Search

Abstract: This paper reports on two-dimensional (2D) and three-dimensional (3D) direct numerical simulations (DNSs) of the auto-ignition process of a lean H2/air mixture with temperature stratification in a constant volume enclosure. Detailed chemistry and transport properties are taken into account in the simulations. The combined propagation of spontaneous ignition front and deflagration front is identified and the relation between the reaction front displacement speed and the temperature gradient is verified. The difference between 2D- and 3D-DNS is investigated by comparing the evolutions of global combustion parameters such as the averaged heat release rate, total reaction front area and the averaged displacement speed of the reaction front. The extra spatial dimension in 3D-DNS has been shown to cause a higher velocity strain rate to enhance the heat transfer process, which leads to a delayed but more rapid ignition of the mixture than the 2D-DNS cases. The 3D reaction front surfaces are examined based on the local mean and Gaussian curvatures. By introducing a cutoff Gaussian curvature two types of 3D surface elements, the small sphere fronts and the strong saddle fronts, are defined. The effect of these fronts on the combustion process is studied in terms of their contribution to the total reaction front area, fuel consumption rate and curvature-induced stretch. [Copyright &y& Elsevier]

The proposed algorithm aims to realize the division of arbitrary fractional frequency. The general frequency divider is comprised of the counter cascade ,and can realize the even frequency division easily, but it is difficult to realize odd frequency divider and fractional frequency divider. This paper proposes a design method of Arbitrary Fractional Frequency Divider (AFFD)based on FPGA circuit cores by an improved Bresenham line-drawing algorithm, which reduces the hardware resource and eases the implementation process. Bresenham line-drawing algorithm is introduced briefly in this paper outside its original applications. The circuit is designed with Verilog Hard Design Language(VHDL), and the program is proposed. The frequency divider is simulated in Quartus II and the divider is examined in FPGA-Based gear hobbling machine CNC system to control the step-servo motor, and the result is steady and reliable. This paper extends the investigation on the frequency division by the improved algorithm. The algorithm designs arbitrary Fractional Frequency Multiplier, and it is also examined by simulation and experiment. [ABSTRACT FROM AUTHOR]

In this paper, numerical simulations were carried out to identify the mode of ignition and ignition delay time of pulverized biomass particles in hot flue gas produced by a methane/air flame. In the experiments, it was observed that for most biomass residues the dominant combustion mode was the staged gas-phase ignition in the surrounding gas followed by surface ignition at the char surface. There were some exceptions to this general trend, e.g. wheat straw particles, which ignited at the surface of the particle under some temperature conditions. Moreover, temporally and spectrally resolved images of the single burning particles were obtained in the experiments and CH* chemiluminescence at different stages of biomass conversion was recorded. In this study, by means of a detailed numerical model for conversion of biomass particles and employing detailed gas chemistry mechanism, the ignition mode and ignition delay time of the particles are studied. The model is able to distinguish between different ignition modes of the particles in agreement with the experimental data. The underlying physics behind shifting ignition mode from homogeneous ignition to heterogeneous ignition for wheat straw are discussed. The ignition delay times for different biomass sources at different conditions are calculated and the results are in good agreement with the experimental data. Apart from the detailed model, CFD simulations are performed to assess the flow and combustion process (temperature, O 2 concentration and velocity difference between the ambient gas and the particle) around the particle. The CFD results show similar trends compared with the CH* chemiluminescence from the particle at different times during the devolatilization stage. [ABSTRACT FROM AUTHOR]

Transient mixing and ignition mechanisms in a simplified pre-chamber/main-chamber system are investigated using direct numerical simulation (DNS) with detailed chemical kinetics. Full ignition and flame propagation processes in the premixed methane/air mixtures are simulated. Ignition, the progress and topology of flame evolution, and the mean burning velocity in the main chamber are analyzed in detail. Four important phases in the ignition and flame propagation processes are identified based on the flame structure development in the main chamber, the pressure and velocity evolution at typical points in both the pre-chamber and main chamber. Results show that the intermediate species OH, CH 2 O, and HO 2 are critical for flame stabilization and propagation in the main chamber due to their high reactivity. This is sorted as the chemical effect that the pre-chamber jet acts on the main chamber. The high temperature jet also brings heat and unburned fuel into the main chamber, which are sorted as thermal and enrichment effect, respectively. The heat release rate is found to be approximately proportional to the product of CH 2 O and OH mass fractions, which could be regarded as a reliable and effective marker for the heat release rate of methane/air mixtures. It is found that the mean burning velocity in the main chamber can be elevated up to 30 times under the condition investigated. [ABSTRACT FROM AUTHOR]

Large Eddy Simulation (LES) was employed to investigate the fuel/oxidizer mixing process in an ethylene fueled scramjet combustor with a rearwall-expansion cavity. The numerical solver was first validated for an experimental flow, the DLR strut-based scramjet combustor case. Shock wave structures and wall-pressure distribution from the numerical simulations were compared with experimental data and the numerical results were shown in good agreement with the available experimental data. Effects of the injection location on the flow and mixing process were then studied. It was found that with a long injection distance upstream the cavity, the fuel is transported much further into the main flow and a smaller subsonic zone is formed inside the cavity. Conversely, with a short injection distance, the fuel is entrained more into the cavity and a larger subsonic zone is formed inside the cavity, which is favorable for ignition in the cavity. For the rearwall-expansion cavity, it is suggested that the optimized ignition location with a long upstream injection distance should be in the bottom wall in the middle part of the cavity, while the optimized ignition location with a short upstream injection distance should be in the bottom wall in the front side of the cavity. By employing a cavity direct injection on the rear wall, the fuel mass fraction inside the cavity and the local turbulent intensity will both be increased due to this fueling, and it will also enhance the mixing process which will also lead to increased mixing efficiency. For the rearwall-expansion cavity, the combined injection scheme is expected to be an optimized injection scheme. [ABSTRACT FROM AUTHOR]

A particle-based transported probability density function (PDF) method with a novel chemistry acceleration technique is developed in this work. The technique is based on the chemistry coordinate mapping (CCM) approach that was proposed in our previous works for accelerating direct numerical simulations (DNS) of partially premixed combustion. The method is first validated using Sandia flames D and F. It is shown that PDF-CCM results converge toward those obtained without CCM as phase-space resolution increases. PDF-CCM is then applied to simulate methane/air lifted jet flames in vitiated coflow reported in experiments by Cabra et al. (2005). It is shown that combustion is initiated in the form of auto-ignition in very fuel-lean gases where the gas velocity is low (the residence time is long) and the gas temperature is high (the ignition delay time is short). The ignition delay of the mixture below the liftoff position scales well with the liftoff height at different coflow temperature conditions. The combustion process above the liftoff height can develop into different modes depending on the coflow temperature. For high-temperature coflow, a premixed-burned combustion is formed above the liftoff height, which involves fuel-lean to fuel-rich burning modes; a triple-flame structure eventually is formed a few nozzle diameters above the liftoff position. For low-temperature coflow, the ignition delay and the liftoff height are sufficiently large to allow premixing between fuel and oxidizer before the onset of high-temperature combustion; in this case, a lean-to-stoichiometric premixed burn combustion is established downstream of the liftoff height, and no obvious triple-flame structure is formed at this condition. [ABSTRACT FROM AUTHOR]

We present a simple design for a broadband tunable terahertz (THz) metamaterial absorber (MMA) consisting of a complementary cross-oval-shaped graphene (CCOSG) structure and dielectric substrate placed on a continuous metal film. Both numerical simulation and theoretical calculation results indicate that the absorbance is greater than 80% from 1.2 to 1.8 THz, and the corresponding relative bandwidth is up to 40%. Simulated electric field and power loss density distributions reveal that the broadband absorption mainly originates from the excitation of continuous surface plasmon resonance (SPR) on the CCOSG. In addition, the MMA is polarization-insensitive for both transverse-electric (TE) and transverse-magnetic (TM) modes due to the geometry rotational symmetry of the unit-cell structure. Furthermore, the broadband absorption properties of the designed MMA can be effectively tunable by varying the geometric parameters of the unit-cell and chemical potential of graphene. Our results may find promising applications in sensing, detecting, and optoelectronic-related devices. [ABSTRACT FROM AUTHOR]