Experimental And Numerical Investigation On Stall Flow Phenomenon In The Centrifugal Compressor With Vaneless Diffuser

Stall and surge in the centrifugal compressor are the ordinary unsteady flow phenomena, which limit compressor steady operation range, impact operation reliability and obstacle stages matching during centrifugal compressor designing and adjusting. Even those can lead to a terrible accident at non-design operation conditions. To increase centrifugal compressor designing efficiency, extent steady operation range and improve operating safety and reliability, it is very important to investigate and master the unsteady flow mechanism in the centrifugal compressor.Both experiment and numerical simulation are main methods to investigate flow fields in the Turbomachinery. With the development of the science and technology, more advanced measurement instruments have been used to measure the flow fields, such as hot wire and particle image velocimeter (PIV); Simultaneously, with the development of computer and computation flow dynamics, some CFD software have been used to numerically simulate the flow fields inside Turbomachinery.In this thesis, unsteady flow phenomena in the centrifugal compressor at small mass flow rates are investigated by both experimental measurement and numerical simulation. Main contents and conclusions of the research are described in the following:1. The experiment facility was designed and installed based on Chinese National Standard GB1236-2000 (Test Methods of Aerodynamics Performance for Industrial Fan). The aerodynamics experiments of the low speed centrifugal compressor were carried out. The performance curves were acquired and the stall operation conditions were confirmed.2. The detailed flow parameters of the low-speed centrifugal compressor at small flow rates were successfully acquired using high-frequency dynamic pressure transducer. Based on the above experimental data, there is a stall cell rotating circumferential in the low-speed centrifugal compressor. The rotating speed is fifty-two percents of the blade rotating speed.3. The detailed internal 2D flow fields of the low-speed centrifugal compressor at small flow rates were successfully acquired using PIV. Relative velocity vector and radial velocity contour near the hub at four different time moments are used to present the flow fields in one stall period. Based on Phase-locked averaging PIV experimental results, there are flow separation regions near the suction side at all small flow rates, which forms whole flow separation vortex. There are two separate regions respectively locating near the hub inlet and the shroud outlet, based on the first measured flow fields on the meridional plane of the vaneless diffuser. The flow structure on the meridional plane of the vaneless diffuser presents a similar"S"shape.4. With a plenum chamber model added at the vaneless diffuser outlet, numerical simulations of stall flow phenomenon inside the low-speed centrifugal compressor were carried on the distributed parallel computational hardware platform by solving three-dimension Reynolds averaged compressible N-S equations using commerce CFD software CFX adopting RNG k ?εturbulence model Though there is static pressure fluctuation difference between the experimental and the calculation, the stall frequencies fairly agree each other through DFT analysis. The separate vortex structure is similar between the experimental measurement and CFD result. These validate the CFD reliability.5. Based on the above numerical method, the stall flow phenomenon inside a turbocharger centrifugal compressor with vaneless diffuser provided by Honeywell Inc. was numerically simulated in detail at small flow rates, with or without volute. It is found that the volute plays an important role in the compressor stage stall. The volute acts like a tuned pipe attenuating flow unsteadiness into a standing wave within the volute. The stall frequency from the numerical result with volute is far less than that from the numerical result without volute. During simulation with volute, the wavelength of pressure and mass flow oscillations are about four times the volute circumferential length.6. During the simulations with/without volute, it is found that the flow fields in the vaneless space and vaneless diffuser are distinctly influenced by the flow separation in the rotor flow passages. There are the obvious low velocity flow regions in the vaneless diffuser during stall period. As the flow rate decreasing, there is the radial reverse flow region at the inlet near the diffuser shroud. However, there is the radial reverse flow region at the outlet near the diffuser hub, only in the simulation results with the volute. The reverse flow region at the inlet near the diffuser shroud can be attributed to the second flow in the flow passages, but that at the outlet near the diffuser hub can be attributed to the reentering flow from the volute. 7. During the stall period, the flow structure in all volute cross sections consists of a single forced vortex at all times. The flow loss is rather great in the center of the forced vortex, which lead to the low total pressure. Because of the distinct reducing of the swirl velocity and the instability of the forced vortex itself, the vortex at the volute discharge section breaks down easily, inducing the reverse flow at the center of volute discharge section. This reverse flow is the important character of the compressor stage stall. The vortex core at the cross section center breaks down into counter rotating pairs under the stall condition, which is the likely energy source that sustains the standing wave in the volute when the volute is in stall.8. Compared the frequency analysis result at the 0.7kg/s with that at 1.1kg/s, besides stall frequency, there are the impeller rotational frequency (736.4Hz) and the sum of the volute stall frequency and impeller rotational frequency. It seems that the impeller is now starting to play important role in deciding the stage stall and the stage is forced to resonate at its frequency. The occurrence of the frequency equal to the sum of the two lower frequencies is also a typical nonlinear behavior. It is suggested that the stage is now so far away from its neutral stability point that a nonlinear analysis is required to describe its behaviors.