Author: Darius Isaiah Craig

Title: Automatic Optimization Methods for Patient-Specific Tissue-Engineered Vascular Grafts

Committee Members:

• Dr. Mark Fuge (Chair/Advisor)
• Dr. Axel Krieger
• Shapour Azarm

Date/Time: Jul 31, 2020 03:00 PM Eastern Time (US and Canada) 

Zoom Link: https://umd.zoom.us/j/92772967497

Abstract: Surgical intervention is sometimes necessary in cases of Coarctation of the Aorta (CoA). The post-repair geometry of the aorta can result in sub-optimal hemodynamics and can have long-term health impacts.
Patient-specific designs for tissue engineered vascular grafts (TEVGs) allow greater control over post-repair geometry. This thesis proposes a method for automatically optimizing patient-specific TEVGs using computational fluid dynamics (CFD) simulations and the ANSYS Fluent adjoint solver. Our method decreases power loss in the graft by 25-60\% compared to the native geometry. As patient-specific graft design can be challenging due to incomplete or uncertain flow and geometry data, this thesis also quantifies the robustness of the optimal designs  with respect to CFD boundary conditions derived from imaging data. We show that using velocity conditions that deviate by more than 20\% of the measured peak systolic velocity, our method produces grafts with deviations on the order of 5\% in predicted power loss performance. Lastly, as one way to accelerate the optimization process, we demonstrate and compare how some established machine learning models (K Nearest Neighbors and Kernel Ridge Regression) predict reasonable starting points for an optimizer on a 2D bifurcated pipe dataset.

Title: Design and Experimental Characterization of Metal Additive Manufactured Heat Exchangers for Aerospace Application

Date/Time: July 24, 2020 – 12-2PM

Committee Members:

• Professor Michael Ohadi, Chair
• Professor Hugh A. Bruck
• Professor Christopher Cadou
• Professor Marino DiMarzo
• Professor  Jungho Kim

Abstract: High temperature heat exchangers are key to the success of emerging high-temperature, high-efficiency solutions in energy conversion, power generation and waste heat recovery applications. When applied to the aerospace applications, the main objective is to develop heat exchangers that can realize significant performance improvement in terms of gravimetric heat exchange density (kW/kg). In the present study, two air-to-air crossflow heat exchangers were designed, built and tested to determine their potential for high performance, pre-cooling heat exchanger for aircraft applications. A novel design based on manifold-microchannel technology was chosen as it provided localized and optimum distribution of the flow among the heat transfer surface micro channels, offering superior heat transfer performance and low pressure drops, when compared to conventional, state of the art heat exchangers for the chosen application. However, fabrication of the manifold microchannel design for high temperature with super alloys as the heat exchanger material presents serious manufacturing challenges fabrication techniques. To overcome this limit, direct metal laser sintering (DMLS) additive manufacturing technique was selected for the fabrication of the Ni-based superalloy manifold-microchannel heat exchangers in the present study. Extensive work was performed to characterize the printing capability of different metal 3D-printers in terms of printing orientation, printing accuracy and structure density. Based on the knowledge acquired, two units were printed, with overall size of 4”x4”x4” and 4.5”x4”x3.5” and fin thickness of 0.220 mm and 0.170 mm, respectively. The printed units were the largest additively printed, superalloy-based manifold-microchannel heat exchangers found in the literature. The experimental characterization was carried at high temperature (600°C) and the model prediction of the performance was updated to characterize the behavior of the heat exchangers in this operational conditions. Based on the experimental results, a gravimetric heat duty of 9.4 kW/kg for an effectiveness (ε) of 78% was achieved, which corresponds to an improvement of more than 50% compared to the conventional designs. The characterization of the performance at high temperature was then completed by analyzing the thermo-mechanical stress generated by the simultaneous presence of temperature gradient and pressures. The current study is the first to characterize the behavior of manifold-microchannel heat exchanger under high temperature in terms of performance prediction and thermo-mechanical analysis.

Title: Electrokinetic transport in nanochannels grafted with backbone charged Polyelectrolyte brushes

Date/Time: August 03, 2020 – 2:00 pm

Examining Committee:

• Dr. Siddhartha Das – Chair/Advisor
• Dr. Amir Riaz 
• Dr. Taylor J Woehl 

Abstract: In this thesis, we study the electrokinetic transport in nanochannels functionalized with pH- responsive backbone charged polyelectrolyte (PE) brushes modeled using thermodynamically self-consistent augmented strong stretching theory. We investigate the electroosmotic (EOS) transport, induced by the application of external electric field, and the diffusioosmotic (DOS) transport due to applied salt concentration gradient induced electroosmotic transport in brush functionalized and brushless nanochannels with equal surface charge density. We find massive enhancement in the electrokinetic transport in PE brush functionalized nanochannels when compared to brushless nanochannels which can be ascribed to the brush induced localization of the EDL and hence the net EOS body force away from the flow retarding walls. Further, we establish that both EOS and DOS transport in nanochannels grafted with backbone charged PE
brushes is larger in magnitude when compared to that in nanochannels grafted with end charged PE brushes.

Title: Experimental Investigations and Scaling Analyses of Whirling Flames

Date: Thursday 23 July, 2020

Time: 3:00pm (Eastern), 12:00 (Pacific)

Advisory Committee

• Professor Michael J. Gollner, Chair & Advisor
•  Professor Elaine S. Oran, Co-Chair & Co-Advisor 
• Professor Arnaud Trouvé
•  Professor Peter B. Sunderland 
• Professor James H. Duncan
•  Professor Christopher Cadou
•  Professor Konstantina Trivisa, Dean’s Representative

Abstract

Swirling flows are ubiquitous in nature, occurring over a large range of length scales – on the order of many tens-of-thousands of kilometers in the case of Sat- urn’s hexagonal polar vortex, to just a few centimeters in dandelion flight. Most instances of swirling flow involve momenta competing in two different directions. Whirling flames (also known as fire whirls) occur at the intersection of vortical flow fields and buoyant, reactive plumes, and they represent a general class of flows that may be considered slender vortices involving axial momentum from heat-release and tangential momentum from air entrainment. In this work, two previously unexplored characteristics of whirling flames are considered over a wide range of scales, spanning three orders of magnitude in length and four orders in heat-release rate.

First, emissions of particulate matter (PM) from fire whirls (FW) were measured and compared to those from free-buoyant pool fires (PF). For different pool diameters and fuels, FWs showed higher burning rate and fuel-consumption efficiency, but lower PM-emission rate, leading to lower PM- emission factors. The lower PM emissions from FWs is attributed to a feedback cycle between higher oxygen consumption from improved entrainment, higher average temperatures, increased heat feedback to the fuel pool, which in turn increases burning rate and entrainment. A scaling analysis showed that the PM emission factor decreased linearly with the ratio of inverse Rossby number to nondimensional heat-release rate.

Second, the structure of the blue whirl (BW), a soot-free, whirling-flame regime, was investigated using dimensional analysis and non-intrusive optical diag- nostics. Experimental data of heat-release rates and circulation for BWs and FWs from the literature were used to define the nondimensional equivalents of buoyant and azimuthal momenta. The combinations of these parameters showed that FWs primarily formed in a buoyancy-dominated regime, and that a circulation-dominated regime was required for a transition to the BW, corroborating hypotheses that the transition was caused by the bubble mode of vortex breakdown, resulting in the formation of a recirculation zone. Finally, optical diagnostics including OH- and PAH-PLIF, OH* and CH* chemiluminescence suggest a triple-flame structure an- chored at the blue ring region of the BW, with the rich branch formed by the lower blue cone, and the lean branch by the upper purple haze. These results show that the mixing process occurs upstream of the conical region and that the recirculation zone is comprised of combustion products.