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"Two-Phase Flow and Boiling Heat Transfer Analysis of Macro and Mini-channels at Atmospheric and Subatmospheric System Pressure"
(Indian Institute of Tehcnology, Jodhpur, 2024-02-29) Kothdia, Hardik
Phase change heat transfer processes find extensive application in heat extraction, harnessing both sensible and latent heat. Flow boiling, driven by rapid fluid vaporization, emerges as a highly effective method for high-flux heat transfer. Its versatile applications span refrigeration systems, thermal power plants, nuclear facilities, thermal desalination processes, and electronics cooling. The dominant forces governing flow boiling encompass surface tension, inertia, buoyancy, and viscous forces, crucial for maintaining thermal and momentum equilibrium. These forces are notably influenced by channel orientation, geometry, hydraulic diameter, and operating parameters. Importantly, the impact of these variables varies distinctly between the subcooled and saturated regions of flow boiling. Existing literature lacks comprehensive insights into the impact of orientation, geometry, diameter, and subatmospheric system pressure on flow boiling phenomena. This study delves into the impact of orientation on flow boiling heat transfer coefficients, employing both conventional straight tubes and helical coils. A spectrum of channel hydraulic diameters, spanning from 2 mm to 18 mm, is scrutinized to determine flow boiling heat transfer coefficients, two-phase pressure variations, and the dynamic behavior of wall temperature under two-phase flow conditions. Spatial and temporal variations in wall temperature are meticulously analyzed using thermal imaging technology. The experimental investigations encompass a wide range of operating conditions, including atmospheric and subatmospheric system pressure, with varying mass flux and heat flux values. The analysis includes an examination of the impact of Froude number and Boiling number. The study further investigates pressure drop and fluctuations within these channels under different operating pressure, and it offers a comparative analysis of existing correlations for flow boiling heat transfer coefficients and two-phase pressure drop. In the study, the flow boiling heat transfer coefficient in straight tube exhibits the highest values for horizontal flow, followed by vertical upward flow, and the lowest for vertical downward flow. Wall temperature displays radial asymmetry for Froude numbers below 0.22 due to gravitational effects but is symmetric for vertical up and downflow at all Froude numbers. Two-phase pressure fluctuations are more pronounced in horizontally oriented straight conventional tubes, and two-phase pressure drop is increased with higher mass flux and vapor quality in smaller diameter tubes at lower subatmospheric system pressure. Subcooled flow boiling heat transfer coefficient is higher at lower subatmospheric system pressure, with surface temperature showing radial symmetry under such conditions at all Froude numbers. For mini-channels, heat transfer stability is observed up to a boiling number of 1.76 × 10-4, but subcooled flow boiling heat transfer coefficient exhibits more fluctuations at higher Boiling numbers. Helical coils show variations in heat transfer distribution, critical heat flux, and pressure drop during flow boiling. Vertical-oriented helical coils display higher local and average heat transfer coefficients, with a higher burnout heat flux value compared to horizontal orientation. Two-phase pressure drop in helical coils is higher at subatmospheric system pressure and increases linearly with higher heat flux. This work contributes valuable insights into optimizing flow boiling processes in various geometries and operating conditions, advancing the understanding of heat transfer phenomena.
Studies on Electrochemical-Assisted Manufacturing Techniques and Associated Applications
(Indian Institute of Tehcnology, Jodhpur, 2024-03-14) Gupta, Ankur
An ever-growing demand for small-scale functional components and the increasing trend of miniaturization in functional systems have led to the rise of microsystems technologies in recent years. Traditional manufacturing processes face limitations in producing precise smallscale tubular and complex structural profiles while maintaining required functional capabilities. Many unconventional manufacturing processes, including electrochemical-assisted techniques, have been revitalized to meet the need for miniaturization. Electroforming, as one of these unconventional processes, enables the low-cost fabrication of high-precision components in cutting-edge micro and nanotechnology domains at room temperature. This work proposes a cost-effective approach for manufacturing metallic and composite tubular structures as well as for metallic deposition on flat and complex surface profiles using an "in-house pulse electroforming" setup. In the upcoming chapter of this thesis, we present the amenable fabrication methodology for small-scale tubular structures, addressing the challenge of fluidic transportation without outflow, applicable in microfluidics, micro heat exchangers, and various biomedical microdevices. We also developed super hydrophobic surfaces over the electroformed structure, achieving a water contact angle of more than 150 degrees on both the interior and exterior surfaces. Additionally, with the widespread use of millions of dyes in the textile industry, there is an engineering challenge to remediate the wastewater generated and protect natural water sources. In this context, we developed nanofunctionalized electroformed tubes for the degradation of Azo dyes, which are considered poorly biodegradable industrial pollutants. Copper/graphene oxide electroform tubes are also developed using an in-built setup and these substrates are utilized for photocatalytic degradation of organic dyes (methyl orange and methylene blue) under sunlight irradiation. Additionally, we developed nano-sized SiC embedded Cu tubular structure, thereby enhancing its mechanical strength and corrosion resistance. We investigated the impact of pulse frequency, duty cycle, bath agitation, and salt concentration with the help of statistical tool (ANN, ANFIS) revealing nuanced effects on surface properties, microhardness, compression, corrosion, and hydrophobicity. Furthermore, the surface micromanufacturing technique is explored to develop porous copper electrode with unique structural and functional properties, holding potential for a wide range of applications in catalysis, sensing, energy storage, and filtration. In addition to it, another work was performed on 3D-printed complex structures to enhance their mechanical strength and functional properties, through a combination of pyrolysis and electrodeposition techniques. Digital light processing (DLP) based additively manufactured structures underwent pyrolysis, transforming into carbon with a 90% of volumetric shrinkage, followed by Ni-Cu bimetallic electrodeposition for hydrogen evolution reaction (HER) and microelectromechanical (MEMS) applications. In another study, we explored electrochemical energy to propose an effective solution for the treatment of textile wastewater before discharging it into natural waterbodies, which includes the real-time photocatalytic degradation using novel ZnO caterpillars along with the electrochemical processing. Additionally, to reduce the processing time, we also performed the analysis by exploring the ultrasonic-assisted technique along with the statistical optimization of parameters.
Application of Clay and Silica Alumina Supported Metal Catalysts for Hydrogenation and Hydrotreatment Reactions
(Indian Institute of Tehcnology, Jodhpur, 2024-01-31) Sharma, Rakesh Kumar
Considering the global quest for environmental sustainability and a dire need for effective energy use, catalytic hydrogenation and hydrotreatment have emerged as a critical process in value-added products, petrochemicals, and environmental industries. The thesis aims to develop efficient green catalytic systems synthesizing fine chemicals for industrial scale and exploring the potential of upgrading model compound methyl oleate and biomass resources such as vegetable oils for large-scale production of diesel-grade hydrocarbons. The main objectives laid out in this thesis are focused on the synthesis of natural clay or silica-alumina supported metal catalysts. The idea is to use these catalysts for various applications relating to value-added products and the production of biofuels. These catalysts frequently exhibit improved catalytic activity, enhancing reaction rates and better selectivity. The study aims to provide insights into sustainable chemistry and engineering practices, leading to industrial applications, catalyst design, and new sustainable catalysts. The study aims to enhance catalytic efficiency, minimize waste, and find economically viable and environmentally benign materials to ensure its wide usability and production. The studies undertaken in this thesis attempt to address some of these issues. The use of clay as a cost-effective, environmentally benign, and abundant support is an ideal choice. Clay-supported metal catalysts are compatible with green chemistry principles, leading to increased productivity and a reduced need for potentially hazardous solvents. The study presents a simple wet-impregnation route for a sustainable natural clay-supported palladium catalytic system for the hydrogenation of imine to amines, feedstock for agrochemicals, and pharmaceuticals. The catalytic system was investigated under mild reaction conditions, and its catalytic activity and effect on reaction conditions were analyzed. Catalyst immobilization on clay supports results in reduced costs and waste generation by allowing the recycling of metal catalysts. The mechanistic details of imine hydrogenation are elucidated, structural, chemical, and morphological properties of these catalysts are examined, and reaction conditions are optimized to achieve high conversion rates and selectivity. The study emphasizes the need for greener technology in synthesizing fine chemicals for industrial scale due to rising energy demand and environmental concerns. Additionally, studies have been done to explore the impact of non-noble metal integration in a SiO2–Al2O3 catalyst on the conversion of methyl oleate into diesel-grade aliphatic hydrocarbons. This study presents the effect of cobalt incorporation into the SiO2-Al2O3 hybrid catalytic system on the conversion, selectivity, and stability of the catalytic conversion of methyl oleate as a model compound for biofuels. The study found that the amount of Co loading, reaction time, temperature, and H2 pressures greatly influence the conversion and selectivity. The complete ester conversion rate and substantial yield towards n-heptadecane / n-octadecane are achieved under solvent-free conditions. Further, iron poisoning on nickel oxide supported on silica-alumina catalysts is investigated. Here, Bimetallic FexNiy/SA catalysts were synthesized using the hydrothermal method followed by chemical deposition strategies. The study also investigated the characterization and tuning of FexNiy/SA catalysts for the low-temperature hydrodeoxygenation (HDO) of methyl oleate and vegetable oils to n-alkanes. Out of all catalysts, The Fe1Ni1/SA catalyst demonstrated outstanding HDO efficiency, conversion, and hydrocarbon selectivity. The Fe2+/Fe3+ ratio in FeaOb species on the FexNiy/SA catalyst regulates the carbon number distribution of generated hydrocarbons, suggesting a viable strategy for designing an efficient HDO catalyst.
On Some Problems For Graph Induced Symbolic Systems
(Indian Institute of Technology Jodhpur, 2024-02-15) Sharma, Puneet
Symbolic dynamics was introduced in the late 19th century by Jacques Hadamard, where he applied the theory of symbolic dynamics to examine the geodesic flows on surfaces of negative curvature [Hadamard, 1898]. Later, Morse and Hedlund used symbolic dynamics as a tool to study the qualitative behavior of a general dynamical system [Morse and Hedlund, 1938]. In 1948, Shannon employed symbolic dynamics to examine certain fundamental problems in communication theory [Shannon, 1948]. The convenience of symbolic representation and easier computability of the system has attracted attention of several researchers around the globe and the topic has found applications in various branches of science and engineering. In particular, the area has found applications in areas like data storage, data transmission and communication systems to name a few [Shannon, 1948; Lind and Marcus, 1995; Kitchens, 1998]. Since it is known that every discrete dynamical system can be embodied in a symbolic dynamical system (with an appropriate number of symbols) [Fu et al., 2001], it is sufficient to study the shift spaces and its subsystems to investigate the dynamics of a general topological dynamical system. In this work, we investigate a d-dimensional shift space arising from a d-dimensional graph G = (G1;G2 :::Gd), where each graph Gi has common set of vertices and i-th graph determines the compatibility of vertices in the i-th direction. In particular, we investigate non-emptiness, finiteness, existence of periodic points and mixing notions for a d-dimensional shift space. We examine the structure of the shift space using generating matrices and establish that a d-dimensional shift of finite type is finite if and only if it is conjugate to a shift generated through permutation matrices. We establish conditions under which a two-dimensional shift space is non-empty and contains periodic points. We introduce the notion of an E-pair for a two-dimensional shift space and use it to derive sufficient conditions for non-emptiness, finiteness and periodicity of the two-dimensional shift space under discussion. Additionally, we study properties such as transitivity, directional transitivity, weak mixing, directional weak mixing and total transitivity for the two-dimensional shift space XG. We assert that if the condition (HV)i j 6= 0 , (VH)i j 6= 0 holds for all i; j 2 V (G), then the irreducibility of any generating matrix guarantees the equivalence of transitivity and directional transitivity for the shift generated by the graph G = (H;V). We present examples demonstrating that weak mixing and totally transitivity are not analogous in two-dimensional shift spaces (it is known that these two notions coincide in one-dimensional case). Further, we characterize directional transitivity (in (r;s)-direction for rs > 0) through the block representation of HrV s. We provide necessary and sufficient criteria to establish horizontal (vertical) transitivity for the shift space XG. We investigate the topological dynamics of a general two-dimensional shift space generated by a graph G = (H;V) through matrices M;N; where M;N are indexed with allowed triangular patterns of form c a b ; x z y respectively. We investigate how the characteristics of matrices M and N are related to one another. We derive sufficient conditions on M and N to exhibit non-emptiness and existence of periodic points for shift space XG. We assert that if the condition MIJ 6= 0 , NI1J1 6= 0 holds for all E-pairs (I;I1) and (J;J1); then XG is doubly (1;1)-transitive ((1;1)-weak mixing) if and only if M is an irreducible (Primitive) matrix. We establish that under imposed conditions, (1;1)-weak mixing implies (r;s)-weak mixing (for rs > 0) for XG. We provide necessary and sufficient conditions for any graph G to be expressed as a graph product of two smaller graphs. We relate the dynamics of one-dimensional shift space XG with the dynamics of component shift spaces XGi, where graph G can be expressed as graph product (Cartesian, Tensor, Lexicographic and Strong Product) of two smaller graphs G1;G2. We investigate structural properties such as non-emptiness problem and the existence of periodic points for shift spaces through graph product of two-dimensional graphs. We assert that a shift arising from Cartesian (Lexicographic, Strong) product of two-dimensional graphs is always non-empty and possesses periodic points, but the shift arising from Tensor graph product is non-empty and contains a non-empty set of periodic points if and only if each component shift space (i.e., XGi) is non-empty and possesses periodic points. Finally, we examine various mixing notions such as transitivity, directional transitivity and weak mixing for two-dimensional shift space XG (under imposed condition) through its component subshifts XGi.
Flexible Organic Transistors for E-Textile and Memory Applications
(Indian Institute of Tehcnology Jodhpur, 2023-11-04) Tiwari, Prakash
Flexible electronics has been explored as a low cost technology for numerous applications such as wearable devices, foldable displays, and E-textiles, etc. and offers advantages such as large area applicability, low temperature processing, and adaptability for heterogeneous integration. Organic field effect transistor (OFET) is a crucial device for flexible electronics, which has been explored for various circuit, sensing, and memory applications. Gate dielectric is a crucial component for optimization to achieve high performance and stability in OFETs. This work demonstrates OFET devices with various dielectric combinations for circuit and memory applications. Moreover, a unique strategy for fabrication of devices on textile substrates is demonstrated. To start with, flexible OFETs were demonstrated with bilayer hybrid gate dielectric with various polymers and polyelectrolyte dielectrics such as P(VDF-TrFE), PVP-co-PMMA and polyelectrolyte polyacrylic acid (PAA) in combination with a thin layer of high-k hafnium oxide (HfOx) grown by atomic layer deposition (ALD). TIPS-Pentacene was used as semiconductor. The optimized devices operated at -10 V with decent Ion/Ioff values ranging from ~104 to ~103. The devices with HfOx/PAA dielectric exhibited better performance compared to other counterparts due to higher capacitance density, along with excellent cyclic stability for continuous 500 cycles. Moreover, these devices showed high bending stability upon different radii up to 5 mm causing tensile stress. Further, a bilayer of polyvinyl alcohol (PVA)/(PAA) was used as gate dielectric to demonstrate low voltage high-performance flexible OFETs on a plastic and paper substrate. A super strong hydrogen bonding between PVA and PAA confirmed by fourier-transform infrared (FTIR) spectroscopy makes it a potential solution-processed bilayer dielectric candidate. Fabricated devices with TIPS-Pentacene: PS blend exhibited -5 V operation with nearly zero threshold voltage and high Ion/Ioff of ∼104. High bending stability was achieved upon successive bending in different ways. These devices were also used to demonstrate resistive-load inverter circuit. The devices fabricated on paper showed a significant level of disintegration in soil with a bio fertilizer. OFET devices on fabric substrate were successfully demonstrated by a simple lamination technique. These devices with TIPS-Pentacene:PS blend as active layer and high-k P(VDF-TrFE) gate dielectric showed maximum and average field effect mobility of ∼1.2 and ∼0.5(±0.3) cm2 V-1 s-1 in the saturation regime, and Ion/Ioff of ∼103 with an low operating voltage of -5 V along with excellent bending stability. Excellent cyclic stability for 500 cycles was performed. Moreover, a high shelf life in ambient for 26 weeks was observed from these devices. Finally, solution-processed flexible non-volatile memory (NVM) based on OFETs (OFETNVMs) were demonstrated with TIPS-Pentacene and P(VDF-TrFE) as gate dielectric. These OFET-NVMs showed excellent memory behaviour with very high memory window (MW) of 12 V for VGS sweep of ±15 V and low VDS of -5 V. Moreover, these devices show memory Ion/Ioff ∼103 for 100 continues cycles alongwith stable retention capability for higher than 104 s. These devices showed fairly stable and reliable NVM behaviour even after subjected to 100 repeated bending cycles. Despite of fact that minimal degradation in performance was observed upon bending. These devices are promising candidate for further exploration into flexible electronics due to overall excellent memory performance of the devices.