UC Riverside

Due to their unique and interesting properties, porous structures have recently gained increasing attention and are widely utilized in many applications. Large surface area to volume ratio and design simplicity and controllability, has made them a suitable candidate for effective thermal enhancement. In this thesis, capability of porous structures as a novel surface treatment method for thermal and hydraulic enhancement in microchannel heat sinks and energy storage units are studied. First, in chapter 1, we perform a comprehensive investigation on utilizing porous substrates on vertical and horizontal solid fins in a microchannel heat sink (MCHS). Three dimensional models of MCHSs with different solid and porous fin thicknesses are constructed and numerically analyzed. Various thermal and hydraulic performance parameters are evaluated and compared for a parametric optimization on the thickness of the solid/porous fins. The results show that given a set of parameters for a conventional MCHS, an optimized porous design can be found that can improve the thermal capacity at the same or even lower level of pumping power. This type of improvement is tested and observed for different Reynolds numbers, heat sink materials, and porosities.

In chapter 2, we evaluate the improved porous setup for double-layered MCHSs and effect of utilizing porous substrates on their thermal and hydraulic performance is analyzed. Similar to the single-layered MCHSs, thermal and pumping power of the new porous double-layered MCHSs are evaluated to retrieve the optimized design. Three dimensional geometries are constructed and conjugate heat transfer between the coolant and the MCHS is numerically simulated. A parametric study is presented on the porous and solid fin thicknesses considering two scenarios where the top and bottom microchannels can be similar or having different porous and solid fin thicknesses. Performance parameters including heat transfer effectiveness and pumping power effectiveness are defined and studied for the proposed double-layered MCHSs along with the Figure of Merit (FOM). The results show enhancement in different Reynolds numbers, heat sink materials, and porosities. Interesting behavior is sound for double-layered MCHSs where the top channel has a different design from the bottom one.

In the last chapter, chapter 3, thermal enhancement capability of porous metal foams is investigated for energy storage applications heat sinks working with phase change materials (PCMs). Porous foams as thermal conductivity enhancers (TCEs) are placed inside a PCM-based system and role of the foam’s pore structure including the pore size and density is comprehensively studied utilizing a new numerical approach. The porous medium with variable properties is modeled using Darcy-Brinkman-Forchheimer equations and the transient phenomenon of PCM’s melting inside the foam is simulated. Positive and negative gradient porous morphologies in different directions of the enclosure are considered the thermal response of the energy storage system is determined. It is found that, the new proposed gradient porous TCEs can considerably affect the thermal performance and the detailed results can be exploited to attain the designs with higher efficiencies. The melting profile and heat transfer rate distribution is studied for various heating configurations at different melting stages. Effect of porosity, pore density, and size of the enclosure on the melting and energy storage rate is discussed in detail.