Experimental and numerical study of flow distribution in compact plate heat exchangers

This PhD work was motivated by the CEA R&D program to provide solid technological basis for the use of Brayton power conversion system in Sodium-cooled Fast nuclear Reactors (SFRs). Multi-channel compact heat exchangers are necessary for the present application because of the low heat transfer capacity of the gas foreseen. In ASTRID project, a minimum size of Na channels section is required to avoid the plugging risk. However, this induces very low pressure losses in the bundle. Considering an additional inlet flow condition, a real risk of bad flow distribution remains. As a result, the thermal performance and thermal loading of the heat exchanger degrades due to it. The main goal of this work was to overcome the flow maldistribution problem by means of an innovative design of sodium distribution system (PATENT FR1657543), the development of a numerical strategy and the construction of an experimental database to validate all theoretical studies. The innovative sodium distribution system consists on an inlet header which tries to guide the evolution of the impinging jet flow while a system of bifurcating pre-distribution channels increases pressure drops in the bundle. Lateral communications between pre-distribution channels are introduced to further homogenize the flow. Two experimental facilities have been conceived to study the flow behavior in bifurcating channels and in the inlet header, respectively. At the same time, their effect on the flow distribution between channels is evaluated. The acquired PIV aerodynamic database allows to validate the numerical models and to prove the design basis for the proposed distribution system. Once having validated the CFD turbulence models and the strategy to study the flow maldistribution in the SGHE module, a decisive and trustworthy optimization of each component of the sodium distribution system has been performed. Finally, an optimal configuration has been proposed for the actual phase of ASTRID project

Oscillatory Flow Driven by Cavity

Flows past a cavity are known to exhibit an oscillatory behavior with an amplitude and frequency dependent on the incoming flow properties and the geometry of the cavity. Experiments and numerical analyses have been performed to determine the effects of a flow passing through an axisymmetric cavity and discharging into a transverse freestream. The study focuses on the mechanisms involved in the generation of pulsatile flow and its influences on a jet in crossflow. Flow characteristics through the cavity and the jet in crossflow interaction were analyzed using the computational fluid dynamics software Siemens Star-CCM+. The experimental analysis utilized a Laser Doppler Velocimetry (LDV) system for measurements of velocity profiles to determine the oscillatory jet flow properties as well at the oscillation frequency. Cavity dimensions with a Length to Depth ratio of 2 was used with an incoming flow mean velocity of 50 m/s resulting in a turbulent jet with a Reynolds number of approximately 33,600. The flow through the cavity emitted an oscillatory flow at 66.68 Hz determined by a Power Spectral Density plot. The oscillatory flow in cross flow exhibited a lower jet trajectory when compared to a steady jet in crossflow and indicates increased vorticity production within the jet, supporting flow recovery immediately downstream of the jet. The passive approach of generating an oscillatory jet in crossflow can aid in mixing of the two flows. Also included in the study is an application of the cavity driven oscillatory flow as it pertains to the upper respiratory system

Bubble Dynamics and Acoustic Droplet Vaporization in Gas Embolotherapy.

Gas embolotherapy is a twist on traditional catheter based embolotherapy approaches. Localized gas bubbles generated are used in place of solid emboli normally used to restrict blood flow. The gas bubbles are formed through targeted vaporization of intravenously injected liquid microdroplets using focused ultrasound, also known as acoustic droplet vaporization (ADV). A greater understanding of the ADV process, bubble transport, and acoustic interactions are essential to devising a safe and effective therapy. This dissertation delves into the dynamics at various time-scales throughout the ADV process from the phase-change process to bubble transport in vessels. The dissertation has been divided into five time-scale events that may occur throughout the ADV process. First, ultra-high speed imaging investigating the initial gas nucleus formation within liquid microdroplets is compared against a numerical model of the acoustic field within the droplet to determine the mechanism behind ADV. The effects of pulse length and acoustic power are correlated with the likelihood of collapsing the newly formed bubble possibly resulting in vessel damage. Next, influences from channel resistance on the ADV bubble expansion rates and wall stress are estimated in idealized microvessels. Once the bubbles have completed expansion, transport phenomena and additional acoustic pulses may influence bubble dynamics and treatment outcomes. The scenario of a finite-sized bubble attached to a vessel wall approaching a bifurcation point is modeled using the boundary element method in order to understand the influences of sticking conditions and bifurcation geometry on bubble lodging or dislodging. Finally, an instability resulting from short acoustic pulses impinging on a bubble attached to a solid boundary resulting in droplet atomization of the bulk liquid in the bubble is characterized. The implications from all of these dynamics are discussed in the context of gas embolotherapy and extended to other ultrasound therapies. It is concluded that potential sources of damage include bubble torus formation, rapid expansion in small vessels, and contact line motion. However, it is revealed that the level of damage can be addressed through the careful choice of acoustic parameters and droplet distribution and functionalization. Furthermore, controlled stress levels can be leveraged for enhanced therapeutic benefits.PhDBiomedical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/107251/1/shicheng_1.pd

Large-eddy simulations of flow and heat transfer for jet impingement on static and vibrating surfaces

The present numerical work serves to understand the flow and thermal characteristics of a turbulent impinging air jet under dynamic flow and geometric conditions using Large-eddy Simulations (LES). A clear relationship between the large-scale structures and resulting heat transfer on the impingement surface exists and was demonstrated through highly-resolved LES of both static and dynamic surface configurations

The State of the Art in Turbulence Modelling in Brazil

The present work discusses at length the current status of turbulent research in Brazil, After eight introductory sections on the subject, where some general aspects of the problem are presented, and a brief review of some scientific and engineering approaches is given, the paper strolls over four specific sections, analyzing all work carried out in Brazil in the past twenty five years on turbulence. In fact, the present compilation is restricted to the main events sponsored by the Brazilian Society of Mechanical Sciences. The present review quotes 284 references, presents 6 tables and ¡6 figures. The paper contents is: Paper Outline, Some Insights, The Traditional Approaches, Some Basic Working Rules, Turbulence Models, One-Point Turbulence Closure Models, Some Other Aproaches to Turbulence Modelling, Some Major Achievements, A Bit of History, Statistics, A Personal View, Gallery, Final Remarks, Cited Bibliography and Compiled Bibliography.Indisponível

A robust immersed boundary method for flow in complex geometries: study of aerosol deposition in the human extrathoracic airways

The flow and the transport of particles in the human respiratory system dictate the effectiveness of therapeutic aerosols used in inhaled drug delivery. The aerosol particles are generally inhaled through the mouth, passing by the throat before reaching the targeted areas in the lungs. Therefore, knowledge of the particle deposition in the mouth-throat region is critical in the design of effective inhalation devices for optimum delivery to the lungs. Numerical simulations offer a non-invasive and cost-effective alternative to in vivo and in vitro tests. However, accurate prediction remains a challenge for numerical models due to the complexity of the flow in the extrathoracic airways. A robust immersed boundary method for flow in complex geometries is proposed. This greatly simplifies the task of grid generation and eliminates the problems associated with grid quality that exist for boundary-fitted grid techniques. The proposed method is an extension to the momentum forcing approach onto curvilinear coordinates and applies an iterative procedure to compute the forcing term implicitly, which stabilizes the scheme for higher Reynolds numbers. The use of a curvilinear grid minimizes the number of unused cells outside the geometry and increases the efficiency of the numerical scheme. The method is validated against numerical and experimental data in the literature for a number of test cases on both Cartesian and curvilinear grids. The results show good agreement with previous studies. Direct numerical simulations were performed in a number of realistic mouth and throat geometries obtained from MRI scans. A Lagrangian particle tracking scheme was employed to advance the particles dynamically, and total and regional deposition efficiencies were determined and compared to in vitro data. The effect of inflow turbulence and intersubject variation on deposition was studied. Geometric variation has a large impact on total deposition whereas the effect of inflow turbulence is confined to oral deposition

Jet Mixing Enhancement by High Amplitude Pulse Fluidic Actuation

Turbulent mixing enhancement has received a great deal of attention in the fluid mechanics community in the last few decades. Generally speaking, mixing enhancement involves the increased dispersion of the fluid that makes up a flow. The current work focuses on mixing enhancement of an axisymmetric jet via high amplitude fluidic pulses applied at the nozzle exit with high aspect ratio actuator nozzles. The work consists of small scale clean jet experiments, small scale micro-turbine engine experiments, and full scale laboratory simulated core exhaust experiments using actuators designed to fit within the engine nacelle of a full scale aircraft. The small scale clean jet experiments show that mixing enhancement compared to the unforced case is likely due to a combination of mechanisms. The first mechanism is the growth of shear layer instabilities, similar to that which occurs with an acoustically excited jet except that, in this case, the forcing is highly nonlinear. The result of the instability is a frequency bucket with an optimal forcing frequency. The second mechanism is the generation of counter rotating vortex pairs similar to those generated by mechanical tabs. The penetration depth determines the extent to which this mechanism acts. The importance of this mechanism is therefore a function of the pulsing amplitude. The key mixing parameters were found to be the actuator to jet momentum ratio (amplitude) and the pulsing frequency, where the optimal frequency depends on the amplitude. The importance of phase, offset, duty cycle, and geometric configuration were also explored. The experiments on the jet engine and full scale simulated core nozzle demonstrated that pulse fluidic mixing enhancement was effective on realistic flows. The same parameters that were important for the cleaner small scale experiments were found to be important for the more realistic cases as well. This suggests that the same mixing mechanisms are at work. Additional work was done to optimize, in real time, mixing on the small jet engine using an evolution strategy.Ph.D.Committee Chair: David Parekh; Committee Member: Ari Glezer; Committee Member: Jeff Jagoda; Committee Member: Richard Gaeta; Committee Member: Samuel Shelto

NAS technical summaries. Numerical aerodynamic simulation program, March 1992 - February 1993

NASA created the Numerical Aerodynamic Simulation (NAS) Program in 1987 to focus resources on solving critical problems in aeroscience and related disciplines by utilizing the power of the most advanced supercomputers available. The NAS Program provides scientists with the necessary computing power to solve today's most demanding computational fluid dynamics problems and serves as a pathfinder in integrating leading-edge supercomputing technologies, thus benefitting other supercomputer centers in government and industry. The 1992-93 operational year concluded with 399 high-speed processor projects and 91 parallel projects representing NASA, the Department of Defense, other government agencies, private industry, and universities. This document provides a glimpse at some of the significant scientific results for the year

Adaptive Control of Axisymmetric Jets by Cavities

Actuators capable of producing large amplitude oscillations at desired frequencies are needed in many flow control applications. In this dissertation, turbulent jets were actuated using self-sustaining oscillations in axisymmetric cavities. Tested geometries include baseline turbulent jets passing through axisymmetric cavities, with and without peripheral walls. Cavity length to depth ratio (Lc/Dc) was varied from 1 to 5 and data were collected at several intermediate steps. Tests were performed with air over a Reynolds number, based on jet exit diameter, range of 40,000 to 225,000 in low to high subsonic jet flow conditions. Pressure signals, 2D velocity data obtained using Particle Image Velocimetry (PIV) and 2D axisymmetric Computational Fluid Dynamics (CFD) modeling were concurrently used to analyze and understand the problem. Measurements on baseline turbulent jets showed that the near field coherent structures and their residence time are two important factors that could be manipulated to influence the initial growth rate of a jet. Frequency of oscillation of the pressure field within the cavity was primarily dependent on the length scale (length to depth ratio and the pipe diameter) of the cavity and the Mach number of the flow. Axisymmetric cavities with cavity length to depth ratio 1.5 –2.0, preferably 1.75, placed immediately after the exit of the jet exhibited a resonant condition between the axial shallow cavity mode and the radial acoustic mode of the pipe) with very high amplitude oscillations (in excess of 180 dB within the cavity) at moderate to high Reynolds number range of this study. Comparison with baseline jets based on centerline velocity decay and lateral jet spread rate showed that mixing characteristics had improved significantly. The potential core length (location of 90% jet exit velocity) for the best case was at 1D as compared to 5D for the baseline jet. Ensemble averages of different phases of an oscillation cycle (along the jet flow direction and in the cross-section) were used to recreate the oscillation cycle (flow visualization images, velocity and vorticity fields) and study the flow structures in the near field. Mode of oscillation (axial / helical) of the jet exiting the cavity was found to be dependent on the inlet and exit boundary conditions of the cavity. Cavities with sharp orifices at the inlet and exit boundaries had complex helical mode structures in the near field of the jet. When the exit boundary of the cavity had a short pipe instead of a sharp orifice, only axial mode vortex rings remained in the near field. However the jet entrainment was not as high, as in the case of sharp orifices

NAS Technical Summaries, March 1993 - February 1994

NASA created the Numerical Aerodynamic Simulation (NAS) Program in 1987 to focus resources on solving critical problems in aeroscience and related disciplines by utilizing the power of the most advanced supercomputers available. The NAS Program provides scientists with the necessary computing power to solve today's most demanding computational fluid dynamics problems and serves as a pathfinder in integrating leading-edge supercomputing technologies, thus benefitting other supercomputer centers in government and industry. The 1993-94 operational year concluded with 448 high-speed processor projects and 95 parallel projects representing NASA, the Department of Defense, other government agencies, private industry, and universities. This document provides a glimpse at some of the significant scientific results for the year