Steady flow in axial one-stage turbines is assessed numerically and experimentally. The simulations are performed on coarse meshes using a standard numerical approach (3D, steady state, kε-turbulence model, wall function at solid boundaries). In order to allow for conclusions drawn from these rapid numerical studies, the approach was compared with an explicit LDA (Laser Doppler anemometry) mapping of the velocity field downstream the rotor on a representative turbine stage. A two-component LDA system allowed for measurements of axial and tangential velocity components at varying depth (radius) in the flow channel, Measurements thus correspond to a full plane at constant axial position in the rotating frame of reference of the rotor. Comparison between LDA velocity mapping and CFD results shows good agreement. A series of subsequent simulations is thus used to judge the impact of varied blade/stage design parameters. Two turbine layouts are defined for identical operating conditions and shaft power. The flow in the unshrouded rotor blade row is analyzed for the influence of varying tip clearance size and the dependency on stage velocity triangles. – Known correlations for tip clearance losses (typically used in mean line predictions) are used, though the blade row geometry considered is beyond the limits the correlations are intended for. The absolute loss level found in CFD simulations differs significantly from what is expected when using loss correlations. Still the variation with tip gap size is predicted well by some of the investigated models. As dependency of tip clearance losses on stage velocity triangles is considered, none of the tested correlations gives results consistent with the numerical simulations. The use of standard correlations ‘beyond the limits’ is thus considered to introduce high uncertainty. Due to the good consistency between LDA and numerical results, the conclusions are considered to be valid for stage designs similar to the ones analyzed.
Temperature inhomogeneities and their influence on proton exchange membrane (PEM) water electrolyzer performance of industrial-scale stacks were investigated. Three temperature differences were examined: between the inlet and outlet, in-between the cells of a stack, and between the cell’s solid materials and the fluids. A validated stack model for temperature and performance is presented which is used to quantify the above-mentioned temperature fields and their influence on current density distribution and cell voltages. For a chosen scenario, with current densities of 2.0 A cm-2, fluid inlet temperatures of 60°C and flow-rates of 0.15 kg s-1m-2, peak temperature differences amount to 8.2 K along-the-channel. This relates to inhomogeneities of current density of up to 10% inside a cell and deviations of cell voltage of 9 mV in-between cells in the center of the stack and outer cells. For higher current densities these differences increased further. More homogeneous temperatures allow operation at elevated average temperatures without exceeding temperature limitations and reduce the spread of degradation mechanisms. Hence, homogenous profiles lead to a more hole-some utilization of electrolysis stacks. Therefore, the ability to homogenize via alternative operation such as higher flow-rate, higher pressure and altered routing of fluid-flow is analyzed.
An upgrade of the lean premixed combustion system installed in the SGT5-8000H in Irsching/Germany was developed for the 50 Hz and 60 Hz versions of the SGTX-8000H gas turbines. It features lower CO and NOx emissions by improving combustion aerodynamics and reduction of the air consumption of the combustion system. Furthermore an improved secondary air managing system increases the amount of air, which can be supplied in a controllable way to the turbine in part load operation and, thus, increases the combustor temperature. This is done in stepwise increasing the air mass flow to the turbine by feeding compressor exit air to different distinct turbine stages. All in all this system extends the turn down capability beyond the level achievable by the new combustion system alone. The new combustion system and the secondary air managing system were installed in full scale and tested in the SGT6-8000H test facility of the Siemens Gas turbine plant in Berlin. The results have subsequently successfully been validated in the first commercial application on a customer site. This paper presents the technical features of the systems, the development program and the test results.
Introduction Large-scale water electrolysis is needed for scaling up the production of hydrogen as a renewable energy carrier. The required size and number of cells of the PEMWE stack exceed laboratory-scale settings, which leads to additional effects that could impact the operation and performance [1]. The interaction of large, neighbouring cells with each other are expected to lead to significant temperature differences within the stack of cells, which cause deviations in the contribution to the loss mechanisms and uneven stresses or loading of the members. Methods This contribution presents a dynamic PEMWE-stack model, which is formulated in two dimensions: through the stack and along the channel. Due to the high electric conductivity of the bipolar plates the cell voltage of each cell is independent of the cell/channel flow coordinate. The model is based on mass and enthalpy balances of the fluids on the anodic and cathodic side. Conductive heat transport along the cell and through the stack is considered as well as convective transport by the fluids. Time-dependency of mass and enthalpy balances is considered to capture unsteady operation of the stack. In addition, a temperature-dependent polarisation model for determining the cell voltage is included in the model. The polarisation model includes the contributions of activation overpotentials, mass transport losses and ionic membrane and contact resistances. The cells on both edges of the stack are modelled with dedicated boundary conditions to capture the physical arrangement and convective heat exchange with the environment. Water flow and electrical settings are prescribed as external boundary condition. The model is validated with experimental data from industrial stacks. Results The temperature profile inside the stack is found unevenly distributed in both spatial directions between the cells with colder boundary and warmer inner cells. The colder outer cells operate at a higher cell voltage in an electrical series connection. For the inner cells, a significant temperature gradient is found in flow direction. As the polarisation loss mechanisms depend on temperature, the share of the losses is unevenly distributed in both coordinates. This leads to higher current densities at the parts of a cell that experience elevated temperatures (Fig. 1). These mechanisms depend on the macroscopic design of the cell, boundary and operating conditions. Load variations induce a dynamic stack response in temperature, cell voltage and the fluids until a steady state is reached. At the same time local temperature and current density variation, can be analyzed as performance drivers, including degradation. Both aspects can be used for design and optimization of industrial scale PEMWE stacks. References [1] S. Siracusano, V. Baglio, N. Briguglio, G. Brunaccini, A. Di Blasi, A. Stassi, R. Ornelas, E. Trifoni, V. Antonucci and A. S. Aricò, International Journal of Hydrogen Energy, 37(2), 1939–1946 (2012). Figure 1
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