Comprendre les auto-oscillations dans le caloduc pulsé mono-branche

Abstract : In this thesis, scientific contributions on the understanding of the self-oscillations in the Single-Branch Pulsating Heat Pipe (SBPHP) and on the Self-Oscillating Heat Engine (SOFHE) are presented. The SBPHP is a tube of small diameter closed at one end in which a vapor bubble is followed by a liquid plug. Surprisingly, heating the closed end over a threshold leads to oscillations of the liquid plug which can be maintained indefinitely. Those self-oscillations can be used for cooling, pumping or energy harvesting when coupled to an electromechanical transducer (SOFHE). However the lack of understanding of the dynamics makes it difficult to control the self-oscillations and to understand how to design good devices. In this thesis, some fundamental questions on the dynamics are answered by a theoretical approach and experimental validation, in order to better understand the phenomenon and to provide guidelines for the design of effective devices. We first look at where the oscillations come from and why the amplitude grows during the startup. We show that the compression and expansion of the vapor coupled with the liquid plug inertia leads to a spring-mass system. We then uncover the existence of an instability mechanism due to the interplay of phase-change which acts as a positive feedback and viscous friction, which dissipates energy. The startup occurs when the phase-change coefficient is greater than the friction coefficient. Both the spring-mass system and the instability mechanism are validated experimentally. We then ask: why does the amplitude saturate during the startup? We show, using nonlinear dynamical techniques, that this is explained by a limiting mechanism produced by the nonlinearities . The system reaches a limit cycle, created through a Poincaré-Andronov-Hopf bifurcation. By controlling the phase-change and the friction, one can promote the instability mechanism and reduce the limiting mechanism such that the oscillations amplitude increases. We also study the dynamics further, from small to large oscillations amplitude. To do so, we use numerical continuation first, and then obtain accurate analytical solutions. We then consider the behavior of a SOFHE. We show how the dynamics, the power output and the efficiency are impacted by the electromechanical transducer. We find that self-oscillating harvesters (as SOFHE) differs qualitatively from forced-oscillating harvesters. Finally, we review our results from a general energy perspective. We show that the instability mechanism and the limiting mechanism can be explained by the phase-change and the friction work rate. One can increase the oscillations amplitude or the power output and the efficiency significantly by increasing the phase-change work rate or reducing the friction work rate. We conclude by suggesting that controlling the magnitude and the timing of the phase-change by engineered tubes seems a promising approach to increase the performance of devices.Dans cette thèse, des contributions scientifiques sur la compréhension des auto-oscillations dans le caloduc auto-oscillant mono-branche (SBPHP) et le moteur fluidique auto-oscillant (SOFHE) sont présentées. Le SBPHP est un tube de faible diamètre fermé à l’une des extrémités, dans lequel une bulle de vapeur est suivie d’une colonne de liquide. Étonnamment, chauffer l’extrémité fermée au-delà d’un certain seuil mène à des oscillations de la colonne de liquide qui peuvent être maintenues indéfiniment. Ces auto-oscillations peuvent être utilisées pour refroidir, pomper ou pour récupérer de l’énergie lorsque couplées à un transducteur électromécanique (SOFHE). Toutefois, parce que la dynamique est mal comprise, il est difficile de contrôler les auto-oscillations et de comprendre comment concevoir des dispositifs performants. Dans cette thèse, des réponses à des questions fondamentales sur la dynamique sont obtenues, par une approche théorique et des validations expérimentales, pour mieux comprendre le phénomène et guider la conception. Nous nous demandons d’abord d’où proviennent les oscillations et pourquoi leur amplitude augmente durant le démarrage. Nous montrons que l’inertie de la colonne de liquide couplée à la compression/ dilatation de la vapeur produit un système masse-ressort. Nous révélons ensuite l’existence d’un mécanisme d’instabilité, dû à l’interaction du changement de phase qui agit comme force de rétroaction positive et à la friction visqueuse, qui dissipe de l’énergie. Le démarrage se produit lorsque le coefficient du changement de phase est supérieur au coefficient de friction. Le système masse-ressort et le mécanisme d’instabilité sont validés expérimentalement. Nous nous demandons ensuite : pourquoi l’amplitude sature durant le démarrage ? Nous montrons, à l’aide de techniques de dynamique non linéaire, que cela s’explique par l’existence d’un mécanisme limitant produit par les non-linéarités. Le système atteint un cycle limite, produit par une bifurcation de Poincaré-Andronov-Hopf. En contrôlant le changement de phase et la friction, il est possible d’augmenter l’instabilité et de réduire la limitation et ainsi, d’augmenter l’amplitude des oscillations. Nous poussons l’étude de la dynamique plus loin, de faibles à grandes amplitudes. Pour ce faire, nous utilisons la continuation numérique d’abord, puis obtenons des solutions analytiques précises. Nous nous intéressons ensuite au comportement du SOFHE. Nous montrons comment la dynamique, la puissance et l’efficacité sont influencées par le transducteur électromécanique. Des récupérateurs auto-oscillants (comme SOFHE) diffèrent qualitativement de récupérateurs forcés. Finalement, nous revisitons nos résultats selon une approche énergétique générale. Nous montrons que le mécanisme d’instabilité et le mécanisme limitant peuvent être expliqués en fonction du travail produit par le changement de phase et par la friction. Il est possible d’augmenter significativement l’amplitude, la puissance ou l’efficacité en augmentant le travail fait par le changement de phase ou en réduisant celui fait par la friction. Nous concluons en suggérant que contrôler l’amplitude et le synchronisme du changement de phase par des tubes modifiés semble être une avenue très prometteuse pour améliorer la performance des dispositifs

Effect of evaporator length on the performance of a self-oscillating fluidic heat engine (SOFHE)

Abstract: This paper reports the effect of evaporator length on the performance of a self-oscillating fluidic heat engine (SOFHE). The SOFHE is a thermal energy harvester, when coupled with an electro-mechanical transducer that was proposed to power wireless sensors widely used in the Internet of Things (IoT). The mechanical power of the SOFHE is in the order of fraction of milliwatts, which makes it a promising power supply for a range of wireless sensors with the power requirements of 10s µW. The SOFHE consists of a vapor bubble trapped by an oscillating liquid plug acting as a piston. The working principle of the SOFHE is similar to a singlebranch pulsating heat pipe. The engine is a small tube (inner diameter of 2 mm) filled with deionized water heated from a closed end and cooled from the opposite open end. By perturbing the equilibrium of the vapor bubble-liquid plug, oscillation start and are sustained by cyclic evaporationcondensation from a thin film in the vapor bubble. To characterize SOFHE’s mechanical power as a function of the evaporator length, measurements of pressure, oscillation amplitude, and frequency are conducted. As the evaporator length decreases (from 7 cm to 1 cm), the oscillation amplitude decreases (from 5.9 mm to 1.5 mm) while the frequency increases (from 27 Hz to 52 Hz). In theory, the power of SOFHE is proportional to the square of frequency and amplitude, so the trend in power is not obvious given the opposing effects. The results show a decrease in the mechanical power from 380 µW to 180 µW, which implies that the negative effect of the amplitude decrease dominates over the increase in frequency. A fourfold decrease was also observed in the net evaporation rate (from 1027 to 242 µg/s), which explains why the amplitude decreases with the evaporator length. The research findings contribute to the design of both SOFHEs and pulsating heat pipes by suggesting that a longer heated zone improves the performance.Communication présentée lors du congrès international tenu conjointement par Canadian Society for Mechanical Engineering (CSME) et Computational Fluid Dynamics Society of Canada (CFD Canada), à l’Université de Sherbrooke (Québec), du 28 au 31 mai 2023

Reactivity of neodymium carriers in deep sea sediments: Implications for boundary exchange and paleoceanography

The dissolved neodymium (Nd) isotopic distribution in the deep oceans is determined by continental weathering inputs, water mass advection, and boundary exchange between particulate and dissolved fractions. Reconstructions of past Nd isotopic variability may therefore provide evidence on temporal changes in continental weathering inputs and/or ocean circulation patterns over a range of timescales. However, such an approach is limited by uncertainty in the mechanisms and importance of the boundary exchange process, and the challenge in reliably recovering past seawater Nd isotopic composition (εNd) from deep sea sediments. This study addresses these questions by investigating the processes involved in particulate–solution interactions and their impact on Nd isotopes. A better understanding of boundary exchange also has wider implications for the oceanic cycling and budgets of other particle-reactive elements. Sequential acid-reductive leaching experiments at pH ∼2–5 on deep sea sediments from the western Indian Ocean enable us to investigate natural boundary exchange processes over a timescale appropriate to laboratory experiments. We provide evidence that both the dissolution of solid phases and exchange processes influence the εNd of leachates, which suggests that both processes may contribute to boundary exchange. We use major element and rare earth element (REE) data to investigate the pools of Nd that are accessed and demonstrate that sediment leachate εNd values cannot always be explained by admixture between an authigenic component and the bulk detrital component. For example, in core WIND 24B, acid-reductive leaching generates εNd values between −11 and −6 as a function of solution/solid ratios and leaching times, whereas the authigenic components have εNd ≈ −11 and the bulk detrital component has εNd ≈ −15. We infer that leaching in the Mascarene Basin accesses authigenic components and a minor radiogenic volcanic component that is more reactive than Madagascan-derived clays. The preferential mobilisation of such a minor component demonstrates that the Nd released by boundary exchange could often have a significantly different εNd composition than the bulk detrital sediment. These experiments further demonstrate certain limitations on the use of acid-reductive leaching to extract the εNd composition of the authigenic fraction of bulk deep sea sediments. For example, the detrital component may contain a reactive fraction which is also acid-extractible, while the incongruent nature of this dissolution suggests that it is often inappropriate to use the bulk detrital sediment elemental chemistry and/or εNd composition when assessing possible detrital contamination of leachates. Based on the highly systematic controls observed, and evidence from REE patterns on the phases extracted, we suggest two approaches that lead to the most reliable extraction of the authigenic εNd component and good agreement with foraminiferal-based approaches; either (i) leaching of sediments without a prior decarbonation step, or (ii) the use of short leaching times and low solution/solid ratios throughout

Fabrication and characterization of RNA aptamer microarrays for the study of protein–aptamer interactions with SPR imaging

RNA microarrays were created on chemically modified gold surfaces using a novel surface ligation methodology and employed in a series of surface plasmon resonance imaging (SPRI) measurements of DNA–RNA hybridization and RNA aptamer–protein binding. Various unmodified single-stranded RNA (ssRNA) oligonucleotides were ligated onto identical 5′-phosphate-terminated ssDNA microarray elements with a T4 RNA ligase surface reaction. A combination of ex situ polarization modulation FTIR measurements of the RNA monolayer and in situ SPRI measurements of DNA hybridization adsorption onto the surface were used to determine an ssRNA surface density of 4.0 × 10(12) molecules/cm(2) and a surface ligation efficiency of 85 ± 10%. The surface ligation methodology was then used to create a five-component RNA microarray of potential aptamers for the protein factor IXa (fIXa). The relative surface coverages of the different aptamers were determined through a novel enzymatic method that employed SPRI measurements of a surface RNase H hydrolysis reaction. SPRI measurements were then used to correctly identify the best aptamer to fIXa, which was previously determined from SELEX measurements. A Langmuir adsorption coefficient of 1.6 × 10(7) M(−1) was determined for fIXa adsorption to this aptamer. Single-base variations from this sequence were shown to completely destroy the aptamer–fIXa binding interaction

COVID-19 symptoms at hospital admission vary with age and sex: results from the ISARIC prospective multinational observational study

Background: The ISARIC prospective multinational observational study is the largest cohort of hospitalized patients with COVID-19. We present relationships of age, sex, and nationality to presenting symptoms. Methods: International, prospective observational study of 60 109 hospitalized symptomatic patients with laboratory-confirmed COVID-19 recruited from 43 countries between 30 January and 3 August 2020. Logistic regression was performed to evaluate relationships of age and sex to published COVID-19 case definitions and the most commonly reported symptoms. Results: ‘Typical’ symptoms of fever (69%), cough (68%) and shortness of breath (66%) were the most commonly reported. 92% of patients experienced at least one of these. Prevalence of typical symptoms was greatest in 30- to 60-year-olds (respectively 80, 79, 69%; at least one 95%). They were reported less frequently in children (≤ 18 years: 69, 48, 23; 85%), older adults (≥ 70 years: 61, 62, 65; 90%), and women (66, 66, 64; 90%; vs. men 71, 70, 67; 93%, each P < 0.001). The most common atypical presentations under 60 years of age were nausea and vomiting and abdominal pain, and over 60 years was confusion. Regression models showed significant differences in symptoms with sex, age and country. Interpretation: This international collaboration has allowed us to report reliable symptom data from the largest cohort of patients admitted to hospital with COVID-19. Adults over 60 and children admitted to hospital with COVID-19 are less likely to present with typical symptoms. Nausea and vomiting are common atypical presentations under 30 years. Confusion is a frequent atypical presentation of COVID-19 in adults over 60 years. Women are less likely to experience typical symptoms than men