상세 화학반응 메커니즘을 이용한 직분식 디젤 분무 연소의 수치적 연구
Numerical Investigation of DI Diesel Spray Combustion Using Detailed Chemical Reaction Mechanism
- 주제(키워드) diesel spray , chemical kinetics , detailed chemical reaction mechanism
- 발행기관 The Graduate School, Ajou University
- 지도교수 최윤호
- 발행년도 2011
- 학위수여년월 2011. 2
- 학위명 박사
- 학과 및 전공 일반대학원 기계공학과
- 파일정보 pdf
- 실제URI http://www.dcollection.net/handler/ajou/000000011644
- 본문언어 영어
- 저작권 아주대학교 논문은 저작권에 의해 보호받습니다.
초록/요약
Diesel engine has been widely used in the vast majority of combustion applications, and its popularity has steadily increased due to inherent benefits in terms of its thermal efficiency, fuel economy and durability. However, the resulting exhaust from the diesel fuel combustion includes PM (particulate matters) and gases, that represent thousands of different chemical substances known to be hazardous air pollutants. Recently, as the interest of environmental pollution is increased all over the world, emission standards are also getting gradually stricter. Therefore, better understanding of various processes inside the cylinder of diesel engine is critical to achieve a strategy for reducing pollutant formation. Diesel engine combustion can be characterized by spray dynamics and its combustion in terms of pollutant formation processes. Due to complex interactions among spray, ignition, combustion and pollutant formation processes, it is very hard to investigate experimentally one process independently from the others. On the other hand, it’s a good engineering practice to study numerically the characteristics of diesel spray and its effects on emission as the first step for understanding the diesel spray combustion. In the present study, the characteristics of fuel spray and the influence of the operating parameters on the resulting exhaust emission are investigated to correlate the spray characteristics with exhaust emissions such as soot and NOx. For this purpose, numerical studies have been made in the spray jet injected into a pressurized, high temperature constant volume vessel using KIVA code. KIVA, which is a multi-dimensional CFD code for analyzing a chemically reacting flow with fuel spray, is modified to implement the reduced and detailed chemical reaction mechanisms as an engineering tool for studying of soot and NOx formation in diesel spray combustion and validated by comparing with a reliable experimental data. The simulations employ an n-heptane chemical reaction mechanism including 66 species and 274 reactions for investigating diesel ignition and emissions formation such as soot and NOx under various ambient gas temperatures. This detailed chemical reaction mechanism of n-heptane as a representative diesel fuel is coupled directly with the CFD calculations through a partially stirred reactor (PaSR) model to consider the interaction of turbulence and chemistry. Using the updated KIVA code, the comparison with the experimental data gave a good reliability in terms of spray penetration length, total maximum soot mass concentration, auto-ignition point and soot formation. The numerical prediction of product emissions gave an insight of the relationship between the spray characteristics and exhaust emissions. However, there was a sensitive grid dependency on the computational results. This severe grid dependency is originated from the O’Rourke’s collision model used in this study. It has a built-in drawback that a computational mesh with small cell size will yield the increase of collision frequency and the decrease of domain meeting two parcels. Compared to the good consistency with the experimental data of temporal distribution of soot formation, soot mass concentration is lower than the measured one. It is because the soot formation process which is represented by particle inception, surface growth and oxidation and coagulation is not modeled phenomenologically. These drawbacks should be updated for the improvements of current numerical methods as future works.
more목차
1. Introduction 1
1.1 Background 1
1.2 Objectives 9
1.3 Literature Survey of Physical Phenomena of Spray Dynamics 11
1.3.1 Spray Structure and Modeling Approach 11
1.3.2 Review of Fuel Spray Formation and Atomization Process 15
1.3.2.1 Fuel Spray Formation 16
1.3.2.2 Spray Atomization 19
1.3.3 Spray Penetration 29
1.3.4 Droplet Size Distributions 30
2. Mathematical Formulations of Fluid and Chemistry Equations 33
2.1 Governing Equations of Fluid Dynamics 34
2.2 Governing Equations of Chemically Reactive Flow with Fuel Spray 36
2.3 Turbulence Models 40
2.3.1 The Standard Model 40
2.3.2 The SGS Turbulence Model (RNG Model) 42
2.4 Chemistry Equations 43
3. Mathematical Formulations of Spray Sub-models 46
3.1 Primary and Secondary Spray Breakup Model 47
3.2 TAB (Taylor Analogy Breakup) Model 49
3.3 Droplet Collision and Coalescence Model 54
3.4 Droplet Vaporization Model 58
3.5 Turbulent Dispersion of Droplets 59
3.6 The Conceptual Description of DI Diesel Spray Flame Structure 61
4. Numerical Scheme 64
4.1 General Structure of KIVA Package 64
4.2 Numerical Algorithm 70
4.2.1 Spatial Discretization 71
4.2.2 Temporal Discretization of Variable Implicit Scheme 73
4.2.3 The SIMPLE algorithm 77
5. Results and Discussion 78
5.1 Boundary Conditions 79
5.2 Numerical Investigation of DI Diesel Spray Combustion 82
5.2.1 Reaction Mechanism of Diesel Combustion 84
5.2.2 Effects of Grid Size 90
5.2.3 Soot Formation 94
5.2.4 NOx Formation 100
5.2.5 Auto-Ignition and Ignition Delay Time 104
5.2.6 Ambient Temperature Impact on Diesel Ignition 109
5.2.7 Turbulent Models 114
5.2.8 Turbulence - Chemistry Interaction 117
6. Conclusions and Recommendations 121
6.1 Conclusions 121
6.2 Recommendations for Future Work 125
References 127
Appendix A 138
Appendix B 142
