Detailed Flow Investigation Using PIV in a Rotating Square-Sectioned Two-Pass Cooling System with Ribbed Walls

Abstract

In a 2-pass cooling system the pressure driven air flow dis-tribution is investigated experimentally using the non-intrusive PIV Technique. The generic model as part of a complex and sophisticated cooling system consists of two square-sectioned ducts with a length of 20 diameters and an inherent 180 degree bend. The system has been investigated basically with smooth walls (case 0) and, later on, with two different kinds of ribbed walls in both legs. Ribs are applied to enhance the cooling performance; they are placed on two opposite walls of both legs in a symmetric (case A) and an asymmetric manner (case B), respectively. The ribs are inclined with an angle of 45 degrees versus the duct axis (i.e. main flow direction). The applied rib lay-out is well-proved and optimized with respect to heat trans-fer improvement and the inherent pressure drop increase. The system rotates about an axis orthogonal to the centreline of the straight passes. The configuration was analyzed with the planar the two-component Particle Image Velocimetry (2C PIV), which is capable of obtaining complete maps of the instantaneous as well as the averaged flow field even at high turbulence levels, which are typically present within duct turns, near ribs and, above all, during rotation. The presented investigations were conducted in stationary and rotating mode. Especially in the bend region separation phenomena and vortices with high local turbulence are apparent. The presence of ribs changes the fluid motion by generating additional vortices impinging the side walls. Flow visualization with injected oil smoke using the laser light sheet visualization technique was helpful to detect vortex structures and separations. Especially in the bend area separation regions and vortices with high local turbulence are apparent. The results shown in this paper demonstrate the effect of the 180 degree bend in combination with the two rib turbulator geometries for isothermal flow conditions excluding any buoy-ancy with and without rotation. Turbulent channel flow was investigated at a Reynolds number of 50,000, derived with the hydraulic diameter of the pass, non-rotating and at a rotation number of 0.02 which was chosen still moderate. Engine rele-vant rotation numbers are in order of .1 or higher. A reconstruc-tion of model mountings will allow higher values for the next tests. Future work will expand to higher rotational speed and, also, will include buoyancy effects. This investigation shall help to clarify the complex flow phenomena due to the interaction of several vortices, present in two-pass cooling systems. The flow maps obtained with PIV are of good quality and high spatial resolution and therefore provide a test case for the development and validation of nu-merical simulation tools like the DLR flow solver TRACE which is not a topic of this paper.