Papers by Keyword: Wall Slip

Abstract: In order to improve the simulation accuracy of single hole nitroguanidine propellant extrusion molding process, the influence of wall slip on the extrusion process was studied. The finite element method was used to simulate the extrusion forming of single pore nitroguanidine. The power law constitutive equation considering the effect of wall slip was constructed to calculate the slip factor and non-Newtonian index , the influence of the slip coefficient on the pressure field, velocity field and shear velocity field in the flow channel were analyzed. compared with the actual results, the error of the middle aperture is 1.37%, the arc thickness error is 0.96% and the outer diameter error is 1.12%.Those errors are less then 1.5%. So, the simulation results basically meet the production needs.

Abstract: In micro-scale flow condition, the flow rate and fluid extrusion expansibility are always affected by the wall slip between the fluid and tube wall. In this research, a wall slip model based on a micro-tube flow was developed, and then the wall slip effect on the viscous fluid micro-extrusion process was explored by computational fluid dynamics (CFD) with different extruding pressures. The results showed that wall slip make the fluid resistance and expansibility decrease, and the fluid mass flow rate increase.

Abstract: The use of conventional lubricant such as hexadecane and toluene in micro-bearings has shown poor performance due to their hydrophilicity. High friction between the lubricated surfaces could lead to the occurrence of stiction which limits the functionality of a micro-bearing. In order to assess this strategy, a lubrication model of a micro-slider bearing with modified surface was used to simulate the technology. Friction, hydrophobic zone and hydrophobicity coefficient were evaluated based on the modified Reynolds equation. Results showed that in general the application of a hydrophobic coating has a significant improvement in reducing friction. Further, particular care must be taken in choosing the hydrophobic coating zone; an inappropriate choice of this zone will lead to a deterioration of the friction. This finding may have useful implications to accelerate the development of micro-bearings.

Abstract: Effect of wall slip on melt filling flow behaviors in micro injection molding are investigated based on analysis of wall slip mechanism for polymer melt flowing at filling stage. By means of comparisons, slip coefficients in different microchannels are confirmed. With finite element method, the relationship between slip velocity and inlet flow rate or length-diameter ratio is analyzed. The results indicate that the wall slip happen for polymeric melt in microchannels, and slip coefficients are related with the size characteristics of microchannels. Moreover, wall slip velocity has the size effect and increase with the decrease of the section size of microchannels, which cause melt velocity distribution to smooth. At the same time, slip velocity is proportional to length-diameter ratio of microchannel, namely when wall shear stress is uniform, the slip velocity rises with increasing length-diameter ratio of microchannel.

Abstract: The apparent wall slip flows of incompressible and viscoplastic fluids in plane pipeline were analyzed assuming that the apparent slip layer consists solely wall slip layershear layerslug flow part and they are independent of the flow rate. Taking coal slurry for example,the experiment is conducted on pressure pipe rheological and resistance characteristic test system. It studies how two key factors (the particle concentration and the pipe diameter) influence the dense paste wall slip velocity. The assumed apparent slip mechanism provides methodologies for the determination of the slip velocity values that are consistent with the traditional Mooney method and furthermore allows the determination of the true shear rate of the dense paste at the wall and the yield stress. The analysis of the slip data of various dense pastes of rigid particles reveals that the apparent slip layer thickness is related to the particle concentration and the pipe diameter.

Abstract: The flow behaviors for polymer melt at the filling stage in micro injection molding are different from those in conventional injection molding due to the miniaturization of plastic parts. This paper focuses on the study of the effects of three main influencing factors, including the microscale viscosity and wall slip, on melt filling flow in microscale neglected those in conventional injection molding process. The theoretical models and the interrelation of these factors in microscale channels were constructed by means of the model correction method. Then, the micro melt flow behaviors were investigated with comparisons of the available experimental data. The results indicate that the dimensions affect the shear rates and viscous dissipation, which in turn affects the apparent viscosity. Finally, the conclusion is that the melt flow behaviors in microchannels are different from those in macrochannels owing to these significant influencing factors.

Abstract: Compared with in the conventional scales, the rheological properties of polymer melts have changed in the micro-scales. Based on the principle of capillary flow and modified with the method of Bagley or zero die micro-die, the rheological characteristics of polypropylene (PP) were investigated using the HAAKE capillary rheometer under the micro-die and conventional die. The results show that: the viscosity of polymer melt modified with the method of bagley or zero die micro-die in the different L/D ratio of micro-die is the same and the shear viscosity of PP decreases with the increase of shear rate; Compared with the viscosity in the conventional scales, the measured viscosity of PP in the micro-scales was reduced by 46% in 0.25mm diameter die, 44% in the 0.20mm diameter die and 88% in the 0.15mm diameter die at the same shear rate; a layer of fixed polymer melt was found in the inwall of the micro-die through the calculation of wall slip rate.

Abstract: Aimed at the wall slip phenomenon of micro flow, the wall slip boundary condition was added to simulate the process of the micro flow by Polyflow based on the traditional flow analysis method. The effect of the wall slip on the micro flow was verified by comparing the pressure difference data obtained from the simulation with the quoted test data. In addition, based on the Generalized Navier’s slip law, the shear stress and slip coefficient were researched by the numerical simulation analyses to find out the influence of the key factors on the phenomenon of wall slip. The results show that the phenomenon of wall slip is an important factor that cannot be ignored in the micro flow. And only under the high shear stress, the wall slip phenomenon will have an obvious influence on the micro flow. Along with the decrease of the slip coefficient, the wall slip phenomenon becomes more apparent and the micro flow tends to be stable.

Abstract: The linear stability of flows confined between two concentric rotating cylinders with wall slip is examined. The curves of marginal stability in the plane of the angular velocities at the walls are obtained as a function of slip coefficient, showing the stabilizing effect of slip on the flow stability. The eigenfunction for the first mode at the onset of instability and the corresponding cell pattern are obtained for different slip coefficients. It is found that the number of cell pattern in the radial direction increases with increasing slip coefficients.

Abstract: The Reynolds-Orr energy equation is generalized to include the velocity slip effect at the walls, for investigating the energy gain or loss in the disturbed slip flow. Taken the Poiseuille flow, typical prototype of parallel shear flows as an example, it is found that for the very weak effect of wall slip, the disturbance energy being transferred from the basic flow overcomes the viscous dissipation, resulting in the growth of disturbance energy and the destabilizing role of wall slip. Otherwise, the viscous dissipation overcomes the energy production resulting in the decay of the disturbance energy and the stabilizing role of wall slip.