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P F Linden - One of the best experts on this subject based on the ideXlab platform.
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contaminant transport by human passage through an air curtain separating two sections of a corridor part ii two zones at different temperatures
Energy and Buildings, 2021Co-Authors: Narsing K Jha, Daria Frank, L Darracq, P F LindenAbstract:Abstract Air curtains are installed in open doorways of a building to reduce Buoyancy-Driven exchange Flows across the doorway. Although an air curtain allows an unhampered passage of humans and vehicles, the interaction of this traffic with an air curtain is not well understood. In this study, we investigate the problem of the simultaneous interaction between the air curtain, the wake of a moving person and the Buoyancy-Driven Flow arising due to the density difference across the doorway. To this end, we conduct small-scale waterbath experiments with fresh water and salt water solutions to achieve different fluid densities. As a model of human passage, a vertical cylinder is pulled through a planar jet representing an air curtain and separating two zones at different densities. For a fixed travel distance of the cylinder before and after the air curtain, the average infiltration flux of dense fluid into the light fluid side increases with increasing cylinder velocity. Remarkably, we find that the infiltration flux is independent of the density difference across the doorway and is mainly due to the interaction between the air curtain and the cylinder wake with negligible effects from the Buoyancy-Driven Flow. Furthermore, the infiltration flux is also independent of the travel direction of the cylinder. As a consequence, the sealing effectiveness of an air curtain reduces with an increasing cylinder speed and this reduction is independent of the direction of the Buoyancy-Driven Flow. We provide a theoretical explanation for the observed changes in the effectiveness curve of the air curtain as the function of the deflection modulus. Dye visualisations of the air curtain and the cylinder wake are used to examine the re-establishment process of the air curtain after its disruption by the cylinder. We observe that the re-establishment time of the air curtain and the infiltration in the cylinder wake increases with an increasing cylinder speed.
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contaminant transport by human passage through an air curtain separating two sections of a corridor part ii two zones at different temperatures
arXiv: Fluid Dynamics, 2020Co-Authors: Narsing K Jha, Daria Frank, L Darracq, P F LindenAbstract:Air curtains are installed in open doorways of a building to reduce Buoyancy-Driven exchange Flows across the doorway. Although an air curtain allows an unhampered passage of humans and vehicles, the interaction of this traffic with an air curtain is not well understood. We study this problem by conducting small-scale waterbath experiments with fresh water and salt water solutions. As a model of human passage, a vertical cylinder is pulled through a planar jet representing an air curtain and separating two zones at different densities. For a fixed travel distance of the cylinder before and after the air curtain, the average infiltration flux of dense fluid in light fluid side increases with increasing cylinder velocity. However, we find that the infiltration flux is independent of density difference across the doorway and the travel direction of the cylinder. As a consequence, the sealing effectiveness of an air curtain reduces with an increasing cylinder speed and this reduction is independent of the direction of the Buoyancy-Driven Flow. Dye visualisations of the air curtain and the cylinder wake are used to examine the re-establishment process of the air curtain after its disruption by the cylinder. We observe that the re-establishment time of the air curtain and the infiltration in the cylinder wake increases with an increasing cylinder speed.
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displacement and mixing ventilation driven by opposing wind and buoyancy
Journal of Fluid Mechanics, 2005Co-Authors: G R Hunt, P F LindenAbstract:The effect of an opposing wind on the stratification and Flow produced by a buoyant plume rising from a heat source on the floor of a ventilated enclosure is investigated. Ventilation openings located at high level on the windward side of the enclosure and at low level on the leeward side allow a wind-driven Flow from high to low level, opposite to the Buoyancy-Driven Flow. One of two stable steady Flow regimes is established depending on a dimensionless parameter F that characterizes the relative magnitudes of the wind-driven and Buoyancy-Driven velocities within the enclosure, and on the time history of the Flow. A third, unstable steady Flow solution is identified. For small opposing winds (small F) a steady, two-layer stratification and displacement ventilation is established. Exterior fluid enters through the lower leeward openings and buoyant interior fluid leaves through the upper windward openings. As the wind speed increases, the opposing wind may cause a reversal in the Flow direction. In this case, cool exterior fluid enters through the high windward openings and mixes the interior fluid, which exits through the leeward openings. There are now two possibilities. If the rate of heat input by the source exceeds the rate of heat loss through the leeward openings, the temperature of the interior increases and this Flow reversal is only maintained temporarily. The buoyancy force increases with time, the Flow reverts to its original direction, and steady two-layer displacement ventilation is re-established and maintained. In this regime, the increase in wind speed increases the depth and temperature of the warm upper layer, and reduces the ventilation Flow rate. If, on the other hand, the heat loss exceeds the heat input, the interior cools and the Buoyancy-Driven Flow decreases. The reversed Flow is maintained, the stratification is destroyed and mixing ventilation occurs. Further increases in wind speed increase the ventilation rate and decrease the interior temperature. The transitions between the two ventilation Flow patterns exhibit hysteresis. The change from displacement ventilation to mixing ventilation occurs at a higher F than the transition from mixing to displacement. Further, we find that the transition from mixing to displacement ventilation occurs at a fixed value of F, whereas the transition from displacement to mixing Flow is dependent on the details of the time history of the Flow and the geometry of the openings, and is not determined solely by the value of F. Theoretical models that predic t the steady stratification profiles and Flow rates for the displacement and mixing ventilation, and the transitions between them, are presented and compared with measurements from laboratory experiments. The transition between these ventilation patterns completely changes the internal environment, and we discuss some of the implications for the natural ventilation of buildings. © 2004 Cambridge University Press.
Narsing K Jha - One of the best experts on this subject based on the ideXlab platform.
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contaminant transport by human passage through an air curtain separating two sections of a corridor part ii two zones at different temperatures
Energy and Buildings, 2021Co-Authors: Narsing K Jha, Daria Frank, L Darracq, P F LindenAbstract:Abstract Air curtains are installed in open doorways of a building to reduce Buoyancy-Driven exchange Flows across the doorway. Although an air curtain allows an unhampered passage of humans and vehicles, the interaction of this traffic with an air curtain is not well understood. In this study, we investigate the problem of the simultaneous interaction between the air curtain, the wake of a moving person and the Buoyancy-Driven Flow arising due to the density difference across the doorway. To this end, we conduct small-scale waterbath experiments with fresh water and salt water solutions to achieve different fluid densities. As a model of human passage, a vertical cylinder is pulled through a planar jet representing an air curtain and separating two zones at different densities. For a fixed travel distance of the cylinder before and after the air curtain, the average infiltration flux of dense fluid into the light fluid side increases with increasing cylinder velocity. Remarkably, we find that the infiltration flux is independent of the density difference across the doorway and is mainly due to the interaction between the air curtain and the cylinder wake with negligible effects from the Buoyancy-Driven Flow. Furthermore, the infiltration flux is also independent of the travel direction of the cylinder. As a consequence, the sealing effectiveness of an air curtain reduces with an increasing cylinder speed and this reduction is independent of the direction of the Buoyancy-Driven Flow. We provide a theoretical explanation for the observed changes in the effectiveness curve of the air curtain as the function of the deflection modulus. Dye visualisations of the air curtain and the cylinder wake are used to examine the re-establishment process of the air curtain after its disruption by the cylinder. We observe that the re-establishment time of the air curtain and the infiltration in the cylinder wake increases with an increasing cylinder speed.
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contaminant transport by human passage through an air curtain separating two sections of a corridor part ii two zones at different temperatures
arXiv: Fluid Dynamics, 2020Co-Authors: Narsing K Jha, Daria Frank, L Darracq, P F LindenAbstract:Air curtains are installed in open doorways of a building to reduce Buoyancy-Driven exchange Flows across the doorway. Although an air curtain allows an unhampered passage of humans and vehicles, the interaction of this traffic with an air curtain is not well understood. We study this problem by conducting small-scale waterbath experiments with fresh water and salt water solutions. As a model of human passage, a vertical cylinder is pulled through a planar jet representing an air curtain and separating two zones at different densities. For a fixed travel distance of the cylinder before and after the air curtain, the average infiltration flux of dense fluid in light fluid side increases with increasing cylinder velocity. However, we find that the infiltration flux is independent of density difference across the doorway and the travel direction of the cylinder. As a consequence, the sealing effectiveness of an air curtain reduces with an increasing cylinder speed and this reduction is independent of the direction of the Buoyancy-Driven Flow. Dye visualisations of the air curtain and the cylinder wake are used to examine the re-establishment process of the air curtain after its disruption by the cylinder. We observe that the re-establishment time of the air curtain and the infiltration in the cylinder wake increases with an increasing cylinder speed.
Nawaf H. Saeid - One of the best experts on this subject based on the ideXlab platform.
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numerical study of mixed convection on jet impingement cooling in a horizontal porous layer using brinkman extended darcy model
International Journal of Thermal Sciences, 2009Co-Authors: Kokcheong Wong, Nawaf H. SaeidAbstract:Abstract In the present study, numerical investigation of jet impingement cooling of a heated surface immersed in a confined porous channel is performed under mixed convection conditions with Brinkman-extended Darcy model, which the Darcian and non-Darcian effects are evaluated. The results are presented in the mixed convection regime with wide ranges of Rayleigh number (Ra), Peclet number (Pe), jet width and Darcy number (Da) in Darcy regime and non-Darcy regime. It is found that, the average Nusselt number ( Nu ¯ ) decreases with the increase in Da for the non-Darcy regime when Pe is low. When Pe is high, Nu ¯ increases with the increase in Da for the non-Darcy regime. Variation of Da in Darcy regime has negligible effect on the heat transfer performance. It is shown that mixed convection mode can cause minimum Nu ¯ unfavorably due to counteraction of jet Flow against buoyancy driven Flow. Minimum Nu ¯ occurs more obviously at higher values of Ra. Therefore, mixed convection conditions should be carefully considered when designing a system of jet impingement cooling through porous medium.
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Jet impingement cooling of a horizontal surface in a confined porous medium : Mixed convection regime
International Journal of Heat and Mass Transfer, 2006Co-Authors: Nawaf H. Saeid, Abdulmajeed A. MohamadAbstract:Abstract In the present article the jet impingement cooling of heated portion of a horizontal surface immersed in a fluid saturated porous media is considered for investigation numerically. The jet direction is considered to be perpendicular from the top to the horizontal heated element; therefore, the external Flow and the buoyancy driven Flow are in opposite directions. The governing parameters in the present problem are Rayleigh number, Peclet number, jet width and the distance between the jet and the heated portion normalized to the length of the heated element. The results are presented in the mixed convection regime with wide ranges of the governing parameters with the limitation of the Darcy model. It is found for high values of Peclet number that increasing either Rayleigh number or jet width lead to increase the average Nusselt number. Narrowing the distance between the jet and the heated portion could increase the average Nusselt number as well. No steady-state solution can be found in some cases; when the external jet Flow and the Flow due to buoyancy are in conflict for domination. The results from the unsteady governing equations in these cases show oscillation of the average Nusselt number along the heated element with the time without reaching steady state.
Jocelyn Bonjour - One of the best experts on this subject based on the ideXlab platform.
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experimental and numerical investigation of the infiltration heat load during the opening of a refrigerated truck body
International Journal of Refrigeration-revue Internationale Du Froid, 2015Co-Authors: Lafaye T De Micheaux, M Ducoulombier, J Moureh, Valerie Sartre, Jocelyn BonjourAbstract:Abstract The present article deals with an experimental and numerical investigation of heat and mass infiltration rates during the opening of a refrigerated truck body. Experiments were carried out for different temperatures and different aperture ratios. The infiltration dynamics was found to involve two distinct phenomena: a buoyancy driven Flow and a boundary layer Flow. The first is a density-driven Flow which gives birth to an important heat load peak. The second phenomenon is due to quasi steady-state natural convection over the inner wall of the truck. In the present work, the infiltration phenomenon was first analysed by means of a computational fluid dynamics model. The infiltration Flow rate is well predicted except at the transition between both Flow regimes. An analytical model using the ideal Flow theory was also developed to model the buoyancy driven Flow. The natural convection boundary layer Flow is then well predicted using a classical method.
Van Cj Hans Duijn - One of the best experts on this subject based on the ideXlab platform.
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buoyancy driven Flow in a peat moss layer as a mechanism for solute transport
Proceedings of the National Academy of Sciences of the United States of America, 2003Co-Authors: C Rappoldt, Gjm Gertjan Pieters, E B Adema, G J Baaijens, Albert Grootjans, Van Cj Hans DuijnAbstract:Transport of nutrients, CO2, methane, and oxygen plays an important ecological role at the surface of wetland ecosystems. A possibly important transport mechanism in a water-saturated peat moss layer (usually Sphagnum cuspidatum) is nocturnal buoyancy Flow, the downward Flow of relatively cold surface water, and the upward Flow of warm water induced by nocturnal cooling. Mathematical stability analysis showed that buoyancy Flow occurs in a cooling porous layer if the system's Rayleigh number (Ra) exceeds 25. For a temperature difference of 10 K between day and night, a typical Ra value for a peat moss layer is 80, which leads to quickly developing buoyancy cells. Numerical simulation demonstrated that fluid Flow leads to a considerable mixing of water. Temperature measurements in a cylindrical peat sample of 50-cm height and 35-cm diameter were in agreement with the theoretical results. The nocturnal Flow and the associated mixing of the water represent a mechanism for solute transport in water-saturated parts of peat land and in other types of terrestrializing vegetation. This mechanism may be particularly important in continental wetlands, where Ra values in summer are often much larger than the threshold for fluid Flow.
