Faculty of Engineering, Mechanical EngineeringTheses / Dissertations submitted to Department of Mechanical Engineeringhttp://dl.lib.uom.lk/handle/123/692024-03-28T09:31:28Z2024-03-28T09:31:28ZA Molecular dynamics study on viscous and thermal properties of NanofluidsSomarathna CNhttp://dl.lib.uom.lk/handle/123/216812023-11-22T03:34:55Z2023-01-01T00:00:00ZA Molecular dynamics study on viscous and thermal properties of Nanofluids
Somarathna CN
The aim of this study is to understand the microscopic behavior of heat and momentum
transfer in nanofluids. With nanofluids reporting enhanced thermal conductivities (𝜅)
and viscosities (𝜂), a microscopic understanding is essential for engineering nanofluids
to be practical in heat transfer applications. Therefore, to study the microscopic
transport behavior, copper-argon nanofluids simulated by classical molecular
dynamics are employed. The Applicability of the Green-Kubo (GK) method in
nanofluid 𝜅 evaluation is questioned as the calculated thermal conductivities through
the GK method are considerably higher than the direct method in Non-EquilibriumMolecular-Dynamics
(NEMD). Green-Kubo calculations are found to be very
sensitive to the ill-defined partial enthalpy computation, resulting in an overestimation
of the 𝜅. However, the Green-Kubo and the direct method viscosity calculations
demonstrate a reasonable agreement.
Following the more reliable method, the NEMD direct approach, 𝜅 of the nanofluids
consisting of spherical nanoparticles with different diameters are investigated. The
computational results are compared with the classical effective medium theories and
no anomalous 𝜅 enhancements are observed in the nanofluids having fully dispersed
spherical particles. Various microscopic mechanisms such as liquid layering and
micro-convection are found to be ineffective for 𝜅 enhancements in nanofluids.
However, greatly enhanced 𝜅 are achieved, a maximum of 63% relative to pure argon,
in nanofluids consisting of chain-like particle arrangements. This demonstrates the
potential origin of anomalous 𝜅 enhancements in experimental measurements and the
capability of nanofluids with extended nanostructures to deliver better 𝜅
enhancements.
Further investigating the capability of extended nanostructures in nanofluid thermal
transport, 𝜅 enhancements of nanofluids consisting of nanowires with different lengths
and diameters are evaluated. It is shown that the heat conduction in the parallelly
arranged liquid and the nanowires exhibit a coupled thermal behavior owing to the
interface thermal resistance (R
b
). This contradicts with the predictions of the classical parallel heat conduction model and therefore, a novel model is proposed taking this
coupled behavior into account. New heat transfer characteristics at the nanoscale are
identified including the R
b
-driven coupled heat conduction, the reduced 𝜅 of suspended
nanowires, and the solid-like liquid layering. Using the new model, the importance of
these microscopic thermal characteristics in accurately predicting the effective 𝜅 is
shown. The sole contribution from the solid-like liquid layer to the 𝜅 enhancement is
found to be in between 20-30% for the nanofluids considered.
Extending the investigation of heat transfer phenomena in nanofluids based on
spherical nanoparticles, 𝜂 of nanofluids with different nanoparticle sizes,
concentrations, and arrangements are evaluated. Both the Green-Kubo and the direct
methods are employed and unlike the 𝜅, both methods show a reasonable agreement
with one another. Viscosity is observed to decrease as the particle diameter increases
in fully dispersed nanofluids. The ratio 𝐶
⁄
shows a decreasing trend indicating
better heat transfer performance in nanofluids with large particles. Nanofluid 𝜂 is
𝜂
𝐶
𝜅
observed to increase rapidly as the concentration increase. This makes 𝐶
⁄
to
increase as well indicating the diminished heat transfer performance in nanofluids with
high particle concentrations. As the particles in the nanofluid arrange into chain-like
structures, 𝜂 remains unaffected. This makes 𝐶
⁄
to decrease rapidly indicating the
greater heat transfer performance in nanofluids with chain-like nanoparticle
arrangements or in general, extended nanostructures.
2023-01-01T00:00:00ZDevelopment of a soft linear actuator to use in wearable assistive exosuitsKulasekera ALhttp://dl.lib.uom.lk/handle/123/221052024-01-26T22:13:40Z2023-01-01T00:00:00ZDevelopment of a soft linear actuator to use in wearable assistive exosuits
Kulasekera AL
Wearable exosuits require flexible, linearly contractile, and lightweight actuators to
provide sufficient force to move the respective limb. This thesis presents the concept,
design, fabrication, experimental performance characterization, and numerical modeling
of two types of respectively thin and low-profile vacuum-driven, soft, linearly
contractile actuators. The proposed soft actuators are made of an inextensible yet flexible
thin-skinned pouch supported by a collapsible skeleton that orients the collapse of
the actuator in the longitudinal axis upon the evacuation of the air within the pouch.
The proposed novel soft, lightweight, contractile actuators are thin (ThinVAc) and lowprofile
(LPVAc). Both these actuators are lightweight (ThinVAc: 0.75 g; LPVAc: 14
g), provide high maximum blocked forces (ThinVAc: 5.2 N; LPVAc: 39 N), provide
maximum stresses similar to that expected from biological muscles (ThinVAc: 184
kPa; LPVAc: 117 kPa) and have high force-to-weight ratios (ThinVAc: 477; LPVAc:
285). The ThinVAc can combine to create multifilament actuators for force scaling.
Combining 15 units of 500 mm ThinVAcs generates a maximum blocked force of 54
N (Max. stress: 62 kPa), 290 times the self-weight. The LPVAc integrates a position
sensor based on an inductive sensor allowing closed-loop control with minimal error at
0.25 Hz. Numerical models for the contraction and blocked force of mono- and multifilament
actuators allow for predicting their behavior independent of external sensors.
The proposed actuators are tested in wearable applications to check their suitability.
The ThinVAc is integrated into a knee rehabilitation assist device, and the LPVAc is
incorporated into a novel mono-articular sit-to-stand transition (StSt) assist exosuit,
helping to reduce muscle activity by 45%. These actuators have the potential to be
integrated into a wide range of assistive devices and orthoses, such as knee or ankle
braces, exoskeletons, and prosthetics, to provide the necessary support for people with
mobility impairments.
2023-01-01T00:00:00ZDeveloping a policy framework for the design and manufacture of environmentally sustainable food packagingJagoda SUMhttp://dl.lib.uom.lk/handle/123/222082024-02-08T22:07:23Z2023-01-01T00:00:00ZDeveloping a policy framework for the design and manufacture of environmentally sustainable food packaging
Jagoda SUM
Food packaging plays a vital role in human existence by eliminating barriers to satisfying food
requirements regardless of geography. Since food is a basic requirement of all human beings, a
billion-dollar industry has been developed surrounding the food supply chain. Providing
protection against chemical (oxygen, moisture, carbon dioxide, etc.), physical (vibration and
shock), and biological (insects, microorganisms) agents while facilitating the handling of food
items in bulk or in appropriate portions for easy and efficient logistics are the primary
requirements of food packaging. Concurrently, the adverse consequences of food packaging have
caused higher resource consumption and waste generation. Food packaging contributes to
significant plastic waste accumulated in landfills, open environments and oceans. Due to the high
contribution to single-use plastic waste, organisations and nations are taking several actions to
minimise the environmental burden caused by food packaging. However, limited efforts are
being made to introduce systematic frameworks that could help packaging designers and policy
developers to design and manufacture food packaging. This research focuses on proposing a
policy framework for designing and manufacturing food packaging that oversees the triple
bottom line of sustainability; environmental, economic, and social. The initial phase of the study
identifies the considerations in the development of food packaging concerning rigid packaging
and how sustainability could be numerically represented as a decision support tool. Multiple
methods, such as public surveys, questionnaires, and focus group interviews, were used for data
collection. Then, Quality Function Deployment (QFD) was used to analyse the data to identify
the prioritised set of design considerations. Based on the analysis, different designs were
developed and evaluated to identify the product characteristics that would influence the
sustainability of food packaging. The next phase focused on developing a policy framework
using the results of the from the analysis of the case studies. The design science research (DSR)
method was used to develop the framework combining different food packaging aspects and
graphically representing them in a diagram. The main outcome of this research is the policy
framework for designing and manufacturing food packaging that integrates the three main
aspects of food packaging. The proposed framework was modified and validated with expert
insight, adding credibility to the research outcome.
2023-01-01T00:00:00ZEvaluation of hourly solar radiation models to estimate radiation on inclined surfaces in day zone of Sri LankaAbeyrarthna, ARMUEhttp://dl.lib.uom.lk/handle/123/183172022-08-24T05:47:16Z2018-02-20T00:00:00ZEvaluation of hourly solar radiation models to estimate radiation on inclined surfaces in day zone of Sri Lanka
Abeyrarthna, ARMUE
An analysis of global, beam and diffuse solar radiation on horizontal and 7° tilt about east west axis and facing due south orientation at Hambanthota was carried out to assess the solar resource potential in dry zone of Sri Lanka. The calculated monthly averaged daily insolation for dry zone was found to be varying between 16.30 MJ/m2/day to 22.75 MJ/m2/day with the annually averaged daily insolation of 20.07 MJ/m2/day. Calculated annually averaged beam horizontal radiation was 10.87 MJ/m2/day and diffuse horizontal radiation was found to be 9.19 MJ/m2/day while 0.56 was the annual average clearness index indicating that partly cloudy sky throughout the year. Horizon brightness coefficients of Perez et al (1990) was modified using diffuse radiation data of Hambanthota. Modified model was used for the estimation of titled radiation on due south faced surfaces. Diffuse tilted daily insolation and global tilted insolation for -45° to +45° inclined surfaces with 1° increments was estimated and monthly and annual optimum tilt angles were derived. The calculated monthly optimum tilt angle varied between -26° to +27° while having annual optimum tilt angle of -2°. Hence, tilting towards due south by same angle as latitude is not the recommended optimum tilt for fixed axis systems. Optimum tilt angle for beam radiation was derived and it was found that annual optimum tilt angle for beam radiation is 6° facing towards the due south. The derived maximum solar resource potential was 2068 kWh/m2 per annum for fixed system at -2° tilt angle and 2169 kWh/m2 per annum for monthly tracking system which is 5% higher than the horizontal potential. It is proposed to assess the solar resource potential for tilted surfaces with different surface azimuth angles by using modified Perez et al (1990) model in future. It is also possible to modify the coefficients of circumsolar brightness components of Perez et al (1990) model for better results.
2018-02-20T00:00:00Z