Numerical simulations of combustion fields are indispensable recently not only for the understanding of combustion physics but also for the efficient development and optimal design of combustion devices. Because of the lack of computing power and the immature state of turbulent combustion model, however, the accuracy of the present numerical simulations of combustion fields are insufficient. It is strongly expected that the K computer, which has taken first place on the 38th TOP500 list announced in November 2011 for a consecutive two terms in a row, will be great use to researches in the combustion community. In the "Researches and Developments of Design Systems for Next Generation Combustion and Gasification Devices" project, we aim to physically understand turbulence, particle/droplet dispersion, combustion and gasification, and to develop numerical design systems for next generation combustion and gasification devices by use of the K computer. Particular emphasis is placed on large-eddy simulation (LES) and direct numerical simulation (DNS) of multiphase combustion and gasification such as spray combustion, pulverized coal combustion and coal gasification.
Using the detailed numerical simulation approach, the physical mechanisms of spray primary atomization and early group combustion formation are investigated. The grid resolution is made fine enough to capture the final droplet generation by surface tension. In the cold flow simulation of liquid jet primary atomization, the injected liquid is subject to the interaction with the gas phase and atomization occurs both from the jet head umbrella edge and from the liquid core surface. Ligaments are first created, and droplets are finally generated from these ligaments. The interface instability development mechanism and droplet generation mechanism can be physically explained in detail by the simulation results. In the evaporating/reacting simulation, the early stage fuel vapor clustering is captured. The interaction between droplets determines the clustering formation process. In the jet head recirculation region where the droplet number density is relatively high, group combustion is expected to occur when ignition finally starts.
Improvements in ab initio calculations based on quantum chemistry are very important in the theoretical modeling for the combustion reactions. Highly accurate and widely adaptive Arrhenius parameters of the elementally reactions are needed to construct “universal” chemical kinetic models. The accuracy of theoretically calculated Arrhenius parameters strongly depends on the accuracy of potential energy surfaces (PES) on the reaction system. Recent developments in the molecular electronic structure theory based on the wave function theory and density functional theory are summarized in this article, and the present state of massively parallelized calculations for large scale molecules is also reported. In addition, some examples of calculations which need to improve or replace the theoretical methods to obtain more accurate results are shown.
With the increase of resources of supercomputer system, handing and visualization of huge DNS data of turbulent combustion become more important. In this paper, current state in the data handing is discussed based on the state of art DNS. Even though the speed of internet is improved, data transfer time of DNS data from the remote supercomputer is still enormous, which prevents active researches. The effective utilization of scientific visualization is also prevented by the data transfer time. To resolve this issue, cloud-type data analysis center will be required to use expensive huge DNS database effectively. As for the visualization of DNS data, several defects in visualization without mature consideration are described especially for contour surfaces and volume rendering. The inappropriate applications of these techniques result in misreading of physics. The developments in acquisition system and laser cause similar problems in experimental researches. The practical realizations of 3D technique and virtual reality may contribute further developments in turbulent combustion researches though the scientific visualization.
Slight amount of hydrogen and oxygen exist in the main steam pipelines of boiling water reactor. These gases are generated by the radiolytic decomposition of the water. The detonation of these gases lead to pipe rupture problems at Hamaoka-1 and Brunsbuttel in 2001. To prevent similar problems, gas accumulation experiments were conducted to investigate accumulation mechanisms of these non-condensable gases. As a result of accumulation experiments, it was shown that non-condensable gas (hydrogen and oxygen) remains in closed pipe due to condensation of steam which caused by cooling effect of a surrounding cold gas. Concentration of accumulated gas is affected by buoyancy force effect to non-condensable gas and gravity effect on condensed water. Therefore, non-condensable gas accumulates easily in vertical pipes, in which non-condensable gas accumulates from the top of the pipes and condensed water is easily vented. Contrary, in case of horizontal pipes, gravity effect to non-condensable gas is small compared to vertical pipes, so non-condensable gas is not likely to accumulate in the pipes. To estimate the failure potential of detonation waves caused by accumulated gases, detonation experiments were conducted with straight tube and 90-degree bend tube. In the straight tube tests, the highest detonation pressure was recorded at the closed end. In the 90-degree bend experiments, pressure time histories revealed pressure loads greater than the straight tube portion at two locations. One is a high pressure peak at the extrados of the bend and the other is a double pressure peak just downstream of the bend outlet. Comparisons of experimental results show that impulse at the closed end of straight tube is greater than two high pressure location of 90-degree bend.
Catalytic reactions have been investigated to remove CH4 efficiently from the off-gas containing moisture of relatively high concentration. While the high temperature above 600 degree C was needed to remove CH4 by 90% conversion by means of catalytic combustion method using precious metal catalysts, the O2 control method could be applied to remove CH4 with high conversion above 90% at 450 degree C under the high moisture condition.The elemental catalytic reactions for the CH4 conversion were studied under the O2-H2O co-existing condition in comparison with the reactions under the H2O non-existing condition.The oxidation reaction of CH4 was strongly inhibited below 350 degree C by the presence of H2O. On the other hand, the steam reforming reaction was occurred so that the CH4 reacted with H2O more preferentially than O2 above 350 degree C. And the CH4 removal was promoted by the oxidation of H2 which resulted of the steam reforming reaction. Oxygen inhibited the steam reforming reaction when the O2 concentration amounted to or exceeded the stoichiometry under the O2-H2O co-existing condition.The cause was considered that oxygen was strongly adsorbed on the catalyst surface when the H2O co-existed.
Flame spread route in fire strongly depends on distribution of combustible areas. Two types of scenario are considered in flame spread when combustible areas randomly distributed; one case is that flame spreads and combustible areas burn out, and the other case is that flame self-extinguishes on the way. The threshold of burning out or self-extinguishing may be determined by quantity of combustible areas and their placement in space. Our objectives are to clarify the characteristics and threshold of flame spread based on the percolation theory. In this paper, we examine two-way flame spread in open air along a thin combustible solid with randomly distributed square pores, which are considered as noncombustible space. Experimental results show that the flame can not spread any way and completely self-extinguish over 65% of porosity. This threshold is higher than that of 60% for one-way flame spread which was previously examined. However, instead of porosity, the threshold of flame spread is decided by the number of slits, which is made by connecting pores each other. The slit for two-way flame spread needs to extend over two-way direction. In addition, we obtained the ratio of unburned area (unburned area / total combustible area) by means of counting the unburned area after flame spread test, which might be useful to predict urban hazards by fire. We found that the ratio of unburned area is linearly decreasing to the flame-spread probability.
Oxygen-enriched combustion has the potential to save energy, hence, reduce carbon dioxide emissions. In this study, an inherently safe technique of rapidly mixed type tubular flame combustion has been experimentally examined to extend it to higher oxygen concentrations. Results show that when the oxygen concentration of the oxidizer is less than about 40%, stable tubular flame combustion can be obtained. Above 40%, however, oscillatory combustion occurs. With an increase of the oxygen concentration, the oscillatory combustion becomes stronger and its lower limit in the overall equivalence ratio becomes smaller. Further decrease in the equivalence ratio, however, results in stable tubular flame combustion and subsequent extinction. For pure oxygen, stable tubular flame combustion can be obtained in the range of the equivalence ratio from 0.11 to 0.18. With an increase of oxygen concentration the equivalence ratio at extinction decreases gradually, while the fuel concentration at extinction takes an almost constant value of 4.7%. The process leading to the oscillatory combustion is different from that in the conventional premixed type tubular flame combustion; under higher oxygen concentrations, diffusion flames are anchored at the slit exits, which inhibit mixing of fuel and oxidizer, resulting in an intense combustion downstream in a strong swirling flow.