Stratified flows occur if at least two fluids with different densities move together in horizontal or slightly inclined channels. In this case both fluids may separate due to the gravity and interfacial areas appear. Depends on the flow velocities and also further parameters like surface tension and viscosity, these areas are more or less stable. In this way various flow regimes develop like e.g. wavy or slug flow.
Slug flow in the HZDR test channel HAWAC
The investigation of such flows is very important for the evaluation of the mass- and energy transfer in technical devices and units. For instance the size of the interfacial area influences the reaction rate in chemical reactors considerably. A further example is the safety-related design or assessment of piping systems in the power plant technology or oil industry. Here it is essential to avoid critical phenomena like water hammer loads, to protect the piping systems. These phenomena may occur, if slug flows interact with pipe walls.
In consideration of the state-of-the-art of science and technology it is aimed to simulate flow phenomena in complex geometries in a realistic way by powerful computers. Especially in the field of two-phase flows the computer codes need additional improvement. For the optimization of the thermal-hydraulic algorithms and models applied in these codes top-quality measurement data with high spatial and temporal resolution are necessary.
Experimental activities and results
The department of Experimental Thermal Fluid Dynamics at the Helmholtz-Zentrum Dresden-Rossendorf is investigated stratified flows by experimental and theoretical works already for many years. Thereby the first step was the design and operation of a transparent horizontal channel with rectangular cross section (HAWAC) that was applied first of all for the analysis of flow regimes in a wide range of gas and water superficial velocities. These works resulted in a flow map for rectangular channels. Furthermore phenomena like hydraulic jump were investigated in detail and local water velocity distributions in slugs were measured by PIV (particle image velocimetry). The HAWAC channel was operated with air water co-current flows at ambient pressure.
The continuation of these activities took place in a multi-functional test basin (DENISE) that also was used for co-current stratified flow investigation but with a steam water mixture. Beside the variation of phase velocities also the pressure was changed up to 5 MPa and the water was sub-cooled in a range between 0 and 150 K. In opposite to the HAWAC experiments energy transfer between the phases occurred during the steam water tests, so that condensation effects were observed. After data evaluation information about flow regimes and condensation rates are available in a wide parameter range.
Visualization of steam water co-current flow in the DENISE test basin at a pressure of 2.5 MPa and 50 K water sub-cooling; flow moves from the left to the right side with variable gas velocities: top: 2.2 m/s; middle: 3.6 m/s and bottom: 5.3 m/s
As in the HAWAC channel as in the DENISE basin undisturbed flows were analyzed. The next step was the investigation of stratified flows in complex geometries. For this a hot leg test rig was designed and erected that was applied for co- and counter current flows. This test rig modelled a part of the main circulation pipe, the “hot leg”, and the following inlet chamber of a vertical steam generator of a Pressurized Water Reactor (Nuclear Power Plant).
In the hot leg test rig as co- as counter-current flows were observed with air/water and steam/water mixtures at pressures up to 5 MPa. The evaluated data and picture sequences of the flow pattern are available for code validation. Furthermore an extensive test series was carried out for the investigation of the partial and complete counter-current flow limitation. These activities resulted in high-quality flooding characteristics that show significant dependency on the pressure. In addition evaluated data of slug frequencies are available.
Another example for the investigation of stratified flows in the frame of nuclear safety research was the analysis of mixing processes of cold emergency cooling water with hot coolant inside the main circulation pipe of a Pressurized Water Reactor that aimed at the risk estimation of the occurrence of material defects at the pressure vessel by thermal stress (Pressurized Thermal Shock or PTS). Here also stratified flow may occur in the main circulation pipe, in which the density differences arise by different temperatures of the same fluid (water). These measurements resulted in temperature and velocity profiles inside the main circulation pipe as well as temperature distributions at its surface. In addition the flow regime of the injected emergency cooling water was recorded by a high-speed camera.
Beside these works condensation experiments in a slightly inclined tube (COSMEA) were conducted in the tomography laboratory of the TOPFLOW facility. Also in this case a stratified flow may develop, if the condensation rate is high enough. Here the water level was determined by a conventional X-ray system in a noninvasive way. Furthermore the azimuthal-distributed heat flux was calculated. These data allow a significant improvement of condensation models applied in the computer codes.
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Counter-current flow limitation for air-water and steam-water flows in a PWR hot leg geometry
Nuclear Engineering and Design,(2017), eingereicht
- Apanasevich, P.; Lucas, D.; Beyer, M.; Szalinski, L.
CFD based approach for modeling direct contact condensation heat transfer in two-phase turbulent stratified flows
International Journal of Thermal Sciences 95(2015), pp. 123-135
- Höhne, T.; Geissler, T.; Bieberle, A.; Hampel, U.
Numerical modeling of a horizontal annular flow experiment using a droplet entrainment model
Annals of Nuclear Energy 77(2015), pp. 351-360
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Experimental CFD grade data for stratified two-phase flows
Nuclear Engineering and Design, Vol. 240/9 (2010), pp. 2347-2356
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Air/water counter-current flow experiments in a model of the hot leg of a pressurized water reactor
Journal of Engineering for Gas Turbines and Power - Trans. of the ASME, Vol. 131/2 (2009), Article 022905
- Vallée, C.; Höhne, T.; Prasser, H.-M.; Sühnel, T.
Experimental investigation and CFD simulation of horizontal stratified two-phase flow phenomena
Nuclear Engineering and Design, Vol. 238/3 (2008), pp. 637-646
This work is based on research projects funded by the German Federal Ministry of Economics and Energy, support codes: 150 1265, 150 1329 und 150 1411. The authors assume the responsibility for the content.