Tiju Thomas

This paper proposes a Phononic crystal (PnC) device formed by embedding spheres in a polymer matrix for energy harvesting, an application that has drawn much research interest recently. A parametric study is performed to evaluate the optimal radius to lattice parameter ratio r/a = 0.4 to obtain a large bandgap as well as gap to mid-gap ratio. By removing a sphere from the central lattice point, a defect is created within an otherwise perfect PnC. The supercell technique is employed to compute defect modes and band structure of defect PnC. These defect bands show strong vibrational modes within the spatial arrangement of the defect point and these defect frequencies appear to be flat (dω/dk = 0) in the band structure confirming energy localization at these modes. Also, while varying the radius of the defect sphere rd in the range of [0.1a–0.5a], the number of defect modes within the bandgap tend to decrease, vanishing when rd equals 0.4a and 0.5a. To utilize the defect mode for energy harvesting, a small patch made of a piezoelectric material is attached onto the PnC device exactly above the defect. Introduction of this patch renders the band structure and defect modes to shift upwards compared to the defect PnC without the patch. A highly localized vibrational eigen mode is observed around 662 kHz in the band structure for the defect PnC with patch. The voltage response of this device is simulated by connecting a high resistance (open-circuit) to the patch and sweeping frequencies around the defect mode. This process produced a voltage of 2.8 V at 656.7 kHz for a 0.3 µm displacement excitation, thus demonstrating the usefulness of the proposed concept.

Heat transport finds use in several applications including heat exchangers, marine units, food processing units, oil and gas industries, petroleum industries and so on. By providing a large surface area and irregular motion in flow, made possible by porous media and enhances the thermal efficiency. The addition of a single type of nanosized particles in the base fluid gives rise to nanofluids. It tends to have higher thermal conductivity than the base fluid. However, currently hybrid nanofluids are introduced to overcome the disadvantages associated with nanofluids such as lower chemical stability, lower corrosion-resistant and so on. Hybrid nanofluids consist of dissimilar nanoparticles. Here, the heat transfer performance of nanofluids and hybrid nanofluids which flow through porous media is discussed. In addition, the role of the external magnetic field on the heat transfer performance of nanofluids with porous media is also explored. The relation between important parameters like Darcy number, porosity, Hartmann number, Reynolds number, Biot number and heat transfer performance of fluids with porous media is duly discussed. This chapter focuses on the role of porous media in enhancing heat transfer performance of nanofluids and hybrid nanofluids when flow occurs through different geometries.