Masashi Katsuki

The science and technology of combustion in highly preheated air

Recent advances in heat-recirculating combustion in industrial furnaces, particularly of the alternating flow type, are reviewed. A large amount of waste heat can be recovered by this type of system. Highly preheated combustion air, typically above 1300 K, is easily obtained due to advanced design and metarials employed. Although preheated air combustion generally produces high nitric oxide emissions, it has been used to generate high-temperature flames for some special applications. The energy saving achieved simultaneously by heat recirculation has become more attractive, from an ecological point of view. However, to enjoy the energy saving brought by a high rate of heat recirculation by applying highly preheated air combustion to generic industrial furnances, a reduction of nitric oxide emission is required. The possibility of low nitric oxide emission from highly preheated air combustion is intensively discussed. Dilution of the air with burned gases and combustion occurring in air with low oxygen concentration are shown to be indispensable factors in realizing low nitric oxide emissions. This has led to advanced furnance technology.

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.

Structure of Spark-Ignited Spherical Flames Propagating in a Droplet Cloud

A droplet cloud of liquid fuel produced by an ultrasonic atomizer was ignited by a spark, and the flame ball propagating outward was observed in order to elucidate the mechanism of flame propagation and complicated group combustion behaviors of spray flames. For that purpose, the instantaneous images of droplet clusters, OH-radical chemiluminescence and C2-band flame luminosity were taken simultaneously. Furthermore, the light emission signals in OH- and CH-bands, Mie-scattering signal from droplets, and the size and velocity of droplets were monitored simultaneously in time series. It was found that a nonluminous flame propagated ahead of a luminous flame, and that droplets disappeared in the luminous flame zone due to rapid evaporation, where a number of small-scaled droplet clusters were burning in diffusion combustion mode associated with solid-body emissions.

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.

Suppression of Combustion Oscillation of Premixed Flames by Active Control

Combustion-induced oscillations in a duct were observed by changing the duct length and the mixture equivalence ratio. The wall pressure fluctuations in the combustion duct and the OH emission intensity of the flame were well-correlated with the free field sound pressure. A closed-loop active control using the above signals and a Open-loop active control were studied to suppress combustion oscillations. Based on the results of the experiments, the possibility of suppression of combustion oscillation by a forced oscillation in the upstream feed tube of the facility was suggested.

A Unified Model of Mean Reaction Rate in Turbulent Premixed Flames

We developed a model of the mean reaction rate in a turbulent premixed flame to predict time-averaged profiles of the velocity, temperature and species concentrations in practical combustion systems. The fundamental features of the model consists of chemistry- and mixing-controlled reaction rates which vary with the local Damkohler number. The comparison of numerical predictions with experiments demonstrated the applicability of the concept to the flames of wide range of Damkohler number.