Internal combustion engines are expected to be utilised for many years in the future; however improvements to limit pollutant emissions and gain in efficiency are required: transiting to alternative power sources is needed. To better reproduce new fuels combustion and to develop new generation engines, predictive and accurate kinetic mechanism have to be implemented. With this aim, different experimental facilities are extensively used to simulate the reacting environment representative of engine conditions: between them, rapid compression machine plays an important role in achieving high pressures and low-to-intermediate temperatures. It is designed in order to isolate chemical kinetic phenomena from other complex effects occurring in real engines. The objective of this work is to propose a new multi-zone model describing combustion behaviours in RCMs, implementing both physical and chemical phenomena. Indeed, mathematical model formulation is based on an onion-like structure, considering heat and mass fluxes, both laminar and turbulent, between adjacent zones. The model also accounts for crevices, a constant-volume zone able to contain the cold boundary layer, causing convective fluxes. A wall function is implemented to reproduce heat losses in the near-wall region, coupled with a turbulence sub-model assuming that the fluid dynamics have a marginal influence. Therefore, the naturally occurring thermal stratification can be reproduced. Since chemistry is controlling auto-ignition process, the model was designed to manage detailed kinetic mechanism. As an additional tool to investigate the system reactivity, sensitivity analysis is implemented. Rapid compression machine from Argonne National Laboratory has been examined and model parameters have been tuned by a combined use of CFD and experimental data. Furthermore, adopting two different fuels and a wide range of operating conditions, differences between multi-zone and zero-D model predictions have been analysed. Since the use of a creviced piston enables attainment of a nearly homogeneous temperature field inside the reaction chamber, it is expected to positively influence the validity of the zero-D modelling approach. This hypothesis seems to be valid for ignition delay time less than 20 ms, for one-stage ignition, that decreases to 10 ms, for two-stage ignition, after that stratification phenomena cannot be caught using the current zero-D approach.
Migliorare l’efficienza dei motori a combustione interna limitandone le emissioni nocive rappresenta una delle sfide tecnologiche più importanti insieme alla necessità di adottare fonti energetiche alternative. Per poter caratterizzare il processo di ignizione di nuovi combustibili, è fondamentale il supporto di meccanismi cinetici accurati. A tal proposito vengono usati differenti macchinari che puntano a ricreare i fenomeni cinetici presenti all’interno dei motori. Tra questi la rapid compression machine ricopre un ruolo importante, potendo sostenere alte pressioni e temperature medio-basse. E’ progettata per isolare i fenomeni cinetici chimici da altri effetti complessi che si verificano nei motori reali. Il presente lavoro di tesi si propone di descrivere l’evoluzione del processo di combustione all’interno della RCM tramite una modellazione multi-zona. La stesura delle equazioni è basata su una struttura a cipolla dove le zone comunicano con le adiacenti tramite flussi di materia e calore di carattere laminare e turbolento. La presenza dei crevices è stata anch’essa considerata: si tratta di una zona dal volume costante che nel contenere il boundary layer provoca la nascita di flussi convettivi. Nell’esaminare le perdite di calore è stata implementata una wall function che, affiancata da un sottomodello turbolento (si suppone la fluidodinamica influisca in modo marginale), risulta essere in grado di riprodurre la stratificazione termica esistente nel macchinario. Infine, essendo la chimica il fattore controllante nei processi di autoignizione, il modello è stato pensato in modo da poter essere implementato con l’utilizzo di meccanismi cinetici dettagliati. Un ulteriore strumento per esaminare il sistema reattivo è l’analisi di sensitività rispetto ai parametri cinetici. E’ stata studiata la rapid compression machine del Argonne National Laboratory e i parametri del modello sono stati tarati attraverso l’utilizzo di analisi CFD e dati sperimentali. In seguito, è stata osservata la diversità tra quanto predetto dal modello adimensionale e dal multi-zona, variando le condizioni operative e i combustibili impiegati. Poiché l’uso del crevice consente di ottenere un campo di temperatura quasi omogeneo all’interno della camera, si prevede che influenzerà positivamente la validità dell’approccio di modellazione zero-D. Questa ipotesi sembra essere valida per un ritardo di accensione inferiore a 20 ms, per l’accensione a uno stadio, che diminuisce a 10 ms, per l’accensione a due stadi, dopodiché i fenomeni di stratificazione non possono colti usando l’attuale approccio zero-D.
Multi-zone model for RCM : tuning of model parameters and comparison with zero-D model
GOBBO, GIACOMO
2018/2019
Abstract
Internal combustion engines are expected to be utilised for many years in the future; however improvements to limit pollutant emissions and gain in efficiency are required: transiting to alternative power sources is needed. To better reproduce new fuels combustion and to develop new generation engines, predictive and accurate kinetic mechanism have to be implemented. With this aim, different experimental facilities are extensively used to simulate the reacting environment representative of engine conditions: between them, rapid compression machine plays an important role in achieving high pressures and low-to-intermediate temperatures. It is designed in order to isolate chemical kinetic phenomena from other complex effects occurring in real engines. The objective of this work is to propose a new multi-zone model describing combustion behaviours in RCMs, implementing both physical and chemical phenomena. Indeed, mathematical model formulation is based on an onion-like structure, considering heat and mass fluxes, both laminar and turbulent, between adjacent zones. The model also accounts for crevices, a constant-volume zone able to contain the cold boundary layer, causing convective fluxes. A wall function is implemented to reproduce heat losses in the near-wall region, coupled with a turbulence sub-model assuming that the fluid dynamics have a marginal influence. Therefore, the naturally occurring thermal stratification can be reproduced. Since chemistry is controlling auto-ignition process, the model was designed to manage detailed kinetic mechanism. As an additional tool to investigate the system reactivity, sensitivity analysis is implemented. Rapid compression machine from Argonne National Laboratory has been examined and model parameters have been tuned by a combined use of CFD and experimental data. Furthermore, adopting two different fuels and a wide range of operating conditions, differences between multi-zone and zero-D model predictions have been analysed. Since the use of a creviced piston enables attainment of a nearly homogeneous temperature field inside the reaction chamber, it is expected to positively influence the validity of the zero-D modelling approach. This hypothesis seems to be valid for ignition delay time less than 20 ms, for one-stage ignition, that decreases to 10 ms, for two-stage ignition, after that stratification phenomena cannot be caught using the current zero-D approach.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/151203