Wall Jet Research Articles

Numerical investigation on optimization of wall jet to reduce high temperature corrosion in 660 MW opposed wall fired boiler

Abstract For an air staged combustion boiler, the rational organization of jets to form closing-to-wall film using as little air as possible plays a key role in resolving the high temperature corrosion problems. In this work, a comprehensive computational fluid dynamics (CFD) model including hydrodynamics and coal combustion is established for a 660 MW opposed wall fired boiler. Based on the grid independence and model validation, the flow field, temperature profile, and species concentration are predicted, and the influences of the structure of nozzles and the operation parameter of jets are further evaluated. The results show that the corrosion area of the side wall is dependent on the jet projection velocity and nozzle structures. The increase of the jet velocity does not always have an active influence on the reduction of corrosive area. Only increasing the nozzle diameter does not always have a positive impact on the improvement of the corrosion. The increase of the jet inclination angle can extend the jet trajectory, contributing to increase the oxygen coverage area. Reasonably adjusting the jet inclination angle of each layer can obtain the lower corrosion area. The increase of jet row number leads to a decrease in the spacing between rows, which enables the downstream jet to penetrate deeper into the cross stream. By increasing the number of jet layers and reducing the jet velocity of each layer, the lowest corrosion area can be obtained.

Two-dimensional solution of steady free jet and wall jet by strip integration method and its comparison with empirical relationships and numerical modeling

Two-dimensional solution of steady free jet and wall jet by strip integration method and its comparison with empirical relationships and numerical modeling

Energy transport in axisymmetric flow on a rotating cylinder with heat source/sink and chemical reaction

In this article, a thermal analysis is conducted for the axisymmetric flow of viscous nanofluid induced by torsional motion of cylinder. Here the rotation of the cylinder is axially dependent. The impression of heat source/sink with chemical reaction is perceived on the thermal and concentration boundary layer, while the consequence of magnetic field is observed on the fluid flow. In addition, we utilized a two-phased model for nanofluids, namely Buongiorno's model to compute the outcomes of the Soret effect and Brownian diffusion. The non-dimensional ordinary differential equations (ODEs) are obtained by employing the similarity transformation into governing partial differential equations (PDEs). We employed a built-in function, viz. bvp5c, a finite difference method in Matlab®, to solve the BVPs. The acquired results showed that the axial component of the velocity field occurred as a wall jet phenomenon, which is due to an axial pressure gradient. The axial flow and energy of the system are lessened; however, the peak of the wall jet is amplified for higher values of Reynolds number, but the converse trend is observed in the case of the magnetic parameter. The influence of pertinent parameters is also scrutinized for the wall-shear stress, local Nusselt, and Sherwood number for a selected range of Reynolds number, i.e., [Formula: see text] Furthermore, the consequences of the magnetic field have been succinctly observed on the flow, temperature, and concentration profiles. It is concluded that the magnetic field creates a resisting force that causes a reduction in the velocity fields, while temperature profile is enhanced because of the thermal conductivity of nanofluid. The impression of heat source/sink elevated the energy of system, whereas chemical reaction reduced the concentration field.

Far Field of a Three-Dimensional Laminar Wall Jet

We consider a steady submerged laminar jet of viscous incompressible fluid flowing out of a tube and propagating along a solid plane surface. The numerical solution of Navier–Stokes equations is obtained in the stationary three-dimensional formulation. The hypothesis that at large distances from the tube exit the flowfield is described by the self-similar solution of the parabolized Navier–Stokes equations is confirmed. The asymptotic expansions of the self-similar solution are obtained for small and large values of the coordinate in the jet cross-section. Using the numerical solution the self-similarity exponent is determined. An explicit dependence of the self-similar solution on the Reynolds number and the conditions in the jet source is determined.

Surface pressure fluctuations in wall jets with different plate lengths

Surface pressure fluctuations in wall jets with different plate lengths

Unsteady analysis of jet impingement under vibration conditions

The vibration of thermodynamic machinery will affect its cooling system. In this research, a high-resolution simulation of jet impingement was performed to quantify the unsteady turbulent convection under vibration conditions. A newly developed Self-Adaptive Turbulence Eddy Simulation (SATES) method was used. The Reynolds number was Re = 23000, the jet-to-wall distance was fixed at H/D = 2, and the vibrating frequency of the impinging wall f varied from 0 to 200 Hz. Compared with the static wall case, the maximum enhancement of the stagnation point and area averaged Nusselt number within r/D = 1 could reach up to 5% due to the larger primary vortices, whereas it could reduce the heat transfer by 10% beyond r/D = 3 due to the suppression of the wall vortices development. Based on the unsteady analysis and Proper Orthogonal Decomposition (POD) pattern, the modes controlled by vibration were recognized and their contributions to the heat transfer performance were also evaluated. The introduction of the vibration promoted the development of the primary vortices and changed the radial alternating motion to a vertical alternating motion at the wall jet region. The former was beneficial for the heat transfer, while the latter was unfavorable.

Layered structure of turbulent plane wall jet

This paper investigates the layered structure of a turbulent plane wall jet at a distance from the nozzle exit. Based on the force balances in the mean momentum equation, the turbulent plane wall jet is divided into three regions: a boundary layer-like region (BLR) adjacent to the wall, a half free jet-like region (HJR) away from the wall, and a plug flow-like region (PFR) in between. In the PFR, the mean streamwise velocity is essentially the maximum velocity, and the simplified mean continuity and mean momentum equations result in a linear variation of the mean wall-normal velocity and Reynolds shear stress. In the HJR, as in a turbulent free jet, a proper scale for the mean wall-normal flow is the mean wall-normal velocity far from the wall and a proper scale for the Reynolds shear stress is the product of the maximum mean streamwise velocity and the velocity scale for the mean wall-normal flow. The BLR region can be divided into four sub-layers, similar to those in a canonical pressure-driven turbulent channel flow or shear-driven turbulent boundary layer flow. Building on the log-law for the mean streamwise velocity in the BLR, a new skin friction law is proposed for a turbulent wall jet. The new prediction agrees well with the correlation of Bradshaw and Gee (1960) over moderate Reynolds numbers, but gives larger skin frictions at higher Reynolds numbers.

Scour downstream of a corrugated apron under wall jets

Abstract Experiments were performed over smooth and corrugated aprons with different corrugation dimensions to study the scour and flow characteristics under submerged wall jets condition. The scour depth and length are significantly lower for corrugated than smooth rigid aprons. The maximum reductions in scour depth and length are 79 and 83%, respectively. Optimum scour depth and length are found for aspect ratio (ratio of corrugation wave length to amplitude) three for corrugated apron. The factors affecting scour depth and length were analyzed graphically, and empirical equations are proposed for predicting maximum scour depth and length, and the point of maximum scour depth for corrugated aprons. Velocity, turbulence characteristics, and Reynolds stress in scour holes for smooth and corrugated aprons were also studied.

Heat Transfer Measurements for Array Jet Impingement With Castellated Wall

Abstract Impingement cooling is one of the powerful cooling methods in high-temperature devices. For the gas turbine applications, impingement cooling is commonly applied in the transition piece of a combustor and in the leading edge, suction and pressure sides of a turbine blade/vane. In the suction side and pressure side, impingement cooling is applied as a form of an array jet. However, due to the small gap between the jet hole and target surface, the wall jet faces a crossflow inside of the gap. This crossflow has an adverse effect on the jets and deteriorates the heat/mass transfer performance. Therefore, several studies have been conducted to minimize the crossflow effect. The present study also investigated the effect of crossflow reduction in the gap by having a castellated hole plate. The heat/mass transfer was measured using the naphthalene sublimation method. Heat/mass transfer data are compared among three different cases. One is the baseline case which is simple array impinging jets. Others are the castellated cases with and without rib structures on the target wall. Jet-to-jet spacing(s/d) and jet-to-target spacing(z/d) are selected as geometrical variables. Also, the experiments were conducted for the Reynolds numbers (based on jet hole diameter) of 5,000, 15,000, and 30,000. The baseline case was named as B case, and the castellated case without rib structure as C case and with rib structure as CR case. Both C and CR cases showed the crossflow reduction effect and resulted high and similar Nusselt number values.

Numerical Investigation of Laminar Impinging Jet Cooling of a Protruded Heat Source

Abstract Thermofluid dynamics of an unconfined steady two-dimensional laminar jet impinging on an isothermal protruded heater is numerically studied for low jet inlet Reynolds number (Re) between 50 and 250. Results are shown for a range of impingement distances (h/W) between 1 and 10 for Prandtl numbers (Pr) 0.71 and 7.56. The volumetric entrainment increases with increasing h/w and decreasing Re. The reattachment distance of the wall jet appears to increase with Re and shows discernible deviation from the backward-facing step flow prediction for Re>150. Correlations are presented for average heater surface and sidewall Nusselt numbers as functions of Re and h/w for Pr=0.71 and Pr=7.56. In an overall convection dominant heat transfer, a relatively warmer and diffusion-dominated recirculation zone is identified adjacent to the sidewall with a low Nusselt number, which enhances significantly at Pr=7.56 when Re is increased above 100. At a low impingement distance, integrated kinetic energy flux shows greater magnitude in the impingement region but with a higher rate of decay. The integrated heat flux is greatly influenced by Re, and the effect is more pronounced at Pr=0.71. Self-similar behavior is observed for the velocity and heat flux profiles throughout the length in the developed region and for the temperature distribution over the heater surface. Both high Re and high h/w seem to adversely affect the self-similar behavior owing to a slower wall jet development.

Vortex Shedding Frequency Scaling of Coherent Structures Induced by a Plasma Actuator

The spatial–temporal evolution of coherent structures generated by an asymmetrical dielectric barrier discharge (DBD) plasma actuator is an intriguing unsteady process with profound implications for revealing the controlling mechanism of the plasma actuator and promoting the control effect of the DBD plasma actuator at high wind speed and high Reynolds number. In this study, the induced coherent structures of the plasma actuator are studied systematically with a particle image velocimetry for a range of force coefficients (). Results indicate that the vortices that shed periodically from the downstream edge of the exposed electrode are a result of the amplification of natural disturbances in the jet shear layer. During later evolution, the adjacent vortices pair up to form a new vortex, leading to a reduction of the dominant frequency in the velocity spectra. Further downstream, the coherent vortical structures decay considerably, correlating with the shear-layer transition process in the plasma wall jet. The vortex shedding frequency is observed to scale with the force coefficient according to , which is of importance to the selection of the dominant frequency by adjusting the actuation parameters of the plasma actuator based on the characteristic frequency of the controlled flowfield and improving the control effect of the plasma actuator. With increasing force coefficient, the propagation speed of the roll-up vortices increases linearly, whereas the streamwise vortex spacing decreases monotonically. It is of great significance that the nondimensional Strouhal number based on the vortex shedding frequency, vortex propagation speed, and streamwise vortex spacing is proposed and varies slightly between 0.045 and 0.065, for all the tested cases, which lays a foundation for promoting the numerical simulation model of the plasma actuator and revealing the controlling mechanism of the flow control using the plasma actuator.

Effect of apron roughness on flow characteristics and scour depth under submerged wall jets

Effect of apron roughness on flow characteristics and scour depth under submerged wall jets

Hydraulic characteristics of countercurrent jets on adverse-sloped beds

Abstract The counter-current jet (CCJ) acts like a reverse surface jet layer covering the free jump surface and has potential applications in the energy dissipation of hydraulic engineering. The present study investigated the hydraulic characteristics of CCJs on adverse-sloped beds. The results showed that, compared to the horizontal bed, the slope didn't increase the energy dissipation rate of CCJ but reduced the return flow length and upstream depth. The velocity distribution along the depth was divided into the boundary layer region, mixing region, and reverse surface jet region. The velocity distribution in the boundary layer region and the mixing region was similar to the classical wall jet. The jet Froude number and the bed slope had no significant effect on the turbulence intensity distribution and turbulence kinetic energy (TKE) distribution of CCJ. The distribution of TKE was similar to that of a submerged jump. The maximum absolute turbulence intensity appeared at exactly half of the maximum velocity. The maximum TKE appeared at the mixing region. Besides, empirical formulas for estimating length scales and maximum velocity attenuation are proposed. The results could provide a reference for the potential application of CCJ in energy dissipation in hydraulic engineering.

Numerical study on the desorption processes of oil droplets inside oil-contaminated sand under cavitation micro-jets

The removal of the adsorbed oil droplet is critical to deoiling treatment of oil-bearing solid waste. Ultrasonic cavitation is regarded as an extremely useful method to assist the oil droplets desorption in the deoiling treatment. In this paper, the effects of cavitation micro-jets on the oil droplets desorption were studied. The adsorbed states of oil droplets in the oil-contaminated sand were investigated using a microscope. Three representative absorbed states of the oil droplets can be summarized as: (1) the individual oil droplet adsorbed on the particle surface (2) the clustered oil droplets adsorbed on the particle surface; (3) the oil droplet adsorbed in a gap between particles. The micro-jet generation during the bubble collapse near a rigid wall under different acoustic pressure amplitudes at an ultrasonic frequency of 20 kHz was investigated numerically. The desorption processes of the oil droplets at the three representative absorbed states under micro-jets were also simulated subsequently. The results showed that the acoustic pressure has a great influence on the velocity of micro-jet, and the initial diameter of cavitation bubbles is significant for the cross-sectional area of micro-jets. The wall jet caused by a micro-jet impacting on the solid wall is the most important factor for the removal of the absorbed oil droplets. The oil droplet is broken by the jet impinging, and then it breaks away from the solid wall due to the shear force generated by the wall jet. In addition to a higher sound pressure, the cavitation bubble at a larger initial diameter is more important for the desorption of the clustered oil droplets. Conversely, the micro-jet generated by the cavitation bubble at a smaller initial diameter (0.1 mm) is more appropriate for the desorption of the oil droplet in a narrow or sharp-angled gap.

Dynamics of wall jet flow under external pulsation

The present study aims to identify the dominant coherent structures in the wall jet flow subjected to external pulsation at Reynolds number 2600 (based on average jet exit velocity and nozzle diameter). The forcing frequency is varied between St = 0 and 0.99 (St is the Strouhal number). Quadrant analysis is employed to identify the relative contribution of different quadrant motions to the total Reynolds shear stress. Unlike boundary layer flows and channel flows, two distinct regions (inner shear region and outer shear region) are observed in the wall jet flows, and the characteristics of different quadrant motions change in these regions. About 70% of the total shear stress is contributed from the first and fourth quadrants in the outer shear region. We observe that ejection motion is more energetic than sweep motion in the downstream direction, although less frequent. The ejection motion is observed to be more violent for St = 0.44 than for the other frequencies. A proper orthogonal decomposition (POD) analysis reveals while the modal structures exist in different regions of the wall for different jet pulsation; there are no dominant modes (30 modes are required to recover about 75% of the total energy), and the energy is fairly distributed over a large number of modes. However, the POD analyses are capable of capturing the response of the wall jet to different jet pulsations. The most dominant and strongest modal structures are found nearer to the impingement region of the wall when St = 0.44 and the jet tends to laminarize for St > 0.9.

Development of a wall jet model dedicated to 1D combustion modelling for CI engines

Diesel engines are becoming smaller as technology advances, which means that the fuel spray (or jet) interacts with the cylinder walls before combustion starts. Most fuel injection 1D models (especially for diesel fuel) do not consider this interaction. Therefore, a wall-jet sub-model was created on an Eulerian 1D diesel spray model. It was calibrated using data from the literature and validated with experimental data from a fuel spray impacting a plate in a constant volume combustion chamber. Results show that the spray moving along the wall has a higher mixing rate but less penetration as an equivalent free jet, therefore they show a similar volume. Spray-wall interaction creates a stagnation zone right before the impact with the wall, and friction of the jet with the wall is relatively low. All these phenomena are well captured by the wall-jet sub-model.

Vorticity transports in turbulent channels under large-scale control via spanwise wall jet forcing

Vorticity transport in turbulent channels under large-scale active drag control (i.e., spanwise opposed jet forcing) is investigated via direct numerical simulation of the Navier–Stokes equation. The skin-friction coefficient is newly expressed by the volume integration average of the mean flow dissipation expressed in terms of the spanwise vorticity (Ωz), and the turbulent transports of the spanwsie vorticity fluctuation (−v′ω′z) and of the wall-normal vorticity fluctuation (w′ω′y). Three Reynolds number cases (i.e., Reτ=180,395, and 550) with notable drag reductions (i.e., 18%, 16%, and 15%, respectively) are examined, following the setup in Yao et al. [Phys. Rev. Fluids 2, 062601(R) (2017)]. The transports of vorticity fluctuations dominate the contributions to the frictional drag, consistent with previous results for the passive drag reduction strategy of slip-wall [Yoon et al., Phys. Fluids. 28, 081702 (2016)]. Specifically, the effect of −v′ω′z is to increase drag (due to sweeping −v′), while w′ω′y decreases the drag. A triple decomposition (mean, coherent, and random) reveals the random −v″ω′′z as the only term adding to drag, while the random w″ω′′y and the coherent −ṽω̃z and w̃ω̃y transports all decrease the drag. Weighted joint probability distribution function (p.d.f.) of v″ and ω′′z shows a transition from the first–third quadrant dominance (hence positive-correlated) near the wall to the second–fourth quadrant dominance (hence negative-correlated) away from wall. In contrast, w″ and ω′′y are negatively correlated in the entire region. The analysis here suggests that the suppression of random spanwise-vorticity transport (v″ω′′z) is the target for more effective drag reduction under the current method.

The Influence of Variable Density on Turbulent Wall Jet: A Numerical Comparative Study

The first objective of this numerical research is to help understand the influence of variable density on the structure of turbulence, through the study of a wall jet, and to validate our results with those of the experimental study of A. Soudani. The source of density variation is the mixture between two different non-reactive fluids, with a fixed temperature and pressure. A mass weighted averaging for different variables is applied to the calculation, using ANSYS FLUENT 15.0 commercial software. The principal experience consists of injecting tangentially and alternatively near the wall a gas (air-helium) different from the external flow, through a slot of height 3mm between two plane walls. Such a process permits to provoke an important density difference. The study reaches the conclusion that turbulence is strong, with a slight increase of velocity near the wall and an evident diminution of skin friction, in the case of light fluid injection. The second aim is to estimate the Kolmogorov and large eddies’ scales to construct LES grid to access instant variables in experience.

Impact of thermophoretic particle deposition on Glauert wall jet slip flow of nanofluid

The heat transfer technology is growing significantly for desired solutions as the need for efficient heating and cooling systems in the aerospace, automotive, and chemical sectors. Bearing this in mind the need for effective cooling/heating systems, the steady, two-dimensional Glauert wall jet flow with thermophoretic particle deposition is investigated in the presence of Al2O3 nanoliquid. The Glauert transformations are used to transform the system of partial differential equations (PDEs) that reflect conservation, continuity, temperature, and concentration, as well as boundary conditions. The fourth fifth order Runge-Kutta-Fehlberg (RKF-45) method along with the shooting approach is used to solve the reduced system of ordinary differential equations (ODEs). The role of physical quantities for the parameters in concern are visually depicted and described in depth. Results reveal that, rise in values of velocity slip parameter upsurges the velocity profile away from the wall and declines near the surface. The increase in radiation parameter inclines the heat transfer. The growing values of both radiation parameter and volume fraction advances the heat transfer rate. The growing values of thermophoretic parameter improves the mass transfer rate.

Thermal mixing in T-junctions with different incident angle of branch pipe

In this paper, a detailed study of mixing characteristics and temperature fluctuations in T-junctions of 90° and 45° is carried out based on visualization experiments and Large Eddy Simulations (LES). In this study, both flow patterns in the T-junctions are wall jets. Unique thermocouples are employed in the experiments to reduce the influence of temperature measurement on the flow field. The LES is validated by the experimental data. The statistical results from LES indicate that the branch flow incidence of 45° makes the mixing uniformity worse. The instantaneous LES results show that the shear layer vortices and quasi Karman vortex series dominate the fluctuations on the thermal stratification interface. The formation and motion of the shear layer vortices depend on the angle of incidence, while the quasi Karman vortex series is almost unaffected. The shear layer vortices in the 90°/45° cases determine the dominant frequency at 5.5 Hz/10 Hz. The dominant frequency of near-wall temperature fluctuations at 3 Hz in both cases is determined by the quasi Karman vortex series.