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An experimental investigation was carried out on high porosity metal foams subjected to array jet impingement with an objective to develop enhanced heat transfer configurations. In this study, we propose an integrated thermal management system (TMS) aimed toward leveraging the conjugate heat transfer capabilities of target plate, metal foam, and the jet plate--all made from aluminum and assembled such that a proper contact between them can be established. Steady-state heat transfer experiments were carried out for 10 and 20 pores per inch (PPI) aluminum foams of 0.93 porosity. Both metal foams were 12.7-mm thick. The normalized jet-to-jet spacing was varied from 2 to 12 times the jet diameter, while the jet diameter was fixed. The ratio of the jet plate thickness and jet diameter (nozzle aspect ratio) was 6.35, which ensured proper development of jets inside the nozzles. Experiments were conducted over a wide range of Reynolds number (based on jet diameter) varied from 100 to 5000. The obtained convective heat transfer coefficient for different configuration was evaluated in context with pressure drop. The analysis of experimental results reveal that large open area ratio jets combined with high porosity metal foams provide highly efficient and high-performance cooling for the investigated range of Reynolds numbers. [ABSTRACT FROM AUTHOR]

Phase change materials (PCMs) are an effective medium for thermal management of avionics because of their ability to absorb, store, and release high heat loads while operating within narrow temperature range in confined spaces. Despite hosting several benefits, PCM systems suffer from low thermal conductivity issue. Strategically placed fins in a volume filled with PCM has the potential to significantly improve the transient thermal performance of PCMs by improving the overall thermal conductivity and providing enhanced heat transfer surface area. Furthermore, avionics can be subjected to varying gravity conditions during flight that can have significant influence on the overall thermal performance of PCMs. This emphasizes the need to characterize the PCM performance under various gravity conditions that can impart different buoyancy induced effects in a confined system. To this end, numerical investigation to study the melting characteristics of PCM-fin configuration subjected to three different gravity conditions, i.e., (a) microgravity, (b) terrestrial gravity, and (c) hypergravity, is conducted. The influence of these three conditions on the performance of PCM-fin combination configuration is investigated and finally the temperature field, melt fraction, and melting time is reported. The performance of PCM-fin system is compared with the corresponding PCM-only configuration to highlight the benefits of adding fins under each investigated gravity condition. [ABSTRACT FROM AUTHOR]

This article presents conjugate heat transfer characteristics of strut-based sandwich-type configurations obtained through reticulation of unit cell topologies of Tetrakaidecahedron, Octet and Rhombic dodecahedron shapes (porosity ∼0.90) with water as working fluid. Tetrakaidecahedron lattice exhibited the highest overall heat transfer coefficient, followed by Octet and Rhombic dodecahedron, respectively. Pressure drop was the highest for Octet, followed by Rhombic dodecahedron, and Tetrakaidecahedron, respectively. Thickness of struts in contact with endwall played a dominant role in conjugate thermal exchange of sandwich configurations. For the investigated conditions, Tetrakaidecahedron-shape based lattice provided the least thermal resistance value at any given pumping power condition. [ABSTRACT FROM AUTHOR]

In typical turbulent flow problems, detailed heat transfer coefficient (h) maps obtained through short-duration experiments are based on inverse heat transfer methods that take the wall temperatures measured via liquid crystals or infrared thermography as input, and an error minimization routine is adopted to determine the best value of h that satisfies the wall temperature temporal evolution under a certain change in fluid temperature. A common practice involves modeling the solid as a one-dimensional semi-infinite medium by selecting the solid material that has low thermal conductivity and low thermal diffusivity. However, in certain flow scenarios, the neglection of the lateral heat diffusion may lead to significant errors in the deduced h values. It is imperative to understand the reasons behind large errors that may be incurred by using the 1D heat conduction assumption in order to accurately determine high-resolution h maps for better heat exchanger designs in a wide range of thermal management applications. This paper presents a computational heat transfer study on different jet impingement scenarios to demonstrate the errors incurred in the determination of h when calculated under the assumption of one-dimensional (1-d) heat conduction into a solid. To this end, three different cases are studied: (a) single jet, (b) array jet (theoretical distribution), (c) array jet (experimental distribution), along with three different mainstream temperature evolution profiles representing step change, moderately fast transient and slow transient nature of flow driving the heat transfer in the solid. A known distribution of heat transfer coefficient ("true h") for each of the three cases is considered, and three-dimensional transient heat diffusion equations were solved to populate temperatures of each node in the solid at every time step. It is found that stagnation zones' h1d calculations were lower than the "true h" while the low heat transfer zones exhibited significantly higher h1d compared to the "true h". For the array jet (experimental distribution) case, it was observed that errors can be as high as 10% in certain low heat transfer zones. Different data reduction procedures, configurations, and conditions explored in this study indicate that a suitable balance can be achieved if shorter time durations in transient experiments are used as a reference for tracking in h1d calculations to keep the deviations from the "true h" low. [ABSTRACT FROM AUTHOR]

Film-cooling technology is used in high-temperature components of gas turbines to extend their service lives. Hot-gas path components are susceptible to damage or failure in the absence of film cooling. Much of the optimization research efforts have been focused on film hole shapes, heat/mass transfer measurement techniques, and film cooling performance under various mainstream and coolant side operating conditions. Due to recent rapid advancements in the areas of measurement techniques (e.g., pressure-sensitive paints and fast high-resolution imaging) and metal additive manufacturing (AM), film cooling technology has undergone significant changes and shows potential new development. In this review, a historical perspective is discussed covering over five decades of innovation: the geometrical effects from injection angle and hole shapes; flow effects from density ratio, momentum-flux ratio, blowing ratio, advective capacity ratio, and freestream conditions; and more items related to AM. The impact of AM on film hole design strategies, the challenges posed by state-of-the-art AM technology, and pathways for future research are discussed. A comparative analysis of AM assisted film hole fabrication and conventionally manufactured film holes is elaborated. [ABSTRACT FROM AUTHOR]

Jet impingement cooling is considered as one of the most effective heat transfer enhancement techniques. The primary mode of heat transfer enhancement is due to the flow stagnation. The effectiveness of jet impingement as a cooling technique is well documented; however, the application of jet impingement to different problems has been hindered by inability of manufacturing methods to incorporate impingement systems easily into cooling designs. Impingement heat transfer effectiveness can be further improved by improving the jet strength by modifying the jet holes, enhancing surface features, or adding swirl. With an increased usage of this cooling technique and additional modifications in geometry to further enhance the heat transfer capacity to suit different applications, this paper provides a much-needed review of the advancements in the effectiveness of this cooling technique. A comprehensive look at impingement cooling over a variety of modifications and applications with a focus on improved manufacturing techniques impacting novel design and implementation is provided for a variety of heat transfer enhancement applications. [ABSTRACT FROM AUTHOR]

Heat and flow characteristics of V-shaped protrusion/concavity combination with 45 ° miniature V-rib has been investigated numerically at flow Reynolds number of 50,000. With suitable baseline studies, a total of six different configurations viz. (a) V-rib only, (b) 'V-rib and V-protrusion', (c) 'V-rib and V-concavity', (d) V-protrusion only, (e) V-concavity only, (f) Sparse V-rib, were studied in a square channel with the above features installed on single wall. With air as the working fluid, the heat transfer mechanism and pressure drop characteristics were compared for all the configurations. The compound configuration of 'V-rib and V-protrusion' exhibited the highest thermal hydraulic performance (THP) of 2.15. [ABSTRACT FROM AUTHOR]

Highlights • Novel micro-roughness shapes studied for jet impingement heat transfer enhancement. • Experiments carried out over wide range of geometrical and flow variables. • Novel concentric shape roughness yielded fin effectiveness up to 1.6. • Correlations have been prescribed for heat transfer and fin effectiveness. Abstract With recent advancements in the field of additive manufacturing, the design domain for development of complicated cooling configurations has significantly expanded. The motivation of the present study is to develop high performance impingement cooling designs catered towards applications requiring high rates of heat removal, e.g. gas turbine blade leading edge and double-wall cooling, air-cooled electronic devices, etc. In the present study, jet impingement is combined with strategic roughening of the target surface, to achieve high heat removal rates. Steady state experiments have been carried out to calculate the heat transfer coefficient for jet impingement onto different target surface configurations. The jet-to-jet spacing (x/d = y/d) was varied from 2 to 5, and jet-to-target distance (z/d) was varied from 1 to 5. The target surface configurations featured cylindrical, cubic and concentric shaped roughness elements, fabricated through binder jetting process. The baseline case for the roughened target surface was a smooth target. Heat transfer and pressure drop experiments were carried out at Reynolds numbers ranging from 2500 to 10,000. Further, numerical simulations were carried out to model flow and heat transfer for all configurations at a representative Reynolds number. Through our experiments and numerical results, we have demonstrated that the novel "concentric" roughness shape was the best in terms of fin effectiveness and Nusselt numbers levels, amongst the investigated shapes. The concentric-shape roughened target resulted in fin effectiveness up to 1.6, whereas the cubic- and cylindrical-shape roughened targets yielded in fin effectiveness up to 1.4 and 1.3, respectively. Further, it was experimentally found that the addition of micro-roughness elements does not result in a discernable increment in pressure losses, compared to the impingement on the smooth target surface. Hence, the demonstrated configuration with the highest heat transfer coefficient also resulted in highest thermal hydraulic performance. [ABSTRACT FROM AUTHOR]

• Recent advancements in the application of ultrasonic vibration in enhanced heat transfer research areas have been reviewed, along with earlier studies on mechanical vibrations. • Ultrasonic vibration helps in achieving significant enhancements in heat transfer and is more beneficial in applications with low-speed flows and involving liquids as working fluid. • Ultrasonic vibration has shown promise in thermal transport performance enhancement of condensation and phase change material-based concepts as well. High-performance thermal management solutions are required in a wide range of industrial applications, which involves natural and forced convection, boiling and condensation heat transfer, phase change materials etc. Enhanced heat transfer research on the above topics has been carried out for several decades, with primary focus on the heat transfer surface modification. In recent years, there has been a growing interest in achieving even higher levels of heat transfer through agitating near-wall flow, promoting acoustic cavitation, and streaming, through the introduction of ultrasonic vibration. This paper reviews the recent advancements in the application of ultrasonic vibration in the above-mentioned enhanced heat transfer research areas, along with the earlier studies on mechanical vibration of the heat transfer surface. In general, ultrasonic vibration helps in achieving significant enhancements in heat transfer and is more beneficial in applications with low-speed flows and involving liquids as working fluid. The rapidly growing interest in this technique shows promise in an increased number of studies in the near future, for which this timely review may serve as a guiding tool. [ABSTRACT FROM AUTHOR]

Jet impingement heat transfer has been studied numerically for a maximum crossflow condition using a 3 × 9 array of jets. Five-hole configurations have been studied for jet average Reynolds numbers ranging from 10,000 to 20,000. Crossflow has been mitigated by varying the jet diameters in the streamwise direction to reduce the impact of crossflow on downstream jet impingement. The design criteria for all five configurations were to keep the average of the jet diameters equal to the constant jet diameter configuration (baseline). It has been found that the configuration with increasing and then decreasing jet diameters provided higher levels of heat transfer with more uniform cooling when compared to the traditional constant diameter configuration and other configurations. [ABSTRACT FROM PUBLISHER]

This paper presents a new cooling design for a typical two-pass channel of a high pressure stage turbine blade. Gas turbine blades are subjected to elevated heat loads on the pressure and suction walls. In order to enhance heat transfer from the surfaces to the relatively colder internal air (gas), rib turbulators are installed on the opposite walls of two-pass channel. Combination of Coriolis force and centrifugal buoyancy force result in increase in heat transfer on trailing walls (along the pressure surface, and radially outward coolant flow) and leading walls (along the suction surface, radially inward flow) and vice versa. This leads to non-uniform cooling in both the passes and hence a non-optimum usage of cooling potential. The present study is focused on utilizing the Coriolis force favorably in both the passes by rotating the typical arrangement of two-pass channels by 90°. Detailed heat transfer coefficients were measured by transient liquid crystal thermography under rotating conditions. In order to match the direction of Buoyancy force as it exists in actual engines, colder air was passed during the transient experiment. The heat transfer experiments were carried out at a Reynolds number of 20000 and Rotation numbers of 0 and 0.1. The Nusselt numbers have been reported in two forms, (a) normalized with respect to Dittus-Boelter correlation for developed turbulent flow in circular duct, (b) normalized with corresponding Nusselt number obtained from smooth channel experiments. In order to understand the heat transfer characteristics of both traditional and new design, numerical simulations were also carried out for all configurations and at all experimental conditions to obtain flow and heat transfer predictions. A combined experimental and numerical discussion has been provided to explain the findings of the present study and to support the proposed design. It has been reported that the new design has 11% higher heat transfer enhancement at 8% lower pumping power compared to the traditional two pass rib roughened duct. [ABSTRACT FROM AUTHOR]

In the current study, a novel cooling channel configuration featuring periodically placed tetrakaidecahedron (TKD) unit cells is proposed and studied computationally. The concept can also be realized through the additive manufacturing process to mitigate the heat transfer deterioration and improve the overall thermal performance of combustor cooling channels with n-decane as the working fuel. The conjugate thermal performance of the TKD channel was investigated and compared with the corresponding smooth channel at 3 MPa operating pressure. Simulations were conducted at a fixed coolant mass flow rate of 2 g/s, wall heat flux of 1.5 MW/m2, and an inlet temperature of 423.15 K. TKD channel provided lower overall wall temperatures and higher heat transfer coefficient values than the smooth channel. Furthermore, the thermal performance of both the smooth and TKD channels were analyzed under accelerated flight conditions by employing accelerations a c =2 g, 4 g, and 6 g in the streamwise direction. For the smooth channel, with increase in the acceleration values, the peak temperature values were reduced, and the location of peak value shifted towards the channel outlet. The thermal performance of TKD channel was, however, not significantly influenced by the accelerated states, which ensures enhanced cooling performance at different operating conditions. [ABSTRACT FROM AUTHOR]

Heat transfer enhancement inside ribbed channels for turbine blades is a critical phenomenon that impacts overall performance and life of the gas turbine. Present study investigates heat and fluid flow in a rectangular duct with heat transfer enhancement features, under rotating and stationary conditions. The heat transfer data obtained experimentally has been explained using numerical prediction of flow features. Detailed heat transfer coefficients have been measured on the walls of two-pass rectangular duct (AR = 1:2) featuring V-shaped rib turbulators, using transient liquid crystal thermography (TLCT). The first pass and second pass featured nine V-shaped ribs each and the bend featured a 90° rib connecting the blade tip underside and the two-pass divider wall. The flow in the first pass was developing in nature. The rib-pitch to rib-height ratio (p/e) was 9.625 and the rib-height to channel hydraulic diameter (e/d h ) was 0.125. The baseline case for the rib roughened duct was geometrically identical smooth duct (with no heat transfer enhancement features). Stationary experiments were carried out for Reynolds numbers ranging from 25000 to 75000. The rotation experiments were carried out at 400 RPM ( Ro = 0.036) and 700 RPM ( Ro = 0.063), at Reynolds number of 25000 ( Ro = Ω d h / V , Re = Vd h / ν ) . Also, numerical simulations were performed for a similar test model under similar flow conditions, using realizable k - ∊ turbulence model. Detailed discussion on rib induced secondary flows and rotational effects on heat transfer in smooth and rib roughened duct are presented in this paper using results obtained from detailed heat transfer measurements from experiments and fluid dynamics predictions from numerical simulations. [ABSTRACT FROM AUTHOR]

Transient heat transfer experiments using liquid crystal thermography have been carried out on a two-pass square channel for testing several unique combinations of ribs and cylindrical dimples. Four different rib shapes, viz. 45° angled, V, W and M have been studied. In compound channels, the rib pitch accommodates cylindrical dimples arranged in the shape of the rib, hence following the direction of rib induced secondary flows. For each rib shape, three different configurations – rib alone, dimple alone and rib–dimpled compound cases were studied. The rib-height-to-channel hydraulic diameter ratio was 0.125 and rib-pitch-to-rib-height ratio was 16. The dimple-depth-to-print diameter ratio was 0.3. The experiments were carried out for a wide range of Reynolds number (19,500–69,000), covering a spectrum typically found in both land-based and air-breathing engines. A total of 52 experiments were carried out to measure detailed heat transfer coefficient on the bottom wall of the two-pass channel. A transient liquid crystal thermography technique was used for heat transfer measurement. Static pressure measurements were carried out to measure the overall pressure drop in the two pass channel. From globally averaged Nusselt number and overall pressure drop, thermal-hydraulic performance of the 13 configurations were determined, compared and analyzed. It has been observed that 45° angled and V compound configurations resulted in higher heat transfer augmentation as well as higher thermal hydraulic performance when compared with their corresponding ribs alone and dimples alone configurations. [ABSTRACT FROM AUTHOR]

In this study, the heat transfer and friction characteristics of four different rib geometries- 45° angled, V-shaped, W-shaped and M-shaped ribs in a two-pass stationary channel have been numerically investigated. The aspect ratio (Height to Width) of the cooling channel was 1:1 (square). The rib pitch-to-rib height ratio (p/e) and the rib-height-to-channel hydraulic diameter ratio (e/D h ) were 16 and 0.125 respectively. The Reynolds number was varied from 20,000 to 70,000. For the computations, the Reynolds averaged Navier–Stokes (RANS) equations were solved with the commercial software ANSYS Fluent using the realizable version of k-ε (RKE) model. The heat transfer results were benchmarked with experiments on a test rig with similar geometries and flow conditions. Detailed analysis of the flow characteristics in the two-pass channel was carried out so as to understand the interaction of the rib-induced secondary flows and the bend-induced secondary flows and their contribution to heat transfer enhancement. The heat transfer enhancement provided by V-shaped ribs was 7% higher than 45° ribs, 28% higher than W-shaped ribs and 35% higher than M-shaped ribs. However, the pressure penalty for V-shaped ribs was 19% higher than 45° ribs, 24% higher than W-shaped ribs and 28% higher than M-shaped ribs. On comparing the overall thermal hydraulic performance, V-shaped and 45° ribs were observed to perform significantly better than W-shaped and M-shaped ribs. [ABSTRACT FROM AUTHOR]

Swirling flows have applications in several areas which includes enhanced heat transfer and combustion. In these applications, swirling jets typically impinge on a target surface either orthogonally or at an angle. This paper presents an experimental and numerical investigation on the effects of jet-to-target spacing (z/d – 0.5 to 4) on heat transfer characteristics of orthogonally impinging swirling jets in comparison to geometrically similar cylindrical jet. Further, obliquely impinging jets (at z/d = 4) with anti-clockwise swirl (when viewing target surface from the jet) have been studied. Transient liquid crystal thermography experiments were carried out for above configurations at jet Reynolds number of 18,000 for orthogonal impingement and for Re ranging from 9000 to 18,000 for obliquely impinging swirling jet. The experimentally obtained local heat transfer when studied in conjunction with the flow-field predictions reveal that swirling jets can result in high heat transfer compared to conventional cylindrical jets at low z/d, where the area coverage of swirling jet is smaller than cylindrical jet. With increasing z/d, swirl jet impingement heat transfer showed significant reduction (while increasing coverage) while cylindrical jet impingement primary heat transfer zone was not very sensitive towards it. This is attributed to increased expansion of swirling jet exiting the nozzle at higher z/d, while cylindrical jet retaining its potential core for the z/d range investigated here. Obliquely impinging swirling jet had reduced stagnation zones due asymmetric arrangement of swirl passages with respect to target surface. [ABSTRACT FROM AUTHOR]

Experimental and numerical study of flow and heat transfer in a two-pass channel featuring different rib geometries has been carried out. The thermal hydraulic performance of four different rib geometries – 45° angled, V, W and M-shaped ribs, have been reported and analyzed. The tests were performed over a Reynolds number range from 19,500 to 69,000. The channel aspect ratio was 1:1 (square), the rib-pitch-to-rib-height ratio ( p / e ) was 16 and the rib-height-to-channel hydraulic diameter ratio ( e / D h ) was 0.125. The detailed Nusselt number distributions on the ribbed wall were obtained using transient liquid crystal thermography. Numerical simulations (using ANSYS Fluent) have been carried out to resolve the complex flow field peculiar to rib turbulator shapes, for detailed understanding of the experimentally measured heat transfer coefficients. For the numerical simulations, realizable version of k - ε model was chosen because of its ability to predict separated flows behind ribs. Also, CFD simulations have been validated with experimentally obtained pressure measurements at several locations in the two pass channel. In addition to flow validation, the numerically obtained heat transfer results are validated and compared with the experiments and discussion has been presented on the role of secondary flows, turbulent kinetic energy etc., on heat transfer augmentation due to the presence of the ribs. [ABSTRACT FROM AUTHOR]

• Two-dimensional heat transfer coefficient maps presented for obliquely impinging jet. • Effect of jet-to-target spacing (z / d), and jet Reynolds number (Re) studied. • z/d had a dominant effect on local heat transfer characteristics. • The upstream shift of the primary peak heat transfer moves closer to the geometric center with increasing z/d. This paper presents an experimental investigation on the primary and secondary peak heat transfer characteristics of an obliquely (45°) impinging jet on a flat and smooth target surface. Local convective heat transfer coefficients have been calculated through a transient liquid crystal thermography technique employing one-dimensional semi-infinite conduction model for the target surface. The effects of jet Reynolds number (Re) and jet-to-target spacings (z/d) have been investigated where Re was varied from 9,000 to 18,000 and z/d was varied from 2.5 to 6. Local heat transfer measurements reveal several important features of oblique jet impingement, e.g., the location of the primary and secondary peaks with respect to the geometric center (GC), its dependence on Re and z/d, and the shape of enhanced heat transfer zone with stagnation, separation, and reattachment features. The two-dimensional heat transfer map is presented in a normalized form Nu/Nu max , where Nu max is the primary peak heat transfer. It has been found that for a certain z/d, the primary peak occurs at a fixed distance from the geometric center for the range of Re investigated, while the secondary peak location varies with Re. For the smallest z/d, the primary peak heat transfer location was offset by one jet diameter from the GC in the uphill direction, whereas for the moderate z/ d = 4, the primary peak occurred near the GC. For the highest z/d, the primary peak was again in the vicinity of GC, however, slight offset on the downhill side. The spent flow reattachment (post the first impingement) which also determined the secondary peak location and heat transfer level was strongly dependent upon the Re. The oblique jet impingement radial variation is also compared against 90° (orthogonal) jet impingement at z/ d = 2.5 and 6 at Re = 18,000. It has been observed that the primary peak for above two z/d configurations were sharper for the 45° case when compared to 90°, and the secondary peak was further offset in the downhill direction with higher heat transfer levels for the 45° impingement case compared to 90°. [ABSTRACT FROM AUTHOR]

Liquid Crystal Thermography is a widely used experimental technique in the gas turbine heat transfer community. In turbine heat transfer, determination of the convective heat transfer coefficient (h) and adiabatic film cooling effectiveness (η) is imperative in order to design hot gas path components that can meet the modern-day engine performance and emission goals. LCT provides valuable information on the local surface temperature, which is used in different experimental methods to arrive at the local h and η. The detailed nature of h and η through LCT sets it apart from conventional thermocouple-based measurements and provides valuable insights into cooling designers for concept development and its further iterations. This article presents a comprehensive review of the state-of-the-art experimental methods employing LCT, where a critical analysis is presented for each, as well as some recent investigations (2016–present) where LCT was used. The goal of this article is to familiarize researchers with the evolving nature of LCT given the advancements in instrumentation and computing capabilities, and its relevance in turbine heat transfer problems in current times. [ABSTRACT FROM AUTHOR]

Impingement heat transfer is considered one of the most effective cooling technologies that yield high localized convective heat transfer coefficients. This paper studies different configurable parameters involved in jet impingement cooling such as, exit orifice shape, crossflow regulation, target surface modification, spent air reuse, impingement channel modification, jet pulsation, and other techniques to understand which of them are critical and how these heat-transfer-enhancement concepts work. The aim of this paper is to excite the thermal sciences community of this efficient cooling technique and instill some thoughts for future innovations. New orifice shapes are becoming feasible due to innovative 3D printing technologies. However, the orifice shape variations show that it is hard to beat a sharp-edged round orifice in heat transfer coefficient, but it comes with a higher pressure drop across the orifice. Any attempt to streamline the hole shape indicated a drop in the Nusselt number, thus giving the designer some control over thermal budgeting of a component. Reduction in crossflow has been attempted with channel modifications. The use of high-porosity conductive foam in the impingement space has shown marked improvement in heat transfer performance. A list of possible research topics based on this discussion is provided in the conclusion. [ABSTRACT FROM AUTHOR]

Recent studies on traditional metal foam thick blocks suggest that much of the foam volume remains unused in heat dissipation and that the thinner configurations are more efficient in forced convection scenarios. Single cell thick lattice-frame configurations are attractive solutions to the problem of unused foam volume and can also be additively manufactured through Direct Metal Laser Sintering process while also offering superior load bearing capabilities. Due to the inherent nature of such single cell thick configurations, their unit cell topologies' footprint at the enclosing walls has significant effect in both local heat transfer coefficient distribution and net overall heat dissipation. This study presents detailed endwall heat transfer coefficient measurements for lattice-frame structure of Octet unit cell topology and compares its thermal performance with two other members of the Octahedron family, viz. Octa and V-Octet. Developing heat transfer was measured through transient liquid crystal thermography at the endwalls of a unity aspect ratio duct featuring five consecutive interconnected unit cells at a Reynolds number of 37,200. Relative to a smooth channel, the cell-based averaged Nusselt numbers for Octet, Octa and V-Octet were 2.96, 2.78 and 3.05 times, respectively. [ABSTRACT FROM AUTHOR]

• Critical review of morphological and thermal-hydraulic performance of the conventionally and additively manufactured lattices is conducted. • Conventionally manufactured wire-meshes can provide heat transfer enhancements similar to stochastic metal foams and relatively easy and cheap to manufacture. • Additively manufactured lattices demonstrate higher interfacial heat transfer coefficient values than stochastic metal foams, but the role of inherent surface roughness needs more investigation. The advantages offered by the stochastic metal foams when used in high-performance compact heat exchangers is evident in the open literature. Recently, there has been a surge in investigations on regular metal lattices which can be manufactured additively, because of their "potential" to provide multifunctional and tunable characteristics to heat exchangers. These lattices are also reported to lend superior mechanical properties as well. The literature, however, lacks in the critical assessment of these additively manufactured regular lattices in terms of the quality of printed parts (defects, surface roughness, etc.), their compatibility in compact heat exchangers, flow and thermal transport characteristics, and whether or not they are viable alternatives to the commercial metal foams conventionally manufactured. This paper reports different manufacturing routes for the fabrication of conventional stochastic and regular lattices and additively manufactured regular lattices, and how these technologies have evolved over the years. The additively manufactured lattices different from the commercial TKD may result in significantly higher interstitial heat transfer coefficient. Further, the design freedom allows different porosities through which higher effective thermal conductivity can be achieved. The usage of regular lattices in tube-heat exchangers, as wicks in heat pipes, heat transfer enhancement features in boiling heat transfer and transpiration cooling mechanisms of turbine blades as an alternative to conventional technologies is promising. The role of inherent surface roughness in dictating the thermal and flow transport in additively manufactured lattices, especially for multi-phase flow applications, should be studied in more detail. [ABSTRACT FROM AUTHOR]

Present study reports flow and thermal transport characteristics of metal-foam based on Octet-shaped unit cell (porosity ~ 0.97) for electronic cooling application with water as the working fluid. Conjugate simulations were carried out on a single cell thick lattice frame comprising of four consecutive connected unit cells in the streamwise direction. Simulations were performed for flow velocity range of 2–8 cm/s with the substrate subjected to a heat flux of 10 W/cm2, for two different solid-phases corresponding to solid-to-fluid thermal conductivity ratios (k s /k f) of 37 and 288. A control-volume-based energy balance method was adopted for calculation of global and cell-based convective heat transfer coefficient in the conjugate simulations, where the global heat transfer coefficient was ~ 2900 W/m2K for k s /k f ~288 and ~ 2100 W/m2K for k s /k f ~37, both for flow velocity of 8 cm/s. The cell-based heat transfer coefficient showed a sharp declining trend typical of developing flow. The interfacial heat transfer coefficient (h sf) on the fibers and the substrate-endwall was also obtained through separate simulations imposing constant heat flux and constant temperature boundary conditions on the walls. The averaged h sf on fiber walls and substrate-endwall was ~ 8700 W/m2K and ~ 2000 W/m2K, respectively, corresponding to average flow velocity of 8 cm/s. [ABSTRACT FROM AUTHOR]

Detailed understanding of hot gas path flow and heat transfer characteristics in gas turbine systems is imperative in order to design cooling strategies to meet the stringent requirements in terms of coolant usage to maintain critical components below a certain temperature. To this end, extensive research has been carried out over the past four decades on advanced thermal diagnostic methods to accurately measure heat transfer quantities such as Nusselt number and adiabatic film cooling effectiveness. The need to capture local heat transfer characteristics of these complex flow systems drives the development of measurement techniques and the experimental test facilities to support such measurements. This article provides a comprehensive overview of the state-of-the-art thermal diagnostic efforts pertaining to heat transfer measurements in rotating gas turbine blade internal and external cooling and rotor-stator disc cavity, all under rotating environments. The major investigation efforts have been identified for each of the above three categories and representative experimental results have been presented and discussed. [ABSTRACT FROM AUTHOR]

• Traditional open-cell Tetrakaidecahedron (TKD) structure is compared with three circular strutbased lattices, viz. Cube, FD - Cube and Octet. • Pressure drop and interfacial heat transfer coefficient was evaluated for periodic unit cells. • Octet provided pressure drop comparable to that of TKD • Octet and FD - Cube topologies exhibited higher interfacial heat transfer coefficients than the TKD and Cub ic counterparts. Advancements in Additive Manufacturing (AM) routes have opened several opportunities for manufacturing complex porous structures which could not have been manufactured by the conventional foaming process that essentially results in unit cells represented as Tetrakaidecahedron (TKD) shape. The regular open-cell lattices belonging to the cubic and octahedron family of structures provide wide spectrum of porosity, pore-density, permeability and surface area-to-volume ratios. These complex cell topologies can potentially be manufactured additively, making the realization of above-mentioned properties possible. For efficient heat exchanger designs, the reticulated structures made from these unit cells are expected to have higher permeability, porosity, effective thermal conductivity and interstitial heat transfer coefficient. A detailed understanding of local flow and heat transport characteristics is necessary to design unit cells with such properties. To this end, four different metal foam unit cell topologies have been investigated in the present study, viz. Tetrakaidecahedron (TKD), Cube, FD-Cube and Octet. The Cube and FD-Cube unit cell topologies investigated in the present study belong to the cubic family, where the fibers are present only along the edges and face of the unit cell volume. The TKD and Octet configurations belong to the octahedron class of lattices, where complex connectivity exists within the unit cell volume. To allow for direct comparison between these three unit cells, circular fibers are chosen, which is different from the general TKD cells (tricuspid cross-sections). The porosity was fixed 0.986 which dictated the fiber dimensions of respective unit cells, with Octet fibers being thinnest. Direct simulations have been carried out with periodic boundary conditions on unit cell faces which were parallel to the flow. Such configurations when reticulated in three dimensions are representative of porous structures which are single-cell thick in the streamwise direction. These foam shapes have been proven to be very efficient as they do not allow the thermal boundary layer development. Flow and heat transfer results obtained for above boundary conditions have also been compared with the periodic boundary condition on all the faces. The direct simulation results include pressure drop per unit length and volumetric Nusselt number (corresponding to constant heat flux and constant wall temperature boundary conditions on fibers). Flow stagnation on fibers was the main contributor for both, pressure drop increase, and interstitial heat transfer enhancement. TKD unit cell had higher flow losses and thermal performance than FD-Cube. Octet (octahedron family) configuration had the highest flow losses as well as Nusselt number compared to all other unit cell topologies, because of the large surface area offered for flow stagnation. [ABSTRACT FROM AUTHOR]

High-porosity metal foam (MF) is a popular option for high-performance heat exchangers as it offers significantly higher heat transfer participation area per unit volume compared to other convection enhancement cooling methods. Further, metal foams provide highly tortuous flow paths resulting in thermal dispersion assisted by enhanced mixing. This paper presents experimental and numerical studies and the detailed underlying physics of jet array impingement onto high-porosity (ε ~ 0.95?) thin aluminum foams. The jet and foam configurations were designed for the maximum utilization of the foam area for heat transfer and reduced penalty on the pumping power requirement. Three different pore density foams were tested with three different array-jet impingement configurations. The minimum possible thickness for each pore density was tested, viz., 5 pores-per-inch (PPI): 19?mm, 10 PPI: 12.7?mm, and 20 PPI: 6.35?mm. The baseline case for these foam-based jet impingement configurations was the corresponding configuration of orthogonal jet impingement onto a smooth heated surface, where the distance between the jet-issuing plane and the heated surface was maintained at the foam thickness level. In general, thinner foams facilitated greater jet penetration and increased foam volume usage, resulting in higher heat transfer rates for a given pore density, especially when combined with jet configurations with larger open areas. Finally, we evaluated the thermal hydraulic performance for different foam configurations and the optimum value of a given PPI was found to be at an intermediate rather than the lowest foam thickness. [ABSTRACT FROM AUTHOR]

Accurate quantification of local heat transfer coefficient (HTC) is imperative for design and development of heat exchangers for high heat flux dissipation applications. Liquid crystal and infrared thermography (IRT) are typically employed to measure detailed surface temperatures, where local HTC values are calculated by employing suitable conduction models, e.g., one-dimensional (1D) semi-infinite conduction model on a material with the low thermal conductivity and low thermal diffusivity. Often times, this assumption of 1D heat diffusion and ignoring its associated lateral conduction effects leads to significant errors in HTC determination. Prior studies have identified this problem and quantified the associated errors in HTC determination for some representative cooling concepts, by accounting for lateral heat diffusion. In this paper, we have presented a procedure for solution of three-dimensional (3D) transient conduction equation using alternating direction implicit (ADI) method and an error minimization routine to find accurate HTCs at relatively lower computational cost. Representative cases of a single jet and an array jet impingement under maximum crossflow condition have been considered here, for IRT and liquid crystal thermography, respectively. Results indicate that the globally averaged HTC obtained using the 3D model was consistently higher than the conventional 1D model by 7--14%, with deviation levels reaching as high as 20% near the stagnation region. Proposed methodology was computationally efficient and is recommended for studies aimed toward local HTC determination. [ABSTRACT FROM AUTHOR]

• A novel V-shaped protrusion is developed as a heat transfer enhancement concept. • Array of V-shaped protrusion and concavity investigated in channel of AR 4:1. • V-shaped protrusion provided the highest thermal-hydraulic performance of approximately 2.23. • V-shaped protrusion exhibited ~ 40.4% higher thermal-hydraulic performance than their concavity counterpart at Re ==10,000. Modern high-pressure stage gas turbine blades are equipped with several high-performance cooling concepts, which vary based on the external heat load and the blade-topology. Blade trailing edge cooling is challenging due to the space constraints, where channel aspect ratio (AR: channel width-to-height ratio) is ~ 4:1. With an aim to enhance the thermal hydraulic performance of such narrow channels, a novel V-shaped protrusion configuration is proposed in the present study. The V-shaped protrusion derives its profile from a known concept V-shaped concavity. Numerical investigation of inline configurations of V-shaped concavities and protrusions has been carried out for a 4:1 AR channel for Reynolds number ranging from 10,000 to 60,000. The configurational parameters of both concavity and protrusion were identical. Concavity/protrusion depth-height-to-diameter (δ/D) ratio was 0.3. The streamwise (S x) and spanwise (S y) pitch values were 3.2D in both the configurations. The fluid flow and heat transport in the two cooling configurations are discussed in detail. Globally averaged Nusselt number, friction factor and thermal-hydraulic performance have been reported for the investigated Re range. Results show that V-shaped protrusion had the highest thermal-hydraulic performance ~ 2.23, which was about 40.4% higher than the V-shaped concavity configuration for a representative Reynolds number of 10,000. [ABSTRACT FROM AUTHOR]

• Experimental and numerical study on array jet impingement on foams flushed with jets presented. • Different pore-densities 10, 20, 40 ppi and jet configurations x / d = 2,3 and 5 studied. • Largest pore density (40 ppi) and largest open area ratio jet configuration (x / d = 2) yield highest heat dissipation. This paper reports an experimental and numerical investigation on flow and thermal transport in high porosity aluminum foams (ε~0.94–0.96) placed in a channel of square cross-section and subjected to jet array (5 × 5) impingement. The plate issuing the jets was flushed with the metal foam face. Experiments were conducted for three different values of jet-to-jet spacing (x/ d j =y/ d j) of 2, 3 and 5, and three foam samples with pore-densities 10, 20, and 40 PPI (pores per inch). Values of steady-state heat transfer rate and pressure drop were calculated for Reynolds number (based on channel hydraulic diameter and channel inlet velocity) ranging from 2500 to 13,500. The main findings are as follows: first, for all the test runs, the impingement configurations had higher heat transfer rate, h , compared to that for the baseline configuration of metal foams in a channel flow. Second, for all values of the jet-to-jet spacing, an increase in pore-density was accompanied by an increase in heat transfer. The enhancement (h / h 0) varied between 26 and 48, with the highest gain observed for the widest jet (x/d=2) and the highest pore density (40 ppi) configuration, where h 0 is the heat transfer coefficient for developed turbulent flow in a circular duct. The numerical computations reveal interesting recirculating flow structures immediately downstream of the jet exits and the extent of flow penetrating in the foam. Collectively, these flow patterns play a dominant role in determining the volume of metal foam volume participating in the thermal transport and the net convective heat transfer rate. [ABSTRACT FROM AUTHOR]

Particle-to-supercritical carbon dioxide (sCO2) heat exchanger is a critical component in next-generation concentrating solar power (CSP) plants. The inherently low heat transfer between falling particles and sCO2 imposes a challenge toward economic justification of levelized cost of electricity produced through solar energy. Introduction of integrated porous media with the walls bounding particle flow has the potential to enhance the overall particle-to-sCO2 heat exchanger performance. This paper presents an experimental study on heat transfer characterization of additively manufactured lattice frame material based on Octet-shaped unit cell with particles and air as working fluids. The lattice structures were additively manufactured in stainless steel (SS) 316L and SS420 (with 40% bronze infiltration) via Binder jetting process, where the lattice porosities were varied between 0.75 and 0.9. The mean particle diameters were varied from 266 µm to 966 µm. The effective thermal conductivity and averaged heat transfer coefficient were determined through steady-state experiments. It was found that the presence of lattice enhances the effective thermal conductivity by 2-4 times when compared to packed bed of particles alone. Furthermore, for gravity-assisted particle flow through lattice panel, significantly high convective heat transfer coefficients ranging from 200 W/m2K to 400 W/m2K were obtained for the range of particle diameters tested. The superior thermal transport properties of Octet-shape-based lattice frame for particle flow makes it a very promising candidate for particle-to-sCO2 heat exchanger for CSP application. [ABSTRACT FROM AUTHOR]

• Detailed parametric study of novel concentric microroughness configurations subjected to array jet impingement • Configuration with largest pin-fin height produced maximum pin-fin effectiveness of 3.23 • Prescribed correlation for pin-fin effectiveness based on 64 configurations Array jet impingement when combined with strategically placed short height (<1.25 mm) roughness elements on the target wall results in enhanced heat dissipation at high fin-efficiency (>95%) without much penalty on pumping power. Advances in metal-powder-based additive manufacturing enables the printing of roughness scales (< 0.5 mm) with acceptable accuracy. This paper presents an experimental and numerical study on the characterization of heat transfer performance of concentric-shaped micro pin-fins subjected to array jet impingement. The experimental study served the dual purpose of establishing manufacturability of such configurations and validating the computational model. Three concentric micro-pin fin configurations were additively manufactured with AlSi10Mg alloy through Direct Metal Laser Sintering (DMLS) with dimensions between 0.25 - 2.25 mm on the target wall subjected to a 5 × 5 array of jets with normalized jet-to-jet spacing of X/D jet =Y/D jet = 3 and normalized jet-to-target spacing Z/D jet of 1. The resultant Nusselt number (Nu) and fin effectiveness (ε) values have been reported for Reynolds number ranging between 3,000 and 12,000. The three target plate configurations produced effectiveness between 1.8 - 2.4 and the configuration with maximum wetted surface area (wall thickness of 250 µm) and void fraction produced highest effectiveness. Further, a validated numerical model was used to perform a parametric study on geometrical parameters of roughness element viz. pin-fin spacing (S), inner diameter (D 1), outer diameter (D 2) and height (H) at a Re Djet = 6,000. Significant enhancement in ε was observed with H being the dominant parameter affecting the heat transfer. The effect of void fraction (α), which denotes the free flow area between pin-fin elements (A b –A pin) was also explored. There exists an α opt at which maximum heat transfer was achieved. A correlation for fin effectiveness is proposed as a function of different non-dimensional variables derived from the parametric study and its predictive capabilities were found to be within ±10%. [ABSTRACT FROM AUTHOR]

• Experimental investigation on additively manufactured lattices with four different topologies, namely, (a) Octet, (b) FD-Cube, (c) Tetrakaidecahedron, and (d) Cube, was conducted to analyze their periodic heat transfer characteristics. • The flow became thermally fully developed after travelling five or six rows of unit cells in the streamwise direction and heat transfer coefficient levels of Octet, FD-Cube, and TKD in this periodic heat transfer zone were similar with values between ∼216 W / m 2 K and 415 W / m 2 K. • Lattices were capable of providing ∼ 10 to 20 times higher heat transfer than the smooth channels and also yielded overall thermal-hydraulic performance value of ∼ 3 at the lowest representative Reynolds number investigated. In the present study, the flow and heat transfer characteristics of lattices with four different unit cell topologies, namely, (a) Octet, (b) FD-Cube, (c) Tetrakaidecahedron (TKD), and (d) Cube were investigated with air as the working fluid. All the samples had the designed porosity of ∼ 0.88 and unit cell size of 10 mm. The distribution of local heat transfer coefficient revealed that depending on the unit cell topology, the flow became thermally developed after travelling over five or six rows of unit cells in streamwise direction. The heat transfer coefficient values in thermally developed zones were similar for Octet, FD-Cube and TKD. Overall heat transfer coefficient provided by Octet, FD-Cube, and TKD was in the range of ∼216 W / m 2 K to 415 W / m 2 K and Cube yielded the lowest heat transfer coefficient values between 167 W / m 2 K and 290 W / m 2 K. Three different characteristic length scales, viz. (a) channel hydraulic diameter, (b) pore-diameter, and (c) fiber-diameter were used to define the Nusselt number. The usage of pore-scale dimensions such as pore-diameter and fiber-diameter provided a very coherent data set for Nusselt number of all the topologies which helped in deriving a single correlation for predicting the thermal performance. The lattices provided heat transfer augmentation of 10-20 times higher than the smooth channel counterparts. Amongst all the investigated topologies, FD-Cube incurred the highest and Cube yielded the lowest pressure drop. It was also observed that convective heat transfer coefficients obtained for different lattices can be conveniently reported in Nusselt number forms with pore diameter and/or fiber diameters as reference length scales, where such Nusselt numbers for different lattice configurations converges to a single trendline when plotted against Reynolds numbers (also based on same reference length scales). We have provided a single correlation for Nusselt numbers as a function of Reynolds numbers which fits all the lattice configurations studied here. Finally, the thermal-hydraulic performance of lattices is reported which had similar magnitude levels for all the investigated topologies and a single correlation has been proposed for THP covering all the configurations of present study. The investigated topologies provided THP of ∼ 3 at the lowest investigated Reynolds number. [ABSTRACT FROM AUTHOR]