Published experimental data obtained in well-defined simple cases are discussed in order to qualitatively test various models of premixed turbulent combustion, utilized in multi-dimensional numerical ...simulations of SI engines. An analysis of such data indicates that there exist several unresolved issues important for flame propagation in SI engines. Two of them, pressure dependence of turbulent flame speed St and turbulent flame development, are discussed in the paper. First, existing experimental data indicate an increase in St by pressure despite the marked decrease in the laminar burning velocity SL by P. Although this well established trend appears to be of substantial importance for SI engine applications, many combustion models utilize SL as the sole mixture characteristic and, hence, predict similar dependencies both of St and of SL on P, contrary to the aforementioned experimental results. The ability of various models to correctly predict the effects of pressure on turbulent combustion is assessed by qualitatively comparing analytical expressions for fully developed, 1D, planar flame speed, resulting from the models, with the available experimental data. The following approaches (1) Flame Speed Closure model 46, (2) CFM2 12, (3) the thickened flamelet model by Peters 9, (4) the pair-exchange model by Kerstein 59, and, maybe, fractal model by Zhao et al. 18, are shown to be the most promising. Second, experimental data indicate the non-stationary nature of turbulent flames: in most laboratory and industrial burners and in SI engines, in particular, both St and mean flame brush thickness δt develop with time after ignition. By analyzing experimental data, phenomenological approximations of St{t) and δt(t) are singled out and a self-similar regime of turbulent flame propagation is emphasized. The ability of various models to describe this regime and, in particular, to yield the qualitatively different time-dependencies of St(t) and of St(t), indicated by experimental data, is assessed both analytically and numerically. The results show that the emphasized difference is a challenge for many current combustion models but the Flame Speed Closure model is well tailored to simulate it and other emphasized trends of turbulent flame development.
An expanding cylindrical laminar flame kernel affected by random external strain rates and diffusivity is numerically simulated in order to gain insight into the influence of small-scale turbulence ...on the combustion variability in engines. In the simulations, the kernel is strained, as a whole, by external velocity gradients randomly generated with either Gaussian or log-normal probability density functions. The influence of smallscale turbulent heat and mass transfer is modeled by turbulent diffusivity, the randomness of which is controlled by the fluctuations in the viscous dissipation averaged over the kernel volume. The computed results show that small-scale phenomena can substantially affect the quenching characteristics of a small flame kernel and the kernel growth history τf(t); the scatter of the computed curves of τf(t) being mainly controlled by the scatter of the duration of the initial stage of kernel development. The results reveal the importance of small-scale turbulent straining for cyclic variability in spark ignition engines.
A model of premixed turbulent combustion is modified for multi-dimensional computations of SI engines. This approach is based on the use of turbulent flame speed in order to suggest a closed balance ...equation for the mean combustion progress variable. The model includes a single unknown input parameter to be tuned. This model is tested against two sets of experimental data obtained by Bradley et al 17-19 and Karpov and Severin 15 in fan-stirred bombs. The model quantitatively predicts the development of the turbulent flame speed, the effects of the initial pressure, temperature, and mixture composition on the turbulent flame speed, and the effects of r.m.s. turbulent velocity and burning mixture composition on the rate of the pressure rise. These results were computed with the same value of the aforementioned unknown input parameter of the model.
The structure of the ordered phase of non-stoichiometric tantalum carbide is studied by means of neutron diffraction. Its characteristics are shown to have an incommensurate superstructure similar to ...a M sub 6 C sub 5 type. The influence of non-stoichiometric and ordered tantalum carbide TaC sub y on the transition temperature in the superconducting state is studied. The stoichiometric carbide is TaC and the hardened non-stoichiometric carbides studied are: TaC sub 0.96 , TaC sub 0.94 , TaC sub 0.92 , TaC sub 0.90 , TaC sub 0.87 and TaC sub 0.83 and the annealed carbides TaC sub 0.87 and TaC sub 0.83 . As y changes from 0.83 to 1 the transition temperature increases from approx 2.0-10.5K.
A comparison has been made of the thermal capacity of TaC sub y specimens in both the non-ordered and ordered states, the latter being induced by quenching, annealing and slow cooling. On a basis of ...Debye and Einshtein temperature measurements, it was shown that the ordered TaC sub y had a greater thermal capacity than the non-ordered material. An equation has been derived for determining the differential between the two conditions. Graphs. 5 ref.--M.R.M.
