The flow in a layer of volatile fluid driven by a horizontal temperature gradient is a fundamental transport model for numerous evaporative passive cooling applications. When a thin film of a volatile liquid is subject to a horizontal temperature gradient, changes in the surface tension at the free surface lead to Marangoni stresses that drive the flow. In a thicker liquid layer, the flow is also affected by buoyancy. This thesis describes experimental studies of convection driven by a combined action of Marangoni stresses and buoyancy in simple and binary volatile liquid layers confined in a sealed rectangular cavity heated at one end and cooled at the other. Experiments with varying concentrations of noncondensables (i.e., air) ca were performed to investigate their effect on the phase change and heat and mass transport. In the simple liquid, thermocapillary stresses drive the liquid near the free surface away from the heated end. Varying ca is shown to strongly affect the stability of this buoyancy-thermocapillary flow for Marangoni numbers Ma = 290 - 3600 and dynamic Bond numbers BoD = 0.56 - 0.82: removing air suppresses transition to multicellular and unsteady flow. The results are compared with numerical simulations and linear stability analysis. In the binary liquid considered here, a methanol-water (MeOH-H2O) mixture, solutocapillary stresses drive the flow near the free surface towards the heated end. Four distinct flow regimes are identified for this complex flow driven by thermocapillarity, solutocapillarity, and buoyancy, and are summarized in a flow regime map as a function of ca and the liquid composition (MeOH concentration). At low ca, solutocapillary effects are strong enough to drive the liquid near the free surface towards the heated end over the entire liquid layer, suggesting that binary-fluid coolants could significantly reduce film dryout.

No description available.

An extension of the current acetone Planar Laser-Induced Fluorescence (PLIF) was formulated for mixing studies of fluids at subcritical and supercritical conditions. The new technique, called Planar Laser-Induced Fluorescence and Phosphorescence (PLIFP), employs the difference in the mass diffusivity of the denser (liquid) to the less dense (vapor/supercritical fluid) to delineate the interface where a phase change occurs. The vapor/supercritical acetone fluorescence signal is utilized to measure of the acetone vapor density, the mixture fractions and liquid acetone phosphorescence signal to determine the location of the phase interface. The application of the technique requires the photophysical properties of vapor and liquid acetone to be known. Therefore, a series of controlled experiments were done to determine their photophysics at elevated temperatures and pressures up to T/TC = 1.2 and p/pC =1.25. The demonstration of the techniques shows it was able to provide quantitative measurements of acetone number density and the overall mixture fraction within the test chamber. Also, the size and mass of droplets that have broken off from the main jet were determined as well, though the ability is limited to small droplets (d~100 μm). In addition, the technique was able to delineate the low diffusivity (subcritical)/high diffusivity (supercritical) interface very well.

No description available.

No description available.

Flows of normal and superfluid 4 He (He I and He II, respectively) are investigated experimentally. Relatively small particles of solid hydrogen and deuterium are suspended in the experimental volume and their motions are tracked in both mechanically and thermally driven flows. A statistical study of the particle velocity and velocity increment distributions is performed at scales smaller and larger than the mean distance between quantized vortices, the quantum length scale of the investigated flows. We show that, at small scales, the observed particle dynamics in He II is greatly influenced by that of quantized vortices. We, additionally, report that this behavior is independent of the imposed large-scale flow. Instead, at large scales, we observe that particle motions are quasiclassical, that is, very similar to those reported to occur in turbulent flows of viscous fluids. The study reinforces therefore the idea of close similarity between viscous flows and large-scale (mechanically-driven) flows of He II, and simultaneously highlights the small-scale differences due to the presence of quantized vortices in He II.

Large information systems are typically implemented using frameworks and libraries. An important property of such systems is data lineage - the flow of data loaded from one system (e.g. database), through the program code, and back to another system. We implemented the Java Resolver tool for data lineage analysis of Java programs based on the Symbolic analysis library for computing data lineage of simple Java applications. The library supports only JDBC and I/O APIs to identify the sources and sinks of data flow. We proposed some archi- tecture changes to the library to make easily extensible by plugins that can add support for new data processing frameworks. We implemented such plugins for few frameworks with different approach for accessing the data, including Spring JDBC, MyBatis and Kafka. Our tests show that this approach works and can be usable in practice. 1

Carbon dioxide (CO2) is a common working fluid for refrigeration systems. The triple point of CO2 (about −56 °C and 0.51 MPa) is often regarded as the lower operating limit for the con-ventional CO2 vapor compression systems, because below this temperature and pressure, solid CO2 could occur and block the system components. However, if the technical issue could be solved and a stable operation of a vapor compression cycle for heat absorption be-low the triple point pressure (or sublimation cycle) could be realized, there would be a great potential for CO2 to replace the common refrigerants with a very high environmental impact such as R-23 for refrigeration applications below −50 °C. The focus of this work is on the dis-cussion of the feasibly of the sublimation cycle regarding the energy efficiency and the block-ing issues. Seven different two-stage and three-stage CO2 sublimation systems are theoretically evalu-ated and compared to a two-stage R-23 system, which serves as a baseline. A calculation model for the systems is developed. The optimum intermediate pressures for each system as well as the high pressure for the systems in transcritical operations are calculated within the given temperature and pressure constraints. Multiple influence factors, such as the ambient temperature, compressor efficiency, are considered in determining the operating limit and evaluating the performance for each system. In order to find out the cause of the blockages in the sublimation system due to the solid CO2, the solid-gas flow is visualized through experiments. Different throttling devices are investi-gated under various inlet conditions. As the sublimator, a heated sight glass assembly is used. It is found that besides the inlet temperature and pressure condition, the tube wall in the down-stream section of the throttling devices has a great influence on the blockages. A larger heat flux also helps to reduce the blockage in the sublimator. Based on the knowledge gained from the theoretical investigation of the cycle variant and preliminary experiments, a cascade sublimation system is designed, constructed and tested. Despite the fact that the system still requires optimization in terms of energy efficiency and operation stability, it is capable of long continuous operation, and thus the basic feasibility of the sublimation cycle is verified. Finally, the further issues and improvement potentials for the heat transfer and sublimator are discussed.:Acknowledgment Abstract Contents Index of figures Index of tables List of abbreviations and symbols 1 Introduction 1.1 Background and Motivation 1.2 Objective and procedure 2 Fundamentals and state of the art 2.1 The R-744 sublimation cycle 2.2 Expansion into solid-gaseous region and critical flow 2.3 Sublimator and solid-gas two-phase flow 2.4 Summary 3 Thermodynamic analysis of sublimation systems 3.1 Definition of the cycle variants 3.1.1 The baseline system 3.1.2 R-744 cascade systems 3.1.3 R-744 booster systems 3.2 Boundary conditions 3.3 Description of the models 3.3.1 Compressor 3.3.2 Heat exchangers 3.3.3 Other components 3.3.4 Fluid properties 3.4 Process calculation and optimization 3.5 Results and discussion 3.5.1 General boundary conditions 3.5.2 Variable temperatures 3.5.3 Variable compressor efficiency 3.5.4 Variable pressure loss and superheating in the sublimator 3.6 Evaluation of the system variants 4 Experimental visualization of the solid-gas flow 4.1 Throttling below the triple point 4.1.1 Experimental setup - test rig I 4.1.2 Results and discussion 4.2 CO2-Sublimation in a horizontal channel 4.2.1 Experimental setup - test rig II 4.2.2 Results and discussion 4.3 Summary 5 Experimental investigation on the performance of a cascade sublimation system 5.1 Experimental setup – test rig III 5.1.1 The refrigerant cycles 5.1.2 The sublimating unit 5.2 Methodology 5.2.1 The measuring procedure 5.2.2 Data evaluation and uncertainty analysis 5.3 Results and discussion 5.3.1 Transient behavior 5.3.2 System performance 5.3.3 Compressor performance 5.3.4 Long period measurements 5.4 Summary 6 Existing issues and optimization potentials 6.1 Blockage-free operation at low wall temperatures 6.1.1 Supplementary experiment 6.1.2 Outlook 6.2 Heat transfer 6.2.1 Supplementary experiment 6.2.2 Outlook 7 Summary Literature Appendix A. Differential evolution A.1 Basics of differential evolution A.2 Convergence of the results for different system variants Appendix B. Mass flow rate from the capillary tubes B.1 Measurement of the mass flow rate B.2. Comparison of the results with the numerical model and correlations Appendix C. Supplement to the measurements of the test rig III C.1 Exemplary measurement of the R-23 operation C.2. Measurement of the air velocity for the sublimator Appendix D. Supplement to the measurements at low wall temperatures D.1. Calculation of the heat transfer coefficients for the airside D.2. Determination of the local sublimation heat transfer coefficients Publications during the PhD study

This dissertation presents the results from an experimental study of three-dimensional turbulent tip gap flow in a linear cascade wind tunnel with 3.3% chord tip clearance with and without moving endwall simulation. Experimental measurements have been completed in Virginia Tech low speed linear cascade wind tunnel. A 24" access laser-Doppler velocimeter (LDV) system was developed to make simultaneous three-velocity-component measurements. The overall size of the probe is 24"à 37"à 24"and measurement spatial resolution is about 100 μm. With 24" optical access distance, the LDV probe allows measurements to be taken from the side of the linear cascade tunnel instead of through the bottom of the tunnel floor. The probe has been tested in a zero-pressure gradient two-dimensional turbulent boundary layer. Experimental measurements (oil flow visualization, pressure measurement, and LDV measurement) for the stationary wall captured the major flow structures of the tip leakage flow in the linear compressor cascade, such as tip leakage vortex, tip leakage vortex separation and tip separation vortex. Large velocity gradients in the tip leakage vortex separation, tip leakage vortex, and tip separation vortex regions generate large production of the Reynolds stresses and turbulent kinetic energy. One of the most interesting features of the tip leakage flow is the bimodal velocity probability histograms of the v component due to the unsteady motion of the flow in the interaction region between the tip leakage vortex and tip leakage jet. The tip separation vortex, tip leakage vortex separation, and tip leakage vortex contain most of turbulent kinetic energy and generate the highest dissipation rate. Relative motion of the endwall significantly affects the tip gap flow structures, especially in the near wall region. Compared to the stationary wall case, velocity gradients in the near wall region for the moving wall case are much smaller and lower velocity gradients in the near wall region cause the low production of Reynolds stresses and turbulent kinetic energy. Similar to the stationary wall case, high Reynolds stresses and turbulent kinetic energy values are mainly located in the vicinity of the tip leakage vortex and tip separation vortex region. The bimodal velocity probability histograms of the v component are also found at the same locations. The tip separation vortex with most of the turbulent kinetic energy generates the highest dissipation rate. The dissipation rate in the tip leakage vortex region is reduced with the decrease of turbulent kinetic energy under the moving wall effect. / Ph. D.

This thesis presents some results from an experimental study of three-dimensional turbulent tip gap flows in the linear cascade wind tunnel, for two different tip gap clearances (t/c=1.65% and 3.3%). The experiments focus on near-wall flow field measurements for the stationary wall and moving wall, and static pressure measurement on the low end-wall for the stationary wall case. The representative flows were pressure driven, three-dimensional turbulent boundary layers in the linear cascade tunnel for the stationary wall case, and the combination of the pressure driven and shear driven flow for the moving wall case. Several experimental techniques are used in the studies: a three-orthogonal-velocity-component fiber-optic laser Doppler anemometer (3D-LDA) system, surface oil flow visualization, and a scanivalve system for static pressure measurement through pressure ports on the end-wall. From the details of the oil flow visualization pattern on the end-wall, some features of the passage flow, cross flow, and the tip leakage vortex in this cascade flow were captured. Oil flow visualization on the blade surface reveals the reattachment of the tip leakage vortex on the blade surface. The static pressure results on the lower end-wall and mid-span of the blade show huge pressure drop on the lower end-wall from the pressure side to the suction side of the blade and from mid-span to the lower end wall. The end-wall skin friction velocity is calculated from near-wall LDA data and pressure gradient data using the near-wall momentum equation. The statistics of Reynolds stresses and triple products in two-dimensional turbulent boundary layer and three-dimensional turbulent boundary layer was examined using a velocity fluctuation octant analysis in three different coordinates (the wall collateral coordinates, the mid tip gap coordinates, and the local mean flow angle coordinates). The velocity fluctuation octant analysis for the two-dimensional turbulent boundary layer reveals that ejections of the low speed streaks outward from the wall and the sweeps of high speed streaks inward toward the wall are the dominant coherent motions. The octant analysis for the three-dimensional turbulent boundary layer in the tip gap shows that the dominant octant events are partially different from those in the two-dimensional turbulent boundary layer, but ejection and sweep motions are still the dominant coherent motions. For the three-dimensional turbulent boundary layer in the moving wall flow, the near-wall shear flow reinforces the sweep motion to the moving wall and weakens the out-ward ejection motion in the shear flow dominant region. Between the passage flow and the shear flow, is the interaction region of the high speed streaks and the low speed streaks. This is the first time that the coherent structure of the three-dimensional turbulent boundary in the linear cascade tip gap has been studied. / Master of Science