Open and closed annular two phase gas-liquid flow in horizontal and slightly inclined pipes
Dr.-Ing. M. Creutz
In cooperation with Tel-Aviv University, Prof. D. Barnea and Prof. Y. Taitel
Toady's parabolic trough solar power plants are based on a conventional Rankine steam cycle which is heated via a primary oil loop by the concentrated solar radiation. In order to improve the efficiency, to reduce the parasitic energy consumption and to reduce the investment costs it is planned to avoid the oil loop. The high pressure steam required to drive the turbine/generator unit shall be produced in the absorber tubes of the solar field directly (DSG: direct steam generation).
Flow patterns of the evaporating, high pressure water/steam two-phase flow in the absorber tubes are of special interest since the tubes will be mounted horizontally or slightly inclined and, therefore, flow stratification may occur due to the influence of gravity together with low flow velocities. Temperature differences around the circumference of the pipe resulting from large differences of the heat transfer coefficients in the case of stratified flow may cause excessive thermal stress in the pipe material and/or bending of the pipe. Measurements at different pipes show that these temperature differences may be larger than 100 K (which is the safety switch off value for the test facility) depending on flow rates, pipe diameter and heat flux. Moreover, the efficiency of the absorber pipe decreases with increasing temperature due to radiative heat losses of the pipe.
Based on adiabatic water/air experience, flow stratification and corresponding poor and uneven heat transfer may be avoided by a slight inclination of the pipe which is set to approximately 8° as a result of an economic optimization. In figure 1 the transitions between annular, stratified and intermittent flow is shown schematically as a function of the superficial liquid and gas velocities for a pipe with an 8 degree. upward inclination.
The boundary between annular and stratified flow dryout is basically connected with the formation of annular flow. The fundamental question in horizontal and near horizontal annular flow is the reason for the liquid being forced towards the top of the pipe against the gravity. Several investigations deal with this problem and a large variety of "supply and spreading mechanisms" was found or postulated. Most experimental investigations on this problem are carried out in horizontal adiabatic gas/water flow. By performing measurements on the three dimensional structure of the film, informations about these supply and spreading mechanisms are obtained. Transient multi-electrode conductance measurements are being carried out. The data processing is based on tomography and two dimensional fourier transformation.
 M. Creutz, Y. Taitel, D. Barnea, M. Müller and D. Mewes, The Transition Region between Intermittent, Stratified and Annular Two-Phase Flow in Upward Inclined Pipes, Proceedings of the European Two-Phase Flow Group Meeting, Den Bosh, The Netherlands, 1995