| Advanced aero-engines and gas turbines require an increasing sealing performance between rotational and stationary parts.The stator well in a compressor is the typical space between the rotor drum and stator vane inner-band.Its sealing characteristic can influence the compressor efficiency and even the performance of aero-engines.The compressor inter-stage seal can be considered as the labyrinth seal with upstream and downstream rotor-stator cavities.The large surface area of rotating wall in the inter-stage seal results in substantial temperature rise and swirl flow development,which can cause quite different flow characteristic from the normal labyrinth seal.In addition,this leakage flow across the stator well would interact with the main passage flow and influence the compressor aerodynamics,leading to overall performance degradation.In order to further improve the compressor efficiency and reliability,it is essential to understand the flow mechanisms and windage heating characteristic of the stator well,as well as to control the leakage flow effectively.In this paper,a comprehensive study into the leakage behavior,swirl characteristic and windage heating characteristic of the inter-stage seal was conducted by experimental measurements,numerical simulations and one-dimensional analyses.In the first,as few publications deal with the swirl flow development and total temperature variation,a rotating(9000rpm)compressor inter-stage seal test rig was designed.Leakage flow rates,windage heating and swirl ratios in the cavities were measured at different rotating speeds.In order to get the working tip clearance accurately,the set up tip clearance was measured with plug gauges,while the radial displacements of rotating disc and stationary casing were measured separately with several high precision laser distance sensors.Numerical simulations were also carried out to provide more detailed insight into the flow physics and the influences of parameters such as working tip clearance,rotation,tooth shape,tooth position,pressure ratio and Reynolds number were discovered.Then,according to the structure of compressor inter-stage seal,a one-dimensional network model consisting of rotating disc,labyrinth seal and local loss elements was developed.The leakge flow rate was gained based on the empirical correlations of elements and one-dimensional basic flow equations were used to get the swirl development and windage heating in the stator well.In addition,the modified structures of seal lips attached to the rotor and stator inside the downstream rim seal and impellers positioned in the cavities were proposed and studied.The flow and windage heating characteristics for these cases were compared with the baseline stator well.Finally,the impact of the stator well leakage flow on the stator vane aerodynamic performance was analyzed by simultaneously simulating the stator well flow path and the main flow path.Moreover,comparisons of the stator vane performances for modified and normal inter-stage seal structures were also made,expecting to provide help to the design of compressor stator wells.For the typical flow mechanisms and windage heating characteristic,due to the swirl flow,the pressure gradient in the inlet cavity along radial direction called the effect of centrifugal booster will suppress the leakage,while the effect of centrifugal booster in the outlet cavity will promote the leakage through the stator well.In addition,stronger swirl flow in the stator well causes more axial flow loss and thus the reduction of leakage.On the other hand,for higher swirl flow,the circumferential velocity difference between flow and rotor is smaller,resulting in less viscous work as well as less windage heating.Due to the presence of rotor-stator cavities,the discharge coefficient of inter-stage seal decreases by 6%-10% compared to normal labyrinth seals.With the decrease of working tip clearance,the leakage flow rate would reduce significantly.However,very small as well as very large tip clearances yield lower discharge coefficient values than medium clearances with a maximum around c=0.8mm.It is clear that increasing rotational speed causes working tip clearance to reduce sharply.When ?=8100rpm,the reductions in working tip clearance and leakage flow find to be 43% and 50% respectively at typical conditions.The leakage reduction caused by the rotation effect accounts for about 15%,while 35% of leakage reduction can be attributed to the variation of the working tip clearance.Moreover,increases in the outlet cavity swirl ratio and system windage heating coefficient are almost linearly or even exponentially proportional to the increase of rotational speed and the decrease of working tip clearance.In the current study,the measured temperature rise reaches about 45 K and the maximum swirl ratio in the outlet cavity is around 0.4.In order to avoid severely rubbing between the teeth and the stator at high rotating speeds,several methods were presented to reduce leakage flow at relatively larger tip clearance in the current study.Changing axial positions of teeth in the stator well can reduce leakage flow 8%-15%,compared to normal three-tooth inter-stage seal.The leakage flow rates of adding seal lips and positioning impellers in the inlet cavity are found 10%-20% and 10%-40% lower than that of baseline model,respectively.For the one-dimensional network model,coupling calculations for the leakage flow rate,swirl development and temperature increase in the stator well were achieved.The comparison between the results of 1D model and test data shows a close agreement.The deviations of leakage flow rate and swirl ratio in the outlet cavity are within 5.5% and 9%,respectively.The windage heating results from the 1D model over predict the experimental values by about 0.5-2.5K.When leakage flow having low tangential velocity is entrained into the mainstream,cross-passage pressure gradient drives it to the suction side of the stator vane where it collects and forms large separated flow region.When the tangential velocity of stator well flow is high enough,the leakage fluid collects on the pressure side of the stator vane as it travels downstream,resulting in a small region of separated flow.This is the main reason for the performance degradation of stator vane.In this paper,the leakage flow degrades stator vane performance significantly as the tip clearance reaches and exceeds 2% of stator vane height.Meanwhile,when the swirl of the injected fluid increases,a reduction in total pressure loss of stator vane occurs and the sensitivity of loss to leakage reduces.Additionally,at c=0.7,the modified inter-stage seal structures were also applied for the main flow path simulations and the improvement of stator vane performance was observed.A 0.65% increase in main flow rate and a 15% drop in loss coefficient occur compared to normal inter-stage seal structure,approximating to the stator vane performance at c=0.5. |
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