot-spots. Inoltre, nel flusso che interagisce con lo statore del primo stadio di turbina sono presenti alcuni disturbi del campo di velocità causati dal moto di swirl indotto nella camera di combustione per favorire il miscelamento del flusso principale con il combustibile. Questi fenomeni generati dalle instazionarietà del combustore sono responsabili di alcuni inconvenienti. Innanzitutto, i disturbi di vorticità interagiscono con i flussi secondari. Tali disturbi inoltre, combinati con hot-spots ed Entropy Waves, complicano la progettazione dei sistemi di raffreddamento delle pale, obbligando i progettisti a tenere conto delle sovratemperature generate dai flussi caldi instazionari. Infine, i tre i risultati dell'instazionarietà discussi comportano un aumento del rumore indiretto. A causa della vicinanza degli aeroporti ai centri abitati e per rendere più confortevole il volo dei passeggeri, ridurre il rumore dei motori aeronautici è una delle principali sfide dei ricercatori e costruttori di tali macchine. Per poter studiare al meglio le interazioni combustore-turbina, le singole forme di instazionarietà devono essere isolate e studiate separatamente le une dalle altre. Per tale motivo, un Generatore di Onde di Entropia (EWG) è stato progettato e testato in questo progetto di tesi, in modo da poterlo utilizzare nel banco prova per turbine di alta pressione del Laboratorio di Fluidodinamica delle Macchine. L'EWG deve essere in grado di produrre un Onda di Entropia che soddisfi i requisiti imposti dai costruttori dei motori aeronautici. Durante la prima fase della tesi sono stati progettati e costruiti differenti prototipi di questo dispositivo. In seguito, mediante l'utilizzo di alcuni strumenti di misura propriamente tarati, i vari prototipi sono stati testati nell'ugello convergente della Galleria del Vento del LFM per simulare l'inserzione dell'EWG nel flusso della turbina. Durante questa fase l'esperienza ha portato a riconsiderare alcune decisioni prese in fase di progetto e a migliorarle. Si è giunti così alla configurazione finale del dispositivo che meglio rispetta le caratteristiche richieste. Durante il lavoro è stato constatato che la fluttuazione di temperatura dell'onda di entropia generata si riduce se la frequenza di iniezione aumenta e se la pressione di alimentazione diminuisce. Nelle prove sull'ugello, alla posizione che simula l'ingresso dello statore, l'onda che si ottiene nelle condizioni rappresentative dei motori è caratterizzata da una frequenza di 120 Hz, un valore di temperatura minima di 50°C, un valore di temperatura picco-picco di 20°C e un numero di Swirl pari a 0.4.
Design and thermo-fluid dynamic analysis of an entropy wave generator
ANTONIOLI, VALENTINA;NOTARISTEFANO, ANDREA
2017/2018
Abstract
Modern aero-engine combustors burn a lean and premixed mixture. These new combustion chamber concepts have been adopted to reduce flame temperature, but they are responsible for the hot flow unsteady character developed after combustion. The turbulent flame so produced induces larger heat release fluctuations which generate Entropy Waves and hot-spots. Furthermore, to enhance the fuel air mixing, combustion air is swirled leading to vorticity disturbances at first stage stator inlet. These instabilities generated during combustion are the cause of some drawbacks. First, vorticity disturbances interact with secondary flows. These, together with hot-spots and Entropy Waves, make more challenging the cooling technique design constraining the designers to take on board blades overheating. Lastly, all these disturbances involve an indirect noise increase. Due to the proximity of airports to residential zones and to make more comfortable passengers flight, the reduction of acoustic emissions is now a major field of research both for industry and research groups. To properly study combustor-turbine interactions, each single disturbance component should be insulated and studied. For this purpose an Entropy Wave Generator (EWG) has been designed and characterized in this Thesis project, so to allow future investigation in the High Pressure Turbine Test Rig of the Laboratorio di Fluidodinamica delle Macchine. The EWG must be able to produce an Entropy Wave that suits with aero-engine constructors requirements. At the beginning of this Thesis work, the design phase is approached and the different manufactured prototypes are illustrated. Afterwards, by means of properly calibrated measuring instruments, the different configurations are tested in the convergent nozzle of LFM Wind Tunnel to simulate EWG insertion in the turbine flux. Thanks to the experience acquired during experimental tests, some decisions taken in the design phase have been revised so to achieve better outcomes. Experiments show that Entropy Wave temperature fluctuation amplitude reduces as the frequency increases and the feeding pressure decreases. EW properties measured in the nozzle at the representative stator inlet position and with the engine-representative frequency of 120 Hz have a minimum temperature of 50°C, a peak-to-peak temperature value of 20°C and a Swirl Number of 0.4. These instabilities generated during combustion are the cause of some drawbacks. First, vorticity disturbances interact with secondary flows. These, together with hot-spots and Entropy Waves, make more challenging the cooling technique design constraining the designers to take on board blades overheating. Lastly, all these disturbances involve an indirect noise increase. Due to the proximity of airports to residential zones and to make more comfortable passengers flight, the reduction of acoustic emissions is now a major field of research both for industry and research groups. To properly study combustor-turbine interactions, each single disturbance component should be insulated and studied. For this purpose an Entropy Wave Generator (EWG) has been designed and characterized in this Thesis project, so to allow future investigation in the High Pressure Turbine Test Rig of Politencico di Milano. The EWG must be able to produce an Entropy Wave that suits with aero-engine constructors requirements. At the beginning of this Thesis work, the design phase is approached and the different manufactured prototypes are illustrated. Afterwards, by means of properly calibrated measuring instruments, the different configurations are tested in the convergent nozzle of LFM Wind Tunnel to simulate EWG insertion in turbine flux. Thanks to the experience acquired during experimental tests, some decisions taken in the design phase have been revised so to achieve better outcomes. Experiments show that Entropy Wave temperature fluctuation amplitude reduces as the frequency increases and the feeding pressure decreases. EW properties measured in the nozzle at the representative stator inlet position and with the engine-representative frequency of 120 Hz have a minimum temperature of 50°C, a peak-to-peak temperature value of 20°C and a Swirl Number of 0.4.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/146199