Xiaocheng Zhu

Numerical Simulations of Onset of Volute Stall Inside a Centrifugal Compressor

In a previous publication (Guo & Chen et al., 2007), the authors solved the unsteady, 3-D Navier-Stokes equations with the k-e turbulence model using CFX software to show that there is a volute stall coincided with the stage stall of a turbocharger centrifugal compressor operated at 423m/s tip speed and the stage stall frequency is dictated by a volute standing wave. This paper presents the flow condition at the vaneless diffuser and volute from the same simulation at various mass flow rates from stage peak efficiency to deep stage stall. Time averaged flow conditions show that (1) the influence of exducer blade passing at the volute inlet rapidly diminishes at the compressor peak pressure ratio point and the influence vanishes when the stage is in stall; (2) only at the peak pressure ratio point, circumferentially averaged, spanwise distribution of radial velocity at the volute inlet has an inflection point and the distribution meets the requirement of the Fjortoft instability theorem; (3) in the volute discharge section, the flow stalls after the stage stalls and the vortex core at the cross sectional center of the section breaks down; (4) impeller total pressure rise curve has a flat region in the middle before the stage stalls and (5) diffuser stall triggers the stage stall and drives the volute into stall.Copyright

A three-dimensional compressible flow model for rotating waves in vaneless diffusers with unparallel walls

A three-dimensional compressible flow model is presented to study the occurrence of weak rotating waves in unparallel wall vaneless diffusers in centrifugal compressors. The model extends the three-dimensional compressible flow model for parallel wall diffusers recently developed by present authors. Linearised three-dimensional compressible Euler equations casted on a rotating frame of reference travelling at the same speeds as the waves are employed and the viscous effects are ignored. Complex functions of the solutions to the linearised Euler equations are then obtained by a second-order finite difference method and the singular value decomposition technique. Undisturbed flow is assumed potential and first solved by numerical method of strongly implicit procedure. Critical inlet flow rate and rotating wave speed of diffusers of three different shroud wall shapes, namely, convergent, convergent then divergent and constant area tapered, are studied for three different diffuser outlet-to-inlet radius ratios and for different inlet Mach numbers, and results compared with those from diffusers with parallel walls. The results show suppression effects on rotating stall by the contracting walls and the suppression effects vary with wall contraction rate, wall shape, inlet Mach number and the diffuser radius ratio. Further, the effects of diffuser inlet contraction are studied and prediction of the model is compared with experimental result.

Surrogate-Based Recurrence Framework Approach to Unsteady Aerodynamic Modeling of Wind Turbine Airfoils

This paper proposes a method for predicting unsteady aerodynamics of wind turbine airfoils using surrogate-based recurrence framework (SBRF) method. Using specified simulation results generated by the CFD method in some conditions, the unsteady aerodynamic model could be established by the Kriging surrogate model. Then, time-domain predictions of unsteady lift, moment, and drag in different conditions can be gained by the SBRF method with minimal computational expense. Some parameters have been set according to the operational condition of wind turbines so as to describe the unsteady aerodynamic modeling problem. The unsteady aerodynamic performance of the wind turbine airfoils in some training conditions is carried out by the commercial CFD simulator CFX, the results of which could be utilized to build the SBRF. Then the predicted time-varying aerodynamic characteristics of wind turbine airfoils in the validated condition could be obtained by the SBRF method and the CFD simulation, respectively. It is revealed from the results that the time-varying aerodynamic characteristics of wind turbine airfoils in most dynamic stall cases can accurately approximate by the SBRF method. In addition, the SBRF method has relatively less computational cost compared with the CFD method. Therefore, it can be used as the foundation of aero-elastic analysis and design optimization studies of wind turbines.Copyright

Optimization of Wind Turbine Blades Using Lifting Surface Method and Genetic Algorithm

This paper describes an optimization method for the design of horizontal axis wind turbines using the lifting surface method as the performance prediction model and a genetic algorithm for optimization. The aerodynamic code for the design method is based on the lifting surface method with a prescribed wake model for the description of the wake. A micro genetic algorithm handles the decision variables of the optimization problem such as the chord and twist distribution of the blade. The scope of the optimization method is to achieve the best trade off of the following objectives: maximum of annual energy production and minimum of blade loads including thrust and blade rood flap-wise moment. To illustrate how the optimization of the blade is carried out the procedure is applied to NREL Phase VI rotor. The result shows the optimization model can provide a more efficient design.Copyright

Influence of wake asymmetry on wind turbine blade aerodynamic and aeroelastic performance in shear/yawed wind

Wind turbines operate in atmospheric shear layer and often in yawed flow condition, which, among others, produce cyclic fluctuating loads on the blades. The unsteady aerodynamics of wind turbine in both shear and yawed wind are studied in the present work. The rotor aeroelastic behaviors considering blade flexibility are also discussed. An advanced aeroelastic model based on free wake lifting surface model and geometrically exact beam theory is established to conduct the study. The aerodynamic simulations show that the wake is asymmetric in both shear and yawed conditions. Comparison is made between the free wake lifting surface model and the skewed wake correction model used in BEM theory for the induced velocity on blade at different azimuth positions. The correction model for yawed flow in BEM theory seems to overpredict the induced velocity variations. The skewed wake induction in yawed condition causes a phase shift of angle of attack variation as a function of azimuth angle, particularly on the outboard of the blade. In addition, the blade aeroelastic deformations are found to further change the phase of circumferential distribution of aerodynamic loads. Rotor moments are influenced by the phase shifts of aerodynamic loads.Wind turbines operate in atmospheric shear layer and often in yawed flow condition, which, among others, produce cyclic fluctuating loads on the blades. The unsteady aerodynamics of wind turbine in both shear and yawed wind are studied in the present work. The rotor aeroelastic behaviors considering blade flexibility are also discussed. An advanced aeroelastic model based on free wake lifting surface model and geometrically exact beam theory is established to conduct the study. The aerodynamic simulations show that the wake is asymmetric in both shear and yawed conditions. Comparison is made between the free wake lifting surface model and the skewed wake correction model used in BEM theory for the induced velocity on blade at different azimuth positions. The correction model for yawed flow in BEM theory seems to overpredict the induced velocity variations. The skewed wake induction in yawed condition causes a phase shift of angle of attack variation as a function of azimuth angle, particularly on the outb...

Reduced-order modelling of wind turbine airfoil unsteady aerodynamic loading

In this paper, three different reduced-order models (auto-regressive and moving average, Volterra series and a surrogate-based recurrence framework model) are presented for the prediction of the unsteady dynamic loading of wind turbine airfoils. A wind turbine blade section can experience unsteady aerodynamic loads when subjected to an unsteady aerodynamic environment. The generations of three reduced-order models for the evaluation of unsteady aerodynamic loads are investigated, with the different models used as a representation of linear or non-linear loading. The validity of the presented reduced-order models is assessed mainly by comparing the model output with unsteady time-accurate computational fluid dynamics (CFD) simulations. The results reveal an encouraging agreement between the computational fluid dynamics simulations and the model predictions under different conditions. All three reduced-order models would therefore be useful in engineering conditioning for aeroelastic analysis and wind turbine design optimization.

Numerical Simulation of Surge in Turbocharger Centrifugal Compressor: Influences of Downstream Plenum

Surge is an important instability seriously affecting compression systems. This paper presents a numerical simulation of surge flow phenomenon inside a turbocharger centrifugal compressor with a vaneless diffuser. The compressor was discharged into a plenum and the effect of the plenum on surge behavior of the compressor system was investigated. The entire geometry of the compressor, including the impeller, vaneless diffuser, volute housing and downstream plenum, were included in the simulation. Three-dimensional Reynolds averaged compressible Navier–Stokes equations were solved with the k–e turbulence model using commercial software CFX and two different sizes of plenum were studied. A new plenum model is proposed which allows temporal variation of temperature inside the plenum. The numerical technique employed to set up CFD (computational fluid dynamics) with such an unstable flow system are described. The results show that when the plenum volume was nearly doubled, the dominating frequency of the system suddenly dropped from 72Hz to 23Hz. During the surge cycle, the compressor characteristic (pressure ratio ν mass flow curve) showed distinct differences. With the smaller plenum, the characteristic showed random traces with little global backflow at the compressor inlet, while with the larger plenum, clear surge cycles are displayed with strong global backflow at the inlet. The flow fields of the two systems are presented as functions of time and show distinct differences. In the case of the smaller plenum, the circumferential flow field inside the impeller is non uniform, showing influences of rotating stall, while in the case of the large plenum, the circumferential uniformity returns and the flow field behaves quasi-steadily during the surge cycle. With the larger plenum, the volute flow synchronises with the inlet mass flow oscillation in time and a completed vortex break down occurs at every volute cross section, but with the smaller plenum the synchronisation disappears and vortex break down only occurs partially at the centers of some volute cross sections.Copyright