Yingzheng Liu

Numerical Analysis of Leakage Flow Through Two Labyrinth Seals

The leakage flow through two labyrinth seals, e.g the interlocking seal and the stepped seal, was numerically investigated. Preliminary calculation of the seal-cavity averaged pressure by using the one-dimensional control volume method showed favorable agreement with the experimental measurements. Subsequently, in-depth understanding of the fluid flow through the labyrinth seals was obtained by employing Computational Fluid Dynamics (CFD) and k-ε turbulence model, which resulted in a potential wealth of information like the streamline pattern, velocity vector field, and distribution of turbulent kinetic energy and static pressure. At the clearance of the seal the turbulent kinetic energy reached the peak value, while in the bulk region of the cavities it decayed fast. The static pressure rapidly dropped as the fluid flow went through the clearance; no distinct difference of the static pressure was inspected in the cavities. Also noted from the numerical results was that the stepped seal showed better sealing performance than the interlocking seal

Study of creep–fatigue behavior in a 1000 MW rotor using a unified viscoplastic constitutive model with damage

A unified viscoplastic constitutive model with damage is presented to describe creep–fatigue deformation behavior. Creep and fatigue tests under different temperature and loading conditions were conducted to validate the material model. Good agreement was achieved between the simulated and measured values. The material model was further applied to a specific part of a steam turbine rotor for numeric investigation of the creep–fatigue behavior. The multiaxial mechanical behavior of the rotor was studied in detail in terms of temperature, von Mises stress, equivalent strain, accumulated plastic strain, and damage. The results illustrate that the stress is concentrated on the notch of the groove. In addition, although the accumulation of damage at the rotor locations is dominated by creep behavior, the results actually disclose that the combination of creep and fatigue deformation behavior further accelerate the damage.

Simulation of Blood Flow in Intracranial ICA-PComA Aneurysm Via Computational Fluid Dymamics Modeling

In the present study, hemodynamics of the internal carotid artery-posterior communicating artery (ICA-PComA) was numerically modeled with CFD approach. The steady and pulsating blood flow in the ICA-PComA was simulated. The main concern was placed on the influence of aneurysm geometry on the local hemodynamics by changing the sac diameter and Aspect Ratio (AR) of the aneurysm. The numerical results show the significantly weakened Wall Shear Stress (WSS) and the intensified wall pressure in the aneurysm as AR is increased. Two factors, i.e., low WSS and high pressure of the aneurysm, may play important roles in the fragile change of the aneurysm and the final rupture. The distributions of Time-Averaged WSS (TAWSS), Oscillatory Shear Index (OSI) and spatial WSS Gradients (WSSG)) were determined and discussed in view of their influences on the evolution of ICA-PComA aneurysm.

Numerical Evaluation of Two-Fluid Mixing in a Swirl Micro-Mixer

A collaborative investigation of two-fluid mixing in a swirl micro-mixer was carried out by the Shanghai Jiao Tong University and the Tokyo Denki University. Pure water and a mixture of glycerol and water were separately injected into branch channels and they were subsequently mixed in the central chamber. The two-fluid flow pattern was numerically modeled, in which the dependence of the mixture viscosity and density on the mass fraction of glycerol in the mixing fluid was carefully taken into consideration. The mixing performance of the two fluids was evaluated by varying the Reynolds numbers and the mass fractions of glycerol in water. The mixing process was extensively analyzed using streamline maps and contour plotting distributions of pressure and glycerol concentration. The numerical results show that the acceptable uniformity of mixing at Re = 0.1 is primarily attributed to the time-consuming molecular diffusion, whereas the cost-effective mixing at Re > 500 was obtained because of the generation of the swirling flow. The increasing mass fraction of glycerol in water was found to attenuate the mixing performance. The preliminary microscopic visualization of the two-fluid mixing at Re=1300 demonstrated the consistence with the numerical results.

Simultaneous Flow Visualization and Wall-Pressure Measurement of the Turbulent Separated and Reattaching Flow Over a Backward-Facing Step

Unsteady behaviors of the large-scale vortical structure superimposed in the turbulent separated and reattaching flow over a backward-facing step were convincingly delineated by performing simultaneous measurements of the wall pressure fluctuations and visualizations of the flow. Toward this end, a synchronized instrumentation system integrated with the microphone array and the high-speed camera was established. The smoke-wire technique was employed to visualize the unsteady events. A thorough analysis based on the wall pressure fluctuations disclosed that the large-scale vortical structure shedding at the frequency of fH/U0= 0.064 gave a primary contribution to the wall pressure fluctuations, and consequently dominated unsteady behaviors of the turbulent shear layer. The convection velocity of the large-scale vortical structure was determined as UC=0.550 U0. The instantaneous flow visualizations and wall pressure were compared in a straightforward manner. Below the separation bubble and the reattachment zone, the negative peak of the time-varying wall pressure was in phase with passage of the local large-scale vortical structure. In the redeveloping turbulent boundary layer, the decaying large-scale vortical structure was clearly revealed.

Multiaxial Creep-Fatigue Life Prediction on the Rotor of a 1000MW Supercritical Steam Turbine

Damage of a high temperature rotor subjected to the creep-fatigue interaction was numerically investigated. Toward that end, a high temperature rotor of a 1000MW supercritical steam turbine was chosen for the study. A continuum damage mechanics model (CDM), which depicts the fatigue-creep interaction, was developed in the present paper. During the practical startup and shutdown processes, the influence of the multiaxial creep-fatigue interaction on strength of the rotor was analyzed in terms of stress, strain and damage. Comparison of the results from linear damage accumulation model (LDA) and CDM demonstrated that CDM was more reasonable to predict the lifetime of the rotor due to the multiaxial creep-fatigue interaction.Copyright

A Continuum Damage Mechanics-Based Viscoplastic Model of Adapted Complexity for High-Temperature Creep–Fatigue Loading

A continuum damage mechanics (CDM) based viscoplastic constitutive model is established in this study to describe the fully coupling of creep and fatigue behavior. The most significant improvement is the introduction of a continuum damage variable into the constitutive equations, instead of considering creep damage and fatigue damage separately. The CDM-based viscoplastic constitutive material model is implemented using a user-defined subroutine (UMAT). A standard specimen is used for carrying out uniaxial creep, fatigue, and creep–fatigue interaction tests to validate the material model. In addition, to further demonstrate the capability of the material model to predict the complex material behavior, a complex strain-control loading test is performed to validate the material model. The simulated and measured results are in good agreement at different temperatures and loadings, in particular for rapid cyclic softening behavior following crack initiation and propagation.

Numerical investigation on influence of real gas properties on nonlinear behavior of labyrinth seal-rotor system

A nonlinear model of fluid-structure interaction between high-pressure methane leakage through interlocking seal and the whirling rotor was proposed. The real gas properties of the methane at the pressure 102Â?bar were considered in the mathematical reduction. Three cases of different pressure ratio 1.7, 2.5 and 3.3 at the constant inlet pressure 250Â?bar were chosen in the present study. Two models, e.g., ideal gas model and real gas model, were employed to investigate the influence of real gas properties of methane leakage on the rotor dynamics. Distribution of thermal parameters in the seal cavities and seal clearance were determined, e.g., density, temperature, compressibility factor and specific heat capacity. The rotor-seal system was modeled as a Jeffcot rotor subject to shear stress and pressure force associated with the methane gas leakage. Spatio-temporal variation of the methane gas forcing on the rotor surface in the coverage of the seal clearance and the cavity volume was calculated by using the Muzynska model and the perturbation analysis, respectively. The governing equation of rotor dynamics which includes the main contribution from the methane leakage forcing was solved by using the fourth-order Runge-Kutta method, resulting in the orbit of the whirling rotor.

WALL PRESSURE FLUCTUATIONS OF TURBULENT FLOW OVER BACKWARD-FACING STEP WITH AND WITHOUT ENTRAINMENT:MICROPHONE ARRAY MEASUREMENT

Wall pressure fluctuations in turbulent boundary layer flow over backward-facing step with and without entrainment were investigated. Digital array pressure sensors and multi-arrayed microphones were employed to acquire the time-averaged static pressure and fluctuating pressure, respectively. The differences of two flows were scrutinized in terms of static pressure characteristics, pressure fluctuations, cross-correlation and coherence of wall pressure. Introduction of the entrainment increased scale of large-scale vortical structure and reduced its convection velocity. However, shedding frequency of large-scale vortical structures was found to be the same for both flows.

Creep-Fatigue Life Prediction of Stop and Regulating Valves on the Intermediate-Pressure Section of a 1000MW Steam Turbine

The stop&regulating integrated valve on the intermediate-pressure (IP) section of a 1000MW steam turbine is presented in this paper. A multiaxial model based on continuum damage mechanics (CDM) is applied to life prediction of the valve. The transient stress and the temperature fields of the valve in a 1000MW supercritical steam turbine are investigated by using finite element method (FEM) for fatigue-creep. Since the turbine typically runs 120 days between starts, a simplified mission profile for a 120-day block was created. Accordingly, the 120-day loading block with plasticity and creep was run repetitively to achieve a 22 years creep-fatigue life prediction. The interaction between creep and fatigue was considered in total damage in proper order. Due to highly complex stress and structure, the multiaxial factors for fatigue and creep are assessed from the temporal and spatial points of view respectively. Furthermore, the creep-fatigue damage of the integrated valve is discussed in relation to the multiaxial factors. The results drawn from the multiaxial CDM model give a satisfactory life prediction on the valve.Copyright