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Effect of Streamwise Fences on Secondary Flows and Losses in a Twodimensional Turbine Rotor Cascade

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Journal of Thermal Science 2006年第4期15卷 296-305页

作者： m.govardhan A.Rajender J.P.Umang Thermal Turbomachines Laboratory Department of Mechanical EngineeringIndian Institute of Technology MadrasChennai 600036India

To control secondary flows, streamwise fences were attached to end wall of a linear turbine rotor cascade. The cascade had 8 blades of 400 mm long and 175 mm chord. The blades deflected the flow by 120°. The fences were made out of 0.7 mm thick brass sheet and the heights of the fences were 14 mm, 18 mm respectively. The curvature of the fences was the same as that of the blade camber line. The fences were fixed normal to the end wall and at half pitch away from the blades. The experimental program consists of total pressure, static pressure measurements at the inlet and outlet of the cascade, by using five-hole probe. In addition, static pressure on the blade suction surface and pressure surface was also obtained. Fences are effective in preventing the movement of the pressure side leg of the horseshoe vortex. Consequently the accumulation of low energy fluid on the suction surface is minimised. End wall losses are reduced by the fences due to weakening of the end wall cross flow.

关键词： Turbine cascade streamwise fences loss core lift coefficient

Investigations on Low Speed Axial Compressor with Forward and Backward Sweep

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Journal of Thermal Science 2007年第2期16卷 121-133页

作者： m.govardhan O.G.Krishna Kumar N.Sitaram Thermal Turbomachines Laboratory Department of Mechanical EngineeringIndian Institute of Technology MadrasChennai 600036India

In the recent past, experimental studies have shown some advantages of blade lean and sweep in axial compressors. As most of the experimental results are combined with other features, it is difficult to determine the effect of individual parameters on the performance of the compressor. The present numerical studies are aimed at understanding the performance and three-dimensional flow pattern at the exit of swept and unswept rotors. Three rotors, namely; unswept, 200 forward swept and 200 backward swept rotors are analysed with a specific intention of understanding the pattern of the blade boundary layer flow. The analysis was done using a fully three-dimensional viscous CFD code CFX-5. Results indicated reduction in pressure rise with sweep. Backward sweep is detrimental as far as the performance near endwalls is considered. On the other hand total pressure loss in the wake in mid span region is less with backward sweep, which favours its application here. However, backward sweep adversely affects the stall margin. The ability of the forward sweep to deflect the streamlines towards hub gets diminished at low flow rates. Forward sweep changes the streamline pattern in such a way that the suction surface streamlines are deflected towards the hub and the pressure surface streamlines are deflected towards the easing. An opposite behaviour is observed in backward swept rotors.

关键词： Sweep pressure rise streamline shift stall margin blade loading

Experimental Investigations of the Three-DimensionalFlow in a Large Deflection Turbine Cascade with Tip Clearance

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Journal of Thermal Science 1998年第3期7卷 149-164页

作者： m.govardhan S.S.S.R.K.Sastri V.S.Vishnubhotla ThermalurbomachinesLab DepartmentofMechanicalEngineeringIndianInstituteofTechnologyMadrasChennai600036IndiaThermalurbomachinesLabDepartmentofMechanicalEngineeringIndianInstituteofTechnologyMadrasChennai600036IndiaThermalurbomachinesLabDepartment

Results obtained from an experbontal study of the threedimensional flow survey within and exit of a large defiection linear turbine cascade are presented for a tip clearance levels of 0.08, 1.5, 3.0 percent of chord and compared with the help of boundary layer probes and that within and exit of a blade passage was done with a miniaturised five hole probe. End wall and blade tip surface static pressures were also obtained, in addition to flow visualisation studies. A strong horse-shoe vortex forms in front of the leading edge for zero clearance whereas this vortex does not appear for 3 percent clearance indicating that for large clearance the pressure forces have dominating infiuence than the viscous forces. In addition to normally known clearance vortex, a small tip separation vortex was noticed on the blade tip surface inside the tip gap. Due to the area contraction caused by the tip separation vortex, the fluid movign towards the tip gap from the pressure side is accelerated. Downstream of the vortex, the endwall pressure increases due to flow mixing. Both vortices increase in size and strength along the chord. The miring is incomplete in the aft portion of the blade. The tip gap velocity profiles exhibit wak like characteristics especially at axial positions where the mixing is incomplete. The passage vortex in the present investigations did not diminish with increase in clearance. The discharge coefhcient and the total pressure loss coefficient within the tip gap show similar tendency with lower values near the leading and trailing edge regions.

关键词： turbine cascade tip clearance vortex tip separation vortex horse-shoe vor-tex total pressure loss coefficient discharge coefficient

Stall Characteristics and Tip Clearance Effects in Forward Swept Axial Compressor Rotors

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Journal of Thermal Science 2009年第1期18卷 40-47页

作者： Ramakrishna PV govardhan m Thermal Turbomachines Laboratory Department of Mechanical EngineeringIndian Institute of Technology Madras

Tilting the blade sections to the flow direction （blade sweep） would increase the operating range of an axial compressor due to modifications in the pressure and velocity fields on the suction surface. On the other hand, blade tip gap, though finite, has great influence on the performance of a turbomachine. The present paper investigates the combined effect of these two factors on various flow characteristics iu＇a low speed axial flow compressor. For this present study, nine computational domains were modeled; three rotor sweep configurations （0°, 20° and 30°）and for three different clearance levels for each rotor. Commercial CFD solver ANSYS CFX 11.0 is used for the simulations. Results indicated that tip chordline sweep is found to improve the stall margin of the compressor by modifying the suction surface boundary layer migration phenomenon. Diffusion Factor （DF） contours showed the severity of stalling with unswept rotor. For the swept rotors, the zones of high probable stall are less severe and they become less in size with increasing sweep. Increment in the tip gap is found to gradually affect the performance of unswept rotor, while the effect is very high for the two swept rotors for the earlier increments. As a minimum clearance is unavoidable, swept rotors suffer relatively higher deviation from the idealistic behavior than the unswept rotor due to tip clearance.

关键词： Sweep Tip clearance Axial compressor Stall margin Tip leakage flows.

Computational Studies of Flow through Cross Flow Fans-Effect of Blade Geometry

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Journal of Thermal Science 2005年第3期14卷 220-229页

作者： m . govardhan D. LAKSHmANA SAmPAT School of mechanical Engineering Universiti Malaysia Sabah Sabah Malaysia ProfessorThermal Turbomachines Laboratory Department of Mechanical Engineering Indian Institute of Technology Madras Chennai India

This present paper describes three dimensional computational analysis of complex internal flow in a cross flow fan. A commercial computational fluid dynamics (CFD) software code CFX was used for the computation. RNG k-ε two equation turbulence model was used to simulate the model with unstructured mesh. Sliding mesh interface was used at the interface between the rotating and stationary domains to capture the unsteady interactions. An accurate assessment of the present investigation is made by comparing various parameters with the available experimental data. Three impeller geometries with different blade angles and radius ratio are used in the present study. maximum energy transfer through the impeller takes place in the region where the flow follows the blade curvature. Radial velocity is not uniform through blade channels. Some blades work in turbine mode at very low flow coefficients. Static pressure is always negative in and around the impeller region.

关键词： cross flow fans computational fluid dynamics quasi steady state multiple frames of reference

Study on the Performance Deterioration of mixed Flow Impeller due to Change in Tip Clearance

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Journal of Thermal Science 2013年第6期22卷 532-538页

作者： Ramesh Rajakumar.D Ramamurthy.S govardhan.m National Aerospace Laboratories-Bangalore-560017 NCAD National Aerospace Laboratories Thermal Turbomachines laboratory Department of Mechanical EngineeringIndian Institute of Technology Madras

Performance of mixed flow compressor with un-shrouded impeller strongly depends upon unsteady, asymmetrical flow fields in the axial directions. The flow through the mixed flow impeller is complex due to three-dimensional nature of geometry. In mixed flow impeller, there are clearances between the rotating impeller blades and the casing as the high pressure ratio compressors are usually open shrouded impellers. As a result, certain amount of reduction in the performance is unavoidable due to clearance flows. In the present investigations, numerical analysis is performed using a commercial code to investigate tip clearance effects on through flow. The performance of mixed flow impeller with four different clearances between impeller and stationary shroud are evaluated and compared with experimental results. The impeller performance map was obtained for different operating speeds and mass flow rates with different tip clearances. The result shows that the tip leakage flow strongly interacts with mainstream and contributes to total pressure loss and performance reduction. The pressure and performance decrement are approximately linearly proportional to the gap between impeller and stationary *** analysis showed scope for improvement in design of the compressor for better performance in terms of efficiency and operating range.

关键词： mixed Flow Compressor Surge Stall Choke CFD Tip Clearance Compressible Flow Periodic Grids and Jet-Wake

Experimental Study of Endwall and Tip Clearance Flows in a Two- Dimensional Turbine Rotor Blade Cascade - Effect of Incidence Angle

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Journal of Thermal Science 2000年第1期9卷 63-76页

作者： m. govardhan B.H.L. Gowda R.m. Wankhade Department of mechanical Engineering Indian Institute of Technology Madras Chennai 600036 India Department of Applied Mechanics Indian Institute of Technology Madras Chennai 600 036 India(Department of Mechanical Engineering Indian Institute of T

Experimental investigations were carried out on a two-dimensional cascade fitted with a 120° deflection rotor blades to study the effect of incidence angle on the endwall flow in the presence of tip clearance. A total of five incidence angles, namely: -10°，-10° -5°, 0°, 5° ， 10° were chosen and for each incidence angle, the experiments were conducted for five tip clearance values at a constant space -chord ratio of 0.79. The results are presented in the form of contours of static pressure coefficient on the endwall and the blade tip surface. In addition, the variation of static pressure coefficient ahead of the blade leading edge and from the pressure surface to the suction surface at various axial stations, and discharge coefficient gi different axial stations are presented. The results indicate that the adverse pressure gradient upstream of the leading edge is reduced as tip clearance is increased. The contours of static pressure coefficient on the endwall indicate a deep low-pressure trough near the suction surface in comparison to the normal trough for zero clearance. Loading also increases as incidence changes from the negative to positive values. Due to area contraction caused by the tip separation vortex, the fluid moving towards the tip gap from the pressure side is accelerated. Downstream of the tip separation vortex, the endwall pressure increases due to flow mixing. The maximum value of discharge coefficient increases and the point at which maximum value occurs shifts towards leading edge when incidence is changed from -10° to 10°.

关键词： endwall blade tip surface tip clearance incidence angle tip separation vortex discharge coefficient.

Effect of Guide Vanes on the Performance of a VariableChord Self-Rectifying Air Turbine

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Journal of Thermal Science 1998年第4期7卷 211-217页

作者： m. govardhan T.S. Dhanasekaran(Thermal Turbomachines Laboratory, Department of mechanical Engineering, Indian institute of Technology madras, Chennai 600036) Thermal Turbomachines Laboratory Department of Mechanical Engineering Indian Institute of Technology Madras Chennai

Wells turbine is a self rectifying air flow turbine capable of converting pneumatic power of the periodically reversing air stream in Oscillating Water Column into mechanical energy. One of the principal reasons for the low efficiency of the Wells turbine is its lower tangential force compared to its axial force. Guide vanes before and after the rotor suggest a means to improve the tangential force, hence its efficiency. Experimental investigations are carried out on the Wells turbine with a variable chord (VACR) blade rotor fitted with inlet and outlet guide vanes to understand the aerodynamics especiallyimprovement in efficiency and starting characteristics. Numerical simulation has been made to clarify the unsteady characteristics of the turbine with guide vanes. Studies are done at various flow coefficients covering the entire range of flow coefficients over which the turbine is operable. The efficiency,starting characteristics of the Wells turbine has improved when compared with the turbine without guide vanes.

关键词： Wells turbine guide vane variable chord

Investigations on an Axial Flow Fan Stage subjected to Circumferential Inlet Flow Distortion and Swirl

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Journal of Thermal Science 1997年第4期6卷 241-250页

作者： m. govardhan K. Viswanath (Thermal Turbomachines Laboratory Department of mechanical Engineering, Indian Institute of Technology, madras 600 036, India ) Thermal Turbomachines Laboratory Department of Mechanical Engineering Indian Institute of Technology Madras India

The combined effects of swirl and circumferential inlet flow distortion on the flow field of an axial flow fan stage are reported in this paper. The study involves measurements at the inlet of the rotor and exit of the rotor and stator at design and off design now conditions. The study indicated that at the design flow condition, swirl had caused deterioration of the performance in addition to that caused by distortfou. Pressure rise imparted in the distortion zone is higher than in the free zone. The attenuation of distortion is high in the presence of swirl.

关键词： circumferential distortion swirl inlet guide vanes distortion index