CONVEXITY OF THE FREE BOUNDARY FOR AN AXISYMMETRIC INCOMPRESSIBLE IMPINGING JET

This paper is devoted to the study of the shape of the free boundary for a threedimensional axisymmetric incompressible impinging jet.To be more precise,we will show that the free boundary is convex to the fluid,provided that the uneven ground is concave to the fluid.

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.

Numerical Analysis of the Influence of the Impinging Distance on the Scouring Efficiency of Submerged Jets

Water jet technology is widely used in submerged buried pipes as a non-traditional trenching process,often invol-ving a complex sediment response.An important adjustable and influential engineering variable in this technol-ogy is represented by the impinging distance.In this study,the FLOW-3D software was used to simulate the jet scouring of sand beds in a submerged environment.In particular,four sets of experimental conditions were con-sidered to discern the relationship between the maximum scour depth and mass and the impinging distance.As shown by the results,a critical impinging distance h0 exists by which the static scour depth can be maximized;the scour mass ratio between dynamic and static conditions decreases as the impinging distance increases.Moreover,the profile contours are similar when the erosion parameter Ec is in the range 0.35<Ec<2.Empirical equations applicable for predicting the jet trenching contour under both dynamic and static scour modes are also provided in this study.

EFFECT OF WALL FUNC TION IN NUMERICAL STUDY ON TURBULENT IMPINGING JET

A low Reynolds number k-ε model is used in the numeri cal study on a circular semi-confined turbulent impinging jet . The result is c ompared with that of the standard k-ε model and a refined k-ε mode l, which re-consi-dered the fluctuating pressure diffusion term in the dissipa tion rate equation (ε-equation) through modeling. It shows that the low Re ynolds number k-ε model and the standard k-ε model yield very poor performance, while the predicting ability of the refined k-ε model is mu ch improved , especially for the turbulent kinetic energy k. So it can be co ncluded that the poor performance of the standard k-ε model is owing to t he incorrect considering the effect of the fluctuating pressure diffusion term r ather than the use of the wall function near the wall just as presumed in the re ference.

Experimental Investigation on Flow and Drying Characteristics of Impinging Stream Drying

Based on the experimental investigation of one-stage semi-circular impinging stream drying, the experiments with the two-stage semi-circular, as well as the vertical and semi-circular combined impinging stream drying were conducted. The variations of system pressure drop, the mean residence time of particles with the mass flow-rate ratio and air velocity etc, were determined. The influences of inlet air temperature, mass flow-rate ratio, initial moisture content of particles and air velocity etc. on drying characteristics were also studied. The results indicate that the vertical and semi-circular combined impinging stream drying can make full use of the advantages of both the vertical and semi-circular impinging stream drying. Reasonable mass flow-rate ratio, air velocity, and higher inlet air temperature should be used for less energy consumption and cost during drying process.

Micromixing in the Submerged Circulative Impinging Stream Reactor

Micromixing in the submerged circulative impinging stream reactor (SCISR) developed by the authors is investigated with the Bourne's reaction scheme. The values measured for the impinging velocity, u0, under the conditions of SCISR normal operation, only is of the order of 0.1m·s^-1, are much slower than that inferred,suggesting low power requirement for operation. The values of the characteristic time constant for micromixing,tM, determined in the impinging velocity range of 0.184m·s^-1 < u0 < 0.326m·s^-1 are ranged from 192ms to 87 ms, showing that impinging streams promotes micromixing very efficiently. The data follow approximately the relationship of tM∝ u0^-1.5. A comparative study shows that the micromixing performance of SCISR is much better than that of the traditional stirred tank reactor. The tM values predicted with the existing theoretical model are systematically longer than those measured by about 2--3 times, implying that the regularity of impinging streams promoting micromixing is unclear yet.

Investigation of erosion behavior of 304 stainless steel under solid–liquid jet flow impinging at 30°

This work carried out liquid-solid two-phase jet experiments and simulations to study the erosion behavior of 304 stainless steel at 30° impingement.The single-phase impinging jet was simulated using dense grid by one-way coupling of solid phase due to its dilute distribution.The simulation results agreed well with experiments.It was found that after impinging particle attrition occurred and particles became round with decreasing length-ratio and particle breakage occurred along the "long" direction.Both experiment and simulations found that the erosion generated on the sample could be divided into three regions that were nominated as stagnant region,cutting transition region and wall jet region.Most particle-wall impacts were found to occur in the cutting transition region and the wall jet region.In the cutting transition region,holes and lip-shaped hogbacks were generated in the same direction as the flow imping.In the wall jet region,furrows and grooves were generated.The averaged grooves depth tended to become constant with the progress of impinging and reach the steady state of erosion in the end.In addition,it was found that impinging effect increased erosion and anti-wear rate.

Pressure Fluctuation in the Submerged Circulative Impinging Stream Reactor

Pressure fluctuation in the submerged circulative impinging stream reactor （SCISR） is studied by measuring the dynamic pressure with micro pressure sensors of high accuracy, with water as the process material. Experimental results show that the maximum amplitude of fluctuation can be up to about 1.6kPa. On the power spectra the fluctuation is relatively concentrated in the range of 〈1000Hz, with some weak peeks in acoustic wave range. The space profile of intensive fluctuation region in the reactor is determined. The region is found to take the form of a couple truncated cones of empty core, with coincided bottoms, and is symmetrical with respect to the impinging plane and approximately symmetrical about the axis, essentially independent of u0. The integral intensity of fluctuation increases as the impinging velocity, Uo increasing.

Optimal Conditions for Preparing Ultra-Fine CeO_2 Powders in A Submerged Circulative Impinging Stream Reactor

Cerium carbonate powders were produced in a submerged circulation impinging stream reactor （SCISR） from Ce（NO3）3· 6H2O. NH4HCO3 was used as a precipitant in the reaction. Cerium carbonate powders were roasted to produce ultra-fine cerium dioxide （CeO2） powders. The optimal conditions of such production process were obtained by orthogonal and one-factor experiments. The results showed that ultra-fine and narrowly distributed cerium carbonate powders were produced under the optimal flowing conditions. The concentrations of Ce（NO3）3 and NH4HCO3 solutions were 02,5 and 0.3 mol · L^-1, respectively. The concentration of PEG4000 added in these two solutions was 4 g · L^-1. The stirring ratio, reaction temperature, feeding time, solution pH, reaction time and digestion time were 900 r · min^- 1,80 ℃, 20 min, 5 - 6, 5 min and 1 h, respectively. The final product, CeO2 powders, was obtained by roasting the produced cerium carbonate in air for 3 h at 500 ℃. The finally produced CeO2 powders were torispherical particles with a narrow size distribution of 0.8 -2.5 μm. The crystal structure of CeO2 powders belonged to cubic crystal system and its space point 5 group was OH^5-FM3M. Under optimal conditions, powders produced by SCISR were finer and more narrowly distributed than that by Stirred Tank Reactor （STR）.

Simulation of Droplet-gas Flow in the Effervescent Atomization Spray with an Impinging Plate

Abstract A comprehensive three-dimensional model of droplet-gas flow was presented to study the evolution of spray in the effervescent atomization spray with an impinging plate. For gas phase, the N-S equation with the κ-ε turbulence model was solved, considering two-way coupling interaction between droplets and gas phase. Dispersed droplet phase is modeled as Lagrangian entities, accounting for the physics of droplet generation from primary and secondary breakup, droplet collision and coalescence, droplet momentum and heat transfer. The mean size and sta- tistical distribution of atomized droplets at various nozzle-to-plate distances were calculated. Some simulation resuits were compared well with experimental data. The results show that the existence of the impinging plate has a pronounced influence on the droplet mean size, size distribution and the droplet spatial distribution. The air-to-liquid ratio has obvious effects on the droplet size and distribution.

Quadrature Method of Moments for Nanoparticle Coagulation and Diffusion in the Planar Impinging Jet Flow

A computational model combining large .eddy simulation with quadrature moment method was em-ployed to study nanoparticle evolution in a confined impinging jet. The investigated particle size is limited in the transient regime, and the particle collision kernel was obtained by using the theory of flux matching. The simulation was validated by comparing it with the experimental results. The numerical results show coherent structure acts to dominate particle number intensity, size and polydispersity distributions, and it also induce particle-laden iet to be diluted by .the ambient.The evolution of particle dynarnics in.the impinging jet flow are strongly related to the Rey-nolds number and nozzle-to-plate distance, and their relationships were analyzed.

CFD analysis of performance improvement of the Savonius water turbine by using an impinging jet duct design

The majority of research on water turbines focuses on design improvement of large-scale hydrokinetic turbines for power generation, which may have delayed the utilization of kinetic energy contained in rivers and canals. The aim of this paper is to improve the efficiency of a two bladed Savonius type cross-flow hydrokinetic turbine, which can be used as an energy converter to harness free-stream kinetic energy of water. An impinging jet duct design is presented for improving performance of the Savonius turbine in wind application as seen from literature. The performance of the modified turbine is evaluated using CFD software Fluent, and is compared with that of a simple two bladed Savonius water turbine and some of the prominent literature designs of the Savonius turbine. It is shown that the present design exhibits improved performance compared to the selected designs of the Savonius turbine.Further an insight of the improved performance of the modified turbine is also obtained from flow physics study.

Theoretical analysis of fluid mixing time in liquid-continuous impinging streams reactor

The mixing time of impact zone in liquid-continuous impinging streams reactor(LISR) is theoretically calculated by empirical model and modern micromixing model of the fluid mixing process, and the variation laws of macromixing time and micromixing time are quantitatively discussed. The results show that under a continuous and stable operating condition, as the paddle speed increases, the macromixing time and micromixing time calculated by the two models both decrease, even in a linkage equilibrium state. Simultaneously, as the paddle speed increases, the results figured by the two models tend to be consistent. It indicates that two models both are more suitable for calculation of mixing time in high paddle speed. Compared with the existing experimental results of this type of reactor, the mixing time computed in the speed of 1500 r/min is closer to it. These conclusions can provide an important reference for systematically studying the strengthening mechanism of LISR under continuous mixing conditions.

Micromixing efficiency in a T-shaped confined impinging jet reactor

Confined impinging jet reactor(CIJR)offers advantages for chemical rapid processes and has become an important new reactor used in the chemical industry.The micromixing efficiency in a T-shaped CIJR for two tubes of inner diameter of 3 mm was studied by using a parallel competing iodide–iodate reaction as the working system.In this work,the effects of different operating conditions,such as impinging velocity and acid concentration,on segregation index were investigated.In addition,the effects of the inner nozzles diameter and the distance L between the jet axis and the top wall of the mixing chamber on the micromixing efficiency were also considered.It is concluded that the best range of L in this CIJR is 6.5–12.5 mm.Based on the incorporation model,the estimated minimum micromixing time tmof CIJR approximately equals to 2×10-4s.These experimental results indicate clearly that CIJR possesses a much better micromixing performance compared with the conventional stirred tank(micromixing time of 2×10-3to 2×10-2s).Hence,it can be envisioned that CIJR has more promising applications in various industrial processes.

Pressure Fluctuation in the Submerged Circulative Impinging Stream Reactor

Pressure fluctuation in the submerged circulative impinging stream reactor (SCISR) is studied by meas- uring the dynamic pressure with micro pressure sensors of high accuracy, with water as the process material. Ex- perimental results show that the maximum amplitude of fluctuation can be up to about 1.6kPa. On the power spectra the fluctuation is relatively concentrated in the range of <1000Hz, with some weak peeks in acoustic wave range. The space profile of intensive fluctuation region in the reactor is determined. The region is found to take the form of a couple truncated cones of empty core, with coincided bottoms, and is symmetrical with respect to the impinging plane and approximately symmetrical about the axis, essentially independent of u0. The integral intensity of fluctua- tion increases as the impinging velocity, u0 increasing.

Numerical studies on four-engine rocket exhaust plume impinging on flame deflectors with afterburning

This paper studies the four-engine liquid rocket flow field during the launching phase.Using threedimensional compressible Navier-Stokes equations and two-equation realizable k-epsilon turbulence model,an impact model is established and flow fields of plume impinging on the two different shapes of flame deflectors,including wedge-shaped flame deflector and cone-shaped flame deflector,are calculated.The finite-rate chemical kinetics is used to track chemical reactions.The simulation results show that afterburning mainly occurs in the mixed layer.And the region of peak pressure occurs directly under the rocket nozzle,which is the result of the direct impact of exhaust plume.Compared with the wedgeshaped flame deflector,the cone-shaped flame deflector has great performance on guiding exhaust gas.The wedge-shaped and cone-shaped flame deflectors guide the supersonic exhaust plume away from the impingement point with two directions and circumferential direction,respectively.The maximum pressure and temperature on the wedge-shaped flame deflector surface are 37.2%and 9.9%higher than those for the cone-shaped flame deflector.The results provide engineering guidance and theoretical significance for design in flame deflector of the launch platforms.

Experimental and Theoretical Analysis of the Impinging Stream Drying

The experiments of one-stage semi-circular and two-stage semi-circular impinging stream drying as well as the vertical and semi-circular combined impinging stream drying were carried out. The velocity distribution and the mean residence time of particles, and the influence of various factors on drying characteristics were studied. A mathematical model of granular material drying in a semi-circular impinging stream dryer was proposed, in which the flow characteristics as well as the heat and mass transfer mechanisms were considered. Reasonable numerical methods were used to solve the equations. Under various conditions, the calculated results agree well with the experimental data. The unsteady-state drying dynamic equation, as well as the variations of drying rate and moisture content versus time were obtained. The results indicate that constant drying rate period does not exist in a semi-circular impinging stream dryer. Appropriate semi-circular stage number and curvature radius, flow-rate ratio, air velocity, and higher inlet air temperature should be used for enhancing the drying process.

Convective Heat Transfer Enhancement of a Rectangular Flat Plate by an Impinging Jet in Cross Flow

Experiments were carried out to study the heat transfer performance of an impinging jet in a cross flow.Several parameters including the jet-to-cross-flow mass ratio(X=2%-8%), the Reynolds number(Red=1434-5735)and the jet diameter(d=2-4 mm) were explored. The heat transfer enhancement factor was found to increase with the jet-to-cross-flow mass ratio and the Reynolds number, but decrease with the jet diameter when other parameters maintain fixed. The presence of a cross flow was observed to degrade the heat transfer performance in respect to the effect of impinging jet to the target surface only. In addition, an impinging jet was confirmed to be capable of enhancing the heat transfer process in considerable amplitude even though the jet was not designed to impinge on the target surface.

Investigation of extraction fraction in confined impinging jet reactors for tri-butyl-phosphate extracting butyric acid process

The extraction fraction E and overall volumetric mass transfer coefficient kka of TBP extracting butyric acid pro- cess in confined impinging jet reactors （CIJR） with two jets were investigated. The main variables tested were the concentration of tri-butyl-phosphate （TBP） and butyric acid, the impinging velocity V, the impinging velocityratio of two phases Vorg/Vaq, the nozzle inner diameter di and the distance L between the jet axes and the top wall of the impinging chamber. The results showed that E and kLa increase with an increase of the impinging velocity V, the concentration ofTBP Corg, and the impinging velocity ratio Vor/Vaq. However, E and kta decrease with an increase of the inner diameter d1 from 1 to 2 mm, the concentration of butyric acid Caq from 0.5% （v/v） to 2% （v/v）. The factor L ranging from 3 to 11 mm has a negligible effect on E and kLa. A correlation on these variables and kLa was proposed based on the experimental data. These results indicated good mass transfer oerformance of CIJR in the extraction operation.

Heat Flux Characterization of DC Laminar-plasma Jets Impinging on a Flat Plate at Atmospheric Pressure

By using steady and transient methods, the total heat fluxes and the distributions of the heat flux were measured experimentally for an argon DC laminar plasma jet impinging normally on a flat plate at atmospheric pressure. Results show that the total heat fluxes measured with a steady method are a little bit higher than those with a transient method. Numerical simulation work was executed to compare with the experimental results.