PhD Theses (Mechanical Engineering)

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Experimental Investigation on Condensation Heat Transfer Coefficient and Frictional Pressure Drop of Refrigerant R1234yf on Brazed Plate Fin Compact Heat Exchanger Surfaces
(2025) Kemprai, Partha Pratim
Compact heat exchanger an improved heat transfer equipment, is being widely used in many industrial applications such as automobiles, aerospace, cryogenic, electronic cooling, oil and gas sectors. Among the various types of compact heat exchangers, brazed plate fin types are mostly preferred due to their compactness, higher effectiveness, reduced space and weight. Brazed plate fin compact heat exchangers generally use plain fins, serrated, perforated, wavy and louvered fins that offer large fin density. The single-phase heat transfer characteristics study in compact heat exchangers are more available when compared to the phase change heat transfer applications. However, there exists an extensive application of phase change heat transfer mechanisms in compact heat exchanger due to their better performance. There are many studies available using refrigerant R134a in literature. But the condensation study of low GWP refrigerant R1234yf is found to be limited in compact heat exchangers design and the works available are mostly for tubes, channels, mini-channels and micro-fins. Literature survey also indicates that the work on condensation heat transfer of refrigerant R1234yf inside brazed plate fin compact heat exchangers with serrated and wavy fin surfaces are not available.
(A) New Strain Gage Technique for the Accurate Determination of Mode-I Notch Stress Intensity Factors of Sharp V-Notches
(2025) Paul, Pranjol
Notch stress intensity factor (NSIF) of a sharp V-notch is a vital parameter in notch fracture mechanics whose limiting value is frequently used in assessing the fracture of V-notched components. The accuracy of NSIFs acts as a regulatory aspect for appropriately making use of principles of linear elastic fracture mechanics in predicting and preventing aforesaid failures.
Bionic Reflex Control for a Tendon-Optimized Underactuated Anthropomorphic Hand: A Reinforcement Learning Approach
(2025) Basumatary, Hirakjyoti
This thesis investigates advanced bionic reflex control in underactuated, tendon-driven anthropomorphic robotic hands tailored for prosthetic applications. The work emphasizes the replication of human-like dexterity by optimizing actuation parameters—such as pulley radius, spring stiffness, and spring preload angles—to generate five distinct, synergy-based grasp types (power, precision, cylindrical, oblique, and pinch) using potential grasp robustness (PGR) and potential contact robustness (PCR) metrics as the optimization objective function. Central to the study is the development of a novel control pipeline that integrates a wavelet transformation technique for real-time slip detection with a reinforcement learning (RL) framework. The RL agent, trained under domain randomization to accommodate unmodeled dynamics and environmental noise, autonomously adjusts grasp forces to prevent slippage without manual intervention. In addition to slip prevention, the thesis presents an innovative approach to deformation control by eliminating the need for pre-defined labeling thresholds, thus enhancing the bionic reflex by seamlessly adapting to the physical properties of grasped objects. To further bolster the system’s resilience, an adaptive sliding mode controller is layered over the pre-trained RL policy. This hierarchical control strategy ensures robustness against matched disturbances and model uncertainties in continuously changing environments. Simulation results confirm that the integrated system can reliably grasp and lift various objects, maintaining high-quality grasps with minimal slippage and deformation. Overall, the proposed methodology represents a significant step forward in developing autonomous bionic reflex controllers that can potentially transform prosthetic hand functionality and robotic grasping research.
Advancing the Understanding of Sandwich Structures through Mechanical Behaviour Modelling and Failure Analysis of FSSW-Joined Honeycomb Core
(2025) Kumar, Avneesh
The modern automotive industry is increasingly focused on addressing global energy and environmental challenges by developing lightweight vehicles, particularly electric cars, whose performance and range are heavily influenced by weight. Sandwich structures, especially those with honeycomb cores, have gained prominence due to their lightweight nature, high bending stiffness, energy absorption, and strength. These structures are highly customizable, offering superior mechanical and crash-worthiness properties. However, joining these components effectively remains a critical challenge. Traditional methods like adhesive bonding (AB) and mechanical fastening have limitations, such as sensitivity to environmental conditions, stress concentrations, and added weight. Additive manufacturing (AM) offers potential but faces material and cost barriers.
Heat Transfer Analysis of a Fin-tube Heat Exchanger using Shearthinning Fluid and Winglet Type Vortex Generators
(2024) Kumar, Dheeraj
The present thesis work investigated the effect of various shapes of winglets, such as delta winglet pairs (DWP), rectangular winglet pairs (RWP), and curved rectangular winglet pairs (CRWP), on the flow behavior of an aqueous solution of carboxymethylcellulose (CMC) through a rectangular channel with and without the inbuilt cylinders. Threedimensional numerical simulations were carried out for a range of Reynolds numbers (50-200) using aqueous carboxymethylcellulose, commonly known as non-Newtonian shear-thinning fluid as a working fluid. Nusselt number (Nu), friction factor (f), and the combined effect of Nu and f, commonly known as quality factor (Qf), were calculated and compared for each case. Additionally, the comparison is also made with the base channel considering a dimensionless number (η) defined as (Num/Nuo)*(fo/f)0.33. As well, an investigation was conducted into the impact of attack angle, winglet placement, and tube placement on the thermohydraulic performance of the channel. The fintube heat exchanger (FTHE) performance parameters were effectively compared and evaluated using an aqueous solution of CMC and water. It was discovered that incorporating DWP in FTHE improves overall performance compared to RWP. Still, when only heat transfer applications are considered, RWP outperforms DWP. When comparing the thermohydraulic performance, the use of CRWP is unequivocally superior to RWP. The former option provides a more efficient and effective means of achieving the desired outcomes. Reducing the attack angle improves performance up to a limit, but further decrease reduces performance. Utilizing an aqueous solution of CMC as a working fluid has been established as a practical approach to augment heat transfer while simultaneously reducing pressure drop. Based on the thorough evaluation of the overall performance, it is evident that FTHE with winglets significantly outperforms the base channel. Staggered tube configurations of FTHE show better thermal performance than inline tube configurations. They generate higher values of critical parameters such as Nu, f, and Qf, which enhances the efficiency and effectiveness of relevant applications.
Drop-drop and dropsurface dynamics during impact and electrohydrodynamic interactions
(2023) Das, Santanu Kumar
The interaction between drop-drop and drop-surface is an essential aspect of fundamental studies in fluid dynamics. Researchers have been interested in studying these dynamics for their applications in various industrial processes. To understand the fundamental aspects of drop-surface interactions, robust computational tools have been utilized. The transition regimes between coalescence and splashing of drops include jet formation with single or multiple secondary drops. Previous studies have demonstrated that the diameter of the secondary drop lies between 0.58 and 0.94 times the diameter of the impacting drop. However, the present study reveals that secondary drops larger than the primary impacting drop can be obtained at higher impact velocities. Due to the recent advancements in microfluidic devices and biological processes, electrohydrodynamic flows became a critical part of these investigations. The dispersion of drops in an emulsion is commonly seen in several chemical, pharmaceutical, and petroleum industries. The stability of these dispersions can be affected by an electric field. Studies with creeping flow approximations have shown that depending on the electrical properties, the drops can repel and move apart or get attracted toward each other. We defined a phase map representing regions for various drop interaction dynamics. Finally, we studied polymer blends essential in food, paint, cosmetics, and processing industries. The microstructure of the polymers resulting from drop deformation, coalescence, and breakup is directly related to the properties of these emulsions. A viscoelastic drop exhibits reduced deformation with higher viscoelasticity, while a Newtonian drop suspended in a viscoelastic medium shows non-monotonic behavior.
Design and Testing of Metal Hydride Reactors for Stationary Hydrogen Storage and Cooling Applications
(2023) Jana, Sayantan
Metal hydrides (MH) are class of materials that offer compact, safe and ultra-pure hydrogen storage for longer periods. Also, by virtue of exothermic formation and endothermic dissociation, a wide variety of thermal devices can be engineered utilizing the MH-H2 pair. However, the design of efficient MH reactors/heat exchangers is the most critical issue limiting hydrogen charging/discharging rate and therefore the concomitant heating/cooling effect.
Aerodynamic Design and Wind Tunnel Tests of Novel Blade Shapes for the Savonius Wind Rotor
(2024) Mohan, Man
Wind turbines are increasingly used worldwide to generate electricity without contributing to global warming. The Savonius wind rotor is widely known for small-scale power generation devices. However, the Savonius rotor, a simple type of vertical-axis wind turbine, suffers from low efficiency due to a high negative torque from the returning blade. To address this issue, extensive research has been done on enhancing various design parameters, geometric parameters, implementing augmentation techniques and other aspects. However, there is always scope for further investigation to contribute better. In view of this, the present study aims at generating blade profile through optimization techniques (OTs). The optimal blade profile is developed considering the power coefficient as an output function using the optimization techniques, viz., simplex search method (SSM), non-dominated sorting genetic algorithm-II (NSGA-II). Moreover, a novel parabolic blade profile is developed analytically considering the sectional cut angle () of the parabola as a design parameter.
Micro Plasma Arc Welding of Similar and Dissimilar Materials and Formability Study of Welded Joint
(2025) Haldar, Vivekananda
This research focuses on the similar and dissimilar joining of austenitic stainless steel (316L) and Inconel 625 using Micro Plasma Arc Welding (MPAW). The study evaluates joint metallurgy and investigates its impact on the formability of welded blanks. Welding experiments with varying parameters are conducted to optimize joint strength and formability. Advanced characterization techniques, including XRD, TEM, and FESEM, are employed to analyse microstructures and phases in the fusion zone. The solution heat treatment improves formability by reducing dislocation density and dissolving secondary phases.
Vortex-Assisted Mixing in Microscale Flows
(2024) Kumar, Dhananjay
Microscale mixing in fluidic devices offers several advantages, including reduced sample volume, portability, cost-effectiveness, safe handling of hazardous materials, compact size, and biodegradability. Efficient mixing is crucial for various microfluidic processes, particularly in biochemical and medical diagnostics. Several studies investigated in this thesis focus on enhancing vortex-assisted mixing in microfluidic devices through both passive and active approaches. The challenge lies in achieving efficient mixing in the confined space of microfluidic/nanofluidic pathways, which has led to exploring secondary flows to boost advection over molecular diffusion. This involves implementing passive methods, such as introducing inlet swirl, and active methods, like applying an external electrical field, all within the constraints of a narrow-fluidic channel. The dissertation outlines objectives and analyzes each application in detail across respective chapters.
Numerical Investigation of Transitional Flow Past Low Reynolds number Airfoils and Wings
(2024) Kulkarni, Dattatraya S
Low Reynolds number (Re) aerodynamics is important for both natural and man-made flying objects. Over the last few decades there is a growing interest in designing Micro Air Vehicles (MAVs) which are used for both civil and military applications. The small length scale and low Reynolds number (104 − 5 × 105 ) flight regime encountered in these classes of air vehicles lead to a complex flow phenomenon. This flow complexity is mainly due to viscous effects, laminar to turbulent transition, formation of the laminar separation bubble (LSB) with turbulent reattachment,which pose a major aerodynamic challenge to designers. The phenomenon of laminar to turbulent flow transition is one of the major area which is not fully understood by the Computational Fluid Dynamics (CFD) community.However, there are different ways in CFD to predict transitional flow namely (i) empirical eN method based on linear stability theory approach (ii) Direct Numerical Simulation (iii) Large Eddy Simulation (iv) the statistical based RANS models. The present work mainly focuses on predicting the aerodynamics and transition characteristics of airfoils and wings used in MAV application and understanding the influence of different flow and geometry parameters on the performance of these airfoils and wings. In this study, the numerical simulations for Re ranging from 4 × 104 to 2 × 105 are carried out using the RANS based transition models viz. kT − kL − ω and γ − Reθ SST available in the in-house multi-block incompressible flow solution code 3D-PURLES (3D Pressure based Unsteady RANS LES). The RANS based transition models are chosen for this study as they are computationally less expensive with their average flow quantities being reasonably accurate and best suited as a designer approach. This report discusses, the systematic numerical study carried out to investigate the effect of geometrical parameters like camber, thickness and planforms, grid parameters, flow parameters, transition models for low Re Selig-Donovan (SD) airfoil series on the aerodynamic and transition characteristics for a wide range of angle of attack. The grid size, grid resolution, grid topology, farfield location and other flow parameter like spatial discretisation schemes, freestream turbulence intensity and inlet eddy viscosity ratio which control the accuracy of the flow solution code is fixed based on the sensitivity study carried out at Re = 6 × 104 for SD7003 airfoil. The aerodynamic performance parameters (stall angle, lift coefficient and drag coefficient) and transition characteristics (transition onset, separation point, reattachment point and length of the LSB) are validated against the available measurement data. This comparative study indicated that the transition and aerodynamic features of the low Re SD7003 airfoil was predicted better by the γ − Reθ SST transition model when compared to the kT − kL − ω transition model. The γ − Reθ SST model because of its better performance was used for all the other simulations for different airfoils and wings carried out in this work. Based on the study carried out to understand the influence of the airfoil camber and thickness in the low Re regime it was inferred that increasing the airfoil camber significantly improved the performance of the airfoil whereas the increasing the thickness did not bring in any significant improvement in the performance of the airfoil. It was observed from this study that increasing the Re form 6 × 104 to 2 × 105 enhanced the aerodynamic performance whereas the transition features were not significantly effected. The numerical simulation past the non-dimensionalized SD7003 rectangular wing using the γ − Reθ SST transition model is also discussed in detail. In this study, the effect of the different wing semi aspect ratio (sAR) of 0.25, 0.5 & 1 in the low Re regime at Re = 6 × 104 is carried out to predict and understand its influence on the aerodynamic and transition characteristics. This study showed that wing with lowest semi aspect ratio of 0.25 is most suited for MAV application as its performance was better in terms of having lower drag, higher maximum lift coefficient, delayed stall angle, negative pitching moment slope and having a larger laminar region as compared to other two wing planforms with higher semi aspect ratio. It was further noted from this study that increasing the Re from 4 × 104 to 1.5 × 105 did not bring a considerable change in the aerodynamic and transition features of the SD7003 rectangular wing with 0.25 semi aspect ratio.
Experiments and Modelling of Hydrogen Gas Generation from the Reaction of Aluminium-NaOH Solution and its Application in Sintering Furnace
(2024) Das, Biswajyoti
Hydrogen gas as an energy carrier has high energy density and almost nil emission during its combustion. The production of hydrogen gas from the reaction of aluminium-NaOH solution is a clean method of production. In the present study 1 g aluminium scrap is reacted with aqueous NaOH concentrations of 1M-5M and temperature of water varying from 303 K- 333 K. Mathematical modelling of the hydrogen generation is carried out by using machine learning techniques for large scale applications. Hydrogen gas produced by the chemical reaction is used to heat a sintering furnace and to determine its thermal efficiency. Approximately, 97 % of stoichiometric hydrogen is produced in the condition 5M/333 K which is 1322 ml.g-1 Al. The activation energy of 57.62 kJ.mol-1 is obtained in this work. The hot combustion product of hydrogen-air mixture is introduced inside the furnace to heat up at different fuel-air ratios. The thermal efficiency (η) of the furnace is measured as 76.22 % at fuel-air ratio of 1:60 corresponds to the fuel-air equivalence ratio (λ) of 0.57. Hydrogen gas is an effective energy carrier for power generation which is clean at the same time.
Dynamic Analysis and Control of String-Stiffened Flexible Robotic Manipulators
(2023) Ranjan, Rajesh
In the present thesis, kinematic and dynamic analyses of a manipulator with a rigid link and flexible joint, a manipulator with flexible links, and manipulators with both link and joint flexibilities have been studied. Kinematic and dynamic modeling of flexible manipulators with the finite element method and assumed mode method both have been done.
Experimental and Numerical Studies on Mixed Mode Fracture and Fatigue Using a New Specimen
(2024) Shukla, Shiv Sahaya
Mixed mode fracture and fatigue failures are one of the most significant causes of failure in materials and structural components. In the present investigation, a simple and efficient specimen geometry and out-of-plane loading fixture that can be used with conventional uniaxial universal testing machines for conducting any combination of mixed mode (I/II/III) fracture and fatigue crack growth studies have been proposed. The proposed specimen is a single edge cracked circular (SECC) specimen which can be employed on both metallic and non-metallic materials under static and fatigue loads.
Development of Laser-based Directed Energy Deposition System with Non-Pneumatic Powder Feedstock Handling and Synthesis of Toolpath Strategies
(2024) Singh, Ambrish
Pneumatic methods of powder feedstock handling are prevalent in laser-based Directed Energy Deposition (DED). These methods use inert gases (mostly Argon) to meter, convey, and inject powder feedstock into the melt pool created by the laser on the substrate. Although several variants of pneumatic feedstock handling exist in the literature, the difference is mainly in the mechanism of powder metering and the geometric design of the powder delivery nozzle. This work identifies several shortcomings of such pneumatic-based feedstock handling systems and offers an easy solution that could be more economical. A gravity-based system, proposed as part of this study, has been demonstrated to be a viable replacement for pneumatic methods. The gravity-based non-pneumatic system utilizes a helical-grooved shaft rotating inside a powder-filled hopper to meter the mass flow rate. This metered influx of powder is subsequently delivered to the powder delivery nozzle, which creates a conical and convergent envelope of powder stream around the fusion source before finally injecting it into the melt pool.
Experimental Investigation of Cusp Magnetic Field assisted GTAW of Similar and Dissimilar Low Carbon Steels and AISI 304 Stainless Steel
(2023) Prasad, Kelli Durga
The research work was carried out to understand the effect of external magnetic field on the mechanical and metallurgical properties of similar and dissimilar weldments. A detailed experimental investigation on similar welding of low carbon steels and AISI 304 stainless steel plates, as well as dissimilar welding of low carbon steels and SS 304, with and without the influence of an external magnetic field were carried out. A specially designed fixture was also developed to accommodate both the permanent magnet setup and the welding torch to generate the external magnetic field. Furthermore, a 3D finite element analysis was carried out to measure the magnetic flux density along the electrode tip and compered with experimental results. With the application of external magnetic field, improvement in weld depth of penetration and reduction in bead widths were achieved. Also, a homogenous distribution of alloying elements was observed in the weld zone as compared to conventional welds. In similar welding of SS 304, due to higher arc energy densities, the thermal energies attained in CMF assisted weld pool is higher and the cooling rate is little slower resulting in favorable conditions for precipitation of austenite. Also, resulted in decrease of columnar dendrites and transformation of columnar dendrites to equiaxed dendrites in the weldment. The decrease in heat affected zone thickness was also observed in all cases. The additional magnetic field caused a stirring action in the weld pool, which enhanced grain refinement and ferrite reduction. A better corrosion rate was achieved due to homogenous distribution of alloying elements in stainless steel weldments.
Analysis of GLARE Laminates Under Low Velocity Impact
(2023) Kakati, Sasanka
Superior impact properties of glass aluminium reinforced epoxy (GLARE) have led to their usage in impact prone structures such as aircraft fuselage, wings and cargo panels. However, these advanced laminated structures are also susceptible to impact induced damages, especially under low velocity impact (LVI) where the damages are sub-surface and barely visible. Finite element analysis (FEA) enables a more in-depth study of the complex nature of the response and damages due to arbitrary LVIs. This dissertation thus presents the FEA of GLARE under LVI, evaluating the response of the target and the associated damage due to LVI. A complete 3D finite element (FE) formulation has been developed using 3D layered solid elements to evaluate the contact impact response of GLARE subjected to arbitrary (normal and oblique) LVIs. A transient dynamic FE code has been developed incorporating the Newmark-β method and implementing suitable normal and tangential contact models for accurate determination of the contact responses and the associated interfacial delamination damages. An important aspect of the present FE modelling is the incorporation of an adjustable contact stiffness based on the impactor to plate mass ratio which is critical for accurate evaluation of the contact response and correctly predicting the associated damages. Influence of important parameters like properties and geometry of the GLARE laminate and the impactor along with the trajectory of the impact on the contact impact response and the delamination damages have been investigated. Results from the present work show that besides the size and geometry of the impactor, the trajectory of the impactor relative to the target and the coefficient of friction between them also significantly influence the contact response as well as the evolution of delamination at the interfaces. Further, for multiple impacts, the interval between the successive impacts greatly influences the magnitude of contact force as well as delamination at the fibre-metal interfaces. The presence of discontinuities in the form of cut-outs or open holes (due to functional requirements) in the GLARE laminate significantly influences how delamination grows around these locations due to increased stress concentrations under impact loads. Results from the analysis of LVI show that the pitch of the cut-outs and their size and shape has significant influence on the evolution of delamination, especially at the metal/composite interfaces.
Evaluation of Surface and Sub-surface Defects in Friction Stir Welding Through Experiments and Coupled Eulerian and Lagrangian Based Finite Element Model
(2023) Das, Debtanay
The manufacturing industry widely accepts friction stir welding (FSW) as an environment friendly technology since it can produce a joint with almost zero emissions to the environment. Moreover, this green technology operates without the need for any shielding gas or filler material. FSW produces good quality weld joint at temperatures lower than the melting point. The welds produced by FSW are free from fusion welding defects such as porosity, distortion, spatter, etc. However, the FSW process is not circumscribed from deleterious defects such as tunnel defects, flash formation, and residual stresses. Extensive research has already been conducted to understand the physical process and material flow in FSW process. A significant amount of research has focused on optimizing the process parameters and tool geometry using experiments and mathematical models. However, there remains a lack of any predictive model to estimate the defect formation and its impact on the weld quality. The successful prediction of weld quality in terms of defect formation will aid to reduce the material wastage and trial-and-error cost for producing a defect free weld. In the present work, a 3D thermo-mechanical model is developed following the Coupled Eulerian-Lagrangian (CEL) approach to evaluate the varying surface morphology, material flow, and defect formation in FSW process. The similar and dissimilar combinations of AA6061 and AZ31B materials are extensively investigated by solid-state FSW process. The tool wear is also estimated to account its influence on the defect formation viz. weld quality. Proper selection of mass scaling factor eliminates the high computational time associated with the CEL approach. The influence of mass scaling technique on the total computational time, weld quality, thermo-mechanical responses, and defect prediction are extensively investigated. Any adverse effect of the artificial mass scaling is kept under check by maintaining the kinetic energy (KE) to internal energy (IE) ratio below 10%. The KE to IE ratio varies as the tool moves from a sound weld region to a defective weld region, but it remains constant if the weld quality is sound or defective for the complete weld length. Therefore, the KE to IE ratio variation cannot be conclusively used to determine the weld quality and can only be used to check any adverse effect of mass scaling. The developed numerical model is validated by comparing the experimentally measured temperature evolution, volumetric defects, and residual stress data. The model can accurately predict the different surface and sub-surface defects, viz., the tunnel defect, flash formation, failed joints, exit hole, and other surface defects, while considering the tool rotation speed, traverse speed, pin height, plunge depth, pin geometry, tool condition and plate position as the process variables. Further, the developed model can estimate the material mixing and weld interface location specifically for dissimilar welding. All these process variables influence the magnitude and distribution of residual stress of FSW joints. However, the residual stress is more generous during dissimilar FSW due to the non-uniformity of the heat flow and material properties. This makes the residual stress estimation more complicated in dissimilar FSW. Hence, the CEL based mechanistic model is coupled with the machine learning (ML) algorithms to predict the temperature distribution and residual stress in dissimilar AA6061-AZ31B FSW. This approach essentially reduces the prediction time of the final result involving large amount of data. Different ML algorithms are compared to estimate their prediction accuracy. The extra tree regressor, XGBoost, and random forest regressor models predict the temperature evolution and residual stress with a maximum deviation of less than 8% for temperature and 10.3% for residual stress, respectively. Essentially, the physics-informed ML algorithm is a reliable process model for predicting the transient state of thermal and mechanical responses during the FSW process, which is more appealing to apply for any other industrial problem of interest.
Numerical and Experimental Investigations of Nonlinear Dynamics and Heat Transfer Deterioration in Supercritical Natural Circulation Loop
(2024) Srivastava, Tanuj
The natural circulation loop (NCL) efficiently transfers energy from a high-temperature source to a low-temperature sink without direct contact. The key driving force in natural circulation systems is buoyancy, caused by density differences. Single-phase NCLs face limitations from saturation temperature and low flow rates, while two-phase loops risk dry-out and complex flow regimes. Supercritical fluids offer an alternative, combining the benefits of both single- and two-phase systems. The concept of a supercritical natural circulation loop (sNCL) is important for Generation-IV nuclear reactors. This thesis evaluates sNCLs using CO2 through both numerical and experimental methods. Numerical studies analyze the steady-state and transient behavior of sNCL, using 3D simulations for steady-state and 2D/1D models for transient cases. Buoyancy and friction forces determine system behavior. As heating power increases, buoyancy dominates, leading to a rise in flow rate and heat transfer. However, friction eventually takes over, reducing flow rate and leading to flow-induced heat transfer deterioration (FiHTD). This phenomenon, which can be delayed but not avoided, is key to ensuring safe operation. Based on fluid temperature, four heat transfer zones were identified: low power, enhanced heat transfer, transition, and deteriorated heat transfer. 3D simulations using ANSYS Fluent confirmed the boundary of safe operation, with data fitting a power-law curve. Changes in friction factor along the heater section also signal the onset of FiHTD. The dissertation further explores static and dynamic instability in sNCL. Steady-state circulation shows a sharp decline, consistent with previous analyses, and exhibits Ledinegg instability at intermediate power levels. Both static and dynamic instabilities were identified, with results aligning across simulations. Under varying heat input, sinusoidal heating caused chaotic oscillations, while ramp heating remained stable due to gradual buoyancy generation. A 2D model explored startup transients, revealing the complex behavior of sNCL near the pseudocritical point. The system's bulk motion is influenced by phenomena like the piston effect, Rayleigh-Taylor instability, and adiabatic heating, which create hot fluid packets that drive system dynamics. Flow reversals and chaotic behavior result from intermittent fluid packet generation and disappearance. Experiments using CO2 examined the effects of sink temperature, pressure, tilt angle, and heating power for FiHTD. Flow rate peaked before declining, with the highest rates in vertical loops. No instability was observed under the test conditions, but the mass flow trends closely matched the simulations.
Experimental and Computational Analysis of Interface Fracture using Extrinsic and Intrinsic Cohesive Zone Modelling
(2024) Saikia, Pran Jyoti
In recent years, material interfaces have become part of numerous engineering and structural applications. Interface failure comprising both cohesive and adhesive failure is one of the shortcomings of bonded structures during service loading conditions. Therefore, predicting interface failures is essential for ensuring the reliability, safety, and cost-effectiveness of systems and processes across various industries. The cohesive zone model (CZM) is a widely used computational technique for analyzing the interface fracture phenomenon within computational fracture mechanics studies. The main objective of the present thesis is to expand the applicability of the CZM for a wide range of material interfaces, ranging from adhesively bonded joints to laminated composites. Additionally, the experimental crack growth studies of isotropic and orthotropic material interfaces augment the proposed numerical methodology within the finite element framework.

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