Experimental Study of the Diffusion of a Confined Wall Jet through a Perforated Plate: Influence of the Porosity and the Geometry

This paper investigated lateral diffusion of a confined two-dimensional wall jet (air inlet height: 5 cm) through a perforated plate. We considered two plates with porosities of and . The plates were positioned at distances of 10 cm and 20 cm below the jet inlet. The experiments were realized using 2D Laser Doppler Anemometer (LDA). Different profiles of mean and fluctuating velocities are presented. The presence of a perforated plate strongly modified the airflow pattern compared to an empty enclosure. The velocities above and below the plate depend on several parameters, including the porosity and the plate’s position relative to the inlet slot and the longitudinal position. The difference between the flow velocity above and below the plates could not be related using a universal formula that depends on these parameters. We also investigated the influence of a porous media of a height of 20 cm (a stack of spheres having a diameter of 3.75 cm) located below the perforated plate. The results highlight that the porous medium strengthens the effects of the perforated plate on the flow.

Effect of wall jet on the bubble dynamics near arigid wall

The bubble dynamic near a rigid wall with a wall jet was investigated by codynamics(CFD)method with the volume of fluid(VOF)model,which had been validated by vious experimental data.The effects of different velocities of the wall jet and ditances on the bubble dynamics were studied.The results show that the bubble is squjet due to more force added on the bubble.When the velocity of the wall jet increa,the wall anthe pressure along the wall at collapse time increase because of the extra push indAs the stand-off distance increases,the pressure along the wall first increases then decrethe distance from the bubble to the wall increases.

Experimental Investigation on Space-dispersed Double-wall Jet Combustion System for DI Diesel Engine

A space-dispersed double-wall jet combustion system was developed by adopting the wall-guiding spray method and the stratification theory.The experimental test was carried out to optimize the structural parameters of the diesel-engine combustion system,including chamber structure,swirl ratio of cylinder head,included angle of jet orifice,number and diameter of jet orifice,fuel injection pressure and timing.The effect of double-wall jet combustion system on combustion and engine performance was tested to obtain the best performance indexes,and the double-wall jet combustion system was compared to the prototype.The results show that NOx is reduced from 712 PPm to 487 PPm at 2 100 r/min,and from 593 PPm to 369 PPm at 3 000 r/min,which are reduced by 31.6% and 37.7%,respectively.The smoke intensity was reduced form 3.67 BSU to 2.1 BSU,and the oil consumption was reduced from 240.5 g/(kW·h) to 225.4 g/(kW·h),which was decreased by 6.3% at low speed.The pressure in the cylinder was obviously reduced from 115 bar to 108 bar,which was reduced by 6%.

Experimental and Numerical Investigation of the Diffusion of a Confined Wall Jet through a Perforated Plate

When performing numerical modeling of fluid flows where a clear medium is adjacent to a porous medium, a degree of difficulty related to the condition at the interface between the two media, where slip velocity exists, is encountered. A similar situation can be found when a jet flow interacts with a perforated plate. The numerical modeling of a perforated plate by meshing in detail each hole is most often impossible in a practical case (many holes with different shapes). Therefore, perforated plates are often modeled as porous zones with a simplified hypothesis based on pressure losses related to the normal flow through the plate. Nevertheless, previous investigations of flow over permeable walls highlight the impossibility of deducing a universal analytical law governing the slip velocity coefficient since the latter depends on many parameters such as the Reynolds number, porosity, interface structure, design of perforations, and flow direction. This makes the modeling of such a configuration difficult. The present study proposes an original numerical interface law for a perforated plate. It is used to model the turbulent jet flow interacting with a perforated plate considered as a fictitious porous medium without a detailed description of the perforations. It considers the normal and tangential effects of the flow over the plate. Validation of the model is realized through comparison with experimental data.

Critical Heat Flux in Forced Convective Boiling with a Wall Jet

The critical heat flux (CHF) in the forced convective boiling with a wall jet has been investigated.The experiments of CHF with a wall jet have been performed over a wide range of ρ l/ρ g=6.6-1 603 and ΔT sub =0-60 K. The mechanism on CHF is discussed and a CHF model based on heat balance in sublayer can provide a good clue for analyzing and deriving CHF.Finally,a generalized correlation is presented, which can predict CHF for saturated and subcooled conditions.

LARGE EDDY SIMULATION OF THE INTERACTION BETWEEN WALL JET AND OFFSET JET

The interaction between a plane wall jet and a parallel offset jet is studied through the Large Eddy Simulation （LES）. In order to compare with the related experimental data, the offset ratio is set to be 1.0 and the Reynolds number Re is 1.0× 104 with respect to the jet height L and the exit velocity U0. The Finite Volume Method （FVM） with orthogonal-mesh （6.17× 106 nodes） is used to discretize governing equations. The large eddies are obtained directly, while the small eddies are simulated by using the Dynamic Smagorinsky-Lily Model （DSLM） and the Dynamic Kinetic energy Subgrid-scale Model （DKSM）. Comparisons between computational results and experimental data show that the DKSM is especially effective in predicting the mean stream-wise velocity, the half-width of the velocity and the decay of the maximum velocity. The variations of the mean stream-wise velocity and the turbulent intensity at several positions are also obtained, and their distributions agree well with the measurements. The further analysis of dilute characteristics focuses on the tracer concentration, such as the distributions of the concentration （i.e., C / C0 or C / C,,）, the boundary layer thickness 6c and the half-width of the concentration b., the decay of the maximum concentration （ C / Co） along the downstream direction. The turbulence mechanism is also analyzed in some aspects, such as the coherent structure, the correlation function and the Probability Density Function （PDF） of the fluctuating velocity. The results show that the interaction between the two jets is strong near the jet exit and they are fully merged after a certain distance.

NUMERICAL SIMULATION OF HORIZONTAL BUOYANT WALL JET

This article applies the realizable k - ω model to simulate the buoyant wall jet and gives the results of cling length, centerline trajectory and temperature dilutions at certain sections. The comparison between the numerical results and Sharp＇s experimental data indicates that the model is effective in estimating velocity distribution and temperature dilutions. The velocity profiles at the cental plane and z-plane both show a strong similarity at certain distance from the nozzle, and the distributions of velocity and temperature dilutions also exhibit a similarity along the axial direction at centerline in the near-field. Based on the results, the article gives the corresponding relationships between the distance and the dilutions of velocity and temperature, which is useful in predicting the behavior of the wall buoyant jet.

NUMERICAL STUDY OF FLOW AND DILUTION BEHAVIOR OF RADIAL WALL JET

The radial wall jet is a flow configuration that combines the radial jet and the wall jet. This article presents a simulation of the radial wall jet by applying the transition Shear-Stress Transport （ SST） model. Tanaka’s experimental data are used for validation. The computed velocity profiles agree well with the experimental ones. The distributions of the velocity on cross-sections show a similarity in the main region and the profiles are different with those of the free radial jet or the wall jet, because the presence of the wall limits the expansion of the jet. By introducing the equivalent nozzle width, the maximum velocity decays and the half-width distributions are normalized, respectively. In addition to compare the flow field with experiments, this paper also analyzes the dilution effect of radial wall jets in terms of the concentration distributions. The concentrations on the wall keep constant within a certain distance from the nozzle. And the concentration distributions also show a similarity in the main region. Both the decays of the maximum concentration and the distributions of the concentration half-width fall into a single curve, respectively. The dilution effect of radial wall jets is thus verified.

Study on three-dimensional expansion characteristics of four wall combustion-gas jets in confined liquid space

To explore further the launch mechanism of the new underwater launching technology proposed in this paper, the expansion characteristics of four wall combustion-gas jets in confined liquid space must be studied firstly. The experimental device is designed, and the high-speed digital photographic system is adopted to obtain the expansion sequence processes of Taylor cavities formed by the four wall jets. Meanwhile, the influence of the injection pressure on the axial expansion property of the four wall jets is discussed. Based on the experiments, a three-dimensional unsteady mathematical model is established to simulate the turbulent flow process of the four wall jets expanding in liquid, and the temporal and spatial distribution laws of phase, pressure, temperature, and velocity and the evolution rules of vortices are illustrated in detail. Results show that, accompanied by the jets expanding downstream, the four wall combustion-gas jets get close to each other and achieve convergence eventually under induction of the interference effect between multiple jets. Meanwhile, the heads of the Taylor cavities separate from the observation chamber wall and offset to the central axis of the observation chamber with time going on. The numerical simulation results of the four wall combustion-gas jets coincide well with the experimental data.

SCOUR OF FINE SEDIMENT BY A TURBULENT WALL JET

A total of 66 experiments were conducted to investigate the scour of fine finesediment by a turbulent wall jet. The independent variables studied were the flow velocity, the jet size, the grain sine, and the water temperature.Three Closely sized grades of bed material were used, and their median diameters were 0. 273mm, 0. 050mm, and 0. 030mm. The jet velocities varied from 0. 30m/s to 1. 10m/s for the coarse sediment (D =0. 273mm), and from 0. 30m/s to 0. 70m/s for the fine grades (D = 0. 050mm , and D= 0. 030mm). The jet size was set to 3. 18mm, 6, 35mm, and 9. 53mm for each grade size, and the water temperature varied from about 60 degrees Fahrenheit to about 85 degrees Fahrenheit.The independent variables were analyzed using dimensional analysis. Three dimentsionless Parameters, namely U(=pu2/ΔpgD), B(b/D), and G(=ΔpgD3/pv2), were obtained. These parameters enabled a close correlation of all experimental results. Other studies were also found to correlate well with these parameters.

Numerical modeling and spatial stability analysis of the wall jet flow of nanofluids with thermophoresis and brownian effects

The present work describes similarity solution to a general scheme for the wall jet flow of nanofluids,accounting both the similarity branches(say upper and lower),allowed with respect to the suction and moving wall conditions in the context of Glauert type e-jets.Before proceeding with this,a spatial stability analysis is performed to check the stability of the similarity modes.Results indicated that the upper similarity branch is possibly stable;whilst,the lower branch is not likely to reside in actual physics.The governing transport equations of mass and energy subject to a general two-phase modeling framework were transformed into similarity equations.The involved equations were then solved numerically employing the standard 4th order Runge-Kutta together with shooting technique.The influence of the involved parameters is shown graphically and in a detailed manner.In the last section,it is presented closed-form algebraic solution to the energy equation for the base fluids with a general convective boundary condition.

Heat Transfer in a Forced Wall Jet on a Heated Rough Surface

In this paper, an experimental investigation of a Iaminar wall jet in the presence of a heated wall with stationary particles on its surface, is reported. The wall jet was submitted to external acoustic vibrations amplifying the coherent structures appearing in the laminar region. A wind tunnel was used at very low Reynolds number. Mean velocity and turbulence intensity were measured by a constant temperature anemometer. Measurements were taken in the transition and turbulent regions. Embedded particles were outside the viscous sublayer and it was observed that their presence modifies significantly the flow characteristics in particular the boundary layer is thickened. This study can bring a better understanding of the structure of a flow when it is heated and forced on a rough wall.

SOME RECENT PROGRESS IN STUDIES ON ACTIVE CONTROL OF WALL-JET FLOWS

Active control of wall his is important in both application and basic researches. In order to establish a solid background for the expected application,it is necessary to perform detailed studies on the base nows,on their stability characteristics and on the identification technique of coherent structures. This paper is a summarizing article rather than a detailed technical report, Its main purpose is just to introduce the recent progress in the related area.

Variation of Transition Characteristics in a Wall Jet

The characteristics of a laminar wall jet submitted to different configurations have been experimentally explored in order to obtain a better knowledge of transition modifications,In fact different parameters can accelerate or delay the transition using a smooth or rough surface,heated or not,submitted to vibrations or not .In this study,significant effects can be noticed.It has been possible to measure the beginning of growth rates of disturbance amplitude using a wind tunnel and comparing the results with the linear stability theory.

Numerical Simulations on Geometrical Properties of Wall-attaching Transonic Jet

Effects of operating conditions and device's geometrical sizes on geometrical properties of wall-attaching transonic jet between two parallel plat plates are numerically simulated. Conclusions are as follows: 1) Upriver part of the wall-attaching jet's center streamline is in good accordance with parabola; 2) When both gas inlet pressure and outlet pressure as well as their ratio are not too high ( the outlet pressure is less than 10 MPa and the pressure ratio is less than 3), the center streamlines of the wall-attaching jet with the same pressure ratio coincide with each other very well, and the deflection degree of the center streamline decreases with rise of the pressure ratio; 3) The deflection degree of the jet's center streamline decreases with either broadening of nozzle's throat or rise of wall offset; 4) With rise of the pressure ratio, attachment distance of the jet increases, but the increase rate descends; 5) The attachment distance ascends with rise of either the nozzle's throat or the wall offset.

Characteristics of a bubble jet near a vertical wall

A numerical model of a coupled bubble jet and wall was built on the assumption of potential flow and calculated by the boundary integral method. A three-dimensional computing program was then developed. Starting with the basic phenomenon of the interaction between a bubble and a wall, the dynamics of bubbles near rigid walls were studied systematically with the program. Calculated results agreed well with experimental results. The relationship between the Bjerknes effect of a wall and characteristic parameters was then studied and the calculated results of various cases were compared and discussed with the Blake criterion based on the Kelvin-impulse theory. Our analyses show that the angle of the jet’s direction and the pressure on the rigid wall have a close relationship with collapse force and the bubble’s characteristic parameters. From this, the application range of Blake criterion can be determined. This paper aims to provide a basis for future research on the dynamics of bubbles near a wall.

Expansion Characteristics of a Plasma Jet in the Stepped-Wall Chamber Filled with Water

The interaction mechanism between the plasma and liquid is a key problem for the electrothermal chemical launch technology. To investigate this problem, a simulated experiment for the expansion process of a plasma jet in the working fluid is carried on. Based on this experiment, a two-dimensional axisymmetric unsteady theoretical model is established to reveal the plasma-liquid interaction flow field pattern. The results show that a typical Taylor cavity forms as the plasma jet expands in liquid. The induction effect of the stepped-wall structure enhances the radial expansion of the plasma jet. An arc-shaped pressure wave is generated at the front of the plasma jet and then evolves into the plane wave. A high-pressure area forms at the head of the plasma jet and then moves downstream. There is a strong plasma-liquid turbulent mixing at the interface, especially near the steps and the nozzle exit area.

不同风场中特高压换流站阀厅屋盖风压特性试验研究

特高压(ultra high voltage,UHV)换流站阀厅的金属屋面系统在风荷载作用下易发生屋面表层风揭事故。为深入探讨该类建筑屋面的风压极值特性,基于风洞试验分别探讨了大气边界层(atmospheric-boundary-layer,ABL)风、壁面射流、均匀湍流三种风场作用下的屋面风压特性,比较了平均风剖面、风速、风向、湍流强度等因素对屋面风压的影响。结果表明:阀厅屋盖迎风前缘负风压最大,且控制风向角在45°左右;壁面射流风场下平均风压系数与脉动风压系数均超过大气边界层风场的结果;风速对阀厅屋盖的负风压系数均值和极值影响较小,而湍流度对风压系数的极值影响较大;大气边界风场时,JGJ/T 481—2019《屋盖结构风荷载标准》的最不利风压系数建议值偏于安全;而在壁面射流风场下,阀厅屋盖全风向最不利风压系数在所有区域都大于JGJ/T 481—2019的建议值,设计中应加以重视。

振动及加热壁面条件下的近壁空泡溃灭可视化研究

采用低压电火花生成空泡的方式系统研究了复杂壁面条件下的近壁空泡溃灭行为。通过不同类型振动壁面及不同温度壁面附近的空泡溃灭行为的可视化试验,发现壁面振动对空泡溃灭有促进作用,空泡溃缩过程更快,体积变化更显著。不同类型壁面对空泡溃灭过程的影响不同,柱形壁面相较于球形壁面,近壁的空泡形态较不规则,溃灭呈现整体塌陷的趋势,在反弹阶段会形成不规则的空泡云。壁面升温会使得近壁空泡产生明显趋向于壁面的溃灭行为,并伴随多次空化的现象,这会对壁面造成严重损伤。

近壁面深水爆炸气泡射流演化特性的研究

近壁面气泡载荷是水中战斗部对障碍物毁伤作用的重要载荷之一。以近壁面深水爆炸气泡脉动及射流载荷特征问题为中心,开展了模拟300 m水深环境下的爆炸实验。通过高速摄影仪得到了气泡射流的演化过程;利用Autodyn轴对称模型对气泡射流的演化过程进行了计算,分析了比例距离对气泡脉动周期、射流演化形成时刻及射流强度特征的影响;探讨了壁面近场范围内的压力特征,总结得到射流载荷的演化规律。可为深水环境下水中战斗部对障碍物的毁伤效应研究提供理论参考。