Mechanical Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2795
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Item Simulation and Analysis of Energy Consumption for Two Complex and Energy intensive Buildings on UMD Campus(2019) Kelly, Jason; Ohadi, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Physical Sciences Complex and Eppley Recreational Center are multi-purpose buildings which are complex in functionality and are among the highest consumers of energy on the UMD campus. Building energy analyses used to identify energy efficiency measures to optimize energy efficiency in the buildings. Detailed building energy models were developed in EnergyPlus and OpenStudio that sought to mimic current operations of the buildings. PSC model results deviated respectively -1.05%, 1.19%, and 5.27% for electricity, steam, and chilled water. ERC model results deviated respectively 0.47%, 5.3%, and 2.2% from annual electricity, hot water, and gas. Four energy efficiency measures for the Physcial Sciences Complex provided energy model predicted energy savings of 3,757 MMBtu or 7.5% of the building’s energy consumption. Four efficiency measures were identified for the Eppley Recreation Center with energy model predicted energy savings of 3,390 MMBtu or 8.4% of the building’s energy consumption.Item ADVANCED MODELING AND REFRIGERANT FLOW PATH OPTIMIZATION FOR AIR-TO-REFRIGERANT HEAT EXCHANGERS WITH GENERALIZED GEOMETRIES(2019) Li, Zhenning; Radermacher, Reinhard K; Aute, Vikrant C; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Air-to-refrigerant heat exchangers are key components of the heating, ventilation, air-conditioning and refrigeration systems. The evolving simulation and manufacturing capabilities have given engineers new opportunities in pursuing complex and cost-efficient heat exchanger designs. Advanced heat exchanger modeling tools are desired to adapt to the industrial transition from conventional refrigerants to low Global Warming Potential (low-GWP) refrigerants. This research presents an advanced heat exchanger performance prediction model which distinguishes itself as a cutting-edge simulation tool in the literature to have capabilities, such as to (i) model heat exchangers with variable tube shape and topology, (ii) improved numerical stability, (iv) multiple dehumidification models to improve evaporator prediction, and (v) CFD-based predictions for airflow maldistribution. Meanwhile, HX performance is significantly influenced by the refrigerant flow path arrangements. The refrigerant flow path is optimized for various reasons such as to (i) mitigate the impact of airflow maldistribution, (ii) reduce material/cost, (iii) balance refrigerant state at the outlet of each circuit, and (iv) ensure overall stable performance under a variety of operating conditions. This problem is particularly challenging due to the large design space which increases faster than n factorial with the increase in the number of tubes. This research presents an integer permutation based Genetic Algorithm (GA) to optimize the refrigerant flow path of air-to-refrigerant heat exchangers. The algorithm has novel features such as to (i) integrate with hybrid initialization approaches to maintain the diversity and feasibility of initial individuals, (ii) use effective chromosome representations and GA operators to guarantee the chromosome (genotype) can be mapped to valid heat exchanger designs (phenotype), and (iii) incorporate real-world manufacturability constraints to ensure the optimal designs are manufacturable with the available tooling. Case studies have demonstrated that the optimal designs obtained from this algorithm can improve performance of heat exchangers under airflow maldistribution, reduce defrost energy and assure stable heat exchanger performance under cooling and heating modes in reversible heat pump applications. Comparison with other algorithms in literature shows that the proposed algorithm exhibits higher quality optimal solutions than other algorithms.Item OPTIMAL SCHEDULING OF RESIDENTIAL DEMAND RESPONSE USING DYNAMIC PROGRAMMING(2019) Moglen, Rachel Lee; Gabriel, Steven A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Electricity price volatility in the Electricity Reliability Council of Texas (ERCOT) poses significant financial threat to many players in the electricity market, though most of the financial burden falls on the retail electric providers (REPs). REPs are contractually obligated to purchase and provide all electricity that the end-user wishes to consume, even when the purchase of this electricity brings them financial losses. Electricity prices in ERCOT can increase from typical ranges ($30/MWh) to $3,000/MWh in as little as 15 minutes, causing REPs to seek mitigation techniques to avoid paying price spike prices for electricity. We explore two techniques for REPs to schedule mitigation techniques: a price prediction-based heuristic approach, as well as an optimal scheduling algorithm using dynamic programming (DP). We aim to optimally schedule these mitigation techniques which shift load from high price periods to times of lower prices, called demand response (DR). To achieve this load shifting, REPs remotely manipulate internet-connected thermostats of residential customers, thereby controlling a fraction of residential HVAC load. We found that the price prediction approach was highly unreliable, even for predicting prices as near as 5 minutes out. We therefore chose to rely on the DP as the primary scheduling model. By applying the DP deterministically to historical electricity price and weather data, the load-shifting technique is shown to potentially improve REP profit margins by 10% to 25% per customer annually. Most of these savings come from a few crucial events, highlighting the usefulness of the DP and the importance of accuracy in the timing of DR events. Due to the uncertainty in electricity prices, we apply a multi-objective approach considering the REP’s conflicting objectives: maximizing savings and minimizing financial risk. Results from this multi-objective formulation point to shorter duration DR events in the evening being the least risky, with additional savings possible through riskier short midday events. To ensure that REPs could apply our DP formulation for use in near real-time decision-making applications, the computation speed was verified to be under one second for 24 stages (i.e., 1-hour intervals for one day.)Item Soot Oxidation in Flames: Nanostructure, Morphology, and Chemical Kinetics(2019) Anderson, Paul Marcus; Sunderland, Peter B.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Soot produced from the combustion of hydrocarbons is of immense scientific interest owing to its deleterious effects on human health and the environment. Despite decades of research, existing soot models are accurate across only a narrow range of combustion conditions. A substantial portion of this inaccuracy is rooted in the multitude of factors affecting soot oxidation that remain ill-understood. In the current work, a novel flame system allowed soot oxidation to be observed in isolation from competing soot formation processes. Measurements tracked evolving soot structures, oxidation rates, temperatures and gas species concentrations. Transmission electron microscopy (TEM) was used to characterize soot structure at aggregate, primary particle, and nanostructural scales. For this, a program called Aerosol Image Analyzer was developed, incorporating new algorithms for processing and measuring TEM images of mass-fractal aerosols, like soot. For the first time, TEM image measurement uncertainties incorporating sample, operator, and random effects, were quantified through gage repeatability and reproducibility analysis. Successful methods for reducing operator bias were presented, and automated measurement methods from literature were tested and found to be unreliable. Measurements of surface area by N2 adsorption validated TEM as a technique for determining soot specific surface area, provided that the polydispersity and partial sintering of primary particles is taken into account. TEM measurements of soot undergoing oxidation showed continuously decreasing primary particle size distributions and increasing specific surface area. Measurements of soot aggregate morphology found a fractal dimension of 1.74 that was unchanged by oxidation. The breakup of aggregates by oxidative fragmentation was observed for the first time using methods that combined TEM analysis with laser extinction. Soot nanostructure was characterized through high resolution TEM measurements of lattice fringe length, tortuosity, orientation, and separation distance. It was observed that primary particles could be divided into an inner 80%, where lattice fringes showed greater graphitic order with increasing radial location, and an outer 20%, where this trend was reversed. While oxidation proceeded in a shrinking-sphere manner at the particle surface, the interior underwent thermal and oxidation-induced graphitization, challenging the assumption that the nanostructure of mature soot is “locked-in.” This results in a surface nanostructure that is effectively unchanging from the perspective of the oxidizing gases and corresponds to a constant collision efficiency kinetics model.Item TRITIATED NITROXIDE FOR BETAVOLTAIC CELL NUCLEAR BATTERY: 3D BETA FLUX MODELING, SYNTHESIS, STABILITY ANALYSIS, AND COATING TECHNIQUES(2019) Russo, John A; Bigio, David; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Beta (β-) radioisotope energy sources, such as tritium (3H), have shown significant potential in satisfying the needs of a sensor-driven world. The limitations of current β- sources include: (i) low beta-flux power, (ii) intrinsic isotope leakage (instability) and (iii) beta self-absorption. The figure of merit is the β--flux power (dPβ/dS), where an optimal portion of incident beta particles penetrates the semiconductor depletion region. The goal of this research was to identify a compound to contain a beta emitter that can permit beta-flux power of at least 733nW/cm2 from one side, where it can be used for both planar and high aspect ratio microstructure technology (HARMST) transducers. Nitroxides were chosen because of previous demonstrated deuteration, ease of synthesis, diversity of structure, and pliability. A 6-membered nitroxide was prepared and tritiated with a specific activity of 103Ci/g. The product was stable after 256 days with only 2% tritium loss. A betavoltaic (βV) cell nuclear battery prototype was demonstrated with the tritiation of a 5-membered nitroxide stable in liquid and solid form and a specific activity (Am) of 635Ci/g, the highest recorded Am for an organic compound. A dPβ/dS of 51 nW/cm2 and 102 nW/cm2 were generated when dispensed on βV (4H-SiC) and PV (InGaP) cells. Improvements to increase the dPβ/dS closer to the theoretical limit were identified and demonstrated such as dispensing with different solvents to reduce evaporation time, and increasing solute (nitroxide) concentration in the dispensed solution. A βV cell nuclear battery model was developed, producing a blueprint on what nitroxide characteristics are required to maximize the dPβ/dS and electrical power density (P_(e,vol)). The percent error and percent difference were less than 6% compared to experimental data and other models. For the planar coupling configuration, increasing Am while increasing mass density increases P_(e,vol). Increasing the surface area interaction with the radioisotope and transducer increases the volume radioactivity (Vm), but does not always generate a higher β- source efficiency nor P_(e,vol) compared to the planar coupling configuration. The rectangular pillar array produced the highest 4.54 mW/cm3 of at the highest Vm where HARMST feature width and gaps are proportionally minimized at 100 nm wide.Item A Methodology to Estimate Retrofit Energy Savings Using A Reduced-Order Energy Modeling Approach(2018) Nagda, Harshil; Srebric, Jelena; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Building energy retrofits help to reduce energy use and improve energy efficiency of the buildings, however, most building owners typically consider retrofit implementation as a financial decision rather than an environmental one. Thus, to upgrade the existing buildings, it is extremely important to make accurate predictions of energy and cost savings which can help the building owners and facility managers to make capital budgeting decisions. The study proposes a methodology using reduced-order energy modeling approach to make rapid and accurate estimations of energy savings from retrofit installations in a building portfolio. A case study of 7 campus buildings undergoing several lighting, envelope and HVAC retrofits, and costing $3.6M to the university, is demonstrated in this thesis. The actual energy savings from the retrofits are compared with the modeled energy savings estimated using the proposed methodology.Item Refrigerant Charge Distribution in Unitary Air-conditioning Units(2018) Qiu, Tianyue; Hwang, Yunho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)To address the global warming, reduction of refrigerants charge in HVAC systems is becoming an essential topic. An experimental setup was designed and built for investigating the cyclic degradation performance and the refrigerant charge distribution of a unitary air-conditioning unit. A code tester featuring two parallel ducts was developed to properly control air flows according to the ASHRAE standard test conditions. First, baseline tests were carried out under ASHRAE standard test conditions. System performance was compared to manufacturer’s specifications to validate the accuracy of the experimental setup. Second, the transient performance of the unit was analyzed and presented, and the degradation coefficient of the unit was evaluated. Third, an approach of refrigerant charge distribution measurement was proposed and implemented. Refrigerant R410A charge distribution tests were conducted under steady-state conditions and the results were compared with published results of the refrigerant charge distribution in various types of air-conditioning units. It was concluded that the system charge distribution was significantly influenced by various component combinations and the length of refrigerant copper tubing.Item COMPACT ABSORBER FOR ADVANCED ABSORPTION HEAT PUMPS(2018) Bangerth, Stefan; Ohadi, Michael M; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Almost half of all energy contained in primary energy carriers is discarded as low temperature waste heat. One of few application areas for low temperature waste heat recovery is to drive absorption cooling systems for conversion of waste heat to cooling energy. However, absorption chillers are often not economical due to their bulky, and hence expensive, heat and mass exchangers; the absorber heat/mass exchanger being the largest among them. This dissertation introduces original contributions to advance next generation, more economical absorption chillers by utilizing a novel, highly compact absorber. The novel absorber designed in this work enhances absorption performance by combining rotation of the heat transfer surface for solution-side heat and mass transfer enhancement with innovative high-performance heat transfer technology on the water-side. A numerical model was developed to describe the absorption process and promote design optimization. The replacement of gravitation force by the stronger centrifugal acceleration thins and mixes the solution film and thereby decreases solution-side thermal and mass transfer resistance. The development of an original adaptation of manifold-microchannel technology leads to significant water-side heat transfer enhancement. This dissertation includes the first publication of an experimental characterization of exothermic absorption on a spinning disk. The overall and film-side heat transfer coefficients were 4.7 and 5.5 times higher, respectively, than conventional horizontal tube banks. The absorption rate increased by a factor of 4 to 10 folds over those of the conventional tube absorbers. The power required for spinning the disk was modest and ranged between 1.1% and 2.3% of the cooling capacity. The results suggest that a spinning disk absorber could substantially reduce the size of absorber in the absorption machines. The technology developed in this dissertation can lead to more compact and hence more economical absorption chillers, thereby easing higher market penetration of absorption chillers which in turn can reduce the amount of primary energy spent on cooling applications. Spinning disk absorbers may be especially useful if combined with a new generation of absorbents that promise improved system efficiency and/or expanded application range but exhibit challenging thermophysical properties.Item Dynamics of Vapor Compression Cycle with Thermal Inertia(2018) Dhumane, Rohit; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The use of heating, ventilation, air conditioning, and refrigeration (HVACR) systems is always increasing. Reducing energy consumption has become necessary in modern times for environmental, economic and legislative reasons. Thus, there is ongoing research to improve the performance and reduce the negative environmental impact of these systems. HVACR systems are normally sized for peak load conditions. As a result, these systems operate under off-peak conditions most of the time by on-off cycling. The average efficiency of the system during cycling is lower due to transient losses caused by refrigerant migration and redistribution. This motivates a detailed understanding of the dynamics of vapor compression systems (VCS) for their improved design and performance. The dissertation contributes towards reducing energy consumption from HVACR by exploring both sides: improving the performance of current systems and developing highly energy efficient personal conditioning systems (PCS) which reduce the load of HVACR systems altogether. PCS reduce the energy consumption of building HVACR by up to 30%. Multi-physics modeling for thermo-fluid, electricity and mechanical domains is conducted to compare performance of four PCS employing different thermal storage options. The dissertation then focuses on vapor compression system based version of PCS called Roving Comforter, operating cyclically between its cooling and recharge mode. Exhaustive study of design space including optimization of thermal storage, operation with a natural refrigerant and alternate recharge modes is conducted to improve its overall coefficient of performance. The dissertation then presents comprehensive dynamic validated modeling of air-conditioning systems operating in cyclic operations to characterize cyclic losses. Parametric study with different operating conditions is carried out to provide guidelines for reduction of these cyclic losses. Secondly, a physically based model of the test setup for quantifying the cyclic losses of air-conditioning systems is developed and used to understand its influence on the cyclic losses. A new term called “Thermal Inertia Factor” is defined to enable more uniform rating of equipment from various test centers and help selection of actual energy efficient air-conditioners.Item ENHANCING THE COMBUSTION CHARACTERISTICS OF ENERGETIC NANOCOMPOSITES THROUGH CONTROLLED MICROSTRUCTURES(2018) Jacob, Rohit; Zachariah, Michael R; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Metastable Intermolecular Composites (MIC’s) are a relatively new class of reactive materials which, through the incorporation of nanoscale metallic fuel and oxidizer, have exhibited multiple orders of magnitude improvement in reactivity. Although considerable research has been undertaken, their reaction mechanism is still poorly understood, primarily due to the complex interplay between chemical, fluid mechanic and thermodynamic processes that happen rapidly at nanoscale. For my dissertation, I have attempted to tackle this problem by employing controlled nanomaterial synthesis routes and optical diagnostics to identify the dominant underlying mechanisms. I begin my investigation by examining the nature of metal nanoparticle combustion wherein, I employed laser ablation to generate size- controlled aggregates of titanium and zirconium nanoparticles and studied their combustion behavior in a hot oxidizing environment. The experiments revealed the dominant role of rapid nanoparticle coalescence, before significant reaction could occur, resulting in a drastic loss of nanostructure. The large-scale effects of sintering on MIC combustion was explored through a forensic analysis of reaction products. Electron microscopy was employed to evaluate the product particle size distributions and focused ion beam milling was used to expose the interior composition of the product particles. The experiments established the predominance of condensed phase reaction at nanoscale and the interior composition revealed the poor extent of reaction due to rapid reactant coalescence before attaining completion. In light of such limitations, the final part of my dissertation proposes a solution to counteract rapid, premature coalescence through the synthesis of smart nanocomposites containing gas generating (GG) polymers. The GG acts as a binder as well as a dispersant, which disintegrates the composite into smaller clusters prior to ignition, thereby avoiding large scale loss of nanostructure. High speed optical diagnostics including an emission spectrometer and a high-speed color camera pyrometer were developed to quantify the enhanced combustion characteristics which indicate an order of magnitude improvement in reactivity over counterparts using commercial nanomaterials. Moreover, thermal pretreatment as a possible bulk processing strategy to improve nanoaluminum reactivity in a MIC is examined, where a 1000% increase in reactivity was observed compared to the untreated case. Finally, composites of nanoaluminum and reactive fluoropolymers (PVDF) are examined as a possible candidate for energetic material additive manufacturing (EMAM) and its viability is demonstrated by 3D printing and characterizing reactive multilayer films.