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The present work studies the enhancement of heat transfer on a cylinder impinged by a rectangular jet in presence of a metallic grid on the slot exit. The experiments are carried out to cool a smooth cylinder, electrically heated, with a rectangular jet of air, at three Reynolds numbers in turbulent flow, i.e. Re = 5100, 10,600 and 15,300, where the Reynolds number is defined with the cylinder diameter. Three metallic grids, identified by the French classification as NF06-10-20, are employed with the solidity ratio ranging from 24.3% to 67.7%, and the same diameter of the mesh filament (0.6 mm). Fluid dynamic measurements, performed with a hot film anemometer, are done to justify the thermal results. Heat transfer measurements are presented as local and mean Nusselt numbers. The comparison with the case without grid, indicated as NF00, is done at the same mass flow rate of the jet and the same power consumption of the wind tunnel to move the jet. The main goal of the paper is to evaluate how the solidity ratio of the metallic grids influences the heat transfer. [ABSTRACT FROM AUTHOR]

The paper is aimed at investigating the influence of the Reynolds number on the instant flow evolution of a rectangular free jet of air in the range of Reynolds numbers from Re = 35,300 to Re = 2,200, where the Reynolds number, Re, is defined according to the hydraulic diameter, D , of a rectangular slot of height H , equal to about D = 2 H . The Particle Image Velocimetry (PIV) technique allows obtaining the instant PIV visualizations on the central symmetry section of the rectangular jet. The visual inspection of the instant frames with one and two vortices, except for Re = 35,300 where only one vortex images are detected, shows that after the jet exit is present the Flow with Constant Instant Height, with a length L CIH which increases with the decrease of the Reynolds number, from a ratio L CIH / H equal to L CIH / H = 0.9 at Re = 35,300 to L CIH / H = 4.0 at Re = 2,200. The instant PIV measurements, carried out at several distances from the jet exit, show that the variations of the ratio U / U ‾ 0 of the centerline instant velocity, U , to the exit average velocity, U ‾ 0 , remain below ±4% for a length L CIV , defining the Flow with Constant Instant Velocity on the centerline. The ratio L CIV / H increases from L CIV / H = 1.1 at Re = 35,300 to L CIV / H = 4.1 at Re = 2,200 and is quite similar to L CIH / H. The instant PIV measurements of the centerline turbulence intensity, Tu , show that its variations remain below ±4% for a length L CIT , defining the Flow with Constant Instant Turbulence on the centerline. The ratio L CIT / H is equal to L CIV / H. The instant PIV velocity profiles in the transverse direction, are practically unchanged for a length L UVP which defines the flow with Unchanged Velocity Profile. The four lengths L UVP = L CIH = L CIV = L CIT are considered equal, defining the Flow with Negligible Disturbances, with length L ND , or first type of flow, L 1 , which is in agreement with the average length of the flow with Constant Height, L CH , in the turbulent and laminar regime, and with the average length of the Undisturbed flow, L U , in the turbulent regime. The instant PIV frames show the presence, after the flow with negligible disturbances, of the Flow with Small Variations of the Height, where the jet height varies slightly without forming the vortex. The length of the flow with Small Variation of the Height is L SVH , and the first vortex appears after the total length ( L ND + L SVH ). The variations of the instant PIV measurements of the centerline turbulence intensity, Tu , increase after the length L CIT , but remaining below ±5–9% for a length L SVT , which defines the Flow with Small Variation of the Turbulence. The total length ( L CIT + L SVT ) is comparable to ( L CIH + L SVH ) in the turbulent and laminar regimes, and can be assumed equal to the sum of the lengths of the flows with negligible and small disturbances, L ND + L SD = L 2 , which is in agreement with the average length of the undisturbed flow, L U , in the laminar regime. The instant PIV frames show the flow with Coherent Vortices after L 2 , with a length L CV , followed by the vortices breakdown. The instant PIV frames allow visualizing the end of the flow with Coherent Vortices, defining the total length of the flows with negligible and small disturbances, and coherent vortices, as L ND + L SD + L CV = L 3 , which is in good agreement with the average length of the potential core, L P . [ABSTRACT FROM AUTHOR]

Highlights: [•] Zone of flow establishment contains a newly identified undisturbed region of flow. [•] In the undisturbed region of flow the velocity profile is similar to the exit one. [•] In undisturbed region of flow the height of average PIV visualizations is constant. [•] In the undisturbed region of flow the turbulence on the centerline is equal to exit. [•] Length of undisturbed region of flow decreases with Reynolds number increase. [ABSTRACT FROM AUTHOR]

• Slot jet of air is a technique to enhance heat transfer over a cylinder. • Aluminum foam-covered cylinder and smooth one are employed as alternative. • Jet flow and full flow (or uniform flow) are experimentally investigated. • Jet flow visualizations are carried out over both cylinders. • Aluminum foam-covered cylinder and jet flow are recommended as enhance techniques. The aim of the present paper is to study experimentally the enhancement of heat transfer, due to the presence of a layer of aluminum foam covering a smooth circular cylinder. The measurements are carried on with the impingement of a rectangular jet of air, which has a transversal dimension equal or smaller than the cylinder diameter, or jet flow, and full flow (or uniform flow, which has a greater dimension than the cylinder diameter). Literature does not report any paper dealing with this multiple problem. Experimental heat transfer and fluid dynamics measurements are carried out at Reynolds number ranging from Re D =2200 to Re D =24,750. The layer of the aluminum foam, 8 mm thick, is glued onto the smooth cylinder, with an external diameter of D = 10 mm. The height of the slot jet is H = 10 mm, with a width of W = 135 mm. Jet flow visualizations are performed for the smooth and the foam-covered cylinder to explain the heat transfer results from the fluid dynamics point of view. The flow visualizations show that the jet flow always penetrates into the foam, wetting it completely, while the reattachment never occurs downstream. In the smooth cylinder, the jet completely envelops the cylinder, at greater Reynolds numbers and a distance of x/H = 7, with a small recirculation zone, showing that the rear surface also participates in the heat transfer. The heat transfer on the foam-covered cylinder increases, mainly, because of the large increase in the exchange surface, while it is somewhat penalized, at the higher Reynolds values and at the greater distance from the slot exit, because the rear surface is little involved in the heat transfer. [ABSTRACT FROM AUTHOR]

Jets are a common way to transfer mass among fluids, or from a fluid to a surface. At moderate Reynolds numbers and low turbulent intensities the jet exhibits a Near Field Region (NFR) several diameters long. The paper presents numerical results and a theoretical model for the passive scalar diffusion of a submerged free jet in the NFR. Large Eddy Simulations (LES), in the Reynolds number range of 5000–40,000 and the Schmidt number range 1–100, are performed obtaining the passive scalar fields. Three mathematical models for the passive scalar diffusion are presented; the first one is valid in the NFR, specifically in the Undisturbed Region of Flow (URF), and the other two, obtained under the hypotheses of Tollmien and Görtler momentum spreading, are valid in the Potential Core Region (PCR). The last two models employ a turbulent Schmidt number inversely proportional to the mean velocity gradient, conclusion obtained from the LES numerical results. The self-similar solutions of the passive scalar show good agreement with the LES results. The wide range of Reynolds and Schmidt numbers investigated gives generality to the results. [ABSTRACT FROM AUTHOR]

The present work investigates the average heat transfer on a finned cylinder cooled by a rectangular jet of height H . The diameter of the external cylinder, without fins, is D and the cylinder to slot ratio D / H is equal to 1. Numerical simulations are carried on with two turbulent models, RNG k – ɛ and SST k – ω . The jet flow can be characterized by the Reynolds number, defined with the diameter of the external cylinder without fins, D , or with the hydraulic diameter of the rectangular slot, D hydr ≈ 2 H . Two turbulent flows are investigated at Re D = 7100, or Re D hydr = 13,200, and at Re D = 19,700, or Re D hydr = 36,700. The main goal of the paper is to evaluate the optimal spacing, s , between the fins, and the optimal slot-to-cylinder distance, S / H , which maximize the average heat transfer on the finned cylinder. The optimization process is performed for a pitch of 3 mm and at a given value of the fin volume. The numerical simulations show that an optimal configuration is obtained when the ratio s / l of the spacing, s , to fin height, l , is maximum, and the cylinder is set at the slot-to-cylinder distance, S / H = 3. [ABSTRACT FROM AUTHOR]

The paper investigates the transient flow evolution, up to the steady state, of a rectangular free jet of air with aspect ratio AR = 6.3. The Reynolds number, Re , defined according to the hydraulic diameter, D , of the rectangular slot of height H , equal to about D = 2 H, spans in the range from Re = 48,000 to Re = 3400. A centrifugal fan generates the jet flow with a velocity increasing, as a ramp, from zero up to a constant steady state value. In the transient state of the flow the jet is composed of a primary vortex (PV) and a quasi-stable stem. In the transient 2D experimental instant images the stem of the jet has a height equal to that of the slot and a length increasing with the time up to a maximum value, which is greater than the lengths observed in the steady state, i.e. Negligible Disturbance Flow (NDF) and Small Disturbance Flow (SDF). The experimental jet evolution, observed with measurements and image processing, show that the primary vortex pinch-off does not occur for all the Reynolds numbers investigated, while the leading vortex structure becomes progressively incoherent and breaks down into turbulence. Two-dimensional (2D) Large Eddy Simulations (LES) of mass transfer and fluid flow evolution confirm the experimental images. The transient LES simulations are in good agreement with the experimental results. • Transient slender rectangular air free jet with Reynolds number 48,000 to 3400. • Transient jet evolution from primary vortex formation up to steady state. • Jet penetration and vortex growth evaluated by image processing. • Large Eddy Simulations (LES) of mass transfer and fluid flow. • Maximum lengths of Negligible Disturbance and Small Disturbance Flow. [ABSTRACT FROM AUTHOR]