Techniques for measuring velocity distribution in gas-liquid two-phase flow are important to elucidate the mechanisms of the flows in various two-phase systems, and to improve desigining industrial facilities. In this report, we focus on recent advanced techniques based on particle imaging velocimetry (PIV), which is known as a contact-free, multi-dimensional, instantaneous measurement tool. The current problems in applying the PIV to gas-liquid two-phase flows are summarized, and several examples of measured results are shown.
This paper presents a review on the state of current measuring techniques for gas-liquid multiphase flow rates.After briefly discussing the basic idea on measuring methods for single-phase and two-phase flows, existing methods for the two-phase flow rates are classified into several types, that is, with or without a homegenizing device, single or combind method of several techniques, with intrusive or non-intrusive sensors, and physical or software method. Each methods are comparatively reviewed in view of measuring accuracy and manageability. Its scope also containes the techniques developed for petroleum-gas-water flow rates.
Because “flow pattern” relates closely to design parameters of plant such as pressure drop and heat transfer coefficient, better knowledge of the flow pattern leads not only to deep understanding of the flow but also to accurate prediction of the parameters or to safe operation of plant. Therefore, “flow pattern” is an important and fundamental parameter as well as void fraction. Nevertheless the flow pattern has been judged mostly on a basis of visual observation. Therefore, it is desired to identify the flow pattern objectively and quantitatively. Here, identification of flow pattern using differential pressure fluctuations is described. Behaviors of statistical parameters are discussed in the statistical parameter spaces. And also recent flow visualization methods using fast X-ray CT and HPIV are described.
This report is a state of the art review of recent developments in measurement techniques for liquid film flows in gas-liquid systems. Our concerns are rapidly moving towards clarifying detailed structures of film flows in spatial and time domains dominating the mechanisms. This report focuses, along with typical conventional methods, recent advancements in film flow measurements mainly brought about by the developments in computer technologies, electronics and optics.
An ultra-fast X-ray computed tomography (CT) scanner has been developed for measurement of gas-liquid two-phase flow. This scanner overcomes problems that occur in a transient or unsettled state, which make the conventional CT scanner inappropriate. To reduce the scanning time, this X-ray CT system uses electronic switching of electron beams for X-ray generation instead of the mechanical motion adopted by conventional CT scanners. A prototype system with a scanning time of 3.6 milliseconds was initially developed and confirmed to measure the dynamic events of two-phase flow. However, an increased scanning speed is generally required for practical use in the thermal-hydraulics research field. Therefore, an advanced type which can operate under the scanning time of 0.5 milliseconds and can measure two-phase flow with a velocity up to 4-5 m/s was developed.
A discrete element method was applied for a two-dimensional fluidized bed. The calculation time rapidly increases as the number of particles increases. In order to decrease calculation time, imaginary spheres were used instead of actual particles. An imaginary sphere has a diameter grater than actual particles and a lower actual density. We compared the bubble diameter and minimum fluidization velocity between calculation and experiment. A good agreement of bubble diameter was obtained by using the assumption of a FCC (Face Centered Cubic) particles structure model.
There has been a strong demand to clarify the microscopic structure of bubbly flows. The authors attempted and succeeded in clarifying the flow structure around bubbles. The Ultrasonic Velocity Profile Monitor (UVP) was used to measure the instantaneous velocity profiles of both phases in bubbly countercurrent flows. From the UVP data, probability density function of the instantaneous velocities was obtained at each measuring point and the first to the fourth order statistical moments of the liquid velocity were calculated. From the distribution of these moments, existence of bubble boundary layers was found. The flow structure around bubbles was proposed to be divided into three regions:(1) a boundary layer, (2) a transition region and (3) a main flow region. In the boundary layer, liquid flow is strongly influenced by the bubble motion but is independent of the main flow. In the transition region adjacent to the boundary layer, liquid phase is influenced by both bubble motion and the liquid main flow. Liquid is dragged strongly by bubbles in the case where the flow rate is relatively high. In the main flow region, liquid phase receives much influence from the liquid main flow compared with other regions. Finally from the obtained data, some correlations were found between the layer thickness, void fraction and bubble Reynolds number.
In some multiphase flows, the phase distribution as well as other flow characteristics may have a periodical or semi-periodical property along the tube axis. In such a flow, the flow structure corresponding to one period is referred to as “fundamental structure” of the flow in this paper. If the real flow structure of multiphase flow is assumed to be composed of repetition of the “fundamental structure”, then the mean value and the fluctuation value of flow characteristics such as the velocity and the pressure drop would be possible to be estimated using those for the “fundamental structure”. This paper deals, as a germinal stage, with concepts of the “fundamental structure” as well as “unit element” and “compound element”, which are basic elements to compose the “fundamental structure”, and also deals with an application of this concept to gas-liquid-large particle three-phase slug flow in which the “fundamental element” and the “compound element” are a large bubble etc.and a combination of a large bubble and a liquid slug etc., respectively.
We have developed an extended version of the Real-Coded Lattice Gas Method, in order to simulate behaviors of multi-phase systems with surfactants. We characterized surfactants by dumbbell-shaped particles, composed by sets of hydrophilic ends and hydrophobic ends connected by rigid rods. By adopting our model, we conducted simulations of micelle formations by surfactants in an aqueous environment. In the simulations, our model provided significant results which reflect some of the properties of actual micelle structures.