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  • In order to better understand and quantify the effect of instabilities in systems utilizing flow boiling heat transfer, the present study explores dynamic results for pressure drop, mass velocity, thermodynamic equilibrium quality, and heated wall temperature to ascertain and analyze the dominant modes in which they oscillate. Flow boiling experiments are conducted for a range of mass velocities with both subcooled and saturated inlet conditions in vertical upflow, vertical downflow, and horizontal flow orientations. High frequency pressure measurements are used to investigate the influence of individual flow loop components (flow boiling module, pump, pre-heater, condenser, etc.) on dynamic behavior of the fluid, with fast Fourier transforms of the same used to provide critical frequency domain information. Conclusions from this analysis are used to isolate instabilities present within the system due to physical interplay between thermodynamic and hydrodynamic effects. Parametric analysis is undertaken to better understand the conditions under which these instabilities form and their impact on system performance. Several prior stability maps are presented, with new stability maps provided to better address contextual trends discovered in the present study.Further, this study utilizes experimental results for vertical upflow boiling of FC-72 in a rectangular channel with finite inlet quality to investigate Density Wave Oscillations (DWOs) and assess their potential impact on design of two-phase systems for future space missions. High-speed flow visualization image sequences are presented and used to directly relate the cyclical passage of High and Low Density Fronts (HDFs and LDFs) to dominant low-frequency oscillations present in transient pressure signals commonly attributed to DWOs. A methodology is presented to determine frequency and amplitude of DWO induced pressure oscillations, which are then plotted for a wide range of relevant operating conditions. Mass velocity (flow inertia) is seen to be the dominant parameter influencing frequency and amplitude of DWOs. Amplitude of pressure oscillations is at most 7% of the time-averaged pressure level for current operating conditions, meaning there is little risk to space missions. Reconstruction of experimental pressure signals using a waveform defined by frequency and amplitude of DWO induced pressure fluctuations is seen to have only moderate agreement with the original signal due to the oversimplifications of treating DWO induced fluctuations as perfectly sinusoidal in nature, assuming they occur at a constant frequency value, and neglecting other transient flow features. This approach is nonetheless determined to have potential value for use as a boundary condition to introduce DWOs in two-phase flow simulations should a model be capable of accurately predicting frequency and amplitude of oscillation.Additionally, this study presents a new mechanistic model for Density Wave Oscillations (DWOs) in vertical upflow boiling using conclusions drawn from analysis of flow visualization images and transient experimental results as a basis from which to begin modeling. Counter to many prior studies attributing DWOs to feedback effects between flow rate, pressure drop, and flow enthalpy causing oscillations in position of the bulk boiling boundary, the present instability mode stems primarily from body force acting on liquid and vapor phases in a separated flow regime leading to liquid accumulation in the near-inlet region of the test section, which eventually departs and moves along the channel, acting to re-wet liquid film along the channel walls and re-establish annular, co-current flow. This process was modeled by dividing the test section into three distinct control volumes and solving transient conservation equations for each, yielding predictions of frequencies at which this process occurs as well as amplitude of associated pressure oscillations. Values for these parameters were validated against an experimental database of 236 FC-72 points and show the model provides good predictive accuracy and capably captures the influence of parametric changes to operating conditions.Also, this study shows analysis of pressure signals in condensing systems reveal the presence of relevant oscillatory phenomena during flow condensation as well, which may impact performance in applications concerned with precise system control. Towards this end, the present study presents results for oscillatory behavior observed in pressure measurements during flow condensation of FC-72 in a smooth circular tube in vertical upflow, vertical downflow, and horizontal flow orientations. Dynamic behavior observed within the test section is determined to be independent of other components within the flow loop, allowing it to be isolated and interpreted as resulting from physical aspects of two-phase flow with condensation. The presence of a peak oscillatory mode (one of significantly larger amplitude than any others present) is seen for 72% ofvertical upflow test cases, 61% of vertical downflow, and 54% of horizontal flow. Relative intensities of this peak oscillatory mode are evaluated through calculation of Q Factor for the corresponding frequency response peak. Frequency and amplitude of peak oscillatory modes are also evaluated. Overall, vertical upflow is seen to exhibit the most significant oscillatory behavior, although in its maximum case amplitude is only seen to be 7.9% of time-averaged module inlet pressure, indicating there is little safety risk posed by oscillations under current operating conditions. Flow visualization image sequences for each orientation are also presented and used to draw parallels between physical characteristics of condensate film behavior under different operating conditions and trends in oscillatory behavior detected in pressure signalsFurther, the present work outlines a new methodology utilizing temperature and pressure measurements to identify condensation flow regimes. For vertical upflow condensation, amplitude of dynamic temperature and pressure oscillations are shown to clearly indicate transition from counter-current flow regimes (i.e., falling film, oscillating film, flooding) to annular, co-current flow (climbing film flow regime). In horizontal flow condensation, standard deviation between multiple thermocouple measurements distributed around the tube circumference was calculated at all axial (stream-wise) measurement locations. High values of standard deviation are present for stratified flow (stratified flow, wavy-stratified, plug flow), while axisymmetric flow regimes (i.e., slug flow, annular flow) yield significantly lower values. Successful development of this technique represents a valuable contribution to literature as it allows condensation flow regime to be identified without the often-costly restriction of designing a test section to allow optical access. Identified flow regimes in both vertical upflow and horizontal flow orientations are compared to regime maps commonly found in the literature in pursuit of optimum performing maps.Finally, the present study aims to better analyze the influence of body force on flow condensation heat transfer by conducting tests at multiple orientations in Earth’s gravity. Dielectric FC-72 is condensed in a smooth stainless-steel tube with 7.12 mm diameter and 574.55 mm condensing length by counterflow of cooling water across the outer surface of the tube. Test conditions span FC-72 mass velocities of 50.3 – 360.3 kg/m2s, test section inlet pressures of 127.0 – 132.1 kPa, and test section inlet thermodynamic equilibrium qualities of 0.13 – 1.15. A subset of data gathered corresponding to axisymmetric, annular condensation heat transfer is identified and a detailed methodology for data reduction to calculate heat transfer coefficient presented. Uncertainty analysis is also presented and indicates channel average heat transfer coefficients are calculated within ±3.6% to ±26.7% (depending on operating conditions). Analysis of parametric trends for condensation heat transfer reveals the dominant influence of mass velocity (flow inertia), secondary influence of vapor mass fraction (thermodynamic equilibrium quality), and strong dependence on orientation (body force) at low mass velocities. At higher mass velocities results for all orientations investigated begin to converge, indicating body force independent annular condensation heat transfer is achieved. Separated Flow Model predictions of vertical downflow condensation heat transfer provide reasonable agreement with experimental results, evidence by a Mean Absolute Error (MAE) of 31.2%. Evaluation of condensation heat transfer correlations for horizontal flow reveal most correlations struggle for cases with high liquid content. Specific correlations are identified for superior accuracy in predicting the measured data.
    Data Types:
    • Document
  • In order to better understand and quantify the effect of instabilities in systems utilizing flow boiling heat transfer, the present study explores dynamic results for pressure drop, mass velocity, thermodynamic equilibrium quality, and heated wall temperature to ascertain and analyze the dominant modes in which they oscillate. Flow boiling experiments are conducted for a range of mass velocities with both subcooled and saturated inlet conditions in vertical upflow, vertical downflow, and horizontal flow orientations. High frequency pressure measurements are used to investigate the influence of individual flow loop components (flow boiling module, pump, pre-heater, condenser, etc.) on dynamic behavior of the fluid, with fast Fourier transforms of the same used to provide critical frequency domain information. Conclusions from this analysis are used to isolate instabilities present within the system due to physical interplay between thermodynamic and hydrodynamic effects. Parametric analysis is undertaken to better understand the conditions under which these instabilities form and their impact on system performance. Several prior stability maps are presented, with new stability maps provided to better address contextual trends discovered in the present study.Further, this study utilizes experimental results for vertical upflow boiling of FC-72 in a rectangular channel with finite inlet quality to investigate Density Wave Oscillations (DWOs) and assess their potential impact on design of two-phase systems for future space missions. High-speed flow visualization image sequences are presented and used to directly relate the cyclical passage of High and Low Density Fronts (HDFs and LDFs) to dominant low-frequency oscillations present in transient pressure signals commonly attributed to DWOs. A methodology is presented to determine frequency and amplitude of DWO induced pressure oscillations, which are then plotted for a wide range of relevant operating conditions. Mass velocity (flow inertia) is seen to be the dominant parameter influencing frequency and amplitude of DWOs. Amplitude of pressure oscillations is at most 7% of the time-averaged pressure level for current operating conditions, meaning there is little risk to space missions. Reconstruction of experimental pressure signals using a waveform defined by frequency and amplitude of DWO induced pressure fluctuations is seen to have only moderate agreement with the original signal due to the oversimplifications of treating DWO induced fluctuations as perfectly sinusoidal in nature, assuming they occur at a constant frequency value, and neglecting other transient flow features. This approach is nonetheless determined to have potential value for use as a boundary condition to introduce DWOs in two-phase flow simulations should a model be capable of accurately predicting frequency and amplitude of oscillation.Additionally, this study presents a new mechanistic model for Density Wave Oscillations (DWOs) in vertical upflow boiling using conclusions drawn from analysis of flow visualization images and transient experimental results as a basis from which to begin modeling. Counter to many prior studies attributing DWOs to feedback effects between flow rate, pressure drop, and flow enthalpy causing oscillations in position of the bulk boiling boundary, the present instability mode stems primarily from body force acting on liquid and vapor phases in a separated flow regime leading to liquid accumulation in the near-inlet region of the test section, which eventually departs and moves along the channel, acting to re-wet liquid film along the channel walls and re-establish annular, co-current flow. This process was modeled by dividing the test section into three distinct control volumes and solving transient conservation equations for each, yielding predictions of frequencies at which this process occurs as well as amplitude of associated pressure oscillations. Values for these parameters were validated against an experimental database of 236 FC-72 points and show the model provides good predictive accuracy and capably captures the influence of parametric changes to operating conditions.Also, this study shows analysis of pressure signals in condensing systems reveal the presence of relevant oscillatory phenomena during flow condensation as well, which may impact performance in applications concerned with precise system control. Towards this end, the present study presents results for oscillatory behavior observed in pressure measurements during flow condensation of FC-72 in a smooth circular tube in vertical upflow, vertical downflow, and horizontal flow orientations. Dynamic behavior observed within the test section is determined to be independent of other components within the flow loop, allowing it to be isolated and interpreted as resulting from physical aspects of two-phase flow with condensation. The presence of a peak oscillatory mode (one of significantly larger amplitude than any others present) is seen for 72% ofvertical upflow test cases, 61% of vertical downflow, and 54% of horizontal flow. Relative intensities of this peak oscillatory mode are evaluated through calculation of Q Factor for the corresponding frequency response peak. Frequency and amplitude of peak oscillatory modes are also evaluated. Overall, vertical upflow is seen to exhibit the most significant oscillatory behavior, although in its maximum case amplitude is only seen to be 7.9% of time-averaged module inlet pressure, indicating there is little safety risk posed by oscillations under current operating conditions. Flow visualization image sequences for each orientation are also presented and used to draw parallels between physical characteristics of condensate film behavior under different operating conditions and trends in oscillatory behavior detected in pressure signalsFurther, the present work outlines a new methodology utilizing temperature and pressure measurements to identify condensation flow regimes. For vertical upflow condensation, amplitude of dynamic temperature and pressure oscillations are shown to clearly indicate transition from counter-current flow regimes (i.e., falling film, oscillating film, flooding) to annular, co-current flow (climbing film flow regime). In horizontal flow condensation, standard deviation between multiple thermocouple measurements distributed around the tube circumference was calculated at all axial (stream-wise) measurement locations. High values of standard deviation are present for stratified flow (stratified flow, wavy-stratified, plug flow), while axisymmetric flow regimes (i.e., slug flow, annular flow) yield significantly lower values. Successful development of this technique represents a valuable contribution to literature as it allows condensation flow regime to be identified without the often-costly restriction of designing a test section to allow optical access. Identified flow regimes in both vertical upflow and horizontal flow orientations are compared to regime maps commonly found in the literature in pursuit of optimum performing maps.Finally, the present study aims to better analyze the influence of body force on flow condensation heat transfer by conducting tests at multiple orientations in Earth’s gravity. Dielectric FC-72 is condensed in a smooth stainless-steel tube with 7.12 mm diameter and 574.55 mm condensing length by counterflow of cooling water across the outer surface of the tube. Test conditions span FC-72 mass velocities of 50.3 – 360.3 kg/m2s, test section inlet pressures of 127.0 – 132.1 kPa, and test section inlet thermodynamic equilibrium qualities of 0.13 – 1.15. A subset of data gathered corresponding to axisymmetric, annular condensation heat transfer is identified and a detailed methodology for data reduction to calculate heat transfer coefficient presented. Uncertainty analysis is also presented and indicates channel average heat transfer coefficients are calculated within ±3.6% to ±26.7% (depending on operating conditions). Analysis of parametric trends for condensation heat transfer reveals the dominant influence of mass velocity (flow inertia), secondary influence of vapor mass fraction (thermodynamic equilibrium quality), and strong dependence on orientation (body force) at low mass velocities. At higher mass velocities results for all orientations investigated begin to converge, indicating body force independent annular condensation heat transfer is achieved. Separated Flow Model predictions of vertical downflow condensation heat transfer provide reasonable agreement with experimental results, evidence by a Mean Absolute Error (MAE) of 31.2%. Evaluation of condensation heat transfer correlations for horizontal flow reveal most correlations struggle for cases with high liquid content. Specific correlations are identified for superior accuracy in predicting the measured data.
    Data Types:
    • Document
  • Passive vibration mitigation via multi-stable, mechanical means is relatively unexplored. In addition, achieving vibration suppression through avoiding resonance is at the forefront of up and coming research. This thesis investigates the application of a purely mechanical, bistable device as a passive method of vibration suppression. A purely mechanical device does not require power, multiple materials, or electrical circuits, and a passive device does not require external interaction or control. Therefore, a passive, mechanical device could be implemented with ease even in physically constrained environments with large dynamic loads, such as turbomachinery. The purely mechanical, bistable device presented herein replicates the two switches per resonance crossing evident in semi-active Resonance Frequency Detuning method. This work explores two different bistable, mass-spring models. The first is a single degree of freedom nonlinear mass spring model aiming to utilize asymmetry in the potential function to change the stiffness of the overall system. The second model is a coupled, two degree of freedom system that combines the nonlinear softening and hardening spring characteristics with the unique stiffnesses of two stable states. The performance is verified by targeting the first mode of a cantilever beam, with the device shifting the resonance away from the excitation frequency. Future research could apply these idealized models to complex, rotating structures and replicate the performance of the passive, mechanical devices in a physical geometry that could be manufactured as a part of a target structure.
    Data Types:
    • Document
  • Passive vibration mitigation via multi-stable, mechanical means is relatively unexplored. In addition, achieving vibration suppression through avoiding resonance is at the forefront of up and coming research. This thesis investigates the application of a purely mechanical, bistable device as a passive method of vibration suppression. A purely mechanical device does not require power, multiple materials, or electrical circuits, and a passive device does not require external interaction or control. Therefore, a passive, mechanical device could be implemented with ease even in physically constrained environments with large dynamic loads, such as turbomachinery. The purely mechanical, bistable device presented herein replicates the two switches per resonance crossing evident in semi-active Resonance Frequency Detuning method. This work explores two different bistable, mass-spring models. The first is a single degree of freedom nonlinear mass spring model aiming to utilize asymmetry in the potential function to change the stiffness of the overall system. The second model is a coupled, two degree of freedom system that combines the nonlinear softening and hardening spring characteristics with the unique stiffnesses of two stable states. The performance is verified by targeting the first mode of a cantilever beam, with the device shifting the resonance away from the excitation frequency. Future research could apply these idealized models to complex, rotating structures and replicate the performance of the passive, mechanical devices in a physical geometry that could be manufactured as a part of a target structure.
    Data Types:
    • Document
  • Neuromorphic computing has gained tremendous interest because of its ability to overcome the limitations of traditional signal processing algorithms in data intensive applications such as image recognition, video analytics, or language translation. The new computing paradigm is built with the goal of achieving high energy efficiency, comparable to biological systems.To achieve such energy efficiency, there is a need to explore new neuro-mimetic devices, circuits, and architecture, along with new learning algorithms. To that effect, we propose two main approaches: First, we explore an energy-efficient hardware implementation of a bio-plausible Spiking Neural Network (SNN). The key highlights of our proposed system for SNNs are 1) addressing connectivity issues arising from Network On Chip (NOC)-based SNNs, and 2) proposing stochastic CMOS binary SNNs using biased random number generator (BRNG). On-chip Power Line Communication (PLC) is proposed to address the connectivity issues in NOC-based SNNs. PLC can use the on-chip power lines augmented with low-overhead receiver and transmitter to communicate data between neurons that are spatially far apart. We also propose a CMOS 'stochastic-bit' with on-chip stochastic Spike Timing Dependent Plasticity (sSTDP) based learning for memory-compressed binary SNNs. A chip was fabricated in 90 nm CMOS process to demonstrate memory-efficient reconfigurable on-chip learning using sSTDP training. Second, we explored coupled oscillatory systems for distance computation and convolution operation. Recent research on nano-oscillators has shown the possibility of using coupled oscillator networks as a core computing primitive for analog/non-Boolean computations. Spin-torque oscillator (STO) can be an attractive candidate for such oscillators because it is CMOS compatible, highly integratable, scalable, and frequency/phase tunable. Based on these promising features, we propose a new coupled-oscillator based architecture for hybrid spintronic/CMOS hardware that computes multi-dimensional norm. The hybrid system composed of an array of four injection-locked STOs and a CMOS detector is experimentally demonstrated. Energy and scaling analysis shows that the proposed STO-based coupled oscillatory system has higher energy efficiency compared to the CMOS-based system, and an order of magnitude faster computation speed in distance computation for high dimensional input vectors.
    Data Types:
    • Document
  • Neuromorphic computing has gained tremendous interest because of its ability to overcome the limitations of traditional signal processing algorithms in data intensive applications such as image recognition, video analytics, or language translation. The new computing paradigm is built with the goal of achieving high energy efficiency, comparable to biological systems.To achieve such energy efficiency, there is a need to explore new neuro-mimetic devices, circuits, and architecture, along with new learning algorithms. To that effect, we propose two main approaches: First, we explore an energy-efficient hardware implementation of a bio-plausible Spiking Neural Network (SNN). The key highlights of our proposed system for SNNs are 1) addressing connectivity issues arising from Network On Chip (NOC)-based SNNs, and 2) proposing stochastic CMOS binary SNNs using biased random number generator (BRNG). On-chip Power Line Communication (PLC) is proposed to address the connectivity issues in NOC-based SNNs. PLC can use the on-chip power lines augmented with low-overhead receiver and transmitter to communicate data between neurons that are spatially far apart. We also propose a CMOS 'stochastic-bit' with on-chip stochastic Spike Timing Dependent Plasticity (sSTDP) based learning for memory-compressed binary SNNs. A chip was fabricated in 90 nm CMOS process to demonstrate memory-efficient reconfigurable on-chip learning using sSTDP training. Second, we explored coupled oscillatory systems for distance computation and convolution operation. Recent research on nano-oscillators has shown the possibility of using coupled oscillator networks as a core computing primitive for analog/non-Boolean computations. Spin-torque oscillator (STO) can be an attractive candidate for such oscillators because it is CMOS compatible, highly integratable, scalable, and frequency/phase tunable. Based on these promising features, we propose a new coupled-oscillator based architecture for hybrid spintronic/CMOS hardware that computes multi-dimensional norm. The hybrid system composed of an array of four injection-locked STOs and a CMOS detector is experimentally demonstrated. Energy and scaling analysis shows that the proposed STO-based coupled oscillatory system has higher energy efficiency compared to the CMOS-based system, and an order of magnitude faster computation speed in distance computation for high dimensional input vectors.
    Data Types:
    • Document
  • Achievements in the growth of ultra-pure III-V semiconductor materials using state of the art molecular beam epitaxy (MBE) machine has led to the discovery of new physics and technological innovations. High mobility two-dimensional electron gas (2DEG) embedded in GaAs/AlxGa1−xAs heterostructures provides an unparalleled platform for many-body physics including fractional quantum Hall effect. On the other hand, single electron devices fabricated on modulation doped GaAs/AlxGa1−xAs heterostructures have been extensively used for fabrication of quantum devices such as spin qubit with application in quantum computing. Furthermore, epitaxial hybrid superconductor-semiconductor heterostructures with ultra clean superconductor-semiconductor interface have been grown using MBE technique to explore rare physical quantum state of the matter namely Majorana zero modes with non-abelian exchange statistics. Chapter 1 in the manuscript starts with description of GaAs MBE system at Purdue University and continues with the modifications have been made to MBE hardware and growth conditions for growing heterostrcutures with 2DEG mobility exceeding 35 × 106 cm−2/V s. Utilizing an ultra-high pure Ga source material and its further purification by thermal evaporation in the vacuum are determined to have major impact on growth of high mobility GaAs/AlxGa1−xAs heterostructures. Chapter 2 reports a systematic study on the effect of silicon doping density on low frequency charge noise and conductance drift in laterally gated nanostructures fabricated on modulation doped GaAs/AlxGa1−xAs heterostructures grown by Molecular Beam Epitaxy (MBE). The primary result of this study is that both charge noise and conductance drift are strongly impacted by the silicon doping used to create the two-dimensional electron gas. These findings shed light on the physical origin of the defect states responsible for charge noise and conductance drift. This is especially significant for spin qubit devices, which require minimization of conductance drift and charge noise for stable operation and good coherence. Chapter 3 demonstrates measurements of the induced superconducting gap in 2D hybrid Al/Al0.15In0.85As/InAs heterostructures which is a promising platform for scaling topological qubits based on Majorana zero modes. The 2DEG lies in an InAs quantum well and is separated from the epitaxial Al layer by a barrier of Al0.15In0.85As with thickness d. Due to hybridization between the wave functions of 2DEG and superconductor, the strength of induced gap in the 2DEG largely depends on the barrier thickness. This chapter presents a systematic study of the strength of the induced gap in hybrid Al/Al0.15In0.85As/InAs superconductor/semiconductor heterostructures as a function of barrier thickness.
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  • The continued miniaturization and demand for improved performance of electronic devices has resulted in the need for transformative thermal management strategies. Flow boiling is an attractive approach for the thermal management of devices generating high heat fluxes. However, designing heat sinks for two-phase operation and predicting their performance is difficult because of, in part, commonly encountered flow boiling instabilities and a lack of experimentally validated physics-based phase change models. This work aims to advance the state of the art by furthering our understanding of flow boiling instabilities and their implications on the operating characteristics of electronic devices. This is of particular interest under transient and non-uniform heating conditions because of recent advancements in embedded cooling techniques, which exacerbate spatial non-uniformities, and the demand for cooling solutions for next-generation electronic devices. Additionally, this work aims to provide a high-fidelity experimental characterization technique for slug flow boiling to enable the validation of physics-based phase change models. To provide a foundation for which the effects of transient and non-uniform heating can be studied, flow boiling instabilities are first studied experimentally in a single, 500 μm-diameter borosilicate glass microchannel. A thin layer of optically transparent and electrically conductive indium tin oxide coated on the outside surface of the microchannel provides a spatially uniform and temporally constant heat flux via Joule heating. The working fluid is degassed, dielectric HFE-7100. Simultaneous high-frequency measurement of reservoir, inlet, and outlet pressures, pressure drop, mass flux, inlet and outlet fluid temperatures, and wall temperature is synchronized to high-speed flow visualizations enabling transient characterization of the thermal-fluidic behavior. The effect of flow inertia and inlet liquid subcooling on the rapid-bubble-growth instability at the onset of boiling is assessed first. The mechanisms underlying the rapid-bubble-growth instability, namely, a large liquid superheat and a large pressure spike, are quantified. This instability is shown to cause flow reversal and can result in large temperature spikes due to starving the heated channel of liquid, which is especially severe at low flow inertia. Next, the effect of flow inertia, inlet liquid subcooling, and heat flux on the hydrodynamic and thermal oscillations and time-averaged performance is assessed. Two predominant dynamic instabilities are observed: a time-periodic series of rapid-bubble-growth instabilities and the pressure drop instability. The heat flux, ratio of flow inertia to upstream compressibility, and degree of inlet liquid subcooling significantly affect the thermal-fluidic characteristics. High inlet liquid subcoolings and low heat fluxes result in time-periodic transitions between single-phase flow and flow boiling that cause large-amplitude wall temperature oscillations and a time-periodic series of rapid-bubble-growth instabilities. Low inlet liquid subcoolings result in small-amplitude thermal-fluidic oscillations and the pressure drop instability. Low flow inertia exacerbates the pressure drop instability and results in large-amplitude thermal-fluidic oscillations whereas high flow inertia reduces their severity. Flow boiling experiments are then performed in a parallel channel test section consisting of two thermally isolated, heated microchannels to study the Ledinegg instability. When the flow in both channels is in the single-phase regime, they have equal wall temperatures due to evenly distributed mass flux delivered to each channel. Boiling incipience in one of the channels triggers the Ledinegg instability which induces a temperature difference between the two channels due to flow maldistribution. The temperature difference between the two channels grows with increasing power. The experimentally observed temperature excursion between the channels due to the Ledinegg instability is reported here for the first time. Time-resolved characterization of flow boiling in a single microchannel is then performed during transient heating conditions. For transient heating tests, three different heat flux levels are selected that exhibit highly contrasting flow behavior during constant heating conditions: a low heat flux corresponding to single-phase flow (15 kW/m2), an intermediate heat flux corresponding to continuous flow boiling (75 kW/m2), and a very high heat flux which would cause critical heat flux if operated at this heat flux continuously (150 kW/m2). Transient testing is first conducted using a single heat flux pulse between these heat flux levels and varying the pulse time. It is observed that any step up/down in the heat flux level that induces/ceases boiling, causes the temperature to temporarily over/under-shoot the eventual steady temperature. Following the single heat flux pulse experiments, a time-periodic series of heat flux pulses is applied. A square wave heating profile is used with pulse frequencies ranging from 0.1 to 100 Hz and three different heat fluxes levels (15, 75, and 150 kW/m2). Three different time-periodic flow boiling fluctuations are observed: flow regime transitions, pressure drop oscillations, and heating pulse propagation. For heating pulse frequencies between approximately 1 and 10 Hz, the thermal and flow fluctuations are heavily coupled to the heating characteristics, forcing the pressure drop instability frequency to match the heating frequency. For heating pulse frequencies above 25 Hz, the microchannel wall attenuates the transient heating profile and the fluid essentially experiences a constant heat flux. To improve our ability to predict the performance of heat sinks for two-phase operation, high-fidelity characterization of key hydrodynamic and heat transfer parameters during microchannel slug flow boiling is performed using a novel experimental test facility that generates an archetypal flow regime, devoid of flow instabilities and flow regime transitions. High-speed flow visualization images are analyzed to quantify the uniformity of the vapor bubbles and liquid slugs generated, as well as the growth of vapor bubbles over a range of heat fluxes. A method is demonstrated for measuring liquid film thickness from the visualizations using a ray-tracing procedure to correct for optical distortions. Characterization of the slug flow boiling regime that is generated demonstrates the unique ability of the facility to precisely control and quantify hydrodynamic and heat transfer characteristics. This work has advanced state-of-the-art technologies for the thermal management of high-heat-flux-dissipation devices by providing an improved understanding on the effects of transient and non-uniform heating on flow boiling and an experimental method for the validation of physics-based flow boiling modeling.
    Data Types:
    • Document
  • The continued miniaturization and demand for improved performance of electronic devices has resulted in the need for transformative thermal management strategies. Flow boiling is an attractive approach for the thermal management of devices generating high heat fluxes. However, designing heat sinks for two-phase operation and predicting their performance is difficult because of, in part, commonly encountered flow boiling instabilities and a lack of experimentally validated physics-based phase change models. This work aims to advance the state of the art by furthering our understanding of flow boiling instabilities and their implications on the operating characteristics of electronic devices. This is of particular interest under transient and non-uniform heating conditions because of recent advancements in embedded cooling techniques, which exacerbate spatial non-uniformities, and the demand for cooling solutions for next-generation electronic devices. Additionally, this work aims to provide a high-fidelity experimental characterization technique for slug flow boiling to enable the validation of physics-based phase change models. To provide a foundation for which the effects of transient and non-uniform heating can be studied, flow boiling instabilities are first studied experimentally in a single, 500 μm-diameter borosilicate glass microchannel. A thin layer of optically transparent and electrically conductive indium tin oxide coated on the outside surface of the microchannel provides a spatially uniform and temporally constant heat flux via Joule heating. The working fluid is degassed, dielectric HFE-7100. Simultaneous high-frequency measurement of reservoir, inlet, and outlet pressures, pressure drop, mass flux, inlet and outlet fluid temperatures, and wall temperature is synchronized to high-speed flow visualizations enabling transient characterization of the thermal-fluidic behavior. The effect of flow inertia and inlet liquid subcooling on the rapid-bubble-growth instability at the onset of boiling is assessed first. The mechanisms underlying the rapid-bubble-growth instability, namely, a large liquid superheat and a large pressure spike, are quantified. This instability is shown to cause flow reversal and can result in large temperature spikes due to starving the heated channel of liquid, which is especially severe at low flow inertia. Next, the effect of flow inertia, inlet liquid subcooling, and heat flux on the hydrodynamic and thermal oscillations and time-averaged performance is assessed. Two predominant dynamic instabilities are observed: a time-periodic series of rapid-bubble-growth instabilities and the pressure drop instability. The heat flux, ratio of flow inertia to upstream compressibility, and degree of inlet liquid subcooling significantly affect the thermal-fluidic characteristics. High inlet liquid subcoolings and low heat fluxes result in time-periodic transitions between single-phase flow and flow boiling that cause large-amplitude wall temperature oscillations and a time-periodic series of rapid-bubble-growth instabilities. Low inlet liquid subcoolings result in small-amplitude thermal-fluidic oscillations and the pressure drop instability. Low flow inertia exacerbates the pressure drop instability and results in large-amplitude thermal-fluidic oscillations whereas high flow inertia reduces their severity. Flow boiling experiments are then performed in a parallel channel test section consisting of two thermally isolated, heated microchannels to study the Ledinegg instability. When the flow in both channels is in the single-phase regime, they have equal wall temperatures due to evenly distributed mass flux delivered to each channel. Boiling incipience in one of the channels triggers the Ledinegg instability which induces a temperature difference between the two channels due to flow maldistribution. The temperature difference between the two channels grows with increasing power. The experimentally observed temperature excursion between the channels due to the Ledinegg instability is reported here for the first time. Time-resolved characterization of flow boiling in a single microchannel is then performed during transient heating conditions. For transient heating tests, three different heat flux levels are selected that exhibit highly contrasting flow behavior during constant heating conditions: a low heat flux corresponding to single-phase flow (15 kW/m2), an intermediate heat flux corresponding to continuous flow boiling (75 kW/m2), and a very high heat flux which would cause critical heat flux if operated at this heat flux continuously (150 kW/m2). Transient testing is first conducted using a single heat flux pulse between these heat flux levels and varying the pulse time. It is observed that any step up/down in the heat flux level that induces/ceases boiling, causes the temperature to temporarily over/under-shoot the eventual steady temperature. Following the single heat flux pulse experiments, a time-periodic series of heat flux pulses is applied. A square wave heating profile is used with pulse frequencies ranging from 0.1 to 100 Hz and three different heat fluxes levels (15, 75, and 150 kW/m2). Three different time-periodic flow boiling fluctuations are observed: flow regime transitions, pressure drop oscillations, and heating pulse propagation. For heating pulse frequencies between approximately 1 and 10 Hz, the thermal and flow fluctuations are heavily coupled to the heating characteristics, forcing the pressure drop instability frequency to match the heating frequency. For heating pulse frequencies above 25 Hz, the microchannel wall attenuates the transient heating profile and the fluid essentially experiences a constant heat flux. To improve our ability to predict the performance of heat sinks for two-phase operation, high-fidelity characterization of key hydrodynamic and heat transfer parameters during microchannel slug flow boiling is performed using a novel experimental test facility that generates an archetypal flow regime, devoid of flow instabilities and flow regime transitions. High-speed flow visualization images are analyzed to quantify the uniformity of the vapor bubbles and liquid slugs generated, as well as the growth of vapor bubbles over a range of heat fluxes. A method is demonstrated for measuring liquid film thickness from the visualizations using a ray-tracing procedure to correct for optical distortions. Characterization of the slug flow boiling regime that is generated demonstrates the unique ability of the facility to precisely control and quantify hydrodynamic and heat transfer characteristics. This work has advanced state-of-the-art technologies for the thermal management of high-heat-flux-dissipation devices by providing an improved understanding on the effects of transient and non-uniform heating on flow boiling and an experimental method for the validation of physics-based flow boiling modeling.
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  • Achievements in the growth of ultra-pure III-V semiconductor materials using state of the art molecular beam epitaxy (MBE) machine has led to the discovery of new physics and technological innovations. High mobility two-dimensional electron gas (2DEG) embedded in GaAs/AlxGa1−xAs heterostructures provides an unparalleled platform for many-body physics including fractional quantum Hall effect. On the other hand, single electron devices fabricated on modulation doped GaAs/AlxGa1−xAs heterostructures have been extensively used for fabrication of quantum devices such as spin qubit with application in quantum computing. Furthermore, epitaxial hybrid superconductor-semiconductor heterostructures with ultra clean superconductor-semiconductor interface have been grown using MBE technique to explore rare physical quantum state of the matter namely Majorana zero modes with non-abelian exchange statistics. Chapter 1 in the manuscript starts with description of GaAs MBE system at Purdue University and continues with the modifications have been made to MBE hardware and growth conditions for growing heterostrcutures with 2DEG mobility exceeding 35 × 106 cm−2/V s. Utilizing an ultra-high pure Ga source material and its further purification by thermal evaporation in the vacuum are determined to have major impact on growth of high mobility GaAs/AlxGa1−xAs heterostructures. Chapter 2 reports a systematic study on the effect of silicon doping density on low frequency charge noise and conductance drift in laterally gated nanostructures fabricated on modulation doped GaAs/AlxGa1−xAs heterostructures grown by Molecular Beam Epitaxy (MBE). The primary result of this study is that both charge noise and conductance drift are strongly impacted by the silicon doping used to create the two-dimensional electron gas. These findings shed light on the physical origin of the defect states responsible for charge noise and conductance drift. This is especially significant for spin qubit devices, which require minimization of conductance drift and charge noise for stable operation and good coherence. Chapter 3 demonstrates measurements of the induced superconducting gap in 2D hybrid Al/Al0.15In0.85As/InAs heterostructures which is a promising platform for scaling topological qubits based on Majorana zero modes. The 2DEG lies in an InAs quantum well and is separated from the epitaxial Al layer by a barrier of Al0.15In0.85As with thickness d. Due to hybridization between the wave functions of 2DEG and superconductor, the strength of induced gap in the 2DEG largely depends on the barrier thickness. This chapter presents a systematic study of the strength of the induced gap in hybrid Al/Al0.15In0.85As/InAs superconductor/semiconductor heterostructures as a function of barrier thickness.
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