Numerical Analysis Of Influence Of Boundary Slip On Micro-scale Gap Flow

The gap flow has a larger number of applications in fluid dynamics system, such as in the hydraulic transmission, hydrostatic bearing, liquid film lubrication. In traditional theory a constant liquid viscosity is always assumed, while with the requirements of high speed precision machinery and equipment, it is a challenge for traditional gap flow theory undergone special working conditions of heavy load, ultra-high temperature, very high speed and unstable, especially for the case that the gap thickness of gap flow is reduced to the scale of microns or even thinner. The internal flow pattern will be significantly different from that of macroscopic fluid flow, which will show out some flow characteristics, such as the non-continuous effect and wall boundary effects and the low Reynolds flow behaviors. So the gap flow in micro-scale has some characteristics similar to that of micro-fluidics. The investigation of slippage at solid-liquid interfaces is the basics topics of microfluidics and the wall slip has become a very important factor. In-depth study the slip law in micro-gap flow will promote the design and application of the gap flow in engineering, and will also provide some new mind for the applications of slippage in microfluidics, which is very important for scientific research:(1)For the slippage phenomenon adjacent to wall in micro-scale gap flow, CFD method was applied to investigate the effects of wall slip on flow behaviors. Navier slip boundary condition was adopted in simulating the gap flow between oil sealing sides in hydrostatic bearing system. After examining consistency of results between simulation and theory, the influence of slip parameter on gap flow behavior was implemented. The influence of wall apparent viscosity coefficient, temperature-viscosity characteristic and Non-newtonian behavior on distributions of flow in gap and the value of wall slip velocity were analyzed deeply. It is shown that micro-scale gap flow with slip condition is different from that of no-slip boundary condition, and the temperature- viscosity characteristic is the significant factor affecting gap flow distribution and slip velocity.(2)For the behavior of micro-fluidics in micro-cavity, CFD numerical simulation was adopt to investigate the slip effect of slip wall on micro-cavity flow. In addition, a new slip model based on concept of boundary layer thickness was derived theoretically and be tested. It's found that boundary wall with Navier slip condition will change the pattern of cavity flow. In mass-based micro-cavity flow, slip wall boundary condition will restrain the growth and development of vortex in cavity, while the new slip model is more applicable to step-wall boundary condition, which will reveal the process of slipping occurred. On the lid-driven micro-cavity flow, slip wall boundary condition can change the position and shape of vortex separation line in cavity, and stronger slip will promote the morphological changes of vortex.(3) In the research of gap flow in rotational symmetry seal disk, both theoretical and numerical analysis were adopted to get the mass and bearing capacity of rotational symmetry disk with the slip boundary conditions. Meanwhile the temperature-viscosity and non-Newtonian fluid properties were also taken into account to get the distribution of wall slip along the radius. For the wall slip condition, the nonlinear slip model was applied for high shear-rate case. Results will provide data reference for the high pressure high shear-rate rotational symmetry seal gap flow's applications and analysis.