Two-phase Flow Facility Research Articles

A mechanistic model for the prediction of flow pattern transitions during separation of liquid-liquid pipe flows

A one-dimensional mechanistic model that predicts the flow pattern transitions during the separation of dispersed liquid-liquid flows in horizontal pipes was developed. The model is able to capture the evolution along the pipe of the four characteristic layers that develop from initially dispersed flows of either oil-in-water or water-in-oil at a range of mixture velocities: a pure water layer at the bottom, a settling (flotation/sedimentation) layer, a dense-packed zone, and a pure oil layer on the top. Coalescence correlations from literature were included in the model to predict the drop growth due to binary drop coalescence and the coalescence rate of drops with their corresponding interface. The model predictions on the evolution of the heights of the different layers were partly compared against available experimental data obtained in a pilot scale two-phase flow facility in a test section of 0.037 m inner diameter using tap water and an oil of density 828 kg m−3 and viscosity 5.5 mPa s as test fluids, and in a 0.1 m inner diameter test section using water and an oil of density 857 kg m−3 and viscosity 13.6 mPa s. It was shown that the evolution of the four characteristic layers depends on the rates of drop settling and drop-interface coalescence. Oil-in-water dispersions separated faster than water-in-oil ones, while dispersions with smaller drop-sizes were more likely to exhibit depletion of the dense-packed zone.

Experimental study of interfacial structure of horizontal air-water two-phase flow in a 101.6 mm ID pipe

The current work seeks to investigate the interfacial structure and establish an extensive experimental database in horizontal air–water two-phase flow in a 101.6 mm inner diameter pipe. A wide range of flow configurations are studied including bubbly, plug and slug flows. A flow visualization study using the high-speed video camera enables qualitative description of bubbly-to-plug and bubbly-to-slug transitions, while the database of local time-averaged two-phase flow parameters obtained by the four-sensor conductivity probe enables quantitative study on the evolution of the flow. Detailed measurements across the flow area are performed for nine test conditions at three different axial locations downstream of the inlet. Using this database, the effects of superficial liquid and gas velocities, and development length on the evolution of interfacial structure are investigated. Similar characteristics are observed as in a counterpart 38 mm ID horizontal two-phase flow facility, which include (1) the bubbles are found to be more concentrated near the top wall in bubbly flow as superficial liquid velocity decreases at a constant superficial gas velocity, while increasing superficial gas velocity promotes the growth of bubble layer thickness; (2) in bubbly-to-plug transition, the void fraction of small bubbles decreases and the size of small bubbles increases, while in bubbly-to-slug transition, opposite trends are observed. Meanwhile, different characteristics on the evolution of the interfacial structure are also observed, which indicates the effect of increasing pipe diameter. The bubbly-to-plug/slug transition is found to shift to higher superficial liquid velocities as pipe diameter increases. It is observed that the gas phase is more concentrated near the top wall in the large diameter pipe. As a result, the distance between bubbles is smaller and there is a higher chance for bubbles to coalesce into large bubbles. The critical void fraction where the bubbly-to-plug/slug transition initiates decreases as pipe size increases.

Comparative study of ultrafast X-ray tomography and wire-mesh sensors for vertical gas–liquid pipe flows

Wire-mesh sensors and ultrafast X-ray tomography were developed to investigate two-phase flows with high spatial and temporal resolution. In this study we present results of a comparative study for both imaging techniques, where disperse gas-phase velocity was available from both techniques for the first time. For the experiments we designed a special test section which was operated at HZDR's two-phase flow facility TOPFLOW. A very small vertical distance between X-ray planes and wire-mesh sensor guarantees that lateral flow changes during passage of the respective planes are almost negligible. Experiments were conducted with varying water and air superficial velocities, giving flow regimes from bubbly flow via slug flow to annular flow. Typical experimental results are presented and discussed in detail in this paper. Eventually, the application ranges for both measurement techniques are briefly discussed.

Natural circulation and stability performance of BWRs (NACUSP)

From the beginning of BWR technology it was realized that a BWR can become unstable under particular circumstances caused by a feedback between the thermal-hydraulics and the neutronics. This instability can result in oscillations of the power and the flow rate, which is an unwanted phenomenon. The NACUSP project addresses the stability issues in current and future BWRs by expanding the basic understanding through well structured testing and analyses of experimental data, by analyses of existing operational stability data from three different European reactors (Forsmark, Leibstadt, Cofrentes), by applying this knowledge via efficient models and validated computer codes to operating reactors and reactor designs, and by developing general guidelines for reactor operation and design on how to avoid BWR instabilities. In order to cover the parameter range as efficiently as possible, four existing, sophisticated thermohydraulic test facilities (CLOTAIRE [Gouirand, J.M., 1988. CLOTAIRE Program, description and manufacturing of the mock-up, CEA Cadarache, DRE/STRE/LGV 88–876.] DESIRE [van de Graaf, R., van der Hagen, T.H.J.J., Mudde, R.F., 1994. Two-phase flow scaling laws for a simulated BWR assembly. Nucl. Eng. Des. 148, 455–462.] CIRCUS [de Kruijf, W.J.M., van der Hagen, T.H.J.J., Mudde, R.F., 2000. CIRCUS; a natural circulation two-phase flow facility, Eurotherm Seminar No. 63, 6–8 September 1999 Genoa, Italy, 391–395] and PANDA [Dreier, J., Huggenberger, M., Aubert, C., Bandurski, T., Fischer, O., Healzer, J., Lomperski, S., Strassberger, H.-J., Varadi, G., Yadigaroglu, G., 1996. The PANDA facility and first test results, Kerntechnik 61, 214–222]) have been selected. To extrapolate from small-scale separate-effect testing conditions to full-scale integral reactor conditions one needs to rely on the performance of computer codes (MONA [Hoyer, N., 1994. MONA, a 7-Equation Transient two-phase flow model for LWR dynamics, Proceedings of the International Conference on New Trends in Nuclear System Thermohydraulics, pp. 271–280], ATHLET [Krepper, E., Prasser, H.-M., 1999. Natural circulation experiments at the ISB-VVER integral test facility and calculations using the thermal-hydraulic code ATHLET. Nucl. Technol. 128, 75–86], RAMONA-3(-5) [Grandi, G., Hoyer, N., Belblidia, L., 1998. Two-fluid thermal hydraulics capabilities of RAMONA-5, Proceedings of the International Conference on the Physics of Nuclear Science and Technology, October 5–8, 1998, Long Island, NY, USA, 1621–1625], LAPUR-V [Otaduy, P.J., March-Leuba, J., 1990. LAPUR User's Guide, NUREG/CR-542, ORNL/TM-11285], DWOS, FLICA [Toumi, I., Bergeron, A., Gallo, D., Royer, E., Caruge, D., 2000. FLICA-4: a three-dimensional two-phase flow computer code with advanced numerical methods for nuclear applications, Nucl. Eng. Des. 200, 139–155], SAPHYR, RELAP5/MOD3 [Idaho National Engineering Laboratory, 1995. RELAP5/MOD3 Code Manual, Vols. 1–6, INEL-95/0174, NUREG/CR-5535], TRAC-BF1 [Idaho National Engineering Laboratory, 1992. TRAC-BF1/MOD1; an advanced best-estimate computer program for BWR accident analysis, Vols. 1–3, EGG-2626, NUREG/CR-4356]. For specific items CFD codes are applied as well. Within NACUSP a linear stability analysis tool is developed. Four of the three experimental facilities within the project have yielded a large, unique database. Natural-circulation and stability characteristics at nominal pressure were collected from extensive experiments at the DESIRE facility. Low-pressure characteristics were measured at the PANDA (large scale) and the CIRCUS (small scale) facility. The phenomena encountered (flow rate and stability trends, the occurrence of flashing induced oscillations) can be explained from basic physical models and are now well understood. Several thermal-hydraulic codes have been benchmarked—some successfully, others with limited success—against these data. The CLOTAIRE (nominal pressure, large scale) facility has been modified and is now ready for experiments. Nuclear power plant data from three BWRs (Cofrentes, Forsmark and Leibstadt) have been collected. A very complete set of reactor data was obtained from measurements at the Leibstadt BWR during cycle-19. During this test, which was performed within the NACUSP project, safe reactor operation was demonstrated at extreme conditions, covering the entire power-flow map. State-of-the-art, coupled thermal-hydraulic—neutronics codes are being benchmarked on these data. The development of an easy to use, rapid and efficient analytical tool for parametric studies on BWR stability is in progress. The last year of NACUSP will be devoted to finalising the before mentioned activities and to use the tools developed and the experience obtained for developing general guidelines for designing and operating BWRs.

A comparative study of phase Doppler and laser diffraction techniques to investigate drop sizes in annular two-phase flow

Drop size information is a common requirement in many parts of industry. Phase Doppler anemometry and laser diffraction techniques are usually employed for this purpose. Both measuring techniques give the drop size whereas the phase Doppler anemometry also provides additional information on the drop velocities. In the present work an inclinable two-phase flow facility has been constructed and the impact of inclination on the drop sizes has been investigated. This paper describes the drop size results obtained from two laser techniques and identifies the difference in drop sizes obtained from two laser systems when measurements were made under similar flow conditions. Possible reasons for this discrepancy have been discussed and a technique has been suggested to modify the new data obtained from the phase Doppler anemometry. The modified data have then been compared with those obtained from the laser diffraction technique. Results from both laser systems reveal similar trends in the measurements made under identical flow conditions. It has been found that the drop sizes are affected by the flow orientation and the effect is more pronounced at angles greater than 50° from the vertical. The effect of liquid mass flux has also been recorded. General problems and common sources of error while employing these two laser techniques have also been identified and necessary steps required to obtain laser results have been discussed in detail.

Developing Lengths in Horizontal Two-Phase Bubbly Flow

An air-water two-phase flow facility with a 19.1 mm i.d. test section has been fabricated. Local measurements of pressure drop for two-phase horizontal bubbly flow and single-phase flow downstream of various orifices have been obtained over a range of flow conditions. The wall shear stress developing length is obtained from the pressure drop profile. A developing length correlation is presented in which the relative deviation is 6 percent. The fully developed vapor volume fraction has also been measured up- and downstream of the orifice. A simple correlation for vapor volume fraction is presented in which the relative deviation is 7 percent. Photographs of the two-phase flow pattern in the developing region reveal that the flow structure is extremely complex and continuously evolves until approximately fully developed flow conditions are achieved.

Controlling Two-Phase Flow Using a Fluidic Vortex Amplifier

This paper describes the results from an investigation into the use of a fluidic vortex amplifier for controlling two-phase gas/liquid flow. A two-phase flow facility has been built and an axial vortex amplifer, designed to suit a 75 mm pipe, has been manufactured and tested. The results show that the vortex amplifier operates satisfactorily in cavitating and two-phase flow but that there is a reduction in the performance as measured by the turn-down ratio. However, it is clearly demonstrated that this is due to the increased resistance of the vortex amplifier to the two-phase supply flow and that the control flow characteristics remain unaffected by the nature of the supply flow. To provide an illustration of how a vortex amplifier might be used for controlling two-phase flow, its application as a separator control valve has been suggested.

Combustion of monodisperse droplet clouds in a reactive environment

In practical spray combustion, the behavior of an individual droplet can be significantly affected by the presence of neighboring droplets and by a reactive environment within the spray. Recently, several theoretical approaches to the problem have been presented. These results suggest that droplet spacing affects the combustion rate of the spray, and in some models a dependence of droplet lifetime and mode of burning on ambient oxygen content is predicted. In the present work, the combustion of monodisperse clouds of droplets in a hot reactive environment is studied experimentally with an emphasis on examining the role of the ambient oxidizer mass fraction, Y0∞, and droplet spacing on the droplet combustion rate. The experiments are conducted in a two-phase flow facility that produces an atmospheric-pressure, laminar, one-dimensional monodisperse hydrocarbon aerosol air flame. The droplet burning rates appear to be correlated by a D2 burning law, but the observed dependence on ambient oxidizer mass fraction is not predicted by a quasi-steady isolated droplet model. In addition, a significant reduction in the burning rate is observed with decreasing droplet spacing. The experimental results suggest that for low ambient oxidizer mass fractions fuel vapor accumulation occurs within the droplet cloud.