Boundary layer laminarization by convex curvature and acceleration

摘要

Particle image velocimetry boundary layer measurements are presented for flows over convex surfaces, and subject to flow acceleration. These mechanisms are present in gas reactor core designs, which is of concern because it is known that they can cause boundary layer laminarization which can have a negative effect on surface heat transfer. A wind tunnel was constructed where flow acceleration and curvature effects were applied separately, and simultaneously to investigate the separate and combinative influence of these mechanisms. Applied acceleration for the different test cases varied from K=1.6 × 10~(-6) to 1 × 10~(-5). Curvature values of δ/R= 0.015 to δ/R=0.05 were tested. The applied acceleration spans across the threshold value when laminarization is expected to occur for flat plate geometries. Measurements were taken using particle image velocimetry. Mean flow parameters including mean velocity profiles, boundary layer thickness, and shape factor are presented as a function of streamwise position during the boundary layer response as it is subjected to the laminarization causing mechanisms. Velocity profiles are normalized by both outer and inner variables. It is observed that as the flow is subjected to these mechanisms, the boundary layer mean flow parameters diverge from their turbulent values and approach laminar characteristics. The linear viscous sublayer increases in thickness, the overall boundary layer thickness is reduced, and the shape factor increases. The response in the boundary layer mean flow parameters is more pronounced and occurs more quickly when subject to both mechanisms simultaneously.