Direct Numerical Simulations of Low Temperature Combustion in IC Engine Related Conditions

Abstract

This thesis deals with direct numerical simulation (DNS) of low temperature combustion in internal combustion engine related conditions. In particular, HCCI (homogeneous charge compression ignition), SACI (spark assisted ho-mogeneous charge compression ignition) and PPC (partially premixed charge combustion) were investigated to gain deeper understanding on the details of the combustion process. In the study of HCCI combustion relevant to direct injection HCCI engines, it was found that auto-ignition is not first started at the stoichiometric mixture, but rather in lean mixtures where the local temperature is higher and scalar dissipation rate is low. The first ignition spots with the shortest ignition delay time were found to depend on the turbulence intensity, initial pressure and temperature. With higher turbulence intensity the ignition delay time is longer, and the ignition duration is longer. With higher combustor pressure, the ignition delay time is also longer but the ignition zone is significantly thinner. Compared with the perfectly homogeneous mixture, the ignition delay time in the turbulence case is shorter, owing to differential diffusion and stratifications in composition and temperature. In the study of SACI combustion, the interaction of the premixed flame and auto-ignition under different fuel and operation conditions were studied in 3D DNS and one-dimensional detailed numerical simulations. Several fuels including hydrogen, syngas and methane are studied. The results revealed that due to preferential diffusion of hydrogen, lean hydrogen/air flames tend to develop to cellular shapes, owing to the well known thermal diffusive instability. Furthermore, the imbalance of heat and mass transfer with respect to hydrogen molecular in the preheat zone was shown to inhibit the ignition process leading to a lower temperature and thereby a slower propagation of the reaction front. In the studies of PPC combustion, the effect of several parameters, e.g. the fuel mass ratio between the second and the first fuel injection, initial temperature, and turbulence intensity, was investigated using 2D and 3D DNS for syngas and PRF70. A tradeoff between NO emission and CO emission was identified when varying the fuel mass ratio. Spatial reaction structures showed that during combustion process, auto-ignition was formed first followed by premixed combustion and at last a diffusion controlled partially premixed combustion. In the homogeneous lean charge region, the fuel is consumed almost completely due to relatively fast combustion and abundant of oxygen. In contrast, in the stoichiometric charge region and fuel-rich region CO is hardly oxidized to CO2 even though the temperature is high.

Abstract (Swedish)

Popular Abstract in English

Internal combustion engine plays an important role in our society. In our daily life, we use them to drive cars, trucks, ships, generating electricity and heat, and in many other applications. Modern internal combustion engines fall into three catalogues: gasoline engines, diesel engines and low temperature compression ignition engine. They are developed to achieve higher efficiency to prolong the use of fossil fuels that have a limited reserve on earth, and to reduce carbon dioxide

emission per kilometre of drive. Our public concern on environment has

promoted more stringent environment legislations that demand internal combustion engines to run with low emissions (carbon monoxide, NOx, unburned hydrocarbons, and particulate matters). In the past two decades, advanced low temperature combustion (LTC) engines, including HCCI (homogeneous charge compression ignition), SACI (spark assisted homogeneous charge compression ignition), PPC (partially premixed charge compression ignition) engines, have been under intensive research and development. These engines operate at low temperatures and high compression ratios to minimize pollutant emissions and

to achieve high efficiency. Engine experiments have been performed in many

university laboratories and internal combustion engine industry and the advantages of the LTC concept have been fully demonstrated. However, to put these low temperature combustion engines into mass production, several issues need to be resolved. Extensive experiments showed that LTC engines tend to have high engine noise and difficulty of combustion controlling. Therefore, different strategies need to be developed to overcome various technological barriers.

This thesis is aimed at gaining theoretical understanding of the fundamental chemical and physical processes in LTC engines. This is done by carrying out direct numerical simulation (DNS) of the combustion process employing high accuracy numerical algorithms and the full set of partial differential equations that govern the physical and chemical process. In DNS all the detailed temporal evolution and spatial structures of ignition and chemical reaction zones occurring

in LTC engines are computed. The results of DNS reveal several important

characteristics of LTC engines. The locations and reasons of the emissions of carbon monoxide, NOx, unburned hydrocarbons in LTC are identified. The origin of the high noise is identified. It is demonstrated that by optimizing the fuel injections one can achieve a compromise among carbon monoxide, NOx,unburned hydrocarbons emissions.

• author: Zhang, Fan
• supervisor: Xue-Song Bai
• opponent: Professor Im, Hong G, Department of Mechanical Engineering, Clean Combustion Research Center, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia
• organization: Fluid Mechanics
• publishing date: 2013
• type: Thesis
• publication status: published
• subject: Fluid Mechanics and Acoustics
• defense location: Lecture hall MA6, Matteannexet, Sölvegatan 20, Lund University Faculty of Engineering
• defense date: 2013-10-03 10:15:00
• language: English
• LU publication?: yes
• id: 55187424-ffe0-4f04-9a47-766eac852f58 (old id 4022802)
• date added to LUP: 2016-04-04 09:16:01
• date last changed: 2018-11-21 20:51:55