Study on unsteady flow collaborative characteristics of semi-open impeller and diffuser in a centrifugal compressor
Centrifugal compressor, as the significant equipment in the energy field, is of very complexity in its structure. Good collaboration of individual components determines the good aerodynamic performance and running security of the compressor. Unsteady simulations were performed, based on the platform of CFX and through the UDF interface program, to evaluate the 3D collaborative aerodynamic characteristics of a semi-open impeller and diffuser in a large centrifugal compressor, and the numerical study was focused on the impeller-diffuser interaction. The result obtained from flow structures shows the development of low-velocity vortexes in the diffuser flow passage is affected by impeller-diffuser interaction. The low-velocity vortex may increase flow resistance of the flow passage, however, improving at some extent the flow condition of the neighbor one, which generates a kind of low-frequency interference. In addition, the strongest interface effect is located on the rotor blade trailing edge and the diffuser vane leading edge. Amplitude-frequency characteristics converted from the pressure pulsation on monitor points indicate that the rotor wake flow and potential repercussion of diffuser work together to generate strong vibration and excitation force. The impeller-diffuser interaction is stronger under winter operating condition than it is under design condition. Meanwhile, the impact of diffuser exerts dominating effects on the excitation force near the impeller, which should be considered when exploring the impeller damage accident causes under winter operating condition.
A Simplified Model of Semi-Open Impeller Stage and Analysis of its Effects on the Transient Flow
The centrifugal compressor is one type of vital energy conversion equipment and its unsteady characteristics are extremely complex in actual operation. A semi-open impeller stage with inlet guide vanes, an impeller and a diffuser in a centrifugal compressor was concerned. For simulation of unsteady flow, the full-passage model of the integrate stage requires much more simulating time and memory space, higher computer configuration. Therefore, a single-passage simplified model was established for unsteady analysis. The internal flow characteristics and aerodynamic load on the blade obtained by the simplified model were also compared with that by the full-passage model. The result shows that the precision of the simplified model can meet the engineering requirement. Compared with the full-passage model, the simplified model can give a relatively true reflection of the local flow characteristics and the aerodynamic load on blade surfaces, but it ignores the unevenness resulted from unsteadiness along circumferential direction. Only high-frequency information is retained in aerodynamic load analysis while low-frequency one is diluted. However, as far as the local flow pattern or high-frequency information resulted from unsteady effects is concerned, the simplified model provides the advantages of higher computational efficiency and lower hardware requirements.
Simulation and control of hybrid renewable energy system connected to the grid
This paper presents a control and simulation of hybrid renewable energy system connected to the grid. The studied system includes a photovoltaic panel, synchronous permanent magnet generator based wind turbine, and a battery for storage energy. The PV array and wind systems are connected to the common DC bus by a boost converter. The battery is connected by a bi-directional DC/DC converter, and then integrated into the AC utility grid via a common DC/AC inverter. In order to extract the maximum energy, a simulation study has been carried out according to the meteorological conditions (wind speed and variable solar irradiance) while maintaining power quality at a satisfactory level. In order to capture the maximum power, a MPPT algorithm is applied for both wind turbine and photovoltaic panel. The modeling and simulation of the whole system has been performed under Matlab/Simulink environment. The obtained results show that the current and the voltage of grid side are sinusoidal and alternative forms, and the power injected to the grid is around the power produced by the hybrid system.