Models for predicting flow pattern transitions during steady gas-liquid flow in vertical tubes are developed, based on physical mechanisms suggested for each transition. These models incorporate the effect of fluid properties and pipe size and thus are largely free of the limitations of empirically based transition maps or correlations.

Modelling flow pattern transitions for steady upward gas-liquid flow in vertical tubes

The performance of heat exchangers can be improved to perform a certain heat-transfer duty by heat transfer enhancement techniques. In general, these techniques can be divided into two groups: active and passive techniques. The active techniques require external forces, e.g. electric field, acoustic or surface vibration, etc. The passive techniques require fluid additives or special surface geometries. Curved tubes have been used as one of the passive heat transfer enhancement techniques and are the most widely used tubes in several heat transfer applications. This article provides a literature review on heat transfer and flow characteristics of single-phase and two-phase flow in curved tubes. Three main categories of curved tubes; helically coiled tubes, spirally coiled tubes, and other coiled tubes, are described. A review of published relevant correlations of single-phase heat transfer coefficients and single-phase, two-phase friction factors are presented.

A review of flow and heat transfer characteristics in curved tubes

A Review of two-phase flow dynamic instabilities in tube boiling systems

The interrelation between void fraction fluctuations and flow patterns in two-phase flow

Two-phase flow instabilities: A review

Flow instabilities in parallel-channel flow systems of gas-liquid two-phase mixtures

Distributions of flow rates and flow stabilities in parallel evaporator tube systems have been experimentally investigated. The evaporator is composed of three parallel copper tubes of 4mm inside diameter and 40 m long. Experiments were carried out by using refrigerant R-113 as the fluid in pressure range from atmospheric pressure to supercritical pressure, in order to simulate phenomena in steam generators. It has been shown that distributions of flow rates, their hysterisis and onset of excursion phenomena can be estimated from pressure drop versus flow rate curves for single tubes. Moreover, description is made of behaviors of thermal oscillative instability which are observed at relatively low flow rate. In addition to the experimental results, by a theoretical analysis of a simple model, stability criteria in parallel tube system have been described.

Study on Distribution of Flow Rates and Flow Stabilities in Parallel Long Evaporators

In this paper the frictional pressure drop, void fraction and flow pattern of an air-water two-phase flow in helically coiled tubes are presented. The coils are constructed of 9.92 mm I.D. copper tube and the ratios of inside diameter to coil diameter are 1/11 and 1/22.7, and the lengths are 4 m and 4.5 m respectively. The experiments were conducted in the range of superficial water velocity wlo: 0.35∼1.16 m/sec and superficial air velocity wgo: 0∼5 m/sec. The frictional pressure drop of the two-phase flow in these coils is 1.1 to 1.5 times as much as that in a straight tube in the range of the experiment. Three types of empirical equations for the frictional pressure drop are proposed, and also the experimental data are correlated by a modified Lockhart-Martinelli method. The values of void fraction in the coils are approximately equal to those in a straight tube for the same water and air flow rates.

Study On a Gas-Liquid Two-Phase Flow in Helically Coiled Tubes

An experimental study on the confluence of two two-dimensional incompressible turbulent jets, issuing from nozzles with angle θ0 against the nozzle exit plane, has been presented. The axial momentum and the energy flux of the jets along the z-axis perpendicular to the exit plane are calculated by using the measured values of velocity and pressure distributions. It has been shown that the axial momentum flux along the z-axis decreases owing to the confluence, that the energy flux also decreases remarkably owing to the confluence, that the energy flux also decreases remarkably owing to the confluence, and that the decreased values are approximately 40% of the efflux energy of the jet, independent of the ratio of a distance between two nozzles to the nozzle width and θ0. A method of predicting the velocity distribution also has been proposed. The predicted values agreed well with the experimental results.

An Experimental Study on Confluence of Two Two-Dimensional Jets

In this paper, the flow configuration of the two-phase slug flow, which contains small bubbles in the liquid slug has been analysed. The experiment has been carried out for air-water two-phase upward flow in 27.6 mm ID glass tube, using the electrode probe which indicates the void ratio fluctuation. The new parameters such as the ratios of the large bubble (Taylor bubble) volume (fdB) and of small bubbles volume in liquid slug (fdS) to a control volume, the void ratio in gas slug section (fd1) and the void ratio in liquid slug section (fd2) have been introduced. These parameters except fd1 are given experimentally as the funtion of mean void ratio fd only. They are as follows : fdB=fd1.8 fdS=fd(1-fd0.3) fd1= fd2=fd1.8

Koji Akagawa

Papers

Flow instabilities in parallel-channel flow systems of gas-liquid two-phase mixtures

Study on Distribution of Flow Rates and Flow Stabilities in Parallel Long Evaporators

Study On a Gas-Liquid Two-Phase Flow in Helically Coiled Tubes

An Experimental Study on Confluence of Two Two-Dimensional Jets

Fluctuation of Void Ratio in Two-Phase Flow : 2nd Report, Analysis of Flow Configuration Considering the Existence of Small Bubbles in Liquid Slugs