Hisashi Umekawa

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.

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.

VISUALIZATION OF BED-MATERIAL MOVEMENTS AROUND BUBBLE IN A FIXED-BED BY NEUTRON RADIOGRAPHY

Abstract

VISUALIZATION OF LARGE-PARTICLE MOVEMENT IN FLUIDIZED-BED BY NEUTRON RADIOGRAPHY

Abstract

CHF in a Circumferentially Nonuniformly Heated Tube Under Low-Pressure and Low-Mass-Flux Condition (Influence of Inclined Angles Under High-Heat-Flux Condition)

Critical heat flux (CHF) is an important design factor for boiling two-phase flow equipment, such as boilers and others. In actual boiling systems, the water tube suffers from the nonuniform heating and/or tube inclinations. The objective of this investigation is to understand the influence of tube inclination on CHF characteristics under such high-heat-flux conditions. The experimental investigation was conducted with a forced convective boiling system by using a uniformly heated tube and a nonuniformly heated tube set at arbitrary inclination angles ϕ. The obtained CHF was strongly influenced by the circumferential location of local maximum heat flux point and tube inclination. In the case of the normal tube, the CHF always occurred by the liquid film dryout at the top of the tube. In the case of the nonuniformly heated tube, the influence of the inclination on the CHF characteristics strongly depended on the circumferential heat flux distribution. When the the heat flux at the bottom was higher than that at the top, two types of CHF mechanism, namely, low-quality CHF upstream of the test section under high-mass-flux condition, and liquid film dryout at the tube exit under low-mass-flux condition, were observed. When the heat flux at the top was higher than that at the bottom, intermittent dryout was observed as the dryout mechanism. These CHF characteristics could be categorized by using the CHF ratio against the value of the vertical upward flow with the modified Froude number, which corresponded to the influence factor of disturbance wave.