Wall Jet Research Articles

Experimental study of convective heat transfer distribution of non-interacting wall and perpendicular air jet impingement cooling on flat surface

An experimental study evaluated heat transfer with perpendicular and wall-impinging air jets on stainless steel foil, for Reynolds numbers Re = 3000, 5000, 8000, and 10000, where the perpendicular jet targets the bottom and the wall jet the top, creating a unique, non-interacting effect. Distances to nozzle diameter ratios for wall jets (S/d = 4, 6, 8, 10) and perpendicular jets (Z/d = 2, 4, 6, 8) were varied. Significant heat transfer increases were noted, with the Nusselt number rising by up to 49.20 % for a Z/d = 6 and S/d = 8 combination at Re = 5000. Improvements ranged from 10.03 % to 49.20 %, peaking when the jets' high heat transfer regions overlapped. Optimal performance for Re = 3000 was at S/d = 10, aligning the wall jet's maximum with the perpendicular jet's stagnation area. For Re = 5000 to 10000, optimal S/d values were 8 and 4 for Z/d = 6, 8 and Z/d = 2, 4, respectively. The Nusselt number increase ranged from 29.21 % to 46.57 % at S/d = 10 for Re = 3000, the highest among all tested values. Wall jet heat transfer downstream increased by 90–105 % over perpendicular jets in corresponding regions. Increasing the wall to perpendicular jet distance improved heat transfer near the stagnation point, suggesting this cooling method for high-density electronics like CPUs and GPUs.

Action Mechanism of Film Extraction on Channel Impingement Cooling for Blade Leading Edge Using Large Eddy Simulation

Abstract Various internal cooling techniques combined with external film cooling are applied to cool gas turbine blades, and the coolant extraction can cause variations in internal flow structures and cooling effectiveness. Effects of film extraction under different film hole diameters and coolant mass flow ratio on heat transfer and flow characteristics of channel impingement cooling are presented using large eddy simulation (LES) in this article. The current work was undertaken based on turbine blade dimensions and turbine operating conditions. The results indicate that film extraction coupled with the curvature-induced instability dominates flow patterns and heat transfer in the channel impingement cooling structure with a film hole, especially in the bend region corresponding to the leading edge of turbine blades. Film coolant extraction can disrupt the streamwise counterrotating flow circulation pair. The latter is caused by the curvature-induced instability and is also the major driver of multilongitudinal vortices. The hole edge vortex generated by coolant bleed amplifies heat transfer at the leading edge, and the effect is more significant as the film hole diameter increases. Conversely, heat transfer coefficient downstream of the cooling channel decreases due to a reduction in cooling air. The increasing film bleed flow increases overall total pressure loss, while flow loss in the cooling channel decreases because of the weakened flow circulations and reduced coolant mass flow. This work provides an in-depth insight into the cooling performance of channel impingement cooling with film extraction, contributing to designing film cooling for turbine blades with multichannel wall jet cooling.

Advancements in predicting scour depth induced by turbulent wall jets: A comparative analysis of mathematical formulations and machine learning models

This study examines the scour depth induced by turbulent wall jets and proposes novel mathematical formulations to predict the depth of scouring. Through a comprehensive gamma test, key parameters influencing the scour depth are identified, including the apron length, densimetric Froude number, median sediment size, tailwater level, Reynolds number, and Froude number of the jet. Regression analysis is subsequently conducted to establish relationships between the dependent parameter and the aforementioned independent variables. A comparative analysis is then undertaken between the measured scour depths and those predicted by existing equations from previous studies. Furthermore, predictive models leveraging the support vector machine, artificial neural network with particle swarm optimization, M5 tree algorithm, gene expression programming, and adaptive neuro-fuzzy inference system (ANFIS) are developed using the collected data. Statistical metrics are employed to evaluate the performance of each model and the regression equation. The effectiveness of each model in predicting scour depth is demonstrated. Notably, ANFIS yields a coefficient of determination of 0.809 and a root mean square error (RMSE) of 1.585. Multi-nonlinear regression analysis exhibits a coefficient of determination of 0.752 and an RMSE of 0.421, while the M5 tree achieves a coefficient of determination of 0.739 and an RMSE of 1.874, demonstrating superior performance compared to other machine learning techniques and regression equations employed in this study.

Self-similar Solutions for Laminar Wall Jets

Self-similar Solutions for Laminar Wall Jets

Virtual leading-edge aero-shaping of an airfoil using a dielectric barrier discharge plasma actuator

Recently, it has been demonstrated that dielectric barrier discharge plasma actuators, which can be capable of generating a quasi-steady wall jet under the effect of a sinusoidal power supply, are well suitable for controlling the flow around the airfoil at a low Reynolds number. To uncover the controlling mechanism of plasma actuators, flow separation control over an SC (2)-0714 supercritical airfoil using an asymmetrical plasma actuator arranged at the leading edge of the airfoil, is studied at Reynolds number of 7.8 × 104. The stall angle of attack is delayed from 9° to 13°, and the maximum lift coefficient is increased by about 27%. In addition to the momentum injection to the boundary layer, which is one of the control mechanisms and can be achieved by the induced spanwise vortex of the plasma actuator, the closed recirculation region, which is similar to superimposing a hump on the leading edge of the airfoil, is another control mechanism, leading to an increase in the effective camber of the airfoil. The localized recirculation zone, which is created by the plasma actuator and modifies the leading-edge aero-shaping and results in the enhancement of the lift coefficient, is first observed, and this effect of the plasma actuator is called the virtual leading-edge aero-shaping.

Numerical study on coarse granular flow and performance characteristics of the Coandă effect-based collector for deep sea mining

The application of the Coandă effect-based method for harvesting polymetallic nodules from the seafloor is a sophisticated technique in underwater exploitation. The primary objective of this paper is to investigate the influence of the non-dimensional curvature radius (h/R) of the jet flowing against the wall on the lift capacity of the Coandă effect-based collector. A computational fluid dynamics approach coupled with the discrete element method is proposed and validated. The high-speed curved jet entrains seawater from the bottom clearance to prevent sediment erosion, resulting in an up-wash flow along the collector tube. The adverse pressure gradient and the impinging kinetic energy of the jet are two factors that provide lift force for the manganese nodules. As the value of h/R increases, the wall becomes more curved, resulting in a larger jet half-width and faster energy diffusion along the jet wall, which gradually dominates the movement of particles. The collection rate for the case with h/R = 0.028 (91.4 %) is slightly higher than that for h/R = 0.0145 (91.8 %), while both cases are higher than that for h/R = 0.0459 (90.3 %). As h/R increases, the lifted particles exhibit sharper trajectories at the particle separation position, leading to reduced collection efficiency.

Reappraisal of plane wall jet self-similarity

There is a general consensus that the characteristic velocity and length scales for a plane wall jet exhibit power law with distance downstream of the jet nozzle/slot, x; however, no definitive values are given for the power-law exponents. In view of this, plane wall jet self-similarity is reappraised using a compilation of experimental and direct numerical data of plane wall jets. The plots of compensated distributions of the maximum velocity Umax and length y1/2, the location where the velocity is equal to Umax/2 in the outer part of the flow, reveal that these variables follow the power laws Umax∼y1/2n and y1/2∼xm, where n∼−1/2 and m∼1, which comply with self-similarity solutions. While n∼−1/2 is in agreement with the existing results, the linear growth of y1/2 contrasts with the common belief that m should be less than 1. It is argued that the limited ranges of x and Reynolds numbers (Re) used in the previous studies prevent this latter behavior to be observed. This behavior emerges when these data are compiled into a single set, allowing a reasonable range of x and Re. It is further observed that only one velocity scale, either Umax or uτ, the friction velocity, is needed for the outer region, which complies with a requirement imposed by self-similarity under a linear growth y1/2. In the inner region, either ν/uτ or y1/2,in (the location where the velocity is equal to Umax/2 in the inner part) can be used as a scaling velocity, while uτ should be the adequate scaling velocity. The present analysis provides some support to the argument that scaling could be universal, at least when the Reynolds number is large enough and the flow is allowed to evolve over a very long distance.

Computational study of thermal and fluid mixing behaviour of a dual jet flow using LES approach

The primary goal of any thermal system which demands efficient cooling is to improve heat transmission. For a turbulent jet, this condition becomes far more significant. The present work has employed large-eddy simulation (LES) for an offset ratio of 2.0 to investigate the transfer in heat and flow characteristics of a dual jet. An isothermal boundary wall condition is considered for a Reynolds number of 7500. The present findings are evaluated by comparing with the published experimental and numerical studies on wall jets. A comparative analysis here exhibits the profiles of Reynolds normal stresses, Reynolds shear stresses, turbulence kinetic energy, Nusselt number and turbulent heat fluxes. A comprehensive investigation of the mixing and flow properties of the dual jet is presented by using the Q-criterion of the iso-surface for mean velocity, pressure and vorticity. The present outcomes show that an efficient thermal transmission from the wall occurs in the recirculation region. The magnitudes of the profiles of Reynolds normal stresses, Reynolds shear stresses, r.m.s. temperature and turbulent heat fluxes are observed to be low in the combined region. Roller-like structure formation and near-wall eddies are noticed which spread in the combined region indicating large heat loss due to the turbulent flow.

Analysis of Confined Jet Impingement in Converging Annular Microchannel Heat Sinks

Jet impingement cooling is deemed an excellent choice for the thermal management of high-power electronics. However, high-pressure drop penalties and low local heat transfer coefficients in regions far from the jet zone are its drawbacks. Although it is reported that recirculation areas appear because of the entrainment, the effects of recirculation size on thermal behavior are not understood well enough. Here, jet impingement heat sinks with converging annular channels are employed in a numerical investigation to minimize the adverse cooling effects associated with an impinging jet in a microchannel. The realizable k−ε turbulent model is used for modeling thermal and turbulent flow fields (Re=5,000 to 25,000). It was found that the different flow recirculation zones in small scales are responsible for the enhanced heat transfer rate. While the thermal performance of a converging wall jet impingement heat sink is higher than its flat wall counterpart at low Re numbers, the thermal performance results are in favor of the flat wall jet impingement heat sink at high Re numbers. The flow recirculation area shrinks in converging channels at high Re numbers, thereby deteriorating the thermal performance of the converging channel compared with a flat wall jet heat sink. Also, it was found that employing steeper converging channels shrinks the flow recirculation region, resulting in up to 59% lower pressure drops at Re=25,000. The present study examines the role of flow recirculation at different Re numbers on the thermohydraulic performance of jet impingement converging annular heat sinks.

Dielectric barrier discharge actuators: Momentum injection into co-flow and counter-flow freestream

In a quiescent environment, dielectric barrier discharge (DBD) plasma actuators generate a wall jet through the interaction of ionized and neutral molecules in an electric field. In external flow, the coupling between electrohydrodynamic (EHD), turbulence, inertial, and viscous effects in the flow boundary layer is more complex and requires additional investigation. We experimentally study momentum injection by DBD actuators into the free stream flow with Re = 35,000 and 75,000 in co-flow and counter-flow scenarios over a range of VAC = 12 kV–19.5 kV peak-to-peak at a frequency of 2 kHz. In co-flow, the momentum injection leads to boundary layer thinning and fluid entrainment from the freestream into the DBD forcing region, while in the counter-flow configuration, flow separation can occur. A separation bubble is observed at Re = 35,000 for the tested condition. The momentum difference in the counter-flow configuration is six times greater than the EHD jet momentum in a quiescent environment. Both co-flow and counter-flow momentum injections show diminishing effects with higher Re. We show that the resulting flow pattern is not a superposition of the EHD jet and the free stream but is determined by the coupling and competition of inertial, viscous, and Coulombic effects between the EHD-driven forcing and the external flow. The proposed non-dimensional momentum ratio (M*) of EHD jet momentum to momentum in the external flow boundary layer can be used to predict the onset of separation; however, additional experimental and numerical studies are required to generalize this concept to other flow scenarios. The velocity profiles and momentum measurements presented here can be used to validate numerical models and inform the design of DBD actuators for active flow control.

Heat Transfer and Flow Characteristics of Channel Impingement Cooling Structure at Leading Edge Inside Turbine Blades Using Large Eddy Simulation

Abstract As a main part of multichannel wall jet cooling structure, channel impingement cooling is a cooling strategy of great concern at the leading edge inside of the turbine blade. In this paper, heat transfer and flow behavior in the channel impingement cooling structure are investigated by large eddy simulation (LES). The results imply that impingement created by curvature-induced centrifugal instabilities in the turning region of the cooling channel is dominated by a streamwise vortex system containing a counter-rotating Dean vortex, which presents high heat transfer streaks along the streamwise direction on the target wall. The intensely unsteady nature of the cooling jet induced by a lack of equilibrium between the pressure gradient and the centrifugal force is precisely captured herein by LES. An attaching-wall jet formed on the outer wall downstream of the cooling channel has highly three-dimensional characteristics not observed by Reynolds-averaged Navier–Stokes equations (RANS). Heat transfer augmentation on the target wall of the cooling channel is mainly due to the intensifying streamwise vortex system developing in the turning region as driven by the centrifugal force. This research work will provide a reference for the optimization and application of multichannel wall jet cooling for gas turbine blades.

Comparison of flow characteristics of plane jet impingement on a solid plate and on a sand bed

As compared to the well-researched case of a plane jet impinging on a solid plate, relatively rare attention was paid to the impingent on an erodible sand bed, which induces continuous bed transformations and interactively affects the jet development. The present study measured the flow of an impinging plane jet on a solid plate and on an erodible sand bed, respectively, by using particle image velocimetry technology, and then comparatively investigated the flow structure, main jet development and downstream wall jet development for the two cases. The results revealed that the jet impingement on the sand bed has a longer free jet region than that on the solid plate due to enlarged separation distance induced by localized scouring. The width of the plane jet impinging on the sand bed is larger than that on the plate by as high as five times the nozzle width, due to intensified interactions with the complex vortical structures in the concave scour hole. The impinging angle even decreases to negative values near the sand bed due to upward deflection of flow induced by bedform transformation. For both cases, however, the normalized streamwise mean velocity profiles exhibit universal self-similarity at different zones of jet development: specifically, the main jet and the wall jet satisfy exactly the same exponential function and the power law function, respectively.

Turbulent mean flow prediction in impinging jets using data assimilation methods

In this paper, a data-driven turbulence model is devised based on data assimilation (DA) for predicting impinging jet characteristics for various Reynolds numbers and nozzle-to-plate distances. The shear stress transport (SST) model with Tam–Thies correction is applied. The SST model with the X term makes accurate predictions in the region of wall jet near the overlying stationary fluid but fails to predict the velocity distribution near the wall. The DA-optimized SST model with the X term is used for predicting the impinging jet to minimize the deviation between the model prediction and experimental data. Only the model constants corresponding to the region near the wall are optimized through DA. The model constants at H/D = 2, 3, and 6 are fitted using logarithmic curves with respect to the nozzle-to-plate distance to obtain a universal formulation for predicting the impinging jet under various flow conditions. The model using the fitted model constants, referred to as the SST-H/D model, accurately predicts the mean flow for different nozzle-to-plate distances, nozzle types, and Reynolds numbers.

Study of the effect of nickel foam synergistic CO2 on low hydrogen ratio methane explosion

ABSTRACT To improve the explosion suppression efficiency of porous media in natural gas hydrogen-doped pipelines, this paper designs and builds an experimental platform with dimensions of 10 cm × 10 cm × 100 cm organ transparent glass explosion pipeline and researches the synergistic effect of the use of nickel foam and C O 2 to inhibit the explosion of hydrogen-doped methane with a hydrogen doping ratio of φ = 10 % . Studies have shown that using a dual explosion suppression mechanism, the synergistic effect of nickel foam and C O 2 acting at different jet positions has a significant inhibitory effect on the flame and overpressure of low-hydrogen-ratio methane explosions. The synergistic C O 2 in porous media has a more potent inhibitory effect on the right-side wall jet than on the left-side and left-side jet. When the synergistic impact of nickel foam and C O 2 acts on the right-side wall jet, the flame is quenched, substantially attenuating the overpressure peak. When the nickel foam synergistic C O 2 acts on the right-side wall jet, the optimal effect is achieved when the C O 2 jet pressure is set at 0.3 MPa, which successfully quenches the flame and significantly reduces the overpressure peak. However, the dual explosion suppression mechanism fails when the C O 2 jet pressure is 0.2 MPa.

Numerical study of flow and thermal characteristics of pulsed impinging jet on a dimpled surface

This research comprehensively investigates the flow and thermal characteristics of a pulsating impinging jet over a dimpled surface. It analyzes the impact of key parameters (e.g., inlet velocity pulsation functions, pulsation frequency, amplitude, dimple pitch, dimple depth, Reynolds number) on flow patterns and heat transfer. Validated computational fluid dynamics and the Re-normalization group turbulence model are employed to accurately simulate complex turbulent flow behavior. Local and average heat transfer coefficients are calculated and compared to steady impingement cases, revealing the potential benefits of pulsation for heat transfer enhancement. The study also examines how pulsation-induced flow modulation and thermal mixing affect heat transfer mechanisms. Results indicate that combining fluctuating flow with a dimpled surface can improve heat transfer rates. In summary, increasing pulsation amplitude consistently enhances heat transfer, while the effect of frequency varies between impinging and wall jet zones.

Flow characteristics in a semi-confined circular-pipe impinging jet

The impinging jet is a complex heat and mass transfer technique that involves several process variables, such as the jet Reynolds number, impingement distance, and jet configuration. In this study, the flow characteristics of a semi-confined circular-pipe impinging jet over different Reynolds numbers and impingement distances were experimentally investigated using a two-dimensional particle image velocimetry technique. The confinement was achieved by positioning a plate parallel to the impinging plate at the nozzle exit. The time-averaged velocity field exhibited a recirculation structure that gradually shifted downstream with increasing Reynolds numbers or impingement distances. Notably, at H/d = 2, this downstream shift of the structure was accompanied by an increase in the vortex intensity. Moreover, the confined plate induced alterations in the overall flow pattern within the confined region, significantly reducing the wall jet decay rate compared with both unconfined and confined radial wall jets for H/d ≥ 3. Conversely, the confinement did not affect the expansion of the wall jet. Unlike the free (unconfined) impinging jets, the semi-confined circular-pipe impinging jet did not exhibit self-similar behavior in the conventional outer-scaled coordinates, particularly concerning the turbulence intensity and Reynolds shear stress. Finally, self-similarity in the time-averaged velocity and various turbulence parameters was achieved using the parameter scale proposed in this study, thereby obtaining the corresponding scaling laws in the wall jet region. Our study results can deepen the current understanding of the flow characteristics of semi-confined circular-pipe impinging jets and are significant for optimizing the performance and efficiency of compact electronic packaging equipment.

Numerical investigation into the blending characteristics of opposed wall jets in a finite field with different stagger distances

Strong turbulence is generated by the blending of opposed staggered jets (OSJs). This turbulence results in fluid mixing and energy dissipation, which are crucial for pollutant dilution and the filling of navigation lock chambers. A renormalization group k-ε turbulence model is adopted to conduct three-dimensional simulations of OSJs at various stagger distances. The blending characteristics of two square water jets at eight stagger distances L* within a finite field are examined; here, L* is defined as the distance between the center lines of the staggered jets divided by the jet diameter. The initial Reynolds number and inlet diameter of the jets for the numerical simulations are set to 2.99 × 106 and 0.6 m, respectively. The numerical results show that there is a linear correlation between the decay exponent and the jet half-width, both of which increase and then gradually stabilize with increasing L*. Intriguingly, the vortex strength and blending length both increase at first before decreasing as L* increases, and the blending effectiveness distribution mirrors these fluctuations. Moreover, a decay model for the axial velocity is formulated in terms of the decay exponent and L*. These investigations yield substantial theoretical results underpinning fluid mixing and orifice arrangement in navigation lock chambers.

Single cavitation bubble dynamics in a stagnation flow

Jetting of collapsing bubbles is a key aspect in cavitation-driven fluid–solid interactions as it shapes the bubble dynamics and additionally due to its direct interaction with the wall. We study experimentally and numerically the near-wall collapse and jetting of a single bubble seeded into the stagnation flow of a wall jet, i.e. a jet that impinges perpendicular onto a solid wall. High-speed imaging shows rich and rather distinct bubble dynamics for different wall jet flow velocities and bubble-to-wall stand-off distances. The simulations use a volume-of-fluid method that allows us to numerically determine the microscopic and transient pressures and shear stresses on the wall. It is shown that a wall jet at moderate flow velocities of a few metres per second already shapes the bubble ellipsoidally inducing a planar and convergent jet flow. The distinct bubble dynamics allow us to tailor the wall interaction. In particular, the shear stresses can be increased by orders of magnitude without increasing impact pressures the same way. Interestingly, at small seeding stand-offs, the bubble during the final collapse stage can lift off the wall and migrate against the flow direction of the wall jet such that the violent collapse occurs away from the wall.

Thermal analysis of a turbulent wall jet over an adiabatic wavy surface

The impact of wavy wall amplitude on the turbulent characteristics of a wall jet has been investigated experimentally, where the wavy wall is given an adiabatic condition. In the experimental study, the mean and turbulent velocity are measured using a constant temperature anemometer manufactured by Dantec Dynamics. For temperature measurement over the wall and within the flow domain, the FLIR camera and k-type thermocouple are used, respectively. For the current study, uniformly heated at △T0 = (T0 − T∞) = 30 ± 1.2 0C with a Reynolds number of 15,000 is utilized to flow over a sinusoidal wavy wall (y = A*sin (ωx)), where “A = Amplitude ⁄ a” is the amplitude normalised by the nozzle height “a". The nozzle height is 20 mm and to maintain the Reynolds number 15,000 at the inlet, velocity is set at 10.95 ± 0.36 m ⁄ s. The amplitude of the wavy wall is varied between 0.2 and 0.6 and the results are compared to those of the plane wall jet. The current problem is also numerically solved using the k − ε RNG and k − ω SST models. Based on the results, it is concluded that the k − ε RNG model predicts the wavy wall results quite well. It is also discovered that the thermal self-similar profile deviates from the Gaussian curve at the crest of the wavy wall. The influence of the wavy surface on the thermal characteristics of the turbulent wall jet is more pronounced in the near flow field. It is observed that the bottom wall temperature, fluctuating temperature and turbulent heat flux decrease with the increase in amplitude. This indicates that for the higher amplitude, the turbulence increases, which increases the intermixing of jet fluid with the colder ambient fluid. The present results can be used as a benchmark for validating numerical models working in this area.

Influence of dual purging jets on interferometric measurement of optical path difference

Incompressible large eddy simulations of dual purging jets at three Reynolds numbers of ReL = 1500, 1000, and 500 are performed, providing the database for simulated interferometric measurements subjected to the spatiotemporally varying temperature fields. The temperature is modeled as a passive scalar, and the index-of-refraction field is calculated from the air temperature and pressure using the modified Edlén equation. Based on wave optics, wavefront distortions of laser beams traversing the flow field are computed, and the optical path difference measurement is simulated. Statistical results of the velocity and temperature indicate distinctly different flow patterns at three Reynolds numbers. Spectral proper orthogonal decomposition is adopted to analyze the relationship between coherent flow structures and temperature wavepackets. There is a dominant frequency (St = 0.29) at ReL = 1500 and 1000, which is attributed to the Kelvin–Helmholtz instability, whereas the two jets have notable flapping behaviors at ReL = 500. The wavefront distortions are found to be insensitive to the Reynolds number. The effects of the flow region and tracing distance are examined, and the optical performances of different beam pairs for interferometry are evaluated. The optical beams passing through the outer shear layers of the dual jets and wall jets are distorted severely, resulting in large systematic and random errors in the interferometric measurement.