The present work focuses on the numerical characterization of the gasoline direct injection process performed in modern spark-ignition engines for vehicle applications. A detailed prediction of injection and air–fuel mixing is in fact a mandatory task to guarantee a stable and efficient combustion process with the aim to minimize pollutant formation. Within this context, computational fluid dynamics simulations represent a powerful tool to assess the main physical phenomena related to breakup and evaporation of the liquid jet, leading to mixture formation by means of the interaction between the vaporized fuel and the charge motion. To ensure the accuracy of the computed results, so that they could be a reliable support for industrial design and research activities, it is mandatory to validate the adopted multidimensional spray sub-models against available experimental data under well-defined operating conditions. To this end, the multi-hole gasoline direct injection Spray G injector made available by the Engine Combustion Network community was chosen as the reference geometry for the validation of the proposed numerical setup. Simulations were carried out with the open source OpenFOAM software coupled with the LibICE framework which consists into a set of libraries and solvers developed by the Internal Combustion Engine Group of Politecnico di Milano and dedicated to the modeling of the most important physical phenomena related to internal combustion engines. In this work, at first a reference numerical spray setup was employed and tested under a wide range of Spray G operating conditions. By validating the results in terms of axial spray penetration, gas velocity, entrainment, spray morphology and Sauter Mean Diameter it was possible to observe strengths and weaknesses of the approach and thus propose specific improvements. An innovative liquid post-processing procedure, developed for the ECN6 Workshop and based on a projected liquid volume Eulerian field, represented the reference validation methodology for the proposed numerical improvements of atomization and secondary breakup which were implemented into the LibICE framework. More in detail, two solutions based on a decoupled approach for the management of primary and secondary atomization under a single spray model were proposed and validated in the present work. Furthermore, a literature-based evolution of a classic secondary breakup model was considered and implemented into the LibICE with the aim to provide a dynamic approach for droplet stripping and catastrophic breakup which could better suit the low-evaporating conditions of early injection events typical of modern gasoline direct injection engines. Two literature-based flash boiling evaporations models were implemented as well and validated on a dedicated Spray G flashing operating condition. The proposed numerical setups were also reproduced under dynamic conditions by performing simulations of a full-cycle of two modern gasoline direct injection optical access engines, one of which was directly coupled to the Spray G injector. Finally, the physical phenomenon of collision between liquid drops during the injection process has been studied by implementing into the LibICE framework a literature model whose main purpose was to reduce the required computational time compared to other available mechanisms. The validation of the model was carried out by running different Spray G simulations and by comparing computed axial vapor penetration and droplet diameter with those calculated without collision and with other dedicated models already available in OpenFOAM. On the basis of the observed results a further model modification was then proposed with the aim to achieve a better compromise between numerical accuracy and minimization of the required computational time.
Il presente lavoro si concentra sulla caratterizzazione numerica del processo di iniezione diretta di benzina tipico dei moderni motori ad accensione comandata per automobili. Una riproduzione dettagliata delle fasi di iniezione e miscelazione aria-combustibile è infatti un aspetto fondamentale per garantire un processo di combustione stabile, efficiente e con l'obiettivo di ridurre al minimo la formazione di inquinanti. In questo contesto, le simulazioni di termofluidodinamica computazionale rappresentano un potente strumento per valutare i principali fenomeni fisici correlati all'atomizzazione e all'evaporazione del getto di liquido che porta alla formazione della miscela mediante l'interazione tra il combustibile vaporizzato e i moti della carica aspirata. Per garantire l'accuratezza dei risultati numerici in modo che possano rappresentare un supporto affidabile per la progettazione industriale e le attività di ricerca, è fondamentale convalidare i sotto-modelli multidimensionali di spray con dati sperimentali disponibili in condizioni operative ben definite. A tal fine l'iniettore Spray G a iniezione diretta multi-foro di benzina, reso disponibile dall'associazione Engine Combustion Network, è stato scelto come geometria di riferimento per la convalida delle configurazioni numeriche proposte. Le simulazioni sono state condotte con il software open source OpenFOAM accoppiato alla LibICE, la quale rappresenta un insieme di librerie e solutori sviluppato dall'Internal Combustion Engine Group del Politecnico di Milano e dedicato alla riproduzione numerica dei più importanti fenomeni fisici legati ai motori a combustione interna. In questo lavoro una configurazione numerica di riferimento è stata testata su un'ampia gamma di condizioni operative relative allo Spray G, verificando i risultati in termini di penetrazione assiale dello spray, velocità del gas, interazione liquido-gas, morfologia dello spray e Sauter Mean Diameter (diametro medio). In questo contesto è stato possibile osservare i punti di forza e di debolezza dell'approccio e quindi, su tale base, proporre miglioramenti specifici. Un'innovativa metodologia di analisi della fase liquida, sviluppata per il Workshop ECN6 e basata su un campo Euleriano denominato "projected liquid volume", rappresenta l'approccio di validazione di riferimento utilizzato per valutare i miglioramenti ottenuti dai modelli di atomizzazione primaria e secondaria specificamente implementati nella LibICE nel corso del lavoro. Più in dettaglio, due soluzioni basate su un approccio disaccoppiato per la gestione di atomizzazione primaria e secondaria sotto un unico modello di spray sono state proposte e validate nel presente lavoro. Inoltre, un'evoluzione presente in letteratura di un classico modello di atomizzazione secondaria è stata implementata nella LibICE con l'obiettivo di fornire un approccio dinamico per breakup catastrofici e di stripping così che fosse possibile ottimizzare la riproduzione numerica dei processi di iniezione caratterizzati da condizioni di bassa evaporazione, tipici dei moderni motori a iniezione diretta di benzina. Sono stati inoltre implementati e validati su una specifica condizione operativa dello Spray G anche due modelli di evaporazione di tipo flash boiling disponibili in letteratura. I setup numerici proposti sono stati riprodotti anche in condizioni dinamiche eseguendo simulazioni di un ciclo completo di due moderni motori a iniezione diretta di benzina di tipo ad accesso ottico, uno dei quali direttamente accoppiato all'iniettore Spray G. Infine, il fenomeno fisico della collisione tra gocce liquide durante il processo di iniezione è stato studiato implementando nella LibiCE un modello di letteratura il cui scopo principale consiste in una riduzione del tempo di calcolo richiesto rispetto ad altri approcci disponibili. La convalida del modello è stata effettuata eseguendo diverse simulazioni Spray G e confrontando la penetrazione assiale di vapore e il diametro delle gocce calcolati con quelli resi disponibili senza collisione e da casi eseguiti adottando altri modelli di collisione già disponibili in OpenFOAM. Sulla base dei risultati ottenuti è stata poi proposta una modifica al modello in modo da poter raggiungere un migliore compromesso tra accuratezza numerica e minimizzazione del tempo di calcolo richiesto.
CFD modeling and validation of spray evolution in gasoline direct injection engines
Paredi, Davide
2019/2020
Abstract
The present work focuses on the numerical characterization of the gasoline direct injection process performed in modern spark-ignition engines for vehicle applications. A detailed prediction of injection and air–fuel mixing is in fact a mandatory task to guarantee a stable and efficient combustion process with the aim to minimize pollutant formation. Within this context, computational fluid dynamics simulations represent a powerful tool to assess the main physical phenomena related to breakup and evaporation of the liquid jet, leading to mixture formation by means of the interaction between the vaporized fuel and the charge motion. To ensure the accuracy of the computed results, so that they could be a reliable support for industrial design and research activities, it is mandatory to validate the adopted multidimensional spray sub-models against available experimental data under well-defined operating conditions. To this end, the multi-hole gasoline direct injection Spray G injector made available by the Engine Combustion Network community was chosen as the reference geometry for the validation of the proposed numerical setup. Simulations were carried out with the open source OpenFOAM software coupled with the LibICE framework which consists into a set of libraries and solvers developed by the Internal Combustion Engine Group of Politecnico di Milano and dedicated to the modeling of the most important physical phenomena related to internal combustion engines. In this work, at first a reference numerical spray setup was employed and tested under a wide range of Spray G operating conditions. By validating the results in terms of axial spray penetration, gas velocity, entrainment, spray morphology and Sauter Mean Diameter it was possible to observe strengths and weaknesses of the approach and thus propose specific improvements. An innovative liquid post-processing procedure, developed for the ECN6 Workshop and based on a projected liquid volume Eulerian field, represented the reference validation methodology for the proposed numerical improvements of atomization and secondary breakup which were implemented into the LibICE framework. More in detail, two solutions based on a decoupled approach for the management of primary and secondary atomization under a single spray model were proposed and validated in the present work. Furthermore, a literature-based evolution of a classic secondary breakup model was considered and implemented into the LibICE with the aim to provide a dynamic approach for droplet stripping and catastrophic breakup which could better suit the low-evaporating conditions of early injection events typical of modern gasoline direct injection engines. Two literature-based flash boiling evaporations models were implemented as well and validated on a dedicated Spray G flashing operating condition. The proposed numerical setups were also reproduced under dynamic conditions by performing simulations of a full-cycle of two modern gasoline direct injection optical access engines, one of which was directly coupled to the Spray G injector. Finally, the physical phenomenon of collision between liquid drops during the injection process has been studied by implementing into the LibICE framework a literature model whose main purpose was to reduce the required computational time compared to other available mechanisms. The validation of the model was carried out by running different Spray G simulations and by comparing computed axial vapor penetration and droplet diameter with those calculated without collision and with other dedicated models already available in OpenFOAM. On the basis of the observed results a further model modification was then proposed with the aim to achieve a better compromise between numerical accuracy and minimization of the required computational time.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/169570