51 research outputs found
Structure tourbillonnaire des jets lobés et pouvoir de mélange
Le contrĂŽle passif des jets turbulents est un moyen de mĂ©lange peu coĂ»teux qui intĂ©resse un large domaine industriel. Il est montrĂ© qu'une buse lobĂ©e droite, permet l'amĂ©lioration du mĂ©lange du jet issu de celle-ci, par rapport Ă un jet circulaire de rĂ©fĂ©rence. Une buse lobĂ©e de mĂȘme plan de soufflage et Ă lobes inclinĂ©s, s'avĂšre plus performante que la prĂ©cĂ©dente. L'analyse de la dynamique tourbillonnaire, montre que les structures azimutales des jets lobĂ©s ne sont pas annulaires comme dans un jet circulaire, mais discontinues en raison du cisaillement de l'Ă©coulement transversal gĂ©nĂ©rĂ© par la variation du rayon de courbure de la frontiĂšre de la buse. Il s'en suit un Ă©panouissement des structures longitudinales aux points de discontinuitĂ©s expliquant probablement le gain d'induction dans le jet lobĂ© droit. L'inclinaison des lobes et des creux accentue le cisaillement de l'Ă©coulement transversal et l'auto-induction des structures longitudinales en devient plus importante
Thermal comfort models for indoor spaces and vehiclesâCurrent capabilities and future perspectives
International audienceThroughout this paper, we reviewed the most popular thermal comfort models and methods of assessing thermal comfort in buildings and vehicular spaces. Most of them are limited to specific steady state, thermally homogenous environments and only a few of them address human responses to both non-uniform and transient conditions with a detailed thermo-regulation model. Some of them are defined by a series of international standards which stayed unchanged for more than a decade. The article proposes a global approach, starting from the physiological reaction of the body in thermal stress conditions and ending with the model implementation. The physiological bases of thermal comfort are presented, followed by the main thermal comfort models and standards and finishing with the current methods of assessing thermal comfort in practice. Within the last part we will focus mainly on thermal manikin experimental studies, and on CFD (computational fluid dynamics) numerical approach, as in our opinion these methods will be mostly considered for future development in this field of researc
Passive control of wall shear stress and mass transfer generated by submerged lobed impinging jet
International audienceParticle image velocimetry was used to investigate the flow field in an impinging lobed daisy hemispherical nozzle jet in comparison to its counterpart round jet, at a Reynolds number of 5620 based on the exit velocity and the equivalent diameter D e of the nozzle. The limitations of the PIV technique in the vicinity of the target wall due to the laser scattering were addressed by using the electrodiffusion (ED) technique to determine the wall shear rate distribution. The distribution of the mass transfer coefficient is also obtained using the ED technique. The target wall is placed at a distance H = 2D e from the plane tangent to the nozzle, at the center of the orifice. The entrainment of ambient fluid in the free jet region, which is larger in the lobed jet compared to the round jet, feeds in turn the wall jet region. The maximum wall shear rate was found significantly higher in the daisy jet, with an excess of 93 % compared to the reference round jet. The maximum mass transfer is 35 % higher in the former compared to the latter. Therefore, the hemispherical daisy nozzle is an excellent candidate in passive strategies to enhance local skin-friction and the subsequent local mass transfer at a constant exit Reynolds numbe
Dynamique tourbillonnaire et entraßnement dans la région initiale d'un jet rond et d'un jet Marguerite à bas Reynolds
LâĂ©tude prĂ©sente lâinfluence de la dynamique
tourbillonnaire sur lâinduction dans la rĂ©gion initiale de deux jets rond et lobĂ©.
Lâentrainement du jet rond est modulĂ© Ă la frĂ©quence de passage des structures de K-H.
Dans le jet lobé, la géométrie introduit un cisaillement transversal et des
discontinuités de la structure de K-H conduisant au libre développement des structures
longitudinales. Il en rĂ©sulte un taux dâentrainement non corrĂ©lĂ© au passage des
structures de K-H et de niveau plus grand
PIV and Electrodiffusion diagnostics of flow field, wall shear stress and mass transfer beneath three round submerged impinging jets
International audienceThis paper reports on measurements of velocities, wall shear rates and mass transfer in an impinging round jet issued from a round nozzle. The effect of the nozzle shape on transfer phenomena was investigated. A round orifice perforated either on a flat plate (RO/P) or on a hemispherical surface (RO/H) was compared to a reference convergent nozzle (CONV). All the nozzles have the same exit diameter D. The exit volumetric flow rate was also conserved and led to the same Reynolds number based on the exit bulk velocity, Reb = 5620. The nozzle-to-wall distance was constant and equal to 2D. The Particle Image Velocimetry technique (PIV) was used to capture the jet flow field. The limitations of the PIV technique in the vicinity of the target disc are addressed by using the electrodiffusion technique (ED) to obtain the wall shear rate distribution. The ED technique was extended for the measurement of local mass transfer distribution and global mass transfer on the target disc. The whole velocity field, wall shear rates and mass transfer in the three impinging round jets were compared. It was shown that at constant volumetric flow rate, the use of an orifice nozzle not only improves wall shear rate, but also increases local and global mass transfer. The global mass transfer on a target disc of a 3.2 D diameter is 25% and 31% higher for RO/H and RO/P nozzles, respectively, compared to the reference CONV nozzle. The orifice nozzles generate narrower exit profiles relatively to the convergent nozzle. The vena contracta effect in orifice jets, more intense with RO/P than with RO/H, generates an increase of the exit centerline velocity. The hemispherical surface of RO/H nozzle stretches the flow at the exit and somewhat attenuates the vena contracta effect. The characteristic scale representation of the data confirms the origin of the observed differences between the three jets. A link between the wall shear stress and the mass transfer is revealed. The wall shear rate and the mass transfer are in a close relationship with the near field flow features, themselves affected by the nozzle geometry. Time-resolved tomographic PIV technique reveals that the wall shear rate fluctuation is related to the dynamics of the jet coherent structures. The instantaneous PIV fields indicates the formation of secondary vortices in the region where a secondary peak in local mass transfer emerges. The level of this secondary peak is sensitive to the nozzle shape. The higher is the jet acceleration, the more intense is the level of the secondary pea
Experimental and numerical study of the air distribution inside a car cabin
The main declared goal of all car manufacturers is to ensure high comfort inside the cabin and to reduce the fossil fuel. It is well-known that the time spent by the people indoor has raised in the last decade. The distance between the home and the workplace increased due to diversity of activities and hence job diversity. The thermal comfort during the travel must to be ensured to reduce the occupantâs thermal stress. The present study is investigating a comparison between the measured data and the numerical simulation results in the case when the ventilation system is functioning. It was evaluated the effect of the boundary conditions air flow and air velocity distribution in a passenger compartment in two cases: first is the general used constant inlet flow and the second is a new approach of importing the measured data obtained during the experimental measurement session as a boundary condition.CFD simulations were made taking as input the measured data obtained during experimental session. We have observed differences between initial simulation results and the measured data, therefore, for more accurate results, a new approach is needed, to impose as boundary conditions the measured data
Passive control of jet flows using lobed nozzle geometries
Passive control of the entrainment phenomenon in the turbulent jets is a
mixing enhancement economical method of wide interest in the industrial
field. A lobed nozzle without lobes inclinations allows improving mixing in
the generated flow compared to a reference circular jet. A second lobed
nozzle, having the same exit plane geometry but with inclined lobes
intensifies, in a considerable manner, the entrainment which reaches up to
four times the one in the reference circular jet. The lobed jets vortical
dynamics analysis shows that the azimuthal structures are not annular like
in the case of a circular jet, but discontinuous, due to the shear of the
transverse flow induced by the curvature variation of the exit plane. The
streamwise structures development at the discontinuities locations is
probably explaining the entrainment benefit observed in the lobed jet
without inclination angles. The lobed jet issued from the second nozzle
presents like the first lobed jet, discontinuous azimuthal structures, but
its remarkable induction benefit is not merely owed to the previous
phenomenon. The intensification and organisation of the streamwise vorticity
field into large scale structures, resulting in a consequent mixing
enhancement, are connected to the increased shear produced by the lobes
inclinations
Cross and clover shaped orifice jets analysis at low Reynolds number
The jet coming from a cross-shaped orifice with an open center has been shown
in the past, to provide substantial increase in the near field convective
transport-mixing, in comparison to a classical round orifice jet. Detailed
information has been reported in previous works on the role played in the jet
mixing enhancement by the crow of vortices composed of counter-rotating pairs
of secondary streamwise structures which are developing in orificeâs troughs.
A trough in the cross-shaped jet generates a local shear like the one
generated by a triangular tab in a square jet. In the present study we are
interested by the modification of local shears in the troughs of the
cross-shaped jet, when orifice geometry is modified, such as the center of
the orifice becomes closed, leading to a clover-shaped orifice. The general
motivation is to understand the effect of using a set of combination of
longitudinal structures, themselves produced by the superposition of local
shear regions, in mixing performance of a cross jet. It is shown that lower
entrainment rates in the clover jet is a results of a additional internal
crown of vortices which opposes the external one due to inner shears
generated by closing the center of the orifice
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