Hiroyasu Saitoh

Observation of combustion characteristics of droplet clusters in a premixed-spray flame by simultaneous monitoring of planar spray images and local chemiluminescence

In order to better understand the combustion behavior of spray flames, simultaneous measurments of droplet cluster visualization using laser tomography and local OH chemiluminescence and CH-band emission using a newly develped optical probe system named the Multi-color Integrated Cassegrain Receiving Optics (MICRO) are applied to a premixed-spray flame. Time-series planar images of droplet clusters and their transient structures during combustion are examined using an Ar-ion laser and a high-speed digital CCD camera. By observing the droplet clusters and local chemiluminescence simultaneously in the premixed-spray flame, it is confirmed that some portions of the spray stream disappear very rapidly due to preferential flame propagation, while other portions of the spray stream survive over a long period to form droplet clusters, disappearing gradually from their outermost portions, which seems similar to a diffusion flame. The disappearance speed of individual droplet clusters in the premixed-spray flame, instead of a conventional evaporation rate of a single droplet, is defined and calculated by processing the obtained droplet-cluster planar images. The disappearance speed for rapid preferential flame propagation through easy-to-burn regions in the upstream region of the flame is about 2.5 m/s. On the other hand, the disappearance speed when droplet clusters burn dominated by a diffusion combustion mode in the downstream region of the flame is approximately 0.45 m/s.

Measurement of instantaneous 2-D velocity field and local chemiluminescence in a premixed-spray flame by PIV and MICRO system

In this article, we demonstrate combined measurements of particle image velocimetry (PIV) and Multi-colour Integrated Cassegrain Receiving Optics (MICRO) in an attempt to observe characteristics of propagating flame in a premixed-spray stream. High-speed images recorded with an intensified CCD camera and cross-correlation PIV method showed the capability in obtaining instantaneous velocity fields in sooty spray flames, where liquid fuel of kerosene was supplied in the form of premixed spray. It enabled us to discuss the influence of fluid turbulent motion on the process of preferential flame propagation. Local chemiluminescence in flames detected by MICRO system was conditionally processed in terms of the distance from the spray boundary that was determined from visualized spray images. The time-averaged one-dimensional structure obtained statistically in the direction of flame propagation showed that two distinct reaction peaks appeared on both sides of the spray boundary, which corresponded to the main vaporization region of a spray.

Spray measurements and numerical analysis on an impinging jet injector

The spray characteristics of an impinging jet injector have been investigated experimentally and numerically. The droplet size distribution, the velocity and the mass flux distribution were measured using Phase-Doppler Anemometry (PDA), and approximated to distribution functions in order to obtain the input data for the numerical simulation by KIVA-3 V. The simulation indicated that the effects of turbulent dispersion on the behavior of droplets must be included in the numerical model. The local droplet size distribution is dominated by the turbulence intensity, because small droplets tend to be transported by the drag of surrounding turbulent gas flow, while the trajectories of large ones are almost straight. Computational grid sizes are also important factor because the turbulence intensity strongly depends on the radial width. Simulations with coarse grids resulted in poor predictions, in which the turbulence intensity was overestimated. It is important to choose proper grid sizes with considering the balance of the accuracy and the computational cost.

高速画像撮影および局所火炎自発光の同時計測によるよどみ流中予混合噴霧火炎構造の検討 : 気相酸素濃度が火炎挙動および構造に及ぼす影響(熱工学,内燃機関,動力など)

We conducted an experimental observation of polydisperse premixed-spray flames of kerosene in a stagnation flow. For monitoring behaviors of fuel droplet and flame we adopted laser sheet visualization consisting of a high-speed digital CCD camera and Ar-ion laser, and OH-radical and CH-band emissions detected from local region in the vicinity of propagating flame by using a light-collecting probe. Three flames with different oxygen concentration in gaseous phase (21%, 19% and 17%) were examined. These flames had the same initial droplet size-distribution measured by Phase Doppler Anemometry at the burner exit, and all the other experimental conditions were kept same except oxygen concentration. In the case of oxygen concentration of 21%, various shape of spray boundary was formed depending on time, and combustion reaction zone overlapped considerably with spray area. On the other hand, in the case of 17%, the flame showed relatively flat-shaped spray boundary. Furthermore, we compared these flame behaviors and structures with those of premixed-spray flames we had observed before.