A novel bidirectional valveless piezoelectric micropump with three chambers using the Coanda effect has been developed. The volume efficiency of the micropump is able to reach 50% which is much higher than that of the traditional diffuser/nozzle valveless micropump and its transport direction is easy to change. The chambers are covered by three piezoelectric actuators, respectively. There are three channels connected with each chamber, respectively. And the other ends of the channels join together and connect with a diffuser. One of the channels has the same centerline with the diffuser. The centerline is perpendicular to the other two channels which situate on the two sides of the diffuser symmetrically. Inlet and outlet channels are connected with the other end of the diffuser. Three piezoelectric actuators are driven by the synchronous voltages with different magnitude. When the fluid is discharged from the pump chambers, the flowrate of one channel situated on the side of the diffuser is smaller than that of the two others. Due to the Coanda effect, the jet flow in the diffuser attaches the wall and all fluid enters the outlet channel. And some fluid is sucked from inlet channel because of the vortex induced by the jet flow. When the fluid is sucked into the pump chambers, the flowrate of fluid sucked into the inlet and outlet channels are same. So the fluid transports from the inlet channel to outlet channel in a cycle. And the micropump could change the transport direction by changing the flowrate of the two channels situated on the two sides of the diffuser. CFX is applied to simulate the flow field of the micropump and the dynamic mesh method is used for the simulation of the piezoelectric actuator. The sinusoidal vibration is applied to the piezoelectric actuator and the frequency is 10Hz. The narrowest width and height of the diffuser is 200μm. The Reynolds number is between 1000 and 1800 in the simulation and the SST model is chosen. Hexahedral mesh is used and the number of elements is about 1.2 million. The result shows that when the amplitude of two actuators is same and the amplitude of the third one is 20% of the former (the maximum Reynolds number of the diffuser is about 1200), the maximum flowrate of the micropump is 2.43 ml/min and the volume efficiency reaches 58%.

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