Session: 37-01 Radial Turbomachinery Experimental

Paper Number: 154076

Detailed Experimental and Numerical Investigation of a Transonic Impeller With a Vaned Diffuser 

Modern high-pressure ratio high-speed centrifugal compressors powering rotorcraft applications are inherently transonic in character, i.e. usually above a certain span of the blades; impeller is subjected to supersonic relative flow approaching even though incoming flow is of subsonic in nature with axial Mach numbers around 0.5. Relative Mach number at the shroud attains the maximum value and it has implications on the compressor efficiency, stability and flow range. Usually, the higher the relative Mach number at the shroud the lower the efficiency, flow range with reduced compressor stall margin at the off-design conditions. At the off-design conditions, relative Mach numbers at the shroud are subsonic below a specific rotational speed and there exists a significant positive incidence on the blades, which can cause a recirculation region extending upstream. This recirculation region can trigger rotating instabilities in the guise of rotating stall.

In the current study, a transonic centrifugal compressor (Mu2>1.5) is developed with a vaned diffuser. The configuration is comprised of an axial S duct type intake with struts , a high-speed impeller and a low solidity cascade type vaned diffuser that are suited for high efficiency applications. A detailed experimental campaign is conducted on the tested compressor. Test data is based on wall static pressure taps distributed along static parts, wall pressure measurements via high-speed pressure sensors at critical locations, total pressure and temperature rake measurements, clearance and wall temperature measurements, 3-hole probe measurements at the impeller exit, etc. acquired at various rotational speeds. Detailed data reduction on test data exhibited a typical aerodynamic character of impeller stall up to 95%Nc and diffuser stall from there onwards at higher speeds. Diffuser choke for most of the operating range followed by a collective impeller-diffuser choke is observed, satisfying impeller-diffuser throat area matching criterion laid out by Casey and Robinson. Detailed investigation of wall pressures around the impeller exit and vaneless space consolidated with mass flow rate and total temperature measurements reveals impeller pressure ratio and efficiency. Impeller efficiency is found to be increasing each speed line at part speed operation from stall to choke, up to rotational speed where relative supersonic flow phenomena start to become prominent. Significant amount of losses, occurring inside the vaned diffuser due to a shock train, reduce the stage efficiency at these speeds towards choking conditions, portrayed through loss coefficient plots and local static pressure taps. Peak efficiency lies around 89-95%Nc as expected from a transonic machine, where incidence losses and shock losses are minimised. Impeller work coefficients are steep and apart from each other at part speeds. Around design speeds, the speedlines are closer to each other and they show a knee in the characteristic due to collective impeller-diffuser choke. Traversed 3 hole probe data shows the variation of absolute flow angle from hub to shroud at the vaneless space and confirms the impeller pressure ratio.

Three-dimensional steady state RANS simulations are carried out with various turbulence models to match experimental test data. A thorough grid independency study is conducted to ensure better capture of detailed flow physics, occurring especially in the vaneless space. A predefined roughness value consistent with machining requirements is also employed to yield a better match in terms of pressure ratio, capacity and stability margin. Firstly, the matching of global parameters are sought and the best matching model is obtained; and afterwards a detailed investigation of component performances is carried out. The detailed results will appear in the final version of the article.

Presenting Author: Erinc Erdem TUSAS ENGINE INDUSTRIES

Presenting Author Biography: After graduating from aerospace engineering in Middle East Technical University (METU) in 2002, Dr. Erdem started working for Roketsan Missile Industries, Inc. as a propulsion engineer, focusing on internal aerodynamics of solid rocket motors. In 2005, Dr. Erdem obtained his research M.Sc. from von Karman Institute for Fluid Dynamics (VKI) on numerical and experimental investigation of internal flows inside a simplified Ariane 5 rocket motor geometry with slag accumulation. In 2006, he obtained his M.Sc. from mechanical engineering in METU on a subject called numerical investigation of secondary gas injection systems for thrust vectoring. In 2011, he obtained a Ph.D. from the University of Manchester on active flow control studies at Mach 5 involving detailed wind tunnel measurements with various measurement techniques and complementary computational effort. Afterwards he carried on pursuing active research as a postdoctoral associate in the same university on low speed flow control using different actuation mechanisms. During his Ph.D. and postdoc studies, several projects on high/low speed wind tunnel testing were completed involving partners such as ESA, DSTL and EU FR7. Upon finishing the studies, Dr. Erdem started working for GE Aviation in Turkey in 2013 as thermal systems design lead engineer specializing on engine bay cooling and rotor-stator cooling in gas turbine engines. As of 2015, he is working for TUSAS Engine Industries (TEI) Inc., responsible for mainly thermal systems design comprised of secondary air systems, thermal analysis and component cooling. In addition, he works on radial compressor aerodynamics and rig testing. Dr. Erdem’s role in Chief Engineers Office involves overseeing/reviewing technical activities for the indigenous Turboshaft Engine Development program related to his expertise.

Authors:

Erinc Erdem TUSAS ENGINE INDUSTRIES (TEI)
Ozan Alican TUSAS ENGINE INDUSTRIES
Gülsevim Sepetci Kutmaral TUSAS ENGINE INDUSTRIES (TEI)
Seyfullah Cay TUSAS ENGINE INDUSTRIES (TEI)
Bora Yazgan TUSAS ENGINE INDUSTRIES (TEI)
Ahmet Arslan TUSAS ENGINE INDUSTRIES (TEI)