On the use of acoustic streaming for heat transfer applications

Abstract: This study aims at investigating a possibility to use surface acoustic waves (SAWs) in order to drive acoustic streaming and other nonlinear effects leading to heat transport through a microchannel. As a beginning in this exploratory study, a solid-fluid interface submitted to a temperature gradient is considered, and three distinct mechanisms of heat transfer are estimated analytically. The type of SAW that could be used for an efficient transfer of heat is also discussed

Nonreciprocal and even Willis couplings in periodic thermoacoustic amplifiers

hermoacoustic amplifiers are analyzed in the framework of nonreciprocal Willis coupling. The closed form expressions of the effective properties are derived, showing that an applied temperature gradient causes the appearance of a nonreciprocal Willis coupling. Even and nonreciprocal Willis couplings are exhibited already in the first-order Taylor expansion of the solution and are of equal modulus but opposite sign, thus suggesting that the even Willis coupling is a reaction to the nonreciprocity introduced by the temperature gradients. These Willis couplings cause a coalescence point in the k space, which deviates from Re(k) = 0 (with k the wave number) and is thus a zero-group-velocity point, as well as the opening of an amplification gap at low frequency. Effective parameters and scattering properties are found in excellent agreement with experimental results. This article paves the way to further control the acoustic waves at very low frequencies with nonreciprocal systems

Etude expérimentale et théorique des processus non linéaires de saturation dans un générateur d'ondes thermoacoustique annulaire

For the transformation of thermal energy into mechanical energy, thermoacoustic prime movers employ the interaction between an inhomogeneously heated stack of solid plates and gas oscillations, which results in the onset of self-excited acoustic oscillations. For the past ten years, an important research effort has been devoted to the study of nonlinear processes which are responsible for the saturation of the acoustic wave amplitude; experimental observations were not only poorly mastered and badly documented, but were also unexplained because of the lack of adequate models to describe such complex processes. Thus, the work presented in this report has been initiated to list, classify, interpret the experimental observations analytically, and to propose a model able to describe various transient regimes in an annular thermoacoustic prime mover.The experimental results obtained for various heating processes show complicated transient regimes : periodic switch on-off of the acoustic wave, low frequency modulation of the acoustic wave amplitude, double-threshold effect during the onset of the thermoacoustic instability and so on. Significant variations of the temperature field have also been observed during these transient regimes indicating that, in addition to the classical cascade process of higher harmonic generation and minor losses (vortex generation at the edges of the stack), the nonlinear processes involving interaction between acoustic and temperature fields play an important role in the dynamics of the transient regime. In particular, the development of a high amplitude acoustic field induces both thermoacoustic enthalpy flow (equivalent to acoustically enhanced thermal conductivity of the stack walls) and acoustic streaming which modifies the shape of the temperature field.The influence of sound amplification on temperature distribution is studied analytically, and shows that the distribution of the acoustic field (pressure, velocity and phase shift), and consequently the thermoacoustic amplification, strongly depends on the shape of the temperature field. Then, the analytical description of the above mentioned nonlinear effects and of the transient interaction between acoustic and temperature fields is proposed : the simulation results are very close to experimental results, thus confirming the major role of acoustically enhanced conductivity and acoustic streaming. Finally, a more qualitative description of the transient regime is proposed as a set of ordinary differential, equations, and allows to extract the predominant physical processes.Because it supplies a new undestanding of fundamental processes which control the acoustic wave saturation, this work contributes, in our opinion, to the progress in designing thermoacoustic engines so that they become much more efficient.Les moteurs thermoacoustiques sont des résonateurs acoustiques dans lesquels l'interaction des oscillations acoustiques et thermiques au voisinage d'un empilement localisé de parois solides (stack) soumises à un fort gradient de température engendre une conversion d'énergie thermique en énergie acoustique, qui se traduit par le déclenchement d'oscillations acoustiques auto-entretenues de fort niveau. Depuis une dizaine d'années, un important effort de recherche a été consacré à l'étude des processus non linéaires responsables de la saturation en amplitude de l'onde acoustique ; les observations expérimentales restaient non seulement mal maîtrisées et limitées, mais encore inexpliquées en raison de l'absence de modèles pertinents en regard de la complexité des phénomènes. C'est ainsi que l'objet des travaux présentés dans ce mémoire est de répertorier expérimentalement, de classifier, d'interpréter de façon analytique et de modéliser les différents régimes transitoires dans un moteur thermoacoustique annulaire.Les résultats expérimentaux, obtenus sous diverses conditions de chauffage, montrent des régimes transitoires complexes: régime périodique de déclenchement-arrêt de l'onde acoustique, oscillations basse-fréquence de l'amplitude de l'onde, double déclenchement de l'instabilité thermoacoustique... Ces observations, auxquelles sont associées des évolutions significatives du champ de température, indiquent qu'en plus des processus classiques de cascade harmonique ou de pertes mineures aux extrémités du stack (génération de tourbillons), les effets non linéaires d'interaction entre champ acoustique et champ de température jouent un rôle important sur la dynamique des régimes transitoires observés. En particulier, le développement d'un champ acoustique de niveau élevé est source d'un flux d'enthalpie thermoacoustique (équivalent à une augmentation acoustiquement induite de la conductivité thermique des parois du stack), et d'un vent acoustique modifiant la forme du champ de température.Une étude analytique de l'influence de la forme du champ de température sur l'amplification thermoacoustique est proposée, qui montre que la distribution du champ acoustique (pression, vitesse et phase), et par suite l'amplification thermoacoustique du son, dépendent de façon critique de la forme du champ de température. Puis une description analytique des effets non linéaires précédemment cités et de l'interaction en régime transitoire entre champ acoustique et champ de température est présentée: elle mène à des de résultats de simulations très proches des résultats expérimentaux, ce qui confirme le rôle majeur joué par la conductivité acoustiquement induite et le vent acoustique. Enfin, une description qualitative du régime transitoire sous forme d'un système d'équations différentielles ordinaires est proposée, qui permet finalement d'extraire les processus physiques prépondérants.Parce qu'ils apportent une compréhension nouvelle des processus fondamentaux qui régissent la saturation de l'onde, ces travaux apportent à nos yeux une contribution aux avancées pratiques qui visent à améliorer l'efficacité des machines thermoacoustiques

Synchronization of a thermoacoustic oscillator by an external sound source

International audienceSince the pioneering works of Christiaan Huygens about the sympathy of pendulum clocks, synchronization phenomena have been widely observed in nature and science. In this paper, we describe a simple experiment which exhibits several aspects of synchronization when making use of a ther-moacoustic oscillator driven by an external sound source. Both the synchronization region of leading order around the natural frequency f0 of the oscillator and the ones of higher order (around f0/2 and f0/3) are measured as functions of the loudspeaker voltage and frequency. We also show that increasing the coupling between the loudspeaker and the thermoacoustic oscillator gives rise under some circumstances to the death of self-sustained oscillations (quenching). Moreover, two additional set of experiments are performed : the first one aims at investigating the effect of a feedback loop for which the signal captured by the microphone is delivered to the loudspeaker through a phase-shifter; the second set of experiments aims at investigating the non-trivial interaction between the loudspeaker and the thermoacoustic oscillator when this last one acts as a relaxation oscillator (spontaneous and periodic onset/damping of self-sustained oscillations). The experiment is easy to build and highly demonstrative; it might be of interest for classroom demonstrations or physics lab dealing with nonlinear dynamics

Synchronization of a standing wave thermoacoustic prime-mover by an external sound source

International audienceSince the early observations made by Christiaan Huygens in his experiments with two pendulum clocks placed on a common support, synchronization phenomena have been extensively studied in the field of nonlinear science. Examples of synchronization phenomena are abundant in many fields of science, from mechanical or electrical engineering to chemistry or living systems, provided that a self- sustained oscillator may be driven by an external force or another oscillator. In this study, we investigated the effect of synchronization of a very simple standing wave thermoacoustic oscillator, when it is driven by an external sound source, i.e. a loudspeaker. Experiments are performed by controlling both frequency detuning and driving amplitude in order to draw the Arnold tongues associated to this particular device. The transition from the synchronized to the quasi-periodic regime is also studied, and different kinds of bifurcation are observed depending on the driving amplitude. In our opinion, the experiments performed in this study are of interest for teachers, because they exhibit universal concepts in a particular device which is both very demonstrative and easy to reproduce with minimum equipment

On the Use of a Complex Frequency for the Description of Thermoacoustic Engines

International audienc

Apports des logiciels de planification dans le bilan pré-implantaire

Les avancées en implantologie et en informatique ont été considérables ces 30 dernières années. L introduction de l informatique en chirurgie dentaire a permis de changer certaines méthodes de travail et a permis aux praticiens d avoir une vision différente de l acte chirurgical implantaire et de l élaboration du plan de traitement. L implantologie assistée par ordinateur, avec la création des logiciels de planification est de plus en plus utilisée. Ces logiciels permettent aux praticiens de réaliser avec une grande précision une planification implantaire virtuelle très proche de la réalité. Dans ce travail, sera développé dans un premier temps le bilan pré-implantaire permettant d élaborer le plan de traitement. Dans un second temps, sera exposé ce que ces logiciels de planification peuvent apporter aux praticiens lors du bilan pré-implantaire. Enfin, les intérêts et les limites de ces logiciels seront discutés.LILLE2-UFR Odontologie (593502202) / SudocLILLE2-BU Santé-Recherche (593502101) / SudocSudocFranceF

Etude expérimentale et théorique des processus non linéaires de saturation dans un générateur d'ondes thermoacoustique annulaire

LE MANS-BU Sciences (721812109) / SudocSudocFranceF

Common features in the thermoacoustics of flames and engines

International audienceThermoacoustic phenomena generated inadvertently during combustion, and those produced by heating and cooling material to generate a temperature gradient, have mostly been studied independently. Indeed, researchers of one phenomenon are seldom familiar with the literature on the other. This paper seeks to remedy this by reviewing the two subjects alongside one another, by comparing and contrasting them where possible. There is a brief account of the historically important papers in the development of both subjects, followed by a description of the nature of the phenomena. A selective number of papers is called upon to illustrate these principles. Techniques for handling the pure acoustics in the two subjects are addressed, before an outline is given of the modelling of the two thermoacoustic systems. Non-linear phenomena in the two systems are then explored. Finally, methods of 'control', of changing the system characteristics, are briefly discussed

Periodic self-modulation of an electrodynamically driven heated wire near resonance

International audienceA thin nichrome wire driven near resonance by the Lorentz force and heated by an alternating electrical current is a popular lecture demonstration. Due to the convective cooling of the portions of the wire moving with the greatest amplitude, only glowing regions near a velocity node will be visible in a darkened room. Nonlinear effects and the thermal expansion coefficient of the wire displace the wire's tensioning mass. By adiabatic invariance, the work done on or by the vibrating wire, due to the changes in the mass's elevation, causes the natural frequency of the standing wave resonance to be shifted. Competition between the thermal inertia of the wire and the convective heat transfer coefficient introduces an exponential thermal relaxation time so that the amplitude of vibration is dependent on the ratio of the drive frequency to the changing resonance frequency at an earlier (retarded) time. These thermal and kinetic effects are incorporated into three coupled nonlinear ordinary differential equations that are separated by the method of multiple time scales and are solved numerically, reproducing both the spontaneous appearance of stable periodic amplitude modulation and the hysteretic behavior observed with increasing or decreasing of the drive frequency