بررسي عددي اثرات جريان نشتي از نوك پره ي توربين گاز محوري

چكيده انگليسي :

Numerical Investigation of Effects of Blade Tip Leakage Flow in Axial Flow Gas Turbines Shahab Jahanfar s jahanfar@iut ac ir Date of Submission 2011 04 25 Department of Mechanical Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language FarsiSupervisor AhmadReza Azimian azimian@cc iut ac irAbstract In accordance with the increasing demand for high efficiency energy production some of which is supplied by the gas turbines numerous researches in this field have been conducted during the last decades However owing to complex flow over the blades experimental and numerical efforts are still in progress in order to further improve the performance of these machines Today after decades of research achieving higher performance for turbine parts has become more and more troublesome needing an elaborate understanding of the flow field and heat transfer The current study aims at looking into the flow nature around the blade tip area by means of numerical simulation Gas turbine blade tips are not only one of the main sources of losses but also a limiting factor for the blade lifetime The inevitable gap between the blade tip and the casing in the presence of the pressure difference between the blade pressure and suction sides forms a leakage flow Impingement of this accelerated and high temperature flow on the blade tip results in a high heat transfer region on the blade tip On the other hand interaction of this flow with the passage flow at the suction side leads to formation of a leakage vortex with an area of high losses In this thesis recessed blade tip as a successful leakage control geometry is compared with a flat one for the gap to span ratios of 1 up to 3 and for different rim heights and incidence angles The distribution of gap mass flow rate total pressure loss along the passage and also at the cascade exit plane exit flow angle and heat transfer coefficient on the tip surface have been analyzed Simulations are carried out in linear cascade arrangement similar to an experimental study The heat transfer coefficient on the tip and the pressure at the mid span are in a reasonable agreement with the existing measured data Comparison of different turbulence models shows that k model behaves better in predicting the blade tip heat transfer Results show that as the clearance increases the overall loss the leakage ratio and the exit flow deviation increase almost linearly The recessed tips reduce these parameters in all gaps with a lower rate of increase whereas decrease in the leakage ratio is higher than the other quantities Furthermore for any clearance size an appropriate rim height provides the lowest loss and the exit flow deviation so that the rim height of 4 2 at the gap of 3 and rim height of 2 1 at shorter gaps show better effectiveness Investigating the heat transfer distributions reveal that for the flat cases the average heat transfer coefficient on the tip increases as clearance increases up to 2 and decreases for larger gaps For squealer cases it increases but the rate of increase is reduced at larger gaps Recessed tips demonstrate a lower average heat transfer coefficient in all conditions and maintain their superiority over the flat ones even at off design incidences Finally for the blade profile studied here the tip heat transfer coefficient is more sensitive to varying the rim height compared to the other aerodynamic parameters Keywords Leakage flow Gas turbine Blade tip Recessed tip Turbulence Loss Heat transfercoefficient

جهان فر، شهاب

بررسي عددي اثرات جريان نشتي از نوك پره ي توربين گاز محوري

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