Numerical and Experimental Study of Microchannel Performance on Flow Maldistribution

Miniaturized heat exchangers are well known for their superior heat transfer capabilities in comparison to macro-scale devices. While in standard microchannel systems the improved performance is provided by miniaturized distances and very small hydraulic diameters, another approach can also be followed, namely, the generation of local turbulences. Localized turbulence enhances the heat exchanger performance in any channel or tube, but also includes an increased pressure loss. Shifting the critical Reynolds number to a lower value by introducing perturbators controls pressure losses and improves thermal efficiency to a considerable extent. The objective of this paper is to investigate in detail collector performance based on reduced-order modelling and validate the numerical model based on experimental observations of flow maldistribution and pressure losses. Two different types of perturbators, Wire-net and S-shape, were analyzed. For the former, a metallic wire mesh was inserted in the flow passages (hot and cold gas flow) to ensure stiffness and enhance microchannel efficiency. The wire-net perturbators were replaced using an S-shaped perturbator model for a comparative study in the second case mentioned above. An optimum mass flow rate could be found when the thermal efficiency reaches a maximum. Investigation of collectors with different microchannel configurations (s-shaped, wire-net and plane channels) showed that mass flow rate deviation decreases with an increase in microchannel resistance. The recirculation zones in the cylindrical collectors also changed the maldistribution pattern. From experiments, it could be observed that microchannels with S-shaped perturbators shifted the onset of turbulent transition to lower Reynolds number values. Experimental studies on pressure losses showed that the pressure losses obtained from numerical studies were in good agreement with the experiments (<4%)

Intéractions de structures localisées dans un écoulement pariétal (nouveau mécanisme de transition by-pass)

L'interaction entre deux perturbations localisées est analysée dans un écoulement en canal plan, en condition souscritique à travers des simulations numériques directes. Les perturbations initiales sont en forme de deux paires de tourbillons contra-rotatifs. La perturbation mère génère des couches de vorticité normales à la paroi qui seront comprimées ou étirées localement par la structure secondaire. La rupture de la symétrie dans le sens transversale permet la génération d'une nouvelle couche de vorticité normale à la paroi. Elle est inclinée par le cisaillement et s'enroule pour former un nouveau tourbillon quasi-longitudinale. Le processus a comme conséquence l'apparition d'un spot turbulent localement. Une analyse détaillée est effectuée pour déterminer le rôle de différents paramètres entrant dans la physique du mécanisme. Plusieurs seuils critiques qui déclenchent le processus de transition interactif sont trouvés et analysés. Les paramètres de similitude résultant de la recherche paramétrique coïncident bien avec ceux qui régissent le cycle de régénération de la contrainte de Reynolds, produisant ainsi des structures cohérentes dans la couche tampon d'un écoulement pariétal turbulent pleinement développé. On suggère que le mécanisme que nous proposons puisse jouer un certain rôle dans le cycle de régénération de la turbulence proche parois, en produisant des structures toute en déviant le mécanisme d'instabilité tridimensionnel. On propose une stratégie de mélange active en forçant l'écoulement par des jets synthétiques, cette stratégie étant basée sur le mécanisme interactif. La faisabilité de cette stratégie est montrée par des simulations numériques directes de haute résolution spatiale et temporelleThe interaction between two localized disturbances is analyzed in a subcritical channel flow through direct numerical simulations. The initial perturbations are of the form of two pairs of counter rotating vortices. One of them interacts with the wall normal vorticity layers set-up near the wall, by compressing or stretching locally part of them through the straining motion it induces. The breakdown of spanwise symmetry leads to the rapid development of a new wall normal vorticity patch that is tilted by the shear and rolls up into a new small-scale streamwise vortex. The process results in a localized turbulent spot at later stages of development. A detailed analysis is carried out to determine the role of different parameters entering in the physics of the mechanism. Several critical thresholds that trigger the interactive bypass transition process are found and analyzed. The similitude parameters resulting from the parametric investigation coincide well with those governing the self-sustaining Reynolds shear stress producing eddies in the buffer layer of a fully developed turbulent wall flow. Tt is suggested that the mechanism we propose may play some role in the regeneration cycle of the near wall turbulence generating structures by precisely bypassing the three-dimensional streak instability mechanism. Based on these results, an active micro-mixing strategy through forcing the flow by synthetic wall jets is proposed. The feasibilitv of this strategy is shown through direct numerical simulations of high spatial and temporal resolutionGRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

By-pass transition through interactions of localized disturbances in wall bounded flows

International audienceA new mechanism bypass in a wall-bounded internal flow is proposed and the proposal is checked by direct numerical simulations of high temporal and spatial resolution. The mechanism is based on the interactions of the localized perturbations, rather than the effect of a single perturbation investigated so far in the classical bypass transition process. It is first shown by theoretical considerations that two pairs of quasistreamwise vortices can interact near the wall in such a manner that the compression (stretching) of the existing wall-normal vorticity induced by one of the pairs can enhance a new streamwise vorticity zone that can lead to new coherent structures and enhance considerably the transition process. Direct numerical simulations confirm this hypothesis

Non-linear interactions of localized disturbances. A new route to turbulence

International audienceThe transition scenario related to the disturbance growth on time scales significantly shorter than typical Tollmien-Schlichting (TS) waves that ‘by-passes’ the spatial and temporal development of the two-dimensional disturbances and their inherent secondary instabilities is the subject of this investigation. The set-up of three-dimensionality leads to the achievement of finite amplitudes and of the non-linear effects. They can mainly be generated by local surface irregularities such as roughness. This scenario has been investigated in detail in the past both in internal and external flows for single localized perturbations. It is well known since a while that there is a large structural similarity between a turbulent spot and developed turbulence in wall layers. One of the key problems in wall turbulence is the generation of Reynolds stress producing eddies. There are a multitude of different hypothesis and conjunctures advanced so far, but most of them are contradictory with observed experimental results. The destabilization caused by large scale eddies is for example contradictory with the observed bursting behavior whose frequency scales with inner rather than the outer variables. The regeneration process in the wall turbulence, and in parallel, the development of turbulent structures through by-pass mechanism should be related in some way to the preexisting structures themselves. The aim of the present investigation is to study the interaction between the localized perturbations to determine whether they rapidly trigger the transition under some circumstances, or not. This aspect has not been investigated before to our knowledge. We show through direct numerical simulations of high spatial and temporal resolution that the genesis of new quasi-streamwise vortices depends upon the capability of the primary structures to regenerate streamwise dependent intense wall normal vorticity. The DNS shows that the impingement of sweep flow caused by a parent structure razes rapidly one of the high speed streak. This leads to a local asymmetry between the streamwise evolutions of wall normal vorticity resulting in a secondary vorticity layer, which, in return is tilted by the shear and regenerates new quasi-streamwise structures. The bypass transition resulting from this process is significantly more rapid compared to the effect of localized single disturbances. [abstract only; no pdf

Enhancement in micro-mixing by synthetic vortical structures

International audienc