Turbulent Slot Jet Research Articles

Numerical simulation of turbulent fluid flow and heat transfer characteristics in metallic porous block subjected to a confined slot jet

The present study demonstrates the numerical simulations of fluid flow and heat transfer characteristics in the metallic porous block subjected to a confined turbulent slot jet. The numerical results indicate that the flow field can be adjusted and the thickness of the boundary layer can be decreased by the inserted porous block so that the heat transfer can be enhanced in the channel. As expected, parametric studies show that the average Nusselt number (Nu) increased with the increase of the Reynolds number but slightly increased with the decrease of Ws/H, and the effect of C/H is found insignificant.

Vortex Structure in the Wall Region of an Impinging Plane Jet

The work presented here comes within a research program dealing with vortex detection in the impingement region of a planar jet. In this study, experiments have been performed for a submerged turbulent water slot jet impinging normally on a flat plate, and an emphasis was put on the flow field characteristics. For this purpose, particle image velocimetry (PIV) have been employed. A comprehensive fluid mechanical data includes instantaneous and mean flow field, variance of normal and cross velocity fluctuations have been presented. The present work is also concerned with the flow structure in the impingement region where the transfers (heat/mass) occur. An attempt has been made to understand the flow structure by employing the vortex detection criteria on the instantaneous velocity vector field. Accordingly, the PIV measurements were carried out for four different Reynolds numbers: 3000, 6000, 11000 and 16000, and at three different planes: a plane parallel to the impingement plate, transverse plane of the jet and a plane perpendicular to the jet. A method of filtration, based on proper orthogonal decomposition (POD) technique was applied first to the instantaneous velocity and filtered velocity database is then used for vortex detection. Further, the results about the size, shape, spatial distribution and energy content of the detected vortices have been provided.

Numerical study of turbulent slot jet impingement cooling on a semi-circular concave surface

An investigation of the flow field and heat transfer characteristics of a slot turbulent jet impinging on a semi-circular concave surface with uniform heat flux has been carried out numerically in this study. The turbulent governing equations are solved by a control-volume-based finite-difference method with a power-law scheme and the well-known k– ε turbulence model and its associate wall function to describe the turbulent structure. In addition, a body-fitted curvilinear coordinate system is employed to transform the physical domain into a computational domain. Numerical computations have been conducted with variations of jet exit Reynolds number Re 2 B (5920 ⩽ Re 2 B ⩽ 23,700), dimensionless jet-to-surface distance H/ B (0.5 ⩽ H/ B ⩽ 12), dimensionless jet width B/ D (0.033 ⩽ B/ D ⩽ 0.05) and the heat flux q″ (1663 W/m 2 ⩽ q″ ⩽ 5663 W/m 2). The theoretical model developed is validated by comparing the numerical predictions with available experimental data in the literature. The variations of local Nusselt numbers along the semi-circular concave surface decrease monotonically from its maximum value at the stagnation point. The numerical results show that the local Nusselt numbers are reasonably predicted with a maximum discrepancy within 15%. As the Reynolds number fixes, the effect of the impingement distance ( H/ B) on the average Nusselt (Nu avg) is not significant except at low H/ B = 0.5. This study provides fundamental insight into turbulent slot jet impingement cooling on the semi-circular concave surface.

Technique for delaying splashing in jet wiping process

The current study presents an experimental investigation of a technique for delaying the occurrence of splashing in jet wiping process by means of a side jet. Gas jet wiping is a hydrodynamic method of controlling the film thickness applied on a substrate in coating processes. It consists of a turbulent slot jet impinging on a moving surface coated with a liquid film. The process is limited by splashing; a rather violent film instability which is characterized by the ejection of droplets from the runback flow and results in the detachment of the film from the substrate. In the present study an additional side jet is used close to the main wiping jet in order to stabilize the runback flow and avoid splashing. The mean film thickness after wiping is measured using a light absorption method and the results are compared for the single jet wiping and two jet configuration. It is shown that the use of the side jet allows for stronger wiping, resulting in lower values of the final film thickness which cannot be achieved with a single jet.

A novel dynamic coherent eddy model and its application to LES of a turbulent jet with free surface

We has developed a novel dynamic coherent eddy model, in which the coherent structure—Q criterion introduced by Hunt et al (1988)—is taken into account in the subgrid-scale turbulent viscosity based on the eddy viscosity model. One proposed method is to combine the resolved-scale velocity-gradient tensor in the classical Smagorinsky model with Q criterion weighted. A kind of dynamic procedure which was averaged in a general process at temporal direction based on the autocorrelations of the characteristic resolved scales of turbulence was taken for the coefficient of subgrid model. The model is implemented in the σ-coordinate and the filtered Navier-Stokes equations are solved by the operator splitting method. The validation was explored to 2-D turbulent slot jet both in ambient environment and in regular waves. The prediction of the present model was compared with the experiment data, including the averaged velocity profiles, the velocity fluctuations and the Reynolds stress. The model performance is shown to be satisfactory.

Macroscopic analysis of gas-jet wiping: Numerical simulation and experimental approach

Coating techniques are frequently used in industrial processes such as paper manufacturing, wire sleeving, and in the iron and steel industry. Depending on the application considered, the thickness of the resulting substrate is controlled by mechanical (scraper), electromagnetic (if the entrained fluid is appropriated), or hydrodynamic (gas-jet wiping) operations. This paper deals with the latter process, referred to as gas-jet wiping, in which a turbulent slot jet is used to wipe the coating film dragged by a moving substrate. This mechanism relies on the gas-jet–liquid film interaction taking place on the moving surface. The aim of this study is to compare the results obtained by a lubrication one–dimensional model, numerical volume of fluid–large eddy simulation (VOF-LES) modeling and an experimental approach. The investigation emphasizes the effect of the controlling wiping parameters, i.e., the pressure gradient and shear stress distributions induced by the jet, on the shape of the liquid film. Those profiles obtained experimentally and numerically for a jet impinging on a dry fixed surface are compared. The effect of the substrate motion and the presence of the dragged liquid film on these actuators are analyzed through numerical simulations. Good agreement is found between the film thickness profile in the wiping zone obtained from the VOF-LES simulations and with the analytical model, provided that a good model for the wiping actuators is used. The effect of the gas-jet nozzle to substrate standoff distance on the final coating thickness is analyzed; the experimental and predicted values are compared for a wide set of conditions. Finally, the occurrence of the splashing phenomenon, which is characterized by the ejection of droplets from the runback film flow at jet impingement, thus limiting the wiping process, is investigated through experiments and numerical simulations.

NUMERICAL SIMULATIONS ON THE HYDRODYNAMICS OF A TURBULENT SLOT JET IMPINGING ON A SEMICYLINDRICAL CONVEX SURFACE

This study presents the numerical simulations of flow characteristics of a turbulent slot jet impinging on a semicylindrical convex surface. The turbulent-governing equations are solved by a control-volume-based finite difference method with power-law scheme, and the well-known k − ϵ turbulence model associated with the wall function is used to describe the turbulent behavior and structure. While the width of the slot nozzle is fixed at 9.38 mm, the diameter of the semicylinder is at 150 mm, and air is the working medium, the adopted modifying parameters here include the Reynolds number of the inlet flow (Re = 6000 ˜ 20000), jet to impingement surface spacing (y / w = 7 ˜ 13), and the entrainment or wall boundary is employed nearby the convex surface. The numerical simulations of flow fields indicate that the velocity distribution of the free jet region departs from the center with increasing y / w. When we increase Reynolds number Re, the variation of the velocity on the convex surface becomes rapid, and the turbulent kinetic energy increases.

A comparative study of five low Reynolds number k– ε models for impingement heat transfer

Five versions of low Reynolds number k– ε models for the prediction of the heat transfer under a two-dimensional turbulent slot jet were analyzed by comparison with the available experimental data. The Yap correction proposed for reducing the turbulence length scale in the near wall region was also tested with low Reynolds number k– ε models and found that for most of the models tested it is capable of improving the predicted local Nusselt number in good agreement with the experimental data in both stagnation and wall jet regions. Effects of the magnitudes of the turbulence model constants were also carried out for two low Reynolds number k– ε models and found that the set of model constants identical to those in the high Reynolds number k– ε model performs better than the original ones for jet impingement configurations. Finally, the effect of three jet inlet velocity profiles on the heat transfer rate was evaluated.

Numerical Study on the Effect of Cross-Flow on Turbulent Flow and Heat Transfer Characteristics Under Normal and Oblique Semi-confined Impinging Slot Jets

Impinging jets are commonly used in industrial dryers and electronics chip cooling. Since in industrial practice it is necessary to use multiple jets, the interaction between jets can have important effect on their heat transfer performance. Hence, the study of cross-flow caused by the spent flow of upstream jets is obviously significant. In this study, a computational fluid dynamics simulation was carried out of the flow and heat transfer characteristics for a single semi-confined turbulent slot jet of air impinging normally or obliquely into an imposed air cross-flow of the same or different temperature. The standard k−ε and the Reynolds stress models were used. Effects of the various flow parameter (e.g., jet-to-cross-flow mass ratio) and geometric parameters (e.g., nozzle-to-target spacing and jet angle) were evaluated at a fixed Reynolds number (11,000 and 12,000) for equal and unequal temperatures of the jet and cross-flow. Results indicate the significant degradation of the impingement heat transfer rates due to cross-flow and a relatively minor influence of the temperature difference between the jet and cross-flow over the ranges of parameters studied. Both the turbulence models produced comparable Nusselt number distributions along the impingement surface.

Simulation of Superheated Steam Turbulent Flows and Heat Transfer in an Impingement Dryer

Numerical simulations are presented for two-dimensional flow field and heat transfer characteristics due to a turbulent single slot jet of superheated steam discharging tangentially into a confined cylinder. A finite volumes method was used to solve the equations that describe the problem. Calculations were performed for steady state turbulent flow and unsteady state heat transfer. Constant velocities and superheated steam temperature are imposed at the inflow. Particle properties were assumed to be the same of a cubic particle of carrot. Numerical tests were performed to ensure that the model solutions were “grid independent” and also independent of the turbulence intensity of stated as boundary condition at the inlet slot. The solution procedure developed is fast and that convergence is reached after a few iterations. The results obtained are relevant to flow and heat transfer behaviors of the impingement dryer and it will be useful future studies considering particles interactions.

Mean flow and turbulence measurements of the impingement wall jet on a semi-circular convex surface

The detailed mean flow and turbulence measurements of a turbulent air slot jet impinging on two different semi-circular convex surfaces were investigated in both free jet and impingement wall jet regions at a jet Reynolds number Rew=12,000, using a hot-wire X-probe anemometer. The parametric effects of dimensionless circumferential distance, S/W=2.79–7.74, slot jet-to-impingement surface distance Y/W=1–13, and surface curvature D/W=10.7 and 16 on the impingement wall jet flow development along a semi-circular convex surface were examined. The results show that the effect of surface curvature D/W increases with increasing S/W. Compared with transverse Reynolds normal stress, \( \overline {v^2 } /U_{\rm m}^2 \) , the streamwise Reynolds normal stress, \( \overline {u^2 } /U_{\rm m}^2 \) , is strongly affected by the examined dimensionless parameters of D/W, Y/W and S/W in the near-wall region. It is also evidenced that the Reynolds shear stress, \( - \overline {uv} /U_{\rm m}^2 \) is much more sensitive to surface curvature, D/W.

EFFECTS OF PRANDTL NUMBER ON IMPINGING JET HEAT TRANSFER UNDER A SEMI-CONFINED TURBULENT SLOT JET

Despite the extensive literature dealing with impinging jet heat transfer, little attention has been directed at the effect of fluid Prandtl number. Almost without exception, earlier studies, both experimental and numerical, deal with air or water as the jet fluids. The objective of this work was to carry out numerical experiments using a computational fluid dynamics simulation code to examine the effect of different thermal properties of the fluid (gas or liquid phase) on heat transfer and to correlate the effect of the fluid Prandtl number on heat transfer under a semi-confined turbulent slot jet. New results are presented for different gases viz. air, argon, nitrogen, hydrogen, helium, ammonia and ethylene and several liquids viz. water, benzene, ethanol, turpentine and isobutyl alcohol. Local, stagnation and average values of the impingement Nusselt number as well as the heat transfer coefficient were reported. Empirical correlations are derived for the stagnation and average Nusselt numbers as a function of the fluid Prandtl number on the basis of extensive numerical experiments

Computational Study of Impingement Heat Transfer under a Turbulent Slot Jet

Impinging jets of various configurations are commonly used in numerous industrial applications such as drying of tissue, paper, textiles, and photographic films and cooling of high power density electronic components, because of their highly favorable heat and mass transfer characteristics. Despite an extensive body of research literature on the subject over the past two decades, impinging jet heat transfer remains an active area of research covering both the experimental and computational fluid dynamics aspects because of its inherently complex fluid and heat flow characteristics. This paper presents simulation results for a single semiconfined turbulent slot jet impinging normally on a flat plate. Effects of turbulence models, near wall functions, jet turbulence, jet Reynolds number, as well as the type of thermal boundary condition at the target surface are discussed in the light of experimental data. The computed results compare favorably with the experimental results for large nozzle-to-target spacin...

Near Wall Measurements for a Turbulent Impinging Slot Jet (Data Bank Contribution)1

The velocity field in the vicinity of a target surface with a turbulent slot jet impinging normally on it is examined. The impingement region is confined by means of a confinement plate that is flush with the slot and parallel to the impingement plate. The distance H of the impingement wall from the slot is varied from 2 to 9.2 slot widths. Jet Reynolds numbers (based on slot width B) of 10,000–30,000 are considered. Mean velocity and root mean square velocity measurements are carried out using hot-wire anemometry. A boundary layer probe is utilized in order to obtain measurements at a wall distance as close as 110 microns 0.0028B. This corresponds to a distance of approximately y+∼2-4 in wall units and is found to be adequate in order to permit an estimate of wall shear under most conditions. The problems of hot wire interference with the wall and calibration at low velocities are solved by calibrating the probe in a known Blasius flow. With the exception of the stagnation region where shear could not be evaluated, it is found that velocity profiles follow a linear behavior in the viscous sublayer everywhere along the wall. Results indicate that the peak in normal stress occurs at y/B∼0.025 to 0.04 at a distance six to eight jet widths away from the jet-axis.

NUMERICAL INVESTIGATION OF HEAT TRANSFER UNDER CONFINED IMPINGING TURBULENT SLOT JETS

This work numerically in vestigates confined impinging turbulent slot jets. Eight turbulence models, including one standard and seven low-Reynolds-number k-epsilon models, are employed and tested to predict the heat transfer performance of multiple impinging jets. Validation results indicate that the prediction by each turbulence model depends on grid distribution and numerical scheme used in spatial discretization. In addition, spent fluid exits are set between impinging jets to reduce the cross-flow effect in degradation of the heat transfer of downstream impinging jets. The overall heat transfer performance can be enhanced by proper spent fluid removal.

Numerical studies of three turbulent slot jets with and without moving surface

This study presents the numerical predictions of the fluid flow and heat transfer characteristics for three turbulent impingement slot jets. The turbulent governing equations are solved by a control-volume-based finite-difference method with power-law scheme, the well knownK-e model, and its associate wall function to describe the turbulent behavior. The velocity and pressure terms of momentum equations are solved by the SIMPLE (Semi-Implicit Method for Pressure-Linked Equation) method. In this study nonuniform staggered grids are used. The parameters interesting include entrance Reynolds number (Re), dimensionless nozzle to surface space (H/W), dimensionless pitch (H/W), and dimensionless velocity ratio (U s /V j ). The computed results show that the dimensionless pitch has a strong influence on the heat transfer characteristics. In the case with surface motion, it is found that the skin friction coefficient of the impinging surface is strongly affected by the surface motion, but the heat transfer characteristic is not significant in the range of 0.05≤U s /V j ≤0.25.

Mass transfer in turbulent impinging slot jets

The mass transfer characteristics of a turbulent slot jet impinging normally on a target wall are examined using numerical simulations. At the location where the slot jet emerges from the nozzle, a confinement plate is placed parallel to the target plate. The jet fluid is the same as the otherwise stagnant fluid into which the jet emerges. Fluid flow is modeled using the k– ω turbulence model of Wilcox [1]. The computations are validated against existing experimental fluid flow, heat transfer and mass transfer data. The range of Reynolds numbers examined is from 450 to 20 000 with Prandtl or Schmidt numbers from 1 to 2400. The distance of the target plate from the slot jet varies between 2 and 8 times the slot jet width. The study reveals computational aspects that are unique to the solution of flow and mass transfer problems with the combination of high Schmidt numbers and turbulent flows.

Modelling of a single confined turbulent slot jet impingement using various k — ϵ turbulence models

Results of numerical simulation of two-dimensional flow field and heat transfer impingement due to a turbulent single heated slot jet discharging normally into a confined channel are presented. Both low-Reynolds and high-Reynolds number versions of k — ϵ turbulence models were used to model the turbulent jet flow. A control-volume-based finite-difference method was used to solve the governing mass, momentum, turbulent kinetic energy, turbulent kinetic energy dissipation rate, and energy equations iteratively. The parameters studied were the jet Reynolds number (5000 < Re < 20,000) and nozzle-to-impingement surface spacing (2.5 < H⧸W < 7.5). From the low-Reynolds number model studied it was found that models presented by Lam-Bremhorst and Launder-Sharma show very good agreement with the available experimental data for the single turbulent jet impingement heat transfer distribution.

Impingement heat transfer under a confined slot jet. Part II: Effects of surface motion and throughflow

AbstractLocal and average heat transfer coefficients were measured for a confined turbulent slot jet impinging on a moving surface at which there may be throughflow. Profiles of the local convective coefficient at the impingement surface were obtained using a fast responding, highly sensitive porous heat flux sensor. The decrease in average heat transfer with surface motion is not negligible, ∼20%, at values of the surface motion parameter, Mvs, comparable to those used in industry. The enhancement of heat transfer by throughflow at a moving impingement surface is linearly additive and, when expressed as δSt, is proportional to only the throughflow parameter, Mus, with a proportionality constant of 0.17 which is independent of Re, Mvs or extent of the heat transfer surface.

Impingement heat transfer under a confined slot jet. Part I: Effect of surface throughflow

AbstractLocal and average heat transfer coefficients were measured for a confined turbulent slot jet impinging on a permeable surface at which there may be throughflow. Local Nusselt number was obtained using a unique porous sensor designed for measurement of local heat transfer at a permeable surface which is subjected to rapidly and widely varying heat transfer. Measurements were performed for a wide range of jet Reynolds number and throughflow rates. Convective heat transfer coefficients was found to be enhanced by throughflow, and the enhancement factor in terms of Stanton number to be independent of jet Reynolds number and of extent of heat transfer area.