Radiative surface properties play a critical role in the analysis, design, and optimization of thermal systems. Geometry has a strong influence on the emission and absorption characteristics of a surface. Modification of radiative surface properties may be achieved by using engineered surfaces or cavities by capitalizing on the cavity effect. A model for the apparent emissivity of an arbitrarily shaped cavity with spectrally and directionally dependent radiative surface properties is presented in this work. This model is verified through comparison with published models of the apparent radiative properties of cylindrical cavities. The merit of this model is demonstrated by obtaining closed-form expressions for the apparent emissivity of ideal, two-surface cavities for six representative geometries (cylindrical, conical, spherical, cylindro-conical, cylindro-inner cone, and double-cone). A comprehensive review of prior research related to the apparent emissivity of these shapes is presented. Results obtained using the simple, closed-form expressions agree well with results obtained using more complex methods when the intrinsic surface emissivity is large. Finally, this work elaborates on applications of radial and angular cavity shapes and describes the potential benefits of using geometric manipulation for dynamic control of radiative surface properties in energy and thermal management systems.
In recent years, liquid metals (LMs) have garnered increasing attention as newly emerging functional materials in thermal management. These metals exhibit fascinating properties such as high surface tension, high electrical and thermal conductivity, phase transition phenomenon, low viscosity and vapor pressure, non-toxicity, and biocompatibility. Notably, there have been significant advancements across different categories. These include high-performance LM-based convection cooling technology, low melting point phase change materials, thermal interface materials, and energy harvesters/heat sinks. LMs, with their remarkable heat extraction and transport capabilities compared to conventional coolants, are poised to overcome the limitations currently faced by existing thermal strategies. This chapter provides a comprehensive review of the LMs at all researcher levels, from beginners to experts. It delves into various aspects of the LMs, including morphology, advances, processes, applications, etc. Furthermore, we offer an in-depth exploration of the progress in the sciences and technologies enabled by LMs in thermal management. Also, the chapter provides an account of the various challenges in this direction, such as corrosion and compatibility, chemical stability, thermal conductivity, low-temperature operations, and wettability. These challenges are crucial from a scientific perspective and the prospects of this exciting field. Further, these insights are pivotal in the development of LM-based devices.
High porosity metal foams have been widely adopted in enhanced heat transfer applications, due to their ability to dissipate large heat flux levels. They offer high surface area-to-volume ratio, promote flow mixing and thermal dispersion due to flow tortuosity. The field of porous media research has experienced a transformative shift, propelled by advancements in manufacturing techniques, non-intrusive diagnostics, and computational capabilities. This shift has led to a widespread exploration of architectured cellular materials, leveraging the manufacturing-enabled design freedom. This article specifically focuses on recent developments in this area, with an emphasis on the pore- and strut-level flow and thermal transport for single-phase flows involving various working fluids such as air, water, hydrocarbons, supercritical carbon dioxide, and particles. The multi-functional attributes of recently developed engineered cellular materials and their wide range of applications are discussed. The article concludes by outlining the future directions in engineered cellular materials.
