| Round jet flow and syngas jet flame were studied via direct numerical simulation technique. The main research subject were the numerical techniques for there dimensional direct numerical simulation of combustion, air jet flow and syngas jet flame.The fully compressible non-dimensional Navier-Stokes conservation equations were solved here. For the spatial discretization eighth-order explicit central finite difference scheme was adopted and fourth-order five stage Runge-Kutta scheme was used for temporal marching. Besides, tenth-order explicit central finite difference scheme was used to eliminate the unwanted oscillation with high wave number. All these numerical techniques perform very well and have strong stability. Non-reflecting characteristic boundary conditions for three dimensional combustion were obtained. Exit zone where low order filtering was implemented near computational domain boundary were used. Proper parallel computational methods were used combined with the explicit difference schemes, and high speed-up was obtained.First we do a DNS for a subsonic round turbulent air jet in a coflow with laminar flow. The jet mean velocity, Reynolds stresses, second and third order velocity moments and turbulent energy spectral were obtained. These simulation results agree very well with experimental data. Vortex rings were found in the upstream of the jet, further downstream these large scale coherent structure started to break down and merge with each other. Image of this evolution process were showed by vortex magnitude and velocity gradient. The decay rate Bu of jet centerline mean velocity is found to be 5.5, which was very close tR RtKHV rHVu(?). u, wKLF(?)tVSrRILOH of the mean velocity along radial direction, was found to be 75.4. Then we calculate the second and third order velocity moments and found that in the axial region of x=17d-20d the jet statistics exhibit self-similar behavior and agree well with published experimental data in fully self-similar region. The one-dimensional energy spectra of longitudinal and lateral velocity are also presented. The inertial region where the spectra decays according to the k-5/3 is observed. At around x=19d on the jet centerline, their was some difference between autospectra of longitudinal and of lateral velocity due to anisotropy.Then we do a direct simulation of wood residue syngas jet flame in a hot coflow environment. In order to investigate the jet flame structure, species distribution and the formation of NO, wood residue syngas with real composition was considered. The reaction mechanism is a multi-step mechanism which include NO reaction. PLIF was used to measure the OH image and species mean concentration along radial direction. Image of the species mass fraction, temperature, mixture fraction, heat release rate and scalar dissipation rate were obtained. The computed OH image was compared with the OH-PLIF image and the flame front structure was very similar. Many physical variables scatter plot in the mixture fraction space were plotted and also the conditional mean value on the mixture fraction. It is found that the peak of OH curve appears on the left side of the maximum heat release rate, which means on the lean side with high temperature. The discrepancy between peak position of OH curve and heat release rate for this partial premixed jet flame indicates that it is not very precise to use OH as the flame front indicator. Then the relationship between scalar dissipation rate and chemical reaction rate was analyzed and it is found that at the upstream position of the jet high heat release rate appears in the place where the scalar dissipation rate is large. Further downstream this phenomenon disappear gradually. At the end we do analysis of the ignition process of the jet flame. Large scalar dissipation promote the mixing between fuel and oxidizer and the heat release rate grow up in the mixture space, and at last ignition take place.
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