Mamoru Ozawa

Flow and temperature measurement of natural convection in a Hele-Shaw cell using a thermo-sensitive liquid-crystal tracer

The temperature and flow field of natural convection in a Hele-Shaw cell is visualized by using a liquid-crystal tracer. The tracer photographs obtained by this method are compared with the interferograms of previous experiments using the same experimental setup, and the applicability of the present methods is validated. Quantitative data of the temperature and velocity were obtained by applying a colour-image-processing technique to the visualized images.

Two-phase flow in an annulus with a rotating inner cylinder (flow pattern in bubbly flow region)

Abstract The two-phase flow pattern in a concentric annulus with a rotating inner cylinder have been investigated experimentally. The observed flow patterns were dispersed bubbly, ring-form, single-spiral, double-spiral, triple-spiral flows and transition regions. When the rotational speed was at a relatively low level, the buoyancy effect of bubbles dominated the flow field and a dispersed bubbly flow was formed. On the other hand, when the rotational speed was at a high level, the vortex motion induced by the rotation dominated the flow field and ring-form and spiral flows were formed.

CHF in Oscillatory Flow Boiling Channels

Recent advanced technologies in boiler manufacturing bring about highly efficient and compact water-tube boilers for gas firing or oil firing. These boilers have very simple water-circulation system, which often brings about flow instabilities followed by critical heat flux, CHF, problems. This paper describes such an oscillatory flow (CHF) at relatively low mass flux and at low pressure. CHF is a decreasing function of the amplitude and period of the flow oscillation, and reaches almost 40 % of the steady state value. When the tube wall is thin, CHF is mainly controlled by the two-phase flow dynamics and is well simulated using a simple lumped-parameter model of boiling channel. At very low mass flux, CHF depends significantly on the flow orientation, and is closely related to the flow pattern transition.

Scaling parameter of CHF under oscillatory flow conditions

Critical heat flux (CHF) is reduced by flow oscillations. The reduction of CHF is significantly influenced by flow oscillation period and amplitude, heat capacity of test tube, and mean inlet mass flux. A scaling parameter of the temperature response of the tube wall was derived based on a lumped-parameter model of the tube walls heat capacity. When this scaling parameter was applied to CHF under flow oscillations, the experimental data were successfully correlated.

Boiling two-phase heat transfer of LN2 downward flow in pipe

Abstract In the field of the air separation technology, the downward-flow-type system has started operation for the main reboiler-condenser instead of the natural-circulation-type system. The downward-flow-type main reboiler-condenser has an advantage in the cost and the ability compared with the natural-circulation-type, while the estimation of heat transfer characteristics has a significant influence on the system performance. In this study, heat transfer experiments have been conducted by using a downward flow of liquid nitrogen (LN 2 ). The comparison of the heat transfer coefficients with existing correlations is discussed. Estimation methods of the flow rate and the pressure drop are also discussed including the prediction of the flow instability.

Bubble behavior and void fraction fluctuation in vertical tube banks immersed in a gas-solid fluidized-bed model

Abstract Heat transfer characteristics of tube-banks immersed in a fluidized bed are dominated by the time averaged as well as fluctuating characteristics of fluidized-particle movement especially in the neighboring region of heat transfer tube. Quantitative flow visualization of the fluidized bed installed vertical tube-banks has been successfully conducted using neutron radiography and image processing technique. The quantitative data of void fraction distribution as well as the fluctuation data are presented. The bubbles formed in the bed rise along by the vertical tubes and observed bubble size is smaller than that in a free bubbling bed without tube-banks. The bubble diameter is well correlated by the modified Mori and Wens correlation. The bubble rise velocity is also well correlated by applying the drift–flux model. These results are consistent for both tested bed materials of Geldards B- and A-particles, while the bubble size is significantly different between two kinds of particles.

Temperature and Velocity Measurement of a 3-D Thermal Flow Field using Thermo-sensitive Liquid Crystals

It is an important challenge to analyze a three-dimensional thermal flow field in engineering, science, and agriculture. For such an analysis, it is essential to measure physical quantities such as temperature and velocity over the entire thermal flow field.This paper presents a measurement system based on color image processing for temperature and velocity vector distributions in a three-dimensional thermal flow field. Flow visualization is accomplished by the use of thermo-sensitive liquid crystal tracers.An algorithm for the color-to-temperature transformation using a multi-layer feed-forward neural network is applied to three-dimensional natural convection in a rotating cylindrical cell. Two-dimensional temperature distributions in a slit plane are obtained by using the algorithm. A three-dimensional temperature distribution is consequently constructed by interpolating the two-dimensional distributions using the B-spline function. In addition, the Spatio-Temporal correlation method is applied to the natural convection to obtain a three-dimensional velocity vector distribution.

VOID FRACTION PROFILE IN TUBE-BANKS OF A SIMULATED FLUIDIZED-BED HEAT EXCHANGER

Abstract Flow visualization of cross-flow tube-banks in a simulated fluidized-bed was conducted by using the neutron radiography system of JRR-3M. Applying the image processing technique to the visualized image, the void fraction profile in the tube-bank was obtained. The observed flow pattern and the void fraction profile demonstrate the importance of the tube arrangement in fluidized-bed heat exchangers.

Dryout and post-dryout heat transfer in a natural circulation loop of liquid nitrogen

In a natural circulation loop, flow instabilities, such as density wave oscillations, occur under certain operating conditions, but the influence of flow oscillations on the heat transfer is not well understood. An experiment was conducted using a natural circulation loop of liquid nitrogen with test tubes of dimensions of 3.0 mm/5.0 mm I.D., and 900 mm heated length. Experimental results indicated that the heat transfer correlations of Schrock-Grossman and Roko-Shiraha were in good agreement with the experimental results in the saturated boiling and postdryouts regions, respectively, even under oscillatory flow conditions. On the other hand, the critical heat flux (CHF) correlation obtained under stable flow conditions, such as Katto, predicted significantly larger values compared with the experimental data under oscillatory flow condition. The CHF value of the natural circulation loop was well predicted by using the numerical simulation based on a lumped-parameter model, taking account of the circulation rate, oscillation period, and amplitude.

Large-particle movement and drag coefficient in a fluidized bed

Flow visualization studies of the behavior of a large particle in a fluidized bed were conducted using neutron radiography. The large particle moved according to the bed materials, especially in the wake of a bubblc. When the density of the large particle was high, the particle moved with a velocity different from that of a bubble, and in some cases the particle moved downward through a bubble. When the density was low enough, the particle moved on the surface of the bubble wake. The drag force of the large particle immersed in a fluidized bed was measured. The drag force increased with an increase in the volumetric gas flux as well as with an increase in the particle diameter. The drag coefficient was derived and was well correlated on the basis of a two-phase model and experimentally determined effective viscosity of the bed materials.