This study investigates experimentally a heated U-shaped mini-channel heat sink using Infrared Thermography and Particle Image Velocimetry for a water coolant flow of Reynolds numbers of 280, 650, and 1350 (based on the hydraulic channel diameter). The choice of this cell geometry is based on its role as a simplified unit of a serpentine heat exchanger, which is proved to be one of the most promising for cooling processes. The use of the infrared camera allows the detection of temperature fields on top of the external surface of the cell. Therefore, aiming to derive the temperature distribution on the channel roof, a dedicated transfer function is implemented. Moreover, we employed the lumped capacitance model for thermal analysis on both infrared measurements and thermocouple data. The latter are recorded to capture the cooling process of the aluminium base and water temperature at the end of the outlet tube. As a result, thermal transient rate, cooling magnitude and equilibrium temperature are obtained. These parameters indicate that higher Reynolds numbers correspond to increased thermal transient rates, enhanced cooling effects, and lower equilibrium temperatures. A non-uniform distribution of heat transfer along the channel is reported, with the most efficient cooling area localized close to the first 90-degree corner. These findings are consistent with numerical simulations and previous experimental observations. PIV results reveal the presence of two fluid acceleration zones following both 90-degree corners, which contribute to improve the water cooling ability in their respective regions. Additionally, the formation of two recirculating bubbles is reported at the inner wall from corners vertices, whose intensity is dependent on Reynolds number, pushing the main flow towards the outer channel wall and reducing the local heat transfer. Turbulent kinetic energy distributions are also investigated, pointing out the presence of intense areas that better match with regions of minimum equilibrium temperature than maximum velocity zones. This suggests the presence of local turbulent unsteadiness and three-dimensional phenomena, which contributes significantly to cooling enhancement.
Water plunging jets issuing horizontally in air from circular, square, triangular, elliptic and rectangular sharp-edged orifices are investigated experimentally for Reynolds number (based on the orifice diameter D) about 18000. Data are collected through planar time-resolved Particle Image Velocimetry on a vertical plane passing through the jet axis. Results show an asymmetric behaviour of triangular, elliptic and rectangular jets, while a more symmetrical one is observed for square and circular jets after a distance x/D≈1.5 below the plunging point. A double jet-like configuration is observed for rectangular and elliptic jets. Nonetheless, in the latter case, symmetry is restored after about 2.5D. Mixing is addressed through axial velocity decay, potential core length, ambient entrainment and velocity fluctuations rms values. As a result, triangular and elliptic jets reveal to be the most promising for fast and efficient mixing.
Water plunging jets issuing horizontally in air from circular, square, triangular, elliptic and rectangular sharp-edged orifices are investigated experimentally for Reynolds number (based on the orifice diameter D) about 18000. Data are collected through planar time-resolved Particle Image Velocimetry on a vertical plane passing through the jet axis. Results show an asymmetric behaviour of triangular, elliptic and rectangular jets, while a more symmetrical one is observed for square and circular jets after a distance x/D≈1.5 below the plunging point. A double jet-like configuration is observed for rectangular and elliptic jets. Nonetheless, in the latter case, symmetry is restored after about 2.5D. Mixing is addressed through axial velocity decay, potential core length, ambient entrainment and velocity fluctuations rms values. As a result, triangular and elliptic jets reveal to be the most promising for fast and efficient mixing.
The behavior of dispersed particles in a turbulent round jet is experimentally investigated. The role of particle-to-fluid density ratio ρp/ρf is analyzed by inspecting particle velocity fields and preferential concentration at four different ratios, from 0.7 to 19.3. The jet near-field region, i.e., up to X/D=11, is analyzed and compared to the unladen case. Particle-to-fluid density ratio is reported to have a strong impact on particle velocity field structure, in terms of jet transition and self-similar region as well as on turbulent fluctuations. Concentrations of particles show that increasing particle density corresponds to larger departure from uniformity. This occurrence is limited to the region X/D<5, where also the largest differences of average and fluctuating velocities with respect to the unladen case are measured.
Plunging jets are used in many industrial and civil applications, as for example in sewage and water treatment plants, in order to enhance aeration and mass transfer of volatile gases. They are also observed in natural processes as rivers self-purification, waterfalls and weirs. Many investigations dealt with the plunging jets in different configurations, but the dependence on Reynolds number and jet geometry were still not sufficiently addressed. For example, Mishra et al. (2020) studied an oblique submerged water impinging jet at different nozzle-to-plate distances and impingement angles, but only at a rather small Reynolds numbers (2600). On the other hand, different jet geometries have been extensively considered, but not for the plunging jet configuration (Mi, 2000; Hashiehbaf &Romano, 2013). In this work, plunging water jets issuing in air from orifices of different shape are considered. The aim of the work is to detail and compare jet behaviors in terms of velocity fields generated after impacting the air-water interface, as a function of Reynolds number and orifice geometry. However, air bubbles entrainment is mainly avoided in order to study the jet characteristics in a simpler case and use it as a reference starting point for future works.
Abstract The present study aims investigating experimentally wing/blade geometries in which the leading edge is modified by the presence of artificial bumps, following examples in nature (“biomimetics”). Specifically, the tubercles observed in humpback whales are considered with a special focus on easy manufacturing and performance improvements, trying to overcome the observed lift coefficient reduction before stall in comparison with a standard wing. To this end, different tubercle geometries are tested, by measuring overall forces acting on the wings and by deriving detailed velocity fields using particle image velocimetry. Measurements indicate performance improvements for all trailing edge tubercle geometries here tested. In addition, the detailed analysis of mechanisms underlying the improvement of performances suggests that a triangular shape of the leading edge combines the advantages of easy manufacturing and improvements of pre-stall behaviour. So far, a simple mathematical model, describing tubercles as delta wings, is presented and verified by experimental data. The objective of the present work is focusing on the basic fluid-mechanics phenomena involved, to show that beneficial effects of tubercles are present even when tubercle details are simplified, in order to couple performance improvement and ease of assembly. Graphical Abstract
In the present work we study the topology, mixing properties, turbulence quantities, dependence on the outlet geometry of a sharp-edged orifice plunging jet which first issues horizontally in air and then plunges in a water pool. The investigated orifices shapes are circular and rectangular. Data are acquired at different Reynolds numbers in the range 11000–25000, based on the orifice diameter (equal to 2 cm) and on the average exit velocity, as derived from flow rate measurements. Velocity fields in vertical and horizontal planes are measured using planar time-resolved Particle Image Velocimetry. Results show a clear asymmetry of the cross-velocity profiles both in circular and rectangular cases, with the latter that revealed a shape which is Reynolds number dependent. Axial velocity decays, potential core lengths and spreading rates highlight an opposite trend between the two jet geometries, thus suggesting a higher mixing for the lowest Reynolds number circular jet and the highest rectangular one. Plunging angle shows a dependency on Reynolds number. Moreover, it seems to play a role in the evolution of the upper and lower side of the jet due to the onset of a co-flow. Ambient mass entrainment points out the different interactions of the two plunging jets with the ambient flow: in circular case, it entrains fluid from the surroundings, from horizontal to vertical planes in streamwise direction, while in rectangular one it ejects flow from vertical to horizontal planes. Finally, Strouhal numbers are derived for main vortices frequencies along jet centerline, other than upper and lower sides.
In the present work, an experimental set-up designed to allow investigations of the carrier fluid local behaviours in terms of velocity and temperature fields is presented. The study is carried out on a U-turn microfluidic channel, to bear a complete view of the thermo-fluidic system. The experiments have been performed by Infrared Thermal Imaging, which allows deriving the thermal field on the microchannel external walls, and inferring the internal thermal profiles through specific transfer functions in laminar and transitional regimes. Maps of thermal transients allow deriving cooling performances, which are used to identify the local thermal efficiency of the microchannel and relate it to the global efficiency. Velocimetry measurements have been conducted with a Micro Particle Image Velocimetry (μPIV) setup at different flow rates. This analysis has been coupled with thermal results to obtain a description of the effects of local fluid flow phenomena in transitional turbulent regime on the global heat transfer efficiency of a U shaped microchannel. The interactions among thermal and flow fields are specifically related to secondary recirculating flows, close to the turns, where high cooling rates and high magnitude of local velocity fluctuations are measured.
