Gas liquid flows in vertical pipes
For many years the Institute of Fluid Dynamics makes significant contributions to the development and optimization of computer codes for advanced flow simulation. To organize these works in an effective way the institute networks with some software developer companies in the frame of cooperation agreements, so for example with ANSYS CFX and the Gesellschaft für Anlagen- und Reaktorsicherheit. In dependence on the complexity of the flow as one- as three-dimensional codes were applied.
Both codes output good results for single-phase flow. But if the requirements increase and two-phase flows even in complex geometries have to be simulated, the quality of flow prediction needs improvement. To enhance this situation, measurement data in high spatial and temporal resolution are required, which allow the development and optimization of models for specific flow phenomena and its validation over a wide parameter range (e.g. pressure, temperature, superficial velocities).
The investigation of two-phase flows in vertical pipes has a high priority for code development and validation. For instance rotation-symmetric flows develop in smooth vertical pipes parallel to the gravitational vector. This kind of flows may be analyzed under consideration of a limited number of flow phenomena, so that the remained effects can be examined systematically and improved individually, that simplify the flow simulation in general.
Experimental work and selected results
In the department of Experimental Thermal Fluid Dynamics the basic investigation of two-phase flow started with the design and erection of the test rig MTLoop in 1995. It was a flow-loop with a useable vertical test section of a length of 5.7 m and a nominal diameter of DN50 that was operated primarily with air water mixtures. A second task of MTLoop was the test of flow measurement technique, such as wire mesh sensors or needle probs. The MTLoop data resulted in qualitative information about flow regimes as well as in profiles of void fraction and gas velocity and in bubble size distributions.
Based on the experience of MTLoop the experimental possibilities of the department were extended by the assembling of the TOPFLOW test facility that was put into operation in 2002. Beside a vertical DN50 pipe the test section loop of TOPFLOW offers a vertical DN200 tube, so that the influence of the pipe diameter on the flow structure could be analyzed. Furthermore the useable length of the vertical pipes was extended to app. 8 m and the operational pressure was increased up to 7 MPa. Beside these improvements TOPFLOW can supply significant higher superficial velocities of air, steam and water, so that the investigation of annular flows is possible even in the thick test section. Just as on the MTLoop facility also in the vertical pipes of TOPFLOW advanced wire mesh sensors are applied to record the void fraction in high spatial and temporal resolution.
Flow visualization of wire mesh sensor data, left hand side DN50 pipe, right hand side DN200 tube; jw: 1 m/s, jg is increased from the left picture to the right one from 0.04 m/s to 1.3 m/s; developed upward flow
The above figure shows very clearly different flow pattern. At equal boundary condition Taylor bubbles develop in the DN50 pipe, whereat in the DN200 tube a churn turbulent flow is visible.
Beside the identification of flow regimes as function of the test condition also diverse void fraction, gas velocity and bubble size distribution may be used for code development. First of all in a special-designed DN200 test section many tests to the flow evolution were carried out started at a level directly after the mixture formation up to the maximal available inlet length. By means of these data many information about bubble forces was revealed. Amongst other the correlation for the Tomyama lift force was confirmed for two-phase flows at parameters relevant to industrial processes.
Also in the region of non-adiabatic flows unique data were measured. So steam water flows were investigated at pressures up to 6.5 MPa and water sub-cooling of maximal 17 K. Beside the analysis of flow regimes these data revealed important knowledge about the condensation behavior in sub-cooled flows that is essentially for the validation of heat transfer and condensation models. Furthermore two types of pressure release experiments successfully were completed in the test section loop of the TOPFLOW facility. On the one hand stationary saturated water was released through a defined annulus with void fraction measurement. Otherwise circulating saturated water was released at defined pressure drops. These tests provided important data for validation and improvement of the evaporation models.Parallel to the experimental activities the development and design of innovative measurement technique for two-phase flows were continued and finally resulted in the successful implementation of an ultra-fast X-ray tomography system for noninvasive two-phase measurements on the TOPFLOW facility in 2010. To operate the X-ray tomograph under optimal condition, a new DN50 vertical test section was designed from Titanium and installed on the TOPFLOW test section loop. Using the X-ray scanner many upward wire mesh sensor tests were repeated. These measurements result in more accurate data that gives the possibility to check the quality of the wire mesh sensors.
Parallel to the experimental activities the development and design of innovative measurement technique for two-phase flows were continued and finally resulted in the successful implementation of an ultra-fast X-ray tomography system for noninvasive two-phase measurements on the TOPFLOW facility in 2010. To operate the X-ray tomograph under optimal condition, a new DN50 vertical test section was designed from Titanium and installed on the TOPFLOW test section loop. Using the X-ray scanner many upward wire mesh sensor tests were repeated. These measurements result in more accurate data that gives the possibility to check the quality of the wire mesh sensors.
Visualization of interfacial areas on the basis of tomography data; jw: 1 m/s, jg is increased from the left to the right from 0,004 m/s up to 3.2 m/s, developed upward flow
A significant advantage of the noninvasive X-ray scanner is that beside upward flow now for the first time also downward and even counter-current flows can be investigated. In these cases the properties of some bubble forces are changing so that these data open new possibilities for code validation. Beside void fraction and gas velocity profiles as well as bubble size distributions now also information about the interfacial area may be provided. Additional the tomographic data allow the estimation of single bubble velocities.
The current progress of the two-phase flow simulation in vertical pipes is presented at this page.
- M. Banowski, Prof. U. Hampel, E. Krepper, M. Beyer, D. Lucas (2018).
Experimental Investigation of Two-Phase Pipe Flow with Ultrafast X-ray Tomography and comparison with state-of-the-art CFD simulation.
Nuclear Engineering and Design, in press.
- M. Banowski, M. Beyer, L. Szalinski, D. Lucas, U. Hampel (2017).
Comparative study of ultrafast X-ray tomography and wire-mesh sensors for vertical gas-liquid pipe flows.
Flow Measurement and Instrumentation 53, 95-106.
- M. Banowski, D. Lucas, L. Szalinski (2015).
A new algorithm for segmentation of ultrafast X-ray tomographed gas-liquid flows.
International Journal of Thermal Sciences 90, 311-322.
- N. K. Omebere-Iyari, B. J. Azzopardi, D. Lucas, M. Beyer, H.-M. Prasser (2008).
Gas/liquid flow in large risers.
International Journal of Multiphase Flow 34, 461-476.
- H.-M. Prasser (2007).
Evolution of interfacial area concentration in a vertical air-water flow measured by wire-mesh sensors.
Nuclear Engineering and Design 237, 1608-1617.
- H.-M. Prasser, M. Beyer, A. Böttger, H. Carl, D. Lucas, A. Schaffrath, P. Schütz, F.-P. Weiß, J. Zschau (2005).
Influence of the pipe diameter on the structure of the gas/liquid interface in a vertical two-phase pipe flow.
Nuclear Technology 152, 3-22.
This work is based on a research project funded by the German Federal Ministry of Economics and Energy, support codes: 150 1215, 150 1265, 150 1329, 150 1411. The authors assume the responsibility for the content.