Nozomu Hashimoto

Opposed-flow flame spread in a circular duct of a solid fuel: Influence of channel height on spread rate

The influence of channel height on flame spread in a circular duct of the solid fuel in an opposed-flow configuration was examined. Polymethylmethacrylate cylinders with a circular duct (diameter of 1, 2, or 3 mm) were used as fuel specimens, and both flame-spreading and stabilized combustion were observed. In the case of stabilized combustion, the flame cannot spread into the duct because of the high oxygen velocity. The flame-traveling velocity is the velocity at which the flame widens the duct by fuel consumption. Therefore, the flame-traveling velocity in stabilized combustion is significantly low compared with flame-spreading combustion. In the case of flame-spreading combustion, the equivalence velocity, which contains channel height information, defines whether the regime is the thermal or the chemical regime. When the equivalent velocity is higher than a certain value, the flame-spread rate is controlled by chemical effects. On the whole, the flame-spread rate decreases with the decrease of channel height in the case of flame-spreading combustion because of the curvature effect. Owing to the curvature effect, the area ratio of the flame to that of the solid surface decreases with decreasing channel height, and this is conspicuous when the channel height is low. The curvature effect is negligible when the channel height is sufficiently large compared with the flame stand-off distance.

Can a spreading flame over electric wire insulation in concurrent flow achieve steady propagation in microgravity

Abstract Concurrent flame spread over electric wire insulation was studied experimentally in microgravity conditions during parabolic flights. Polyethylene insulated Nickel-Chrome wires and Copper wires were examined for external flow velocities ranging from 50 mm/s to 200 mm/s. The experimental results showed that steady state flame spread over wire insulation in microgravity could be achieved, even for concurrent flow. A theoretical analysis on the balance of heat supply from the flame to the unburned region, radiation heat loss from the surface to the ambient and required energy to sustain the flame propagation was carried out to explain the presence of steady spread over insulated wire under concurrent flow. Based on the theory, the change in heat input (defined by the balance between heat supply from flame and radiation heat loss) was drawn as a function of the flame spread rate. The curve intersected the linear line of the required energy to sustain the flame. This balance point evidences the existence of steady propagation in concurrent flow. Moreover, the estimated steady spread rate (1.2 mm/s) was consistent with the experimental result by considering the ratio of the actual flame length to the theoretical to be 0.5. Further experimental results showed that the concurrent flame spread rate increased with the external flow velocity. In addition, the steady spread rate was found to be faster for Copper wires than for Nickel-Chrome wires. The experimental results for upward spreading (concurrent spreading) in normal gravity were compared with the microgravity results. In normal gravity, the flame did not reach a steady state within the investigated parameter range. This is due to the fact that the fairly large flame spread rate prevented the aforementioned heat balance to be reached, which meant that such a spread rate could not be attained within the length of the tested sample.

Large-eddy simulation of pulverized coal combustion in swirling flow

Large-eddy simulation (LES) is applied to both a pulverized coal combustion and non-combustion field in a combustion test furnace with a practical advanced low NOx burner called CI-α burner, and investigated the effect to coal combustion on flow field and ignition mechanism. The results show that predicted flow field and coal particle behavior are in general agreement with the experiment. Coal combustion strongly affect flow filed. Primary air jet and swirling flow is enhanced by the burned gas expansion. Moreover, a recirculation flow formed by strong swirling flow is observed near the burner region and keeps stable coal combustion by transporting hot gas and increasing coal particle residence time.

Effects of Fuel Species on Soot Formation in Laminar Counterflow: Comparison Between Diesel Fuel and Palm Methyl Ester as an Alternative Fuel

Spray combustion of liquid fuel is utilized in many combustion systems. There are, however, still remained-problems. One of the most important problems is to clarify the combustion characteristics, especially soot formation process. At the same time, the liquid fuel of biomass is getting a lot of attention as an alternative fuel in recent years from the viewpoint of the environmental issues and the exhaustion of fossil fuels. In this study, we focused on the palm methyl ester (PME), which has a large production capacity and oxygen content in its molecular structure. The aim of this study is to clarify the combustion characteristics and soot formation characteristics of PME spray flame for the effective utilization to the conventional combustion systems as the alternative fuel. In order to clarify the soot formation characteristics of PME spray flame, measurements of Sauter mean diameter (SMD) and droplet size distribution by using phase Doppler anemometry (PDA) and measurement of two dimensional soot formation characteristics by using Laser Induced Incandescence technique (LII) are conducted in laminar counterflow field. In addition, since the PME has a normal chain structure and oxygen content in its molecular structure, it needs to clarify effect of the oxygen content and normal chain structure. The comparison with diesel fuel and n-dodecane are also conducted. Results of LII measurements show that the PME and the n-dodecane spray have similar spray flame structures, time-averaged soot volume fraction and instantaneous structure of soot formation while PME has the oxygen content in the molecular structure. On the other hand, the time-averaged soot volume fraction and soot formation area of PME are smaller than those of diesel fuel. It is because the diesel fuel has some components with the aromatic ring. These results indicate that it is not the oxygen content but the normal chain structure in the PME play an essential role in influencing soot formation characteristics.Copyright