Study On Dynamic Characteristics Of A Valve-Less Micropump Accompanying Gas Bubbles And Cavitation

Being as an essential component in microfluid control systems, a piezoelectric valve-less nozzle/diffuser micropump can transport micro flow rate accurately. Its application is getting wider and wider in micro electro-mechanical systems, medical devices and biochemical engineering areas due to the advantages of small size, simple construction and relatively high responding speed. The appearance of gas bubbles and cavitation in a micropump is generally undesirable as they can lead to performance deterioration, life reduction and even complete failure of the pump. It has been a key problem for the valve-less micropump. Therefore, it is necessary to study the dynamic characteristics of valve-less micropump accompanying gas bubbles and cavitation.According to the principle of fluid dynamics and generation mechanism of gas bubbles and cavitation in liquid, the dynamic mathematical models of the valve-less micropump accompanying gas bubbles and cavitation are studied. Based on the gas re-solution and releasing physical procedure, the models of gas bubble and cavitation volumes are developed. On the basis of the cone tube characteristics, the flow rate equations and the resistance coefficients of the nozzle and diffuser are presented, respectively.During pressure pulsation transients in a strait hydraulic pipeline accompanying gas bubbles and cavitation, the mathematical models of gas bubbles and cavitation are validated. The models of pressure transients accompanying gas bubbles and cavitation, including basic equations and friction items, are presented. Using finite difference method, the transients in hydraulic pipeline accompanying gas bubbles and cavitation are simulated. Two piezoelectric pressure transducers, fitted to the pipe at different locations, are used to record pressure transients. The growing and collapsing of gas bubbles and cavitation are recorded using a high speed video camera. Therefore, the validity of the gas bubbles and cavitation model is verified.The model of gas bubble involves three unknown parameters, including the initial gas bubble volume Vib, gas re-solution time constantτ_in and gas releasing time constantτ_out. By making use of genetic algorithms (GAs), the parameters in the gas bubble mathematical model are identified by comparing the pipeline transient simulation results and experimental data. Gas bubble model, accompanying the pipeline pressure transients, with identified parameters from GAs is obtained. Based on the valve-less micropump dynamic mathematical model, the dynamic characteristics of micropump accompanying growth and collapse of cavitation and re-solution and releasing of gas bubble is simulated. The influence of different cone tube geometry, chamber size, vibration amplitude and frequency, as well as the initial gas bubble volume on the micropump dynamic characteristics are analyzed,including chamber pressure, outlet flow rate, outlet accumulated flow rate, cavitation volume and gas bubble volume. Using finite element analysis, the diaphragm maximum volumetric deflection, stiffness and natural frequency are calculated.The experiments of the piezoelectric valve-less nozzle/diffuser micropump are carried out, including the micropump performance test, dynamic test of pressure pulsations in the pump chamber and the observation of gas bubbles. Meanwhile, the modified model for the diaphragm maximum volumetric deflection is presented. The micropump performance test includes outlet accumulated flow rate and back pressure test. A piezoresistive micro pressure transducer fitted on the top of the chamber is used to measure the pressure pulsations in the micropump chamber. In order to validate the mathematical model and the simulation method, the actions of gas bubbles, including movement, separation and combination in the chamber, are recorded by a high speed video camera.