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Experimental characterisation of dual-mass vibration energy harvesters employing velocity amplification
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The thermal behaviour of segregated heat sink structures in natural convection
No full textRemotely deployed wireless devices such as tower-top active antenna arrays, remote radio heads, and pico or femto-cells are an increasingly prevalent feature of communications systems. Environmental standards which govern outdoor wireless equipment stipulate stringent conditions: high solar loads (up to 1 kW/m2), ambient temperatures as high as 55ºC and negligible wind speeds (0 m/s). These challenges result in restrictions on power dissipation within a given envelope, due to the limited heat transfer rates achievable with natural convection.
There are two main objectives of this thesis; the first is to compare the results from a numerically modelled natural convection heat sink, representative of a remote radio head, with and without a solar shield subject to industrial standards and an environmentally defined worst case condition (a daytime condition in a hot location). The impact of solar shield length and location of the heat sink within the solar shield are investigated. The second main objective of this thesis is to segregate the components within a remote radio head across two heat sinks to improve the heat dissipation of the temperature sensitive components. The primary heat sink, representative of the temperature sensitive components, is experimentally and numerically investigated with and without a shield and without wind or solar loading. A secondary heat sink, representative of high power devices, is added to the shielded configuration in order to create a segregated structure. The secondary heat sink is investigated to determine the impact of the location and the number of fins to maximise the heat transfer on the primary heat sink.
The deployment of a solar shield which is longer than the device that it contains was found to increase heat dissipation through a ‘chimney flow’ effect and it was determined that the heat sink should be located at the base of the solar shield. For a segregated structure, a secondary heat sink with a base plate temperature of 65°C above ambient was found to increase the Nusselt number of the primary heat sink by over 50 % in comparison to an unshielded case. The optimization of the number of fins on the secondary heat sink was found to offer an enhancement of 80 % over an unshielded heat sink.
The findings of this thesis are of practical relevance for the thermal design of outdoor communications equipment: in particular, it is evident that the use of standard environmental conditions is conservative in comparison with a more comprehensive design process which references representative worst case data
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
On the development of a contactless thermal characterisation technique for micro-scale thermoelectric modules (μTEMs)
No full textContemporary internet consumer usage, in the form of social media and wide scale video
streaming, has induced an exponential rise in the demand for high speed data. Optical
communications infrastructure has had to evolve at a rapid pace to meet the appetite for data.
Photonic Integrated Circuits (PICs) are critical components of optical communications
equipment that transmit and receive coded light signals of specified wavelengths to transfer
high volumes of data over optical fibres. Wavelength is sensitive to thermal fluctuations,
however, with variations of as low as ± 0.1°C shifting the wavelength of the encoded signal
outside design specifications. Conventional macro-scale thermoelectric modules (TEMs) are
currently employed to maintain tight thermal control of PICs, but shrinking device footprints
and the resultant higher heat fluxes are driving the need for smaller, micro-scale TEMs.
Determining the thermal characteristics (temperature difference across the TEM, ΔT, and heat
pumped by the TEM, Qc, for a range of electrical currents through the TEM, I) of these micro
TEMs (μTEMs) has emerged as a challenge, however, primarily due to their poor compressive
strength (~200 MPa). Conventional characterisation techniques typically use a heat stack
configuration, which involves compressing the TEM between a controlled heat source and sink
combination to measure ΔT and Qc with minimum losses at the thermal interfaces.
The objective of this thesis is to design, commission and demonstrate a novel contactless
apparatus to thermally characterise a μTEM (ΔT ~ 20 K, Qc ~ 0.3 W, I ~ 1 A) in a compressionfree
fashion. Compressive forces on the upper surface of the thermoelectric devices were
obviated by using an infra-red (IR) source to apply a heat load to the upper surface of the
TEMs, and an IR sensor was used to measure the upper surface temperature. A calorimeter was
used to control the temperature of the lower surface of the TEMs as a constant reference and to
determine Qc. Measures were implemented to minimise errors due to reflected radiation within
the setup, and an extensive calibration was undertaken on all measurements to minimise
uncertainty. The contactless apparatus was benchmarked against a high precision conventional
compression apparatus using a macro scale TEM (15 mm x 15 mm x 3 mm) in order to validate
the methodology. Then, an array of twelve μTEMs on an aluminium nitride substrate (15 mm x
15 mm x 0.6 mm) was manufactured (4 x 3) in order to produce sufficient heat flow for
accurate measurement. The array was characterised in both apparatuses, allowing the thermal
characteristics of a single μTEM (0.83 mm x 2.14 mm 1.05 mm) to be isolated from the
performance data for the array.
The contactless characterisation technique produced values for Qc within 15 – 357 mW (1 –
25.5%) and values for ΔT within 0.4 – 6.2 K (0.5 – 7.6%) of the conventional characterisation
apparatus for the macro-scale TEM. The characteristics extracted for a single μTEM measured
within 15 – 100 mW (2.5 – 15%) of the conventional characterisation apparatus for Qc, while
the values for ΔT were within 0.5 – 1.9 K (1.5 – 6.5%). The thermoelectric figure of merit ZT
(0.292), calculated from the Seebeck coefficient (0.0147 V/K), conductance (0.07782 W/K) and
resistance (2.83 W) of the μTEM, was within ± 5.2% of the conventional compression method
(0.308). It was concluded that the novel contactless characterisation method developed in this
thesis could be used to accurately characterise the thermal performance of micro-scale
thermoelectric devices in a compression-free manner
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
The thermal and hydrodynamic behaviour of submerged and confined, normally-impinging, laminar slot jets
No full textOptical networks are a critical element of contemporary communications infrastructure, due to their efficacy in transmitting high-speed data over large distances.
Photonic integrated circuits (PICs) offer compelling advantages in terms of performance and miniaturization, but increasing the power density of these components,
coupled with shrinking packaging footprint, presents a significant thermal management
challenge. This has driven the need for the integration of liquid-based micro
fluidic
cooling artefacts into next generation PIC packages. Liquid microjets are emerging
as a candidate primary or secondary heat exchanger for such packages, however, the
thermal-hydraulic behaviour of confined, low Reynolds number liquid slot jets is not
comprehensively understood. The objective of this thesis is to characterise the inf
uence of slot jet geometry modi cations as a technique for the passive control and
enhancement of single-phase convective heat transfer.
The slot jets investigated featured five different nozzle aspect ratios (L/W), and five
different passive outlet structures in the form of tabs and chevrons. The investigation
was carried out for slot jets in the laminar
ow regime, over a Reynolds number range
of 100 ≤ ReDh ≤ 500 { and with a xed con nement height to hydraulic diameter
ratio (H=Dh) of 1. Particle - Image Velocimetry (PIV) and an iso
ux foil technique
were utilised to identify the
uidic mechanisms associated with each geometry and to
understand their corresponding in
uence on the spatial temperature distributions along
the heated surface. The hydrodynamic penalty of any enhancements in heat transfer
achieved were then determined through the measurement of the pressure drop across
each nozzle geometry.
It was found that increasing the jet nozzle aspect ratio from 1 to 8 resulted in
enhancements in area-averaged Nusselt number (NuAvg) of up to 68%, and a corre-
sponding decrease in head loss coe cient (K) of 12%. Within the stagnation zone,
correlations extrapolated from the experimental results showed that the stagnation
point Nusselt number (Nu0) had a very weak dependency on the jet nozzle aspect
ratio, but scaled with Re0:55
Dh . This Reynolds number scaling was indicative of the potential core of the jet striking the impingement surface. O -center peaks observed in
the velocity
ow elds of the impinging jets were postulated to be as a result of the
stagnation zone
uid dynamics and local
ow entrainment. When compared to the
baseline case, all outlet tab geometries resulted in increased local and area-averaged
heat transfer and, for the same pumping power, enhancements in NuAvg of up to 29%
were achieved through the application of the major triangle outlet tab geometry. It
was also determined that the geometry and location of the outlet tabs were found to
in
uence the local heat transfer coe cients within both the stagnation and wall jet
zones. It was concluded that the passive control and enhancement of an integrated
microjet cooling solution could be achieved through geometry modification { without
compromising the stringent design constraints of an integrated microf
uidic package,
such as confinement height,
flow rate, and the required pumping power
On the influence of ag content on the creep behaviour of sn-ag-cu solder alloys
No full textThis thesis presents an investigation of the influence of silver content on
the creep behaviour of ternary tin-silver-copper (SAC) solder alloys. To this
end, the creep behaviour of Sn98.5Ag1.0Cu0.5 (SAC105) solder alloy was
characterised and compared to that of Sn96.5Ag3.0Cu0.5 (SAC305). Both
solder alloys were tested at the joint-scale (~180 μm) using specially designed
specimens on a shear testing facility developed at the Stokes Institute.
With the use of new lead-free solder alloys in electronic manufacturing
processes, there are two critical considerations: reliability and cost. The
reliability of electronic assemblies is linked to the mechanical performance of
solder joints. SAC105 is more cost effective than SAC305 due to its lower
silver content than SAC305, but it is known to exhibit inferior reliability to
SAC305 under thermal fatigue.
An Anand viscoplastic constitutive model is developed in this thesis for
thermally preconditioned SAC105 from a series of creep tests under varying
constant shear stresses (5MPa to 15MPa), constant shear strain rates (1E-6
(1/s) to 1E-2 (1/s)) and temperatures (20ºC to 100ºC), in order to increase the
understanding of the creep behaviour of the alloy. An extensive comparison is
made of the creep characteristics of SAC105 solder alloy to that of SAC305.
The microstructure of both alloys is found to be a beta-Sn matrix with
precipitates of Ag3Sn and Cu6Sn5; SAC305 is noted to feature more Ag3Sn
precipitates and smaller Sn dendrites than SAC105, which are the primary
reasons for its superior creep resistance.
In terms of practical usage, SAC105 is found to be preferable for
applications which may experience high strain rate stimuli (portable electronic
devices) as the alloy undergoes plastic flow at lower stress levels than
SAC305, minimizing the risk of interfacial failures. SAC305 is superior for
applications which feature thermo-mechanical fatigue (desktop computers, for
example, or servers) as it accumulates less strain than SAC105.
SAC105 is found to be significantly less creep resistant than SAC305. With
the Anand model parameters developed here, however, practitioners using
finite element modelling can determine the suitability of the lower cost
SAC105 alloy for applications which induce thermal fatigue
On the thermal and hydrodynamic characteristics of liquid-liquid Taylor flows
No full textTwo phase liquid-liquid flows offer significant heat and mass transfer enhancements over
single phase flows and, as a result, have found use in numerous emerging technologies
employing microfluidics. Such technologies include lab-on-chip devices for chemical and
biological diagnostics, and biosensors. Liquid-liquid flows have also shown potential for use
in high-heat flux removal systems. Although these flows are found in numerous applications,
little is known about the complex fluid mechanics that govern them. Consequently, there is a
need for a greater knowledge base to serve as a foundation for future system design and
characterisation. This thesis presents a fundamental investigation of the hydrodynamic and
thermal characteristics of liquid-liquid slug or Taylor flows confined to minichannel
geometries.
There were three principal aspects to this thesis, which encompassed the measurement of film
thickness, pressure drop and heat transfer in liquid-liquid Taylor flows. Experiments were
carried out using a number of different carrier fluids – while maintaining water as the
dispersed phase throughout. Dimensionless slug length, Capillary and Reynolds numbers were
varied over several orders of magnitude.
High speed imaging was used in conjunction with microscopy to measure the mean slug
velocity and liquid film thickness. Images of the dispersed slugs revealed that the thickness of
the liquid film was not constant along the length of the slug. However, above a threshold
dispersed slug length, a region of constant film thickness existed. The thickness of the film
was found to be heavily dependent on the Capillary number. Analysis of the experimental data
revealed that it fell into two distinct flow regimes: a visco-capillary regime and a visco-inertial
regime. A modified Taylor’s Law is proposed for flows in the visco-capillary regime, while a
novel correlation – based on the Capillary and Weber numbers – is put forward for flows in
the visco-inertial regime.
The pressure drop induced by the liquid-liquid flow regimes was measured using a differential
pressure transducer, and the results were compared to the most referenced correlations in the
literature. Comparisons highlighted a lack of robustness in the liquid-liquid pressure drop
correlations. Interpretation of the data using liquid-gas Taylor flow correlations unearthed a
threshold viscosity ratio, above which liquid-gas correlations may be used to model the flow.
Below this threshold, a modification to an existing correlation is proposed, where the
interfacial pressure drop is normalised by the volumetric channel fraction occupied by the
carrier phase.
A heat transfer facility was designed and commissioned to subject the flow to a constant wall
heat flux boundary condition. Local temperature measurements were acquired using a high
resolution infrared thermography system. Slug length and film thickness were found to have a
significant effect on the local heat transfer rates, with enhancements up to 600% over
conventional Poiseuille flow noted. Nusselt number oscillations were observed in the lower
Capillary number flows. However, these oscillations damped out as the Capillary number, and
hence film thickness, increased. Based on the characteristics identified, a novel correlation is
proposed to model the flow in the thermal entrance and fully developed regions.
The findings of this thesis are of fundamental and practical relevance for the design of systems
and devices incorporating liquid-liquid Taylor flow regimes
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