Direct numerical simulations of bubbly multiphase flows are used to find closure terms for a simple model of the average flow, using Neural Networks (NNs). The flow considered consists of several nearly spherical bubbles rising in a periodic domain where the initial vertical velocity and the average bubble density are homogeneous in two directions but non-uniform in one of the horizontal directions. After an initial transient motion the average void fraction and vertical velocity become approximately uniform. The NN is trained on a dataset from one simulation and then used to simulate the evolution of other initial conditions. Overall, the resulting model predicts the evolution of the various initial conditions reasonably well.
Data generated by direct numerical simulations (DNS) of bubbly up-flow in a periodic vertical channel is used to generate closure relationships for a simplified two-fluid model for the average flow. Nearly spherical bubbles, initially placed in a fully developed parabolic flow, are driven relatively quickly to the walls, where they increase drag and slowly reduce the flow rate. Once the flow rate has been decreased enough, some of the bubbles move back into the channel interior and the void fraction there approaches the value needed to balance the weight of the mixture and the imposed pressure gradient. A database is generated by averaging the DNS results over planes parallel to the walls, and a Model Averaging Neural Network (MANN) is used to find the relationships between unknown closure terms in a simple model equations for the average flow and the resolved variables. The closure relations are then tested, by following the evolution of different initial conditions, and it is found that the model predictions are in reasonably good agreement with DNS results.
Photovoltaic virtual synchronous generator (PV-VSG) technology, by way of simulating the external characteristics of a synchronous generator (SG), gives the PV energy integrated into the power grid through the power electronic equipment the characteristics of inertial response and active frequency response (FR)—this attracts much attention. Due to the high volatility and low adjustability of PV energy output, it does not have the characteristics of a prime mover (PM), so it must be equipped with energy storage systems (ESSs) in the DC or AC side to realize the PV-VSG technology. However, excessive reliance on ESSs will inevitably affect the application of VSG technology in practical PV power plants (PV-PPs). In view of this, this paper proposes the PV power reserve control type VSG (PV-PRC-VSG) control strategy. By reducing the active power output of part of the PV-PPs, the internal PV-PPs can maintain a part of the active power up/down-regulation ability in real time, instead of relying on external ESSs. By adjusting the active reserve power of this part, the output of the PV-PPs can be controlled within a certain range, and the PV-PPs can better simulate the PM characteristics and realize the FR of the grid by combining the VSG technology. At the same time, the factors affecting the reserve ratio are analyzed, and the position of the voltage operating point in PRC mode is deduced. Finally, the simulation results show that the proposed control strategy is effective and correct.
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