Separation Control And Applied Research In Unsteady Flow Of Low-speed Fan

With the development of national economy and the harsh energy and request of the environment, higher and higher performance requests, such as wider operating range, high efficiency, had been put forward to the design of low-speed fan. There was complex three-dimensional and unsteady viscous flow problems in low speed fan, so controlling the separating flow in the fan was one of the hotspots in the research. It would improve the internal flow structure and the aerodynamic performance by the modification technology for blade profile, and the reasonable match between the dynamic components and static ones also could help to control pressure gradient distribution in the channel, inhibit the development of separating flow and accumulate the low energy fluid.This article carried out some work about the separating flow at the end of the blades in semi open axial flow fan and forward curve centrifugal fan, while the retrofit design of blade trailing edge and tongue of multi-blade centrifugal fan were also considered. Besides, it explored the internal flow mechanism and the ways to improve performance with the method of numerical simulation. The main work and research results are as follows:1. In the γ-Reθ model, strain rate parameters of Reynolds number was introduced and it was help to improve the model based on the local variables. The solved intermittent factor was coupled into the energy equation of SST turbulence model and was used to the numerical calculation of separation flow on the surface of the airfoil with three different thickness. Based on the research on the front improvement and flow separation at low Reynolds number condition, the vortex variation at leading edge of NACA0006and flow separation at trailing-edge of NACA0012had been investigated, and the performance and the boundary-layer flow characteristic of three different front improvement approaches also had been analyzed.2. A new method, named leading edge section with convexity-preserving had been proposed for the optimization design of the axial fan blade based on the three dimensional approach. This new method adopted the Bezier surface and the Casteljau algorithm to draw the convexity-preserving surface.The inner flow field of the fan with convexity-preserving had been studied, and the effect of leading edge section on the flow separation also had been investigated. From the comparison of the areo-acoustic performance between the initial fan and the improved fan, it showed that the design of leading edge section with convexity-preserving can obviously promote the areo-acoustic performance of the fan.3. Based on the SST k-ω turbulence model and rotating inertial coordinate system, a new approach was proposed for studying the nonlinear turbulence flow with strong swirl and curved surface conditions.The CFD numerical method was applied to simulate the inner flow behavior of90°bend pipe, and the precious was also approved from the comparison between the simulation results and the experiment results. Using this improved turbulence model, the inner flow and the performance of forward curved blade fan with long and narrow channel was studied, and the vorticity distribution at leading edge was investigated. The result showed that the flow separation present a non-uniform phenomenon. Therefore, a improve treatment applied the inlet nozzle with eccentric installation to change the inflows distribution at leading edge was implemented. It showed that the method was benefit for the aero-acoustic performance.4. In view of velocity distribution at the outlet of low speed centrifugal impeller, the wake model was established. The dimensionless velocity and circumferential position factor was using to fit speed wake curve. Firstly, the potential problem of bionic simplify process was analyzed. It was regarded that the real structure at the trailing-edge of the blade should be considered in the bionic simplify process. The velocity distribution of modified fan at outlet became more uniform. It didn’t appear ups and downs distribution as the original fan. The pressure values of modified fan around the design flow rate was higher than the original ones, and there was about2%increasing of highest efficiency of modified fan.Secondly, the improved method was presented by combing the curve fitting technology on the tooth shape trailing edge of the blade. Moreover, the leading edge lines of owl wing was adopted for the design of the airfoil volute tongue with different angle. The noise test was implemented in a semi-anechoic chamber. The results showed that the optimization design would obviously promote the areo-acoustic performance of the fan. Compared with the initial fan, the noise of the new fan with bionic tongue decreases by2.0dB.