Dissertations/Theses - Department of Mechanical Engineering
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Item Molecular dynamics study of heat transfer characteristics of bubble induced nanochannel flow(Department of Mechanical Engineering (ME), BUET, 2025-04-07) Nurannabi Miah, Md; Nasim Hasan, Dr. MohammadThe study of phase change heat transfer in liquids has become increasingly significant due to the growing demand for efficient thermal management in compact systems, particularly in applications requiring high heat flux in small-scale devices. However, addressing the challenges posed by small dimensions and high heat densities requires advanced approaches. Boiling heat transfer in bubble induced nanochannel flows has thus emerged as a critical area of research. This study utilizes molecular dynamics simulations to examine the effects of surface interaction potential (W), channel width (D), heater intensity (T) and length (H) on phase change heat transfer in nanochannels. The findings reveal that higher heat source temperatures under hydrophilic conditions promote earlier bubble nucleation, while bubble propagation transitions from gradual at lower temperatures to erratic at higher ones. Multiple nucleation sites are activated for bubble nucleation for a higher heater length due to the lack of accumulation of heat within a single region, as occurs for the case of lower heater length. During heating, a reverse liquid flow towards the liquid pool is observed, driven by a sudden temperature surge at the heater. Additionally, higher surface interaction potentials lead to increased liquid atom accumulation at the heater surface, enhancing energy transfer and bubble propagation, thereby improving heat transfer efficiency. In contrast, under hydrophobic conditions, bubble nucleation is delayed, and the vapor shielding effect which is the formation of a vapor layer between the heat source and liquid, becomes more prominent. This insulating barrier significantly reduces heat transfer efficiency. For identical conditions, hydrophilic surfaces exhibit up to 90% enhancement in heat transfer compared to hydrophobic surfaces. Location of bubble nucleation shifts from middle of the channel to the closest of the heater wall as the wetting condition changes from hydrophilic to hydrophobic. After forming stable bubble nucleus, it propagates gradually in both directions, with upward propagation dominating over time, due to the cooling effect of liquid pool. Lower channel width facilitates bubble nucleation earlier due to the distribution of available heat within a small number of liquid atoms compared to the higher width. By reducing the channel width, nucleation time can be significantly advanced without changing the heating condition and wetting condition. These findings provide valuable insights into phase change heat transfer in bubble induced nanochannel flows, underscoring the pivotal roles of heat source temperature and surface wetting conditions regarding thermal performance of the system.Item Numerical investigation on effects of microporous transport layer in polymer electrolyte membrane water electrolyzers(Department of Mechanical Engineering (ME), BUET, 2025-01-07) Imtiaz Rais, Ahmed; Aman Uddin, Dr. Md.The Polymer Electrolyte Membrane Water Electrolyzer (PEMWE) is pivotal for efficient green hydrogen production through electrolysis. Cost is a major barrier to the widespread commercialization of PEMWE. Introducing a microporous layer (MPL) into PEMWE has immense potential to improve its performance, which could eventually reduce the overall cost. This study numerically investigates the effects of MPL on PEMWE performance after thorough validation of the model with recent experimental data. The study considers the role of MPL in liquid, gas, and charge transport, as well as the energy interactions within the system under various operational conditions. The findings indicate that the implementation of MPL in PEMWE improves liquid, gas, and charge transport, especially at higher current densities. Moreover, the study reports that the MPL enhances capillary pressure distribution, liquid water saturation, and the dissolved water content in the porous media. The MPL is found to significantly accelerate electrochemical reaction kinetics by increasing the triple-phase contact area. The findings also highlight the enhancement of gas pressure distribution across PEMWE due to the MPL. Additionally, lower MPL thickness, higher permeability, and increased exchange current densities are favorable for amplifying PEMWE performance. In essence, this study reveals the core mechanisms and interactions of governing parameters that optimize the performance of PEMWE with MPL.Item CHARACTERIZING THE ROLE OF ELECTRON-PHONON INTERACTIONS IN ULTRAFAST LASER ABLATION OF METAL(Department of Mechanical Engineering (ME), BUET, 2025-03-04) Mustakim Hayder, Md.; Arafat Rahman, Dr. KaziThis thesis investigates electron-phonon interactions in ultrafast laser ablation of nickel using hybrid two-temperature model with molecular dynamics (TTM-MD) simulations. The functional definition of subsystem properties and electronic-lattice coupling influences the accuracy of these simulations. The first phase determines the optimal description of thermophysical properties in the electron subsystem by comparing empirical definitions and Density Functional Theory (DFT) based values within the Two-Temperature Model (TTM). Results showed that with simplistic Beer-Lambert’s optical absorption modeling, empirically derived temperature-dependent parameterizations matched previous studies. Subsequently, hybrid TTM-MD ablation simulations were performed using two electron-phonon coupling approaches: temperature difference scaled coupling and Langevin thermostat. Analyses indicated that temperature difference scaled coupling leads to increased tensile stress due to artificial enhancement of the collision cascade, based on the deterministic nature of the coupling force. In contrast, the Langevin thermostat’s probabilistic nature predicts ablation threshold (115 mJ/cm², absorbed fluence) and phase explosion onset (270 ~ 300 mJ/cm²) for 1 picosecond laser pulses more accurately. Building on these insights, the second phase employs a sophisticated TTM-MD framework incorporating Generalized Langevin Dynamics (GLD) for wavevector-dependent coupling and a Helmholtz solver for precise optical absorption modeling. This optical model addresses the inferior performance of DFT-derived electronic properties, revealing that Beer-Lambert's law concentrates energy deposition near the surface, causing inaccurate elevated temperatures. The primary contribution of this research is the first implementation of TTM-MD with GLD coupling for laser ablation modeling, revealing anisotropic effects in ablation characteristics through phonon mode-dependent interactions. This approach predicts an ablation threshold of 300 mJ/cm² (incident fluence), closely aligning with experimental results. The model shows a yield decrease of approximately 20% when the laser propagates along the <110> versus <100> crystallographic direction at near-threshold fluences. These findings enhance understanding of electron-phonon interactions in ultrafast laser processes and highlight the crucial role of crystallographic orientation in ablation outcomes, with potential implications for optimizing laser ablation processes.Item Thermophysical and phase change characteristics of R152a/R1234ze(E) refrigerant blends through molecular dynamics simulation(Department of Mechanical Engineering (ME), BUET, 2025-03-11) Aminul Islam, Md.; Nasim Hasan, Dr. MohammadThe study of refrigerants and their blends has always been important due to their wide use and environmental impact. In this study, the thermophysical properties and nanoscale phase change characteristics of R152a/R1234ze(E) blends with varying molar fractions have been investigated using non-equilibrium molecular dynamics simulation (NEMD) under non-equilibrium heating conditions. In total, five different compositions of the refrigerant blend have been considered with the blends containing 100%, 67%, 50%, 33% and 0% molar fraction of R152a. Thermophysical properties such as density, liquid thermal conductivity and isobaric specific heat have been computed for the refrigerant blends considered. To study phase characteristics a three-phase domain is considered where liquid and vapor refrigerant molecules are placed over the solid platinum (Pt) surface. Two different heating conditions for the wall have been applied to induce various phase change modes. The first condition involved a linear increase in wall temperature, where temperature rise rates of 80, 160, and 240 K/ns are employed. The second condition maintained constant wall temperatures of 300 K and 320 K respectively. The phase change characteristics of different refrigerant blends under various heating conditions have been reported utilizing various parameters such as net evaporation number, time averaged heat flux, bubble growth, mean square displacement, and energy contours. Three distinct boiling modes are observed, depending on the heating rate and the mixture composition of the refrigerant blend. Moreover, under isothermal heating conditions, diffusive evaporation of the refrigerant blends took place. The results indicate that the increase in percentage of R152a in the refrigerant blend enhances the heat transfer performance. However, considering both environmental impact and thermal performance, refrigerant blend with %R152a of 67% exhibit better overall performance. Furthermore, if environmental considerations are stricter refrigerant blend up to 50% R152a percentage can be considered without significant decline in phase change performance. In addition, in this study the bubble nucleation process and interfacial characteristics of the refrigerant blends have also been examined to elucidate the mechanisms behind the phase change process. The interfacial behavior is analyzed in terms of interfacial thermal resistance, potential energy contours, and spatial density distribution. It is found that R1234ze(E) molecules exhibit a stronger attraction to the solid surface, resulting in lower interfacial thermal resistance.Item Thermomechanical behavior of reinforced 3D printed PLA polymer(Department of Mechanical Engineering (ME), BUET, 2025-03) Fabliha Mayesha; Chowdhury, Dr. ShahereenFused Deposition Modeling (FDM) 3D printing has become increasingly popular for producing parts that are both affordable and customizable. Among the materials commonly used in FDM, Polylactic Acid (PLA) is notable, but its mechanical performance is somewhat limited. To address these limitations, Carbon Fiber Reinforced PLA (CFRPLA) has been developed, incorporating a 15% weight fraction of carbon fiber. This study explores the effects of various critical FDM process parameters—such as raster angle, infill pattern, extruder temperature, layer thickness, and printing speed—on the mechanical properties of both PLA and CFRPLA. The findings from the experiments show that PLA offers higher strength and ductility compared to CFRPLA. However, CFRPLA demonstrates superior hardness and toughness in certain cases. Of all the parameters examined, raster angle and infill pattern were found to have the most significant impact on the tensile properties of both materials. To further investigate the materials' behavior, post-processing was performed through annealing at temperatures of 90°C, 100°C, and 120°C for durations of 3, 4, and 6 hours. The results show that annealing PLA and CFRPLA at 100°C for 4 hours optimizes strength and toughness with minimal standard deviation. CFRPLA also exhibits better thermal and dimensional stability. Further, Scanning Electron Microscopy (SEM) analysis of both the pre- and post-processed specimens highlighted microstructural changes influencing mechanical properties, while Differential Scanning Calorimetry (DSC) assessed thermal behavior and stability under heat treatment This research provides a comparative analysis of the effects of FDM process parameters and annealing on the properties of PLA and CFRPLA, offering valuable insights into the benefits and drawbacks of commercially available CFRPLA. Additionally, it demonstrates the potential of post-processing techniques in optimizing mechanical and thermal properties. By integrating mechanical testing with SEM and DSC analyses, this study contributes to a deeper understanding of material behavior, advancing FDM 3D printing in terms of material and process optimization.Item Ffect of inium and silver on microstructure, mechanical and thermal properties of tin-bismuth solder alloy(Department of Materials and Metallurgical Engineering (MME), BUET, 2024-11-05) Asif Mahmud Noman, Md.; Muktadir Billah, Dr. Md.This thesis explores the effects of adding indium (In) and silver (Ag) to tin-bismuth (Sn-Bi) solder alloys, focusing on their microstructural, thermal, and mechanical properties. As the electronics industry moves towards lead-free alternatives, optimizing solder alloy compositions becomes critical for performance and reliability. Through comprehensive analysis using Differential Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM), and mechanical testing, the study highlights significant enhancements brought by indium and silver additions. Indium contributes to a refined microstructure by promoting uniform phase distribution and stabilizing grain boundaries, leading to reduced segregation and smaller grain sizes. This uniformity enhances the alloy’s ductility and mechanical stability, making it less prone to cracking under stress. Silver, on the other hand, enhances tensile strength and hardness through the formation of intermetallic compounds such as Ag3Sn. These compounds improve load-bearing capacity and contribute to the alloy’s overall mechanical strength. Combined, the additions of indium and silver result in superior thermal fatigue resistance, crucial for electronic components subjected to repeated thermal cycles. Moreover, the improved joint reliability in soldered connections ensures more robust and durable electronic assemblies. This research provides valuable insights into the synergistic effects of indium and silver in Sn-Bi solder alloys, offering a pathway to developing more reliable and efficient lead-free solders for modern electronic applications. The findings pave the way for further innovations in solder alloy formulations, ensuring better performance and longevity of electronic devices in an increasingly demanding technological landscape.Item Numerical study of thermal and hydraulic performance of a two-phase inter-connected microchannel heat sink(Department of Mechanical Engineering (ME), BUET, 2025-01-18) Asaduzzaman Sourov, Md.; Morshed, Dr. A.K.M. MonjurThe increasing demand for efficient cooling solutions in high-performance electronics highlights the critical role of microchannel heat sinks (MCHS) with interconnected channels for advanced thermal management. Despite advancements, limited research has explored the influence of interconnectors design on flow distribution and their impact on optimizing thermal and hydraulic performance. This study addressed this gap by numerically investigating the hydrothermal behavior of interconnected MCHS with orthogonal and oblique interconnectors, using HFE-7100 as the working fluid, focusing on enhancing heat transfer and optimizing hydraulic performance. The investigation utilized the Lee mass transfer model and the Volume of Fluid (VOF) approach to capture phase-change dynamics and two-phase flow behavior under varying mass fluxes (280.4–1121.6 kg/m2s) and heat fluxes (40–60 W/cm2). Nine geometric configurations were analyzed and compared against a baseline parallel microchannel design, varying interconnector width and orientation while maintaining a constant aspect ratio of 1. The interconnectors promoted continuous thermal boundary layer disruption and improved mixing, reducing pressure drop penalties. Key findings revealed two distinct flow regimes: suction-induced flows that enhanced thermal performance and disruptive flows that impaired it. Practical designs use the mix of these two types, where both types of flow field co-exist. Case with 45° orientation and 50 μm interconnector width demonstrated superior performance, achieving a ~28% higher heat transfer coefficient and ~23% lower thermal resistance relative to the baseline, though with a ~15.5% increase in pressure drop. Conversely, Case with 135° and 100 μm interconnector width demonstrated reduced performance, achieving a ~15% lower heat transfer coefficient and ~14% higher thermal resistance relative to the baseline, with a ~19% decrease in pressure. These enhancements facilitated the effective dissipation of high heat fluxes while balancing the pumping power requirements. The study provides a comprehensive analysis of flow phenomena through contour visualizations of vapor fraction and velocity fields, as well as plots of thermal resistance, heat transfer co-efficient, wall superheat and other parameters, and flow instabilities emphasizing the critical influence of interconnector orientation and geometry. These findings underscore the potential of interconnectors design in balancing thermal and hydraulic performance and offer valuable design guidelines for the development of next-generation MCHS for high-performance thermal management applications.Item Numerical study on unsteady shock-induced mixing by partially modulated pulsating fuel jets in supersonic airflow(Department of Mechanical Engineering (ME), BUET, 2025-01-18) Aziz, Sarfaraz; Ali, Dr. MohammadThe supersonic combustion ramjet (Scramjet) engine is an engineering marvel for next-generation hypersonic flights. It typically operates at speeds greater than Mach 5 and utilizes the forward momentum of the flight to compress the incoming air. Even after compression, the air stream remains supersonic, and fuel-air mixing and combustion occur at the supersonic speed. Due to high streamwise momentum and short residence time, efficient mixing of fuel jets with incoming air has been a long concern for the development of scramjet-powered vehicles. In this thesis, the mixing of steady and partially modulated pulsating fuel jets in supersonic airflow is numerically investigated under different jet oscillating conditions. Gaseous hydrogen is injected as fuel and the pulsation is achieved by sinusoidally perturbing the injection pressure profile. The frequency and amplitude of pressure perturbation are varied in such a way that the jets are never off and hence can be categorized as partially modulated jets. The base pressure of oscillation is set as that of the equivalent steady jet to produce the same cycle-averaged mass flow rate of fuel for the direct comparison of two injection schemes. Injection frequencies of 4, 8, 16, 32, and 64 kHz are studied with amplitudes of 0.25, 0.50, and 0.75 times the base pressure. The flow fields are mathematically modeled using the Reynolds-Averaged Navier-Stokes (RANS) equations with additional equations for species conservation. Turbulence closure is obtained by applying the SST k − ω model. The results show complicated shock structures inside the combustor due to the fuel injection in supersonic airflow. Vorticity is produced at the jet shear layer and at the interface where the shock wave impinges the mixing plume. The shock waves become unsteady in response to the pulsation in fuel injection pressure. The concentration of fuel increases during the increasing phases of the pulse and quickly decays due to the back-and-forth oscillation of the bow shock, indicating unsteady shock-induced mixing inside the combustor. Both the penetration and mixing for the pulsed jets are higher than the equivalent steady jet irrespective of the injection frequency and amplitude. For the fuel injection frequency of 32 kHz with an amplitude of 0.75 times the base pressure, about 39% fuel-air mixing is obtained at the combustor exit which is 5% higher than the equivalent steady jet. The time-averaged results also reveal that the pulsed jets introduce no additional penalty in total pressure and thus unveil a minimally intrusive fuel delivery technique in supersonic airflow.Item Study of Mixing of Binary Mixtures of Granular Materials in a Rotating Drum by Image Analysis(Department of Mechanical Engineering (ME), BUET, 2024-03-10) Saiful Islam, Md.; Razzaque, Dr. Muhammed MahbuburIn this work an experiment study has been conducted to investigate the effects of the physical properties of two granular materials and the operating condition of the process of mixing of the binary mixture in a rotating drum on the quality of mixing. Four types of agricultural granular materials of different shapes were used in these experiments. These are rice, lentils, mung beans, and chickpea. The physical properties of the granular materials, such as particle size, shape, bulk density, static angle of repose, and dynamic angle of repose are measured. At least one dimension of the particles is about 4 - 5 mm. The shapes are characterized by a term called sphericity and its values for rice, mung beans, lentils and chickpeas are 0.43, 0.64, 0.78 and 0.92, respectively. The bulk densities are almost close to each other ranging from 1.565 gm/cc for lentils to 1.666 gm/cc for rice. The static angles of repose of rice, mung beans, lentils and chickpeas are 35.9, 32.8, 27.9 and 30.8, respectively. The angle of repose is found to decrease as the sphericity increases. However, in case of chickpeas with a high value of sphericity, the angle of repose is found to be higher than that expected, most likely due to the presence of a spike on the surface of chickpeas. The dynamic angle of repose of the granular particles is determined using a rotary drum apparatus of 14-inch diameter and 6 inch width. It has been found that the sphericity influences the dynamic angles of repose in the same fashion it influences the static angles of repose. In general, the dynamic angles of repose increase with the increase of speed for spherical and almost spherical particles. For particles with a low value of sphericity, the trend is opposite. In the second part of experiments, tests are conducted by mixing two granular materials placed in horizontal layers in the rotary drum. The experiments are performed in three different drum fill levels: 25%, 50%, and 75% and three rotational speeds: 1, 2 and 5 rpm and mixing time and mixing index are determined in each run. Among the operational parameters, fill level has the most dominant effect on the mixing time. At 25% fill level, the mixing time is the shortest for all four binary systems at all rpms. At a 50% fill level, the mixing time is slightly longer. However, mixing time is significantly longer at the 75 percent fill level in all experiments. The values of mixing index follow the same trend in reverse order. The particles of the granular materials have enough free space for avalanching and mixing at 25% fill level. During avalanching, granular particles attain enough kinetic energy, which helps them mix quickly. At 75% fill level, the granular materials have less free space for avalanching and have low kinetic energy during avalanching. In addition, a stagnant zone is developed around the center of the rotating drum. The mixing is slowed down by these two factors. As a result, mixing time at 75% fill level is significantly longer than that at 25% and 50% fill levels. For the very same reason mixing index is highest at 25% fill level and lowest at 75% fill level. With higher RPM, the avalanching frequency of the granular materials inside the rotating drum increases. This causes faster and good mixing and hence, less mixing time and higher mixing index. In case of the mixing of materials of dissimilar shapes, the mixing index is lower. Binary systems with smaller difference between the values of the sphericity as well as the static angle of repose have higher mixing index. In case of mixing of same particles, there is zero difference between the values of sphericity as well as the values of the angle of repose and very high values of mixing index are achieved.Item Exergy and thermo-economic study of A 412 MW natural gas-fired combined cycle power plant(Department of Mechanical Engineering (ME), BUET, 2023-07-26) Shariful islam, Md.; Haq, Dr. Md. ZahurulCombined cycle power plants are highly favored for their superior power output and efficiency. However, evaluating and optimizing these plants require more than just first law-based energy analysis; a combination of energy, exergy, and thermoeconomic analyses is necessary. This thesis presents such an analysis for the 412 MW Haripur combined cycle power plant, utilizing over 3.7 million real-time operational data points collected over a year. This extensive dataset allowed for the examination of seasonal impacts on performance parameters and the development of a correlation table for key metrics. The analysis found that gas turbines (GT) perform optimally at lower ambient temperatures, while steam turbines (ST) achieve their highest efficiency in hot, dry conditions. The plant’s peak efficiency was 60.19% Exergy analysis identified the combustion chamber as the primary source of irreversibility, accounting for about 70% of total exergy destruction, with an exergoeconomic factor of 3.82%. To mitigate this, it is recommended to match the compressor outlet and fuel temperatures before combustion. Although the condenser is a major source of heat loss in energy analysis, exergy analysis showed it has a high exergoeconomic factor of 95.32%, indicating minimal economic loss per unit of exergy destroyed. The thesis concludes with key observations and recommendations for optimizing the power plant under various ambient conditions.