Critical slowing down and fading away of the piston effect in porous media

We investigate the critical speeding up of heat equilibration by the piston effect (PE) in a nearly supercritical van der Waals (vdW) fluid confined in a homogeneous porous medium. We perform an asymptotic analysis of the averaged linearized mass, momentum and energy equations to describe the response of the medium to a boundary heat flux. While nearing the critical point (CP), we find two universal crossovers depending on porosity, intrinsic permeability and viscosity. Closer to the CP than the first crossover, a pressure gradient appears in the bulk due to viscous effects, the PE characteristic time scale stops decreasing and tends to a constant. In infinitly long samples the temperature penetration depth is larger than the diffusion one indicating that the PE in porous media is not a finite size effect as it is in pure fluids. Closer to the CP, a second cross over appears which is characterized by a pressure gradient in the thermal boundary layer (BL). Beyond this second crossover, the PE time remains constant, the expansion of the fluid in the BL drops down and the PE ultimately fades away

Simulation numérique de l'Effet Piston avec prise en compte des échanges thermiques pariétaux

The Piston Effect and heat transfer at the boundaries of the near-critical fluid are modelled together. At the non-heated wall, the conditions at the edge of the fluid are close to a thermostat. Heat diffusion into the heating solid leads to much lower heating of the fluid by Piston Effect. This discrepancy increases when moving away from the critical point. Regarding the critical speeding up, our results do not agree with the theory

Rapid Thermal Relaxation in Near-Critical Fluids and Critical Speeding Up: Discrepancies Caused by Boundary Effects.

We present one-dimensional numerical simulations reporting the temperature evolution of a pure fluid subjected to heating near its liquid-vapor critical point under weightlessness. In this model, thermal boundary conditions are imposed at the outer edges of the solids in contact with the fluid. Our investigations concern the thermal conditions at the edges of the fluid and their consequences on the fluid's global response. The results for piston effect heating are shown to be significantly affected by the simulation of the solid boundaries. Concerning critical speeding up, it is even found that taking conductive solids into account can make the bulk fluid temperature change in a way opposed to that predicted in their absence

Transition vers la turbulence dans un fluide supercritique en configuration de Rayleigh-Bénard

Des simulations numériques directes de la convection de Rayleigh-Bénard dans un fluide, légèrement au dessus de son point critique liquide-vapeur, sont présentées pour des nombres de Rayleigh compris entre 10^6 and 10^8. Une description spatio-temporelle de l'écoulement est réalisée depuis le début du régime convectif jusqu'à l'établissement du régime turbulent. Cette description concerne principalement la structure du champ de température ainsi que l'équilibre thermique global de la cavité. On insiste sur l'influence de la forte stratification du fluide induite par sa compressibilité élevée sur la pénétrabilité des structures convectives et sur l'équilibre thermique global de la cavité, et une comparaison avec le cas d'un gaz parfait, à nombre de Rayleigh équivalent, est présentée

Evaporation d'une goutte de liquide pur dans sa vapeur près du point critique : pertinence de l'hypothèse quasi-stationnaire et corrections au premier ordre

International audienceAbstract : The quasi-steady hypothesis applied to the vaporization process of a pure critical droplet in its vapor at the critical pressure leads to theoretical results that differ quite significantly from the numerical ones. Two kinds of unsteadyness can be involved in order to explain such results : the presence of an unsteady area far from the droplet and the occurrence of initial data relaxation. Corrections to the quasi-steady results that take into account those two kind of unsteadiness are worked out. These analytical results are in good agreement with the numerical results.L'hypothèse quasi-stationnaire appliquée au processus d'évaporation d'une goutte de liquide pur critique dans sa vapeurà la pression critique conduità des résultats qui diffèrent assez significativement des résultats numériques. Deux sources d'instationnarités peuvent être invoquées pour expliquer ces écarts : la présence d'une zone insta-tionnaire loin de la goutte et la relaxation des conditions initiales. Des corrections aux résultats quasi-stationnaires prenant en compte ces deux effets sont déterminées. La comparaison avec les résultats numériques s'avère satis-faisante

Supercritical Water Mixture (SCWM) Experiment in the High Temperature Insert-Reflight (HTI-R)

Current research on supercritical water processes on board the International Space Station (ISS) focuses on salt precipitation and transport in a test cell designed for supercritical water. This study, known as the Supercritical Water Mixture Experiment (SCWM) serves as a precursor experiment for developing a better understanding of inorganic salt precipitation and transport during supercritical water oxidation (SCWO) processes for the eventual application of this technology for waste management and resource reclamation in microgravity conditions. During typical SCWO reactions any inorganic salts present in the reactant stream will precipitate and begin to coat reactor surfaces and control mechanisms (e.g., valves) often severely impacting the systems performance. The SCWM experiment employs a Sample Cell Unit (SCU) filled with an aqueous solution of Na2SO4 0.5-w at the critical density and uses a refurbished High Temperature Insert, which was used in an earlier ISS experiment designed to study pure water at near-critical conditions. The insert, designated as the HTI-Reflight (HTI-R) will be deployed in the DECLIC (Device for the Study of Critical Liquids and Crystallization) Facility on the International Space Station (ISS). Objectives of the study include measurement of the shift in critical temperature due to the presence of the inorganic salt, assessment of the predominant mode of precipitation (i.e., heterogeneously on SCU surfaces or homogeneously in the bulk fluid), determination of the salt morphology including size and shapes of particulate clusters, and the determination of the dominant mode of transport of salt particles in the presence of an imposed temperature gradient. Initial results from the ISS experiments will be presented and compared to findings from laboratory experiments on the ground

Thermoacoustic heating and cooling in near-critical fluids in the presence of a thermal plume

This work brings new insight to the question of heat transfer in near–critical fluids under Earth gravity conditions. The interplay between buoyant convection and thermoacoustic heat transfer (piston effect) is investigated in a two-dimensional non-insulated cavity containing a local heat source, to reproduce the conditions used in recent experiments. The results were obtained by means of a finite-volume numerical code solving the Navier–Stokes equations written for a low-heat-diffusing near-critical van der Waals fluid. They show that hydrodynamics greatly affects thermoacoustics in the vicinity of the upper thermostated wall, leading to a rather singular heat transfer mechanism. Heat losses through this wall govern a cooling piston effect. Thus, the thermal plume rising from the heat source triggers a strong enhancement of the cooling piston effect when it strikes the middle of the top boundary. During the spreading of the thermal plume, the cooling piston effect drives a rapid thermal quasi-equilibrium in the bulk fluid since it is further enhanced so as to balance the heating piston effect generated by the heat source. Then, homogeneous fluid heating is cancelled and the bulk temperature stops increasing. Moreover, diffusive and convective heat transfers into the bulk are very weak in such a low-heat-diffusing fluid. Thus, even though a steady state is not obtained owing to the strong and seemingly continuous instabilities present in the flow, the bulk temperature is expected to remain quasi-constant. Comparisons performed with a supercritical fluid at initial conditions further from the critical point show that this thermalization process is peculiar to near-critical fluids. Even enhanced by the thermal plume, the cooling piston effect does not balance the heating piston effect. Thus, overall piston-effect heating lasts much longer, while convection and diffusion progressively affect the thermal field much more significantly. Ultimately, a classical two-roll convective-diffusive structure is obtained in a perfect gas, without thermoacoustic heat transfer playing any role

Low-frequency vibrations in a near-critical fluid

The response of a near-critical fluid to low-frequency vibrations is investigated by means of numerical simulations. Its characteristics are first established by one-dimensional analysis. It is shown that the strong thermo-mechanical coupling occurring in the boundary layers tends to make the fluid oscillate homogeneously at low frequencies, and with a larger amplitude than in a normal gas. The numerical results obtained in this first part are found to confirm earlier predictions made in pioneering theoretical work. Then, the study is extended to a two-dimensional configuration. In a square cavity, the wall shear stresses developing along the longitudinal boundaries do not affect the one-dimensional regime, since the viscous layer present in these areas behaves like the Stokes boundary layer. By contrast, thermostatting these boundaries, like the others, generates local curvature of the stream lines. The fluid response to the homogeneous acceleration field then takes some more pronounced two-dimensional patterns, but remains driven by the strong alternating expansions and retractions of the fluid in the thermal boundary layers, which are specific to near-critical fluid