• Volumetric heat transfer via constructal theory, and its applications in permafrost and hydrogen energy storage

      Kukkapalli, Vamsi Krishna; Kim, Sun Woo; Lin, Chuen-Sen; Das, Debendra (2016-05)
      Constructal theory is widely used as a powerful tool in designing of engineering systems (flow configurations, patterns, geometry). This theory is observed in nature and its principles are applicable to general engineering. Constructal theory encompasses a wide range of space in the "design", drawing from each and every field from engineering to biology. The universal design of nature and the constructal law unify all animate schemata such as human blood circulatory systems, and inanimate systems, such as urban traffic and river basins. The proceeding research applies the overlying theories of constructal theory to the two different systems in order to achieve best thermal performance phenomena. The first is stabilization of roadway embankments in the permafrost regions with design modifications in existing thermosyphon evaporators with tree structure designs, and defining the optimal spacing between two neighboring thermosyphons based on thermal cooling phenomena. This research utilizes constructal law to the generation of tree-shaped layouts for fluid flow, so that the flow structures use the available space in optimally. The intention here is the optimization of geometry of the flow system. This begins with the most simple cases of tree-shaped flows: T- and Y-shaped constructs, the purpose of which is to create a flow connection between one point (defined as a "source" or "sink") to an infinity of points (via a line/area/volume). Empirically speaking, tree-shaped flows are natural examples of selforganization and optimization. By contrast, constructal law is theory which states that flow architectures such as these are the evolutionary results of nature which tend toward greater global flow access. Tree-shaped flows can be derived from this constructal law. The mathematical simulation revealed that there exists an optimal spacing between two neighboring thermosyphons, and the tree structures perform better than the existing configuration in terms of thermal cooling. The second part of the research is to find an effective way to reject heat released from the metal hydride powder to the outer environment during the hydrogen absorption process. The main objective of this investigation is to minimize the time required for the absorption process, and to reduce the hotspot temperature by determining the optimal aspect ratio of rectangular fins, while the total volume of fins used is kept constant. The intension of using constructal theory in this part of research is to find the optimal geometrical parameters (length, width) of the fin structure for better thermal performance of the metal hydride reactor system. The simulations revealed that there exists an optimum aspect ratio of rectangular fins for accelerating heat rejection and lowering the hotspot temperature in a cylindrical metal hydride reactor. Constructal theory is supremely adapted for use in 2-dimensional and 3-dimensional design for heat transfer structures, as it allows for incorporation of minute analysis of the interior structure with the goal of optimizing for heat transfer. In its application in the realm of engineering, every multidimensional solid structure that is to be cooled, heated or serviced by fluid streams must be vascularized. By this definition, 'vascularization' includes, however is not limited to, structures such as trees, geometrical spacing, and solid walls. Here, every geometric detail will be sized and positioned to achieve maximum efficacy from an engineering design point of view. Furthermore, via design morphing we can achieve low resistances in flow structures which are applicable in cooling and heating applications. An example is that of a ground-source heat pump design where the piping design is morphed by constructal law and spaced in an optimal way to achieve maximum thermal efficiency when extracting heat from the ground.