Non-axisymmetric Casing Research Articles

Impact of non-axisymmetric tip clearance on the aerodynamic and aeroelastic stabilities of a transonic compressor

The aerodynamic and flutter effects caused by the circumferential non-uniform tip clearances from the non-axisymmetric casing geometry, which are more representative of real operational conditions, should receive more attention. In this paper, sensitivity analyses on clearance and non-uniformity regarding rotor performance are conducted. The isentropic efficiency, total pressure ratio and stability margin of the rotor are reduced by 1.96%, 1.06% and 6.76% for an increase of tip clearance by 1% tip chord, respectively. The non-axisymmetric configuration reconfigures the interaction between leakage flow and mainstream in each rotor passage and alters the strength and influence range of the low-speed separation region generated by shock wave. The phase lag phenomenon observed in the aerodynamics of the non-axisymmetric layout also manifests in the aeroelastic effects but requires a larger phase shift at the maximum clearance sector, owing to the longer timescales for flow adjustment relative to the local tip gap considering the flow-induced vibration. Clearance sensitivity on flutter stability varies with the direction of the traveling wave: under forward traveling conditions, damping values initially decrease and then increase with increasing rotor tip clearance; while under backward traveling condition, blade damping linearly decreases with increasing rotor tip clearance. Additionally, variations in flow conditions and nodal diameters affect flutter stability by altering the position of shock waves and their work on the suction surface of the rotors. The mechanism responsible for unsteady flows induced by non-axisymmetric clearances from the casing ovalization on flutter stability is discussed for the first time in this paper, which can guide the industry to evaluate its impact.

Off-design performance of a liquefied natural gas plant with an axial turbine of novel endwall design

A design optimization workflow for the casing of a 1.5 stage axial turbine is implemented through a novel endwall surface definition, towards improving the turbine efficiency. The new non-axisymmetric casing design compares favourably to an established diffusion design technique. The workflow uses an axial turbine three-dimensional Reynolds Averaged Navier-Stokes model built in OpenFOAM Extend 3.2 with the k-ω Shear Stress Transport turbulence closure. Computer-based optimization of the surface topology using a Kriging surrogate model automates the design process. The designs are optimized using the total pressure loss across the full stage as the target function. Axial turbine performance gains are obtained from the workflow, which persist both at the design condition and off-design. These gains are used to project the impact of equivalent design improvements to the power turbine of a representative Natural Gas liquefaction plant cycle. Cycle Coefficient of Performance enhancements between 2.05% and 2.923% are obtained, at design and at off design conditions. Implementing these performance improvements has the potential to reduce carbon dioxide emissions by 165.54 tonnes per year at design and by 108.18 tonnes per year at off design, in a representative Natural Gas liquefaction plant.

Design optimization workflow and performance analysis for contoured endwalls of axial turbines

Advances in computer-based optimization techniques can be used to enhance the efficiency of energy conversions processes, such as by reducing the aerodynamic loss in thermal power plant turbomachines. One viable approach for reducing this flow energy loss is by endwall contouring. This paper implements a design optimization workflow for the casing geometry of a 1.5 stage axial turbine, towards mitigating secondary flows. Two different parametric casing surface definitions are used in the optimization process. The first method is a new nonaxisymmetric casing design using a novel surface definition. The second method is an established diffusion design technique. The designs are tested on a three-dimensional axial turbine RANS model. Computer-based optimization of the surface topology is demonstrated towards automating the design process. This is implemented using Automated Process and Optimization Workbench (APOW) software. Kriging is used to accelerate the optimization process. The optimization and its sensitivity analysis give confidence that a good predictive ability is obtained by the Kriging surrogate model used in the prototype design process tested in this work. A flow analysis confirms the positive impact of the optimized casing groove design on the stage isentropic efficiency compared to the diffusion design and compared to the benchmark axisymmetric design.

Optimization of the non-axisymmetric stator casing of a 1.5 stage axial turbine

The interaction of secondary flows with the main passage flow in turbomachines results in entropy generation and in aerodynamic loss. This loss source is most relevant to low aspect ratio blades. One approach for reducing this flow energy loss is by endwall contouring. However, limited work has been reported on using non-axisymmetric endwalls at the stator casing and on its interaction with the tip leakage flow. In this paper, a non-axisymmetric endwall design method for the stator casing is implemented through a novel surface definition, towards mitigating secondary flow losses. This design is tested on a three-dimensional axial turbine RANS model built in OpenFOAM 3.2 Extend, with k−ω SST turbulence closure. The flow analysis confirms the foundations of the new surface definition approach, which is implemented using Alstom Process and Optimization Workbench (APOW) software. Computer-based optimization of the surface topology is demonstrated towards automating the design process of axial turbines in an industrial design workflow. The design is optimized using the total pressure loss across the first stator and across the full stage, as the target function. Numerical predictions of the 1.5 stage axial turbine show the positive impact of the optimized casing design on the efficiency that increases by 0.69% against the benchmark axisymmetric stage from RWTH Aachen, which is validated against experiment. The new non-axisymmetric casing is also beneficial at off-design condition. The effective mitigation of the secondary flows is predicted to give a 0.73% efficiency gain off-design.

Effects of Non-axisymmetric Casing Grooves Combined with Airflow Injection on Stability Enhancement of an Axial Compressor

AbstractThis paper presents a performance evaluation of non-axisymmetric casing grooves combined with airflow injection in a transonic axial compressor with NASA Rotor 37, using three-dimensional Reynolds-averaged Navier-Stokes equations with the k-ε turbulence model. An axisymmetric casing groove was divided circumferentially into 36 non-axisymmetric grooves. The numerical results for adiabatic efficiency and total pressure ratio were validated with experimental data. A parametric study for stall margin, stable range extension, peak adiabatic efficiency, and total pressure ratio at peak adiabatic efficiency of the compressor was performed using five parameters: the front and rear lengths, the height of the casing groove, the injection mass flow rate, and the injection angle. The non-axisymmetric casing grooves combined with injection improve greatly the stall margin and stable range extension of the transonic axial compressor, but reduce only slightly the peak adiabatic efficiency in all cases, compared to the results for a smooth casing.

Unsteady Measurements of Periodic Effects in a Transonic Compressor With Casing Treatments

Application of nonaxisymmetric casing treatments (CTs) can extend the operating range of a transonic compressor significantly. Recent CT designs have proven successful at achieving operating range extension without efficiency loss under design conditions. Two different CT designs were investigated on a high-speed one and a half stage test rig using extensive instrumentation. The stage setup is representative of the front stage of a modern high-pressure compressor. Results of particle image velocimetry (PIV) measurements taken in the blade tip region underneath the CT show a significantly modified flow structure compared to the smooth casing reference case. Blockage zone, secondary flow, and shock structures are affected by the CT, especially in highly throttled operating conditions. The stall inception process of the system with axial slots shows unexpected behavior, with modal activities that are not observed without CT. These activities are resolved using unsteady wall pressure (WP) and hot wire measurements.

Self-Excited Blade Vibration Experimentally Investigated in Transonic Compressors: Rotating Instabilities and Flutter

This paper investigates the vibrations that occurred on the blisk rotor of a 1.5-stage transonic research compressor designed for aerodynamic performance validation and tested in various configurations at Technische Universität Darmstadt. During the experimental test campaign, self-excited blade vibrations were found near the aerodynamic stability limit of the compressor. The vibration was identified as flutter of the first torsion mode and occurred at design speed as well as in the part-speed region. Numerical investigations of the flutter event at design speed confirmed negative aerodynamic damping for the first torsion mode, but showed a strong dependency of aerodynamic damping on blade tip clearance (BTC). In order to experimentally validate the relation between BTC and aerodynamic damping, the compressor tests were repeated with enlarged BTC for which stability of the torsion mode was predicted. During this second experimental campaign, strong vibrations of a different mode limited compressor operation. An investigation of this second type of vibration found rotating instabilities to be the source of the vibration. The rotating instabilities first occur as an aerodynamic phenomenon and then develop into self-excited vibration of critical amplitude. In a third experimental campaign, the same compressor was tested with reference BTC and a nonaxisymmetric casing treatment (NASCT). Performance evaluation of this configuration repeatedly showed a significant gain in operating range and pressure ratio. The gain in operating range means that the casing treatment successfully suppresses the previously encountered flutter onset. The aeroelastic potential of the NASCT is validated by means of the unsteady compressor data. By giving a description of all of the above configurations and the corresponding vibratory behavior, this paper contains a comprehensive summary of the different types of blade vibration encountered with a single transonic compressor rotor. By investigating the mechanisms behind the vibrations, this paper contributes to the understanding of flow-induced blade vibration. It also gives evidence to the dominant role of the tip clearance vortex in the fluid–structure-interaction of tip critical transonic compressors. The aeroelastic evaluation of the NASCT is beneficial for the design of next generation casing treatments for vibration control.

An advanced axial-slot casing treatment on a transonic compressor: A close look with computational fluid dynamics and experimental validation

The numerical and experimental investigation of non-axisymmetric casing treatments is complex compared to axisymmetric ones like circumferential grooves. Hence they are still rarely investigated, at least regarding combinations of numerical and experimental work on the same configuration. The casing treatment under investigation is capable of significantly broadening the operating range of a tip-critical transonic compressor. This is accomplished without an efficiency penalty at design conditions. It is demonstrated that the application of a casing treatment over the rotor might impose negative effects on the downstream stator that would have to be considered in future designs. Earlier work by the authors was focused on the experimental investigation, in particular of transient effects near stall. In this paper, RANS and URANS simulations are presented in detail. These are used to further study and explain the causes of the effects that were found experimentally. Additional experimental data are added where appropriate, continuing the validation of the numerical methods.

Overset Grid Method for Simulation of Compressors with Nonaxisymmetric Casing Treatment

This paper describes a computational fluid dynamics method enabling unsteady Reynolds averaged Navier–Stokes computations of compressor configurations including nonaxisymmetric casing treatments. The interaction between the casing treatment and the rotor is simulated thanks to an overset grid approach, in which the meshes of the rotor blade channel and of the slot are independent. The method uses a phase-lag-type approach, which enables reduction of the computational domain to one single slot and one rotor blade channel. The present method is evaluated on an experimental configuration of casing treatments at two rotating speeds.

Numerical Analysis of Flow around Blades in Axial Flow Small Fan

Numerical simulations were conducted by using LES (Large Eddy Simulation) with DSM (Dynamic Smagorinsky Model) about two kinds of axial flow small fan for cooling electronic devices. In this study, the tested fans have non-axisymmetric casing (Normal fan) and axisymmetric casing (Silent fan). The numerical simulations of the flow around those fan blades were compared. Therefore, the both of fans have low pressure (Cp≤-0.6) area in center of suction side surface in blade tip. Especially in normal fan, the pressure level of low pressure areas fluctuate by fluctuation of tip vortex. This fluctuation of tip vortex is caused by fan blades passing small radius parts with separating flow and large radius parts without separating flow alternately.