Itaru Michiyoshi

Turbulence structure of air-water bubbly flow—II. local properties

Abstract Microstructure was studied experimentally in air-water two-phase bubbly flow flowing upward in a vertical pipe of 60 mm diameter under atmospheric pressure. The results indicate that over a large portion of fully-developed bubbly flow the phases, the velocities of bubbles and water, and the ratio between the velocities of the phases have fairly flat radial profiles. In the wall region a maximum void fraction was observed. Spectra of the velocities of bubbles and water showed a Poisson distribution and a normal distribution function, respectively. The experimental evidence indicated a trend for the turbulent intensity to decrease first with increasing gas flow rate for constant water velocity and to increase again with a further increase in the gas flow rate. This phenomenon was more significant for a higher water velocity.

Turbulence structure of air-water bubbly flow—I. measuring techniques

This paper, the first in a series describing our work on the turbulence structure of air-water bubbly flow, describes the principles of measurement and specially developed electronic instrumentation for determining various important local parameters, and the rates of turbulent transport of heat and bubbles in air-water two-phase bubbly flow. These instruments indicate the phase distribution, the bubble velocity and its spectrum, the water velocity and the turbulent intensity, and the turbulent dispersion coefficient of bubbles. Brief discussions are also presented on the accuracy of these techniques.

Turbulence structure of air-water bubbly flow—III. transport properties

An experimental study of heat and bubble transport in turbulent air-water bubbly flow was carried out by means of tracer techniques. Helium tracer gas concentration data and temperature distributions were used to extract bubble and heat diffusivity information. The results indicated that the turbulent velocity components of the liquid phase play a predominant role in the turbulent transport process. A systematic increase of diffusivity of heat, ϵH, with quality and water velocity was observed. An empirical correlation for the diffusivity ratio ϵH,TP/ϵH,SP is presented. The Peclet number, u′covbar|dφ, for bubble dispersion can be approximated by 2.0, independent of the flow variables. The bubble-to-heat diffusivity ratio, φ/ϵH, approaches unity with increasing quality and water velocity. Momentum transport is also discussed, based on a mixing length theory.

MHD effect on NaK-nitrogen two-phase flow and heat transfer in a vertical round tube

Abstract The magnetohydrodynamic (MHD) effect on NaK-nitrogen two-phase flow and heat transfer has been experimentally studied in a vertical round tube in a transverse magnetic field. This study covered a wide range of two-phase flow patterns from bubbly flow to annular/annular-dispersed flow, including flow pattern observation and measurements of phase distributions, liquid film behavior and heat transfer coefficients. MHD effects on local flow structures and heat transfer mechanisms have been discussed based on these measurements. It has been found that the application of a magnetic field brought about a significant asymmetric behavior both in two-phase flow structures and heat transfer. The angular dependence of heat transfer in annular/annular-dispersed flow is particularly stressed.

Minimum film boiling temperature for cooldown of insulated metals in saturated liquid

Abstract A theoretical study has been conducted to investigate an earlier transition from the film to the transition boiling, which may occur during the rapid cooldown of hot metals coated with a thin layer of insulating (low thermal conductivity) material. Direct liquid-solid contacts are assumed to occur intermittently in the film boiling regime. The transient contacting and subsequent evaporation of a thin microlayer of liquid bring forth a rapid decrease in the local surface temperature. If the local surface temperature falls below the lowest limit, an earlier onset of transition boiling occurs though the average wall temperature is quite high. The minimum film boiling temperature becomes higher with increasing thickness of insulating layer. The calculated results are in good agreement with experimental data for liquid nitrogen boiling on Teflon-coated copper plates.

Local properties of vertical mercury-argon two-phase flow in a circular tube under transverse magnetic field

Abstract Experimental data are presented in this paper on the profiles of local void fraction, bubble impaction rate, bubble velocity and its spectrum, and also bubble length and its spectrum, of mercury-argon two-phase slug flow flowing upwards in a vertical circular tube in the presence of a transverse magnetic field. Decrease in void fraction and increase in bubble velocity are significant when the magnetic flux density is larger than 0.3∼0.4 T (Ha ⋍ 100) . This effect is discussed by analyzing the bubble size distribution. Recovery of local void fraction profile in the downstream of an obstacle and diffusion of void injected from only one nozzle in the presence of magnetic field are also discussed.

Pool-Boiling Heat Transfer in Liquid Nitrogen

An experimental study was conducted on pool-boiling heat transfer from an electrically heated horizontal wire to saturated liquid nitrogen at atmospheric pressure. Experimental results of heat transfer characteristics in both nucleate- and film-boiling regimes, critical heat flux and minimum heat flux were analyzed and compared with various correlations. In addition, photography was used to obtain information concerning the vapor-bubble and vapor-film behaviors around the heating wire. These data of microcharacteristics were utilized for evaluating theoretical models of nucleate-boiling mechanism proposed by other investigators. Transient conduction to the relatively cold (saturated) liquid, which came into the space vacated by the departing bubble and contacted with the heating surface, was found to be important for high heat transfer rates associated with nucleate boiling.

Comparison of Langmuir probe and spectroscopic electron temperature measurements

Deals with an experimental confirmation of the deviation from LTE in a low density argon plasma and of the applicability of the theory on the deviation from LTE developed by Griem. The excitation temperature determined spectroscopically using relative intensities of the resonance lines 4000-7000 AA of argon atoms was compared with the electron temperature measured by a conventional Langmuir probe. As a result it was found that the excitation temperature was 1000- 1500 degrees C higher than the electron temperature and that the difference between them increased along the radius of the plasma stream. It implies that the plasma was not in LTE. In fact the excitation temperature determined from relative populations of the 5p states of the argon atoms was found to agree quite well with electron temperature across the plasma stream.

Heat Transfer in Turbulent Flow with Internal Heat Generation in Concentric Annulus, (I):Fully Developed Thermal Situation

This paper deals with the problem of heat transfer in fully developed turbulent flow with uniform internal heat generation in a concentric annulus under conditions of uniform but different heat flu...

Mercury-argon two-phase heat transfer in a vertical annulus under transverse magnetic field

Abstract This paper presents experimental data of local properties and heat transfer of mercury-argon two-phase bubbly flow with small gas flow rate in a vertical annulus in the presence of transverse magnetic field. The two-phase Nusselt number decreases as well as single phase mercury heat transfer with increasing Hartmann number at the fixed Peclet number, but its decrease becomes smaller than the single phase as the quality is increased. Discussions are presented on contributions of effective thermal conductivity, eddy diffusivity of heat and liquid velocity to the two-phase heat transfer.