Mechanical Engineering Graduate Office (meGRAD)

Title: EXPERIMENTAL CHARACTERIZATION AND MODELING OF FLAME HEAT FEEDBACK AND OXIDATIVE PYROLYSIS FOR SIMULATION OF BENCH SCALE FIRE TESTS

Author: Conor McCoy

Date/Time: Thursday, August 5th | 9:30am

Examining Committee:
Professor Stanislav I. Stoliarov, Chair
Professor Mohamad Al-Sheikhly, Dean’s Representative
Dr. Richard E. Lyon
Professor Arnaud Trouvé
Professor Bao Yang

Abstract: Two important bench scale fire tests, the cone calorimeter test and UL-94V, were characterized experimentally to allow for predictions using a numerical pyrolysis solver, ThermaKin2Ds with pyrolysis parameter sets. Flame heat feedback was measured in cone calorimeter tests for several polymers to develop a generalized flame model. Flame heat flux was measured in the center and near one side and was found to be 11–23 kW m^-2 and 32–49 kW m^-2, respectively. Based on the difference in measured heat flux, a center zone and a side zone were defined and separate models developed. The final model was an area-weighted combination of the center and side zone simulations. Heat release rate data were predicted well by the final model. Ignition times for low irradiation were not predicted well initially but a correction was made to account for the effect of oxygen. The UL-94V test required characterization of the flame heat feedback but also of the burner flame (temperature, heat flux, and oxygen content). UL-94V tests were performed using polymers of different flammability ratings to evaluate the model; some samples had insulated sides to investigate edge effects. Additional UL-94V tests performed with an embedded heat flux gauge served to measure polymer flame heat feedback. All UL-94V tests were recorded on video using a 900-nm narrow-band filter to focus on emissions from soot for tracking flame length over time. Flame heat fluxes of insulated PMMA samples confirmed a previously developed wall flame submodel, while non-insulated PMMA samples had significantly greater heat fluxes; the wall flame submodel was scaled accordingly. Burner flame oxygen content was measured to be about 5 vol% and was found to enhance decomposition of two materials; oxidation submodels were then developed accordingly. Overall, the model predicted flame spread on insulated UL-94V samples reasonably well but significantly underpredicted the results on non-insulated samples. Discrepancies were attributed to burning and spread on the edges which were not modeled explicitly. Finally, given the importance of oxidation on predictions of ignition time, oxidative pyrolysis was studied both in mg-scale and gram-scale pyrolysis experiments. Kinetic parameters were first developed based on inverse analysis of TGA tests in atmospheres of varied oxygen content. Two models were developed: a surface reaction model and a volumetric model. Mass flux data from gram-scale gasification tests were used to evaluate the models. The anaerobic model gave the best predictions of mass flux for 15 kW m^-2 gasification tests but the oxidative models gave better predictions for the 25 kW m^-2 gasification tests. The volumetric model gives better predictions unless mass transport of oxygen is considered in which case, the surface model gives better predictions.

Title: Experiments and Semi-empirical Modeling of Buoyancy-driven, Turbulent Frame Spread over Combustible Solids in a Corner Configuration.

Author: Dushyant Chaudhari

Day/Time: Aug 4, 2021 09:00 AM Eastern Time (US and Canada)

Examining Committee:
Professor Stanislav I. Stoliarov, Chair
Professor Christopher Cadou, Dean’s Representative
Professor Arnaud Trouvé
Professor Johan Larsson 
Dr. Isaac Leventon

Abstract: The increased use of engineered complex polymeric materials in the construction industry has highlighted their fire hazard. Standardized testing of materials, especially those in the developmental stage, is necessary for screening them for safe commercial application. However, testing can be expensive, hindering the process of development. This research aims to investigate the possibility of utilization of computational capability to predict fire hazard for facilitating screening of wall-lining materials in an important standardized configuration – a corner geometry without a ceiling. It also aims to fundamentally understand the dynamics of interactions between condensed-phase pyrolysis, gas-phase combustion, and flame heat feedback during concurrent, buoyancy-driven flame spread. Consequently, a series of hierarchical experiments and modeling from small-scale (to develop comprehensive pyrolysis models) to large-scale scenarios (to study flame spread fire dynamics) using samples having mass between a milligram to a kilogram were performed. Small-scale experimental data were inversely analyzed using a hill-climbing optimization technique in a comprehensive pyrolysis solver, ThermaKin. Large-scale experiments performed over a non-charring, non-swelling material with well-characterized condensed-phase pyrolysis – Poly (methyl methacrylate) (PMMA) – provided valuable data for fast-response (13 s response) calorimetry, well-resolved flame heat feedback at 28 locations, and radiation intensities at spectrally-resolved narrowband wavelength corresponding to soot emissions during the flame spread. An empirical flame heat feedback model obtained from large-scale experiments conducted over PMMA was then coupled with the pyrolysis model to develop a low-cost, fast, semi-empirical model for simulating fire dynamics during flame spread. The hierarchical experiments and modeling framework was further applied to two important wall-lining materials – Polyisocyanurate (PIR) foam and Oriented Strand Board (OSB) to scrutinize the robustness of the developed modeling framework. The study has presented a systematic methodology that reasonably predicted the fire dynamics in the large-scale tests over the three studied materials and can be judiciously extended to other materials. It has also emphasized the importance of significantly reducing pyrolysis parameter uncertainties, of understanding convection-radiation contribution to the flame heat feedback, and of investigating the use of an empirical flame heat feedback model as being fuel-independent to further improve the large-scale modeling predictions.

Title: MODEL-BASED ESTIMATION AND CONTROL FOR LITHIUM-SULFUR BATTERIES

Author: Chu Xu

Date/Time: 08/02/2021  12:00pm-2:00pm

Zoom: https://umd.zoom.us/j/9255782074

Examining Committee:
Dr. Hosam K. Fathy, Chair/Advisor
Dr. Chunsheng Wang, Dean’s representative
Dr. Balakumar Balachandran
Dr. Miao Yu
Dr. Michael G. Pecht
Dr. Paul Stephen Albertus

Abstract:
This dissertation examines the challenge of (i) estimating the internal states of lithium-sulfur (Li-S) batteries based on an experimentally-parameterized physics-based model, and (ii) optimizing the discharge trajectory to maximize the energy release of a Li-S battery over a fixed time horizon.
This research is motivated both by the potential of Li-S batteries to provide higher energy densities compared to traditional lithium-ion batteries and the potential of model-based estimation/control to improve the performance of a Li-S battery. Existing literature examines the problem of optimizing the underlying materials in Li-S batteries and develops models to furnish a fundamental understanding of the underlying reactions. The dissertation builds on the insights from the existing literature, and focuses on the control-oriented study/analysis of Li-S batteries.
This dissertation first explores the problem of parameterizing multiple zero-dimensional physics-based Li-S models, representing different sequences of reduction reactions, from experimental data. One of these models is found to offer the best tradeoff between fidelity and complexity. This model is used for online state estimation taking into consideration the multiplicity of active species in Li-S batteries. Accurate state estimation is found to be challenging in the low plateau region of the Li-S battery discharge curve due to the shallow slope of open circuit voltage with respect to state of charge (SOC) in this region. Fisher information analysis helps address this challenge by demonstrating the fundamental insight that battery SOC estimation accuracy can benefit from the dependence of battery resistance on SOC. Finally, this dissertation examines the problem of optimizing the discharge trajectory of a Li-S battery to maximize its energy release over a fixed time horizon. The overall outcomes of this dissertation include insights/algorithms that can be implemented into battery management systems to improve the performance of Li-S batteries.

Title: COUPLED OSCILLATOR ARRAYS: DYNAMICS AND INFLUENCE OF NOISE

Author: Abdulrahman Alofi

Day/Time: Wednesday July 28^th from 11:00 am to 1:00 p.m.

Examining Committee:
Professor Balakumar Balachandran, Chair and Advisor
Professor Amr Baz, Department of Mechanical Engineering
Professor Abhijit Dasgupta, Department of Mechanical Engineering
Professor Nikhil Chopra, Department of Mechanical Engineering
Professor Sung Lee, Department of Aerospace Engineering (Dean’s Representative)

Abstract: Coupled oscillator arrays can be used to model several natural systems and engineering systems including mechanical systems. In this dissertation work, the influence of noise on the dynamics of coupled mono-stable oscillators arrays is inves tigated by using numerical and experimental methods. This work is an extension of recent efforts, including those at the University of Maryland, on the use of noise to alter a nonlinear system’s response. A chain of coupled oscillators is of interest for this work. This dissertation research is guided by the following questions: i) how can noise be used to create or quench spatial energy localization in a system of coupled, nonlinear oscillators, and ii) how can noise be used to move the energy localization from one oscillator to another. The coupled oscillator systems of inter est were harmonically excited and found experimentally and numerically to have a multi-stability region (MR) in the respective frequency response curves. Relative to this region, it has been found that the influence of noise depends highly on where the excitation frequency is in the MR. Near either end of the MR, the oscillator re-sponses were found to be sensitive to noise addition in the input and it was observed that the change in system dynamics through movement amongst the stable branches in the deterministic system could be anticipated from the corresponding frequency response curves. The system response is found to be robust to the influence of noise as the excitation frequency is shifted toward the middle of the MR. Also, the effects of noise on different response modes of the coupled oscillators arrays were investigated. A method for predicting the behavior is based on so-called basins of attractions of high dimensional systems. Through the findings of this work, many unique phenomena are introduced under the influence of noise, including spatial movement of an energy localization to a neighboring oscillator, response movement gradually up the energy branches, and generation of energy cascades from a localized mode.

Title: Design and Characterization of Additively Manufactured Lightweight Metal Structures with Equivalent Compliance and Fatigue Resistance

Author: Luis Santos

Day/Time: Monday, July 19th, 2021 at 3:00 PM EST

Committee members: 
Professor Hugh A. Bruck (Chair)
Professor Abhijit Dasgupta
Professor Mark D. Fuge
Professor F. Patrick McCluskey
Professor Sreeramamurthy Ankem (Dean’s Representative)

Abstract:
Additive Manufacturing (AM) has been a disruptive manufacturing technology allowing for control of geometric features and material distributions, potentially starting at the atomistic level, to realize structures with lighter weights. However, it is still begin used primarily as a rapid prototyping tool due to challenges arising from various issues that need to be addressed before commercial parts can be deployed. Three of those issues are: (1) characterization of mechanical properties that may vary spatially, (2) identification of novel defects in the parts, and (3) new design approaches that account for the unique capabilities of AM processes and their impact on fatigue resistance.

This dissertation addresses these three issues by developing a cyclical indentation technique to characterize the fatigue properties of geometric features only capable with AM. The method produces the degradation of the material stiffness as the number of cyclic loads increases and is capable of generating an entire S-N curve with a single test at sub-millimeter scales. Geometric features are then analyzed by running a thermal and mechanical simulation of a Direct Metal Laser Sintering (DMLS) printing process. The new simulation can account for buckling of features with high aspect ratios, such as low percentage infills or high levels of unit cell porosity, and predicts distortions with less than 5% error. This computational approach is useful for analyzing parts before printing and informs designers about regions in the part that may need modification to prevent buckling. Finally, the experimental and computational techniques are combined to design structures with macroscale topological features and microscale unit cell features that are fatigue resistant.

Title: Studies of Inclined Flame Instabilities and the Relationship Between Wildland Fire Exposure and Structure Destruction

Day/Time: Friday, July 23, 2021 at 12:00 pm EST

Committee members: 
Dr. Michael Gollner (Chair)
Dr. Anya Jones (Dean’s rep)
Dr. Ken Kiger
Dr. Johan Larsson
Dr. Elaine Oran, Arnaud Trouvé

Abstract: The present work investigates two aspects of the wildland fire problem: the structure and stability of inclined flames and the application of reconstruction fire modeling to a new methodology of structure level risk analysis in the wildland-urban interface. The work analyzes the structure and stability of a laminar diffusion flame that forms either on the top of or beneath a semi-infinite inclined fuel surface. Experiments have found substantial structural differences between flames developing on the upper and lower sides of an inclined fuel surface. These differences cannot be explained with current analytical models of steady semi-infinite flames, which provide identical solutions for both configurations. The role of instabilities is investigated as they influence the development of structures, such as peaks and troughs in the flame, observed downstream after the transition to turbulence. These structures influence heat transfer processes that govern pre-heating of downstream fuels and, as a result, drive flame spread. The formulation used to investigate instability formation utilizes the limit of infinitely fast reaction, taking into account the non-unity Lewis number of the fuel vapor. The solution of the stability eigenvalue problem determines the downstream location at which the flow becomes unstable, characterized by a critical Grashof number. The analytical solution finds that instabilities emerge downstream in the flame, developing farther downstream on the lower side of the incline as compared with the flames developing on the upper side of the inclined surface, in agreement with existing experimental observations.

The latter study investigates the use of reconstruction modeling to reproduce exposure conditions to structures from a wildland fire using the 2017 Northern California Tubbs Fire as a case study. The reconstruction simulates the distribution of embers, small pieces of burning material or “firebrands” lofted in the fire plume, which can ignite upon deposition. Including embers expands the ability of fire reconstruction to represent conditions during the fire event which are not represented by the flaming fire front. Results from the Tubbs Fire simulation are used to provide exposure conditions in a subsequent study investigating the relation between exposure conditions, structure characteristics, and the damage sustained by a structure in the fire event. A methodology using fragility curves to estimate the probability of destruction, used for risk analysis in other disaster fields, is modified and developed here for application to wildland-urban interface fires. Results of the fragility analysis find that increased fire exposure (represented as flame length) and ember exposure increase the likelihood of damage or destruction; however, there is a stronger relationship between ember exposure and destruction than between flame length and damage or destruction. It is also found that relatively low levels of ember exposure still result in relatively high likelihoods of destruction, highlighting the importance of ember spread. Limitations still exist, such as the inability to model structure-to-structure fire spread, but are highlighted as needed for future work.

Title: Computational Modeling of Nanoscale Vibrational Energy Transfer in Crystalline RDX

Author: Gaurav Kumar

Date/Time: 20th July 2021 10:00 AM – 12:30 PM

Examining Committee:

Title: Advanced Packaging and Thermal Management of DC-DC Converters and Novel Correlations for Manifold Microchannel Heatsinks

Author: Sevket Umut Yuruker

Date/Time: July 2nd, 3:00pm – 5:00pm

Zoom link: https://umd.zoom.us/j/4052654764

Committee Members:
Professor Michael Ohadi, Chair
Professor Patrick McCluskey
Professor Jungho Kim
Professor Amir Riaz
Professor Christopher Cadou, Dean’s Representative

Abstract: An advanced packaging configuration of a dual-active-bridge 10 kW DC-DC converter module is introduced in this dissertation. Through utilization of novel heatsinks for the power switches and the transformer assembly, ~20 kW/Lit converter volumetric power density based on numerical and experimental analysis is obtained. Through a unique placement of the high power/high frequency SiC switches on the printed circuit board, many beneficial features such as double-sided cooling, complete elimination of wirebonding, circumvention of the need for TIM layers between the switches and the heatsinks, and multi functioning heatsinks as electrical busbars is achieved.

A Vertically Enhanced Manifold Microchannel System (VEMMS) cooler is developed to address the thermal challenges of a pair of power switches, simultaneously. Both air and liquid cooled versions of VEMMS cooler is presented, thermal resistances of 1.1 K/W and 0.3 K/W for the air and liquid cooled versions, respectively, at reasonable flow rates and pressure drops was obtained. Besides the power switches, thermal management of the transformer assembly is accomplished via Combined Core and Coil (C3) Coolers, where both the magnetic core and coils are liquid cooled simultaneously with electrically insulating but thermally conductive 3D printed Alumina heatsinks, where thermal resistances as low as 0.3 K/W for the magnetic core and 0.09 K/W for the transformer windings is experimentally demonstrated. Furthermore, a system level model was built to investigate the effect of various components in the cooling loop on each other, and what are the limiting factors to prevent a possible thermal runaway failure.

Lastly, using a metamodeling approach, closed form pressure drop and heat transfer correlations are developed for thermo-fluidic performance prediction of manifold microchannel heatsinks. Due to complexity and vastness of design variables present in manifold microchannel systems, adequate CFD analysis and optimization require significant computational power. Through utilization of the developed correlations, orders of magnitude reduction in computational time (from days to milliseconds) in prediction of pressure drop and heat transfer coefficient is demonstrated. Extensive mesh independence and residual convergence algorithms are developed to increase the accuracy of the created database. Between the correlation predictions and mesh independent CFD results for the entire metamodel range, a mean error of 3.9% and max error of 24% for Nusselt number, and a mean error of 4.6% and max error of 37% for Poiseuille Number are achieved.