A 2d-Numerical Study on Slot Jet Applied to a Wind Turbine as a Circulation Control Technique

The file attached to this record is the author's final peer reviewed version.A study on the feasibility of the Circulation Control (CC) technique for wind turbines is proposed. The CC was born in aeronautic field to improve the lift force on the wings, allowing the short take-off and landing of aircraft. It consists in blowing air at a relatively high speed over a rounded trailing edge. The thin jet of air remains attached to the convex curved surface, imposing a certain curvature to the outer streamlines, and, hence, increasing the lift force of the airfoil. Aim of this study is to numerically investigate the advantages on a wind turbine, based on the S809 airfoil, taking into account the energy related considerations, as the cost of the jet production. The paper, after a thorough evaluation of the increase of the generated power, finds that this technique could be promising in the energy harvesting aim

On the effect of the slot height in the cooling of a circular cylinder with a rectangular jet

The two main problems in the cooling of a cylinder with a rectangular jet of the same width are the choice of the jet height and the distance between the jet exit and the cylinder. The present paper presents experimental measurements of the heat transfer on a cylinder, with diameter D = 10 mm, cooled by rectangular jets with low turbulence, the same width of the cylinder but different height, H, 2.5 and 5 mm, respectively, in order to study their influence on the local and the mean cooling rates. The cylinder to be cooled is heated by electric current and is set on the symmetrical plane of symmetry, i.e. the geometry is two-dimensional, at several distances from the jet exit, x, in order to find the position which realizes the maximum heat transfer. The experimental measurements of the local heat transfer are performed along the circumference of the cylinder at several angles from the impingement for Reynolds number, defined with the cylinder diameter, D, and the average velocity, in the range ReD = 5000–22,000. The comparison between the two slots is performed at the same Reynolds number, ReD, which means that the smaller slot has a slightly greater maximum velocity. The experiments show that on the impinging point the local Nusselt number is greater for the slot with H = 2.5 mm, while at greater angles from the impingement, i.e. 45°, 90°, 135° and 180°, the local Nusselt numbers have different behaviors. The mean Nusselt numbers are similar for the two slots, at the same Reynolds number, ReD. Local and mean Nusselt numbers, obtained with the two jets, are in qualitative agreement if they are compared with the dimensionless distance, x/H, which can be interpreted as the ratio of the Reynolds numbers, Rex/ReH, independent of the mass flow rate or per unit of mass flow rate. The maximum of the local Nusselt number on the impinging point is obtained at greater distances for smaller Reynolds numbers, e.g. in the range from x/H = 15 at Re = 5000 to x/H = 9 at Re = 22,000, while the distance is shorter along the cylinder at greater angles from the impingement. The maximum of the mean Nusselt number is similar to that on the impingement for the two jets. The experimental heat transfer is also examined from the point of view of the slot efficiency, i.e. taking into account the mechanical power necessary to move the fluid in the slot jet. The conclusion is that the smaller slot, H = 2.5 mm or D/H = 4, has a greater efficiency as cooling system because it removes more heat at the same mechanical powe

Thermal and fluid dynamics in Jet Flow

The fluid dynamics of a free submerged jet in the literature has been reviewed. Different conclusions for what concerns the lengths of the potential core region and the line source of the jet have been found among different researches. The submerged slot jet of air has been the subject of several works of the present authors in order to establish its evolution with the distance from the slot exit. Hot anemometry was used to measure velocity and turbulence distributions on the exit as well as at several distances from it. The shadowgraph visualization technique was also employed to confirm the hot anemometry measurements. The existence of the undisturbed region of flow, where the fluid dynamics characteristics are equal to those on the exit, was found in a preliminary experiment with a first small wind tunnel at a turbulent Reynolds number, based on the hydraulic diameter, equal to Re=12,100 in the measurements and Re=10,600 in the visualizations. In a slot of height H=4.25 mm the dimensionless length of the undisturbed region was found equal to about Lu/H=1.6-2. It was confirmed later in systematic experiments with the same wind tunnel and slot. The length of the undisturbed region was shown to decrease with the increase of the Reynolds number from the laminar to the turbulent regime. The velocity and turbulence measurements, carried on in the Reynolds number range from Re=13,500 to Re=22,100, confirmed the dimensionless length of the undisturbed region was in the range Lu/H=1.6–2. The length of the undisturbed region of flow was evaluated also with the shadowgraph visualization technique in the Reynolds range from Re=1,700 until Re=22,100. The length of the undisturbed region was found equal to Lu/H>3 for the laminar Reynolds number of Re=1,700, decreasing to Lu/H=1 for the turbulent Reynolds number of Re=22,100. The velocity and turbulence measurements were performed in a second small wind tunnel with a slot of height 10 mm in the range from Re=7,600 to Re=37,300. At Re=7,600 the undisturbed length was equal to Lu/H=3–4, at Re=23,300 it was equal to Lu/H=1–2 and it was zero at Re=37,300. The flow visualizations with the shadowgraph technique were performed in the same wind tunnel and slot in the range from Re=7,100 to Re=33,400 confirming the dimensionless length of the undisturbed region of flow was decreasing from Lu/H=3.8 for Re=7,100, to Lu/H=0 for Re =33,400. The flow visualizations were also performed in the first wind tunnel with a slot of height H=17 mm. The length of the undisturbed region was decreasing from Lu/H=2.35 for Re=8,400, to Lu/H=1 for Re=31,200 and to Lu/H=0 for Re=52,900. The introduction of a metallic grid inside the wind tunnel, between the converging duct and the slot, and on the slot exit, showed a shorter length of the undisturbed region due to the higher turbulence. Hot anemometry was used also to measure velocity and turbulence evolution in a free jet up to an impinged cylinder, set at the two distances from the slot exit equal to L/H=4 and 6, for the Reynolds number of Re=23,300, showing the undisturbed region was equal to Lu/H=1 when the cylinder is set at L/H=4 and Lu/H=2 when the cylinder is set at L/H=6. The experimental results of the present authors on the length of the undisturbed region of flow, obtained in the turbulent as well as in the laminar regime with hot anemometer and shadowgraph visualizations, have been finally collected together with previous results of the literature, obtained with shadow, Schlieren and PIV visualization in laminar regime, with an interesting good agreement. The local heat transfer, or Nusselt number, experimental data of the present authors are reviewed in the present paper at the light of the evolution of a submerged jet, i.e., to verify the influence of the length of the undisturbed and the potential core regions. Preliminary experiments of heat transfer with a first small wind tunnel without the three metallic screens, i.e., at relatively higher level of turbulence, was used to cool a cylinder of diameter D=10 mm with a slot of height H=5 mm. The experiments showed a qualitative increase of the average and local Nusselt number at Re=17,000. Heat transfer of air jets with the same wind tunnel, slot and circular cylinder was investigated more systematically for Reynolds number ranging from Re=4,000 to Re=20,000. The influence of the distance between the exit of the slot jet and the cylinder, X, was investigated in the range of X/H=2–10. The local Nusselt number was shown to be a function of the X/H ratio, of the angle from the impinging point and of the Reynolds number. The local heat transfer was investigated with the same wind tunnel at reduced turbulence. Slot jets of different heights were used to cool a circular cylinder heated by electric current. The slot jet with the highest ratio between cylinder diameter and slot height, i.e., D/H, realized the highest mean Nusselt number at the same Reynolds number, defined with the hydraulic diameter. The distance from the slot exit which realized the highest mean Nusselt number is equal to X/H=8 for D/H=4. The similar distance observed in a slot jet with D/H=2 is X/H=6. An empirical expression was proposed to correlate mean Nusselt numbers for the three slot jets experimented. The local heat transfer along the cylinder perimeter was measured in an upgraded wind tunnel with lower turbulence level for a large range of Reynolds numbers, i.e., from Re=7,400 to Re=37,300. The local Nusselt number measured at different distances from the slot exit, along the perimeter of the cylinder presented interesting features from the point of view of the presence of the undisturbed region. Similar considerations were done about the mean Nusselt number

Flow evolution of a turbulent submerged two-dimensional rectangular free jet of air. Average Particle Image Velocimetry (PIV) visualizations and measurements

The paper presents average flow visualizations and measurements, obtained with the Particle Image Velocimetry (PIV) technique, of a submerged rectangular free jet of air in the range of Reynolds numbers from Re = 35,300 to Re = 2200, where the Reynolds number is defined according to the hydraulic diameter of a rectangular slot of height H. According to the literature, just after the exit of the jet there is a zone of flow, called zone of flow establishment, containing the region of mixing fluid, at the border with the stagnant fluid, and the potential core, where velocity on the centerline maintains a value almost equal to the exit one. After this zone is present the zone of established flow or fully developed region. The goal of the paper is to show, with average PIV visualizations and measurements, that, before the zone of flow establishment is present a region of flow, never mentioned by the literature and called undisturbed region of flow, with a length, LU, which decreases with the increase of the Reynolds number. The main characteristics of the undisturbed region is the fact that the velocity profile maintains almost equal to the exit one, and can also be identified by a constant height of the average PIV visualizations, with length, LCH, or by a constant turbulence on the centerline, with length LCT. The average PIV velocity and turbulence measurements are compared to those performed with the Hot Film Anemometry (HFA) technique. The average PIV visualizations show that the region of constant height has a length LCH which increases from LCH = H at Re = 35,300 to LCH = 4–5H at Re = 2200. The PIV measurements on the centerline of the jet show that turbulence remains constant at the level of the exit for a length, LCT, which increases from LCT = H at Re = 35,300 to LCT = 4–5H at Re = 2200. The PIV measurements show that velocity remains constant at the exit level for a length, LU, which increases from LU = H at Re = 35,300 to LU = 6H at Re = 2200 and is called undisturbed region of flow. In turbulent flow the length LU is almost equal to the lengths of the regions of constant height, LCH, and constant turbulence, LCT. In laminar flow, Re = 2200, the length of the undisturbed region of flow, LU, is greater than the lengths of the regions of constant height and turbulence, LCT = LCH = 4–5H. The average PIV and HFA velocity measurements confirm that the length of potential core, LP, increases from LP = 4–5H at Re = 35,300 to LP = 7–8H at Re = 2200, and are compared to the previous experimental and theoretical results of the literature in the zone of mixing fluid and in the fully developed region with a good agreement

Numerical simulation of the optimal spacing for a radial finned tube cooled by a rectangular jet. I-Average thermal results

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 ke 3 and SST keu. 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, Dhydr z 2H. Two turbulent flows are investigated at ReD = 7100, or ReDhydr = 13,200, and at ReD = 19,700, or ReDhydr = 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

Influence of the Reynolds number on the instant flow evolution of a turbulent rectangular free jet of air. Average Particle Image Velocimetry (PIV) visualizations and measurements

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 = 2H. 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 LCIH which increases with the decrease of the Reynolds number, from a ratio LCIH/H equal to LCIH/H = 0.9 at Re = 35,300 to LCIH/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=U0 of the centerline instant velocity, U, to the exit average velocity, U0, remain below ±4% for a length LCIV, defining the Flow with Constant Instant Velocity on the centerline. The ratio LCIV/H increases from LCIV/H = 1.1 at Re = 35,300 to LCIV/H = 4.1 at Re = 2,200 and is quite similar to LCIH/H. The instant PIV measurements of the centerline turbulence intensity, Tu, show that its variations remain below ±4% for a length LCIT, defining the Flow with Constant Instant Turbulence on the centerline. The ratio LCIT/H is equal to LCIV/H. The instant PIV velocity profiles in the transverse direction, are practically unchanged for a length LUVP which defines the flow with Unchanged Velocity Profile. The four lengths LUVP = LCIH = LCIV = LCIT are considered equal, defining the Flow with Negligible Disturbances, with length LND, or first type of flow, L1, which is in agreement with the average length of the flow with Constant Height, LCH, in the turbulent and laminar regime, and with the average length of the Undisturbed flow, LU, 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 LSVH, and the first vortex appears after the total length (LND + LSVH). The variations of the instant PIV measurements of the centerline turbulence intensity, Tu, increase after the length LCIT, but remaining below ±5–9% for a length LSVT, which defines the Flow with Small Variation of the Turbulence. The total length (LCIT + LSVT) is comparable to (LCIH + LSVH) 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, LND + LSD = L2, which is in agreement with the average length of the undisturbed flow, LU, in the laminar regime. The instant PIV frames show the flow with Coherent Vortices after L2, with a length LCV, 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 LND + LSD + LCV = L3, which is in good agreement with the average length of the potential core, LP

Preliminary numerical solutions of the evolution of free jets. IMECE2012-86730

The present paper presents preliminary numerical solutions of the flow evolution of a two dimensional rectangular free jet. The numerical simulations in a two-dimensional domain are carried out with Open-FOAM, the open-source code, and compare the numerical results with the experimental visualizations performed in the same laboratory with the shadowgraph technique. The evolution of a two-dimensional submerged free jet is reported in the literature by the presence of two regions of flow: the potential core, where the centerline velocity maintains equal to that on the slot exit, and the turbulent or mixing region, where the centerline velocity decreases with the distance from the exit. Previous anemometric measurements, carried out in this laboratory with an air jet emerging from a rectangular channel, showed the presence of a region of flow, just outside the exit and before the potential core, where velocity and turbulence remain almost equal to those measured on the exit, and it has been called “undisturbed region of flow” because is present also in turbulent conditions. Previous and present shadowgraph visualizations show a jet which has the same height along the undisturbed region of flow and increases its height afterwards. The length of the undisturbed region depends on the Reynolds number of the flow and on the presence of turbulence promoters, e.g. metallic grids, at the exit of the slot. The undisturbed region is becoming nil with the increase of the Reynolds numbers, in agreement to the literature. The present two dimensional numerical solutions, carried out at Re numbers equal to 25,000 and 60,000 confirm the results obtained with the shadow visualizations