Shengcai Li

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.

The secondary Bjerknes force between two gas bubbles under dual-frequency acoustic excitation

The secondary Bjerknes force is one of the essential mechanisms of mutual interactions between bubbles oscillating in a sound field. The dual-frequency acoustic excitation has been applied in several fields such as sonochemistry, biomedicine and material engineering. In this paper, the secondary Bjerknes force under dual-frequency excitation is investigated both analytically and numerically within a large parameter zone. The unique characteristics (i.e., the complicated patterns of the parameter zone for sign change and the combination resonances) of the secondary Bjerknes force under dual-frequency excitation are revealed. Moreover, the influence of several parameters (e.g., the pressure amplitude, the bubble distance and the phase difference between sound waves) on the secondary Bjerknes force is also investigated numerically.

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.

Notes on radial oscillations of gas bubbles in liquids: thermal effects.

For oscillations of gas bubbles in liquids, the polytropic exponent and thermal damping constant for the high frequency region have been re-evaluated based on the framework by Prosperetti [J. Acoust. Soc. Am. 61, 17-27 (2007)]. It is seen that the approximation of G(1)≪ 1 in Prosperetti (1977) should be dropped for G(1)≥ 10(-2). The ratios of bubble radii to wavelengths are the paramount parameters categorizing the behavior into three different regions.

A General Approach for Rectified Mass Diffusion of Gas Bubbles in Liquids Under Acoustic Excitation

Rectified mass diffusion serves as an important mechanism for dissolution or growth of gas bubbles under acoustic excitation with many applications in acoustical, chemical and biomedical engineering. In this paper, a general approach for predicting rectified mass diffusion phenomenon is proposed based on the equation of bubble motion with liquid compressibility. Nonuniform pressure inside gas bubbles is considered in the approach through employing a well-established framework relating with thermal effects during gas bubble oscillations. Energy dissipation mechanisms (i.e., viscous, thermal, and acoustic dissipation) and surface tension are also included in the approach. Comparing with previous analytical investigations, present approach mainly improves the predictions of rectified mass diffusion in the regions far above resonance and regions with frequencies megahertz and above. Mechanisms for the improvements are shown and discussed together with valid regions and limitations of present approach.

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.

Instability of large-scale prototype Francis turbines of Three Gorges power station at part load

The rapid increase of renewable energies (e.g. wind and solar energies) requires hydroturbines (e.g. large-scale Francis turbines) to be operated at part load more frequently in order to improve the stability and flexibility of the power supply system. A device named as guide plate is currently being introduced into Francis turbines in order to shrink the size of the unit for the cost reduction. However, the effect of the guide plate on the instability of Francis turbines (especially at part load) is still an open question. This paper aims to elucidate the influence of the guide plate on the instability (e.g. in terms of large pressure fluctuation and its propagation) of the prototype Three Gorges turbines and its generation mechanisms. Computational simulations of the prototype Francis turbine have been performed and validated using on-site measurement. The results reveal that the addition of the guide plate induces an extremely low-frequency and high-amplitude pressure fluctuation at part load (e.g. guide vane opening 16°). This low-frequency pressure fluctuation initially generates in the draft tube, and then further propagates upstream (e.g. runner, guide vanes), acting as the dominant frequency. Our data analysis shows that the above pressure fluctuation is generated by the interactions between the vortex in the spiral casing induced by the guide plate and the swirling vortex rope in the draft tube.

Tiny bubbles challenge giant turbines: Three Gorges puzzle

Since the birth of the first prototype of the modern reaction turbine, cavitation as conjectured by Euler in 1754 always presents as a challenge. Following his theory, the evolution of modern reaction (Francis and Kaplan) turbines has been completed by adding the final piece of the element ‘draft-tube’ that enables turbines to explore water energy at efficiencies of almost 100%. However, during the last two and a half centuries, with increasing unit capacity and specific speed, the problem of cavitation has been manifested and complicated by the draft-tube surges rather than being solved. Particularly, during the last 20 years, the fierce competition in the international market for extremely large turbines with compact design has encouraged the development of giant Francis turbines of 700–1000 MW. The first group (24 units) of such giant turbines of 700 MW each was installed in the Three Gorges project. Immediately after commission, a strange erosion phenomenon appeared on the guide vane of the machines that has puzzled professionals. From a multi-disciplinary analysis, this Three Gorges puzzle could reflect an unknown type of cavitation inception presumably triggered by turbulence production from the boundary-layer streak transitional process. It thus presents a fresh challenge not only to this old turbine industry, but also to the fundamental sciences.

A new type of cavitation damage triggered by boundary-layer turbulent production

A new type of cavitation damage has been observed on the turbines installed at the Three Gorges Power Station despite no cavitation detected during model tests. Metallurgical and fluid dynamic analysis suggests that this cavitation is triggered by boundary-layer turbulent production; the damaged (roughened) spot in turn triggers subsequent cavitation (damage) immediately down stream. This forms a sustainable dynamic process, resulting in long and equal-width streamwise damage-strips with spanwise regularity reflecting the spanwise stochastic characteristics of turbulent production. Owing to the heat effect of cavitation, intergranular corrosion takes place through sensitization process, leaving the damaged surface with a corrosion appearance. Also, bluing presents at the damaged tails, owing to the nature of low-intensity damage. Extremely large turbines are much more susceptible to this type of cavitation (damage) owing to the similarity laws currently employed for turbine development not concerning the freestream turbulence and the boundary-layer dynamics.

Fundamentals of cavitation and bubble dynamics with engineering applications

Cavitation and bubble dynamics are important topics of fluid mechanics. Generally, cavitation phenomenon plays a negative role in the hydraulic machineries (e.g. hydroturbines, pumps, space engines, and marine propellers) through various kinds of mechanisms. First, the cavitation could cause serious damage on the surfaces of the materials (e.g. patterned erosions with pits) during its final collapse near the boundary. Second, flow inside/outside the machines will be further deteriorated by the vortices generated by the cavitating flows. Third, significant unstable flow could persist for a long time due to the generation of cavitation. As a result, both the efficiency and stable operation of the machines will be challenged by the cavitation.