Matrix cooling is gaining attention as an advanced internal cooling strategy for gas turbines, offering both enhanced mechanical integrity and superior heat transfer. However, the aerothermal behavior in bend regions, especially downstream of the matrix, remains insufficiently explored. This study presents a detailed experimental investigation of flow dynamics and heat transfer across the bend section that occurs downstream of the matrix channel. Two matrix configurations, i.e., (i) 3-subchannel (3SC) and (ii) 4-subchannel (4SC), are systematically studied at Reynolds numbers (Re) of 5,000, 7,500, and 10,000. High-resolution particle image velocimetry (PIV) and liquid crystal thermography (LCT) techniques have been used for multi-plane aerothermal measurements to elucidate the correlation of complex flow with thermal fields. Results indicate that the matrix emanates the corotating vortices, leading to spatially non-uniform secondary flows, including the emergence of Dean-type vortices at the downstream bend. In particular, the 4SC matrix promotes stronger vortex-driven mixing, yielding significant local and average heat transfer enhancement in the bend region. At Re = 10,000, the 4SC channel achieves a peak heat transfer augmentation of 133 % relative to a smooth baseline, while incurring only a slight increase in friction factor. Across all conditions, the 3SC and 4SC configurations deliver thermal performance factors (TPF) of up to 2.09 and 2.34, respectively. These findings support the potential of matrix cooling to deliver high thermal effectiveness in next-generation internal cooling applications.

Matrix or latticework cooling is commonly known for offering an excellent heat transfer performance and structural strength to modern gas turbine's blades. Detailed fluid flow studies inside matrix subchannels are still very limited, mainly due to experimental challenges. This work is a forward step in this theme in which particle image velocimetry (PIV) and Liquid Crystal Thermography (LCT) is used to capture the complex flow patterns and heat transfer across the matrix subchannels at Reynolds numbers 800 and 6500. The outcome of the study shows that a swirl commences at the entry of subchannels, which evolves in terms of a full-scale streamwise vortex in downstream. The evolved vortical structures deteriorate during turning and impingement and subsequently re-develop while propagating through subchannels. Mean turbulent kinetic energy (k¯) distribution shows that turning and impingement offer a sharp turbulence augmentation. i.e., the k¯ values after first turn shows an increment of ∼175 % (for Re = 800) and ∼100 % (for Re = 6500). The average augmentation Nusselt number ([Formula presented]) is found to closely correlated with k¯, consequently the first turning offers an increment ∼125 % (Re = 800) and ∼200 % (Re = 6500) in [Formula presented].

Vortex merging and evolution mechanisms have been characterized across the 180° sharp bend region of matrix-cooled serpentine channel with the help of stereo Particle Image Velocimetry (PIV) system. Complete evolution of flow phenomenon is captured through measurements in multiple vertical and horizontal planes, and the mechanism responsible for evolution of vortices at a typical Reynolds number (Re) of 6500 has been explored. It was observed that the matrix subchannel typically produces a kind of swirling pattern downstream of the matrix structure, which is similar to the swirling flow occurring in various other application areas. Two co-rotating vortices emanate out of matrix subchannels (observed in secondary plane), which immediately merge into a single vortex at the onset of the bend and propagate further downstream of the bend section, where a pair of side-by-side counter-rotating vortices are observed later. Numerical simulations have also been performed to get the pressure distribution and flow characteristics within oblique/inclined planes along the bend, which are difficult to obtain experimentally through PIV. Results demonstrate that the vortical structures lose their strength during merging and turning, significantly reducing turbulence while passing through the bend. Based on the combination of experimental and numerical observations, a conceptual diagram depicting flow dynamics has been proposed, which provides the overall perspective of vortices evolution and merging across the bend.

Many thermal applications involve fluid flow across the sharp bend. The fluid flow across the sharp bend causes entire modification due to back pressure and flow separation, etc. Quantifying pressure across the sharp turn due to the modification of flow behaviour is always a complex challenge. One of the difficulties associated with the intrusive techniques of pressure measurements is to disturb the flow and the lack of pressure field estimation in the entire domain. This article presents a complete methodology for pressure estimation before and at the sharp bend using stereo-particle image velocimetry (PIV) data. The method relies on knowledge of the PIV velocity field, which is used to calculate the pressure field analytically and numerically. The outcomes of the study show that how the sharp bend in the fluid path results in a sharp pressure drop. Pressure at the entry of the bend is found to be ~5 Pa (absolute), and at the exit side, it drops to ~3 Pa for a turbulent in-flow condition (Reynolds number = 5500). The time-averaged streamlines are also superimposed over velocity magnitude and pressure data for a better understanding of flow and pressure simultaneously.

A matrix structure can offer a high heat transfer and improved structural rigidity to the gas turbine's aerofoils. The earlier matrix studies are limited to presenting aerothermal behavior within the matrix structures. Thus, there is a scope to investigate the aerothermal characteristics downstream of a matrix geometry. The present experimental effort seeks to characterize the detailed flow and heat transfer downstream of the matrix and at the bend for Re = 800 and 6500. The heat transfer and flow fields are captured in various planes using liquid crystal thermography and stereoscopic particle image velocimetry techniques. The results show that the matrix ejects a highly turbulent vortical flow in the form of two co-rotating vortices, which further diffuse and merge in downstream. The matrix offers a very high average augmentation (Nu¯/Nu¯₀) in the first pass for Re = 800 (varies from 12 to 4), but it shows a poor heat transfer (Nu¯/Nu¯₀ < 1) at the bend region, which can generate local hot spots for Re = 800. In contrast, for Re = 6500, the matrix offers a stable augmentation throughout the first pass and bend region, i.e., Nu¯/Nu¯₀≈ 2.4–1.

This paper presents the flow patterns and heat transfer distribution across the bend region in a two-pass rectangular duct for an inlet turbulent flow regime (Re = 6500). The results are presented at various vertical and horizontal planes located at different positions across the bend along the flow progression. Two divider wall configurations (in-between two-pass of the duct) are studied, i.e., (1) sharp-corner turn and (2) smooth curved turn. It has been noticed that flow gets completely modified because of divider wall shape. The numerical results reveal that primary and secondary flow in turn regime displays combined features of a bend-induced, Dean-type circulation. The flow dynamics and local heat transfer vary significantly with different divider wall configurations. For the duct with a sharp divider wall, a pair of counter-rotating Dean vortices induce after 90° near the end-wall of bend region. In contrast, for the duct with a smooth curved divider wall, vortices pair is induced close to the divider wall and stretched across the entire width. The results show that Dean vortices play an important role in enhancing localized heat transfer across the bend regime. Interestingly, the duct with a sharp divider wall exhibits higher localized heat transfer, whereas the duct with a smooth, curved divider wall exhibits lower localized heat transfer but more uniform distribution.

The present study deals with the fundamentals of vortex evolution and merging across a 180° sharp bend for laminar inflow (Re = 800). A three-dimensional flow and pressure distribution across the bend are analyzed by using particle image velocimetry measurements. The results show that the sudden flow redirection and adverse pressure gradient create a recirculation region in the vicinity of the divider wall. The vorticity and turbulent kinetic energy distribution highlight the presence of a shear layer between the recirculation and mainstream turning flow. A Rayleigh instability analysis shows the existence of centrifugal instabilities outside the shear layer in the mainstream turning flow. The imbalance of centrifugal and pressure forces act in unison to produce a net force that creates Dean instability across the mainstream turning flow. Consequently, two pairs of counter-rotating Dean vortices develop after the first turn, possibly merging into a single pair after the subsequent 90° turn. Finally, the evolution and merging of Dean vortices are explained with the help of a conceptual diagram illustrating the force interaction and the formation of counter-rotating currents.

This work presents a detailed insight into the flow progression and surface heat transfer distribution across the sharp 180° bend of a two-pass rectangular duct for laminar (Re = 800) and turbulent (Re = 6500) in-flow conditions. Stereoscopic particle image velocimetry (stereo PIV) as well as two-dimensional and two-component PIV measurements and liquid crystal thermography techniques are appropriately used for flow and heat transfer characterization across the complete sharp 180° bend. The centrifugal instabilities arise due to the sharp bend, which induces the secondary flows in the form of counter-rotating vortex pairs commonly known as Dean vortices. These secondary vortices play a significant role in the localized laminar-turbulent transition and turbulence augmentations for 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 V ¯ T | rms increase by 298% and 186% for respective flow conditions. Augmentation of ∼287% (laminar) and 260% (turbulent) in the wall-normal RMS fluctuations (V ¯ N | rms) 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.