Understandings heat transfer across a solid/liquid interface is crucial for establishing novel thermal control pathways in a range of energy applications. One of the major problems raised in this context is the impact of the three-phase contact line between solid, liquid, and gas on heat flux perturbations at the nanoscale. The focus of this research is the thermal transport via nanosized meniscus restricted between two solid walls. The molecular dynamics approach was used to consider different wetting states of the meniscus by varying the interaction potential between atoms of the substrate and the liquid. The influence of the meniscus size on the energy exchange between two solid walls was also studied. It was discovered that possessing a three-phase contact line reduces the interfacial boundary resistance between solid and liquid. Furthermore, the finite element method was employed to connect atomistic simulations with continuum mechanics. We show that the wetting angle and interfacial boundary resistance are essential important parameters for multiscale analysis of thermal engineering issues with precise microscale parametrization.

The characteristics of thin liquid films flowing down a uniformly heated and inclined plane are investigated, with heat transfer across the wavy films quantified using up-to-date optical measurement techniques based on laser-induced fluorescence (LIF). A planar two-colour LIF technique provides the temperature distribution inside the films, but requires a high degree of wave regularity for the spatial reconstruction. A pointwise adaptation of the aforementioned technique, with much finer temporal sampling, provides simultaneous measurements of the average temperature over the film height and of the film thickness. Despite the loss of spatial resolution, the latter technique can be applied to diverse situations, especially when the waves lose their regularity and have large amplitudes. With these two approaches, the enhancement of heat transfer due to surface waves is traced along the film flow. A growing thermal boundary layer is found close to the inlet of the flow (i.e., first few cm), but its thickness remains small relative to the film thickness. Therefore, the heat transfer coefficient (HTC) is observed to be insensitive to the shape and amplitude of the waves at the free surface. A critical distance is necessary for the thermal boundary layer to be thick enough to interact with the flow structures associated with the waves, and the critical length scales with the Peclet number of the flow based on the specific flow rate. Several experiments are conducted to quantify the influence of the main flow parameters that control the HTC, such as the Reynolds number, the inclination angle and the wave frequency. For moderate wave amplitudes, the internal structure of the film is insensitive to the wave dynamics, and the temperature distribution is essentially dominated by thermal diffusion in the direction normal to the heated wall. Classical Nusselt theory is found to be applicable to the unperturbed (flat) film flows with some limited adjustments to predict the heat transfer rate. However, for the waves that have a larger amplitude, the classical Nusselt theory diverges from the experimental results. A sharp increase of the HTC by several tens of percent is observed over just a few cm, compared to an equivalent undisturbed liquid film with the same Reynolds number. Refined images of the temperature field are then used to better understand the mechanisms by which heat transfer is enhanced. Mixing appears in regions close to the wave front, then progressively extends to other film regions, tending to make the temperature more homogeneous. This has a strong effect on the local HTC in the troughs of the waves, with deviations of up to 40% relative to the flat film theory. Finally, a loss of wave regularity, observed after a few tens of cm from the inlet, accelerates the mixing by further altering the distribution of the temperature field over the entire liquid domain. (C) 2022 Elsevier Ltd. All rights reserved.

Droplets temperature is a key parameter for the study of heat and mass transfers in many spray applications (spray cooling, spray combustion, spray drying, spray scrubber for cleaning air and other gases of various pollutants and dust particles…). In this study, we develop a new method based on the measurement of the fluorescence lifetime to characterize the droplet temperature in sprays. The method is tested in the mixing region of two water sprays, which are injected with a significantly temperature difference. Time Correlated Single Photons (TCPSC) is applied to characterize the fluorescence decay in the time domain which is normally mono exponential for a fluorescent dye at a given temperature. For some well-chosen fluorescent dyes, like rhodamine B (RhB), the fluorescence lifetime strongly varies with the temperature. Hence, in the mixing region of two non-isothermal sprays, the fluorescence is expected to follow a biexponential decay. In this study, we discuss the possibility of using the fluorescence decay to determine the temperature of the two sprays as well as their respective volume fraction. In a first step, a simplified configuration is considered where the two sprays are seeded with eosin Y and rhodamine 6G separately. Since these dyes have very different lifetimes and no temperature dependence, the fluorescence decay changes with the mixing fraction alone. A calibration is necessary to evaluate the amount of liquid originating from each of the two sprays. The femtosecond laser used for the ultrashort excitation of the fluorescent molecules is focused inside the spray using a long-distance microscope lens to obtain two-photon absorption within a small volume of a few tens of microns. The out-of-field fluorescence usually affecting the measurements in dense sprays when one-photon absorption is used, is suppressed using this approach. In a second step, both the temperature and the mixing fraction are measured seeding the two sprays by rhodamine B only. The fluorescence decay is further analysed to determine the lifetime and the signal contribution of both sprays. Results show that the volume fraction of a given spray must exceed about 10% to make it possible to determine its temperature and its volume fraction with a reasonable accuracy (typically a few percent for the volume fraction and 2°C for the temperature). Simultaneous measurements of the two sprays temperatures and volume fractions provide a means to calculate the mixing temperature (the average between the temperatures of the two sprays weighted by their volume fraction). One of the benefits of the newly developed technique is to remove the problems inherent to intensity-based measurements. Given that rhodamine B has a more intense emission of fluorescence at low temperatures, intensity-based measurements are known to be deficient for evaluating the mixing temperature, the coldest spray always has a larger signal and therefore a greater importance in the measured temperature.

An experimental setup is implemented in order to study the solidification of a drop impinging a subcooled substrate. One of the major problems relying the study of droplet solidification consist of the visualization of the solidification front within the droplet. Indeed, shadowgraphy measurement only allows the observation of the solidification front along the tri-junction liquid/solid/air. The use of the Laser Induced Fluorescence provides information on the evolution of the horizontal solidification front in a droplet over t ime. The images reconstruction of the solidification front is made by using two high-speed cameras (side view and top view). The measurements allow for the first time to observe the evolution of the solidification front geometry over t ime. The measurements, carried out on a duralumin substrate, were then compared with a 2D numerical model taking into account the heat transfers conjugated with the substrate.

Thin wavy liquid films a reused in many applications as heat a nd mass transfer promoters thanks to their great performances for a small liquid consumption. To further understand the exact mechanisms of coupling between the wavy fluid dynamics and the heat transfer, a laser-induced fluorescence technique has been developed. The technique enables a measurement of the temperature field inside of a wavy film flowing down an inclined plane with a high spatial resolution. This paper reports on the development of the technique. Water liquid films were investigated under two-dimensional waves conditions. The measurements reveal the regions where mixing is enhanced by convective circulation zones and enables the evaluation of the intensification of the heat transfer due to the waves.

Thin wavy liquid films are used in many applications as heat and mass transfer promoters thanks to their great performances for a small liquid consumption. To evaluate the influence of the coupling between the wavy fluid dynamics and the heat transfer in a 2D waves regime, a pointwise laser-induced fluorescence technique has been developed. The technique enables a simultaneous measurement of both the thickness and the temperature of a wavy film flowing down an inclined plane with a high temporal resolution thanks to an optical probe placed directly above the liquid film. The fluorescence intensity is integrated over the thickness of the film, allowing a robust measurement of temperature and thickness for strongly perturbed wave fronts. This paper reports on the development of the technique and discuss the capabilities and limits of the current design. Water liquid films were investigated under two-dimensional waves conditions. The measurements, and the evaluation of the local heat transfer coefficient they permit, reveal the regions where mixing is enhanced by convective circulation zones.

The characteristics of thin liquid films flowing down a uniformly heated and inclined plane are investigated, with heat transfer across the wavy films quantified using up-to-date optical measurement techniques based on laser-induced fluorescence (LIF). A planar two-colour LIF technique provides the temperature distribution inside the films, but requires a high degree of wave regularity for the spatial reconstruction. A pointwise adaptation of the aforementioned technique, with much finer temporal sampling, provides simultaneous measurements of the average temperature over the film height and of the film thickness. Despite the loss of spatial resolution, the latter technique can be applied to diverse situations, especially when the waves lose their regularity and have large amplitudes. With these two approaches, the enhancement of heat transfer due to surface waves is traced along the film flow. A growing thermal boundary layer is found close to the inlet of the flow (i.e., first few cm), but its thickness remains small relative to the film thickness. Therefore, the heat transfer coefficient (HTC) is observed to be insensitive to the shape and amplitude of the waves at the free surface. A critical distance is necessary for the thermal boundary layer to be thick enough to interact with the flow structures associated with the waves, and the critical length scales with the Peclet number of the flow based on the specific flow rate. Several experiments are conducted to quantify the influence of the main flow parameters that control the HTC, such as the Reynolds number, the inclination angle and the wave frequency. For moderate wave amplitudes, the internal structure of the film is insensitive to the wave dynamics, and the temperature distribution is essentially dominated by thermal diffusion in the direction normal to the heated wall. Classical Nusselt theory is found to be applicable to the unperturbed (flat) film flows with some limited adjustments to predict the heat transfer rate. However, for the waves that have a larger amplitude, the classical Nusselt theory diverges from the experimental results. A sharp increase of the HTC by several tens of percent is observed over just a few cm, compared to an equivalent undisturbed liquid film with the same Reynolds number. Refined images of the temperature field are then used to better understand the mechanisms by which heat transfer is enhanced. Mixing appears in regions close to the wave front, then progressively extends to other film regions, tending to make the temperature more homogeneous. This has a strong effect on the local HTC in the troughs of the waves, with deviations of up to 40\% relative to the flat film theory. Finally, a loss of wave regularity, observed after a few tens of cm from the inlet, accelerates the mixing by further altering the distribution of the temperature field over the entire liquid domain.

Obtaining accurate droplet temperature is essential to study heat and mass transfers in a wide range of spray applications such as spray cooling and spray combustion. A novel measurement technique based on the fluorescence lifetime is developed to tackle the challenge of measuring the droplet temperature in sprays. Similarly, to the intensity of the fluorescence signal, the fluorescence lifetime can vary with the temperature for some specific organic dyes, such as rhodamine B and kiton red. In the past, applications of laser-induced fluorescence (LIF) to the measurement of the temperature in sprays have been based exclusively on the fluorescence intensity by using intensity ratios of the signal detected by two or three photodetectors (PMTs, Cameras) operating at different spectral regions. Nonetheless, this approach is not straightforward, and corrections are usually required in dense sprays to mitigate some biases, which are arising in particular from the out-of-field fluorescence and multiple light scattering by the droplets. In contrast, measuring the fluorescence lifetime makes use of only a single detection spectral band and provides an absolute measurement unlike intensity-based measurements, which are always relative. In the present study, the time-correlated single-photon counting (TCSPC) is applied for the first time to measure the lifetime of the fluorescence emitted by droplets in a spray. The measurement technique provides the mean temperature of the liquid phase. No evidence of measurement biases could be pointed out during the tests performed at different injection conditions. Besides, a very high signal-to-noise ratio could be achieved even at a relatively far distance from the injection nozzle, resulting in an absolute error on the measured temperature that typically does not exceed ± 1 °C.

An innovative application of the laser-induced fluorescence (LIF) technique is developed to characterize droplets in an icing wind tunnel (IWT). Two fluorescent dyes are dissolved in water, and fluorescence detection is made in three spectral bands allowing to calculate two fluorescence ratios. A first ratio is used for measuring the temperature of the supercooled droplets with a rather good temperature sensitivity of 2.56%/°C. The second ratio is used to characterize the droplet phase change, namely discriminating a supercooled droplet from a fully or a partially solidified droplet and in this latter case estimating the ice fraction in the droplet. Due to the dissolution of fluorescent molecules in water, it is essential to assess the influence of the dyes on both the physical properties of the liquid and the supercooled (degree) state. Thus, experiments allowing the determination of these different parameters were carried out on de-ionized and seeded water. It was thus observed that only the surface tension of seeded water is affected with a decrease of 25% compared to de-ionized water. Measurements conducted with droplets deposited on a subcooled surface have demonstrated that the droplet solidification occurs with the same probability regardless of whether they are seeded by the dyes or not. Experiments firstly conducted at laboratory scale, involving droplet velocities up to 10 m/s, demonstrated that the second fluorescence ratio makes it possible to estimate the ice fraction for a given droplet population. Finally, the technique was applied to supercooled droplets generated in an icing wind tunnel (IWT) for air flow up to 200 m/s, close to the conditions encountered in aircraft flights. Temperature measurements, derived from the first fluorescence ratio, demonstrate that droplets are not in thermal equilibrium with the cold air flow, even when they are supercooled. The second fluorescence ratio made it possible to detect the phase transition in the droplets and to estimate the ice fraction within the droplet population. Visualization of the phase change of supercooled droplets evolving in an icing wind tunnel and instantaneous evolution of their volume ice fraction.

Wave instabilities of falling liquid films are crucial in many applications to enhance heat and mass transfer. Despite the importance of this issue, the interplay between the heat transfer and the wavy dynamics of falling films is still not completely understood. To get more insight, a planar laser-induced fluorescence technique has been developed for imaging the temperature distribution in the cross section of thin liquid films (approximately 0.5–1 mm thick), which are falling down an inclined heated surface. This study reports on the implementation of this imaging technique. It also discusses its advantages and limitations for the investigation of the heat transfer in the falling liquid films. Two-dimensional flow conditions and regular waves are considered for the reconstruction of a complete temperature field in the waves. Measurements provide new understanding of the wave ability to generate mixing within the film. Temperature maps reveal preferential regions where mixing occurs first, before eventually spreading to the rest of the film if the wave amplitude and the travel distance are large enough. The increase in the heat transfer coefficient is directly related to the internal mixing observed in the temperature images.

En este trabajo estudiamos hasta que punto el vector energetico que constituye el hidrogeno puede ayudar a resolver los problemas de la transicion energetica. Presentamos el sector del hidrogeno desde sus metodos de produccion y sus posibilidades de insercion en la red electrica hasta sus usos (industrial, transporte, calefaccion, etc.), con el fin de analizar su potencial como energia del futuro para construir una nueva economia sostenible. Demostramos que existen varias formas de economia del hidrogeno que implican una dinamica mas o menos rapida de transicion energetica y ecologica, unos costes mas o menos elevados y diferentes niveles de rentabilidad. Probablemente sera necesario un compromiso intertemporal entre la reduccion de las emisiones medioambientales y el desarrollo economico del sector. No existe un unico sector del hidrogeno y puede ser necesario comenzar por el llamado hidrogeno «azul». Por ultimo, los usos futuros cambiaran cuando se aprovechen plenamente las ventajas de la tecnologia del hidrogeno. El hidrogeno, ?un pequeno paso o una solucion real para la transicion energetica? Clasificacion JEL: 40, Q43, Q48, Q54, Q55

In this work, we study to what extent the energy vector of hydrogen is likely to help solve the problems of the energy transition. We present an overview of the hydrogen sector, from its production methods and its potential integration into the electricity network all the way through to its final uses (industrial, transport, heating, etc.), in order to discuss its potential to become the energy of the future and to help build a new sustainable economy. We show that there are several ways to build a hydrogen economy: these involve a more or less rapid completion of the energy and ecological transition, more or less significant costs, and different returns on investments. An intertemporal trade-off between reduction of environmental emissions and economic development of the sector will no doubt be necessary. There is no single hydrogen economy pathway, and it may be necessary to prime the pathway with so-called “blue” hydrogen. Finally, the uses of the future may be very different in the future once the benefits of hydrogen technology will be fully exploited. Does hydrogen represent a small step or a real solution for the energy transition?

Water droplets impinge on a sapphire wall heated to a temperature ranging from 300 degrees C to 700 degrees C. Advanced measurement techniques are used to characterize the thermal processes associated with the drop impact. IR thermography, implemented by coating the impacted surface with an opaque and emissive material in the IR domain, makes it possible to measure the temperature of the solid surface during the impact process. Laser-induced fluorescence imaging is used to characterize the temperature field in the spreading droplet. At the onset of film boiling, the temperature distribution on the solid surface is marked by the formation of a fingering pattern. This latter corresponds to spatial fluctuations in the thickness of the vapor film. When a water droplet hits an overheated wall with a significant impact velocity, the thermal contact is so rapid and intense that the liquid temperature can largely overtake the saturation temperature and reach the spinodal temperature, i.e. the highest temperature at which water can exist in the liquid state. In this situation, experiments show that the dynamic Leidenfrost point is directly linked to the spinodal temperature. A superheating of the liquid by several hundred of degrees C and the subsequent homogeneous nucleation, have to be considered to describe the heat transfer in the film boiling regime. (C) 2020 Elsevier Ltd. All rights reserved.

Une technique de mesure optique a été développée afin de caractériser les transferts de chaleur dans un film ruisselant mince tombant sur un plan incliné.Le film est perturbé par des vagues excitées harmoniquement et se propageant à sa surface libre.Une technique de Fluorescence Induite par Laser est spécialement développée afin de pouvoir mesurer de façon simultanée l'épaisseur et la température dans les vagues au cours du temps.Les mesures révèlent ainsi l'évolution de la température moyenne dans l'épaisseur du film au passage des vagues dont la fréquence peut être contrôlée.ABSTRACT.An optical measurement technique has been developped to characterize the heat transfer in a falling liquid film with surface instabilities, which is flowing down an inclined plane.The waves traveling at the free surface are generated harmonically.The novel technique based on Laser Induced Fluorescence enables to provide simultaneously a measurement of the thickness and the temperature of the liquid film.The experimental results reveal the temporal evolution of the average temperature in the thickness of the film, when the film is travelled by waves of controlled frequency.MOTS-CLÉS.Transferts de chaleur, Fluorescence induite par laser, Film ruisselant mince, Instabilités de surface.

The photoacoustic method with piezoelectric detection for the simultaneous evaluation of the thermophysical properties is proposed. The approach is based on the settling of an additional heat sink for redistribution of heat fluxes deposited on the sample surface. First, the approach was tested on the porous silicon with well-defined morphology and well-studied properties. Then, heat capacity and thermal conductivity of silicon nanowire arrays were investigated by recovering the experimental data through numerical simulations. The decrease in heat capacity and effective thermal conductivity of the samples upon increasing thickness and porosity of the sample was observed. Such a behavior could be caused by the increase of the structure heterogeneity. In particular, this can be related to a larger disorder (increased density of broken nanowires and larger porosity) that appears during the etching process of the thick layers.

Molecular dynamics simulations describing the equilibrium shape of a nanodroplet located on the solid substrate are presented for the cases of a “cylindrical water droplet” on silicon substrates. Several examples of the structuration of the solid substrate surface are simulated, i.e.: atomistic flat substrate and substrates with ordered nanopillars and nanopores. The adhesives forces between molecules of the substrate and the fluid are modified to change the wettability. Three wetting configurations are considered in this work for the smooth surface: (i) hydrophilic (0 = 30∘), (ii) hydrophobic (0 = 136∘), and (iii) an intermediate regime (0 = 80∘). Further, the dependence of the wetting angle as a function of the surface state is studied in details for the above-mentioned configurations.

Dans ce travail, nous étudions dans quelle mesure le vecteur énergétique que constitue l’hydrogène est susceptible de permettre de résoudre les problèmes de la transition énergétique. Nous présentons la filière hydrogène de ses modes de production, de sa potentielle insertion dans le réseau électrique, à ses usages (industriels, transport, chauffage etc…), afin de discuter de son potentiel en tant qu’énergie du futur pour construire une nouvelle économie soutenable. Nous montrons qu’il existe plusieurs formes d’économie de l’hydrogène : ces dernières impliquent des dynamiques d’achèvement de la transition énergétique et écologique plus ou moins rapides, des coûts plus ou moins importants et des rentabilités différentes. Un arbitrage intertemporel entre réduction des émissions environnementales et développement économique de la filière sera sans doute nécessaire. Il n’existe pas une filière hydrogène unique et il sera peut-être nécessaire d’amorcer la filière par de l’hydrogène dit « bleu ». Enfin, les usages du futur sont amenés à changer lorsqu’ils exploiteront complètement l’intérêt de la technologie hydrogène. L’hydrogène, petit pas ou véritable solution pour la transition énergétique ? Classification JEL : Q 40, Q43, Q48 Q54, Q55