Yasushige Ujiie

Experimental study on high-pressure droplet evaporation using microgravity conditions

Evaporation of an individual fuel droplet at high pressures and high temperatures has been studied experimentally under microgravity conditions. A suspended n -heptane droplet was used in the experiments at pressures in the range of 0.1–5.0 MPa and temperatures varying from 400 to 800 K. Temporal variations of the droplet diameter were measured with a computer-aided image analysis system. Microgravity conditions, which were produced by using 5-m and 110-m drop towers and parabolic flights, were employed to prevent natural convection that complicates the phenomena. It was observed that dense fuel vapor surrounded a droplet and the droplet surface became obscure at high pressures and high temperatures. The slope of the temporal variations of the squared droplet diameter initially increases but later becomes approximately constant at ambient pressures below the critical pressure of the fuel. At a pressure of 5.0 MPa and temperatures below the critical temperature, the slope becomes less in the latter half of the evaporation lifetime. The ratio of the initial heat-up time to the evaporation lifetime was used as a measure of unsteadiness of droplet evaporation. The ratio is almost independent of ambient temperature at an ambient pressure of 0.1 MPa, but, as ambient pressure is increased, its tendency to rise with ambient temperature becomes noticeable. Corrected evaporation lifetime t c decreases monotonically as ambient temperature is increased. The slope of its curve becomes steeper as ambient pressure increases. Dependence of t c on ambient pressure changes according to ambient temperature. Above 550 K, t c decreases as ambient pressure is increased. Below 450 K, t c tends to increase as ambient pressure is increased. It is suggested that there exists a certain ambient temperature at which ambient pressure has little effect on t c .

Microgravity experiments of flame propagation in ethanol droplet-vapor-air mixture

A basic study of spray combustion has been made with a rapid expansion apparatus that can produce monodispersed fuel droplet clouds under microgravity conditions. The effects of droplets on flame propagation were investigated for ethanol droplet-vapor-air mixtures. The pressure of the fuel droplet-vapor-air mixtures was set at 0.2 MPa for all experiments. The total equivalence ratio varied in the range of 0.6–1.6. The ratio of the liquid fuel mass to the total fuel mass varied from 0% to 60%, and the mean droplet diameter ranged from 7 to 45 μm. It was found that the flame speed of fuel droplet-vapor-air mixtures exceeded that of premixed gases of the same total equivalence ratio in two regions of the total equivalence ratio. One region exists on the fuel-lean side, the other exists on the fuel-rich side. For mixtures of 0.3 in the liquid equivalence ratio, the increase in the mean droplet diameter from 7.5 to 11 μm caused an increase in the flame speed in the region where the flame speed increases with the increase in the total equivalence ratio. In contrast, in the region where the flame speed decreases with the increase in the total equivalence ratio, an increase in the mean droplet diameter caused the flame speed to decrease. For mixtures with a mean droplet diameter of 11 μm, an increase in the liquid equivalence ratio from 0.2 to 0.3 caused an increase in the flame speed in the region where the flame speed decreases with the increase in the total equivalence ratio. For mixtures with a total equivalence ratio of 0.8 the flame speed reached its maximum value when the mean droplet diameter was about 11 μm and the liquid equivalence ratio was about 0.2.

Effects of energy deposition schedule on minimum ignition energy in spark ignition of methane/air mixtures

A two-dimensional numerical analysis with elementary reactions including ion-molecule reactions has been performed to investigate the effect of energy deposition schedule on the minimum ignition energy in the composite spark ignition of methane/air mixtures. The chemical reaction scheme for a methane/air mixture, which has 27 species including 5 ion molecules and electrons and 81 elementary reactions with ion-molecule reactions, is employed in the calculation. Experiments are also carried out, and a qualitative comparison between experiment and calculation is made. The calculated result showed that the optimum ratio of capacity spark energy, for which the minimum ignition energy has a lowest value, exists. The increase in the capacity spark energy leads to a decrease in the formation of radicals during the spark discharge, while it also leads to an active reaction after spark discharge, which is due to the enhanced supply of the unburned mixture into the flame kernel by the inward flow along electrodes. The qualitative trend of calculated and experimental results shows good agreement for the relationship between the ratio of capacity spark energy and the minimum ignition energy. The minimum ignition energy obtained from the calculation with ion-molecule reactions is slightly larger than that without ion-molecule reactions. This may be due to the consumption of energy for ionization.

Control of Flame-Holding in Supersonic Airflow by Secondary Air Injection

Active control tests were performed to improve e ame-holding in a e xed-geometry rectangular scramjet combustor at an off-design point and to improve self-ignition in the combustor. With a e xed-geometry combustor, the pressure oscillation originated from unstable e ame-holding, and/or blowoff were measured at an off-design point. Eight air injectors were installed in the combustor wall, and secondary air was injected into the boundary layer, which developed downstream of a step that controlled the effective cross-sectional area of the combustor. Signals from pressure transducers on the combustor wall were used as feedback signals for the control. This control system was simple to install in a e xed-geometry combustor. The control system was effective in improving combustion oscillation and/or blowoff at an off-design point and was responsive to a rapid condition change. It was also valid for improving the selfignition limit.

An experimental study on flame propagation in lean fuel droplet-vapor-air mixtures by using microgravity conditions

A basic study of spray combustion has been made with a rapid expansion apparatus that can produce monodispersed fuel droplet clouds. The rapid expansion apparatus, which is essentially Wilsons cloud chamber, generates droplet clouds by reducing the pressure of saturated fuel vapor air mixtures. For droplet clouds more than 15 μm in mean droplet diameter, experiments were performed under microgravity conditions to prevent droplets from falling down by gravity. The mean diameter of droplet clouds was obtained from the laser lights scattered by droplets. Ethanol was used as a fuel. Pressure and total equivalence ratio were set 0.2 MPa and 0.8. The mean droplet diameter d m and the ratio of the liquid fuel mass to the total fuel mass were varied in the range of 8–24 μm and 0%–45%, respectively. Almost monodispersed and mono-sized droplet clouds were generated and ignited successfully under normal and microgravity conditions. Two types of flame propagation were observed. For the mixtures less than about 20 μm in mean droplet diameter, flames propagate in a smooth spherical shape like premixed gas flames. For the mixtures more than about 20 μm in mean droplet diameter, flames propagate in a rough spherical shape after the flames grow up to about 20 mm in diameter. The flame speed of the fuel droplet-vapor-air mixtures decreases with the increase in the mean droplet diameter of the mixtures. In the mixtures of d m

Effect of Fuel Vapor Concentration of an Ambient Gas on Flame Spread along a Fuel Droplet Array Combustion and Effects of Fuel Droplet Array on Gas-Phase Flame Propagation

Combustion experiments of fuel droplet array in a fuel vapor-air mixture were performed at normal and microgravities to investigate growth mechanism of group combustion of fuel droplets. The purpose of this research is to examine the effect of fuel vapor concentration of an ambient gas on the flame spread and the effect of fuel droplet array on gas-phase flame propagation. The initial droplet diameter was 0.32, 0.8 or 1.0 mm, and the droplet spacing was varied from 1.6 to 15 mm. Behavior of flame spread along a fuel droplet array and gas-phase flame propagation were observed and flame speed was measured for various droplet spacing and fuel vapor concentrations of an ambient gas. As a low-volatile fuel, normal decane was used. As a high-volatile fuel, ethanol was employed. In the case of the gas equivalence rations below the flammability limit, the flame spread speed increases with the increase in the gas equivalence ratio. The flame spread speed of ethanol droplet array was about two times as large as that of decane droplet array when the droplet spacing was 1.6 mm. A decane droplet array did not influence the gas-phase flame propagation. On the other hand, an ethanol droplet array disturbed a flame propagation in the gas phase, and increased slightly flame propagation speed at the gas equivalence rations near the lower flammability limit.

Fundamental study on effects of residual fuel droplets on flame propagation in SI engines

Experimental studies on flame propagation in fuel droplet-vapor-air mixtures were performed with a constant-volume chamber. Monodispersed and mono-sized fuel droplet clouds were used for experiments as a simple model of residual fuel droplets in a gasoline SI engine. The results show that, on the fuel-lean side, the flame speed and the maximum burning pressure of the fine droplet-vapor-air mixtures exceed those of the completely gaseous premixtures with the same total equivalence ratio. It was found that the evaporation rate of fuel droplets and the slip velocity between fuel droplets and unburned gas play an important role in determining the flame speed.