Abstract
This study aims to investigate acoustic streaming effect on heat transfer enhancement in microchannel flow by using staggered structures. The microchannel devices were made of PDMS material, with staggered structures were positioned on the side wall of microchannel. Microheaters were fabricated by using MEMS technology and placed at the bottom of the microchannel to provide constant heat flux thermal boundary condition. After investigation of heat loss in the microchannel experiment with ANSYS simulation and experiment, the experimental arrangement was adjusted by expanding the PDMS bulk and the heat loss was successfully reduced from 89 % to 44 %. In this study, the structures were positioned at the side wall and they were driven by piezoelectric actuators to introduce acoustic streaming. The using of the solid structures would resolve the problem of acoustic straming with air bubbles, which could be easily expanded during the acoustic streaming. For the velocity experiments and flow visualization, red fluorescent particles of 3.2 micrometer diameter were seeded into deionized water to be used as tracker particles. The velocity profiles inside the microchannel devices were successfully obtained by Micro-Particle-Image-Velocimetry (Micro-PIV) technique. For the temperature field experiment, luminescence sensor of Rubpy was dissolved in DI water as working fluid for temperature measurements. The temperature field and heat transfer analysis were successfully obtained by Temperature Sensitive Paint (TSP) technique. In this study, the velocity profiles and temperature fields have been successfully acquired with single structure and staggered structures with different aspect ratios (a=0.2 and a=0.4) of structure length and channel width as well as different Reynolds number (Re) conditions. From the experimental results of velocity profiles, it can be seen that microchannel flow with staggered structures of a=0.2 can show the effect of acoustic streaming and it can be propogated downstram at Re number of 2, 4 and 6. However, the results of microchannel flow with staggered structures of a=0.4 is different and the effect has been suppressed due to the narrow pathway for the main flow. Comparing the heat transfer enhancement with acoustic streaming under the constant heat flux (0.2 W/mm2) thermal boundary condition, both microchannel devices with staggered structure designs of a=0.2 and a=0.4 could increase the fluid temperature at microchannel outlet. It could be seen that the staggered structures of a=0.2 has the highest heat transfer enhancement at Re number of 4, with the enthalpy increament up to 22.53 %. It could be found that there were greater temperature differences along downstream with the effect of acoustic streaming, which was similar to the observation of velocity experiments.