Announcements

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.

Title: Fabrication and Characterization of Nanoscale Shape Memory Alloy Mems Actuators

Author: Cory Ray Knick

Date & Time: July 1, 2020 – 2:30-3:30pm

Committee: Dr. Hugh Bruck, Associate Dean for Faculty Affairs, Chair

Dr. Christopher Morris, Branch Chief, US Army Research Laboratory

Dr. Patrick McCluskey, Professor, Dept. of Mechanical Engineering

Dr. Don DeVoe, Professor and Associate Chair, Dept. of Mechanical Engineering

Dr. Miao Yu, Professor, Dept. of Mechanical Engineering

Dr. Ichiro Takeuchi, Deans Representative, Professor of Materials Science and Engineering

Abstract: The miniaturization of engineering devices has created interest in new actuation methods capable of large displacements and high frequency responses. Shape memory alloy (SMA) thin films have exhibited one of the highest power densities of any material used in these actuation schemes with thermally recovery strains of up to 10%. With the use of a biasing force, such as from a passive layer in a bilayer structure, homogenous SMA films can experience reversible shape memory effects provided they are thick enough that the crystal structure is capable of transforming. However, thick films exhibit lower actuation displacements and speeds because of the larger inertial resistance. Therefore, there is a need to find a way to process thinner SMA films with grain structures that are capable of transformation in order to realize larger actuation displacements at higher speeds. 

In this work, a near-equiatomic NiTi magnetron co-sputtering process was developed to create nanoscale thick SMA films. By using a metallic seed layer, it was possible to induce the crystallization of epitaxial, columnar grains exhibiting the shape memory effects in nanoscale films. It was also possible to crystalize these SMA films at lower processing temperatures compared to directly sputtering thicker films onto Si wafers. The transformation behavior associated with the SME in these films were characterized using x-ray diffraction (XRD), differential scanning calorimetry (DSC), and stress-temperature measurements at wafer level. After quantifying the shape memory effects at wafer-level, the SMA films were used to fabricate various microscale MEMS actuators. The SMA films were mated in several “bimorph” configurations to induce out of plane curvature in the low-temperature martensite phase. The curvature radius vs. temperature was characterized on MEMS cantilever structures to elucidate a relationship between residual stress, recovery stress, radius of curvature, and degree of unfolding. SMA MEMS actuators were fabricated and tested using joule heating to demonstrate rapid electrical actuation of NiTi MEMS devices at some of the lowest powers (5 – 15 mW) and operating frequencies (1 – 3 kHz) ever reported for SMA actuators. By developing a process to create nanoscale thickness NiTi SMA films, we enabled the fabrication of MEMS devices with full, reversible, actuation as low as 0.5 V. This indicated the potential of these devices to be used for high frequency, low power, and large displacement applications in power constrained environments (i.e. on chip).

Title: Development and In-Silico Evaluation of a Closed-Loop Fluid Resuscitation Control Algorithm with Mean Arterial Pressure Feedback.

Author: Mohammad Alsalti

Date & Time: June 26, 2020 – 2:00pm

Committee: 

• Dr. Jin-Oh Hahn (Chair) 
• Dr. Nikhil Chopra
• Dr. Axel Krieger 

Abstract: A model-based closed-loop fluid resuscitation controller using mean arterial pressure (MAP) feedback is designed and later evaluated on an in-silico test-bed. The controller is based on a subject specific model of blood volume and MAP response to fluid infusion. This simple hemodynamic model is described using five parameters only. The model was able to reproduce blood volume and blood pressure response to fluid infusion using an experimental data set collected from 23 sheep and is therefore suitable to use for control design purposes. A model-reference adaptive control scheme was chosen to account for inter-subject variability captured in the parametric uncertainties of the underlying physiological model. Three versions of the control algorithm were studied under different measurement availability scenarios. In-silico evaluation of the three controllers was done based on a comprehensive cardiovascular physiology model on a cohort of 100 virtually generated patients.In-silico results showed consistent tracking performance across all patients unlike estimation performance, which varied depending on measurement availability. 

Title: Personal cooling system with phase change material

Author: Yiyuan Qiao

Committee members:

Dr. Reinhard Radermacher(Chair)

Dr. Ichiro Takeuchi

Dr. Jelena Srebric

Dr. Bao Yang 

Dr. Amir Riaz

 Dr. Yunho Hwang

Date: June 26, 2020, 2:00 pm

Abstract: Personal cooling systems are attracting more attention recently since they can set back building thermostat setpoint to achieve energy savings and provide high-level human comfort by focusing on micro-environment conditions around occupants rather than the entire building space. Thus, a vapor compression cycle (VCC)-based personal cooling system with a condenser integrated with the phase change material (PCM) is proposed. The PCM heat exchanger (PCMHX) works as a condenser to store waste heat from the refrigerant in the cooling cycle, in which the PCM melting process can affect the system performance significantly. Different from most of the previous study, various refrigerant heat transfer characteristics along the condenser flow path can result in the uneven PCM melting, leading to the degradation of the system performance. Therefore, enhancing heat transfer in the PCM, investigating the proposed personal cooling system performance, improving PCMHX latent heat utilization in terms of the distribution of PCM melting, and developing a general-purpose PCM model are the objectives of this dissertation. 

Five PCMHX designs with different heat transfer enhancement including increasing heat-transfer area, embedding conductive structures, and using uniform refrigerant distribution among condenser branches are introduced first. Compared with non-enhanced PCM, the graphite-matrix-enhanced PCMHX performs the best with 5.5 times higher heat transfer coefficient and 49% increased coefficient of performance (COP). To investigate the proposed system performance, a system-level experimental parametric study regarding the thermostat setting, PCM recharge rates, and cooling time was conducted. Results show that the PCS can work properly with a stable cooling capacity of 160 W for 4.5 hours. A transient PCM-coupled system model was also developed for detailed system performance, PCM melting process and heat transfer analysis. From both experiment and simulation work, the uneven PCM melting was presented, which could result in an increase of condenser temperature and a degradation of system COP with time. One significant reason for the uneven PCM melting is the variation of the refrigerant temperature and heat transfer coefficient. Therefore, through experimental analysis, several solutions were proposed to minimize the negative effect of the uneven PCM melting. Results show that the less subcooling is recommended, and the new design with the PCM mass maldistribution can increase the cooling time by 28%. In addition, to extend the PCMHX application, a multi-tube PCMHX model was developed for general-purpose design. A new multi-tube heat transfer algorithm was proposed, and variable tube shape, connection, and topology for tubes and PCM blocks were considered. The comparison with other PCMHX models in the literature shows that the proposed model exhibits much higher flexibility and feasibility for comprehensive multi-tube configurations. The personal cooling system coupled with PCMHX could achieve energy savings for a range of 8-36% depending on the climate and building types in the U.S..