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 analysis of mixing ventilation efficiency using a vortex diffuser - comparison to a lobed multi-cone diffuser

International audienceIn a recent work, an innovative multi-cone diffuser integrating lobed inserts as vortices promoters was proposed, and was compared to its counterpart without lobed inserts. The former offers better mixing of the jet with the ambient air, and significantly improves thermal comfort.In this study, the innovative diffuser is compared to a vortex diffuser under the same experimental conditions. The vortex diffuser is described in the scientific and technical literature as the best diffuser in terms of mixing efficiency due to the swirling effect, being thereby a reference for the innovative lobed diffuser.The results demonstrate that the vortex diffuser does not produce the expected swirling effect. The rotation of the jet is prevented by the immediate attachment of the flow to the ceiling due to the Coanda effect. The jet behaves as a radial ceiling-attached flow having a poor mixing performance. It is believed that such a diffuser would be efficient when mounted on a long duct far away from the ceiling. Hence, it will be better suited for high ceilings, such as in industrial or commercial spaces. In this case, the proposed lobed diffuser becomes an appropriate and efficient mixing ventilation solution for rooms having low ceiling heights

Passive Control Strategy for Mixing Ventilation in Heating and Cooling Modes Using Lobed Inserts

International audienceMixing ventilation is commonly used to control thermal comfort in a room by means of air jets. The jet diffusers should distribute the fresh air and energy, for heating or cooling, in the entire occupied zone. Therefore, the design of the diffusers must aim, beyond aesthetic aspect, the ability to provide a good mixing between the jet and the ambient air. Enhancement of jet mixing with the ambient air by means of lobed inserts into a diffuser, was recently proposed in an European patent, as a promising and low-cost solution for improving the performance of HVAC systems. In this paper, an experimental investigation on thermal comfort generated by a classical multi-cone ceiling-mounted diffuser is proposed. Its performance is compared with the same diffuser equipped with lobed inserts. A simplified heated manikin simulates the presence of a human body in the test room. Thermal comfort was analyzed based on traditional pointwise measuring probes and on the standard ISO 7730. It is revealed that the thermal comfort was significantly improved using the lobed diffuser compared to the conventional one, without increase of pressure drop and sound pressure levels

Mixing Ventilation in a Full Scale Model Room Using a Multi-Orifice Nozzle

International audienceA commercial multi-orifice diffuser (OD) is compared under the same experimental conditions to a previously validated innovative lobed diffuser (LD). The OD head is a cylindro-spherical surface pierced with 20 orifices arranged in three rows leading to efficient jet diffusion. The jet analysis reveals that the flow from the closest two orifice-rows to the ceiling attach to it by Coanda effect whereas the orifice jets from the lower orifice-row are directed towards the occupied zone. Thermal comfort analysis reveals less satisfactory conditions for OD than for LD. In the former case, higher velocities in the occupied zone are responsible of a slightly cold sensation. The recorded pressure loss is higher in OD than in LD, whereas no significant difference was observed in terms of sound pressure level in the occupied zone

Analyse de l'écoulement et du confort thermique généré par un diffuseur commercial multiorifices

International audienceDans une chambre climatique de Laboratoire thermiquement gardée, un diffuseur multiorifices (OD) commercial à haute induction est comparé dans les mêmes conditions expérimentales à un diffuseur lobé innovant (LD) préalablement validé. Le diffuseur OD a une surface cylindro-sphérique percée de 20 orifices disposés en trois rangées permettant une diffusion efficace du jet d'air. L'analyse des jets révèle que le flux des deux rangées d'orifices les plus proches du plafond s'y attache par effet Coanda. Quant aux jets d'orifices de la rangée d'orifices inférieure, ils sont dirigés vers la zone d'occupation. L'analyse du confort thermique révèle des conditions moins satisfaisantes pour OD que pour LD. Dans le premier cas, des vitesses plus élevées dans la zone d'occupation sont responsables d'une sensation de refroidissement. La perte de charge enregistrée est presque deux fois plus élevée pour OD que pour LD, alors qu'il n'est pas observé de différence significative en termes de niveau de pression acoustique dans la zone d'occupation

Impinging cross-shaped submerged jet on a flat plate: a comparison of plane and hemispherical orifice nozzles

International audienceIt is well known that the transfer of heat, mass and momentum to a wall by an impinging jet is partially linked to the way in which jet generation is realized. The organization of the vortices at the jet exit depends on the upstream conditions and on the geometry of the nozzle. Particle image velocimetry (PIV) and electrodiffusion techniques were used to investigate the characteristics of different impinging jets and the resulting wall shear rates and mass transfer. Two cross-shaped orifice jets, one produced by a plane orifice nozzle (i.e. a cross-shaped orifice made on a flat plate, CO/P) and the second by a hemispherical orifice nozzle (i.e. a cross-shaped orifice made on a hemisphere, CO/H), were compared to a reference round jet produced by a convergent nozzle. The distance between the jet exit and the target wall was equal to two nozzle equivalent diameters (D e ), based on the free orifice area. The Reynolds number, based on D e and on the exit bulk-velocity, was 5620 for each flow. PIV measurements give an overall view of the flow characteristics in their free and wall regions. The switching-over phenomena observed in the CO/P nozzle case, and already described in the literature with similar nozzles, did not occur in the jet from the CO/H nozzle. Electrodiffusion measurements showed differences in the shape and level of radial distribution of the wall shear rates and mass transfer. One of the most important observations is the large difference in wall shear stress between the three jets. For the same exit bulk-velocity, the maximum wall shear rate in the CO/P and CO/H nozzle jets was almost two and three times higher, respectively, than that of the reference convergent jet. This higher wall shear rate is accompanied by higher mass transfer rate. It is demonstrated that the cross-shaped orifices enhance the mass transfer not only locally but also globall