Four-stage low pressure model steam turbine tests are carried out under the low load conditions of 0% to 20% load. In such low load conditions, the reverse flow is generated from turbine exit. Steady pressure measurements using multi-hole pneumatic probes are made to specify the outer boundary of the reverse flow region. The reverse flow regions are determined from the flow angles measured by the multi-hole pneumatic probes, traversing in the radial direction which rotates 360 deg around the longitudinal axis. The outer boundary of the reverse flow regions varies depending on turbine loads and has good agreement with the results of the numerical analyses. The pressure fluctuations are measured using unsteady pressure transducers installed on both the inner and outer side walls of the outlet stage and on the next-stage stationary blade surfaces to investigate the relation between pressure fluctuation and volumetric flow. It is found that the pressure fluctuations, which are defined by the standard deviation of unsteady pressure, become larger with decreased volumetric flow at the outer side as well as the inner side which is the same as the tendency seen for blade dynamic stress characteristics. The authors have previously reported good agreement between the experimental and numerical results. The unsteady pressure probe as another measurement technique is employed to investigate the spanwise pressure fluctuations at the outlet of the moving blade. The results show that as the load decreases, large pressure fluctuations are observed in the vicinity of the outer side after the stages where the reverse flow is observed. This is the same tendency as the results of wall pressure measurements. The generation of large pressure fluctuations, detected by the two different measurement techniques, might have a relationship with the effects of not only the vortex motion in the reverse flow region but also the overall flow field (including main forward flow) oscillated by the multiple vortex motions in the reverse flow region as seen in both experiments and computations. The large pressure fluctuations in the vicinity of the outer side after the blade lead to the increase of exciting force and vibration stress on moving blades. Detailed aerodynamic investigations of these part-load conditions are needed to analyze a blade excitation for further improvement of reliability and availability of steam turbines. The complicated flow structures at low load conditions in a steam turbine can be understood with the aid of both the steady and unsteady flow measurements and calculations.
A computational technique for multistage steam turbines, which can allow for thermodynamic properties of steam, is presented. Conventional three-dimensional multistage calculations for unsteady flows have two main problems. One is the long computation time and the other is how to include the thermodynamic properties of steam. Ideal gas is assumed in most computational techniques for compressible flows. To shorten the computational time, a quasi-three-dimensional flow calculation technique is developed. In the analysis, conservation laws for compressible fluid in axisymmetric cylindrical coordinates are solved using a finite volume method based on an approximate Riemann solver. Blade forces are calculated from the camber and lean angles of blades with momentum equations. The axisymmetric assumption and the blade force model enable the effective calculation for multistage flows, even when the flow is strongly unsteady under off-design conditions. To take into account .steam properties including effects of the gas-liquid phase change and two-phase flow, a flux-splitting procedure of compressible flow is generalized for real fluid. Density and intemal energy per unit volume are selected as independent thermodynamic variables. Pressure and temperature in a superheated region or wetness mass fraction in a wet region are calculated by using a steam table. To improve computational efficiency, a discretized steam table matrix is made in which the density and specific intemal energy are independent variables. For accuracy and continuity of steam properties, the second order Taylor expansion and linear interpolation are introduced. The computed results of the last four-stage low-pressure steam turbine at low load conditions show that there is a reverse flow near the hub region of the last .stage bucket and the flow concentrates in the tip region due to the centrifugal force. At a very low load condition, the reverse flow region extends to the former stages and the unsteadiness of flow gets larger due to many vortices. Four-stage low-pressure steam turbine tests are also carried out at low load. The radial distributions of flow direction down.stream from each stage are measured by traversing pneumatic probes. Additionally, pressure transducers are installed in the side wall to measure unsteady pressure. The regions of reverse flow are compared between computations and experiments at different load conditions, and their agreement is good. Further, the computation can follow the trends of standard deviation of unsteadv pressure on the wail to volumetric flow rate of experiments.
The basic principle of a distinct idea to reduce an aerodynamic mixing loss induced by the difference in tangential velocity between mainstream flow and rotor shroud leakage flow is presented in “Part I – Design Concept and Typical Performance of a Swirl Breaker” The design concept offers an effective geometry for improving steam turbine stage efficiency. When the swirl breaker is installed in the circulating region of leakage flow at the rotor shroud exit cavity, the axial distance between the swirl breaker and rotor shroud is a crucial factor to trap the leakage flow into the swirl breaker cavity. In this Part II of the study, five cases of swirl breaker geometry with different axial distances between the swirl breaker and rotor shroud, which covered a range for the stage axial distance of actual high and intermediate (HIP) pressure steam turbines, were investigated using computational fluid dynamics (CFD) analysis and tests. Compared to a conventional single-stage CFD analysis, by conducting an additional single-rotor analysis with the modified shear stress transport (SST) model coefficient, the prediction accuracy for typical improvements in stage efficiency was increased in comparison to the single-stage analysis with the default SST model. Based on CFD results, the verification tests were conducted in a 1.5-stage air model turbine. By decreasing the axial distance between the swirl breaker and rotor shroud, the tangential velocity and the mixing region in the tip side which is influenced by the rotor shroud leakage flow were decreased and the stage efficiency was increased. The case of the shortest axial distance between the swirl breaker and rotor shroud increased turbine stage efficiency by 0.7% compared to the conventional cavity geometry. In addition, the unsteady pressure was measured in the swirl breaker cavity to evaluate the structural reliability of the swirl breaker. These results showed the maximum pressure fluctuation was only 0.7% of the entire flow pressure. Consequently, both performance characteristics and structural reliability of swirl breaker were verified for application to real steam turbines.
The exhaust hood performance of LP turbine plays an important role in the efficiency of steam turbine. By improving the exhaust performance, the kinetic energy of the last stage rotating blades can be converted to the potential energy and it becomes possible to improve the turbine efficiency. However, the flow field in the diffuser is closely related to the flow pattern of the last stage rotating blade, and the flow field inside the exhaust chamber afterward has a complicated three dimensional flow field. Therefore, in this study, it conducted a scaled model steam turbine test using two types of diffusers and CFD, and evaluated exhaust performance and flow pattern. The verification test was carried out using a test turbine (4 stages) of × 0.33 scale, the velocity field and the pressure field were evaluated by traverse and the wall pressure measurements. The corresponding CFD was calculated by ANSYS CFX. All four stages of blades and seals, exhaust chambers were accurately modeled. Due to the detailed CFD, the internal flow of the exhaust chamber exhibiting complicated three-dimensionality was visualized and the flow pattern was evaluated. The verification test results and the corresponding CFD results were compared and evaluated, and it has been found that the overall performance predicted by CFD is well showing the verification test result. Therefore, it has been found that CFD can help to understand the internal flow of the exhaust chamber exhibiting complex three-dimensional characteristics.
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