Abstract
Since the rapid development of microfluidic systems, the bubble-induced acoustic streaming has been extensively applied in many fields of research, such as the flow manipulation in micro-biochip because of its great capability on the fluid mixing. As a result, this study aims to investigate the flow field in bubble-induced acoustic streaming and the feasibility of heat transfer enhancement. For the first time, not only flow visualization (FV) technique but also Micro Particle Image Velocimetry (µ-PIV) and Temperature-Sensitive Paint (TSP) techniques are utilized for the measurement of temperature and velocity in bubble-induced acoustic streaming. A PDMS rectangular microchannel has been fabricated and cavities were positioned at the side walls of the microchannel. Bubble-induced acoustic streaming is mainly constructed by trapping air bubbles in cavities, which is caused by surface tension, and exciting the air bubbles by a piezo-actuator. The trapped air bubble will start to resonate during the excitation driven by a piezo-actuator and a pair of vortex will appear which rotates clockwise and counterclockwise. The resonance frequency of the trapped air bubble can be calculated by theoretical equations and verified by the experiments. The resonance frequency of 89 kHz has been identified as the resonance frequency of the bubble-induced streaming in current study. A PDMS microchannel, a piezo-actuator and a micro-heater are integrated in a microfluidic device for the experimental investigation of temperature profiles and flow field. µ-PIV technique and flow visualization are utilized to measure the velocity profiles of bubble-induced acoustic streaming. The vorticity distributions have been analyzed from the velocity profile as quantified information in the flow field. The bubble-induced acoustic streaming has been investigated with different aspect ratios of cavities (L/W) which equals to 5, 2 and 1. The strongest bubble-induced acoustic streaming has been identified while the aspect ratio of cavity equals to 2. However, it is also observed that the vortex pattern of bubble-induced acoustic streaming is highly asymmetrical. Only 13% possibility of symmetrical vortex pattern would be observed even in the flow field using identical cavity structure. In addition, the height of the bubble and fluid interface is also a factor that changes the magnitude of vortex pattern in bubble-induced acoustic streaming. The magnitude of bubble-induced acoustic streaming reduces if the bubble/fluid interface becomes higher. With the main stream flowing in the main channel, the vortex pair formed by bubble-induced acoustic streaming will evolve to two attraction points located at upstream and downstream of the cavity. The attraction point at downstream of the cavity would become a small vortex with recirculation flow. The disturbance as lateral momentum difference in the flow field induced by bubble-induced acoustic streaming are almost the same at Reynolds number varying from 2 to 8. The greatest disturbance and the large affected region from bubble-induced acoustic streaming is identified as the Re=2 of the main flow. At last, a microchannel with multi-cavity structure is designed which is based on the information acquired from the experiment with single pair cavity. Fluid and surface temperature profiles at Reynolds number of 2 and bottom side heated at constant heat flux of 0.12 W/mm2. Up to 12.4% heat transfer enhancement has been observed from the enthalpy change after bubble-induced acoustic streaming applied. From the Nusselt number map around the cavity region in the microchannel, up to 15~20 % increase of Nusselt number have been identified in the region near the cavity structure with the excitation power of 18 mW applied to the piezo-actuator.