VORTICES DEVELOPMENT AND INTERACTION STUDY ACROSS 180° SHARP BEND OF TWO-PASS RECTANGULAR DUCT WITH AND WITHOUT MATRIX-SUBCHANNELS

Abstract

The global population's rapid growth has led to a substantial increase in personal energy consumption. This surge in energy demand, combined with concerns about fossil fuel depletion and environmental damage, poses a grave threat to humanity's long-term sustainability. Consequently, there has been a significant emphasis on research aimed at creating cleaner and more efficient energy systems. Advancements in technologies for cleaner and more fuel-efficient systems hold immense potential, particularly in automotive, aviation, and naval applications. The energy transformation, utilization, and recuperation techniques aimed to develop cleaner and more efficient systems involve fluid flow and heat exchange processes around sharp bends or curvatures at different temperatures. Some typical applications where fluid encounters sharp bends include cooling ducts for alternators and gas turbine blades, heat exchangers, nuclear fusion reactor blankets, blood flow in heart arteries, and electronic chip cooling. The fluid flow across the bends/curvatures is characterized by challenges such as minimizing pressure drops and preventing local hotspots caused by heat accumulation. Flow and heat transfer across sharp curvatures/bends are significantly altered due to the development of secondary flow structures, resulting in complex, periodic, unsteady, and three-dimensional flow patterns. Among the wide range of applications in which fluid encounters sharp curvature/bends in its path, there exists a lot of potential for enhancing the cooling performance of gas turbine blades. Gas turbines are vital for aircraft propulsion, power generation, and industrial applications and have considerable scope for achieving better performance. The thermal efficiency and power output of gas turbines are known to increase with higher turbine rotor inlet temperatures (RIT). However, the higher RIT can address fuel consumption, cost, and environmental concerns, but it can significantly lead to premature failure of the blades if not properly handled. Consequently, researchers are working towards developing high-temperature materials and sophisticated cooling schemes to enhance gas turbine performance and power output. Different cooling schemes are typically used for the different blade sections, i.e., the internal surfaces of turbine blades have impingement, rib turbulators, and pin-fin cooling for the leading, middle, and trailing sections, respectively. In addition to these conventional cooling techniques, an efficient matrix cooling technique has also emerged in passing decades. In both the rib turbulator and matrix cooling schemes, the internal fluid paths commonly have 180° sharp bends. The intricate regions play a vital role in cooling schemes related to turbine blade cooling; thus, fundamental aerothermal characterization is required across the two-pass smooth and matrix cooling channels. The present study focuses on investigating the fundamentals of vortices development (by Dean instability and/or by coplanar matrix), interaction (merging and progression) across the two-pass rectangular duct (with a 180° sharp bend) with and without matrix subchannels. The present study not only addresses the critical issue of gas turbine blade cooling but also investigates the fundamentals of vortices development and their impact on heat transfer across a two-pass duct with a 180° sharp bend under various inflow conditions. Thus, the outcomes of the work can be extended to many engineering applications in which the sharp bend/curvature exists. Primarily, a detailed insight into the fundamentals of vortex evolution, merging, and its effect on heat transfer distribution across the smooth 180° sharp bend (referred as undisturbed inflow, UF) for laminar (Re = 800) and turbulent (Re = 6500) in-flow conditions has been presented and discussed. Stereoscopic particle image velocimetry (stereo PIV), as well as twodimensional and two-component PIV measurements and liquid crystal thermography techniques, are appropriately used for flow and heat transfer characterization across the complete 180° sharp bend. The sudden flow redirection and adverse pressure gradient create a recirculation region near the divider wall. The RMS and Reynolds shear stress ( u'w' ) distribution at the horizontal plane highlight the presence of a shear layer between the recirculation and mainstream turning flow. The results show the evolution of secondary flows in the form of counter-rotating vortex pairs, commonly known as Dean vortices. These secondary vortices are found to play a significant role in the localized laminar–turbulent transition and turbulence augmentations for respective laminar and turbulent inflow conditions. Subsequently, quantitative analysis shows that complete 180° turning of flow resulted in intense augmentation of spatially averaged turbulence quantities. Root mean square (RMS) fluctuations in the transverse direction ( T |rms V ) increase by 298 % and 186 % for respective flow conditions. Augmentation of 287% (laminar) and 260% (turbulent) in the wall-normal RMS fluctuations ( N |rms V ) are observed. These augments in transverse and wall-normal velocity fluctuations result in a very sharp amplification of spatially averaged turbulent kinetic energy ( k ), that is, 1825% for inlet laminar and 928% for inlet turbulent flow regimes.