http://hdl.handle.net/10016/961 2013-05-23T23:24:12Z http://hdl.handle.net/10016/14859 Title: An explicit reduced mechanism for H2–air combustion Author(s): Boivin, Pierre; Jiménez, Carmen; Sánchez, Antonio Luis; Williams, Forman Arthur Abstract: For hydrogen–oxygen–inert systems, just as for other fuel–oxidizer mixtures, systematically reduced chemistry has in the past been developed separately for premixed and diffusion flames and for autoignition. In computational work that addresses turbulent combustion or the transition from deflagration to detonation, however, autoignition and flames both may occur, and reduced chemistry may be required because of computer limitations. To fill that need, systematically reduced chemistry is presented here that encompasses autoignition and flames. The description involves three global steps among five reacting species, H2;O2;H2O;H and HO2, being based on approximations to chemical-kinetic steady states for O, OH and H2O2. These steady states apply well under all conditions except during autoignition in lean and stoichiometric mixtures, where they underpredict induction times substantially. To remedy this deficiency, which occurs only when HO2 is not in steady state, an autoignition analysis is employed to derive a correction factor that reduces the value of the reaction rates to produce agreement of calculated ignition delays. Introduction of a criterion for inclusion of this correction factor, based on a test for the HO2 steady state, results in a generally applicable three-step chemical-kinetic description for hydrogen–air combustion that possesses reasonable accuracy for most computational purposes. Description: Proceedings of: 13th International Conference on Numerical Combustion, 27-29 April, 2011, Corfu (Greece) in conjunction with the 3rd International Workshop on Model Reduction in Reacting Flows 2010-12-31T23:00:00Z http://hdl.handle.net/10016/14852 Title: Explicit analytic prediction for hydrogen–oxygen ignition times at temperatures below crossover Author(s): Boivin, Pierre; Sánchez, Antonio Luis; Williams, Forman Arthur Abstract: This paper addresses homogeneous ignition of hydrogen-oxygen mixtures when the initial conditions of temperature and pressure place the system below the crossover temperature associated with the second explosion limit. A three-step reduced mechanism involving H2, O2, H2O, H2O2 and HO2, derived previously from a skeletal mechanism of eight elementary steps by assuming O, OH and H to follow steady state, is seen to describe accurately the associated thermal explosion. At sufficiently low temperatures, HO2 consumption through HO2 + HO2 → H2O2 + O2 is fast enough to place this intermediate in steady state after a short build-up period, thereby reducing further the chemistry description to the two global steps 2H2 + O2 → 2H2O and 2H2O → H2O2 + H2. The strong temperature sensitivity of the corresponding overall rates enables activation-energy asymptotics to be used in describing the resulting thermal runaway, yielding an explicit expression that predicts with excellent accuracy the ignition time for different conditions of initial temperature, composition, and pressure. 2012-01-31T23:00:00Z http://hdl.handle.net/10016/14850 Title: A four-step reduced mechanism for syngas combustion Author(s): Boivin, Pierre; Jiménez, Carmen; Sánchez, Antonio Luis; Williams, Forman Arthur Abstract: A four-step reduced chemical-kinetic mechanism for syngas combustion is proposed for use under conditions of interest for gas-turbine operation. The mechanism builds upon our recently published threestep mechanism for H2-air combustion (Boivin et al., Proc. Comb. Inst. 33, 2010), which was derived from a 12-step skeletal mechanism by assuming O, OH, and H2O2 to be in chemical-kinetic steady state and includes a correction to account for the failure of the O and OH steady states during autoignition. The analysis begins by appropriately extending the number of elementary steps in the skeletal description to enable computation of the CO chemistry for mixtures with appreciable H2 content, giving a total of 16 elementary steps. It is seen that the formyl radical HCO, which appears as the only additional relevant intermediate in the extended chemical description, follows accurately a steady-state approximation, which can be used along with the steady-state approximations for O, OH, and H2O2 to derive the reduced description, involving the three global steps of our previous H2-air mechanism, 3H2+O2 2H2O+2H, 2H+M H2+M, and H2+O2 HO2+H, along with the additional step CO+ H2O CO2+H2. Expressions are given for the rates of the four global reactions in terms of those of the elementary steps of the skeletal mechanism, with concentrations of the different steady-state species also given in explicit form. Comparisons of results of computations of laminar burning velocities and induction times with published experimental data for H2/CO/O2 mixtures with different diluents at atmospheric and elevated pressures and for varying equivalence ratios and initial temperatures indicate that the reduced description can be applied with reasonable accuracy in numerical studies of gas-turbine syngas combustion. 2011-05-31T22:00:00Z http://hdl.handle.net/10016/14848 Title: Simulation of a supersonic hydrogen-air autoignition-stabilized flame using reduced chemistry Author(s): Boivin, Pierre; Dauptain, Antoine; Jiménez, Carmen; Cuenot, Bénédicte Abstract: A three-step mechanism for H2-air combustion (Boivin et al., Proc. Comb. Inst. 33, 2010) was recently designed to reproduce both autoignition and flame propagation, essential in lifted flame stabilization. To study the implications of the use of this reduced chemistry in the context of a turbulent flame simulation, this mechanism has been implemented in a compressible explicit code and applied to the simulation of a supersonic lifted co-flowing hydrogen-air flame. Results are compared with experimental measurements (Cheng et al. C&F 1994) and simulations using detailed chemistry, showing that the reduced chemistry is very accurate. A new explicit diagnostic to readily identify autoignition regions in the post-processing of a turbulent hydrogen flame simulation is also proposed, based on variables introduced in the development of the reduced chemical mechanism. 2012-03-31T22:00:00Z