Jun Hu’s research while affiliated with Nanjing University of Aeronautics and Astronautics and other places

A quantitative model to predict the boundary of instability of axial compressors based on their maximum loading capability is proposed in this paper, which is an improved version of the classic method of stalling pressure rise. The original model correlates the maximum pressure rise of a compressor to a characteristic geometric parameter, which is an analogue of the normalized length of diffusion in two-dimensional diffusers. However, the influence of the aspect ratio of the passage is overlooked in this analogy, which leads to significant discrepancies in its predictions for compressors, especially those with varying blade aspect ratios. Our model contains two improvements to address this issue. The first involves refining in the definition of the normalized length of diffusion, whereas the second introduces a supplementary correction factor for the aspect ratio of the blades. Nearly 20 low-speed compressor configurations, with variations in solidity, aspect ratio, tip clearance, and axial spacing, were tested to develop the proposed model. It can reduce error in the predicted stalling static pressure rise from 10% to 5%. Experimental data robustly verify the accuracy of our model, making it a more reliable predictive tool of instability boundary in the preliminary design of axial compressors.

In this paper, a method for simulating the instability transient process of the axial compression system based on the body-force model is developed, and a corresponding simulation program is developed. Simulations of the transient process of instability were carried out on a high-speed four-stage compressor and compared with experimental data. At 50% of the design rotational speed, the type of instability was rotating stall, and the simulated and experimental stall cell propagation speed were very close to each other. At 70% of the design rotational speed, the type of instability was surge. A “surge loop” was simulated, and the surge period and the percentage of time spent in each phase were consistent with the experiments. The simulation successfully predicted the blockage in the surge re-pressurization phase, proving the reliability of the simulation results. In addition, the computation yields more information about the flow field. By summing the blade forces of all grids on a blade row by volume, the surge loadings are obtained. The analysis of the axial momentum equation shows that the obtained blade force variations are reasonable. The simulation time of the multistage axial compressor is greatly reduced compared to the full annulus three-dimensional unsteady Reynolds-averaged Navier–Stokes method, demonstrating its great advantage in the design phase of the compressor.

The complete operating range of the compressor includes stable and unstable conditions, and the boundary between the two is the stall and surge line. Previously, the two were modeled in isolation from each other. The unsteady flow equation of the compression system is constructed based on the idea of the body-force model. It is possible to obtain the steady characteristics and transient simulation of post-stall, realizing the unified simulation of the full working conditions of the compression system. Among them, by using and developing the elementary cascade method, the continuity of correlation method for the body-force source term in different flow regions is realized. The model was validated on four single-stage compressors as well as one two-stage compressor, and the results showed that the steady-state characteristics, stable boundary, and rotating stall characteristics obtained by the developed model were in good agreement with the experimental results. The experiments showed that when the blade solidity was increased, the compressor's work capacity increased and its stability improved, and the rotational frequency of the stall cell increased slightly. The calculated results always agreed with the trend of the experimental results. In addition, the variation of blade forces during rotating stall can be easily obtained using the concept of body force and more details of the flow field can be obtained using the model.

To make measurement of end-wall flow between blade rows in a compact multistage configuration possible, a miniature L-shaped five-hole probe was employed in this paper. This compact tip structure, realized by laser-printing instead of the conventional machining technique, reduces the blockage effect of this intrusive measurement on the flow and ensures high spatial resolution. The zonal method is introduced to extend the usable flow angle range up to 60 degrees. A local least-squares interpolation technique is utilized to acquire flow angle and static/total pressure. In order to improve accuracy for the points located at the sector boundary, the overlap region method is included in the interpolation. Additional test data indicate that the maximum error in flow angle is nearly within 1 degree, and the maximum errors of total pressure and static pressure are 0.56% and 1.9% respectively. The application in a low-speed multistage axial compressor indicates that the zonal method can decrease the number of points exceeding the measurable flow range and is of great significance for the end-wall flow measurement, especially for the near-stall condition. Compared with the traditional method, the proportion of available data for the near-stall state measurement was increased by 18% by using the zonal method.

Despite the extensive application of three-dimensional Reynolds-averaged Navier-Stokes equation (RANS) in axial compressor numerical simulations, body-force model (BFM) also plays its own role profiting from its low computation cost. However, the computation accuracy highly depends on the modeling of blade force, which usually involves several parameter constants. In this work, data assimilation based on Ensemble Kalman Filter (EnKF) was employed to optimize these model constants in BFM. Previous work associated with data assimilation mainly focus on employing only one data source. Considering the various measurement quantities in engineering practice, disparate data were incorporated in assimilation method to improve the prediction. The test case of a low-speed axial compressor was provided. Only one single data source, i.e., total pressure ratio, was first employed as the observation data in EnKF. And to reveal the superiority of the disparate data assimilation, total pressure ratio and isentropic efficiency were then incorporated to improve the performance prediction. The converged results reveal the robustness of disparate data assimilation based on EnKF. At last, the optimized constants were adopted to predict the performance of the axial compressor at another rotational speed for further verification and application. The results showed that errors comparing with the experimental data are nearly within 2.5%.

Shock wave–turbulent boundary layer interaction (SWTBLI) exists widely in the internal and external flow fields of supersonic and hypersonic vehicles, and has a significant influence on the aerodynamic performance. Separation scaling analysis for SWTBLIs has been pursued by researchers for decades. This paper took into consideration the downstream expansion fan to present a generalized scaling analysis of the separation zones induced by three types of interactions—incident SWTBLIs, compression corner SWTBLIs, and normal SWTBLIs. To establish this scaling correlation, SWTBLI flow-field data under the condition of free-stream Ma0 of 3.5 and Rδ of 2.34×105 were simulated numerically. The influence of the expansion fan on the wall pressure distributions and the scaling of the separation zone was analyzed in detail. The size of the separation zone was found to be related closely to the pressure increase at the reattachment point. The scaling of the separation zone and the reattachment pressure increase are rendered dimensionless by the displacement thickness of the incoming boundary layer and the dynamic pressure of the incoming free stream, respectively. The generalized scaling of the separation zone was found to follow a simple power function against the reattachment pressure coefficient. This generalized scaling correlation fits a wide range of published experimental data and numerical simulation results.

The inlet swirl distortion and non-uniform tip clearance have great effects on aero-engine performance and stall margin. In this paper, the effects of paired swirl distortion on the aerodynamic stability and stall inception of a single stage axial compressor with non-uniform tip clearance are quantitatively analyzed by using the swirl distortion descriptors. The experimental results show that the paired swirl distortion dominated by co-rotating swirl improves the stability of the axial compressor. For a single-stage axial compressor with eccentricity of 100%, the stall inception starts at the maximum tip clearance with clean inlet. The initial position of the stall inception is determined by the maximum tip clearance when the small intensity paired swirl distortion exists at the compressor inlet. As the swirl intensity increases, it shifts towards the position of the counter rotating swirl vortex core. The inlet swirl will not change the type of stall inception.

The onset of rotating stall and surge in compressors limits the operating range of aero-engines. Accurately predicting the key features during these events is critical in the engine design process. In this paper, a three-dimensional computational model for transient simulation of multi-stage axial compressors during stall is proposed. The kinetic equations describing the dynamic process of the compression system are constructed, with a 3D through-flow model for the compression part and a 1D gas collector model for the outlet part. The calculation of the source term is performed using the developed body-force model, which realizes the correlation between the deviation angle and the loss coefficient with the inlet parameters in various flow regions. Validated on a single-stage compressor and a single-rotor fan, the results show that the method is capable of capturing the stall and surge features correctly and that the three-dimensional structure of the stall cell can be captured. In addition, this model could be used for the analysis of the surge load, which is significant for the structural integrity of the compressor.