Local Flow And Heat Transfer Characteristics Along Transverse Roughness Surfaces

The variety of near-wall flow patterns related with complicated roughness plays an importance role in convective heat transfer, meanwhile a critical factor leading to fluid flow dissipation. By analyzing in detail the flow and heat transfer structures near to wall surface among artificial roughness elements, disciplines concerning roughness effect were discovered and discussed. At the same time, evolving mechanisms of the fluid flow and associated heat transfer enhancement along a rough surface were disclosed in the form of dissipation resulted by roughness effect. The investigation in this thesis includes three parts—numerical simulation, PIV experimental observation and mechanism analysis.Local flow and heat transfer characteristics were numerically simulated and studied for different Reynolds numbers and roughness arrangements. The results revealed that flow structure, especially the distribution of vortices close to the wall, plays an important factor affecting the characteristics of flow resistance and heat transfer performance. For the roughness passage with p/e=20, flow structures in the roughness cavities are highly dependent upon Reynolds numbers, single stable eddy at low Reynolds number, one top vortex and two induced corner vortices forming as Reynolds number being increased to moderate values, only two corner vortices existing at high Reynolds number. For cases of p/e=20 and p/e=10 at a specified Reynolds number of 4×103, flow patterns show very little difference in each rib spacing along the roughened surface, two corner vortices for the former and single vortex for the latter. While for the case of p/e=5, flow pattern or structure in different rib spacing varies along flow direction, and three flow regimes in rough zone of the passage are classified---entrance, middle and exit regions. In entrance and exit region flow structure varies quickly, and particularly vortex distribution close to the wall in rib spacing shows quite different from each other. In middle region, the flow became relatively stable and the flow structure was a single vortex.An experimental investigation was conducted to observe the evolution of flow patterns near-wall along the roughened surface in a rectangular duct and the effects of Reynolds number on the flow structure using a particle image velocimetry system (PIV). Experimental results show that for p/e=5 and Re=4000, the flow structure in the roughness cavities would evolve in the flow direction, which shows a very reasonable agreement with corresponding numerical results. When Reynolds number is increased to 1.8×104, flow pattern turns into a single vortex structure, with the outer vortex-induced velocity more close to the mean velocity of the whole passage. This might indicates that higher Reynolds number increases the velocity uniformity of the bulk flow. When Reynolds number is increased to 2.4×104, the flow structure remained single vortex and it is observed that the bulk flow puts the vortex further into the cavity to decrease its influence.An analysis was also conducted to understand the flow dissipation, and different local dissipation mechanisms for three near-wall structures were discussed. The ratio of flow dissipation to heat transfer enhancement is responsible for the good performance of the ducts with transverse roughness surfaces.