Large Eddy Simulation of Turbulent Reactive Flows under HCCI Engine Conditions

Abstract

Large Eddy Simulation (LES) modeling is developed to study homogeneous charge compression ignition (HCCI) combustion under different engine operation and mixture conditions, including real engine configurations and generic test cases. A previously developed HCCI model has been validated by comparing the simulation results with engine experiments. A new spark-assisted HCCI (SACI) combustion model has been developed and tested. The tumble flow dynamics and turbulence eddies under an experimental low speed engine condition are simulated to improve the understanding of the engine flow dynamics. The onset of temperature inhomgeneity is systematically investigated and compared with laser diagnostic data. The ignition kernel/eddy interaction is simulated to gain insight into the HCCI ignition process in different turbulence conditions. It is found that HCCI combustion is primarily dictated by the in-cylinder temperature stratification. For a given combustion phasing, e.g. a given crank angle at which 10% of heat has been released (CA10), with large temperature stratification the combustion process is slower and the pressure-rise-rate is slower. This can offer an opportunity to run the engine at high load. LES results show that turbulence can affect the HCCI combustion process under certain conditions. Due to rapid propagation of the ignition front, under typical engine conditions turbulence cannot directly affect the reaction zones, e.g., by wrinkling the reaction front or by differential diffusion to adjust the radical levels. Turbulence affects HCCI combustion mainly through modifying the temperature field. Turbulence has two effects on the process: one is to generate temperature stratification by heat transfer between the wall and the in-cylinder gas; another is to smear out the temperature stratification in the gas mixture by turbulence eddy action. If the size of hot zone or the cold zone in the mixture is large, e.g. larger than the integral length scale, turbulence heat transfer may not smoothen the temperature stratification quickly. As a result, the effect of turbulence on the ignition process is less significant. On the other hand if the scales of the hot/cold zones are small, turbulence can play an important effect on the HCCI combustion process. It is shown that the stratification of temperature in an engine is generated through three main mechanisms: mixing of the cold intake gas with the hot residual gas, wall heat transfer, and compression of the mixture. The last mechanism is less well known. By having large amount of residual gas or exhaust gas recirculation (EGR) in the cylinder the temperature stratification can be enhanced and thereby affect the HCCI combustion process. SACI combustion is shown to be sensitive to both turbulence and temperature stratification. The operation window for SACI is narrow: if the temperature is low the process is mainly SI, and if the temperature is high, the process is HCCI. As a result using SACI to control HCCI engine may not be easy.

Abstract (Swedish)

Popular Abstract in English

In the past ten years, homogeneous charge compression ignition (HCCI) combustion has been one of the subjects receiving great research interests in the combustion research community and the engine industry. This is because HCCI combustion processes have the potential of not only capable providing high efficiency like diesel engine combustion, but also producing much lower emissions of NOx and soot than diesel engines. HCCI combustion suffers from several technical difficulties. One is the control of ignition timing and the combustion process; another is the reduction of emissions of unburned hydrocarbons as well as carbon monoxide. In this thesis I carry out numerical studies on the fundamental physics of HCCI combustion, namely the fundamental structure of the reaction fronts and the fundamental interaction between turbulence eddies and ignition kernels. The goals are to gain deeper understanding on how the ignition kernels develop in turbulence conditions and to develop models that are able to predict the combustion process so that strategies of controlling the combustion process can be developed and emissions of fuel and carbon monoxide can be minimized. HCCI combustion is fundamentally different from the two well-known combustion processes, namely the premixed flame propagation found in spark ignition gasoline engines and diffusion flames found in diesel engines. In premixed flame propagation, a thin reaction layer propagates in the fuel/air mixture; the propagation is governed by reaction and diffusion. In diffusion flames chemical reaction occurs also in very thin layers and combustion is governed by the mixing of fuel and air. In turbulent premixed and diffusion flames, turbulence length scales are often larger than the reaction layer thickness, so the effect of turbulence is to wrinkle the reaction layers and thereby it increases the reaction layer area (flame surface density) and the fuel conversion rate. HCCI combustion represents a third mode of combustion: fuel and air are premixed before ignition just like premixed flames; the ignition is however by compression of the fuel/air mixture so that ignition of the mixture can theoretically occur homogeneously in the entire combustor. At the present, fundamental understanding of the details about HCCI combustion structures and effect of turbulence and temperature field is still lacking.

In this thesis Large eddy simulation modeling is developed to study HCCI combustion under different engine operation and mixture conditions, including real engine configurations and generic test cases. A previously developed HCCI model has been validated by comparing the simulation results with engine experiments carried out in KC-FP projects. A new SACI combustion model has been developed and tested. The tumble flow dynamics and turbulence eddies under an experimental low speed engine condition are simulated to improve the understanding of the engine flow dynamics. The onset of temperature inhomgeneity is systematically investigated and compared with laser diagnostic data. The ignition kernel/eddy interaction is systematically simulated to gain insight into the HCCI ignition process in different turbulence conditions. The main findings of this thesis can be summarized as follows. (a) HCCI combustion is primarily dictated by the in-cylinder temperature stratification. For a given combustion phasing, e.g. a given CA10, with large temperature stratification the combustion process is slower and the pressure-rise-rate is slower. This can offer an opportunity to run the engine at high load. (b) Turbulence can affect the HCCI combustion process under certain conditions. Due to rapid propagation of the ignition front, under typical engine conditions turbulence cannot directly affect the reaction zones, e.g., by wrinkling the reaction front or by differential diffusion to adjust the radical levels. Turbulence affects HCCI combustion mainly through modifying the temperature field. Turbulence has two effects on the process: one is to generate temperature stratification by heat transfer between the wall and the in-cylinder gas; another is to smear out the temperature stratification in the gas mixture by turbulence eddy action. If the size of hot zone or the cold zone in the mixture is large, e.g. larger than the integral length scale, turbulence heat transfer may not smoothen the temperature stratification quickly. As a result, the effect of turbulence on the ignition process is less significant. On the other hand if the scales of the hot/cold zones are small, turbulence can play an important effect on the HCCI combustion process. (c) The stratification of temperature is generated through three main mechanisms: mixing of the cold intake gas with the hot residual gas, wall heat transfer, and compression of the mixture. The last mechanism is less well known. By having large amount of residual gas or EGR in the cylinder the temperature stratification can be enhanced and thereby affect the HCCI combustion process. (d) Spark-assisted HCCI combustion is sensitive to both turbulence and temperature stratification. The operation window for SACI is narrow: if the temperature is low the process is mainly SI, and if the temperature is high, the process is HCCI. As a result using SACI to control HCCI engine may not be easy.

• author: Joelsson, Tobias
• supervisor: Xue-Song Bai
• opponent: Prof. Dr.-Ing. Hasse, Christian, VIRTUHCON - TU Bergakademie Freiberg, Germany
• organization: Fluid Mechanics; eSSENCE: The e-Science Collaboration
• publishing date: 2011
• type: Thesis
• publication status: published
• subject: Fluid Mechanics and Acoustics
• pages: 68 pages
• defense location: M:B M-building LTH, Ole Römers väg 1, Lund University Faculty of Engineering
• defense date: 2011-05-12 10:15:00
• ISBN: 987-91-7473-174-3
• project: Competence Centre for Combustion Processes
• language: English
• LU publication?: yes
• id: 7ba97308-c96f-4b71-b66a-534289324123 (old id 1895110)
• date added to LUP: 2016-04-04 13:21:31
• date last changed: 2018-11-21 21:13:27