Effects Of Small Surface Structure On Fluid Dynamics And Heat Transfer

Effects of small surface structure on fluid dynamics and the associated heat transfer have been investigated through CFD and experiments.Experimentally,detailed comparisons of flow structure have been made for configurations with and without multiple small scale ribs on the endwall of 90°turning ducts.The flow visualization of the passage flow system is performed by oil film techniques.It indicates that the addition of small surface structure reduces endwall secondary flow.Furthermore,a comparison of different configurations with no rib,half-length ribs and full-length ribs has been carried out.The results show that passage vortex is generated later and the height of separation line decreases with the length of rib increases;During the development of passage flow along the duct,the height of separation line increases gradually for no-rib configuration,while the height of separation line for both half-length ribs configuration and full-length ribs configuration see a drop around the exit plane of the curve duct.In addition,heat transfer coefficient(HTC)distributions from 45~o section to 90~o section of the endwall surface for configurations with and without small surface structure are obtained by transient thermal measurements using infrared thermography.The results show that there is a low heat transfer level region in inner radius region,as well as a high heat transfer level region in outer radius region for half-length ribs configuration(compared to no-rib configuration).Furthermore,studies of different inlet boundary layer thickness have been carried out to investigate its effects on heat transfer.It indicates that the region of heat transfer enhancement becomes larger with the increase of the inlet boundary layer thickness.As the scale of small surface structure is much smaller than the scale of mainstream flow field,it remains challenging to perform direct numerical simulation(DNS).Numerically,a newly developed numerical algorithm based on heterogeneous multiscale method(HMM)is adopted to explore the scale decoupling approach for multiscale problems,which is more efficiency than direct numerical simulation(DNS).