Shigeyasu Nakanishi

The Modes of Flow Oscillation in Multi-Channel Two-Phase Flow Systems

In this paper, some results of the experimental investigations on oscillation modes of multi-channel two-phase flow systems are reported. In the first part, phase relation between oscillating channels is studied using a Freon-113 test rig; the number of parallel channels is varied up to five. It is found that, if the thermohydraulic characteristics of all channels are identical, they oscillate with the same amplitude and equally out of phase with each other, and that if the characteristics are not identical various modes become possible. In the second part, interaction between oscillating channels is studied in detail at a two-channel air-water test rig. In this rig, it is indispensable to equip every air and water supply lines with compressible capacities in order to realize flow oscillation, and only the oscillations originated from Ledinegg instability, i.e. some kinds of pressure drop oscillation, can be observed. Effects of the compressible capacities and the hydraulic states of the channels on the modes of oscillation are examined. Other than the types of oscillation observed in single channel systems, one inherent to multi-channel systems is observed which is named as modulation wave one. It is clear that this stems from channel interaction.

An approximation method for construction of a stability map of density-wave oscillations

Abstract The D-partition method for the density-wave oscillation in evaporating tubes is elaborated to investigate parametric effects. It is established that the neutral curve is well approximated by its first asymptote in the inlet gag versus exit gag plane and that the oscillation frequency is also approximated well by the one which corresponds to the first asymptote. To illustrate our method, an evaporating tube with uniform heat input is analyzed assuming the homogeneous flow model. A new simple formula is obtained for the oscillation frequency, which shows that the length of the superheating zone has no effect. The frequency given by this formula can be used to calculate an approximation of the neutral curve. The predicted frequencies agree well with the data from a high pressure test rig, though those of the threshold heating power are a little more optimistic. Finally, for the cases without gagging at the inlet and exit, we tried to develop a simpler method for construction of the stability map which might give us clearer insights into the effects of the important parameters.

An Experimental Study on Chaotic Behavior of Thermohydraulic Oscillation in an Evaporating Tube.

The characteristics of flow oscillations in density wave instabilities are experimentally investigated by means of the deterministic manner based on chaotic dynamics. The results are as follows. ( 1 ) The least number of significant figures of the time-series data is two, but it is plausible to take more than three if possible. ( 2 ) The flow oscillation analyzed in this paper is chaotic and obeys a dynamics with a small degree of freedom. ( 3 ) Given the mass flux, the dimension of the attractor in the unstable region decreases with the heat flux, which means that the randomness of the dynamics gradually attenuates with departure from the stability threshold and the structure of the attractor becomes simpler. ( 4 ) The dimension of the attractor can be related to the fundamental frequency, which means that the characteristics of the chaotic dynamics generating the density wave instability depend mainly on the fundamental frequency in the flow oscillation independent of the differences in the experimental conditions.

Visualization of Flow Pattern of Self Oscillated Turbulent Jet in Enclosed Room

A new method based on the Coanda effect for self oscillation of a circular jet bounded by rectangular enclosure is suggested. The flow pattern is observed by an experiment for water flow by using a light sheet and a domestic movie camera. And the experiments in both air and water reveal regions of stable oscillation wherein relationships are obtained between geometrical dimensions of the enclosure.

Analysis of the Flow Reversal in Natural-Circulation Boiler

Flow reversal phenomena in natural-circulation boilers are studied analytically according to both the homogeneous-flow model and the slip-flow model. In the latter model Thoms pressure drop correlation is used. The pressure drop between the upper steam-water drum and the lower header is assumed not to be affected by inception of reverse flow in an evaporator tube which will be analyzed later.The mechanism of flow reversal is examined through analysis of the static flow characteristics of the evaporator tube in the non-dimensional form. Some important dimensionless parameters are introduced and their effects on inception of reverse flow are studied.It was found that the criterion of flow reversal is correlated practically with only three dimensionless parameters. Finally, an approximate method to predict flow reversal is put forward.

An experimental study of disturbance waves in boiling two-phase flow.

Behaviors of the disturbance waves in an evaporation tube were experimentally investigated in the high quality region using a conductance probe method, under the following conditions; pressure 2.95 MPa, mass flux 300, 500 and 700 kg/m/sup 2/s, heat flux up to 1.2 MW/m/sup 2/ and thermodynamic qualities larger than 0.80. By comparing the results with air-water adiabatic data, it was found that the disturbance waves in the boiling steam-water system had the following outstanding features: a) the wave was not so sharp and had a long tail, b) scattering of the propagation velocity was larger than that of the air-water system and c) the spatial separation of two successive waves did not vary significantly with the quality. It was also confirmed that the disturbance waves had a close connection with the wall temperature fluctuations in the dryout region.

Investigations on Two-Phase Flow in Steam Accumulators

The characteristics of the steam accumulator system in the electricity utility system in comparison with the pumped storage hydraulic system demands that the steam accumulator system be used for shaving off peak loads of shorter duration, say one or two hours, than the pumped storage. Responce of the bulk water and the steam space to rapid discharging of steam was investigated up to 2 m/s average steam velocity out of the water surface. Bulk water flow in the charging period was found dependent on the baffle configuration and a method of prediction was presentd.

Separation Performance of Wire-mesh Demister being held perpendicular to the Horizontai Air Flow

In this paper, separation of water droplets from air including water droplets by wire-mesh demisters is investigated through experiments.Wire-mesh demisters used here are constructed by galvanized iron wires which are held perpendicular to the air flow direction and which have one layer. Moreover, the demisters have three different specific surface area, 11.06, 14.43 and 39.78 cm2 /cm3 respectively. For all of the demisters mentioned above, mean air velocity is varied over the range of 1.0 to 6.0 m/s, with the size of water droplets being varied over the range of 5 to about 200μm.The values of separation efficiencies of these demisters are larger by those predicted by theoretical analysis which has been presented by A. Burkholz, et al. or C. Carpenter, et al.Characteristics of size distribution of water droplets at the inlet and the outlet to the demister, and furthermore resistance coefficients of these demisters are also investigated.