Department News
How Microscopic Particles can Affect Macroscopic Turbulence, and the Effect of Turbulence on Intern
Seminar Date
2005-07-13
Author
임아주
Date
2005-07-01
Views
1888
1. 제 목 :
How Microscopic Particles can Affect Macroscopic Turbulence, and the Effect of Turbulence on Internal Combustion Engines
2. 연 사 :
Wontae Hwang, Ph.D.
Engine Combustion Department
Sandia National Laboratories
3. 일 시 : 2005년 7월 13일(수) 오후 4시
4. 장 소 : 301동 1512호 세미나실
5. 내 용 : Dilute loadings of heavy microscopic particles can often attenuate the carrier-phase turbulence levels (up to 80% in some cases) of particle-laden flows. The underlying physics of this complex phenomenon are not well understood. A novel flow facility that can create homogeneous and isotropic turbulence with no mean flow was developed at Stanford University to study basic particle/turbulence interaction. The air turbulence was created by synthetic jet actuators mounted on the corners of a confined cubical chamber. Loudspeakers were used as the actuators. A custom 2D particle image velocimetry (PIV) system that can be used in multiphase flow environments was set up to measure gas-phase statistics. Flow qualification results of stationary turbulence and natural decaying turbulence indicated that the turbulence was homogeneous throughout the measurement domain and isotropic as well. The particle-laden flow experiments were conducted in the lab and also in micro-gravity, aboard NASA’s KC-135 parabolic flight aircraft. Results from the laboratory experiments showed that microscopic particles on the order of the Kolmogorov length scale attenuated the turbulent kinetic energy and dissipation rate with increasing particle mass loadings. The main source of energy production in the chamber was the speakers, but the loss of potential energy of the settling particles also resulted in a significant amount of production of extra turbulence for the flow. The sink of energy in the chamber was due to ordinary fluid viscous dissipation and also extra dissipation caused by particles. The micro-gravity results showed that the absence of particle potential energy loss and particle wakes caused greater levels of turbulence attenuation, up to 30% for a particle mass loading ratio of only 0.08.
Turbulence has also been known to play an important role in internal combustion engines (especially spark ignition and diesel engines). Homogeneous charge compression ignition (HCCI), which is a relatively new engine combustion process, can provide high efficiencies similar to a diesel engine with very low NOx and particulate emissions. Even though the fuel/air mixture is nominally homogeneous, thermal stratification can naturally occur due to heat transfer and turbulence. Chemiluminescence imaging in an optical HCCI engine at Sandia National Laboratories shows that this stratification leads to sequential autoignition, which slows the pressure rise rate and reduces knock. Utilizing the stratification provides an avenue for increasing the maximum allowable fueling rate of HCCI.
6. 문 의 : 기계항공공학부 최 해 천 교수 (☏ 880-8361)
How Microscopic Particles can Affect Macroscopic Turbulence, and the Effect of Turbulence on Internal Combustion Engines
2. 연 사 :
Wontae Hwang, Ph.D.
Engine Combustion Department
Sandia National Laboratories
3. 일 시 : 2005년 7월 13일(수) 오후 4시
4. 장 소 : 301동 1512호 세미나실
5. 내 용 : Dilute loadings of heavy microscopic particles can often attenuate the carrier-phase turbulence levels (up to 80% in some cases) of particle-laden flows. The underlying physics of this complex phenomenon are not well understood. A novel flow facility that can create homogeneous and isotropic turbulence with no mean flow was developed at Stanford University to study basic particle/turbulence interaction. The air turbulence was created by synthetic jet actuators mounted on the corners of a confined cubical chamber. Loudspeakers were used as the actuators. A custom 2D particle image velocimetry (PIV) system that can be used in multiphase flow environments was set up to measure gas-phase statistics. Flow qualification results of stationary turbulence and natural decaying turbulence indicated that the turbulence was homogeneous throughout the measurement domain and isotropic as well. The particle-laden flow experiments were conducted in the lab and also in micro-gravity, aboard NASA’s KC-135 parabolic flight aircraft. Results from the laboratory experiments showed that microscopic particles on the order of the Kolmogorov length scale attenuated the turbulent kinetic energy and dissipation rate with increasing particle mass loadings. The main source of energy production in the chamber was the speakers, but the loss of potential energy of the settling particles also resulted in a significant amount of production of extra turbulence for the flow. The sink of energy in the chamber was due to ordinary fluid viscous dissipation and also extra dissipation caused by particles. The micro-gravity results showed that the absence of particle potential energy loss and particle wakes caused greater levels of turbulence attenuation, up to 30% for a particle mass loading ratio of only 0.08.
Turbulence has also been known to play an important role in internal combustion engines (especially spark ignition and diesel engines). Homogeneous charge compression ignition (HCCI), which is a relatively new engine combustion process, can provide high efficiencies similar to a diesel engine with very low NOx and particulate emissions. Even though the fuel/air mixture is nominally homogeneous, thermal stratification can naturally occur due to heat transfer and turbulence. Chemiluminescence imaging in an optical HCCI engine at Sandia National Laboratories shows that this stratification leads to sequential autoignition, which slows the pressure rise rate and reduces knock. Utilizing the stratification provides an avenue for increasing the maximum allowable fueling rate of HCCI.
6. 문 의 : 기계항공공학부 최 해 천 교수 (☏ 880-8361)
