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Research Papers

J. Eng. Gas Turbines Power. 2019;141(9):091001-091001-13. doi:10.1115/1.4043545.

Plasma actuators may be successfully employed as virtual control surfaces, located at the trailing edge (TE) of blades, both on the pressure and on the suction side, to control the aeroelastic response of a compressor cascade. Actuators generate an induced flow against the direction of the freestream. As a result, actuating on the pressure side yields an increase in lift and nose down pitching moment, whereas the opposite is obtained by operating on the suction side. A properly phased alternate pressure/suction side actuation allows to reduce vibration and to delay the flutter onset. This paper presents the development of a linear frequency domain reduced order model (ROM) for lift and pitching moment of the plasma-equipped cascade. Specifically, an equivalent thin airfoil model is used as a physically consistent basis for the model. Modifications in the geometry of the thin airfoil are generated to account for the effective chord and camber changes induced by the plasma actuators, as well as for the effects of the neighboring blades. The model reproduces and predicts correctly the mean and the unsteady loads, along with the aerodynamic damping on the plasma equipped cascade. The relationship between the parameters of the ROM with the flow physics is highlighted.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(9):091002-091002-10. doi:10.1115/1.4043555.

Turbine attachments in the aero-engine are generally subjected to combined high and low cycle fatigue (CCF) loadings, i.e., low cycle fatigue (LCF) loading due to centrifugal and thermal loading stresses superimposed to the aerodynamically induced high cycle fatigue (HCF) loading. The primary focus of this study is to predict the crack growth life for the actual full-scale turbine attachment through experimentally examining the crack growth behavior under CCF loading at elevated temperature. The crack closure effect was first investigated by using the corner-notched (CN) specimen cut from the turbine attachment since the stress state of CN specimen is more similar to turbine attachment than compact tension (CT) specimen. Employing digital image correlation (DIC) technique, the level of crack closure of CN specimen was clarified under different stress ratios (R) for LCF loading. Afterward, a CCF crack growth model for the full-scale turbine attachment was proposed, which takes the crack closure effect, time-independent crack increment, and transient vibrational analysis into account. In order to verify the proposed method, a Ferris wheel system was established to conduct CCF test on the full-scale turbine attachment at elevated temperature. This study provides an effective methodology to predict the fatigue crack growth (FCG) life of full-scale turbine attachment under CCF loading.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(9):091003-091003-10. doi:10.1115/1.4043430.

Radial inflow turbines, characterized by a low specific speed, are a candidate architecture for the supercritical CO2 Brayton cycle at small scale, i.e., less than 5 MW. Prior cycle studies have identified the importance of turbine efficiency to cycle performance; hence, well-designed turbines are key in realizing this new cycle. With operation at high Reynolds numbers, and small scales, the relative importance of loss mechanisms in supercritical CO2 turbines is not known. This paper presents a numerical loss investigation of a 300 kW low specific speed radial inflow turbine operating on supercritical CO2. A combination of steady-state and transient calculations is used to determine the source of loss within the turbine stage. Losses are compared with preliminary design approaches, and geometric variations to address high loss regions of stator and rotor are trialed. Analysis shows stage losses to be dominated by endwall viscous losses in the stator. These losses are more significant than predicted using gas turbine derived preliminary design methods. A reduction in stator–rotor interspace and modification of the blade profile showed a significant improvement in stage efficiency. An investigation into rotor blading shows favorable performance gains through the inclusion of splitter blades. Through these, and other modifications, a stage efficiency of 81% is possible, with an improvement of 7.5 points over the baseline design.

Commentary by Dr. Valentin Fuster

Research Papers: Research Papers

J. Eng. Gas Turbines Power. 2019;141(9):091004-091004-16. doi:10.1115/1.4043611.

Shock vector control (SVC) based on transverse jet injection is one of the fluidic thrust vectoring (FTV) technologies, and is considered as a promising candidate for the future exhaust system working at high nozzle pressure ratio (NPR). However, the low vector efficiency (η) of the SVC nozzle remains an important problem. In the paper, a new method, named as the improved SVC, was proposed to improve the vector efficiency (η) of a SVC nozzle, which enhances the vector control of primary supersonic flow by adopting a bypass injection. It needs less secondary flow from high pressure component of an aero-engine and has smaller influence on the working character of an aero-engine. The flow mechanism of the improved SVC nozzle was investigated by solving three-dimensional Reynolds-averaged Navier--Stokes with shear stress transport (SST) κ–ω turbulence model. The shock waves, jets-primary flow interactions, flow separation, and vector performance were analyzed. The influences of aerodynamic and geometric parameters, namely, NPR, secondary pressure ratio (SPR), and bypass injection position (Xj.ad.) on flow characteristics and vector performance were investigated. Based on the design of experiment (DOE), the response surface methodology (RSM) and the simulation model of an aero-engine, a method to estimate the coupling performance of the improved SVC nozzle and an aero-engine was studied, and a new balance relationship between the improved SVC nozzle and an aero-engine was established. Results shows that (1) with the assistance of bypass injection, the jet penetration and the capability of vector control are largely improved, resulting in a vector efficiency (η) of 1.98 deg/%-ω at the designed NPRD = 13.88; (2) in a wide range of operating conditions, larger vector angle (δp), higher thrust coefficient (Cfg), and higher vector efficiency (η) of the improved SVC nozzle were obtained, (3) in the coupling process of the improved SVC nozzle and an aero-engine, a δp of 18.1 deg was achieved at corrected secondary flow ratio of 10% and corrected bypass ratio of 6.98%, and the change of the thrust and the specific fuel consumption (SFC) were within 12%, which is better than the coupling performance of a SVC nozzle and an aero-engine.

Commentary by Dr. Valentin Fuster

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