Study On Flow And Mixing Technique In Microfluidic Systems

As one of the main branches of MEMS, micro flow systems have achieved a great development and wide application. However, there are many unsolved academic problems and challenges in the flow and transport processes of the micro fluidic systems. Focusing on the microscale flow, heat and mass transfer, in the present dissertation, the flow characteristics and thrust performance of the micronozzles, mixing enhancement technique for micro channel flows were studied in detail. Based on analytically and numerically results, micronozzles and micromixers were designed and fabricated. Their performances were evaluated numerically and experimentally.Numerical methods based on the continuum hypothesis and molecule kinetic theory are performed to investigate the gaseous flow in the microchannels. The validity of different boundary conditions based on continuum model is examined by DSMC method under different Knudsen number. It is found that the impact of rarefied and compressibility effects are different.DSMC method and Navier-Stokes equation (with no-slip and slip conditions) are applied to study the effects of configuration, operating parameters and temperature boundary conditions on flow characteristics and thrust performance of micronozzles in detail. The optimization proposals for micronozzle design are put forward based on the numerical data and analytical results. Numerical results show that the rarefied effects and non-equilibrium effects are strengthened due to larger Mach number in the divergent section of micronozzle. The Reynolds number is the key parameter for the micronozzle performance. To improve the propulsion efficiency, the appropriate Reynolds number could be obtained by either regulating the operating pressure or nozzle size. Special impulse is increased by the higher propellant temperature, which results in the loss of thrust efficiency.Due to the laminar nature of the flow, the mixing rate is strongly diffusion limited and long mixing channel or time may be required to attain full mixing. As a result, the rapid mixing is difficult to achieve in these micro channels. Strategies to increase stream-stream mixing can be broken into two main categories: active and passive. For passive mixing, the Reynolds number and Schmidt number are the similarity parameters. On the other hand, Reynolds number, Strouhal number, peclet number or Womersley number and Peclet number are the similarity parameters for active mixing.For different application purposes, active mixer based on synthetic jet and passive mixer with baffles at the inlet of confluent channel are proposed. To investigate the effect of the inlet velocity and the jet parameters on the mixing efficiency, Navier-Stokes equations with species diffusion equation are solved to study the mixing process in the active mixer. The similarity theory is applied to study the mixing performance of the passive mixers with different channel configuration and inlet conditions. It is found that active mixer with asymmetric structure has better mixing effectiveness; For passive mixing, as the baffle height increased, the mixing effectiveness improved significantly, while the pressure loss due to extra baffles increased a lot accordingly. The mixing performance improvement and pressure loss depended strongly on the operating Reynolds number. The mixing performances of passive mixer are also experimentally studied by the flow-field visualization. The results validate the numerical data.