<div>Rotating Detonation Engines (RDEs) provide a promising avenue for reducing greenhouse gas emissions from combustion-based propulsion and power systems by improving their thermodynamic efficiency through the application of pressure-gain combustion. However, the thermodynamic and systems-level advantages remain unrealized due to the challenge of harnessing the tightly coupled physics and nonlinear detonation dynamics inherent to RDEs, particularly for the less-detonable reactants characteristic of applications. Therefore, a RDE was developed to operate with natural gas and air as the primary reactants at elevated chamber pressures and air preheat temperatures, providing a platform to study RDEs with the less-detonable reactants and flow conditions representative of land-based power generation gas turbine engines. The RDE was tested with two injector configurations in a broad, parametric survey of flow conditions to determine the effect of operating parameters on the propagation of detonation waves in the combustor and delivered performance. Measurements of chamber wave dynamics were performed using high-frequency pressure transducers and high-speed imaging of broadband combustion chemiluminescence, while thrust measurements were used to characterize the work output potential.</div><div><br></div><div>The detonation dynamics were first studied to characterize RDE operability for the target application. Wave propagation speeds of up to 70% of the mixture Chapman-Jouguet detonation velocity and chamber pressure fluctuations greater than 4 times the mean chamber pressure were observed. Supplementing the air with additional oxygen, varying the equivalence ratio, and enriching the fuel with hydrogen revealed that combustor operability is sensitive to the chemical kinetics of the reactant mixture. While most test conditions exhibited counter-rotating detonation waves within the chamber, one injector design was able to support single wave propagation. A thermodynamic performance model was developed to aid analysis of RDE performance by making comparisons of net pressure gain for identical flow conditions. While the injector that supported a single wave operating mode better followed the trends predicted by the model, neither injector achieved the desire stagnation pressure gain relative to the reactant manifold pressure. Application of the model to a generic RDE revealed the necessity of normalizing any RDE performance parameter by the driving system potential and identified the area ratio between the exhaust and injection throats as the primary parameter affecting delivered pressure gain. A pair of test conditions with distinct wave dynamics were selected from the parametric survey to qualitatively and quantitatively analyze the exhaust flow using high-speed particle image velocimetry. A single detonation wave with an intermittent counter-rotating wave was characterized in the first test case, while a steady counter-rotating mode was studied in the second. The velocity measurements were phase averaged with respect to the instantaneous wave location to reveal contrasting flowfields for the two cases. The total pressure and temperature of flow exiting the combustor were computed using the phase-resolved velocity measurements along with the measured reactant flowrate and thrust to close the global balance of mass and momentum, providing an improved method of quantifying RDE performance. Finally, a reduced order model for studying RDE operability and mode selection was developed. The circumferential detonation wave dynamics are simulated and permitted to naturally evolve into the quasi-steady state operating modes observed in RDEs. Preliminary verification studies are presented and areas for further development are identified to enable the model to reach a broader level of applicability.</div><div><br></div><div>The experimental component of this work has advanced understanding of RDE operation with less-detonable reactants and developed improved methods for quantifying RDE performance. The accompanying modeling has elucidated the design parameters and flow conditions that influence RDE performance and provided a framework to investigate the factors that govern RDE mode selection and operability.<br></div>

A research-scale solid-fuel ramjet test article has been developed to study the secondary combustion zone of solid fuel ramjets. Tests were performed at a constant core air mass flowrate of 0.77 kg/s with 0%, 15%, and 30% bypass ratios. The propulsive performance analysis results indicate that the 0% bypass case had the highest regression rate and fuel mass flowrate. The regression rate and fuel mass flowrate of fuel without carbon black was the lowest. The specific impulse with air mass flowrate included was highest for the 0% bypass case reaching 130 s and lowest for the 30% bypass case reaching 110 s. For specific impulse with air mass flowrate excluded, the 30% bypass case achieved 2,800 s while the 0% bypass case achieved 1,800 s. The characteristic velocity was greatest for 0% bypass reaching 1,025 m/s and lowest for 30% bypass reaching 900 m/s. The combustion efficiency was highest for the 15% bypass case with carbon black addition approaching 0.82. 50 kHz and 75 kHz CH* chemiluminescence imaging was performed. Analyzing thin slivers of the images over 40,001 frames with frequency-domain techniques showed that most of the high amplitude content occurred below 1-5kHz with small peaks near 20 kHz and 30 kHz. Dynamic mode decomposition (DMD) was performed on sets of 10,001 spatially-calibrated images and their corresponding uncalibrated, uncropped images. Most of the tests exhibited low-frequency axial pumping, transverse modes, and other mode shapes indicative of the secondary injection. The prominence of transverse and other jet-related modes over axial modes appeared to be related to increasing bypass ratio. High-frequency axial modes also appeared in a case thought to have high core-flow momentum that did not appear at these high frequencies for other cases. The DMD modes for 0% bypass were indiscernible due to high soot content. Most of the modes corresponding to the calibrated images also appeared in the uncalibrated images, however, with different mode amplitude rankings. PIV was performed at 5 kHz for one test at 15% bypass. The instantaneous vector fields for these tests displayed local velocities up to 600 m/s. The mean images showed velocities up to 250 m/s. The two-dimensional turbulent kinetic energies reached 200 m2/s2 in several regions throughout the flowfield. The turbulence intensity exceeded 0.20 near the bottom of the flowfield.

<div>The continued advancement of gas turbine combustion technology for power generation and propulsion applications requires novel techniques to increase the overall engine cycle efficiency and improved methods for mitigating combustion instabilities. To help address these problems, high-speed optical diagnostics were applied to two different experiments that replicate relevant physics in gas turbine combustors. The focus of the measurements was to elucidate the effect of various operating parameters on combustion dynamics occurring over a wide range of spatio-temporal flow and chemical scales. The first experiment, VIPER-M, enabled the investigation of coupling mechanisms for transverse instabilities in a multi-element, premixed combustor that maintains key similarities with gas turbine combustors for land based power generation. The second experiment, COMRAD, facilitated the study of the effect of fuel heating on the combustion performance and dynamics in a liquid-fueled, piloted swirl flame typical of aviation engine combustors. </div><div> </div><div><br></div><div>Two different injector lengths were tested in the VIPER-M experiment, and high-speed CH* chemiluminescence imaging and an array of high-frequency pressure transducers were used to characterize the overall combustor dynamics. For all conditions tested, the longer injector length configuration exhibited high-amplitude instabilities, with pressure fluctuations greater than 100% of the mean chamber pressure. This was due to the excitation of the fundamental transverse mode, with a frequency around 1800 Hz, as well as multiple harmonics. Shortening the injector length significantly lowered the instability amplitudes at all conditions and excited an additional mode near 1550 Hz for lower equivalence ratio cases. The delineating feature controlling the growth of the instabilities in the two injector configurations was shown to be the coupling between the transverse modes in the chamber and axial pressure fluctuations in the injectors.</div><div> </div><div><br></div><div>Heated fuels were introduced into the COMRAD experiment, and simultaneous 10 kHz stereoscopic particle image velocimetry and OH* chemiluminescence imaging were performed over a range of equivalence ratios and combustor pressures to study the influence of fuel temperature on the flow and flame structure. The main flame was found to move upstream as the fuel was heated, while no changes in the pilot flame location were observed in the field of view at the exit of the injector. The upstream shift of the main flame corresponded to a local increase in the axial velocity, which caused the shear layer between the pilot/main flames and the central recirculation zone to move downstream. Direct comparison of the mean velocity fields relative to the mean flame location showed that heating the fuel caused the velocity normal to the flame front to increase, which is indicative of an increase in flame speed. The changes to the fuel injection and chemical kinetics help explain the local changes to the flow and flame structure, which contribute to an overall increase in combustion efficiency as well as NO_x emissions.</div><div> </div><div><br></div><div>Lastly, the effect of fuel injection temperature on the presence of an 800 Hz combustion instability in the COMRAD experiment was investigated. High-frequency pressure and high-speed chemiluminescence measurements revealed a decrease in the instability amplitude as the fuel was heated. The coupling between the fuel flow and the unsteady heat release was studied using independent 10 kHz stereoscopic particle image velocimetry and 10 kHz Mie scattering measurements. The variations in the fuel flow entering the combustor over the acoustic cycle decreased as the instability amplitude weakened. 100 kHz burst-mode, two-component particle image velocimetry was then applied to the unstable condition with ambient temperature fuel. This measurement was capable of resolving both the large-scale changes to the structure of the inner recirculation zone occurring at 800 Hz as well as the time-evolution of small-scale vortex structures. The vortices were shown to correspond to a characteristic frequency in the range of 4-5.5 kHz, and the strength of the vortex structures fluctuated with the global 800 Hz combustion dynamics. These results highlight the importance of performing measurements capable of resolving the wide range of scales present in the flow-fields typical of gas turbine combustors to improve current understanding of flame-flow coupling mechanisms.</div>

<p>Microfluidics has established itself as a key technology in a wide range of fields including pharmaceuticals, point-of-care diagnostics, thermal management, and space technology. Most of these applications involve manipulation of small quantities (micro – nanoliters) of fluids and various particles or biological cells suspended in them. These platforms employ mechanical, thermal, acoustic, magnetic, optical, electric and many other means for creating particle and fluid motion. Many biological applications require handling cells that are vulnerable to getting damaged if proper physiological conditions are not maintained or if excessive force is applied on them. The non-invasive nature of optical and electrical micro-manipulation techniques such as rapid electrokinetic patterning (REP) has proven to be of great importance in such applications. These techniques enable handling, transportation, sorting and arrangement of fragile synthetic micro/nanoparticles and biological cells without compromising their structure and surface properties.</p> <p>REP is a recently developed micro-manipulation tool that employs optically induced electrothermal vortices to create custom flow patterns. Particle suspensions are entrained in these vortices and are trapped on an electrode surface through AC electrokinetic mechanisms. This work focuses on characterizing a REP trap and discusses its potential applications in handling biological cells. Polystyrene microparticles are confined in a REP trap and a MATLAB program is used to track their motion inside the trap. The tracked particle trajectory reveals that the potential energy of the trapped particle is parabolic and hence the trap is Hookean in nature. The trap is modelled as a spring-mass system and the stiffness coefficient of that system is found to be of the order of 10^-15 N/μm. The origin of the restoring force in the spring-mass model is found to be the drag force created by the electrothermal vortex. The ability to exert ultra-small forces in a stable trap enables REP to be used in various non-invasive particle manipulation applications.</p> <p>The transient nature of REP is studied using numerical modeling and particle image velocimetry (PIV) analysis of a vortex created by a moving laser spot. A numerical model suggests that custom-shaped steady state REP vortices can be created via superposition of multiple axisymmetric circular shaped vortices. However, the method of superposition cannot be extended to transient traps and a more involved 3D model is required to simulate them. The laser spot is scanned back-and-forth in a line with different speeds to create transient REP vortices. The PIV analysis, in agreement with the numerical model, shows that the location of the moving vortex is undiscernible at high speeds. Moreover, the circular shaped vortex is stretched out into a line when the laser scanning frequencies are more than 15 Hz.</p> <p>The particle-electrode attraction force, which entraps the particles at the electrode surface, is characterized using particle diffusometry (PD) and defocusing particle tracking. PD is used to measure the diffusion coefficient of polystyrene particles under different electric field parameters near an electrode surface. It is found that the particle diffusivity decreases with a decrease in the electric field frequency from 150 – 30 kHz and with an increase in the applied voltage from 4 – 8 V_pp. A MATLAB program is used to track the number of in-focus particles and their distance from the electrode surface. A histogram of the particles’ distance from the electrode surface shows an increase in the particle concentration near the electrode at low frequencies (30 – 60 kHz). These observations suggest that the average height of an entrapped particle decreases with a decrease in applied field frequency and an increase in applied voltage. This suggests that the attractive trapping force is significant at 30 kHz but diminishes at around 150 kHz.</p> <p>Salt and sugar-based isotonic media used for cell suspensions pose several challenges for electrokinetic mechanisms such as REP. Various solutions to overcome these challenges for bio-manipulation applications are discussed in this work. The presence of DC offset in the AC electric field is found to enhance particle entrapment in sugar-based media. The effect of DC offset on trapping performance in bio-relevant media is assessed by measuring the stability of the REP trap. This work also shows entrapment and manipulation of Mice pancreatic cancer cells (KPC2) suspended in the sugar-based isotonic media using REP. The biological applications of the REP technology are highly promising, but they have not yet been well-explored. This work lays the foundation of understanding how REP can be operated in high osmolarity media for bio-manipulation applications.</p>

<p>Cavitation is ubiquitous in nature and scientific application where it might hinder through noise, vibration or erosion which eventually leads to reduced performance. Similarly, rising bubbles are employed in several industrial applications to homogenize the fluid. This thesis sheds light on special applications of these two phenomena. </p> <p>Once a bubble has been captured by a vortex core, the low (sometimes negative) pressure in the core causes the cavitation bubble to elongate axially while the radius of the bubble oscillates with time. Three dimensional compressible Navier-Stokes equations with surface tension are numerically solved using an all-mach solver on Basilisk software. The bubble dynamics can be categorised into separate stages: spherical growth, pinching, elongation and fragmentation. As the cylindrical bubble grows, it increases the vortex core radius. The flow and the bubble dynamics are strongly coupled. The effect of changing cavitation number and bubble to vortex size ratio has been explored. The bubble sizes and dynamics at different time steps have also been recorded. When the pressure in the core is negative, the bubble continues to grow axially forming a long tube, which is also observed in experiments. In oceans, density varies with depth due to varying salinity and temperature gradient, which prevents the vertical exchange of heat, carbon, dissolved oxygen, and nutrients as well as blooms the population of harmful bacteria such as cyanobacteria. The rising motion of a single or cluster of bubbles creates an upflow that can cause homogenization or destratification. Confined bubble columns are used for microelectronic cooling as well as in chemical reactors for mixing stratified fluids without any mechanical agitation or power. To begin realizing this complex multi-phase flow system to better understand mixing, we start with a simplified problem of a single air bubble rising in a confined Hele-Shaw channel. We performed a time-resolved stereoscopic Particle Image Velocimetry (PIV) measurement to characterize the bubble wake. Pure water and varying salt concentration were used to achieve a linear density stratification corresponding to Froude numbers (Fr) ranging from 22.1 to 40.7. Due to the large velocity dynamic range for PIV, we enhanced the signal to noise ratio of our correlation planes with pyramid correlation. We found a significant out of plane velocity component in both homogenous and stratified fluid in the vicinity of the bubble, which was assumed to be negligible in previous studies with confined fluid. The wake of the bubble carries the higher density fluid to the top, which later releases from the wake to form the reverse jet. This buoyant jet has been characterized for different Fr. Eulerian coherent structures are also considered to describe the flow. The rising bubble generates vortices that shed downstream and decay with varying timescales for different Fr. The difference in the coherent structures and decay coefficient leads to a different level of mixing with Fr. The scope of this research is in applications homogenizing the stratified flow using rising bubbles. </p>

An examination of parameters affecting the control of a jet vectoring technique used in the Coanda-assisted Spray Manipulation (CSM) is presented. The CSM makes use of an enhanced Coanda effect on axisymmetric geometries through the interaction of a high volume primary jet flowing through the center of a collar and a secondary high-momentum jet parallel to the first and adjacent to the convex collar. The control jet attaches to the convex wall and vectors according to known Coanda effect principles, entraining and vectoring the primary jet, resulting in controllable r-θ directional spraying. Several control slots (both annular and unique sizes) and expansion radii were tested over a range of momentum flux ratios to determine the effects of these variables on the vectored jet angle and profile. Two- and three-component Particle Image Velocimetry (PIV) was used to determine the vectoring angle and the profile of the primary jet in each experiment. The experiments show that the control slot and expansion radius, along with the momentum ratios of the two jets, predominantly affected the vectoring angle and profile of the primary jet. The Reynolds number range for the primary jet at the exit plane was between 20,000 and 80,000. The flow was in the incompressible Mach number range (Mach< 0.3).

The uncertainty of any measurement is the interval in which one believes the actual error lies. Particle Image Velocimetry (PIV) measurement error depends on the PIV algorithm used, a wide range of user inputs, flow characteristics, and the experimental setup. Since these factors vary in time and space, they lead to nonuniform error throughout the flow field. As such, a universal PIV uncertainty estimate is not adequate and can be misleading. This is of particular interest when PIV data are used for comparison with computational or experimental data. A method to estimate the uncertainty due to the PIV calculation of each individual velocity measurement is presented. The relationship between four error sources and their contribution to PIV error is first determined. The sources, or parameters, considered are particle image diameter, particle density, particle displacement, and velocity gradient, although this choice in parameters is arbitrary and may not be complete. This information provides a four-dimensional "uncertainty surface" for the PIV algorithm used. After PIV processing, our code "measures" the value of each of these parameters and estimates the velocity uncertainty for each vector in the flow field. The reliability of the methodology is validated using known flow fields so the actual error can be determined. Analysis shows that, for most flows, the uncertainty distribution obtained using this method fits the confidence interval. The method is general and can be adapted to any PIV analysis.

A particle image velocimetry (PIV) computer software is analyzed in this work by applying automatic differentiation on it. We create two artificial images that contained particles that where moved with a known velocity field over time. These artificial images were created with parameters that we would have on real PIV experiments. Then we applied a PIV software to find the velocity output vectors. As we mentioned before, we applied automatic differentiation through all the algorithm to track the derivatives of the output vectors regarding interesting parameters declared as inputs. By analyzing these derivatives we analyze the sensitivity of the output vectors to changes on each one of the parameters analyzed. One of the most important derivatives calculated in this project was the derivative of the output regarding the image intensity. In future work we plan to use this derivative combined with the intensity probability distribution of each image pixel, to find PIV uncertainties. If we achieve this goal we will find an uncertainty method that will save computational power and will give uncertainty values with computer accuracy.

No description available.

A left ventricular assist device is a mechanical pump implanted in patients with heart failure that continuously takes blood from the left ventricle and delivers it to the aorta, thus decreasing ventricular load. The device is typically considered as a ‘bridge to transplant’, i.e. as a temporary therapy, and involves several risks. Modified ventricular hemodynamics due to a heart pump implantation is studied in-vitro using an elastic ventricle. The ventricle is incorporated into a pulse duplicator setup, which prescribes realistic pulsatile inflow/outflow to mimic a weak ejection fraction. A continuous axial pump mimics a ventricular assist device and its effect on the ventricular hemodynamics is investigated as a function of the pump flow suction. Using particle image velocimetry, pump flow effectiveness at providing unloading on the ventricle and increasing ejection is observed and understanding if proper recirculation of the myocardium down to the apex is restored under varying flow rate.