Study on the Formation and Oxidation of Soot in Diesel Combustion by Using Optical Diagnostics : 광계측 진단법을 이용한 디젤연소에서의 Soot 생성과 산화에 관한 연구

국문 초록 (Abstract)

디젤엔진은 저급연료의 사용이 가능하고 높은 열효율을 가지고 있어서 산업에 많은 기여를 하고 있다. 그러나, 인체와 환경에 유해한 soot이 다량으로 발생하고, 앞으로 더욱 강화되는 규제치를 만족하기 위해서 soot의 저감책이 더욱 요구되고 있다. Soot을 저감하기 위해서는 그에 대한 정확한 생성과 산화를 고찰하여 근본적인 정보를 얻어야 하고, 이를 위해서 시간
적 · 공간적 분해능이 우수한 방법으로 soot 농도를 계측해야 한다. 따라서, 본 연구에서는 이러한 조건을 만족할 수 있는 광계측 진단법을 이용하여 디젤 연소장에서 soot의 농도 분포를 순간적으로 계측한다.
본 연구에서는 이색법과 레이저 시트법의 두 가지 광계측법을 통하여 세 개의 실험을 행하였다. 첫 번째 실험에서는, 디젤분무를 이루는 자유연료액적열을 대상으로 하여 이색법으로 soot의 생성 특성을 고찰하였다.
두 번째 실험에서는, 동축이중관을 가진 층류확산화염에서 LIS법과 LII 법을 동시에 사용하였다.
세 번째 실험에서는, 실엔진에서의 soot 분포를 계측하기 위해 가시화 디젤엔진을 제작하고, soot로 부터의 LIS와 LII 강도를 계측하여, soot 농도의 2차원 가시화를 확립하고 광계측의 신뢰성을 확보함으로써 디젤엔진의 기존 soot 계측방법 보다 명확한 soot의 생성 · 산화과정에 대한 정보를 제공함에 목적을 두었다. 연구의 주요결과에서 결론을 도출하면 다음과 같다.
1. 분위기온도가 증가할수록 연료의 착화지연과 예혼합량에 영향을 미쳐 soot의 생성량은 증가한다. Soot을 저감하기 위해서는 초기 예혼합기의 양과 화염온도를 증가시켜야 한다.
2. 액적간격이 좁을수록 당량비는 농후해져서 연료의 공기이용률이 낮아진다. Soot을 저감하기 위해서는 화염 내부로 산화제를 활발히 유입시켜야 한다.
3. 확산연소에서 액적 표면적의 증가는 열전달과 연료의 증발을 촉진함으로써 soot을 저감시키는데 도움이 된다.
4. 레이저여기산란법(LIS)과 레이저여기적열법(LII)은 soot 농도를 정량적으로 계측하기 위해 동시에 사용되었으며, 광강도가 강한 LIS법은 층류확산화염과 같이 분위기가 청정한 대상에서 soot 농도 분포를 계측하는데 유리하였다.
6. 연료의 C/H와 확산화염 주위의 평행공기유속이 증가할수록 soot의 농도는 증가하였다.
7. 디젤엔진의 초기 연소시 캐비티 내에서의 soot은 직경이 작으며 수밀도는 크다. 팽창행정시에 낮은 온도의 실린더 벽면 근처로 갈수록 soot의 직경은 커지고 수밀도는 감소하였다.
8. Soot의 평균직경은 ATDC 20도에서 110도까지 37 % 정도도 완만히 증가하며, 평균 수밀도는 ATDC 20도와 70도 사이에서 91%정도로 급격히 감소하였다.

다국어 초록 (Multilingual Abstract)

As diesel engines are able to use low-level fuels and have thermal efficiency, they have contributed largely to the industry. However, the low emissive diesel engine is required because soot particles from them are h m f d to the human and the environment. In order to reduce soot, it is necessary to measure soot concentration instantaneously with superior temporary and spatial resolutions. Therefore, in order to measure soot concentration in diesel combustion, the optical measurement techniques with superiority of temporary and spatial resolutions must be used. In this study, three experiments are performed to observe the formation and oxidation of soot using two optical diagnostics.
In the first experiment, the characteristics of soot formation with the ambient temperature and condition of local place in diffusion flame of free fuel droplet array is studied by means of two-color method.
In the second experiment, the distribution of soot concentration is taken as a two-dimensional image by LIS and LII methods in a diffusion flame of a burner. The relative diameter and number density of soot with quantity of air flow around flame and C/H ratio of fuels are obtained.
In the last experiment, an optically accessible diesel engine is built in order to measure distribution of soot concentration. The two-dimensional images of soot concentration were obtained by simultaneous measurement of LIS and LII methods. Conclusions were obtained as follows.
1. As the ambient temperature increases, quantity of soot formation increases. In order to reduce soot, quantity of the early premixed mixture and flame temperature have to be increased.
2. As interdroplet spacing is narrow, the coefficient of air utilization is low due to rich equivalent ratio. In order to reduce soot in the diffusion combustion, it is desirable to introduce an oxidizer inside the flame.
3. The increase of the surface of fuel brings the increase of heat transfer and fuel evaporation, and helps the reduction of soot in the diffusion combustion.
4. The LIS and LII methods are used simultaneously for quantitative measurement of soot concentration, and the simultaneous measurement method is established successfully.
5. LIS method having a strong light intensity is advantageous in soot measurement method for object having a steady state environment like a laminar diffusion flame.
6. The concentration of soot increases with the C/H ratio of fuel and parallel air velocity around diffusion flame.
7. Soot clouds have small diameter and a large of number density in the cavity. During expansion strokes, the diameter near the cylinder wall increases and then the number density decreases.
8. The mean diameter of soot increase about 37% from ATDC 20 degree to ATDC 110 degree gently. The number density of soot with crank angle has large values between ATDC 20 degree and ATDC 40 degree and the decrease rate of about 91% into ATDC 70 degree.

목차 (Table of Contents)

• 목차
• Ⅰ. Introduction = 1
• 1. Background = 1
• 2. The tendencies of domestic and foreign studies = 5
• 3. The purpose of this study = 8
• Ⅱ. A Study on the Characteristics of Soot Formation in Free Fuel Droplet Array Using Two-Color Method = 13
• 1. Introduction = 13
• 1) The purpose of this study = 13
• 2) The studies using two-color method = 14
• 2. Theory of the two-color method = 16
• 3. Experimental apparatus and methods = 19
• 1) Experimental apparatus = 19
• 2) The experimental conditions and methods = 21
• 4. The results and discussion = 26
• 1) The ignition delay = 26
• 2) The influence of the ambient temperature on the soot formation = 27
• 3) The influence of the equivalent ratio of local area on the soot formation = 29
• 4) The influence of droplet size on the soot formation = 30
• 5. Conclusions = 33
• Ⅲ. A Study on the Measurement of Soot Concentration in Diffusion Flames Using Laser Sheet = 48
• 1. Introduction = 48
• 1) The purpose of this study = 48
• 2) The studies using laser diagnostics = 49
• 2. Soot measurements using laser sheet = 54
• 1) Laser induced scattering (LIS) method = 54
• 2) Laser induced incandescence (LII) method = 56
• 3) Soot particle sizing using LIS and LII = 62
• 3. Experimental apparatus and methods = 63
• 1) Test fuels = 63
• 2) The part of Combustion equipments = 63
• 3) Laser optical measurement system for soot concentration = 64
• 4) Synchronization for a laser and an ICCD = 65
• 4. The results and discussion = 67
• 1) General characteristics of diffusion flame = 67
• 2) Distribution of soot concentration for C/H ratio of fuel = 68
• 3) Soot concentration with surrounding air velocity around a diffusion flame = 72
• 5. Conclusions = 74
• Ⅳ. A Study on the Formation and Oxidation of Soot Using LIS and LII Simultaneous Measurement Methods in an Optically Accessible Diesel Engine = 100
• 1. Introduction = 100
• 2. Experiment apparatus and methods = 101
• 1) The design of optically accessible diesel engine and combustion characteristics = 101
• 2) Laser diagnostics system for soot measurement = 103
• 3) Synchronization circuit and devices control = 105
• 3. Results and discussion = 107
• 4. Conclusions = 111
• Ⅴ. Conclusions = 134
• 1. Study on the characteristics of soot formation in a free fuel droplet array using two-color method = 135
• 2. The simultaneous measurement method by using both LIS and Lll for the distribution of soot concetration in a diffusion flames = 136
• 3. The simultaneous measurement method by using both LIS and Lll methods for the formation and oxidation of soot in an optically accessible diesel engine = 137
• APPENDIX = 139
• REFERENCES = 150
• ABSTRACT = 156