14 research outputs found
Application de la stéréo PIV pour la mesures des écoulements bidirectionnels de fumée d'incendie traversant une trémie horizontale
International audienceLâĂ©coulement des fumĂ©es Ă un passage dâune ouverture horizontale reliant deux compartiments superposĂ©s est Ă©tudiĂ© expĂ©rimentalement. Dans cette Ă©tude la technique de stĂ©rĂ©o PIV est appliquĂ©e pour lâĂ©coulement en convection mixte et en convection naturelle. Cet Ă©change est gouvernĂ© Ă la fois par les effets de flottabilitĂ© dĂ»s Ă une diffĂ©rence de tempĂ©rature du fluide contenu dans les deux compartiments, et les effets dâune ventilation mĂ©canique appliquĂ© dans le compartiment infĂ©rieur. Une telle configuration entraĂźne un Ă©change uni- ou bi-directionnel Ă travers lâorifice. Dans les expĂ©riences, la flottabilitĂ© est induite dans le compartiment infĂ©rieur grĂące Ă une rĂ©sistance Ă©lectrique. Les rĂ©sultats en convection naturelle montrent une forte similitude avec les rĂ©sultats en convection mixte dans le rĂ©gime bi-directionel
Experimental study of the mixed convection flow through a horizontal orifice linking two compartments
Afin de rĂ©pondre Ă des problĂ©matiques bĂątimentaires et des enjeux de sĂ©curitĂ© incendie, cette thĂšse aborde lâĂ©coulement de convection mixte Ă travers un orifice horizontal reliant deux compartiments. Lâobjectif est dâamĂ©liorer la connaissance et la modĂ©lisation de lâĂ©change de gaz de masse volumique variable Ă travers lâorifice. Une Ă©tude expĂ©rimentale Ă Ă©chelle rĂ©duite couplĂ©e Ă une approche thĂ©orique est proposĂ©e. L'Ă©tude est d'abord focalisĂ©e sur lâinfluence du rapport gĂ©omĂ©trique L/D de lâorifice sur la variation de dĂ©bit Ă©changĂ© pour un rĂ©gime de convection naturelle. Les mesures non intrusives de ces dĂ©bits, par suivit de l'interface entre deux liquides non miscibles lors d'une premiĂšre approche densimĂ©trique, ainsi que par StĂ©rĂ©o PIV en sortie d'orifice dans une approche thermique, permettent de dĂ©crire le processus d'Ă©change bidirectionnel et de conforter les corrĂ©lations existantes.Des expĂ©riences en rĂ©gime de convection mixte visent ensuite Ă caractĂ©riser lâinfluence dâune ventilation mĂ©canique (en soufflage et en extraction) sur les dĂ©bits Ă©changĂ©s. La confrontation des corrĂ©lations existantes avec les points expĂ©rimentaux montre des Ă©carts importants. Une modification de la corrĂ©lation de Cooper 89 est proposĂ©e et permet d'en accroĂźtre la prĂ©cision. En parallĂšle, une approche thĂ©orique issue des Ă©quations de Navier Stokes simplifiĂ©es et sous lâapproximation de Boussinesq permet de discuter la construction des corrĂ©lations existantes. L'ajustement de coefficients de pertes de charge Ă partir des points expĂ©rimentaux permet de proposer un modĂšle plus performant que ceux disponibles dans la littĂ©rature.To answer to building issues and fire safety challenges, this thesis deals with the mixed convection flow through a horizontal orifice linking two compartments. The aim is to improve the understanding and modeling of the exchange of variable density gas through the opening. A small scale experimental study and a theoretical approach are proposed.The study is first focussed on the impact of the geometrical ratio L/D of the opening on the exchanged flow rate variation for free convection regime. Non-intrusive measurements of these flow rates, via the tracking of the interface between two non miscible liquids in an isothermal approach, and thanks to the SPIV in a thermal approach, permit to describe the bidirectional exchange process and to consolidate existing correlations.Experiments in mixed convection regime aim to study the impact of mechanical ventilation (in blowing and extracting mode) on the exchanged flow rates. The comparison between existing correlations and experimental data shows large differences. A change making the coorelation of Cooper 89 more accurate is proposed. A theoretical approach from the simplified Navier Stokes equations and with the Boussinesq approximation permits to discuss the construction of existing correlations. From this theory, a model more accurate than those available in the literature is proposed thanks to an adjustment of discharge coefficients from experimental data
Theoretical model of continuous inertial gravity currents including a jump condition
International audienceThis paper examines the theoretical modeling of a steady horizontal gravity currentinvolving miscible fluids. The main objective is to determine the longitudinal evolution of the current characteristic quantities: its mean velocity U, its height h, its mean density Ï, and the local Richardson number Ri = gÏh/ÏU2, which characterizes the flow regime.The theory developed by Ellison and Turner [J. Fluid Mech. 6, 423 (1959)] for Boussinesqgravity currents is first extended to the general non-Boussinesq case. In this theoreticalapproach, the differential equations derived from the conservation equations reveal amathematical singularity which no longer allows them to be solved when the current passes from a supercritical to a subcritical regime, i.e., when the Richardson number reaches unity. To circumvent this problem, we propose to introduce a jump condition into the model which leads to a sudden transition from a supercritical to a subcritical regime when needed. The jump location is set to satisfy the boundary condition at the exit. Numerical simulations are carried out using a large-eddy simulation code in order to obtain reference results. These results are first used to select a suitable entrainment model among those proposed in the literature. The simulations are then compared with the theoretical model. In the case of a supercritical current without regime change, the agreement between simulation and theory is good. In the case of a supercritical current turning subcritical before the exit, the discontinuity imposed in the model by the jump is clearly abrupt in comparison with the physical reality, but it allows us to reproduce the nonmonotonic evolution of the velocity, height, and Richardson number of the current and to obtain an acceptable estimation of these quantities
Analytical solutions for long-time steady state Boussinesq gravity currents flowing along a horizontal boundary of finite length
ModĂšle thĂ©orique de courant de gravitĂ© issu dâun rejet continu sur un plan inclinable dans un milieu ouvert
National audienceIn this study, we are interested in the theoretical modelling of a gravity current on a tiltable plane. Thistype of flow is formed when a fluid moves, mainly longitudinally, in a surrounding fluid of differentdensity (Ungarish [16]). To model the dynamics of this type of flow, we consider a gravity flow from acontinuous discharge of a light fluid of controlled velocity and density.The gravity flow reaches a steady state after an initial development stage during which the gravity flowpropagates along the wall to its end. In this stationary regime, the objective is to establish a theoreticalmodel and to determine the longitudinal evolution of the characteristic quantities (velocity, density andthickness) and the associated Richardson number (corresponding to the buoyancy/inertia ratio) of thegravity current along the entire length of the tiltable plane.In a first step, we have extended the theoretical model of Ellison and Turner [7], based on balance equations (mass, momentum and energy), developed in the Boussinesq case (i.e. when the relative differencebetween the density of the current and the one of the ambient air is not relevant) to the general nonBoussinesq framework. This new model enables to theoretically estimate the variation of the main threequantities of the current (velocity, height and density). In the case of an initially inertial flow (Ri 1) and a more accurate transcriptionof the dynamics of the gravity current.Secondly, to compare this theoretical approach with reference gravity flows, we have carried out LargeEddy Simulations with the CALIF3S-Isis software. These simulations confirmed the transition betweenthe inertial and subcritical regimes. They also highlight the influence of the slope angle on the behaviourof the flow. Indeed, in the case of an initially inertial flow becoming subcritical, the increase of the slopeangle reduces the amplitude of the jump until it disappears for a defined theoretical angle. For steeperslopes, we observe a change in the behaviour of the flow from the injection.The slope angle thus plays a preponderant role in this type of configuration and its role, analogous tothe entrainment in the governing equations, increases the mixing between the current and the ambientfluid. Although this model is simple with an abrupt transition, it can reproduce with an acceptable levelof accuracy the evolution of the velocity, density and thickness of the layerDans cette Ă©tude, nous nous intĂ©ressons Ă la modĂ©lisation thĂ©orique dâun courant de gravitĂ© sur un planinclinable. Ce type dâĂ©coulement, communĂ©ment rencontrĂ© dans des situations naturelles ou accidentelles (Ă©coulement de fumĂ©e issue dâun incendie, rejets de polluants, etc.), se forme lorsquâun fluide sedĂ©place, principalement de maniĂšre longitudinale, dans un fluide environnant de masse volumique diffĂ©rente (Ungarish [16]). Afin de modĂ©liser la dynamique de ce type dâĂ©coulement, nous considĂ©rons uncourant de gravitĂ© issu dâun rejet continu dâun fluide lĂ©ger (mĂ©lange air/hĂ©lium) de vitesse et de massevolumique contrĂŽlĂ©es. AprĂšs une phase initiale de dĂ©veloppement au cours de laquelle le courant degravitĂ© se propage le long de la paroi jusquâĂ son extrĂ©mitĂ©, le courant de gravitĂ© atteint alors un rĂ©gime stationnaire.Dans le cadre du rĂ©gime stationnaire, lâobjectif est dâĂ©tablir un modĂšle thĂ©orique et de dĂ©terminer lâĂ©volution longitudinale des grandeurs caractĂ©ristiques (vitesse, masse volumique et Ă©paisseur) et du nombrede Richardson associĂ© (correspondant au rapport flottabilitĂ© / inertie) du courant de gravitĂ© sur toutela longueur du plan inclinable.Dans un premier temps, nous avons Ă©tendu le modĂšle thĂ©orique dâEllison et Turner [7], basĂ© sur desĂ©quations bilans (masse, quantitĂ© de mouvement et Ă©nergie) dans un volume Ă©lĂ©mentaire du courant degravitĂ© dans le cadre Boussinesq (i.e. Ă©cart relatif faible entre la masse volumique du courant et celle delâair ambiant), au cadre gĂ©nĂ©ral non-Boussinesq. Ce nouveau modĂšle, composĂ© dâĂ©quations diffĂ©rentielles couplĂ©es, nous permet dâestimer thĂ©oriquement la variation des trois quantitĂ©s dâintĂ©rĂȘt (vitesse,hauteur et masse volumique) Ă lâaide dâune rĂ©solution numĂ©rique. Dans le cas dâun courant initialementinertiel (Ri 1) et une retranscription plus fidĂšle de la dynamique du courant de gravitĂ© Ă©tudiĂ©.Dans un second temps, afin de confronter cette approche thĂ©orique Ă des courants de gravitĂ© de rĂ©fĂ©rence, nous avons rĂ©alisĂ© des simulations aux grandes Ă©chelles avec le logiciel CALIF3S-Isis quiconfirment lâexistence de la transition entre rĂ©gimes inertiel et sous-critique ainsi que lâinfluence delâangle dâinclinaison du plan sur le comportement de lâĂ©coulement. En effet, dans le cas dâun courantinitialement inertiel devenant sous-critique, lâaugmentation de lâangle dâinclinaison rĂ©duit lâamplitudedu saut jusquâĂ la disparition de ce dernier pour un angle thĂ©orique dĂ©fini. Pour des angles encore plusimportants, il est Ă©galement possible dâobserver un changement de comportement de lâĂ©coulement dĂšslâinjection. En effet, lâĂ©coulement initialement inertiel, devient de plus en plus inertiel dĂšs lâinjectioncontrairement aux cas Ă©voquĂ©s prĂ©cĂ©demment.Lâangle dâinclinaison joue donc un rĂŽle prĂ©pondĂ©rant dans ce type de configuration et son rĂŽle, analogue Ă lâentraĂźnement dans les Ă©quations du modĂšle, permet dâaugmenter le mĂ©lange entre le courantet le fluide ambiant. Bien que ce modĂšle soit simple et que la transition soit brutale, ce dernier permetde reproduire, avec un niveau de prĂ©cision acceptable, lâĂ©volution de la vitesse, de la masse volumiqueet de lâĂ©paisseur de la couche
Analytical solutions for long-time steady state Boussinesq gravity currents flowing along a horizontal boundary of finite length
ModĂšle thĂ©orique de courant de gravitĂ© issu dâun rejet continu sur un plan inclinable dans un milieu ouvert
National audienceIn this study, we are interested in the theoretical modelling of a gravity current on a tiltable plane. Thistype of flow is formed when a fluid moves, mainly longitudinally, in a surrounding fluid of differentdensity (Ungarish [16]). To model the dynamics of this type of flow, we consider a gravity flow from acontinuous discharge of a light fluid of controlled velocity and density.The gravity flow reaches a steady state after an initial development stage during which the gravity flowpropagates along the wall to its end. In this stationary regime, the objective is to establish a theoreticalmodel and to determine the longitudinal evolution of the characteristic quantities (velocity, density andthickness) and the associated Richardson number (corresponding to the buoyancy/inertia ratio) of thegravity current along the entire length of the tiltable plane.In a first step, we have extended the theoretical model of Ellison and Turner [7], based on balance equations (mass, momentum and energy), developed in the Boussinesq case (i.e. when the relative differencebetween the density of the current and the one of the ambient air is not relevant) to the general nonBoussinesq framework. This new model enables to theoretically estimate the variation of the main threequantities of the current (velocity, height and density). In the case of an initially inertial flow (Ri 1) and a more accurate transcriptionof the dynamics of the gravity current.Secondly, to compare this theoretical approach with reference gravity flows, we have carried out LargeEddy Simulations with the CALIF3S-Isis software. These simulations confirmed the transition betweenthe inertial and subcritical regimes. They also highlight the influence of the slope angle on the behaviourof the flow. Indeed, in the case of an initially inertial flow becoming subcritical, the increase of the slopeangle reduces the amplitude of the jump until it disappears for a defined theoretical angle. For steeperslopes, we observe a change in the behaviour of the flow from the injection.The slope angle thus plays a preponderant role in this type of configuration and its role, analogous tothe entrainment in the governing equations, increases the mixing between the current and the ambientfluid. Although this model is simple with an abrupt transition, it can reproduce with an acceptable levelof accuracy the evolution of the velocity, density and thickness of the layerDans cette Ă©tude, nous nous intĂ©ressons Ă la modĂ©lisation thĂ©orique dâun courant de gravitĂ© sur un planinclinable. Ce type dâĂ©coulement, communĂ©ment rencontrĂ© dans des situations naturelles ou accidentelles (Ă©coulement de fumĂ©e issue dâun incendie, rejets de polluants, etc.), se forme lorsquâun fluide sedĂ©place, principalement de maniĂšre longitudinale, dans un fluide environnant de masse volumique diffĂ©rente (Ungarish [16]). Afin de modĂ©liser la dynamique de ce type dâĂ©coulement, nous considĂ©rons uncourant de gravitĂ© issu dâun rejet continu dâun fluide lĂ©ger (mĂ©lange air/hĂ©lium) de vitesse et de massevolumique contrĂŽlĂ©es. AprĂšs une phase initiale de dĂ©veloppement au cours de laquelle le courant degravitĂ© se propage le long de la paroi jusquâĂ son extrĂ©mitĂ©, le courant de gravitĂ© atteint alors un rĂ©gime stationnaire.Dans le cadre du rĂ©gime stationnaire, lâobjectif est dâĂ©tablir un modĂšle thĂ©orique et de dĂ©terminer lâĂ©volution longitudinale des grandeurs caractĂ©ristiques (vitesse, masse volumique et Ă©paisseur) et du nombrede Richardson associĂ© (correspondant au rapport flottabilitĂ© / inertie) du courant de gravitĂ© sur toutela longueur du plan inclinable.Dans un premier temps, nous avons Ă©tendu le modĂšle thĂ©orique dâEllison et Turner [7], basĂ© sur desĂ©quations bilans (masse, quantitĂ© de mouvement et Ă©nergie) dans un volume Ă©lĂ©mentaire du courant degravitĂ© dans le cadre Boussinesq (i.e. Ă©cart relatif faible entre la masse volumique du courant et celle delâair ambiant), au cadre gĂ©nĂ©ral non-Boussinesq. Ce nouveau modĂšle, composĂ© dâĂ©quations diffĂ©rentielles couplĂ©es, nous permet dâestimer thĂ©oriquement la variation des trois quantitĂ©s dâintĂ©rĂȘt (vitesse,hauteur et masse volumique) Ă lâaide dâune rĂ©solution numĂ©rique. Dans le cas dâun courant initialementinertiel (Ri 1) et une retranscription plus fidĂšle de la dynamique du courant de gravitĂ© Ă©tudiĂ©.Dans un second temps, afin de confronter cette approche thĂ©orique Ă des courants de gravitĂ© de rĂ©fĂ©rence, nous avons rĂ©alisĂ© des simulations aux grandes Ă©chelles avec le logiciel CALIF3S-Isis quiconfirment lâexistence de la transition entre rĂ©gimes inertiel et sous-critique ainsi que lâinfluence delâangle dâinclinaison du plan sur le comportement de lâĂ©coulement. En effet, dans le cas dâun courantinitialement inertiel devenant sous-critique, lâaugmentation de lâangle dâinclinaison rĂ©duit lâamplitudedu saut jusquâĂ la disparition de ce dernier pour un angle thĂ©orique dĂ©fini. Pour des angles encore plusimportants, il est Ă©galement possible dâobserver un changement de comportement de lâĂ©coulement dĂšslâinjection. En effet, lâĂ©coulement initialement inertiel, devient de plus en plus inertiel dĂšs lâinjectioncontrairement aux cas Ă©voquĂ©s prĂ©cĂ©demment.Lâangle dâinclinaison joue donc un rĂŽle prĂ©pondĂ©rant dans ce type de configuration et son rĂŽle, analogue Ă lâentraĂźnement dans les Ă©quations du modĂšle, permet dâaugmenter le mĂ©lange entre le courantet le fluide ambiant. Bien que ce modĂšle soit simple et que la transition soit brutale, ce dernier permetde reproduire, avec un niveau de prĂ©cision acceptable, lâĂ©volution de la vitesse, de la masse volumiqueet de lâĂ©paisseur de la couche
Burning rate of elevated pool fire in a well-ventilated compartment: Effects of radiative heat fluxes
International audienceElevated pool fires are potentially exposed to the feedback of radiative heat fluxes from the ceiling that can modify the mass loss rate. In this regard, an experimental study was carried-out in a reduced-scale enclosure with a pool fire brought increasingly closer to the ceiling of the compartment. The objective is to study the effect on the mass loss rate of the increase in radiative heat fluxes without any air vitiation effect. The device is equipped with a mechanical ventilation system to allow the flame to develop in a well-oxygenated environment. The fire sources used are circular pans with diameters between 0.09 and 0.175 m, filled with dodecane. Results show that, as long as the flame does not impinge on the ceiling, the mass loss rate remains quasi-constant. On the other hand, when the flame impinges on the ceiling, the mass loss rate increases drastically and can be multiplied by a factor 2â3 compared to the reference value when the pool fire is located on the ground. Scaling parameters, showing the generic aspect of the results, are finally proposed considering a dimensionless elevation h*. It allows to gather all experimental data on the same curve for all the fire sources tested
Stereoscopic Particle Image Velocimetry Investigation of the Bidirectional Natural Convection Flow Through a Horizontal Vent
International audienceAn experimental study investigates the natural convection flow through a horizontal ceiling vent of a fire compartment. This flow is governed by the buoyancy forces and the friction forces, and is bidirectional. The orifice used is of circular cross section and is characterized by the aspect ratio L/D of its thickness L over its diameter D. The reduced scale apparatus consist of two superimposed compartments of for the lower and for the upper. These two rooms are connected by a 0.038 m length circular orifice with different diameters available ( mm]). For the study of natural convection, only the lower compartment is used; the upper compartment is left at atmospheric conditions by removing all its walls. The temperature gradient (C between the bottom and the top of the orifice) is generated by a 2 kW electrical resistor located in the lower compartment. This electrical resistor makes it possible to generate, without mass inlet, a plume with no soot and to reach a thermal steady state in the ``fire room''. The optical stereoscopic particle image velocimetry measurement technique provides the velocity field through the opening for different diameters. This technique, based on the finding of the displacement of a set of particles between two consecutive snapshots of a seeded fluid, makes it possible to obtain all three components of the velocity vector field by non-intrusive measurement. The use of this technique is a novelty for such vent flows and allows accessing information not only on the variation of the flow rate but also on the distribution of the fluids moving upward and downward through the orifice. Despite an unsteady dynamic regime, the analysis of averaged fields shows a geometrical organization of the bidirectional flow through the orifice, with a central area occupied by the hot fluid flowing upward and a peripheral zone where the cold fluid flows downward. The flow section occupancy ratio reverses with the extrema of flow velocities, for L/D approximate to 0.41
Experimental study of the indoor flow behaviour transitions in a naturally ventilated single-zone building opposing wind and buoyancy
Indoor flow behaviour can strongly impact safety in the case of a fire event. Indeed, when air inlet and outlet are correctly placed, stratification allows a fresh air layer to be maintained in the room that can be favourable for the egress. However, in the case of a naturally ventilated building, the presence of the wind will interact with the indoor flow pattern. Hence, fire smoke extraction in naturally ventilated buildings can strongly be influenced by the wind. Indeed, when wind opposed buoyancy, a change in the ratio between buoyancy forces and wind forces can impact the internal flow pattern resulting in a mixed ventilation mode, which is unfavourable for egress, in the case of a fire event. Flow pattern inside a room and their transitions are then a safety purpose. In this paper, natural ventilation of a singular room with two asymmetrical and opposed openings was studied experimentally when wind opposes buoyancy. The buoyant source was generated by an injection of an air/he mix. Varying wind and injection conditions, the ventilation regimes change as well as the indoor flow dynamic. Three ventilation regimes are experimentally observed depending on the balance between J and Fr :buoyancy-driven, bidirectional and wind-driven. From dimensional analysis, we shown that the ventilation regimes can be described via the Froude number, based on the injection flow rate Fr, and the momentum flux ratio J. in the literature, transitions between these regimes is still difficult to estimate as far as it depends on the assumption made on the initial indoor flow pattern (layered or fully mixed). Hence, an experimental investigation on the transition from each extreme regime (buoyancy-driven or wind-driven) to the bi-directional one, has been conducted. The transitions are found to follow a power low in the form Fr â J3/4.A discussion on the inner flow pattern for this bi-directional regime is also proposed and a focus on how the knowledge of the behaviour of the inflowing flow contributes to improve the modelling is made