Mechanical Engineering
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Item DEVELOPMENT OF VARIABLE TUBE GEOMETRY HEAT EXCHANGERS USING ADJOINT METHOD WITH PERFORMANCE EVALUATION OF AN ADDITIVELY MANUFACTURED PROTOTYPE(2023) Klein, Ellery; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Air-to-refrigerant heat exchangers are a key component for heating, ventilation, air conditioning, and refrigeration (HVAC&R) systems. The performance of these heat exchangers is limited by their air-side thermal resistance. Finless non-round bare tube designs have the potential to improve the air-side thermal-hydraulic performance over their finned counterparts and consequently improve the coefficient of performance (COP) of air-conditioning systems. Previous researchers have used heuristic methods such as multi-objective genetic algorithms (MOGA) with approximation-assisted optimization (AAO) methods utilizing computational fluid dynamics (CFD) based metamodels to shape and topology optimize non-round bare tube heat exchangers. A rather unexplored optimization technique used for heat exchanger optimizations is the gradient based adjoint method. CFD solvers utilizing discrete adjoint methods can be used to shape optimize bare tube heat exchangers and can reveal unintuitive, organic, and potentially superior designs. Additionally, additive manufacturing technology has the capability of building these previously unrealizable heat exchanger designs.The objectives of this dissertation are to experimentally evaluate the performance of shape and topology optimized compact bare tube heat exchangers with non-round bare tubes on a 1) component level, and 2) system level integrated into an air conditioner. Plus, 3) develop new shape optimized variable geometry compact bare tube heat exchangers using discrete adjoint methods for HVAC&R applications. First, a comprehensive experimental investigation of multiple shape and topology optimized compact non-round bare tube heat exchangers was conducted under dry and wet evaporator, condenser, and radiator conditions. For all heat exchangers, air-side pressure drop and heat transfer capacity were predicted within 37% and 15%, respectively. Next, an experimental test facility capable of evaluating the system level performance of a 7.03-8.79 kW commercial packaged air conditioning unit was designed and constructed. The performance of the air conditioning unit was evaluated before and after its conventional tube-fin evaporator was replaced with a shape and topology optimized bare tube evaporator. Results are presented and discussed. Lastly, an ε-constraint and penalty method optimization scheme was implemented which utilizes a commercial CFD software with a built-in discrete adjoint solver to perform multi-objective shape optimizations of 2D bare tube heat exchangers. Critical solver/mesh set-up to best optimize heat exchangers with 0.5-10.0 mm diameter bare tubes were identified and established. The optimized designs can achieve a 40-50% reduction in air-side pressure drop with at least the same heat transfer capacity compared to the initial circular bare tube geometry. An adjoint shape optimized 500 W bare tube radiator was additively manufactured in polymer and experimentally tested. Air-side pressure drop and heat transfer capacity were predicted within 15% and 10%, respectively. The experimental performance confirms the adjoint method shape optimized designs improve the thermal-hydraulic performance over the initial circular bare tube geometry.Item DATA-DRIVEN ANALYSIS OF INDIVIDUAL THERMAL COMFORT WITH PERSONALIZED COOLING(2018) Dalgo, Daniel Alejandro; Srebric, Jelena; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents numerical and experimental results on the effects of Personal Cooling Devices (PCDs) on the energy consumption of buildings and the thermal comfort of occupants. The objective of this analysis was to quantify the tradeoffs of thermal comfort and energy savings associated with PCD technology. Furthermore, this investigation included an electrical cost analysis associated with PCDs at the building level for different cities across the United States. The results of energy and cost analyses, at the building level, indicated the potential for cooling energy and cost savings associated with shifting the electricity consumption during the peak hours to the off-peak hours of the day. The numerical analysis of human thermal comfort demonstrated the potential for PCDs to regulate human thermal comfort at warm environmental conditions. The thermal comfort level achieved in the numerical simulations were within the limits recommended by ASHRAE Standard 55. In addition, the numerical simulations permitted the evaluation of PCD performance based on thermal comfort, and the amount of sensible heat remove from the human body. The experimental work evaluated the performance of PCDs using both subjective and objective measurements of thermal comfort for 14 human subjects. The results demonstrated the ability of a PCD to change and maintain acceptable thermal comfort micro-environments for human subjects under warm conditions. Furthermore, the results showed that a PCD had measurable effects on physiological variables that control the thermoregulatory process of the human body. Specifically, variables such as skin temperature and heart rate variability in the time and frequency domain responded to the micro-environment created by the PCD. This research established a relationship between skin temperature, heart rate variability, and thermal comfort. Overall, this investigation performed a comprehensive analysis of the interaction of PCDs with: building energy consumption, human subjects, and human physiological processes; and demonstrated the potential to recognize human subjects’ thermal comfort based on physiological signals.Item Predicting Fire Sprinkler Sprays(2018) Myers, Taylor; Marshall, Andre W; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Accurate representation of fire sprinkler spray enables quantitative engineering analysis of fire suppression performance. Increasingly, fire sprinkler systems are evaluated using computer fire models in which sprinkler spray is simulated with Lagrangian particles. However, limited guidance exists as to how to predict the formation of complex, spatio-stochastic fire sprinkler spray or how to accurately represent the dispersion of spray in terms of Lagrangian particles. The current work predicts the fire sprinkler spray generated by a canonical axisymmetric sprinkler using a Deflection Atomization Dispersion (DAD) framework, developed as a predictive modeling approach generalizable to typical fire sprinklers. In a DAD framework, spray evolution is divided into three stages: deflection of the water jet by the sprinkler deflector, atomization of the resulting thin fluid sheets into an initial spray, and dispersion of the initial spray into far-field spray. Deflection is described as a free-surface flow and is modeled deterministically using a boundary integral method (BIM). Atomization of the deflected fluid sheet is described by linear-stability theory to develop scaling laws relating sheet characteristics to statistically distributed, spatially resolved initial spray characteristics including breakup radius, volume flux, drop size, and drop velocity. The resulting initial spray is then described by a multivariate probability distribution function that varies over the predicted initialization surface. This function is stochastically sampled to generate Lagrangian particles representative of the near-field spray and the dispersion of these Lagrangian particles is in turn simulated in FireFOAM (an open source computational fluid dynamics fire model) to predict the far-field spray. Modeled results are compared to highly resolved near- and far-field measurements of axisymmetric sprinkler sprays generated by the Spatially-Resolved Spray Scanning System (4S). The end results shows agreement across all three stages of modeling with less than 10\% error when compared to experimental measurements. Further, the newly implemented model shows a stronger ability to capture spray induced airflow when compared to a baseline model. This work is the first to predict sprinkler spray dispersion entirely from sprinkler deflector geometry and operating pressure.Item Quantifying the impacts of urban wind sheltering on the building energy consumption(2016) Khoshdel Nikkho, Saber; Srebric, Jelena; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Common building energy modeling approaches do not account for the influence of surrounding neighborhood on the energy consumption patterns. This thesis develops a framework to quantify the neighborhood impact on a building energy consumption based on the local wind flow. The airflow in the neighborhood is predicted using Computational Fluid Dynamics (CFD) in eight principal wind directions. The developed framework in this study benefits from wind multipliers to adjust the wind velocity encountering the target building. The input weather data transfers the adjusted wind velocities to the building energy model. In a case study, the CFD method is validated by comparing with on-site temperature measurements, and the building energy model is calibrated using utilities data. A comparison between using the adjusted and original weather data shows that the building energy consumption and air system heat gain decreased by 5% and 37%, respectively, while the cooling gain increased by 4% annually.Item Large Eddy Simulation of Boundary Layer Combustion(2013) Bravo, Luis Giovanni; Trouve, Arnaud C; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Numerical simulations of turbulent non-premixed flames occurring in the presence of solid surfaces is a prevalent topic of interest due to the complexity of the near wall physics and the technical modeling challenges it presents. Near wall combustion phenomena is relevant in a variety of combusting environments including but not limited to the occurrence of fire spread, as a result of a heating load to a flammable wall leading to fire growth in enclosure settings; and in engine combustion configurations where the interaction with a cooled surface combined with occurrences of short flame wall distances can lead to extinction events adversely affecting combustion performance. The interaction between the flame and surface can result in a reduction of flame strength near the cold wall region while gas phase heat fluxes can take peak values at flame contact. To address the aforementioned modeling challenges, an advanced computational fluid dynamics (CFD) solver has been developed by adapting a preexisting numerical simulation solver from a boundary layer code to a code with variable mass density and combustion capabilities to produce high-fidelity simulations of turbulent non-premixed wall-flames. A series of verification studies have been developed using several benchmark laminar flow problems for the following canonical configurations: a binary diffusion controlled mixing problem, Poiseuille flow with heat transfer, and classical Blasius boundary layer flow. The turbulence LES modeling capability is validated by performing wall-resolved heated/non-heated turbulent channel flow and transpired boundary layer simulations to capture the effects of heat and mass transfer on the turbulent eddy structure and statistics. Lastly, an application of a simplified non-premixed wall flame configuration is presented in which the fuel corresponds to pyrolysis products supplied by a thermally-degrading flat sample of polymethyl methacrylate (PMMA) and the oxidizer corresponds to a cross-flow of ambient air with controlled mean velocity and turbulence intensities. Comparisons between numerical results and experimental data are made in terms of flame length, wall surface heat flux and flame structure and the ability of the solver in modeling non-premixed turbulent wall-flames is successfully demonstrated. The solver extends the present state of the art in fire modeling (limited to laminar flows) by providing a high quality numerical tool to study the heat transfer aspects of turbulent wall flame phenomena