Dissertations and Theses - Mechanical Science and Engineering
http://hdl.handle.net/2142/14787
Thu, 23 Nov 2017 14:40:09 GMT2017-11-23T14:40:09ZInstability of flow boiling in parallel microchannels
http://hdl.handle.net/2142/97649
Instability of flow boiling in parallel microchannels
Khovalyg, Dolaana
Heat exchangers are key components of energy conversion systems, including HVAC&R equipment. The compact microchannel design of a heat exchanger (MCHX) allows for significant reduction of its volume, weight, and raw material, in comparison to conventional fin-and-tube heat exchangers in HVAC&R systems. The current generation of microchannel evaporators along with advantages, such as high heat transfer rate and reduced refrigerant charge, encounters important problems related to flow maldistribution and instabilities in parallel-channels flow. The thermal and hydrodynamic performance of a microchannel evaporator can drastically decrease, due to the presence of instabilities, and maintaining a set point for operational parameters can become challenging.
Microchannels are characterized by a large ratio of surface area to fluid volume, and rapid growth of bubbles in a confined space causes flow parameter oscillations. Fluctuations in pressure, pressure drop, temperature and mass flux can be triggered in the individual channels (ports) and they can affect neighboring channels. For instance, flow oscillations in parallel channels can lead to premature initiation of dryout that reduces the overall heat transfer. This research is motivated by the challenge of predicting flow boiling instabilities in parallel channels, since understanding the nature of these instabilities and their relationship to the operational parameters can be advantageous for the engineering community. Analysis of the pressure drop behavior in parallel non-uniformly heated microchannels is chosen as the primary method to explore instabilities.
The possible nonuniformity of heat flux from channel to channel was studied by solving the conjugate, three-dimensional, transient heat transfer problem of louvered fins bounded with multiport aluminum plates using commercial software (ANSYS FLUENT). While the fin geometry was kept constant in all simulations, two different multiport plate configurations (11 round ports, D=1.2 mm; and 22 square ports, 0.54x0.54 mm) were analyzed at two air face velocities, 1m/s (Re_Lp = 82) and 5m/s (Re_Lp = 410), and two temperature differences, 10K and 20K, between the incoming air and the inside walls of the channels that have constant temperature of 10oC. Air flow was louver directed in both cases, while the large scale vortex shedding from the plate, in addition to the unstable wake of the exit louver, was observed at Re_Lp = 410. The magnitude of the heat flux difference between ΔT=10K and ΔT=20K cases was two times. The results show that the first channel, facing the flow, has the highest heat flux in all cases. The variation of the channel-to-channel heat flux downstream from the leading edge was dependent on the incoming flow velocity and air flow morphology. The overall heat flux difference between the leading channel and the trailing one was 73% at the incoming air velocity of 5 m/s, while this difference was almost 96% at lower velocity of 1 m/s. It might be concluded that a higher air velocity (mass flow rate) corresponds to a lower temperature drop for the air stream, and less variation in the temperature driving potential (port to port) causes less heat flux variation. Overall, the results of numerical simulations prove the presence of heat flux variation between neighboring channels; therefore, the effects of channel-to-channel heat flux variations on flow maldistribution and flow boiling instabilities between neighboring microchannels were considered.
The region of significant flow boiling instabilities in multiple, nonuniformly heated channels bounded by constant pressure drop is predicted by modeling the pressure drop behavior in each individual channel using the internal characteristic or ΔPi-Gi curve. Combination of parallel channels ΔPi-Gi curves and definition of possible flow rate solutions at a given constant pressure drop across all channels can be used to demarcate regions of possible instabilities. In order to accomplish this, theoretical modeling of a single channel ΔP-G curve is undertaken in this research. Two-phase pressure drop was modeled based on semi-empirical correlations of the frictional two-phase pressure drop by Kim & Mudawar (2014), and the void fraction model by Xu & Fang (2014). A single channel characteristic curve model was experimentally validated for two channel sizes 2 mm and 1 mm using refrigerant R245fa at T_sat=24.5oC. The theoretical model consistently predicted the trends in the data very well, and it predicted pressure drop within 19.3% for the 2 mm tube and within 32.5% for the 1 mm tube. Furthermore, the effect of fluid properties, operational parameters, and geometrical parameters of a channel on a single channel ΔP-G curve behavior is theoretically analyzed. The span of the negative slope region (where instability is manifested) depends on with saturation conditions, inlet subcooling, heat flux, channel size and length, and fluid type. The negative slope region decreases with decreasing heat flux, liquid and vapor densities ratio, and as channel becomes shorter and smaller due to the reduced vapor generation. The negative slope region also decreases with increasing saturation pressure, specific heat and degree of subcooling.
Multiple channel instabilities are analyzed by combining individual ΔPi-Gi curves of 2-6 unevenly heated microchannels and seeking flow rate solutions at a given constant pressure drop across multiple channels. Theoretical results show that for a given total flow rate the flow may split among parallel pipes in various ways satisfying the equal pressure drop condition in all channels; there exist a range of the incoming flow rates where maldistribution is the only possible solution. Furthermore, linear stability analysis was performed to differentiate between stable and unstable solutions. The analysis enabled the demarcation of unstable regions on the total ΔP-G curve. Therefore, it is possible to anticipate unstable regions if the inlet flow rate, number of channels, and operational parameters are known.
In conclusion, this research is focused on the study of flow boiling in parallel microchannels subjected to uneven heat flux. Understanding the single channel pressure drop versus flow rate (ΔP-G) characteristic curve, and understanding the interactions between channels leads to the development of a map that demarks unstable regions. This map can provide guidance to engineers in choosing operational conditions and developing compact evaporators. Therefore, the results of this work have significant impact on understanding flow boiling behavior in multiple microchannels that could lead to practical applications.
Microchannel; Instabilities; Two-phase flow; Boiling; Non-uniform heat flux; Pressure drop; Louvered fins; Stability analysis; Characteristic curve
Fri, 06 Jan 2017 00:00:00 GMThttp://hdl.handle.net/2142/976492017-01-06T00:00:00ZKhovalyg, DolaanaAvalanches, percolation, and stochastic damage evolution in disordered media
http://hdl.handle.net/2142/97647
Avalanches, percolation, and stochastic damage evolution in disordered media
Kale, Sohan Sudhir
Disordered materials are ubiquitous in nature and engineering. The physical properties of such random heterogeneous materials are strongly coupled with the strength and spatial distribution of the disorder. Disorder leads to fluctuations in material properties which become relevant for mesoscales below the representative volume element (RVE) size. The overarching theme of this dissertation is modeling apparent properties of disordered systems on sub-RVE lengthscales using numerical techniques that are based on an explicit representation of the material microstructure.
The classical picture of elastic-plastic transition as a smooth process is challenged by the compression experiments on micro/nano pillars where plastic strain was observed to accumulate intermittently. Using a discrete spring lattice modeling approach, the intermittent plastic strain avalanche behavior is well captured such that the event sizes follow a power-law distribution with an exponent that is in agreement with experiments, theory, and other models. Then using finite-size scaling analysis, the elastic-plastic transition is shown to belong to the long-range correlated percolation class, a second-order phase transition. Interestingly, this behavior is in contrast to the elastic-brittle transition in disordered media which is abrupt, akin to a first-order phase transition.
The elastic-brittle transition in disordered media is characterized by foreshadowing of the final macroscopic failure by accumulation of significant amount of distributed damage which results in precursory events observed as avalanches in experiments and simulations. We use a jump Markov process to model the stochastic evolution of damage. The Markov process is informed by the avalanche size distribution for a given quasibrittle material. The fiber bundle model (FBM) is used as an example to test the viability of the proposed approach. The stochasticity and size-dependence of the damage evolution process are inherently captured through the inputs provided for the jump Markov process. The avalanche and failure strength distributions are used to describe the effect of microscopic information present in the form of the disorder on the macroscopic damage evolution behavior.
We also investigated the effective thermal conductivity of spatially correlated two-phase microstructures. The presence of such spatial correlations is observed in interpenetrating phase composites (IPCs) where either phase is interconnected throughout the microstructure. A Gaussian correlation function based method is employed to generate numerical microstructures that are statistically similar to the experimentally captured micrographs. Scale-dependent bounds on the effective thermal conductivity are then obtained using the Hill-Mandel macrohomogeneity condition. A scaling function is formulated to describe the transition from statistical volume element (SVE) to representative volume element (RVE), as a function of the mesoscale, the spatial correlation length of the microstructure, the volume fraction, and the contrast between the phases. A material scaling diagram is also constructed which allows estimation of the RVE size, to within a chosen accuracy, of a given microstructure with short-range spatial correlations.
Conductive polymer nanocomposites have emerged as an important class of (disordered) materials with a wide range of conductive, semiconductive, and static dissipative applications. Dramatic improvement in the electrical conductivity can be obtained by adding marginal amounts of nanofillers such as carbon nanotubes (CNTs), graphene nanoplatelets (GNPs), and carbon black (CB). This phenomenon is induced by the formation of a percolating network of nanofillers interconnected electrically by the quantum tunneling effect. A continuum percolation model along with the critical path approximation is used to investigate the effect of various filler attributes such as filler size polydispersity and alignment on the effective electrical behavior of the nanocomposite. The model proves to be an effective tool to understand the limitations of theoretical models and analyze experimental data to extract key parameters which would improve the predictive capability of this approach.
Disordered media; Stochastic damage evolution; Plastic strain avalanches; Nanocomposites; Homogenization
Tue, 13 Dec 2016 00:00:00 GMThttp://hdl.handle.net/2142/976472016-12-13T00:00:00ZKale, Sohan SudhirDimensionally reduced modeling and gradient-based design of microchannel cooling networks
http://hdl.handle.net/2142/97540
Dimensionally reduced modeling and gradient-based design of microchannel cooling networks
Tan, Marcus Hwai Yik
Microvascular composites constitute a novel class of biomemetic materials with the ability to perform multiple functions such as dynamic tuning of electromagnetic properties, self-healing and thermal management depending on the fluid circulated in the embedded microchannels. Recent breakthroughs in the vaporization of sacrificial component (VaSC) manufacturing technique have allowed for the creation of intricate microchannel networks and large scale production of these composites. As the design of these networks is key to the performance of the composites and designer's intuition is insufficient to achieve optimal performance, the development of ``automated" design tools is of paramount importance.
The primary goal of this work is to fulfill that need in the specific application of thermal management. To that end, we develop three ingredients: dimensionally reduced thermal and hydraulic models, a numerical solver and a shape optimization scheme. Another goal of this project is to verify and validate the dimensionally reduced models against a commercial computational fluid dynamics software package and experiments. The final goal is to apply the design tool to various 2D and 3D problems.
In the dimensionally reduced thermal model, the microchannels are collapsed into lines/curves to simplify mesh generation and their thermal impacts are added to the heat equation. Two versions of the thermal model are considered: (i) a linear model that does not involve radiative heat exchange or linearizes the Stefan-Boltzman radiation equation and (ii) a nonlinear model that incorporates the original radiation equation. The hydraulic model uses the Hagen-Poiseuille law to describe the flow rates and pressure drops in the microchannel networks.
To capture the gradient discontinuity in the temperature field due to the microchannels, we employ the interface-enriched generalized finite element method (IGFEM) as the numerical solver, which greatly simplifies mesh generation by allowing for the use of meshes that do not conform to the microchannel network. While previous IGFEM works are based on polynomial enrichment functions, we demonstrate the flexibility of the IGFEM by developing non-uniform rational B-splines (NURBS) enrichment functions for branched network of curved microchannels.
We then develop a method to address the convergence issue due to the singularity associated with the thermal model in 3D and combine that method with polynomial IGFEM. The thermal fields obtained from the resulting modified IGFEM agree with those of the significantly more complex and costly ANSYS FLUENT conjugate heat transfer simulations.
The final ingredient involves the development of analytical IGFEM-based shape sensitivity analyses for both linear and nonlinear models. These analyses allow the design tool to efficiently exploit existing powerful gradient-based optimization algorithms, especially for large number of design parameters.
We then apply the gradient-based shape optimization scheme to solve a diverse range of problems, which demonstrate two key advantages of the scheme due to the use of stationary non-conforming meshes by (i) eliminating the cumulative mesh generation cost and (ii) avoiding severe mesh distortion issues as the microchannel geometry evolves during the optimization process. The first problem involves parallel networks of microchannels for 2D microvascular composite battery cooling panels. Using a differentiable alternative to the maximum temperature (the $p$-norm of the temperature field) of a cooling panel as an objective function, we obtain optimized designs superior to the reference designs in terms of cooling performance. We also extensively validate the IGFEM solutions associated with the designs against ANSYS FLUENT simulations and experiments.
We further extend the uses of the tool to include multi-objective optimization, pressure drop as objective function, channel diameters as design parameters and localized heat sources. In the multi-objective optimization, the Pareto fronts of the maximum temperatures and the pressure drops across the networks are generated using the normalized normal constraint method.
Next, we apply the optimization scheme to design blockage-tolerant cooling networks embedded in 2D PDMS panels. In this novel application, a minmax problem that minimizes the worst case of a set of predetermined blockage scenarios is formulated and converted to a simpler single-objective optimization problem. In the worst blockage scenario, the designs optimized in this manner exhibit substantial reduction of cooling performance loss compared with designs optimized without considering blockages, with greater reduction as the redundancy of the network decreases. The designs are also validated against experiments.
Another novel application of the optimization scheme is related to the design of 2D microvascular panels for nanosatellite. In this application, the sensitivity analysis based on the nonlinear thermal model is used since the nonlinear effect of radiation cannot be neglected. Taking advantage of the optimization tool, two formulations are proposed to satisfy the design constraints. We perform extensive benchmarking of the results obtained from the dimensionally reduced models against those from ANSYS FLUENT, and provide analytical estimates of the thermal performance of optimized designs.
In the final application, we design multiple parallel microchannels embedded in 3D microvascular panels using the modified-IGFEM-based optimization scheme. Due to the importance of the straight microchannel design, we propose a semi-analytical model of the maximum temperature in a panel with multiple straight channels.
Microvascular composite; Dimensionally reduced model; Thermal model; Hydraulic model; Microchannel; Interface-enriched generalized finite element; Gradient-based; Optimization; Multi-objective; Pareto front; Blockage; Battery cooling; Nanosatellite radiator panel
Mon, 20 Mar 2017 00:00:00 GMThttp://hdl.handle.net/2142/975402017-03-20T00:00:00ZTan, Marcus Hwai YikPatient-specific technology for in vivo assessment of 3-D spinal motion
http://hdl.handle.net/2142/97527
Patient-specific technology for in vivo assessment of 3-D spinal motion
Goodsitt, Jeremy Edward
One of the most common musculoskeletal problems affecting people is neck and low back pain. Traditional clinical diagnostic techniques such as fluoroscopic imaging or CT scans are limited due to their static and/or planar measurements which may not be able to capture all neurological pathologies. More advanced diagnostics have proven successful in assessing 3-D patient-specific spinal kinematics by combining a patient-specific 3-D spine model (CT or MRI) with bi-planar fluoroscopic imaging; however, custom, not clinically available advanced imaging equipment as well as an increase in radiation exposure is required to acquire a complete patient-specific spinal kinematic description. Hence, the purpose of this research was to develop a clinically viable bi-planar fluoroscopic imaging technique which acquires a complete patient-specific kinematic description of the spine with reduced radiation exposure.
Development of the proposed technique required evaluating the accuracy of 3-D kinematic interpolation techniques in reconstructing spinal kinematic data in order to reduce radiation exposure from bi-planar fluoroscopic diagnostic techniques. Several interpolation and sampling algorithms were evaluated in reconstructing cadaveric lumbar (L2-S1) flexion-extension motion data; ultimately, a new interpolation algorithm was proposed. Similarly, the success of the interpolation algorithm was evaluated in reconstructing spine-specific kinematic parameters.
Next, the interpolation algorithm was combined with a CT-based bi-planar fluoroscopic method. Accuracy of the proposed diagnostic technique was evaluated against previously validated work on an ex vivo optoelectronic 3-D kinematic assessment technique. Bi-planar fluoroscopic images were acquired during both flexion-extension and lateral bending motions of cadaveric cervical (C4-T1) and lumbar (L2-S1) spine. Registration of the bi-planar fluoroscopic images to the CT-based 3-D model was optimized using a gradient derived similarity function. Additionally, a stochastic approach, covariance matrix adaptive evolution strategy, was used as the optimizing function. The newly developed interpolation algorithm was used to reduce the sample size of the bi-planar fluoroscopic images which reduces radiation exposure. Experimental results illustrate the potential success of the technique, but ultimately improvements in registration and validation methods are needed before becoming clinically viable.
Bi-planar registration; Bi-planar; Spine; Kinematics; Interpolation; Joint diagnostics; Biomechanics
Tue, 21 Feb 2017 00:00:00 GMThttp://hdl.handle.net/2142/975272017-02-21T00:00:00ZGoodsitt, Jeremy EdwardNon-lithographic and scalable manufacturing of silicon micro & nanomaterials
http://hdl.handle.net/2142/97526
Non-lithographic and scalable manufacturing of silicon micro & nanomaterials
Azeredo, Bruno Pavanelli
In the last decade, accelerated discovery of novel processing schemes has enabled rapid prototyping and large volume production of parts with extended and programmable functionality - such as biodegradable electronics, high-emissivity surfaces, optoelectronics circuits, fast-discharging batteries, meta-materials, energy conversion systems, and light-weight composites. Novel micro/nanomanufacturing methods - such as 2D and 3D printing, imprinting, self-assembly, deterministic micro-assembly, flexography, and roll-to-roll systems - are among a growing set of new platforms that allow for integration of metals, polymers, oxides, composites, nanomaterials and biomaterials hierarchically across length scales ranging from centimeters down to nanometers. However, nanomanufacturing platforms are still plagued by a trade-off between throughput and resolution, ultimately hindering the scaling of nanomaterial production.
In this context, this thesis introduces two novel processes for manufacturing hybrid and hierarchical silicon nanomaterials: (a) 1D nanostructure fabrication via thin-film dewetting and electrochemical etching, and (b) arbitrary-shaped 3D surfaces via direct electrochemical imprinting. Uniquely, they offer 3D dimensional control with sub-20 nm lateral and vertical resolution, mirror surface finish (i.e. RMS < 5 nm), high-aspect ratio (i.e. >20), low-defect density (i.e. no porous formation), and large-area patterning (>1 cm2). In each of these approaches, a fundamental understanding of ionic diffusion, reaction kinetics, and nucleation dynamics are directly correlated to manufacturing outputs - such as morphology defect density, material removal rate, patterning fidelity and resolution. Three scientific contributions have been made to the existing literature: (a) correlating electrochemical rate of reduction and oxidation to sidewall profiling in catalyst-based etching of silicon, (b) evidencing of diffusion limitations in solid metal catalysts, and (c) measuring the effect of porous substrate and catalysts in controlling the morphology of MACE-fabricated nanostructures. Overall, the techniques developed in this thesis bypass the need for dry etching and lithography, and are potentially compatible with amorphous and polycystalline silicon and III-V semiconductors. In turn, they may pave the way for the manufacturing of complex 3D hybrid objects on semiconductors for use in microoptics and nanophotonics, energy harvesting, and biosensing.
Nanomanufacturing; Semiconductor manufacturing; Metal-assisted chemical etching; Micro-optics; Wet etching; Electrochemical imprinting; Hierarchical assembly; Thin-film dewetting; Self-assembly; Electrochemical methods
Tue, 03 Jan 2017 00:00:00 GMThttp://hdl.handle.net/2142/975262017-01-03T00:00:00ZAzeredo, Bruno PavanelliTransient wave propagation on random fields with fractal and Hurst effects
http://hdl.handle.net/2142/97524
Transient wave propagation on random fields with fractal and Hurst effects
Nishawala, Vinesh Vijay
Due to its significance in natural sciences and engineering fields, wave propagation through random heterogeneous media is a significant area of fundamental and applied research. Recently two models have been developed, Cauchy and Dagum models, that can simulate random fields with fractal and Hurst characteristics. Not only can fractal and Hurst characteristics be captured with these models, but they are decoupled. We evaluate the impact of these random fields on linear and nonlinear wave propagation using cellular automata, a local computational method, and propagation of acceleration waves.
In this study, we evaluate cellular automata's response to a normal, impulse line load on a half-space. We first evaluate the surface response for homogeneous material properties by comparing cellular automata to the theoretical, analytical solution from classical elasticity and experimental results. We also include the response of peridynamics, a non-local continuum mechanics theory which is based on an integro-differential governing equation.
We then introduce disorder to the mass-density. We first evaluate the surface response of cellular automata to uncorrelated mass-density fields, known as white noise. The random fields vary in coarseness as compared to cellular automata's node density. Then, we evaluate the response of cellular automata to Dagum and Cauchy random fields using the Monte Carlo method.
For the propagation of acceleration waves, we apply Dagum and Cauchy random fields to dissipation and elastic non-linearity. We study how the fractal and Hurst characteristics alter the probability of shock formation as well as the distance to form a shock.
Lastly, in our studies of peridynamics, we found that peridynamic problems are typically solved via numerical simulations. Some analytical solutions exist for one-dimensional systems. Here, we propose an alternative method to find analytical solutions by assuming a form for displacement and determine the loading function required to achieve that deformation. Our analytical peridynamic solutions are presented.
Peridynamics; Cellular automata; Wave motion; Computational mechanics; Fractals; Hurst coefficient
Tue, 20 Dec 2016 00:00:00 GMThttp://hdl.handle.net/2142/975242016-12-20T00:00:00ZNishawala, Vinesh VijayBlast diagnostic tools and techniques: a review
http://hdl.handle.net/2142/97456
Blast diagnostic tools and techniques: a review
Desanti, Young Wayne
Pressure measurements are essential in determining the energy output from shock waves generated by high explosives. Thus, it is imperative to choose appropriate sensors and measurement techniques to consistently acquire useful data. Past studies conducted in diagnostics of energetic materials were focused on the energy release and the material properties, but very few, if any, placed an emphasis on the actual diagnostic tools and techniques. There are two main types of pressure transducers utilized in the industry today: piezoresistive and piezoelectric. Piezoresistive sensors experience a change in internal resistance when the sensing material is subjected to mechanical strain, while piezoelectric sensors generate an electric charge when placed under a similar condition. In addition to the two industry standards the Manganin pressure sensor also plays an important role in blast diagnostics. This type of sensor represents a niche part of the pressure transducer market and are primarily used to capture the detonation pressure for high explosives. In this study, appropriate measurement techniques, in addition to the tools utilized, were examined to achieve seamless data collection. Electric noise reduction and data loss prevention techniques were explored in this study. Some of these techniques include: adding feed-through terminator to reduce signal output, creating protective barriers surrounding signal cables, and reducing amplifier-to-gauge cable length. Through preparation and application of appropriate techniques, valuable data can be adequately acquired on a consistent basis with minimal disturbances.
Explosives; Energetic materials; Pressure transducer; Optic diagnostics; Piezoelectric; Piezoresistive; Manganin
Tue, 25 Apr 2017 00:00:00 GMThttp://hdl.handle.net/2142/974562017-04-25T00:00:00ZDesanti, Young WayneDynamic modeling and hardware in the loop testing of chemical processes
http://hdl.handle.net/2142/97451
Dynamic modeling and hardware in the loop testing of chemical processes
Kawamura, Malia L
This thesis presents a framework for hardware-in-the-loop (HIL) testing of chemical plants. HIL testing is a widespread tool used in industry and academia to bridge the gap between computer simulations and physical experimentation. It provides many advantages to the standard development path of building a physical prototype and then running tests. Benefits of HIL testing include decreased development time, reduced cost, increased safety, and better control algorithm development. For this work, HIL testing consists of an emulated real-time chemical plant and a real physical controller.
This HIL testing setup requires two main thrusts – the development of a dynamic model for chemical plants and the implementation of an emulated real-time plant and a real physical controller in hardware. A user-friendly, modular, scalable, dynamic, and nonlinear first principles modeling toolkit is developed in Matlab Simulink. The toolkit contains individual chemical plant components such as a continuous stirred tank reactor (CSTR), pump, and valve. Experimental validation of the chemical concentration models and an example plant model are included. Next, for the hardware and control implementation tasks, National Instrument’s VeriStand software is used to integrate a Simulink model to run in real-time on NI hardware. A myRIO is used as a real physical controller, programmed in LabVIEW, to control the emulated real-time chemical plant running on a CompactRIO. A chemical plant which forms propylene glycol in a CSTR is utilized as a case study. However, the HIL testing framework developed is widely applicable to real physical systems to decrease development time and increase safety.
Hardware-in-the-loop; Dynamic modeling; Controls
Tue, 25 Apr 2017 00:00:00 GMThttp://hdl.handle.net/2142/974512017-04-25T00:00:00ZKawamura, Malia LWettability on nanoparticle modified surface: for thermal engineering
http://hdl.handle.net/2142/97428
Wettability on nanoparticle modified surface: for thermal engineering
Zhang, Feini
In many thermal engineering applications, manipulation of the wetting behavior of liquids is used as a heat transfer enhancement strategy. Hydrophilicity, implying better wettability of liquid on solid surface, is preferred in some processes of air-conditioning and power-generation systems, such as dehumidification, evaporative cooled condensers, and pool boiling at high heat flux. Recent research in nanofluid boiling has revealed a manufacturing technique that is promising for air-conditioning and refrigeration applications. Nanofluid boiling on a solid surface induces deposition of the particles on the boiling surface, and these surfaces exhibit enhanced surface wettability. The properties of the deposited nanoporous layer are affected by the nanofluid boiling parameters such as nanoparticle concentration, nanoparticle type and size, solvent liquid type, boiling surface roughness, heat flux, boiling deposition duration and so on. In this thesis, an investigation of the effect of the nanofluid boiling conditions on the resulting wetting behavior of the treated surface is presented. Understanding how the fabrication process influences the wettability enhancement will guide the design of a surface treatment technique to achieve super-hydrophilicity. Experimental results show that boiling duration positively affects wettability, but little additional enhancement occurs for durations beyond 10 minutes of NBND. Surface wettability change by NBND is independent of boiling heat flux if the particle concentration is 1 wt.%, while at a low nanoparticle concentration of 0.01 wt.%, heat flux has some random influence. The overall systematic trend observed in the experimental study is that, the higher the nanoparticle concentration, the higher the wettability after NBND process, and at the same time the rougher the surface. The goal is to obtain superhydrophilic surface, which is achieved at high particle concentration (1%wt) NBND. Microscopic analysis gives evidence of particle deposition after NBND. Nano-micro structures were studied using SEM. The images show the growth of “nano-grass” like pseudoboehmite on aluminum surfaces after NBND using alumina nanofluids. Surface roughness factor was obtained from AFM scan and contact angle measurements independently, but show good agreement. If the coating was to be applied on fins to enhance their wettability, the height of a droplet on the fin surface would be a parameter that would affect the optimization of fin spacing. The higher the wettability, the lower the height of a droplet of fixed volume, so close fin spacing could be for dehumidification. Also, wetting experiments on rough surfaces with a porous coating by NBND at high nanoparticle concentration reveals the involvement of imbibition effect. This suggests the Hemi-wicking mode of wetting. In this thesis, the solid fraction is determined by contact angle data analysis and confirmed by linear fitting of data. Durability of the treated surface under dry conditions was studied by exposure to air. Air-borne contamination reduces surface wettability, but the NBND treated surface remained more hydrophilic than the untreated surfaces. Eventually, the surface treatment loss its ability to enhance wettability. A possible solution is recommendation as a future work. Overall, this study provides an understanding of wettability changes by nanofluid boiling nanoparticle deposition, and provides a guidance to the wettability treatment for thermal engineering applications.
Surface wettability; Contact angle; Nanofluid boiling; Experiment; Model; Scanning electron microscope (SEM)
Fri, 21 Apr 2017 00:00:00 GMThttp://hdl.handle.net/2142/974282017-04-21T00:00:00ZZhang, FeiniWhen stick fluids do and do not stick: yield-stress fluid drop impacts
http://hdl.handle.net/2142/97429
When stick fluids do and do not stick: yield-stress fluid drop impacts
Blackwell, Brendan C
Here we use high-speed video to experimentally study yield-stress fluids impacting three types of surfaces that exhibit distinct and fascinating behaviors: coated surfaces, permeable surfaces, and hot surfaces. A variety of common materials such as peanut butter, toothpaste, paints, foams, printing ink, cement, and biological fluids can be described as “yield-stress fluids.” Their defining behavior is that they are effectively fluid at high stress and solid at low stress. Many applications take advantage of this duality, utilizing fluid-like behavior to distribute material (e.g. flowing paint through a spray nozzle), and solid-like behavior to hold material where it is placed (e.g. paint building up a coating layer that is stable under its own weight). In some applications, including firefighting and coating processes, the distribution of material involves drops of yield-stress fluids impacting surfaces. During these impact events several factors (material properties, drop size, drop speed, and surface properties amongst others) combine to cause a drop either to stick where it hits, or to display a range of diverse flow phenomena. For drop impacts on pre-coated, permeable, and heated surfaces we perform experiments at varying values of these parameters, and develop a dimensionless group that characterizes drop sticking behavior.
When impacting a solid surface pre-coated with the same material, a drop of yield-stress fluid can calmly deposit itself on the surface, or experience a large splash event. When incoming drop energy (set by drop size and speed) is small compared to arresting forces (set by the material yield stress and surface geometry) all motion is quickly halted, and the drop becomes a lump on the surface. When incoming drop energy is large compared to arresting forces the drop splashes, creating a long-lifetime, evolving ejection sheet that can breakup to eject interestingly shaped droplets. We study these extremes and the transition between them by observing impact events at varying drop size, drop speed, yield stress, and pre-coating thickness, and we develop a dimensionless group that characterizes stick/splash behavior.
When impacting permeable solid meshes (rigid surfaces with small, evenly spaced openings), yield-stress fluids can either stick to the mesh and accumulate large volumes, or pass through the mesh matrix. When incoming drop energy is small compared to arresting forces a drop will be completely stopped, as though it were hitting an impermeable solid surface. When incoming drop energy is large compared to arresting forces the drop can traverse the mesh, flowing through the openings and breaking into smaller fluid particles with varying shapes, sizes, and velocities in the process. We study these extremes and the transition between them by observing impact events at varying drop size, drop speed, yield stress, and mesh geometry. We find that the same dimensionless group that characterizes stick/splash behavior on coated surfaces also effectively predicts material transmission through permeable surfaces.
When impacting a dry, solid surface at sufficiently high temperatures, the Leidenfrost effect can be observed, wherein a layer of vapor is created between the material and the surface due to rapid boiling, which can prevent a drop from sticking to the surface. We report the unexpected result that at high temperatures yield-stress fluids are less prone to sticking than Newtonian fluids. As yield stress increases, the temperature required to prevent material adhesion decreases, and this critical temperature of all the aqueous yield-stress fluids we tested is lower than that of water. We study possible explanations for this counterintuitive trend using high-speed color interferometry. For all three types of surfaces our results and analysis characterize drop impact behavior as a function of a variety of input parameters, creating tools that enable design with and design of these complex materials for specific applications.
In addition, we more closely examine one specific application of these fluids: firefighting. We present experimental data for pressure drops and flowrates of yield-stress fluids in hose flow, and establish design criteria based on equipment specs. For the same materials we also discuss design criteria for forming and maintaining a surface coating. Finally, we expand on the author’s prior analytical work in thixotropic-viscoelastic constitutive modeling, which predicted a unique model signature in asymptotically-nonlinear large-amplitude oscillatory shear. Here we provide experimental data that demonstrates the predicted unique scaling
Yield-stress fluid; Drop impact; Splash; Non-Newtonian; Leidenfrost; Wetted surface; Complex topography
Fri, 21 Apr 2017 00:00:00 GMThttp://hdl.handle.net/2142/974292017-04-21T00:00:00ZBlackwell, Brendan C