Exponential growth of heat flux densities in commercial and industrial electronics, and compact heat exchangers demand surfaces and heat sinks with high dissipation rate capabilities. Among different technologies proposed to meet these demands, high-porosity metal foams have attracted the attention of many investigators due to their higher surface area densities as well as higher thermal performance due to the turbulence and tortuosity generated in the flow due to their structure. One of the disadvantages of such metal foams, however, is the attendant higher pressure drop or pumping power penalty.
This thesis was undertaken to investigate whether channels partially filled with metal foams can reduce the required pumping power with a minimal loss in thermal performance. The thermo-hydraulic (T-H) performance factor J/F<sup>1/3, where J is the Colburn-J factor and F is the friction factor, was used to compare the relative performance of foams for various values of blocking fractions (B), where B is defined as the ratio of the height of the foam to the height of the channel.
The metal foam samples considered were 10 PPI (pores per inch) 6101-T6 Aluminum, with porosity of ∼ 94 − 96%, and B of 1/6, 1/3, 2/3, 5/6, and 1. Each of these samples was attached to an aluminum slab embedded in one of the walls, which had a patch heater that acted as a heat source. A modification was made to all B < 1 configurations by attaching an aluminum plate on top, which then separated the foam-free and the foam-filled flows completely. These configurations are denoted by a 'P' in their names (e.g. B = 1/3P is the plated modification of B = 1/3). Experiments were conducted in an in-house designed wind tunnel, with a test section of 45" in length and a cross-section of 3"X3". Reynolds number (based on channel hydraulic diameter and inlet velocity) was varied from 1,000 to 15,000 to capture the flow domains from laminar to turbulent.
The data obtained for the three scenarios namely - 1. Controlled-Flow Scenario 2. Pumping Power Variation with Temperature Difference, and 3. Fan-Based System were analyzed for their thermo-hydraulic performance. The extreme low blocking fractions are evaluated and compared against the dimpled/protruded surfaces, and were found to give superior performance, hence displaying potential as good turbulators. The plated configurations were found to perform better in almost all scenarios when compared to their non-plated counterparts. Furthermore, a new simplified analytical model is introduced that considers the flow in the partially-blocked region as two separate 'parallel' flows, one in the foam-free region and the other in the foam-filled region. The comparison between this novel approach and the analytical solution from the literature shows good agreement, suggesting that this simplified model may be appropriate. This model is then used for determining the foam-filled region flow ratios for the performed experiments, and a correlation is presented. / Master of Science / Portable devices, such as laptops, and mobile phones are trending towards miniaturization and simultaneously becoming more power-hungry, leading to ever-increasing heat flux densities. Growing energy and technology demands require high thermal dissipation rates to be achieved in equipment such as industrial and commercial electronics, data centers, heat exchangers in automobiles, and power plants - both renewable and non-renewable. One of the best ways to enhance convective heat transfer is by increasing the heat transfer surface area. This is traditionally done using fins. A much higher surface area can be achieved using a metal foam instead, along with improving the turbulent mixing of the fluid. The flow through the metal foam, however, faces a higher pressure drop penalty which is one of the major reasons for a continued preference for fins.
In this experimental study, we aim at minimizing this pressure drop penalty of a metal-foam heat-sink along with maintaining a respectable heat transfer performance through 'partial-blocking' (filling) of the channel, where the height of the foam is lower than the total channel height. The ratio of metal foam height to the channel height is named as blocking fraction B. A general comparison of the hydraulic, thermal, and thermo-hydraulic (T-H) performance reveals that the ∼ 83.3% plated configuration is capable of superseding the baseline of full blockage. The 'plating' here denotes a slight modification - a solid plate rests on top of the metal foam, separating the foam-free and foam-filled flow. For applications with Re > 10000, ∼ 33.3% plated configuration is highly recommended. For fan-based systems, ∼ 83.3% plated, ∼ 33.3% plated, and 33.3% non-plated configurations emerge as possible alternatives to the fully-blocked case. Furthermore, while considering partial configurations, it is shown that one should go for lower PPI metal foams to improve the flow ratio inside the metal foam. For pressure-drop critical equipment, ∼ 16.7% configuration is found to perform better than the conventional double-protruded walls and other turbulence-enhancing surface treatments. Finally, this thesis presents a novel and simplified approach for estimating the flow ratios for partially-blocked channels using scaling analysis.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/113602 |
| Date | 31 January 2023 |
| Creators | Sonavane, Prasad Deepak |
| Contributors | Mechanical Engineering, Mahajan, Roop L., Boreyko, Jonathan Barton, Huxtable, Scott T. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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