Low Reynolds Number Flow Through Nozzle-Diffuser Elements in Valveless Micropumps

Flow characteristics of low Reynolds number laminar flow through gradually expanding conical and planar diffusers were investigated. Such diffusers are used in valveless micropumps to effect flow rectification and thus lead to pumping action in one preferential direction. Four different types of diffuser flows are considered: fully developed and thin inlet boundary layer flows through conical and planar diffusers. The results from the numerical analysis have been quantified in terms of pressure loss coefficient. The variation of pressure loss coefficient with diffuser angle is presented for Reynolds numbers of 200, 500 and 1000. The pressure loss coefficients have been used to calculate the diffuser efficiency for two different types of nozzle-diffuser elements. The general trend of variation of pressure loss coefficient with diffuser angle was found to be similar to that for high Reynolds number turbulent flow. However, unlike at high Reynolds numbers, pressure loss coefficients at low Reynolds numbers vary significantly with Reynolds number. It was also observed that trends of variation in the pressure loss coefficient with Reynolds number are different for small and large diffuser angles. Also, at low Reynolds numbers, the pressure loss coefficients for a thin inlet boundary layer are not always smaller than those for fully developed inlet boundary layer, in contrast to the behavior for high Reynolds number flows. Contrary to past claims, flow rectification is shown to be indeed possible for laminar flows. The two different types of nozzle-diffuser elements considered led to pumping action in opposite directions. Further it was observed that flow rectification properties of both kinds of nozzle-diffuser elements improved with increasing Reynolds number.