Kishore Ramakrishnan

Open Rotor Designs for Low Noise and High Efficiency

Building upon the successes of the UDF® program in the 1980’s, open rotor designs for high flight speed efficiency and low community noise have been developed at GE in collaboration with NASA and the FAA. Targeting a narrow body aircraft with 0.78 cruise Mach number, the cost-share program leveraged computational fluid dynamics (CFD), computational aero-acoustics (CAA), and rig scale testing to generate designs that achieved significant noise reductions well beyond what was attained in the 1980’s while substantially retaining cruise performance. This paper presents overall propeller net efficiency and acoustic assessments of GE’s modern open rotor designs based on measured rig data and the progression of the technology from the 1980’s through the present. Also discussed are the effects of aft rotor clipping, inter-rotor spacing, and disk loading. This paper shows how the two-phase design and scale model wind tunnel test program allowed for test results of the first design phase to feed back into the second design phase, resulting in 2–3% improvement in overall propeller net efficiency than the best efficiency design of the 1980’s while nominally achieving 15–17 EPNdB noise margin to Chapter 4 (when projected to full scale for a prescribed aircraft trajectory and installation). Accounting for trades and near term advancements, such a propulsion system is projected to meet the goal of 26% fuel burn reduction relative to CFM56-7B powered narrow body aircraft.Copyright

Multi-Blade Row Interactions in a Low Pressure Ratio Centrifugal Compressor Stage With a Vaned Diffuser

Previous experimental and CFD investigation of a GE Oil and Gas centrifugal compressor stage with a vaneless diffuser revealed a complex excitation mechanism caused by an aero-acoustic interaction between three blade rows. In stages with vaned diffusers, additional sources of aeromechanical excitation on the impeller can be expected. This unsteady CFD investigation is a follow-up from the previous vaneless diffuser study to identify any additional sources of excitation that arise in the presence of a vaned diffuser in preparation for aeromechanic tests to be conducted later. The study confirms that excitation from impeller-diffuser interaction generated acoustic modes can dominate the potential field excitation from the diffuser vanes. In addition, a significant aero-acoustic excitation to the impeller at a vane pass frequency corresponding to the sum of the vane counts in the two downstream vane rows is observed, and its origination is discussed. The latter excitation is different from that observed in the vaneless diffuser stage where the vane pass frequency observed by the impeller corresponds to the sum of the vane counts in the upstream and downstream vane rows.Copyright

Effect of Cavities on Impeller Aeromechanical Forcing in a Low Pressure Ratio Centrifugal Compressor Stage

Previous experimental and CFD investigation on a GE Oil & Gas centrifugal compressor stage with a vaneless diffuser revealed a complex excitation mechanism caused by aero-acoustic interaction between three blade rows. Recent published studies have indicated that cavities enclosing shrouded impellers may strongly amplify the acoustic excitation of the impeller by Tyler-Sofrin modes. In this current research therefore, the previous CFD study is expanded to include a model of the disk and shroud cavities. A linearized Navier-Stokes frequency sweep in forced response mode shows a well-defined peak in cavity acoustic activity at a certain frequency. It also shows that the inclusion of cavities noticeably increases the modal forcing on the impeller, qualitatively confirming findings in existing literature. A significant difference in modal force magnitude is found between the shroud-disk out-of-phase mode and the in-phase mode, which is consistent with experimental measurements of vibratory response.Copyright

Unsteady Acoustic Forcing on an Impeller Due to Coupled Blade Row Interactions

This article contains an investigation of the unsteady acoustic forcing on a centrifugal impeller due to coupled blade row interactions. Selected results from an aeromechanical test campaign on a GE Oil and Gas centrifugal compressor stage with a vaneless diffuser are presented. The most commonly encountered sources of impeller excitation due to upstream wake interaction were identified and observed in the testing campaign. A 30/rev excitation corresponding to the sum of upstream and downstream vane counts caused significant trailing edge vibratory stress amplitudes. Due to the large spacing between the impeller and the return channel vanes, this 30/rev excitation was suspected to be caused by an aero-acoustic excitation rather than a potential disturbance. The origin of this aero-acoustic excitation was deduced from an acoustic analysis of the unsteady compressor flow derived from CFD. The analysis revealed a complex excitation mechanism caused by impeller interaction with the upstream vane row wakes and subsequent acoustic wave reflection from the downstream return channel vanes. The findings show it is important to account for aero-acoustic forcing in the aeromechanical design of low pressure ratio centrifugal compressor stages.

Prediction and Validation of High-Performance Centrifugal Compressor Impeller Forced Response

High-performance centrifugal compressors call for a reduction in weight to relax rotor-dynamic constraints, and a reduction in blade thickness as a way to improve efficiency. In this tight design space, synchronous excitations on impellers are often unavoidable and may represent a potential aeromechanic risk, especially for high speed stages operated at high pressures. Therefore, forced response analysis has become a fundamental design step. The aero/structure unsteady interaction in impellers, mainly driven by both stationary row wakes and potential fields, and the possible overlap with structural frequencies, must be carefully analyzed and quantified since they may lead to high cycle fatigue failures under critical operating conditions. This paper focuses on the forced response analysis of an unshrouded high-speed centrifugal impeller incorporating aircraft engine technology designed for hydrocarbon processing. The methodology focuses on the effects of generic disturbances (inlet distortions, potential fields, aero-acoustic excitations) superimposed in amplitude and frequency on the time averaged flow field, and follows a rigorous numerical approach supported by validation against dedicated aeromechanic test data. Aerodynamic forcing frequencies and amplitudes, and the corresponding aero-mechanic damping jointly contribute to the vibratory response. An unsteady CFD analysis determines the impeller aerodynamic load which, in conjunction with FE (Finite Elements) modal analysis, allows for computing the modal force. A time-linearized Navier-Stokes computation predicts the aerodynamic damping, and allows mapping the mode shapes onto the CFD grid in order to supply the displacement boundary conditions. A purely numerical Goodman diagram has been drawn and the response level for a specific structural mode shape analyzed. The predictions compare favorably with test data, providing confidence in this method for practical high-speed radial compressor design.