Experimental Studies of Separated Flow and Heat Transfer in a Ribbed Channel

Abstract: The main concern of this thesis is to experimentally investigate the turbulent flow and heat transfer in a ribbed channel. Such flows are encountered in many engineering applications, e.g., gas turbine cooling. In order to highlight the physical mechanism of flow separation, only one wall of the channel is fitted with periodic ribs. The inter-rib spacing is set such that the reattachment is allowed to take place on the portion between consecutive ribs and a distinct post-reattachment-redevelopment region is introduced prior to a re-separation over the next rib. With aid of liquid crystal thermography (LCT) and particle image velocimetry (PIV), respectively, the temperature and velocity fields can be obtained with high resolution. Measurement of local heat transfer coefficients shows that the flow separation induced by ribs greatly enhances the heat transfer rate in comparison with the smooth channel. In addition, it is found that the position of the maximum Nusselt number corresponds well to the reattachment point. In the corner immediately downstream of a rib, however, the heat transfer coefficients decrease sharply because the fluid is almost stagnant and heat conduction is dominant. In the recovery region, the agreement between the Nusselt number and skin-friction coefficient is poor, which indicates that the Reynolds analogy fails to predict the heat transfer coefficients. In the re-separation region downstream reattachment, the heat transfer rates rise again due to strong turbulent momentum exchanges from mainstream flow to the inner boundary layer. The separated shear layer is dominated by the roller vortices that are generated due to the Kelvin-Holmholtz instability. Similar to a plane-mixing layer, the growth rate of the separated shear layer is linear with respect to the streamwise direction. Moreover, it is found that the inflection point plays a crucial role in the production of turbulence. Near the reattachment point three interesting features are characterized; first, the maximum shear stresses decay rapidly just downstream of reattachment; second, the anisotropy parameter is close to unity; third, the flatness factor is high. In addition, the two-point correlations are presented to give the spatial structure of the flow. In order to visualize the coherent structures, which are the backbones of turbulent motions, various decomposition methods are employed in this thesis, including Reynolds decomposition, Galilean decomposition, large eddy simulation (LES) decomposition and proper orthogonal decomposition (POD). Among these methods, the proper orthogonal decomposition is discussed in detail. The result shows that the leading edge and the separated shear layer contain most of the turbulent kinetic energy. In the less energetic eigenmodes the coherent structures disappear gradually and turbulence becomes homogeneously chaotic. This means the symmetries are restored in the less energetic POD modes. List of papers 1. Lei Wang, Jiri Hejcik, and Bengt Sundén, PIV Measurement of Separated Flow in a Square Channel with Streamwise Periodic Ribs on One Wall, accepted for publication in ASME J. Fluids Engineering, to appear July 2007, Vol. 129. 2. L. Wang, S. S. Burgers, M. M. J. F. Verbeek, and B. Sundén, Experimental Investigation of Flow Fields in a Square Channel Roughened with Various Ribs on One Wall, Proceedings of 2006 ASME International Mechanical Engineering Congress and Exposition, IMECE2006?13659. 3. Lei Wang and Bengt Sundén, (2005). Experimental Investigation of Local Heat Transfer in Square Duct with Continuous and Truncated Ribs, Experimental Heat Transfer, Vol. 18, pp. 179-197. 4. Lei Wang and Bengt Sundén, (2007). Experimental Investigation of Local Heat Transfer in a Square Duct with Various-Shaped Ribs, J. Heat Mass Transfer, Vol. 43, pp. 759-766.