A multi-objective optimization of a coupled casing treatment for an axial transonic compressor is performed in this study. The coupled casing treatment is the basis axial slots with a circumferential groove located at various positions along the slots. During the optimization stage, five important parameters to control the geometry are used as the optimal variables. The stall margin and the peak efficiency are selected as the optimal objectives. Nondominated sorting genetic algorithm II coupled with radial basis function approximation is used to search for Pareto-optimal solutions. Then, four optimal configurations are selected from Pareto-front for further analysis. As shown in the simulation results with and without the coupled casing treatments, the leakage flow is reenergized and the blocking region near the blade leading edge at rotor tip is decreased by use of these structures under the low flow rate condition, which is the main reason for stability enhancement. Besides, a coupled casing treatment with the groove settled near the end of the basis slots have potential to generate more injection flow and extend the operating range of compressor further.
The unsteady characteristic in the tip region of an axial compressor has been numerically studied with the help of the dynamic mode decomposition analysis. The characteristics of frequency and dynamic modes are compared and discussed under different operating points and different parameters, such as tip clearance and rotating speeds. For the flowfield structures in the tip region, such as tip leakage flow, separation flow and shock wave, their relationships with the unsteadiness are studied in detail. Except for the unsteadiness caused by the interaction between rotating rotor and the stationary boundaries, it is found that the unsteadiness is attributed to the moving of the low-velocity cell. Based on the generation and the development of the low-velocity cell, the unsteady characteristics in tip region are divided into 4 types: BPF-dominated, shedding-dominated, self-induced and separation-dominated. When the tip leakage flow is weak, the unsteadiness in the tip region is only triggered by the blade sweeping. As the tip leakage flow gets stronger to a certain extent, the low-velocity cell is shed into the flow passage and mixed with the main-flow. When the main-flow is weaker under the low flowrate condition, the interaction between the low-velocity cell and the pressure side occurs and generates a new low-velocity cell near the leading-edge of the neighboring blade. The frequency of the new cell generation is actually the self-induced frequency. In the zero and small clearance model, the low-velocity is shed by the separation in the leading-edge and the casing-suction corner. By understanding these unsteady characteristics, the change tendency of the leading frequency in the rotor tip is easily explained and forecasted. Furthermore, under the transonic operation condition, the low-velocity cell is decelerated and eliminated by the shock wave in the unsteadiness of the self-induced type and the separation-dominated type, respectively. Thus, the leading frequency in the tip flow field is moderated.
The unsteady flow at small flow rates is always the most important of typical unsteady phenomena in centrifugal compressors, since it is closely related to the operating safety and efficiency. To study the mechanism of stall and surge generation, an experimental research on an industrial centrifugal compressor with variable vaned diffuser is carried out to study the unsteady flow structure from design point to surge. A multi-phase dynamic pressure measurement is conducted, based on 23 dynamic pressure sensors mounted on the shroud side casing surface of the compressor. The sensors are circumferentially distributed in a non-uniform manner at seven different radial positions, including the impeller region, the vaneless region and the diffuser throat region. Real-time data is recorded during the whole valve-adjusting process. The characteristics of pressure fields at some specific operating conditions are focused on, especially the pre-stall, stall precursor, stall and surge conditions. According to the multiphase data association, the originating position of the stall precursor can be determined. The features of the unsteady flow structure are also obtained, such as the surge pattern and the propagation direction of stall cells. In addition, when the diffuser vane setting angle (OGV) is turned up, the core factors to trigger total instability will change. In order to visually show how the tip leakage and separation vortex in the impeller gradually affect the flow structure in the vaned diffuser region and even the whole machine, numerical simulation and dynamic mode decomposition (DMD) method are used to study the flow mechanisms. The numerical simulation result is well matched with the experimental result. With the help of the DMD method, a few low-frequency tip leakage vortex structures are extracted from the unsteady numerical result over a period of time, which correlate with the experimental result. Meanwhile, on this issue, the feasibility of dynamic experimental analysis combined with multi-channel numerical simulation analysis is verified and discussed. Through the two analytic methods, a detailed understanding of the unsteady flow structure in the centrifugal compressor with variable vaned diffuser is obtained.
For the predicting of compressor noise, Lighthill analogy method based on numerical simulation has usually been adopted. Therefore, accuracy of the numerical simulation of the flowing filed is very important to the noise estimation. In this paper, nonlinear harmonic (NLH) method, which is one of promising numerical simulation methods used for the noise prediction, is compared with dual time step (DTS) method based on the calculating of the flowing field of one and a half stage compressor. It is found that the time consumption of NLH is actually decreased by about 70% compared with DTS. All the same, pressure fluctuation in the results obtained from NLH is more obvious than that of DTS in the upstream channel and downstream channel. Meanwhile, the two series of numerical simulation results are compared respectively with experiment data measured by dynamic pressure sensors. It is observed that the results from DTS is much closer to experimental data in both time domain and frequency domain. It is believed that the overmuch fluctuation found in NLH method is mainly caused by the contradiction between actual flowing pattern and periodical assumption made by NLH method initially. In addition, the pressure distribution around rotor blade surface, which is one of important tonal noise sources, is studied in detail as well.
Reliability of water-sealed conditions is crucial to the safety of water-sealed oil-storage. With a case study of underground water-sealed oil storage cavern in Huizhou, a reliability evaluation index system of water-sealed conditions is estabilished based on the main factors influencing water-sealed conditions. Also, a zonal evaluation of water-sealed conditions of the cave rock mass ranging from 0m to -70m is made by Fuzzy Analytical Hierarchy Process. Moreover, a three-dimensional numerical seepage model is established to study the seepage laws of the groundwater before and after the cavern excavation, and the water inflow during excavation and operation can be forecasted too. The results prove that the cavern site is suitable for the construction of large-scale underground oil storage caverns for its good water-sealed conditions and high reliability.
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