The concept of a valveless acoustic micropump was investigated. Two-dimensional, time-varying, axisymmetric, incompressible viscous flows through a planar diffuser-nozzle element were analyzed for applications in valveless acoustic micropumps. The diffuser divergence half-angles (h), and the maximum pressure amplitudes (P) were independently varied. The inflow was periodic and the excitation frequency (f) was varied over the range 10 kHz B f B 30 kHz. The net time-averaged volume flux and the rectification capability of the diffuser were found as functions of h, f, and P. The phase difference between pressure and velocity waveforms, the life time and the size of large scale flow recirculation regions inside the microdiffuser, and energy losses were found to be strongly frequency dependent. Keywords Acoustic micropumps Á Acoustic standing waves Á Microdiffuser Á Numerical analysis List of symbols DP Pressure drop (Pa) d Stokes layer thickness (m) g Rectification capability k Acoustic wavelength (m) l Shear viscosity (kg/m s) m Kinematic viscosity (m 2 /s) x Angular frequency (rad/s) q Density (kg/m 3 ) h Diffuser divergence half-angle (°) n d Pressure loss coefficient in the diffuser direction n n Pressure loss coefficient in the nozzle direction D h Hydraulic diameter of the microdiffuser at the inlet (m) f Excitation frequency (Hz) I Flow inductance (kg/m 4 ) I d Flow inductance in the diffuser direction (kg/m 4 ) I n Flow inductance in the nozzle direction (kg/m 4 ) N Cell numbers P Maximum pressure (Pa) p Pressure (Pa) p 0 Static pressure (Pa) Q Volumetric flow rate (m 3 /s) Q net Net flow rate (m 3 /s) R Flow resistance (kg/m 4 s) R d Flow resistance in the diffuser direction (kg/m 4 s) R n Flow resistance in the nozzle direction (kg/m 4 s) Re Reynolds number T Excitation period (s) t Time (s) U Maximum velocity (m/s) u Velocity in x direction (m/s) v Velocity in y direction (m/s) V net Sectional net velocities (m/s) Wo Womersley number x Axial position (m) x max Maximum fluid displacement during one oscillation cycle (m) y Transverse position (m) Z Flow impedance (kg/m 4 s) Z d Flow impedance in the diffuser direction (kg/m 4 s) Z n Flow impedance in the nozzle direction (kg/m 4 s)