Hiroyoshi Naito

Water Droplets Behavior in Extinguishing the Methane-Air Counterflow Diffusion Flame

The behavior of fine water droplets was investigated in a methane-air counterflow diffusion flame, specifically when the water droplets were added to the inlet air feed and used to extinguish the diffusion flame. The water droplets studied had a relatively broad size distribution (from 1 to 60 m), with the number mean diameter of 15 m and Sauter mean diameter of 25 m. With an increase in the velocity gradient, the flame approaches towards the stagnation plane and flame thickness decreases. The flame properties, such as flame location and flame thickness are insensitive to the water mass loading. Along the stagnation stream line, the velocity decreases towards a local minimum just before the flame front. In contrast, the velocity increases in the flame zone due to the thermal expansion, and then decreases towards the stagnation point. In the decelerating zone, the droplet mass flux decreases on approach to the flame zone due to a non-equilibrium in velocity of large droplets between liquid and gas phases. Furthermore, as the droplet-laden air approaches the flame front, the flow stream begins to diverge in the counterflow field. Since the small droplets move away from the burner axis, the divergence of the air flow acts to reduce the water droplet mass flux along the stagnation streamline. Large droplets moving faster than the air flow „catch up‟ to slower ones just before the flame front, and the mass flux of the droplets tends to increase, resulting in the droplets accumulating just before the flame zone. Measurements of the velocities of individual droplets show that large droplets move faster than the small droplets which are in equilibrium (in velocity) between liquid and gas phases. Droplet behavior was found to be controlled by the Stokes number. Equilibrium in velocity is re-established just in front of the flame zone. Closer to the flame front, evaporation occurs and the mass flux of the droplets decreases drastically. On the other hand, thermophoresis acts on extremely small droplets to move them along the steep temperature gradient and these droplets are forced from the high to low temperature regions against the convection velocity. The thermophoretic velocity was estimated in the present study to be compared with the measured convection velocity and it was concluded that the thermophoretic effect is negligibly small over all sizes of droplets.

Inhibition of Propane/Air Premixed Flame by Water Mist

The effects of fine water mist on flame temperature and laminar flame speed of propane-air mixtures were investigated both experimentally and numerically. In experiments, flame temperature and laminar flame speeds were measured using a single jet-plate configuration for the cases with and without water mist. The numerical simulation was also performed using the OPPDIF code in CHEMKIN package. To include the phase change with evaporation, the evaporation process was assumed as a chemical reaction of which rate constant follows the Arrhenius law. For the case without water mist, experiments showed that the flame speeds increase with the stretch rate toward the limit of extinguishment. This tendency was fairly reproducible by the numerical simulation with Davis-Law-Wang kinetic mechanism. The flame temperature was measured by a fine wire thermocouple. The flame temperature decreases with the water mist addition toward the limit of extinguishment within the range of φ from 0.8 to 1.2. The reduction of the flame temperature is more enhanced for lean and rich mixtures than the stoichiometric one. On the other hand, in terms of inhibition of the flame speed, the water mist is more effective for lean mixture than rich one. Reduction of flame temperature in rich mixture does not contribute to decrease the flame speed. Additionally, the stretch rate also decreases the flame temperature. Therefore, for lean mixtures, an appropriate combination of water mist and stretch rate can enhance the suppression effectiveness of water mist.

Flame Structure and Emission Characteristics of a Jet Stirred Reactor

A jet stirred reactor was designed for the study of high-intensity combustion using highly preheated mixture of methane and air. NOx emission characteristics were measured experimentally. These measurements revealed that the high intensity and lean combustion with extremely low NOx emission could be achieved. Schlieren photographs were taken to visualize the flame structure. Also the mean temperature, species concentration and ionization measurements were made. Schlieren photograph and mean temperature showed the uniformity of the temperature field, whereas species concentration and ion current showed that the main reaction zone is localized at the interface of high speed reacting mixture jet and recirculating combustion gas. However, the thickness of the mean reaction zone is thicker than that of the laminar flamelets. The assumption of the thin laminar flamelets is no more valid and the volumetric combustion must be achieved.

Effect of water mist on temperature and burning velocity of stretched propane-air premixed flames

Effects of fine water mist on the flame temperature and laminar burning velocity of propane/air premixed flame were investigated by using a single jet-plate configuration. For the case without water mist, the measured laminar burning velocities are in reasonably good agreement with previously reported data and numerical simulation. The dependence of the burning velocity on the stretch rate for the case without water mist is positive for all mixtures tested, resulting in the negative Markstein length, which coincides with previous experimental and theoretical studies. With water mist addition, the laminar burning velocity is lowered significantly and its dependence on the stretch rate is changed to negative. This leads to apparently positive Markstein lengths. Concurrently, the flame temperature decreases linearly with the water mist mass loading. Also, it decreases with the stretch rate, even if the water mist mass loading is kept constant. The positive Markstein length was discussed on the basis of the mist droplets behavior in the stagnation flow field. Even if the mist mass loading is kept constant, the mist droplets do not follow the diverting flow field when the stretch rate is high and the droplets accumulation occurs in the stagnation region where the burning velocities were measured. This fact results in the lower burning velocity and lower flame temperature as compared to those measured for uniformly dispersed water mist.