PIV Investigation Of The Laminar Pulsating Flow In Triangular And Rectangular Grooved Channels

The channel with periodic variations in cross-section has been widely used in compact heat exchangers, microelectronic systems, bio-engineering and other fields. When the fluid of such a channel is in the state of low Reynolds laminar flow, the fluid viscosity plays a dominant role. This will result in the poor fluid mixing and the low heat transfer capacity. Applying pulsation disturbance is an effective method to improve the heat transfer of laminar flow in such a channel. Researches show that the fluid mixing enhancement is a key factor on heat transfer enhancement of pulsating flow fields. Therefore researching the laminar pulsating flow field and analyzing the fluid mixing have important theoretical and practical meanings.In this research, the PIV technology was utilized to investigate the flow fields of triangular and rectangular grooved channels. The instantaneous velocity fields were obtained with various pulsation frequencies f, groove depths a and channel widths f. Then the characteristics of above-mentioned flow fields, especially the vortices characteristics, were studied. Finally the PIV results and heat transfer results were compared to study the heat transfer enhancement mechanism from the point of fluid mixing.The PIV results show that both in triangular and rectangular grooved channels, the flow pulsation brings about the periodic variations of the vortex diameter, the vortex angular velocity and the vortex center location. In the flow acceleration phase, the vortex is generated close to the mainstream in the groove leeward and then propelled into the inside groove rotating. As the flow velocity decreases, the vortex entrains the fluid of the mainstream which flows relatively slow to expand. As a result, the mixing between the groove inside fluid and the mainstream is promoted. The evolution of the vortex in triangular grooved channel can be divided into formation, growth, expansion and pushed downstream by the mainstream. While in the rectangular grooved channel, the vortex finally disappears in the groove inside before the acceleration phase.The influence of pulsation frequency on the fluid mixing varies in the same trend for the triangular and rectangular grooved channels. Specifically, the mixing effect first increases and then decreases with increasing pulsation frequency. That is, the mixing effect at f=0.52Hz is better than the effect at f=0.27Hz and f=0.77Hz. In the triangular grooved channels, the mixing degree and area at a=12mm are both larger than these at a=9mm and a=6mm. While in the rectangular grooved channels, the mixing effect enhancement becomes worse with the increase groove depth. That is to say, the mixing enhancement reaches a higher level at a=6mm. At the same time, the groove width also influences the mixing enhancement between the fluid inside the rectangular grooved channels and the mainstream. The mixing enhancement first increases and then decreases with increasing groove width. That is, the mixing enhancement at l=18mm performs better than l=12mm and l=24mm.