Title

Evaluation of Alumina Nanofluids and Surfactant Drag Reducing Solutions to Improve Heat Transfer for Aircraft Cooling Systems

Date of Award

1-1-2018

Degree Name

Ph.D. in Materials Engineering

Department

Department of Chemical, Materials and Bioengineering

Advisor/Chair

Advisor: Robert Wilkens

Abstract

There is a critical need for improved coolants for military aircraft applications. Research in this area has focused on either synthesizing novel coolants with improved thermal properties or developing additives for current coolants that can improve heat transfer capability in recirculating systems. This dissertation focuses on the latter alternative investigating the effect of nanoparticles or drag-reducing surfactant additives on effective heat transfer coefficients.Nanoparticles are reported to not only enhance thermal conductivity of coolants, but also increase the heat transfer coefficient. Some setbacks related to the use of nanofluids have been identified, such as increased pressure loss, erosion, and fluid instability. Surfactant drag-reducing additives greatly reduce skin friction, thereby reducing resistance to flow in tubes. Lower flow resistance means that the volumetric flow rate can be greatly increased at constant pumping power. Under these circumstances, the heat transfer coefficient can be enhanced. However, researchers have found that the heat transfer coefficient in tube heat exchangers is reduced by the addition of drag-reducing additives. Moreover, the percentage of heat transfer reduction in tube heat exchangers is greater than the corresponding drag reduction. The reason for the loss of heat transfer is that surfactant drag reducers lower flow resistance by damping turbulent eddies, which are known to drive heat transfer. Several researchers have tried to overcome heat transfer reduction in heat exchangers by different means described in this dissertation. However, despite their efforts, the improvements they achieved were not enough to eliminate the heat transfer reduction. Instead of focusing on turbulence, this dissertation explored the impact of increased heat transfer area per volume within microchannel devices, as the flow regime is typically laminar. The objective of this dissertation is to evaluate two approaches for heat transfer enhancement by additives - nanoparticles and surfactants - in aircraft cooling systems. For proof-of-concept experiments carried out in this work, the base fluid selected was DI water. A computational fluid dynamics study on the pressure, velocity, and temperature profiles was performed to analyze the flow and temperature patterns inside a cold plate, the microcooling device used in this research. A small study on the flow inside an elbow was also performed to analyze secondary flows. Alumina/DI water nanofluids were evaluated at system level. It was observed that, at the same volumetric flow, there was no significant improvement in convective heat transfer coefficients. Problems such as increase of pressure loss, particle settling, and especially vaporization were observed. Next, an aqueous surfactant solution was also tested within the heat exchanger system. A small reduction in both pressure loss and heat transfer coefficient at the system level was found. The relatively high pressure loss was due to the large ratio of form friction to total friction. Other problems associated with the use of surfactants as a heat transfer enhancer were surfactant poisoning and chemical degradation. Finally, alternatives to improve heat transfer coefficient by nanoparticles and surfactant additives are proposed, based on the analysis identified by this study.

Keywords

Chemical Engineering, Computer Engineering, Mechanical Engineering, Materials Science, Nanotechnology, alumina nanofluids, computational fluid dynamics, CFD, surfactant drag-reducing agents, heat transfer, drag reduction

Rights Statement

Copyright 2018, author

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