Yulin Wu

Vibration-Based Condition Monitoring

Condition monitoring is the process of monitoring a condition parameter in machinery, so that a significant change is indicative of a developing failure. The use of conditional monitoring allows maintenance to be scheduled, or other actions taken to avoid the consequences of failure before it actually occurs.

Unsteady Numerical Simulation of Cavitating Turbulent Flow Around a Highly Skewed Model Marine Propeller

The cavitating flows around a highly skewed model marine propeller in both uniform flow and wake flow have been simulated by applying a mass transfer cavitation model based on Rayleigh‐Plesset equation and k- shear stress transport (SST) turbulence model. From comparison of numerical results with the experiment, it is seen that the thrust and torque coefficients of the propeller are predicted satisfactory. It is also clarified from unsteady simulation of cavitating flow around the propeller in wake flow that the whole process of cavitating-flow evolution can be reasonably reproduced including sheet cavitation and tip vortex cavitation observed in the experiments. Furthermore, to study the effect of pressure fluctuation on the surrounding, pressure fluctuations induced by the cavitation as well as the propeller rotation are predicted at three reference positions above the propeller for comparison with the experimental data: The amplitudes of the dominant components corresponding to the first, second, and third blade passing frequencies were satisfactorily predicted. It is noted that the maximum difference of pressure fluctuation between the calculation and experiment reached 20%, which might be acceptable by usual engineering applications. DOI: 10.1115/1.4003355

Simulations of unsteady cavitating turbulent flow in a Francis turbine using the RANS method and the improved mixture model of two-phase flows

This paper reports the simulation results for the unsteady cavitating turbulent flow in a Francis turbine using the mixture model for cavity–liquid two-phase flows. The RNG k–ε turbulence model is employed in the Reynolds averaged Navier–Stokes equations in this study. In the mixture model, an improved expression for the mass transfer is employed which is based on evaporation and condensation mechanisms with considering the effects of the non-dissolved gas, the turbulence, the tension of interface at cavity and the effect of phase change rate and so on. The computing domain includes the guide vanes, the runner, and the draft tube, which is discretized with a full three-dimensional mesh system of unstructured tetrahedral shapes. The finite volume method is used to solve the governing equations of the mixture model and a full coupled method is combined into the algorithm to accelerate the solution. The computing results with the mixture model have been compared with those by the single-phase flow model as well as the experimental data. The simulation results show that the cavitating flow computation based on the improved mixture model agrees much better with experimental data than that by the single-phase flow calculation, in terms of the amplitude and dominated frequency of the pressure fluctuation. It is also observed from the present simulations that the amplitude of the pressure fluctuation at small flow rate is larger than that at large flow rate, which accords with the experimental data.

Cavitating Turbulent Flow Simulation in a Francis Turbine Based on Mixture Model

As a numerical method to study the cavitation performance of a Francis turbine, the mixture model for the cavity/liquid two-phase flow is adopted in the cavitating turbulent flow analysis together with the re-normalization group (RNG) k-e turbulence model in the present paper. The direct coupling numerical technique is used to solve the governing equations of the mixture model for the two-phase flow. Unsteady cavitating flow simulation around a hydrofoil of ALE15 is conducted as preliminary evaluation. Then, the cavitating flow in a Francis turbine is treated from the steady flow simulation since the feasibility of the cavitation model to the performance prediction of the turbine is the present major concern. Comparisons of the computational results with the model test data, i.e., the cavitation characteristics of hydraulic efficiency and the overload vortex rope at the draft tube inlet being reproduced reasonably, indicate that the present method has sufficient potential to simulate the cavitating flow in hydraulic turbines. Further, the unsteady cavitating flow simulation through the Francis turbine is conducted as well to study the pressure fluctuation characters caused by the vortex rope in the draft tube at partial load operation.

Pressure Fluctuation Prediction of a Model Kaplan Turbine by Unsteady Turbulent Flow Simulation

While larger and larger turbines are being developed, hydraulic stability has become one of the key issues for their performance assessments. An accurate prediction of their pressure fluctuations is vital to the success of new model development. In this paper, we briefly introduced the method, i.e., the three-dimensional unsteady turbulent flow simulation of the complete flow passage, which we used for predicting the pressure fluctuations of a model Kaplan turbine. In order to verify the prediction, the model turbine was tested on the test rig at the Harbin Electric Machinery Co., Ltd. (HEC), China, which meets all the international standards. Our main findings from this numerical prediction of pressure fluctuations for a model Kaplan turbine are as follows. (1) The approach by using 3D unsteady turbulent flow including rotor-stator interaction for the whole flow passage is a feasible way for predicting model turbine hydraulic instability. The predicted values at different points along its flow passage all agree well with the test data in terms of their frequencies and amplitudes. (2) The low-frequency pressure fluctuation originating from the draft tube is maximal and influences the stability of the turbine operation mostly. The whole flow passage analysis shows that the swirling vortex rope in the draft tube is the major source generating the pressure fluctuations in this model turbine. (3) The second harmonic of the rotational frequency 2f(n) is more dominant than the blade passing frequency Zf(n) in the draft tube. This prediction, including the turbulence model, computational methods, and the boundary conditions, is valid either for performance prediction at design stage and/or for operation optimization after commissioning.

Distribution of Pressure Fluctuations in a Prototype Pump Turbine at Pump Mode

Pressure fluctuations are very important characteristics in pump turbines operation. Many researches have focused on the characteristics (amplitude and frequencies) of pressure fluctuations at specific locations, but little researches mentioned the distribution of pressure fluctuations in a pump turbine. In this paper, 3D numerical simulations using SSTk − ω turbulence model were carried out to predict the pressure fluctuations distribution in a prototype pump turbine at pump mode. Three operating points with different mass flow rates and different guide vanes’ openings were simulated. The numerical results show how pressure fluctuations at blade passing frequency (BPF) and its harmonics vary along the whole flow path direction, as well as along the circumferential direction. BPF is the first dominant frequency in vaneless space. Pressure fluctuation component at this frequency rapidly decays towards upstream (to draft tube) and downstream (to spiral casing). In contrast, pressure fluctuations component at 3BPF spreads to upstream and downstream with almost constant amplitude. Amplitude and frequencies of pressure fluctuations also vary along different circumferential locations in vaneless space. When the mass flow and guide vanes’ opening are different, the distribution of pressure fluctuations along the two directions is different basically.

Runaway transient simulation of a model Kaplan turbine

The runaway transient is a typical transient process of a hydro power unit, where the rotational speed of a turbine runner rapidly increases up to the runaway speed under a working head as the guide vanes cannot be closed due to some reason at the load rejection. In the present paper, the characteristics of the runaway transient of a model Kaplan turbine having ns = 479(m-kW) is simulated by using a time-dependent CFD technique where equation of rotational motion of runner, continuity equation and unsteady RANS equations with RNG k- turbulence model are solved iteratively. In the calculation, unstructured mesh is used to the whole flow passage, which consists of several sub-domains: entrance, casing, stay vanes + guide vanes, guide section, runner and draft tube. And variable speed sliding mesh technique is used to exchange interface flow information between moving part and stationary part, and three-dimensional unstructured dynamic mesh technique is also adopted to ensure mesh quality. Two cases were treated in the simulation of runaway transient characteristics after load rejection: one is the rated operating condition as the initial condition, and the other is the condition at the maximum head. Regarding the runaway speed, the experimental speed is 1.45 times the initial speed and the calculation is 1.47 times the initial for the former case. In the latter case, the experiment and the calculation are 1.67 times and 1.69 times respectively. From these results, it is recognized that satisfactorily prediction will be possible by using the present numerical method. Further, numerical results show that the swirl in the draft-tube flow becomes stronger in the latter part of the transient process so that a vortex rope will occur in the draft tube and its precession will cause the pressure fluctuations which sometimes affect the stability of hydro power system considerably.

Numerical investigation of unsteady cavitating turbulent flow around a full scale marine propeller

This paper treats the unsteady cavitating turbulent flow around a full scale marine propeller operated in non-uniform ship wake. The RANS method combined with k−ω SST turbulence model and the mass transfer cavitation model was applied for the flow simulation. It is noted that both the propeller performance and the unsteady features of cavitating turbulent flow around the propeller predicted by the numerical calculation agreed well with the experimental data. Due to the non-uniform wake inflow and gravity effect, there occurred periodical procedure for cavity development such as cavitation inception, growth, shrinking, etc near the blade tip for the propeller. The study also indicated that there was considerably large pressure fluctuation near the propeller during the operation. The 1st order frequency of pressure fluctuation predicted by numerical simulation equaled the rotating frequency of propeller blades. Both amplitude and frequency agreed with the experimental results fairly well.

Experimental Study on Internal Flow of a Mini Centrifugal Pump by PIV Measurement

Abstract The internal flow field in a centrifugal pump working at the several flow conditions has been measured by using the particle image velocimetry (PIV) technique with the laser induced fluorescence (LIF) particles and the refractive index matched (RIM) facilities. The impeller of the centrifugal pump has an outlet diameter in 100mm, and consists of six two-dimensional curvature backward swept blades of constant thickness. Measured results give reliable flow patterns in the pump. It is obvious that application of LIF particle and RIM are the key methods to obtain the right PIV measured results in pump internal flow Keywords : Centrifugal pump, PIV, laser induced fluorescence, refractive index matched. 1. Introduction Experimental flow studies are often conducted using optical diagnostic techniques including qualitative flow visualization methods, laser Doppler velocimetry (LDV), and more recently full field methods such as particle image velocimetry (PIV) for velocity field measurements, and laser induced fluorescence (LIF) for concentration field measurements. A difficulty common to all these methods is the refraction of light passing through gas-solid or liquid-solid interfaces of model and/or test section walls (Budwig, 1994)

A mixture model with modified mass transfer expression for cavitating turbulent flow simulation

Purpose – The purpose of this paper is to provide a mixture model with modified mass transfer expression for calculating cavitating (two‐phase) flow.Design/methodology/approach – The mass transfer relations are derived based on the mechanics of evaporation and condensation, in which the mass and momentum transfer count for factors such as non‐dissolved gas, turbulence, surface tension, phase‐change rate, etc.Findings – As shown by two calculation examples, the modified model can predict the cavitating flow with high accuracy, agreeing well with experimental results.Originality/value – The methods described are of value in improving stability in numerical calculations.