Initial-value ordinary differential equation solution via variable order Adams method (SIVA/DIVA) package is collection of subroutines for solution of nonstiff ordinary differential equations. There are versions for single-precision and double-precision arithmetic. Requires fewer evaluations of derivatives than other variable-order Adams predictor/ corrector methods. Option for direct integration of second-order equations makes integration of trajectory problems significantly more efficient. Written in FORTRAN 77.

Solving Ordinary Differential Equations

乱流予混合火炎のDirect Numerical Simulation

In the flamelet approach of turbulent premixed combustion, the flames are modeled as a wrinkled surface whose propagation speed, termed the {open_quotes}displacement speed,{close_quotes} is prescribed in terms of the local flow field and flame geometry. Theoretical studies suggest a linear relation between the flame speed and stretch for small values of stretch, S{sub L}/S{sub L}{sup 0} = 1 - MaKa, where S{sub L}{sup 0} is the laminar flame speed, Ka = {kappa}{delta}{sub F}/S{sub L}{sup 0} is the nondimensional stretch or the Karlovitz number, and Ma = L/{delta}{sub F} is the Markstein number. The nominal flame thickness, {delta}{sub F}, is determined as the ratio of the mass diffusivity of the unburnt mixture to the laminar flame speed. Thus, the turbulent flame model relies on an accurate estimate of the Markstein number in specific flame configurations. Experimental measurement of flame speed and stretch in turbulent flames, however, is extremely difficult. As a result, measurement of flame speeds under strained flow fields has been made in simpler geometries, in which the effect of flame curvature is often omitted. In this study we present results of direct numerical simulations of unsteady turbulent flames with detailed methane/air chemistry, thereby providing an alternative method of obtaining flame structure and propagation statistics. The objective is to determine the correlation between the displacement speed and stretch over a broad range of Karlovitz numbers. The observed response of the displacement speed is then interpreted in terms of local tangential strain rate and curvature effects. 13 refs., 3 figs.

https://digital.library.unt.edu/ark:/67531/metadc695872/m2/1/high_res_d/555263.pdf

Correlation of Flame Speed with Stretch in Turbulent Premixed Methane/Air Flames

With the increasing use of natural gas, improving the thermal efficiency and reducing emissions has become the major goals in its combustion. The objective of the present work is to investigate the effect of hydrogen addition on the combustion characteristics of natural gas. A ChemkinⅡ/Premix Code with the detailed chemical reaction mechanisms was employed with the Soret effect taken into account in all the calculations. With the mole fraction of hydrogen in the fuel varied from 0 to 40% at different initial temperatures (298–500 K) and pressures (1–8 atm), the results showed that the laminar burning velocities (LBVs) and the adiabatic flame temperatures of the C1 C4 four alkanes increased with increasing hydrogen-doping ratio. The LBV and the adiabatic flame temperature of methane displayed the maximum increase with the hydrogen-doping ratio. Additionally, the generation of active radicals H, O, and OH during the combustion process was strongly correlated with the LBV. The sensitivity of the flame temperature in four alkane fuels present in the natural gas at the maximum temperature gradient was analyzed. At a constant hydrogen-doping ratio, the LBV and the adiabatic flame temperature increased significantly with the increasing initial temperature. With increasing the pressure, the LBV gradually decreased while the adiabatic flame temperature increased.

Effect of hydrogen addition on the laminar premixed combustion characteristics the main components of natural gas

A three-dimensional (3D) direct numerical simulation (DNS) of an experimental turbulent premixed jet flame at high Karlovitz number was studied. The DNS resolution adequately resolves both the flame and turbulence structures. A reduced chemical mechanism for premixed CH4/air flames with NOx based on GRI-Mech3.0 was used, including 268 elementary reactions, and 28 transported species. Consistent post-processing methods were applied to both the DNS and experimental data to evaluate turbulent burning velocity-related statistics, namely the flame surface density (FSD), and the flame curvature. Good agreement was achieved for the 2D comparisons. The DNS data were further analysed and provide 3D statistics unattainable from the experiment. The ratio of the 3D and 2D flame surface densities was estimated. The results are comparable with other values reported for various experimental flames. The 3D and 2D flame curvatures were also compared and their distributions are shown to be quite different owing to the round on-average geometry. Instantaneous images of the heat release surrogate, [CH2O][OH], between the DNS and experiment agreed qualitatively. Various other experimentally obtainable surrogates for heat release rate including [CH2O][H], [CH2O][O], [HCO], and [CH] are also evaluated and compared using the DNS. The inner structure of the flame was compared between the DNS and experiment in terms of the joint PDFs of OH concentration and temperature. Generally good agreement was obtained; discrepancies may be due to the inconsistency of assumed equilibrium levels of OH concentration in the co-flow.

/pdf/a-comparison-between-direct-numerical-simulation-and-1h6ek2pukr.pdf

A comparison between direct numerical simulation and experiment of the turbulent burning velocity-related statistics in a turbulent methane-air premixed jet flame at high Karlovitz number

Zacharias M. Nikolaou

Papers

An optimisation framework for the development of explicit discrete forward and inverse filters