Impingement Wall Research Articles

Effect of Anisotropy on Particle Deposition in Impinging Jet Flow

This study presents a numerical prediction of particle deposition on an impingement wall for nozzle-tosurface distance, L/D = 2. The continuous phase flow was solved using Reynolds-averaged Navier Stokes (RANS) along with Baseline Reynolds stress turbulence model (RSM-BSL). The particulate phase was simulated using a one-way coupling Lagrangian random-walk eddy-interaction model (EIM). The particle deposition density using turbulent tracking and mean flow tracking was predicted and the effect of the near-wall correction of the normal Reynolds stress component was evaluated. The effect of anisotropic flow using the minimum eddy lifetime is examined. To assess the accuracy of EIM in framework of RANS, large eddy simulation (LES) with Lagrangian particle tracking was used to predict the particle deposition in impinging jet flow. LES prediction was compared to RANS/EIM predictions and experimental data. Moreover, simulation findings demonstrate the superiority of LES compared to RANS/EIM in predicting the particle deposition. The results obtained using RANS/EIM showed that the deposition of the particles using the minimum eddy lifetime and near-wall correction yields close results to LES prediction and the experimental data. In addition, the deposition exhibits a ring-like pattern similar to experiments.

Experimental investigation of characteristics of transient low pressure wall-impinging gas jet

This paper describes an investigation of the jet structure and mixture formation process of wall-impinging gas jet injected by a low pressure gas injector in a constant volume chamber at room conditions. The tracer-based planar laser-induced fluorescence (PLIF) technique is applied to qualitatively evaluate the mixture formation process. The macroscopic structure and concentration distribution of wall-impinging jet were studied based on a series of time evolution high-definition images. In particular, the effects of injection pressure on characteristics of turbulence were investigated. Experimental results show that vortex structure with large scale is one of important characteristics for wall-impinging jet, and the interaction among jet flow, impingement wall, and surrounding air plays a dominant role in the mixture formation. The comparative study about the effect of injection pressure on wall-impinging jet reveals higher injection leads to higher mixing efficiency and better mixture formation.

Flow Asymmetry in Symmetric Multiple Impinging Jets: A Large Eddy Simulation Approach

A numerical study on in-line arrays of multiple turbulent round impinging jets on a flat heated plate was conducted. The Large Eddy Simulation turbulence model was used to capture details of the instantaneous and mean flow fields. The Reynolds number, based on the jets diameter, was equal to 20,000. In addition to flow features known from single jets, the interaction between the neighboring jets was successfully elucidated. Symmetry boundary conditions were imposed to reduce the computational domain to only a quarter. In accordance with previous numerical and experimental works, the asymmetry in the velocity field near to the impingement plate was also found to exist. LES showed oval imprints of the Nusselt number similar to experiments but with some discrepancies on the symmetry boundaries. The asymmetry, observed in previous experimental and numerical results, in the horizontal planes, parallel and close to the impingement wall, was confirmed. The recirculation zone responsible for asymmetry, known to develop due to the wall jets interaction, was seen in only one side of the diagonal formed by the central and the farthest jets.

A suggestion of a new method for the calculation of the coating thickness in continuous hot-dip galvanizing

It is known that the distributions of the impinging pressure gradient and the shear stress at the strip surface play a decisive key role in the decision of the coating thickness in hot-dip galvanizing. So, to predict the exact coating thickness, it is essential that the distributions of the impinging wall jet pressure and the shear stress acting between the liquid film and jet stream are measured (or calculated) exactly for each specific coating condition. So far, to obtain the impinging wall jet pressure, it was assumed that the jet issuing from an air-knife is similar to the Hiemenz plane stagnation flow, and the wall shear stress could be predicted by an equation using the assumption of a non-negative Gaussian profile in impinging wall jet pressure in general, so that it cannot be reliable for some impinging wall jet regions and nozzle systems intrinsically. Nevertheless, one cannot find a suitable method to cope with the difficulties in measuring/calculating of the shear stress and the impinging wall jet pressure. Such a difficulty which will cause an inaccuracy in the coating thickness prediction. With these connections, in the present study, we suggest a new method named as a two-step calculation method to calculate the final coating thickness, which consists of the air jet analysis and coating thickness calculation. And, from the comparison of the results one may confirm the validation of the new suggested method.

Computational Analysis of Conjugate Heat Transfer and Particulate Deposition on a High Pressure Turbine Vane

Numerical computations were conducted to simulate flash deposition experiments on gas turbine disk samples with internal impingement and film cooling using a computational fluid dynamics (CFD) code (FLUENT). The standard k-ω turbulence model and Reynolds-averaged Navier–Stokes were employed to compute the flow field and heat transfer. The boundary conditions were specified to be in agreement with the conditions measured in experiments performed in the BYU turbine accelerated deposition facility (TADF). A Lagrangian particle method was utilized to predict the ash particulate deposition. User-defined subroutines were linked with FLUENT to build the deposition model. The model includes particle sticking/rebounding and particle detachment, which are applied to the interaction of particles with the impinged wall surface to describe the particle behavior. Conjugate heat transfer calculations were performed to determine the temperature distribution and heat transfer coefficient in the region close to the film cooling hole and in the regions further downstream of a row of film cooling holes. Computational and experimental results were compared to understand the effect of film hole spacing, hole size, and TBC on surface heat transfer. Calculated capture efficiencies compare well with experimental results.

Heat Transfer Characteristics of an Oblique Jet Impingement Configuration in a Passage With Ribbed Surfaces

Heat transfer measurements of a confined impingement cooling configuration with ribs on the target surfaces are presented. The assembly consists of four nonperpendicular walls of which one holds two rows of staggered inclined jets, each impinging on a different adjacent wall. The ribs are aligned with the inclined jet axes, have the same pitch, and are staggered to the impinging jets. The flow exhausts through two staggered rows of holes opposing the impingement wall. The passage geometry is related to a modern gas turbine blade cooling configuration. A transient liquid crystal technique was used to take spatially resolved surface heat transfer measurements for the ground area between the ribs. A comparison with the smooth baseline configuration reveals local differences and a generally reduced heat transfer for the rib-roughened case. Furthermore, lumped heat capacity measurements of the ribs yielded area averaged heat transfer information for the ribs. From the combination of ground and rib heat transfer measurements, it is concluded that the overall performance of the ribbed configuration depends on the Reynolds number. Of the five investigated jet Reynolds numbers from 10,000 to 75,000, only for the highest Re the averaged Nusselt numbers increase slightly compared with the smooth baseline configuration.

Heat Transfer Experiments in a Confined Jet Impingement Configuration Using Transient Techniques

A confined jet impingement configuration has been investigated in which the matter of interest is the convective heat transfer from the air flow to the passage walls. The geometry is similar to gas turbine blade cooling applications. The setup is distinct from usual cooling passages by the fact that no crossflow and no bulk flow directions are present. The flow exhausts through two staggered rows of holes opposing the impingement wall. Hence, a complex 3-D vortex system arises, which entails a complex heat transfer situation. The transient thermochromic liquid crystal (TLC) method was used in previous studies to measure the heat transfer on the passage walls. Due to the nature of these experiments, the fluid as well as the wall temperature vary with location and time. As a prerequisite of the transient TLC technique, the heat transfer coefficient is assumed to be constant over the transient experiment. Therefore, it is the scope of this article to qualify this assumption and to validate the results at discrete locations. For this purpose, fast response surface thermocouples and heat flux sensors were applied, in order to gain information on the temporal evolution of the wall heat fluxes. The linear relation between heat flux and temperature difference could be verified for all measurement sites. This validates the assumption of a constant heat transfer coefficient. Nusselt number evaluations from independent techniques show a good agreement, considering the respective uncertainty ranges. For all investigated sites, the Nusselt numbers range within ±9% of the values gained from the TLC measurement.

Development and Evaporation of Group-Hole Nozzle Sprays under Various Surrounding and Impinging Conditions of Direct-Injection Diesel Engines

In the present study, penetration and evaporation of the diesel sprays injected by a group of two closely spaced orifices (a group-hole nozzle) were investigated and compared with those of conventional single-hole nozzle sprays under various engine loads and wall-impinging conditions of direct-injection (DI) diesel engines. Both free and wall-impinging conditions were considered. The experiments were performed inside a constant-volume vessel under simulated ambient conditions for low and high engine loads of DI diesel engines. To investigate the effect of spray targeting, two impinging conditions (impingement angles of 45° and 90° with the same impingement distance) were applied. Geometry and liquid/vapour mass distributions of the evaporating diesel sprays were analysed using a laser absorption scattering (LAS) technique. Under a free spray condition, fuel evaporation of the group-hole nozzle spray was improved compared with that of the single-hole nozzle spray at low load conditions, while it showed simultaneous deterioration in fuel evaporation and spray tip penetration at high load conditions. Jet axes deflection of the two jets from the group-hole nozzle, which generated a dense liquid region at the central region of the spray, was responsible for this deteriorated evaporation at high load conditions. Under a vertical impingement condition (impingement angle of 90°), the group-hole nozzle spray showed simultaneous improvement in spray tip penetration and fuel evaporation at both low and high load conditions from strong momentum interaction of the two jets on the impingement wall. However, this improvement from the group-hole nozzle did not appear at an inclined wall-impingement condition (impingement angle of 45°) owing to weakened and delayed momentum interaction of the two jets. Both spray tip penetration and fuel evaporation of the group-hole nozzle spray were deteriorated at the inclined wall-impinging condition.

Numerical Simulation of Turbulent Flow and Heat Transfer in an Anti- Gravity Blind Duct with Tangential Entry Jet

This numerical study investigates the flow and heat transfer characteristics in a vertical square blind duct with the coolant fed by a tangential entry jet. The detailed Nusselt number (Nu) distributions over the five constituent walls of the blind duct are calculated at duct (jet) Reynolds numbers Re(Rej) of 5000(20000), 7000(28000) and 10000(40000) us- ing the commercial CFD Star CD code. As an attempt to explore the buoyancy effect on heat transfer performances, three different heat fluxes, which vary the gravitational Grashof numbers (Grg) at the fixed Re, are imposed on each duct wall to vary the buoyancy levels. The jet-induced flow phenomena in the blind duct exhibit various heat transfer impacts on the five duct walls over which the different near-wall flow structures are generated. This is demonstrated by cross-examining the detailed Nu distributions and the area-averaged Nusselt number ( Nu ) over the five duct walls at the tested Re and Grg. The cross-plane swirls induced by the tangential entry jet together with the impinging jet flows considerably elevate the Heat Transfer Enhancement (HTE) performances in the blind duct. Within the parametric conditions simulated, the ratios of Nu to the Dittus-Boelter levels (Nu� ) over the jet wall, back wall, impingement wall, side wall and end wall are re- spectively raised to 3-5.7, 2.8-5.6, 3-5.8, 2.7-4.8 and 3.1-6.1; while the HTE ratios ( Nu /Nu� ) over these duct walls con- sistently decrease as Re increases.

A Mechanism for the Rapid Development of Intracranial Aneurysms: A Case Study

Despite technical and diagnostic progress there are still open questions in the understanding of the pathophysiology of intracranial aneurysms. Within 44 days we observed the de novo genesis and rupture of an aneurysm of the basilar artery in a patient. We performed computational fluid dynamics on 3-dimensional (3D) models of the inconspicuous vessel and the same vessel with aneurysm. Based on the simulations we propose a mechanism of genesis of fast-growing aneurysms. Three-dimensional mesh models were built using computed tomography-angiography slices. Flow was modeled as a non-Newtonian blood model with shear-dependent dynamic viscosity. We investigated flow velocity, wall pressure, impingement point, wall shear stress (WSS), and asymmetric flows in 3D models of the vessel tree of the basilar artery. Impingement point and wall pressure had no clear relation to the origin of the aneurysm. The impingement point faded away during aneurysm growth. Instead we found an area of permanently low WSS in the original basilar artery. This location corresponded to the origin of the later developing aneurysm. Aneurysm growth was facilitated by an increasing overall expansion of the basilar tip and a constant decrease of WSS. Assuming a preexisting reduced resistibility of the vessel wall to pressure changes and an area of permanently low WSS, an increase in pressure induces geometrical changes. These cause changes of intravascular flow distribution, lowering the already low WSS in specific locations. This leads to endothelial damage in this area and to a decreasing stability of the vessel wall, causing aneurysm development, growth, and rupture.

Improving diesel mixture preparation by optimization of orifice arrangements in a group-hole nozzle

The factors affecting the free and wall-impinging spray characteristics of closely spaced micro-orifices (group-hole nozzles) are investigated to determine the optimal orifice arrangement for improved in-cylinder air utilization. To determine the optimized arrangement of the nozzles, different types of group-hole nozzles with different intervals and angles between the orifices have been studied. The spray tip penetration and fuel evaporation were analysed using a laser absorption scattering technique for non-axisymmetric sprays. In the case of a free spray, the spray tip penetration of the group-hole nozzle increased as the included angle and the distance between orifices decreased. The shortest spray tip penetration was observed when the two jets injected by a group-hole nozzle began to separate, but the periphery of each jet still touched. On the contrary, fuel evaporation deteriorated when the distance and angle between the two orifices decreased since the severe collision/coalescence of the two jets hindered evaporation. In the case of a wall-impinging spray, the spray tip penetration of the group-hole nozzle was strongly related to the distance χ between the arbitrary centres of each jet at the impingement wall. Maximum spray tip penetration of the group-hole nozzle was observed at a specific range of χ. In this range, the strong momentum regions of the jets began to meet just after wall impingement, therefore collision/coalescence before wall impingement was minimized and the interaction between the two wall-impinging jets was maximized. The fuel evaporation in the wall-impinging spray also improved at a distance of χ for maximum tip penetration. The mechanism causing this phenomenon is discussed in terms of near-field spray interaction and wall-impinging jet interaction after wall impingement.

Modeling the effect of spray/wall impingement on combustion process and emission of DI diesel engine

This work is presented to study the effect of spray impinging on the combustion process and emissions in a direct injection diesel engine at various engine speeds. Computations are carried out using a three-dimensional modeling for sprays, spray-wall interactions, flow field, emission, and combustion process. Results indicate an increase in engine speed leads to increased spray impinging (wall film formation), turbulence intensity and average wall temperature in cylinder. The enhanced air/fuel mixing and intensified evaporation of wall film decreases soot emission by reducing the extent of the fuel rich regions specially in impinging zones. Also at higher engine speeds, combustion is delayed and fuel is consumed in a shorter time period by the enhanced air and fuel mixing. The shorter combustion duration provides less available time for soot and NOx formations. However, only a few attempts have been made to address the effect of impingement of spray with piston walls on the emissions and combustion process. The results of model in addition to approving the corresponding data in the literature are also compared with the experimental data and shown good agreement.

Enhancing impinging jet heat or mass transfer by fluidically generated flow pulsation

Periodic pulsation of high intensity generated by a no-moving-part fluidic oscillator located upstream from jet generating nozzles was demonstrated to enhance heating (or cooling and, because of the analogy of governing equations, also drying) by impinging hybrid-synthetic jets. The test setup was built for an envisaged application in food processing by baking or roasting. It has two nozzles, each connected to one of the two outlets of the diverter-type oscillator. The nozzles were of annular exit cross-section and operated in anti-parallel (outflow from one nozzle simultaneous with suction into the other nozzle). The investigation, focused on study of the temperature fields on a plane impingement wall visualised by thermochromic liquid crystals, revealed some unexpected effects.

Relationship between shear stress on the walls of a pipe and an impinging jet

In order to relate a shear stress controlled-corrosion at the impinged jet wall to that in a pipe the pressure drop along a jet tube and the mass transfer rate at eight locations on the jet-impinged wall were measured concurrently using microelectrodes. The shear stress on the impinged jet wall increased radially giving stagnation and jet wall regions in the order of 2 and 2.9 times the jet radius, respectively. The mean of the jet-impinged wall shear stresses at all radial positions was 50–120 times that at the pipe wall at any given Re. A factor of about 80 may be used where the jet impinged wall corrosion rates are to be used to estimate corrosion in pipe flow at the same Re.

Wall Shear Stress on Ruptured and Unruptured Intracranial Aneurysms at the Internal Carotid Artery

Hemodynamics is often recognized as one of the major factors in aneurysm rupture. Flow impingement, greater pressure, and abnormal wall shear stress are all indications for aneurysm rupture. Characterizing wall shear stress for intracranial aneurysms at similar anatomic locations may help in understanding its role. Twenty-six intracranial aneurysms at the paraclinoid and superclinoid segments of the internal carotid artery from 25 patients between July 2006 and July 2007 were studied retrospectively. Among them, 8 aneurysms were ruptured and 18 were unruptured. Computational fluid dynamics was used to determine the wall shear distribution. Morphologic and hemodynamic variables was analyzed by using the Mann-Whitney rank sum test. Wall shear stress was qualitatively the same throughout the cardiac cycle; thus, only wall shear stress at the end of diastole was compared. Both ruptured and unruptured aneurysms have similar maximal wall shear stress (26 versus 23 N/m(2)), and mean wall shear stress is shown to be a function of the aneurysm area. Ruptured aneurysms also have a greater portion of aneurysm under low wall shear stress (27% versus 11% for unruptured aneurysms, P = .03). For intracranial aneurysms at the internal carotid artery, an area of low wall shear is associated with aneurysm rupture.

Analysis of Evaporation Process of Non-Axisymmetric Wall-Impinging Spray by Means of Laser Absorption Scattering Technique

An evaporation process plays an important role on the combustion and emission formation processes in a direct injection (D. I.) gasoline engines. An experimental study was conducted on the evaporation process of the non-axisymmetric wall-impinging spray injected by the hole type injector for a D. I. gasoline engine. For the quantitative measurement of the vapor of the non-axisymmetric spray, the image processing technique using the Laser Absorption Scattering (LAS) principle was newly developed. The total amount of fuel vapor in the whole spray was obtained from the optical thickness image of the vapor and the Lambert Beer's law. The amount of fuel vapor in the wall-impinging spray is decreased compared with the free spray. In addition, the amount of fuel vapor is decreased as the distance between nozzle tip and the impingement wall is decreased.

Experimental study of turbulent round jet flow impinging on a square cylinder laid on a flat plate

A turbulent axisymmetric air jet impinging on a square cylinder mounted on a flat plate has been studied experimentally. Turbulence statistics and flow’s topology were investigated. When the surface was heated through uniform heat flux, local heat transfer coefficient was measured. The jet from a long round pipe, 75 pipe diameters ( D) in length, at Reynolds number of 23,000, impinged vertically on the square cylinder (3 D × 3 D × 43 D). Measurements were performed using particle image velocimetry, flow visualization using fluorescent dye and infrared thermography. The flow’s topology demonstrated a three-dimensional recirculation after separating from the square cylinder and a presence of foci between the bottom corner and the recirculation’s detachment line. The distribution of heat transfer coefficient was explained by the influence of these flow’s structures and the advection of kinetic energy. On the impingement wall of the square cylinder, a secondary peak in heat transfer coefficient was observed. Its origin can be attributed to very pronounced shear production coupled with the external turbulence coming from the free jet.

Study on hydrodynamic vibration in fluidic flowmeter

The characteristics of the fluidic flowmeter, which is a combination of impinged concave wall and bistable fluid amplifier, is investigated by experimental studies and numerical simulations. The numerical approaches are utilized to examine the time dependent flow field and pressure field inside the proposed flowmeter. The effect of varying structural parameters on flow characteristics of the proposed fluidic flowmeter is investigated by computational simulations for the optimization. Both the simulation and experimental results disclose that the hydrodynamic vibration, with the same intensity, frequency and 180° phase shift, occurs at axisymmetric points in the feedback channel of the fluidic flowmeter. Using the structural combination of impinged concave wall and bistable fluid amplifier and differential signal processing technique, a novel fluidic flowmeter with excellent immunity and improved sensibility is developed.

B107 タービンシュラウドの傾斜インピンジメント冷却に関する実験的研究

Very effective cooling methods have been required for high-temperature gas turbines. Reduction of cooling air also contributes to the improvement of jet engine performance, because the cooling air is bled from compressor of jet engine. Therefore, from this point of view, Multiple inclined impingement cooling for turbine shroud cooling has been invented by IHI, JAXA and authors' group. This cooling structure realizes 50% reduction of cooling air compared with conventional one maintaining same cooling efficiency. However detailed internal cooling behavior has not been clarified. This study is one of the researches in collaboration with IET, JAXA and us. The main purpose is to clarify the cooling behavior by Particle Image Velocimetry (PIV) and heat transfer measurement on the impingement wall by IR camera. In this paper, we describe the results of PIV for 4 types of acrylic test pieces (impingement angle=90°, 45°, 35°, 25°). The characteristics of time-averaged velocity, turbulence intensity and pressure drop were considered.

Ignition Position of a Diesel Spray Impinging on a Wall(Effect of Wall Angle, Injection Pressure and Fuel Quantity)

Ignition and combustion characteristics of diesel spray impinging on a wall were experimentally investigated. Ignition position and appearance position of the luminous flame kernel were stereoscopically observed using a two-way fiber optical system. Flat impingement wall was fixed in a high temperature, high pressure combustion chamber. Inclined angle of the flat wall was set at 30 degees or normal against the center axis of the injection spray. Distance from nozzle tip to the impingement point on the wall was set at 50 mm. Effects of injection pressure and fuel quantity on ignition position were investigated. As the result, ignition positions were observed near the spray periphery. And the radial position of ignition was shifted to the larger radial position when the injection pressure increased. Luminous flame appeared near the wall surface in the both cases of 30 degrees and normal impingements. And the region of luminous flame appearance was not changed significantly with an increase of injection pressure.