Control and Prevention of Ice Formation on the Surface of an Aluminum Alloy

Maral Rahimi

Research output: Book/ReportPh.D. thesisResearch

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Abstract

In cold climates, mechanical ventilation systems with heat recovery, e.g. air-to-air exchangers, are often used to reduce energy demand for heating by recovering the heat from the exhausted air. This, however, creates a risk of ice accretion on the fins of the heat exchanger as warm and humid exhausted air cools down. Due to the reduction in heat exchanger efficiency due to ice formation, this phenomenon has been studied for many decades. There are two approaches to controlling ice formation on heat exchangers: active and passive. The active methods, e.g. bypass, recirculation, preheating etc., require energy and consequently reduce the overall efficiency of the system. They are not addressed in this work and have already been studied extensively by many researchers. The passive methods, which are related to the surface characteristics of the heat exchanger fins and their effect on the initial phases of ice formation, are the main focus of this PhD study. Since aluminum alloys are commonly used to build air-to-air heat exchangers, their surface characteristics play a crucial role in ice nucleation, formation and accretion. This study is specifically focused on aluminum alloy 8011.
Aluminum and its alloys are expected to possess a high energy surface; however, measurements show that the actual surface exhibits a rather high contact angle of about 78 degrees, which is presumably related to surface contamination. In this PhD study, several types of surface modifications were developed that allowed us to obtain stable hydrophilic and hydrophobic surfaces with the contact angles varying from 12° to more than 120°. The effects of these modifications on surface morphology and wettability—the main parameters determining ice nucleation, formation, accretion and freezing delay—were studied comprehensively. In particular, it was found in the first part of study that flat hydrophobic surfaces exhibit slower ice growth and denser ice layers, making this type of treatment preferable for aluminum heat exchangers. Moreover, observations show that the bare aluminum surfaces are characterized by faster ice growth and less dense ice layer as compared to hydrophilically and hydrophobically modified aluminum surfaces. This commonly observed phenomenon can be attributed to the heterogeneous character of bare aluminum surfaces, leading to a broad distribution of surface energies on the microscopic scale. Upon even minor cooling below the freezing point, this leads to the nucleation of widely separated water droplets/ice crystals on high-energy nucleation centers and the formation of low-density feather-like ice structures, hence this significantly deteriorates the performance of heat exchangers with aluminum fins.
Furthermore, the freezing delay and wettability of chemically modified aluminum surface as a function of the substrate temperature was studied. Comparison of the observed behavior with the predictions of the heterogeneous ice nucleation theory showed that a slightly hydrophilic substrate modified with (3-aminopropyl) triethoxy silane (APTES) exhibited longer freezing delays as compared to both more hydrophilic and more hydrophobic substrates. This is attributed to a particular surface chemistry of the APTES modification that prevents ice formation at the interface of the substrate due to presence of high local ion concentration (amino groups), hence leading to significant freezing point suppression. Furthermore, the results suggest that surface topography and wettability determine the freezing kinetics of a droplet placed on a precooled sample. Therefore, surface chemistry which may change these surface characteristics can be used as a tool to control the actual wetting properties of a cold surface in a humid atmosphere.
On the basis of the findings and observations of this study, we find that tailoring the surface characteristics through the application of different chemical or mechanical methods is an effective method for changing the icing properties of a surface. Future studies might focus on studying the effect of different surface coatings with ion concentration on ice formation kinetics.
Original languageEnglish
PublisherAalborg Universitetsforlag
Number of pages125
ISBN (Electronic)978-87-7112-845-1
DOIs
Publication statusPublished - 2016
SeriesPh.d.-serien for Det Teknisk-Naturvidenskabelige Fakultet, Aalborg Universitet
ISSN2246-1248

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