Nobuki Nagai

Spray Characteristics of Liquid Jet Traversing Subsonic Airstreams

The spray characteristics of a liquid jet traversing subsonic airstreams were experimentally investigated. The disintegration phenomena of liquid jets were observed by instantaneous photographs and high-speed video movies. The waves on the liquid jet surface are three dimensional and show very complicated behaviors. These waves are an important cause of the liquid jet disintegration. Droplet mass e uxes were measured using an isokinetic sampling probe. Empirical equations of their distributions that are expressed by the standard normal function were deduced. Droplet sizes and droplet velocities were measured by a phase Doppler particle analyzer. At low air velocity, the mean droplet diameter reaches its maximum in the peripheral mixing region. At high air velocity, however, the mean droplet diameter reaches its maximum in the core region. The droplet velocity peaks in the peripheral mixing region over the whole range of the air velocity.

Liquid Jet Disintegration and Spray Characteristics in Subsonic Air Streams.

The present study focuses on the problem of a liquid jet injected transversely into a subsonic air stream. Droplet sizes and droplet velocities were measured, and empirical equations of mean droplet size and mean droplet velocity profile were deduced. On the basis of the observation, the spray formation mechanism is classified into two mechanisms : one due to the instability of the liquid jet surface and the other due to the instability of the liquid jet itself. The maximum size of the mean droplet size profile in the liquid injection direction decreases with increasing air velocity and decreasing momentum ratio of the liquid and the air. The location in the liquid injection direction where the mean droplet velocity becomes minimum is coincident with the location where the mean droplet size becomes maximum. There is a strong correlation between the droplet size and droplet velocity, and larger particles have smaller velocities.

Mass Flux Distribution of Spray Formed by Liquid Jet Disintegration in Subsonic Air Streams

The mass flux distribution of spray formed by liquid jet disintegration in subsonic air streams was measured experimentally, and the empirical equation of the mass flux distribution was deduced. The configuration of the upper profile of the spray mass flux in the liquid injection direction above the peak of the profile is different with that of the lower profile. With increasing the momentum ratio of the liquid and the air, the location in the liquid injection direction where the mass flux becomes maximum is elevated, and the maximum mass flux decreases. With increasing momentum ratio, the number of spray droplets formed at the liquid jet surface before the liquid jet breakup point increases, and contour lines of the spray mass flux show a kidney-shaped configuration near the liquid jet breakup point. Downstream of the liquid jet breakup point the maximum mass flux decreases, and the mass flux profile becomes flatter.

Measurement of Spray Characteristics by Direct Photography Method

Double flash photographs of liquid atomization process by color CCD camera was used for velocity measurement. The intensities of blue image and green image are reversed, and their cross correlation coefficients are calculated by using fast Fourier transfer. Then, the displacement of those images is obtained, and 2-dimensional velocity is measured. Velocities of droplets or liquid sheets with complex shape were measured well. But the peaks of cross correlation coefficients for liquid sheets image are not so clear as those for droplets image. Velocities of liquid jet surface wave could not be measured well, but enforcing the boundaries between liquid and air using differential processing enables the velocity masurement.

Combustion processes of a suspended single droplet of coal water mixture.

Combustion behavior of a suspended CWM droplet on a silica fiber was observed in the combustion chamber and compared with that of dried CWM droplets and coal particles in order to elucidate the combustion features of actual CWM fuel. The combustion phenomena in the present experiment can be classified into three types. For small droplets at low ambient temperature, gas flames made by the emission of volatile matters are not observed preceding char combustion. With increasing ambient temperature, not only a large droplet but also small one begins to burn with gas flames, and bursting of some larger droplets occurs at high temperature when volatile matter is burning. The combustion processes of CWM droplets are almost similar to those of coal particles, but there are some difference between them. The ignition delay time of CWM is much longer than that of coal because of water evaporation. The quantity of volatile matter and char burning rate constants of CWM are smaller than those of coal at high ambient temperature.

Disintegration of Liquid Droplets of High Concentration Coal-Water Slurry by Air Flow

Experiments on the disintegration of coal-water slurry (CWM) drops by parallel air flow have been carried out in order to clarify the disintegrating mechanism of CWM using a rectangular wind tunnel. Experiments were conducted with varying kinds of liquid and coal concentrations of CWM. From the experiments the following results were obtained. Liquid atomization processes are classified into three types of disintegration, namely, separation-wise, ligament-wise and dropwise disintegration. Generated CWM spray is composed of irregular and spherical particles. Irregular particles increase as air velocity increases and they are mostly generated by separation-wise and ligament-wise disintegration. On the other hand spherical particles are mostly generated by dropwise disintegration. Finally, a model of the disintegrating mechanism, which can well explain the formation mechanism of these particles, was proposed.

A Study on the Twin-Fluid Atomization of a Highly Concentrated Coal-Water Mixture : Heat Transfer, Power, Combustion, Thermophysical Properties

A study has been carried out for the purpose of clarifying spray performances of highly concentrated coal-water mixtures (CWM) and obtaining a guide for the design of practical twin-fluid atomizers. Disintegration mechanisms and spray characteristics were investigated on the simple twin-fluid atomization of CWM, and compared with those of single-phase liquids. Furthermore, these disintegration model. In the disintegration of a CWM jet by high-speed air, atomization phenomena are similar to those of low-viscosity liquids. Spray is generated basically by the tearing-off of irregularly shaped small drops from the jet surface and by the splitting-up of the unstably fluctuating jet into spherical drops. With the increase of air velocity, Sauters mean diameter of spray decreases steeply, but its manner depends on the disintegration mechanism.