Reaction mechanism reduction for higher hydrocarbons by the ILDM method

Because of its generality and simple concept, the intrinsic low-dimensional manifolds (ILDM) method is used to reduce the number of concentration variables from about 50-100 to only 1-3. Typical results for flame propagation and premixed flame-front structure are presented for n-heptane and n-dodecane combustion. The new features are that (1) ILDMs for higher hydrocarbons can be determined die first time, with sufficient accuracy to determine NO (which is sensitively dependent on the O-atom concentration) by use of a newly developed stiff-stable algorithm; (2) an effective in situ tabulation has been developed, which can save orders of magnitude of computer time and storage; and (3) use of a set of a key species leads to a very simple interface to (even commercial) codes where source terms determined from the ILDM simply replace source terms calculated from a reaction mechanism, so that the problematic projection of the conservations to the ILDM is no longer necessary. A secondary result is that flames at increasing pressure (1 bar, 20 bar, and 80 bar are considered in this paper) become more and more determined by a thermal rather than a chain-branching mechanism. One important consequence is that at elevated pressures, similar results for Lewis numbers Le = 1 and Le not equal 1 are produced, leading to a considerable simplification of the transport model to be applied. ILDMs generated in this way have been successfully applied to turbulent combustion simulation and pollutant formation in diesel and two-stroke engines.