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.

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 towards 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 266-966 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 200-400 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.

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.

Single unit cell thick lattice frame materials have applications in efficient heat exchangers. This study is focused on strut-based sandwich-type configurations obtained through reticulation of unit cell topologies of tetrakaidecahedron (TKD), octet, and rhombic dodecahedron (DDC) shapes at a porosity of 0.9 with water as the working fluid. Interfacial heat transfer coefficient values on struts and endwalls were determined by imposing constant temperature boundary condition. Averaged heat transfer coefficient on the endwall was the highest for tetrakaidecahedron lattice whereas rhombic dodecahedron lattice exhibited the highest average interfacial heat transfer coefficients on the struts. Flow analysis showed the presence of strong secondary flow features on planes normal to the mean flow direction that demonstrated the unique flow mixing capabilities of these lattices. Reported interfacial heat transfer coefficient at struts and endwall can be used in volume-averaged computations of metal foams (representative of lattices' flow and thermal properties) under local thermal nonequilibrium.

Moving packed-bed heat exchangers in concentrated solar power (CSP) plants involves heat transfer between heated falling particles and supercritical carbon dioxide. The overall effective thermal conductivity of the moving packed bed and particle-side channel contact resistances are still the bottlenecks in achieving the desirable thermal transport levels. To this end, a novel moving packed bed heat exchanger consisting of an Octet lattice packed between the walls of the particle-side channel is proposed in this study. Granular flow analysis in Octet lattice moving packed bed heat exchanger (OLHX) was conducted through experiments and discrete element method (DEM)-based numerical simulations. The experimental images clearly demonstrated stagnation regions upstream of lattice fibers, void regions downstream of the fiber junctions, and wavy-type unobstructed flow on the lateral sides of the fibers. DEM simulations were successful in capturing all these critical flow phenomena. Larger flow velocities were observed on the lateral sides of the fibers in the simulations. Also, when the particles in the silo were emptied, the final images showed an accumulation of particles on the inter-fiber as well as fiber–channel wall junctions. Moreover, the fiber connections resulted in some regions devoid of particle contact on the channel endwall, which means that these regions would suffer from poor thermal exchange. The overall mass flowrate increased with increasing porosity for a fixed particle diameter.