Modeling Ignition and Premixed Combustion Including Flame Stretch Effects

Objective of this work is the incorporation of the flame stretch effects in an Eulerian-Lagrangian model for premixed SI combustion in order to describe ignition and flame propagation under highly inhomogeneous flow conditions. To this end, effects of energy transfer from electrical circuit and turbulent flame propagation were fully decoupled. The first ones are taken into account by Lagrangian particles whose main purpose is to generate an initial burned field in the computational domain. Turbulent flame development is instead considered only in the Eulerian gas phase for a better description of the local flow effects. To improve the model predictive capabilities, flame stretch effects were introduced in the turbulent combustion model by using formulations coming from the asymptotic theory and recently verified by means of DNS studies. Experiments carried out at Michigan Tech University in a pressurized, constant-volume vessel were used to validate the proposed approach. In the vessel, a shrouded fan blows fresh mixture directly at the spark-gap generating highly inhomogeneous flow and turbulence conditions close to the ignition zone. Experimental and computed data of gas flow velocity profiles and flame radius were compared under different turbulence, air/fuel ratio and pressure conditions

수치해석 기법을 이용한 가솔린 엔진에서 난류 유동이 연소의 사이클 편차에 미치는 영향에 대한 연구

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 기계항공공학부, 2020. 8. 민경덕.Abstract The Effect of Turbulent Flow on the Combustion Cyclic Variation in a Spark Ignition Engine using Large-Eddy Simulation Insuk Ko Department of Mechanical and Aerospace Engineering The Graduate School Seoul National University At the present, the problem of worldwide air pollution has emerged as an important issue and many countries are trying to solve the problem. Emission regulations have been tightened around the world in an effort to reduce emissions from internal combustion engine (ICE) vehicles. From 2014, Tier 3 emissions standards in the United States (U.S.) and EURO6 regulations in the European Union (EU) are adopted. Currently, CO2 is also being strongly enforced annually. To meet the tightened CO2 regulations, the development of high efficiency engines is actively being carried out by each vehicle manufacturer. In the development of high efficiency engines, the key point is the increase in thermal efficiency. Many technologies have been developed to increase thermal efficiency and are being applied to mass-production engines. However, there is currently a cycle-to-cycle variation (CCV) of combustion as the biggest obstacle to engine development. Therefore, research on the CCV is also being actively carried out. Because the causes that affect the cycle deviation are various and complex, it is difficult to conduct detailed research on the source of the CCV through experimental studies. Therefore, the 3D simulation is actively carried out as an alternative. In the present study, the CCV phenomenon of combustion was reproduced using large-eddy simulation (LES) approach and the investigation on the source of CCV are conducted. Currently, the engine simulation using LES is immature. Therefore, it is necessary to consider each sub-model for accurate simulation. First, three Sub-grid scale (SGS) turbulence models were evaluated with particle image velocimetry (PIV) data from the single-cylinder transparent combustion chamber (TCC-III) engine. The dynamic structure model (DSM) was adopted for this study, based on the analysis of the flow field and the predicted SGS turbulent velocity compared to the PIV data. Secondly, the G-equation was employed as a combustion model. The model can be used in the corrugated flamelets regime and the thin reaction flamelets regime. The turbulent burning velocity of the model is quite complicated to simulate the turbulent flame included in the two regimes. Therefore, in this study, the combustion regime of the target engine operating condition was found by using Reynolds averaged navier-stokes equation (RANS) approach and was identified to the corrugated flamelets regime. Thus, the G-equation was modified for the corrugated flamelets regime. Thirdly, an ignition model reflecting the characteristics of LES was developed. The lagrangian particles were employed to realize the ignition channel and the secondary electric circuit model was implemented to predict the spark energy, restrikes phenomena and the end of ignition time. The one of the key features of the ignition model developed in this study is that a simplified empirical function is implemented to realize the thermal diffusion during arc phase. After ignition phase, the channel grows by chemical reaction and the flame propagation progresses. The turbulent flame brush thickness term is introduced to predict the transition state between the laminar flame propagation and the turbulent flame propagation. Finally, when the channel is grown sufficiently, flame is propagated in the 3D field by the G-equation Finally, 30 LES cycles were performed to identify the cause of the CCV and validated against the experimental data. The sources of the CCV are mainly from the small scale turbulent flow and the large scale turbulent flow. The small scale turbulent flow effect was investigated and the fact that the small scale turbulent flow is related to the tumble motion is identified. In terms of the large scale turbulent flow, the effect of the local vortex on the flame propagation was found through the detailed analysis of the flow field. In particular, the vortex produced by wall flow on the secondary tumble plane is an important factor. A new piston shape was designed to strengthen the vortex formation by wall flow. The result of new piston case shows the reduced combustion CCV than the base case. This research provides the guide how to investigate the sources of the combustion CCV and how to reduce the combustion CCV for the future engine development Keywords: SI engine, LES, CFD (Computational Fluid Dynamics), CCV (Cycle-to-cycle variation), Ignition model, SGS model Student Number: 2013-20641국 문 초 록 현재 전 세계 대기오염 문제가 중요한 이슈로 떠오르고 많은 나라들이 이 문제를 해결하기 위해 노력하고 있다. 내연기관 차량의 배기 가스 배출량을 줄이기 위해 전 세계적으로 배출가스 규제가 강화되었다. 2014년부터 미국은 Tier 3 배기배출물 규정을 유럽연합은 EURO 6 규정을 채택하고 있다. 현재 연비 규제인 CO2도 매년 강력하게 강화되고 있다. 강화된 CO2 규정을 충족시키기 위해, 고효율 엔진의 개발은 각 차량 제조사에 의해 활발하게 이루어지고 있다. 고효율 엔진 개발에서 핵심은 열효율 증가이다. 열효율을 높이기 위해 많은 기술이 개발되어 양산 엔진에 적용되고 있다. 그러나 현재 엔진 개발에 가장 큰 장애물로 연소 사이클 간 편차가 있다. 따라서 사이클 편차에 대한 연구도 활발히 진행되고 있다. 사이클 편차에 영향을 미치는 원인은 다양하고 복잡하기 때문에, 실험 연구를 통해 사이클 편차의 근본 원인에 대한 상세한 연구를 실시하기 어렵다. 따라서 대안으로 3D 시뮬레이션을 활용한 연구가 활발히 진행되고 있다. 본 연구에서는, 연소의 사이클 편차 현상을 Large-Eddy Simulation (LES) 유동 해석 방법을 이용하여 재현하고 사이클 편차의 원인에 대한 연구를 진행한다. 현재 LES를 이용한 엔진 시뮬레이션은 아직까지 미숙한 단계이다. 따라서 정확한 시뮬레이션을 위해 각 물리적 현상을 구현할 수 있는 모델을 구현해야 한다. 먼저, 3개의 sub-grid scale (SGS) 난류 모델을 단기통 광학 엔진의 (TCC-III) particle image velocimetry (PIV) 측정 결과로 평가하였다. PIV 데이터와 비교한 유동장 및 예측된 SGS 난류속도에 대한 분석을 바탕으로 본 연구에서는 dynamic structure model (DSM)이 채택되었다. 둘째로, G-equation 모델을 연소 모델로 선택하였다. G-equation 모델은 Pitsch[1]에 의해 LES 적용 가능 하도록 개발되었다. 이 모델은 corrugated flamelets regime과 thin reaction flamelets regime에서 사용될 수 있다. 연소 속도 모델은 두 연소 환경에 포함된 난류 연소를 모사하기 위해 상당히 복잡하다. 따라서 본 연구에서는 RANS 를 이용하여 대상 엔진 작동 조건의 연소 환경을 찾아 내었고, 연소 환경은 corrugated flamelets regime에 속한 것을 확인 하였다. 따라서 기존의 G-equation 연소 모델을 corrugated flamelets regime에 맞도록 변경 하였다. 셋째로, LES의 특성을 반영한 점화 모델이 개발되었다. Lagrangian 개념을 이용하여 점화 채널을 구현하고, 2차 전기 회로 모델을 이용하여 점화 에너지, 리스트라이크, 점화 시간 종료 등을 예측하였다. 본 연구에서 개발된 점화 모델의 주요 특징 중 하나는 아크 페이즈 중 열 팽창 현상을 구현을 위해 간단한 경험 함수를 이용한다는 것이다. 아크 페이즈 후, 점화 해널은 화학 반응에 성장하고 화염 전파가 진행된다. 난류 화염 두께는 층류 화염 전파와 난류 화염 전파 사이의 천이 상태를 예측하기 위해 도입되었다. 마지막으로 점화 채널이 충분히 커지면 G-equation 의해 3D 계산 영역에서 화염 전파가 구현된다. 마지막으로 30개의 LES 사이클을 수행하여 연소의 사이클 편차 원인을 분석하고 실험 데이터를 이용하여 시뮬레이션의 정확도를 검증하였다. 연소의 사이클 편차의 원인은 주로 작은 규모의 난류 유동과 큰 규모의 난류 유동에서 나온다. 난류 모델로 구현된 작은 규모의 난류 유동과 큰 규모의 난류 유동에 속한 텀블 값을 같이 분석 하였다. 작은 규모의 난류 유동은 텀블 값과 관계가 있다는 사실을 파악 하였다. 큰 규모 난류 유동 측면에서는 국부적인 유동의 소용돌이가 화염 전파에 미치는 영향을 유동장을 상세히 분석하여 확인되었다. 특히 2차 텀블면에서 벽면 유동에 의해 생성되는 소용돌이가 연소의 사이클 편차에 미치는 중요한 요인임을 밝혀 내었다. 벽면 유동에 의한 소용돌이 형성을 강화하기 위해 새로운 피스톤 현상을 설계 하였다. 새로운 피스톤 형상의 결과는 베이스 피스톤보다 연소의 CCV가 줄어들었다. 본 연구는 향후 엔진 개발을 위해 연소 CCV의 원인을 조사하는 방법과 연소 CCV를 줄이는 방법에 대한 방법론을 제시한다. 주요어: 전기점화 엔진, LES, 전산유체역학, 사이클 편차, 점화모델, 난류모델 학 번: 2013-20641Chapter 1. Introduction 1 1.1 Background and Motivation 1 1.2 Literature Review 10 1.2.2 Turbulence Modeling 12 1.2.3 Combustion Modeling 16 1.3 Research Objective 20 1.4 Structure of the Thesis 22 Chapter 2. Sub-grid Scale Turbulence Model 24 2.1 The Fundamentals of Turbulent Flow 23 2.1.1 The Energy Cascade 23 2.1.2 The Energy Spectrum 27 2.2 Sub-grid Scale Turbulence Model 29 2.2.1 Zero-equation Model 31 2.2.1.1 Smagorinsky Model 31 2.2.1.2 Dynamic Smagorinsky Model 32 2.2.2 One-equation and Non-viscosity Model 34 2.2.2.1 Dynamic Structure Model 34 2.3 Evaluation of Turbulence Models 37 2.3.1 Numerical Configuration 40 2.3.2 Comparison of Sub-grid Scale Model 45 Chapter 3. Modeling of Gasoline Surrogate Fuel 59 3.1 Literature Review 55 3.2 Determination of Surrogate Component 56 Chapter 4. Combustion Model for LES 63 4.1 The Laminar Burning Velocity 59 4.1.1 Literature Review 59 4.1.2 The Correlation for the Laminar Flame Speed 62 4.2 G-equation Model for LES 69 4.3 Sub-filter Turbulent Burning Velocity 73 Chapter 5. Lagrangian Ignition Model 82 5.1 Literature Review 77 5.2 Modeling of Ignition 81 5.2.1 Initialization of Particles 82 5.2.2 Channel elongation 83 5.2.3 Electric circuit model 83 5.2.4 Plasma channel expansion. 88 5.2.5 Ignition channel development 94 5.2.6 Restrike 95 5.2.7 Transition between ignition and flame propagation 96 Chapter 6. Experimental and Numerical Setup 106 6.1 Experimental Setup 99 6.2 Numerical Setup 104 Chapter 7. Simulation Results of Combustion CCV 116 7.1 Validation of Simulation Results 109 7.2 Correlation between Combustion Phase and Peak Pressure 115 7.3 Investigation of turbulent flow effect on CCV 121 7.3.1 Small Scale Turbulent Flow Effect on CCV 121 7.3.2 Large Scale Turbulent Flow Effect on CCV 127 7.4 Method for Reduction of CCV 156 7.4.1 Investigation of the Controllable Source of CCV 156 7.4.2 Result of New Designed Piston 166 Chapter 8. Conclusions 182 Chapter 9. Bibliography 186 국 문 초 록 201Docto

Development of combustion models for RANS and LES applications in SI engines

Prediction of flow and combustion in IC engines remains a challenging task. Traditional Reynolds Averaged Navier Stokes (RANS) methods and emerging Large Eddy Simulation (LES) techniques are being used as reliable mathematical tools for such predictions. However, RANS models have to be further refined to make them more predictive by eliminating or reducing the requirement for application based fine tuning. LES holds a great potential for more accurate predictions in engine related unsteady combustion and associated cycle-tocycle variations. Accordingly, in the present work, new advanced CFD based flow models were developed and validated for RANS and LES modelling of turbulent premixed combustion in SI engines. In the research undertaken for RANS modelling, theoretical and experimental based modifications have been investigated, such that the Bray-Moss-Libby (BML) model can be applied to wall-bounded combustion modelling, eliminating its inherent wall flame acceleration problem. Estimation of integral length scale of turbulence has been made dynamic providing allowances for spatial inhomogeneity of turbulence. A new dynamic formulation has been proposed to evaluate the mean flame wrinkling scale based on the Kolmogorov Pertovsky Piskunow (KPP) analysis and fractal geometry. In addition, a novel empirical correlation to quantify the quenching rates in the influenced zone of the quenching region near solid boundaries has been derived based on experimentally estimated flame image data. Moreover, to model the spark ignition and early stage of flame kernel formation, an improved version of the Discrete Particle Ignition Kernel (DPIK) model was developed, accounting for local bulk flow convection effects. These models were first verified against published benchmark test cases. Subsequently, full cycle combustion in a Ricardo E6 engine for different operating conditions was simulated. An experimental programme was conducted to obtain engine data and operating conditions of the Ricardo E6 engine and the formulated model was validated using the obtained experimental data. Results show that, the present improvements have been successful in eliminating the wall flame acceleration problem, while accurately predicting the in-cylinder pressure rise and flame propagation characteristics throughout the combustion period. In the LES work carried out in this research, the KIVA-4 RANS code was modified to incorporate the LES capability. Various turbulence models were implemented and validated in engine applications. The flame surface density approach was implemented to model the combustion process. A new ignition and flame kernel formation model was also developed to simulate the early stage of flame propagation in the context of LES. A dynamic procedure was formulated, where all model coefficients were locally evaluated using the resolved and test filtered flow properties during the fully turbulent phase of combustion. A test filtering technique was adopted to use in wall bounded systems. The developed methodology was then applied to simulate the combustion and associated unsteady effects in Ricardo E6 spark ignition engine at different operating conditions. Results show that, present LES model has been able to resolve the evolution of a large number of in-cylinder flow structures, which are more influential for engine performance. Predicted heat release rates, flame propagation characteristics, in-cylinder pressure rise and their cyclic variations are also in good agreement with measurements

Analysis and Simulation of Non-Flamelet Turbulent Combustion in a Research Optical Engine

In recent years, the research community devoted many resources to define accurate methodologies to model the real physics behind turbulent combustion. Such effort aims at reducing the need for case-by-case calibration in internal combustion engine simulations. In the present work two of the most widespread combustion models in the engine modelling community are compared, namely ECFM-3Z and G-equation. The interaction of turbulent flows with combustion chemistry is investigated and understood. In particular, the heat release rate characterizing combustion, and therefore the identification of a flame front, is analysed based on flame surface density concept rather than algebraic correlations for turbulent burn rate. In the first part, spark-ignition (S.I.) combustion is simulated in an optically accessible GDI single-cylinder research engine in firing conditions. The turbulent combustion regime is mapped on the Borghi-Peters diagram for all the conditions experienced by the engine flame, and the consistency of the two combustion models is critically analysed. In the second part, a simple test case is defined to test the two combustion models in an ideally turbulence-controlled environment: this allows to fully understand the main differences between the two combustion models under well-monitored conditions. and results are compared against experimental databases of turbulent burn rate for wide ranges of Damkohler (Da) and Karlovitz (Ka) numbers. The joint experimental and numerical study presented in this paper evaluates different approaches within the unified flamelet/non-flamelet framework for modelling turbulent combustion in SI engines. It also indicates guidelines for reduced calibration effort in widespread combustion models

Large eddy simulation of premixed combustion in spark ignited engines using a dynamic flame surface density model

In this work, cyclic combustion simulations of a spark ignition engine were performed using the Large Eddy Simulation techniques. The KIVA-4 RANS code was modified to incorporate the LES capability. The flame surface density approach was implemented to model the combustion process. Ignition and flame kernel models were also developed to simulate the early stage of flame propagation. A dynamic procedure was formulated where all model coefficients were locally evaluated using the resolved and test filtered flow properties during the fully developed phase of combustion. A test filtering technique was adopted to use in wall bounded systems. The developed methodology was then applied to simulate the combustion and associated unsteady effects in a spark ignition engine. The implementation was validated using the experimental data taken from the same engine. Results show that, even with relatively coarser meshes used in this work, present LES implementation has been able to resolve the evolution of a large number of in-cylinder flow structures, which are more influential for engine performance. Predicted combustion rate and pressure rise is also in good agreement with the measurements. The limits of cyclic variations are well within the experimentally observed range. It has also been able to demonstrate the limits of cyclic fluctuations to a reasonable degree even with a fewer number of simulation cycles. A significant variation of flame propagation has also been predicted by the simulations

Computer-aided engineering and design of internal combustion engines to support operation on non-traditional fuels

2020 Fall.Includes bibliographical references.Traditional fuels like gasoline and diesel make up ~37 % of the US energy production; because of that, they are rapidly depleting their finite resources. These traditional fuels are also primary contributors to greenhouse gases, global warming, and particulate matter, which are bad for the environment and human beings. For that reason, research in non-traditional fuels (e.g., Carbon neutral biofuels, low GHG emitting gaseous fuels including NG and hydrogen) that achieve greater if not similar efficiencies compared to traditional fuels is gaining traction. On top of that, emission requirements are becoming even more strenuous. Engineers must find new ways to investigate non-traditional fuels and their performance in internal combustion engines while permitting the engine-fuel system's low-cost design. This being the case, Computer-Aided Engineering (CAE) tools like Computational Fluid Dynamics (CFD) and chemical kinetics solvers are being taken advantage of to assist in the research of these non-traditional fuel applications. This thesis describes the use of CONVERGE CFD to investigate two different non-traditional fuel applications, namely, the retrofitting of a premixed gasoline two-stroke spark-ignited (SI) engine to function with multiple injections of JP-8 fuel and to retrofit a diesel compression-ignited engine into a premixed anode tail-gas SI engine. The first application described herein uses a solid oxide fuel cell "Anode Tail-gas," which has similar syngas characteristics in a spark-ignited engine. Anode Tail-gas is a byproduct from an underutilized Metal Supported Solid Oxide Fuel Cell (MS-SOFC) used in a high efficiency distributed power (~100 kWe) system. Gas turbines or reciprocating ICEs typically drive distributed power systems of this capacity because they can quickly react to change in demand but traditionally have lower thermal efficiencies than a large-scale Rankine cycle plant. However, with the MS-SOFC, it may be possible to design a 125 kWe system with 70 % efficiency while keeping the system cost-competitive (below $1000/kW). The system requires a ~14 kW engine that can operate at 35 % efficiency with the highly dilute (17.7% H2, 4.90 % CO, 0.40% CH4, 28.3 % CO2, 48.7 % H2O) Anode Tail-gas to meet these lofty targets. CAE approaches were developed and used to identify high-efficiency operation pathways with the highly diluted anode tail-gas fuel. The fuel was first tested and modeled in a Cooperative Fuel Research (CFR) engine to investigate the anode tail gas's combustibility within an IC engine and to provide validation data with highly specified boundary conditions (Compression Ratio (CR), fuel compositions, intake temperature/pressure, and spark timing). A chemical mechanism was selected through CAE tools to represent the highly diluted fuel combustion best based on the CFR data. Five experimental test points were used to validate the CFD model, which all were within a maximum relative error of less than 8 % for IMEP and less than 4 crank angle degrees for CA10 and CA50. The knowledge gained from the CFR engine experiments and associated model validation helped direct the design of a retrofitted Kohler diesel engine to operate as a spark-ignited engine on the anode tail gas fuel. CFD Investigations into spark plug and piston bowl designs were performed to identify combustion chamber design improvements to boost the Kohler engine's efficiency. Studies revealed that piston designs incorporating small clearance heights, large squish areas, and deep bowl depths could enhance efficiency by 5.41 pts with additional efficiency gain possible through piston rotation. The second fuel investigation was a jet propellant fuel called "JP-8," which was deemed non-tradition when used in a two-stroke UAV engine to satisfy the military's single fuel policy requirements. The JP-8 fuel proved challenging in this application due to its significantly lower octane number and volatility than gasoline and experienced knock when used as a homogeneous premixed mixture within the simulated UAV platform. Although with CFD modeling, it was possible to reduce the severity of knock by using eight rapid direct injections of JP-8 at 20 µm diameter droplets. With further investigation, it might be possible to reduce further the severity of knock using CFD through more advanced injection strategies

A phenomenological model for predicting the early development of the flame kernel in spark-ignition engines

This work presents a simple and effective phenomenological model for the prediction of the early growth of the flame kernel in SI engines, including its initiation as a result of the electrical breakdown of the fuel/air mixture between the spark plug electrodes. The present model aims to provide an improved description of the ignition-affected early phases of flame kernel development compared to the majority of models currently available in literature. In particular, these models focus on electrical energy supply and turbulence, whereas the stretch-induced kernel growth slowdown is quantified with linear models that are inconsistent with the small kernel radius. For the flame kernel initiation, this model replaces the current methods that rely on 1D heat diffusion within a plasma column with a more consistent analysis of post-breakdown conditions. Concerning the kernel growth, the present model couples the mass and energy conservation equations of a spherical kernel with the species and temperature profiles outside of it. This combination leads to a non-linear description of the flame stretch, according to which the kernel development is controlled by the Lewis-number-dependent balance between the heat gained via combustion and the heat lost via thermal diffusion. As a result, the kernel temperature differs from the adiabatic flame temperature, causing the laminar flame speed to change from its adiabatic value and ultimately affecting the overall kernel development. Kernel growth predictions are conducted for laminar flames and compared to literature data, showing a satisfactory agreement and highlighting the ability to describe the stretch-induced kernel slowdown, up to its possible extinction. A good agreement with literature data is also obtained for kernel expansions under moderately turbulent conditions, typical of internal combustion engines. The simple formulation of the present model enables swift integration into phenomenological combustion models for spark-ignition engines, while simultaneously offering useful insight into the early kernel development even for CFD-based approaches

An improved formulation of the Bray-Moss-Libby (BML) model for SI engine combustion modelling

In this paper an improved version of the BML model has been developed so that it could be applied to wall-bounded combustion modelling, eliminating the wall flame acceleration problem. Based on the Kolmogorov-Petrovski-Piskunov (KPP) analysis and fractal theory, a new dynamic formulation has been proposed to evaluate the mean flame wrinkling scale making necessary allowance for spatial inhomogeneity of turbulence. A novel empirical correlation has been derived based on experimentally estimated flame image data to quantify the quenching rates near solid boundaries. The proposed modifications were then applied to simulate premixed combustion in two spark ignition engines with different operating conditions. Results show that the present improvements have been successful in eliminating the wall flame acceleration problem found with the original BML model, while accurately predicting the in-cylinder pressure rise, mass burn rates and heat release rates

Modeling and Numerical Simulations of Two-Phase Ignition in Gas Turbine

In order to meet the new international environmental regulations while maintaining a strong economic competitiveness, innovative technologies of aeronautical combustion chambers are developed. These technologies must guarantee fast relight in case of extinction, which is one of the most critical and complex aspects of engine design. Control of this phase involves a thorough understanding of the physical phenomena involved. In this thesis the full two-phase ignition sequence of an aeronautical engine has been studied, from the breakdown of the spark plug to thepropagation of the flame in the complete engine. For this purpose, Large-Eddy Simulations (LES) using a detailed description of the liquid phase (Euler-Lagrange formalism) and of the combustion process (Analytically Reduced Chemistry) were performed. The results also led to the development of a simplified model for the prediction of ignition probability map, which is particularly useful for the design of combustion chambers