In this work, the performance characteristics and the flow field of a centrifugal compressor operating with supercritical CO2 are investigated by means of three-dimensional CFD. The considered geometry is based on main dimensions of the centrifugal compressor installed in the supercritical CO2 compression test-loop operated by Sandia National Laboratories. All numerical simulations are performed with a recently developed in-house hybrid CPU/GPU compressible CFD solver. Thermodynamic properties are computed through an efficient and accurate tabulation technique, the Spline-Based Table Look-Up Method (SBTL), particularly optimised for the applied density-based solution procedure. Numerical results are compared with available experimental data and accuracy as well as potentials in computational speedup of the solution method in combination with the SBTL are evaluated in the context of supercritical CO2 turbomachinery.
The Organic Rankine Cycle (ORC) represents an emerging technology aimed at exploiting lower temperature heat sources, like waste heat in industrial processes or exhaust heat in combustion engines. One key aspect of this technology is an efficient and economical operation at part load, typically realized by a partial admission control, which is challenging to predict numerically. Full annulus computation can only be avoided applying empirical partial admission loss models to conventional full-admission computations. This article aims at assessing the reliability of such a loss model under real-gas and supersonic conditions as a first step towards knowledge-based improved loss models. Three different operating points of an 18.3 kW ORC turbine working with an ethanol-water mixture with two open stator passages (2 x 36°) are considered. Full annulus CFD computations are compared to experimental data and results of simulations in a conventional, full admission, periodic 72°-sector model with application of a 1D partial admission loss model. The experimentally obtained mass flow rate and efficiency are matched overall within their measurements accuracy. By highest inlet total pressure, the computed efficiency deviates about 4 % from the experiments. Predictions of efficiency based on the full admission and loss model correction deviate from full annulus computations less than 1 %. These findings suggest that the used empirical correlations for partial admission losses can provide acceptable results in the configuration under investigation.
A GPU based 3D RANS multi-block slover with the capability of predicting heat transfer in fully resolved boundary layer flows based on the turbomachinery CFD code TBLOCK has been developed. A diabatic wall treatment and the scalar dissipation scheme have been integrated. The standard one-equation Spalart-Allmaras turbulence model with trip term is used for turbulence modeling. For validation, steady CFD results from the developed GPU version of TBLOCK are compared to a well-documented experiment of a highly loaded turbine cascade. The experiment covers a range of different Mach numbers and Reynolds numbers with static pressure and heat flux distributions on suction and pressure side of the blade. The predicted pressure and heat flux distribution show good agreement to the experimental data. A significant reduction of calculation time could be achieved compared to the authors’ CPU based MPI version of TBLOCK.
With the growing importance of regenerative power generation and especially of a hydrogen-based economy, the full potential of gas turbines of the smaller output class (< 10 MW) can be ideally exploited to provide peak coverage of the energy need whilst stabilising the electric grids in the mid- and low-voltage range. Such machines can be typically started in a relatively short time (similarly to aero engines) and are capable, at the same time, of delivering dispatchable power-on-demand. A safe, stable and profitable operation under highly unsteady conditions poses renewed challenges for an optimal thermal management (especially in the HP stages) as well as control and surveillance of the machines. The understanding and hence predictability of the propagation of the temperature inhomogeneities originating at the combustor outlet remains hence a primary objective of current research, as persistent distortion patterns could be adopted at the turbine exhaust as diagnostic indications of a malfunction of the combustor, for example. In the present study low-frequency disturbances introduced by a periodic load variation have been simulated and superimposed to the inhomogeneous, unsteady flow entering a 3-stage, high-pressure industrial gas turbine fed by a can-type combustion chamber comprising 6 silo-burners. The effects of the unsteadiness realized at the combustor exit have been investigated by means of Detached Eddy Simulations, whereby a density-based solution approach with detailed thermodynamics has been employed. The periodic disturbances at the turbine inlet have been obtained by means of an artificially generated, unsteady field, resulting from a two-dimensional snapshot of the flow field at the combustor exit. Also, a combustor failure has been mimicked by reducing (respectively increasing) the mean temperature in some of the turbine inlet regions corresponding to the outlet of two burners. The propagation and amplitude changes of temperature fluctuations have been analyzed in the frequency domain. Tracking of the temperature fluctuations’ maxima at the lowest frequencies revealed characteristic migration patterns indicating that the corresponding fluctuations persist with a non-negligible amplitude up to the last rows. A distinct footprint could also be observed at the same locations when a combustor failure was simulated, showing that, in principle, the early detection of combustor failures is indeed possible.
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