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.

Experimental Investigation on Flow and Heat Transfer of Jet Impingement inside a Semi-Confined Smooth Channel

Experimental investigation is conducted to investigate the flow and heat transfer performances of jet impingement cooling inside a semi-confined smooth channel.Effects of jet Reynolds number(varied from 10 000to 45000),orifice-to-target spacing(zn=1d—4d)and jet-to-jet pitches(xn=3d—5d,yn=3d—5d)on the convective heat transfer coefficient and discharge coefficient are revealed.For a single-row jets normal impingement,the impingement heat transfer is enhanced with the increase of impingement Reynolds number or the decrease of spanwise jet-to-jet pitch.The highest local heat transfer is achieved when zn/dis 2.For the double-row jets normal impingement,the laterally-averaged Nusselt number distributions in the vicinity of the first row jets impinging stagnation do not fit well with the single-row case.The highest local heat transfer is obtained when zn/dis 1.A smaller jetto-jet pitch generally results in a lower discharge coefficient.The discharge coefficient in the double-row case is decreased relative to the single-row case at the same impingement Reynolds number.

3-D NUMERICAL COMPUTATION OF JET ARRAY IMPINGEMENT WITH INITIAL CROSSFLOW

The 3-D numerical computation of the flow and temperature fields for jet array impingement with initial crossflow investigates the effects of the jet-to-surface spacing, the impinging hole arrangement and the jet-to-crossflow mass flux ratio on heat transfer characteristics. The study shows that: (1) under the different jet-to-surface spacing, the impingement cooling with inline arrangement is better than that with staggered arrangement for a given jet-to-crossflow mass flux ratio;( 2 ) the value of jet-to-surface spacing impacts a complicated effect on the flow and heat transfer for jet array impingement; (3) as the ratio of crossflow-to-jet mass flux ratio increases, the cooling effectiveness on monotonous decrease for both inline and staggered arrangements at the same jet-to-surface spacing.

Combined effects of local curvature and elasticity of an isothermal wall for jet impingement cooling under magnetic field effects

The aim of this study is to examine the effects of local curvature and elastic wall effects of an isothermal hot wall for the purpose of jet impingement cooling performance.Finite element method was used with ALE.Different important parametric effects such as Re number(between 100 and 700),Ha number(between 0 and 20),elasticity(between 104 and 109),curvature of the surface(elliptic,radius ratio between 1 and 0.25) and nanoparticle volume fraction(between 0 and 0.05) on the cooling performance were investigated numerically.The results showed that the average Nu number enhances for higher Hartmann number,higher values of elastic modulus of partly flexible wall and higher nanoparticle volume fraction.When the magnetic field is imposed at the highest strength,there is an increase of3.85% in the average Nu for the curved elastic wall whereas it is 89.22% for the hot part above it,which is due to the vortex suppression effects.Nanoparticle inclusion in the base fluid improves the heat transfer rate by about 27.6% in the absence of magnetic field whereas it is 20.5% under the effects of magnetic field at Ha=20.Curvature effects become important for higher Re numbers and at Re=700,there is 14.11% variation in the average Nu between the cases with the lowest and highest radius ratio.The elastic wall effects on the heat transfer are reduced with the increased curvature of the bottom wall.

Planar collisionless jet impingement on a specular reflective plate

This paper presents a fundamental gas-kinetic study on a high speed planar rarefied jet impinging on a flat plate of specular reflections. Based on previous collisionless planar free jet results, it is straightforward to obtain jet impingement flowfield solutions, and jet impingement for specular reflective plate surface properties. Several direct simulation Monte Carlo simulation results are provided and they validate these analytical solutions of rarefied planar jet flows. The results can find applications in many disciplines, such as materials processing, molecular beams, and space engineering.

CFD study of turbulent jet impingement on curved surface

The heat transfer and flow characteristics of air jet impingement on a curved surface are investigated with computational fluid dynamics(CFD)approach.The first applied model is a one-equation SGS model for large eddy simulation(LES)and the second one is the SST-SAS hybrid RANS-LES.These models are utilized to study the flow physics in impinging process on a curved surface for different jet-to-surface(h/B)distances at two Reynolds numbers namely,2960 and 4740 based on the jet exit velocity(U_e)and the hydraulic diameter(2B).The predictions are compared with the experimental data in the literature and also the results from RANS k-εmodel.Comparisons show that both models can produce relatively good results.However,one-equation model(OEM)produced more accurate results especially at impingement region at lower jet-to-surface distances.In terms of heat transfer,the OEM also predicted better at different jet-to-surface spacings.It is also observed that both models show similar performance at higher h/B ratios.

Modeling and Simulation of a Hybrid Jet-Impingement/Micro-Channel Heat Sink

With the progressive increase in the number of transistors that can be accommodated on a single integrated circuit,new strategies are needed to extract heat from these devices in an efficient way.In this regard methods based on the combination of the so-called“jet impingement”and“micro-channel”approaches seem extremely promising for possible improvement and future applications in electronics as well as the aerospace and biomedical fields.In this paper,a hybrid heat sink based on these two technologies is analysed in the frame of an integrated model.Dedicated CFD simulation of the coupled flow/temperature fields and orthogonal tests are performed in order to optimize the overall design.The influence of different sets of structural parameters on the cooling performance is examined.It is shown that an optimal scheme exists for which favourable performance can be obtained in terms of hot spot temperature decrease and thermal uniformity improvement.

Experimental Study of Heat Transfer for a Rotating Cylinder Water Jet Impingement Quenching

Transient heat transfer has been experimentally investigated for subcooled water jet impingement quenching of a hot rotating stainless steel cylinder. Temperatures beneath the impinged surface were measured during quenching and used to estimate surface temperature and surface heat flux by using a developed numerical inverse solution of heat conduction. Heat flux reached its maximum value just after the WF （wetting front） （visible leading edge of boiling region） started moving from stagnation towards the circumferential region. WF moved in a non-uniform manner in angular direction on the hot rotating surface. With the increase of surface velocity, heat flux decreased. Higher surface velocity moved away the produced vapor bubbles and reduced the solid-liquid contact time which made it one-dimensional heat conduction from multi-dimensional, that reduced heat flux. The generated boiling curve from the estimated heat flux showed a reasonable agreement with existing studies. The surface maximum heat flux （maximum value in each cycle） distribution trend with radial position is entirely comparable with the static surface critical heat flux in literature. An explosive to a sheet like flow patterns were observed with the decrease of surface temperature. The flow patterns were followed by the intensity of sound during quenching.

EXPERIMENTAL RESEARCH ON SLOTTED & PERFORATED ELECTROPLATED CBN GRINDING WHEEL WITH RADIAL JET

A creative conception is proposed to enhance heat transfer in grinding contact zone through jet impinging on the basis of analysis on the mechanism of burn during creep feed grinding, and a new apparatus of slotted & perforated electroplated CBN grinding wheel with radial jet is developed, the effect on heat transfer is studied through the experiment of intermitted creep feed grinding. Experimental results show that the technology of enhancing heat transfer through jet impinging is valid to raise the efficiency of heat transfer in grinding contact zone and it is widely applied to solve the problem in grinding burn for difficult to machine materials.

Effects of using sinusoidal porous object(SPO)and perforated porous object(PPO)on the cooling performance of nano-enhanced multiple slot jet impingement for a conductive panel system

Cooling system design for thermal management of electronic equipment,batteries and photovoltaic(PV)modules is important for increasing the efficiency,safety operation,and long life span the products.In the present study,two different cooling systems are proposed with nano-enhanced multiple impinging jets for a conductive panel.The present cooling systems can be used in electronic cooling and PV modules.Perforated porous object(PPO)and sinusoidal porous object(SPO)are used in the jet cooling system.2D numerical analysis usingfinite volume method is conducted considering different values of permeability of the objects(Darcy number(Da)between 10^(-6) and 10^(-1)).When PPO is used in the cooling system,num-ber of cylinders(between 1 and 6),and size of the cylinders(between 0.015 and 0.075)are considered.In the case of using SPO,amplitude(between 0.1 and 2)and wave number(be-tween 1 and 12)are varied.Alumina-water nanofluid with cylindrical shaped nanoparticles is used as the heat transferfluid.When permeability is changed for PPO,the average temper-ature increases by roughly 3.89℃ for a single cylinder and drops by roughly 0.57℃ for a six-cylinder cases.Increasing the size of the cylinder in the PPO case at highest permeability results in temperature drop of 5.3℃.When changing the number of cylinders,cooling rate varies by about 3.6%.Wave number of SPO is more influential on the cooling performance enhancement as compared to amplitude and permeability of the SPO.The average surface temperature drops by 12.4℃ when the wave number is increased to 12.As compared to reference case of jet impingement cooling without porous object,using PPO and SPO along with the nanofluid result in temperature drop of 12.3℃ and 14.4℃.

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.

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.

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.

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.

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.

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.

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.

Experimental and numerical investigations on the vortical structures of an impinging jet in crossflow

The objective of this dissertation is to investigate the impinging jet under the influence of crossflow. It has been known that there exist jet shear layer, impingement on the bottom wall, interactions between the induced wall jet and the ambient crossflow in near field. There are few intensive studies of the impinging jet in crossflow at home and abroad due to the complexities of flow, such as the formation and evolution of the vortical structures, interactions among vortices, while researches on the temporal and spatial evolution of these vortical structures can promote the practical applications in environment engineering, hydroelectricity engineering, etc., and provide the basis for flow control and improvement through revealing the inherent mechanism and development of the vortical structures.

Numerical simulation of circular jet impinging on hot steel plate

Flow structure and heat transfer characteristics of an axisymmetric circularjet impinging on a hot 1Cr18Ni9Ti medium plate have been simulated numerically using computationalfluid dynamic (CFD) code. The relation between flow field of jet impingement and its heat transfercapability is analyzed, and the phenomenon that heat transfer at stagnation point is smaller thanthat of points directly around is discussed. The simulation result provides boundary conditions forthermal analysis of medium plate quenching.