Wang Xilin

Large-eddy Simulation of Near-field Dynamics in a Particle-laden Round Turbulent Jet

This article investigates the near-field dynamics in a particle-laden round turbulent jet in a large-eddy simulation (LES). A point-force two-way coupling model is adopted in the simulation to reveal the particle modulation of turbulence. The particles mainly excite the initial instability of the jet and bring about the earlier breakup of vortex rings in the near-field. The flow fluctuating intensity either in the axial or in the radial directions is hence increased by particles. The article also describes the mean velocity modulated by particles. The changing statistical velocity induced by particle modulation implies the effects of modulation of the local flow structures. This study is expected to be useful to the control of two-phase turbulent jets.

Large Eddy Simulation of Turbulent Channel Flow with 3D Roughness Using a Roughness Element Model

Large eddy simulation of turbulent channel flow with dense and small 3D roughness elements is carried out using a roughness element model. Profiles of mean Reynolds stress, mean velocity and rms velocity as well as turbulent structures near the wall are obtained. The shear stress in the rough wall is larger than that in the smooth wall side and the rough wall has a larger influence on the channel flow. Profiles of mean streamwise velocity near the wall have logarithmic velocity distributions for both smooth and roughness walls, while there is a velocity decrease for the rough wall due to larger fractional drag. All the three components of rms velocities in the rough wall region are larger than that in the smooth wall region, and the roughness elements on the wall increase turbulent intensity in all directions. The streak spacing and average diameter of near wall quasi-streamwise vortices increase with the presence of roughness elements on the wall and it is shown that the rough wall induces complex and strong streamwise vortices. Results of dense and small 3D roughness elements in both turbulent statistics and structure, obtained with a relatively simple method, are found to be comparable to related experiments.

Study on the influencing factors of natural degradation of liquid silicone rubber used for external insulation

In recent years, liquid silicone rubber (LSR) has been widely used in the external insulation field. However, further study on the degradation mechanism and the influencing factors of liquid silicone rubber is still required. Especially when it is used in southern parts of China, of which the high temperature and humidity environment could accelerate the degradation process. It is reported that cracking and chalking phenomena were observed on the liquid silicone rubber sheath of PT and CT. In this paper, a series of ageing experiments were designed and carried out to test the performance of liquid silicone rubber when exposed to factors such as high temperature, high humidity, sun exposure and corona discharge. After the experiments, characteristics such as hardness, hydrophobicity, glossiness and mechanical properties were tested and SEM was used to study the change of micro structure. It is concluded that high temperature, high humidity and sun exposure play significant roles in the degradation of liquid silicone rubber, which could lead to the rise of hardness, loss of hydrophobicity, decrease of glossiness and surface cracking. The results also show that the change of mechanical properties of liquid silicone rubber were tiny.

Experimental Investigation of Flow Drag and Turbulence Intensity of a Channel Flow with Rough Walls

Turbulent flow over rough walls is investigated through acoustic doppler velocimeter measurements. Smooth rods with a diameter of 6 mm are used as roughness elements. The rods are arranged at the channel bottom wall in three ways: longitudinally (along the flow direction); transversely (orthogonally to flow direction); and mesh-shaped (in a staggered mesh). The transverse roughness elements produce higher disturbance and flow drag than longitudinal roughness. Both turbulence intensity and flow drag for mesh-shaped roughness are not significantly different from those of transverse roughness, indicating that the transverse roughness elements mainly affect turbulence characteristics. Both turbulence intensity and flow drag are greatest for transverse rough walls at w/k = 7; likewise, both increase with decreasing w/k for longitudinal rough walls. Compared with channel flow over a smooth wall, the turbulence intensity increases considerably, while the flow drag only increases slightly when w/k is small for the three arrangements. This is beneficial for enhancing heat transfer and mixing in channel flows with relatively small flow resistance.

Large Eddy Simulation of SGS Turbulent Kinetic Energy and SGS Turbulent Dissipation in a Backward-Facing Step Turbulent Flow

The instantaneous and time-averaged statistic characteristics of the sub-grid scale (SGS) turbulent kinetic energy and SGS dissipation in a backward-facing step turbulent flow have been studied by large eddy simulation. The SGS turbulent kinetic energy and SGS turbulent dissipation vary in different flow regions and decrease with the flow developing spatially. The fluid molecular dissipation shares about 14% to 28% of the whole dissipation.

Visualization and analysis of longitudinal vortices at curved walls of 2D laminar and turbulent channel flows

The 3D structure of longitudinal vortices at the curved walls of both laminar and turbulent channel flows are visualized by the hydrogen bubble technique. Together with the conditional sampling of the turbulent characteristics at the wall processed by the VITA method the dynamics of the near-wall structure is discussed.

A new method to remove the aging RTV coatings on glass insulators

RTV coatings had been widely used in electric insulators to prevent pollution flashover performance at external insulation. For the bad weatherability of polymers, RTV coatings will be aging during the long serve time. To remove the aging coatings, many methods had been used, such as solvent dissolve, high pressure water flushing, and dry ice flushing. But it is not easy for these methods to remove the coatings entirely and easily. In this paper we described a new method-laser cleaning, which is a very easy and useful method. We used the continuous laser with Ytterbium Laser System (YLS-2000) and the pulsed laser with IPG fiber laser system (Nd:YAG) in the experiment. Different scanning speed of the laser irradiating and different energy density had been tested to find Optimization parameters which could remove the coatings and not damage the surface of the glass insulators. The both laser irradiating can remove the coatings clearly in the experiment. But the continuous laser will leave the carbon clusters on the surface, which is easy to remove but bad for the insulators. The scanning speed can be higher than 10 mm/s that means the glass insulators with 20mm diameter can be dealt in 3 minutes with pulsed laser. The laser cleaning is a good method to remove the RTV coatings on the glass insulator surfaces up and down. With the cost of laser technology decreases, it will be a better method as a coating removing method.

Study of nonlinear nanoparticles doped silicone rubber composites and its dielectric properties

For the purpose of changing dielectric behavior of silicone rubber composites to obtain desirable dielectric properties, variety of inorganic particles fillers were doped into silicone rubber composites. In this paper we researched the dielectric properties of silicone rubber composites doped by nonlinear fillers, including zinc oxide(ZnO) with the size of 40nm and barium strontium titanate (BST) with the size of 200nm and measured the dielectric constant (εr), dielectric loss angle tangent (tanδ) and resistivity(ρ). Various mass fractions (0%,10% and 24%) of nanosized nonlinear powder (BST and ZnO)are doped into silicone rubber to obtain multi-phase dielectric composites. The parameters were measured at room temperature and different electric field with the intensity increased from 103V m-1 to 107 V m-1 was applied. The result showed that BST and ZnO fillers tended to exhibit enhanced dielectric constant and lower tan because of the increase of effective mass fraction. The influence mechanism were discussed in accordance with the mass fractions, electric intensity and polarization mode of each kind of nonlinear fillers.

Research on shed crack of composite insulators used in strong wind area

Composite insulators promoted development of ultra-high voltage transmission lines and study of the composite insulators is a very important topic at the electric engineering. For the wide area and variable weather of China, the composite insulators will endure several terrible conditions. Strong wind area is very special environment at China or the worldwide. Now the transmission lines problems caused by are mainly focusing on the tower strength, conductor galloping and external insulation selection. The sheds cracks of the composite insulators at 750 kV AC lines in Xinjiang province is very new topic. It is found that when the wind speed is higher than 30 m/s, the biggest sheds of the tested composite insulator would vibrate at high frequency and great amplitude. This paper obtained the reason of the shed crack at high speed wind. With the ANSYS simulation, the process between the strong wind and the sheds of composite insulators was studied. The mechanical properties were researched. To indicate the mechanical properties of HTV materials, we tested several composite insulators used in 750 kV lines. The tearing fatigue strength of HTV should be considered as one important factor when select the HTV.