Impingement Process Research Articles

Experimental Investigation on Wall Film Distribution of Dimethyl Ether/Diesel Blended Fuels Formed during Spray Wall Impingement

Dimethyl ether (DME)/diesel blended fuels are used to improve the emissions caused by spray wall impingement during the early injection period. However, experimental results have showed that the spray wall impingement still cannot be avoided due to the engine structure and low density of the in-cylinder charge at the early injection timing. Furthermore, the wall film formed in the spray wall impingement process directly affects fuel/air mixture formation, combustion, exhaust emissions and oil quality subsequently. In this paper, the wall film distribution of DME/diesel blended fuels formed during the spray wall impingement process has been experimentally investigated. The variations of wall film distribution, wall film area and average thickness with different injection pressures, impingement distances, impingement angles and blending ratios have been discussed under both dry wall and wet wall conditions. Results showed that the wall film distribution styles were mainly determined by the spray impingement momentum. The variation of the wall film area and average thickness were affected by three factors including the impingement momentum, wall film mass and fuel properties. Correlation analysis was introduced in order to evaluate the effect of each impact factor on the variation of wall film area and average thickness.

Experimental investigation on wall film ratio of diesel, butanol/diesel, DME/diesel and gasoline/diesel blended fuels during the spray wall impingement process

Spray wall impingement in the early injection period has been proved be unavoidable in the diesel engine using early injection strategy. The formation of wall film directly affects fuel/air mixture formation, combustion, exhaust emissions and oil quality. In this study, experiments were carried out to investigate the wall film ratio of pure diesel, butanol/diesel, DME (Di Methyl Ether)/diesel and gasoline/diesel blended fuels. The variations of wall film ratio with different injection pressures, impingement distances, impingement angles and blending ratios were compared under both dry wall and wet wall conditions. For both dry wall and wet wall conditions, with increasing injection pressure and blending ratio, wall film ratio decreased, which is the opposite trend to impingement distance and impingement angle variations. Some dimensionless numbers also have been introduced in order to evaluate the effect of each impact factor on the variation of wall film ratio. Results showed that impingement momentum is the major impact factor on wall film ratio when varying the injection pressure or impingement distance. Impingement flow mass is the major impact factor when varying the impingement angle. For different blending ratios, the wall film ratio is mainly influenced by surface tension, followed by viscosity and saturated vapor pressure. In addition, the equations between the wall film ratio and Weber number of the impingement droplet were derived for each test fuel.

Vortex dynamics and wall shear stress behaviour associated with an elliptic jet impinging upon a flat plate

Vortical structures and dynamics of a Re h = 2100 elliptic jet impinging upon a flat plate were studied at H/d h = 1, 2 and 4 jet-to-plate separation distances. Flow investigations were conducted along both its major and minor planes using laser-induced fluorescence and digital particle image velocimetry techniques. Results show that the impingement process along the major plane largely consists of primary jet ring-vortex and wall-separated secondary vortex formations, where they subsequently separate from the flat plate at smaller H/d h = 1 and 2 separation distances. Key vortex formation locations occur closer to the impingement point as the separation distance increases. Interestingly, braid vortices and rib structures begin to take part in the impingement process at H/d h = 4 and wave instabilities dominate the flow field. In contrast, significantly more coherent primary and secondary vortices with physically larger vortex core sizes and higher vortex strengths are observed along the minor plane, with no signs of braid vortices and rib structures. Lastly, influences of these different flow dynamics on the major and minor plane instantaneous and mean skin friction coefficient levels are investigated to shed light on the effects of separation distance on the wall shear stress distributions.

Research into the process of impingement of two plane jets of an ideal fluid with free boundaries

The problem of finding parameters stationary jets, outgoing from a place of impact of two incoming flat ideal jets with free boundaries and possessing the identical speed, but various width, has no decision. Various models are widely used for a conclusion of the missing equation now, but they lead to contradictory results. The new model is offered. Adequacy to the offered model was checked by comparison results with data of the numerical calculations in ANSYS AUTODYN. The dependence approximating results of numerical calculations is developed to increase accuracy in calculations of angular provision of the internal outgoing jet.First of all, the executed researches are interesting to experts, who works on behavior low- value technological errors in shaped charges.

Lattice Boltzmann study on the dynamics of successive droplets impact on a solid surface

Three dimensional computations on successive droplet impingement on a solid surface are reported. A high-density ratio based lattice Boltzmann model is employed for the present two phase computations and a geometric-based contact angle formulation is used to model the moving contact line. The focus of the present work is on the impact behaviour and the interaction dynamics between the impinging droplets with an initial gap. The effects of trailing drop velocity, surface wettability, drop viscosity and surface tension on the time-dependent dynamics are investigated during the impingement process. Two different transient interaction modes are observed for different velocity ratios. With increase in advancing contact angle, it is observed that primary spreading increases in the case of in-phase mode of collision. Effect of eccentricity, through the offset between the droplets, is found to result in an asymmetric evolution of droplet morphologies. Passive particle tracers are seeded inside the droplets to visualize mixing process as a time sequence of closely interacting droplets during the impact with a solid surface.

Effects of High Frequency Droplet Train Impingement on Crown Propagation Dynamics and Heat Transfer

The objective of this study is to investigate the hydrodynamics and heat transfer phenomena due to high frequency droplet train impingement on a pre-wetted solid surface for electronic cooling applications. The effects of crown propagation dynamics and surface heat transfer were investigated experimentally and numerically. Experimentally, a single stream of mono-dispersed HFE-7100 droplets was generated using a piezo-electric droplet generator at a frequency ( f ) of 6000 Hz with a droplet Weber number (We) of 280. Droplet-induced crater and crown were imaged using a high speed camera system. Numerically, the ANSYS Fluent CFD tool was used to simulate the droplet train impingement process. A reasonable agreement was reached between experimental and numerical data in terms of crown propagation dynamics. Numerical simulations reveal that at the instant of initial spot formation, the magnitude of droplet velocity is almost identical to the crown's radial velocity. The instantaneous temperature field obtained by numerical simulations shows that heat transfer was most effective within the crown propagation region due to the radial momentum generated by the droplets, which leads to a large velocity gradient within the liquid film. A significant increase in surface temperature was observed beyond a radial position of 500 μm. In summary, high frequency droplet impingement leads to a very small temperature gradient in the radial direction within the droplet-induced impact crater. This study will benefit in understanding the relationship between the droplet parameters and surface heat transfer for different cooling applications involving impinging droplets.

Enhanced combustion model with fuel-wall impingement oriented to injection pattern tuning in automotive Diesel engines

Nowadays the high competition reached by the automotive market forces original equipment manufacturers (OEMs) towards innovative solutions. Strict emission standards and fuel economy targets make the work hard to be accomplished. Therefore modern engines feature complex architecture and embed new devices for exhaust gas recirculation (EGR), turbocharging (e.g. multi-stage compressors), gas after-treatment (e.g. selective catalyst reduction (SCR)) and fuel injection (either high or low pressure). In this context the engine management system (EMS) plays a fundamental role to optimize engine operation. The paper deals with fuel spray and combustion simulation by a multi-zone phenomenological model aimed at the optimal tuning of the injection pattern. The fuel spray model simulates the injection process, the fuel-air mixture formation, the in-cylinder gas mixture evolution and accounts for the fuel-wall impingement that usually occurs in case of low-medium injection pressure or advanced injection timing. This feature is fundamental to investigate a wide range of injection pattern set-points, as those applied for advanced combustion concepts (i.e. Premixed Charge Compression Ignition (PCCI)). The impingement of the spray on cylinder walls is modeled by a zero-dimensional approach, simulating dynamics and evaporation of the fuel film with two different semi-empirical models. The numerical and experimental analyses have been performed with respect to an automotive Common-Rail Diesel engine, equipped with high pressure EGR and VGT turbocharger. The experimental measurements have been carried out at the engine test stand, investigating operating conditions with highly advanced injection timing and low rail pressure, in order to induce the fuel impingement process. Model validation against the experimental data exhibits satisfactory accuracy in predicting the impact of injection pattern control on impingement.

Simulation of double droplets impact on liquid film by a simplified lattice Boltzmann model

A simplified method is proposed to reduce the calculated quantity of two-phase lattice Boltzmann (LB) model for large density ratio. The simplified model not only apparently reduces the calculated quantity but also has a good stability for two-phase flow with large density ratio. Based on the simplified two-phase LB model, the impingement of droplet onto a stationary liquid film is simulated. The simulation of single droplet impacts onto liquid film is used to verify the applicability of the simplified model for the impingement process. It shows that the results calculated by the simplified model are in good agreement with experimental data in published paper. Then, the impingement process of double droplets with a horizontal distance and time interval onto a liquid film is simulated by two-phase lattice Boltzmann model. Different horizontal distances, droplet diameters, impact velocities, Reynolds numbers, relative film thicknesses and time intervals between the droplets are considered. The mechanism of the impingement process is analyzed. The rule of middle splash height is obtained. The changing rule of the middle splash height and the horizontal distance has a relationship with the droplet diameter. Larger Reynolds number makes height rise faster. Thicker liquid film leads a higher height but a slower rising rate. The time interval between the droplets would destroy the symmetry and make the flow become complex. It changes the middle splash direction and lowers the splash height. The flow mechanism of the impingement process is elaborated.

Numerical and experimental investigations of crown propagation dynamics induced by droplet train impingement

In this study, hydrodynamics of HFE-7100 droplet train impinging on a pre-wetted solid surface was investigated experimentally and numerically. Experimentally, single stream of mono-dispersed droplets were produced using a piezoelectric droplet generator with the ability to adjust parameters such as droplet impingement frequency, droplet diameter and droplet velocity. Crown propagation events were imaged using a high-speed camera system given the high-frequency of droplet impingement. Relationships between droplet-induced crown propagation and crater formation were investigated experimentally. The high-frequency droplet impingement process was simulated numerically using CFD tool. Crown propagation dynamics were evaluated and analyzed experimentally and numerically, with reasonable agreement between the two methods. A revised theoretical crown propagation model based on numerical results is proposed in this paper, which takes into account the impinged liquid velocity distribution and film thickness at the moment of initial spot formation. The revised theoretical crown propagation model gives predictions with improved accuracy, which are in better agreement with the numerical results.

Quasi-molecular modeling of a single supercooled large droplet impact

A mesoscale model for droplet dynamics based on a quasi-molecular approach is proposed. It considers the interaction between quasi-molecules within a single liquid droplet, each quasi-molecule representing an agglomeration of a large number of actual water molecules. The goal is to improve the understanding of the dynamics of large droplet collisions over dry or wet surfaces at velocities typical of aeronautical applications. This detailed analysis will eventually be used to refine the macroscopic Eulerian description of the water impingement process by providing numerical correlations for splashing and bouncing phenomena relevant for in-flight icing applications. Based on the Equipartition Theorem, approaches for extracting macroscopic quantities such as temperature and transport coefficients from the quasi-molecular method are discussed. A proper choice of the free parameters of the model that leads to accurate values of the macroscopic properties is also addressed.

Effects of isotropic and anisotropic slip on droplet impingement on a superhydrophobic surface

The dynamics of single droplet impingement on micro-textured superhydrophobic surfaces with isotropic and anisotropic slip are investigated. While several analytical models exist to predict droplet impact on superhydrophobic surfaces, no previous model has rigorously considered the effect of the shear-free region above the gas cavities resulting in an apparent slip that is inherent for many of these surfaces. This paper presents a model that accounts for slip during spreading and recoiling. A broad range of Weber numbers and slip length values were investigated at low Ohnesorge numbers. The results show that surface slip exerts negligible influence throughout the impingement process for low Weber numbers but can exert significant influence for high Weber numbers (on the order of 102). When anisotropic slip prevails, the droplet exhibits an elliptical shape at the point of maximum spread, with greater eccentricity for increasing slip and increasing Weber number. Experiments were performed on isotropic and anisotropic micro-structured superhydrophobic surfaces and the agreement between the experimental results and the model is very good.

Modeling of aluminum particle ignition behavior in open atmosphere rocket propellant fires

The study of aluminum particle behavior in an open atmosphere propellant burn is of particular interest when considering accident scenarios for rockets carrying high-value payloads. One of the major difficulties in analyzing the thermal environment that a payload will see is the behavior of aluminum particles, which are a component in the propellant formulation. This research activity focuses on the development of models to rationalize observed aluminum behavior beginning with developing an understanding of particle–wall interaction and wall impingement process using STAR-CCM+, a commercial CFD code. Importance of implementation of aluminum evaporation models and radiation heat transfer are discussed.

Molecular Dynamics–Continuum Hybrid Simulation for the Impingement of Droplet on a Liquid Film

A molecular dynamics–continuum hybrid method is used to study the droplet impingement process on a liquid film. The hybrid code is validated by simulating the sudden-start Couette flow and unsteady heat transfer problem. The impingement process is strongly affected by Weber number. At low Weber number, the evolution of the crown after droplet impingement is stable, while at high Weber number the secondary droplets emerge and the splash phenomenon occurs. The effect of liquid film thickness on the evolution of crown diameter is also investigated.

Visualization of High Speed Liquid Jet Impaction on a Moving Surface

Two apparatuses for examining liquid jet impingement on a high-speed moving surface are described: an air cannon device (for examining surface speeds between 0 and 25 m/sec) and a spinning disk device (for examining surface speeds between 15 and 100 m/sec). The air cannon linear traverse is a pneumatic energy-powered system that is designed to accelerate a metal rail surface mounted on top of a wooden projectile. A pressurized cylinder fitted with a solenoid valve rapidly releases pressurized air into the barrel, forcing the projectile down the cannon barrel. The projectile travels beneath a spray nozzle, which impinges a liquid jet onto its metal upper surface, and the projectile then hits a stopping mechanism. A camera records the jet impingement, and a pressure transducer records the spray nozzle backpressure. The spinning disk set-up consists of a steel disk that reaches speeds of 500 to 3,000 rpm via a variable frequency drive (VFD) motor. A spray system similar to that of the air cannon generates a liquid jet that impinges onto the spinning disc, and cameras placed at several optical access points record the jet impingement. Video recordings of jet impingement processes are recorded and examined to determine whether the outcome of impingement is splash, splatter, or deposition. The apparatuses are the first that involve the high speed impingement of low-Reynolds-number liquid jets on high speed moving surfaces. In addition to its rail industry applications, the described technique may be used for technical and industrial purposes such as steelmaking and may be relevant to high-speed 3D printing.

Numerical study of molten and semi-molten ceramic impingement by using coupled Eulerian and Lagrangian method

Large temperature gradients are present within ceramic powder particles during plasma spray deposition due to their low thermal conductivity. The particles often impinge at the substrate in a semi-molten form which in turn substantially affects the final characteristics of the coating being formed. This study is dedicated to a novel modeling approach of a coupled Eulerian and Lagrangian (CEL) method for both fully molten and semi-molten droplet impingement processes. The simulation provides an insight to the deformation mechanism of the solid core YSZ and illustrates the freezing-induced break-up and spreading at the splat periphery. A 30μm fully molten YSZ particle and an 80μm semi-molten YSZ particle with different core sizes and initial velocity ranging from 100 to 240m/s were examined. The flattened degree for both cases were obtained and compared with experimental and analytical data.

A Numerical Investigation of Impinging Jet Cooling with Nanofluids

Nanofluids exhibit interesting heat transfer enhancement capabilities that may render them good candidates for use in industrial applications. They can be used as a cooling medium to spray onto the bottom of a piston to reduce the overall heat load during the operation of an engine. However, the flow and temperature fields in an impinging jet of nanofluid are rather complex in the multiphase turbulent state. In order to accurately describe the flow and heat transfer processes in an impinging jet, the current article explores several different turbulence models and wall functions, discusses the velocity and temperature fields predicted by the single-phase model and various multiphase models, and analyzes the influence of nanoparticles on the base fluid. On this basis, the effects of physical properties, concentration, Reynolds number, and geometrical configuration on the process of impingement cooling with nanofluids are investigated. The results may provide a reliable reference for the transient spray cooling of pistons with nanofluids.

Effect of surface wettability on carbon nanotube water-based nanofluid droplet impingement heat transfer

Recent studies into droplet impingement heat transfer have demonstrated that it has great potential for providing high heat flux cooling in areas such as thermal management of electronics. The wettability of the surface affects the flow dynamics of the impingement process and the resulting heat transfer. In this study, the effect of surface wettability on carbon nanotube water-based nanofluid droplet impingement heat transfer has been studied and compared with water. Superhydrophobic or hydrophilic coatings are applied on one face of monocrystalline silicon wafers (the drop impinges on this face) while the other face is painted matt black to permit infrared thermography. The silicon wafer is preheated to 40 °C and a single droplet impinges normally on the top facing coated surface of the monocrystalline silicon wafer. The inverse heat conduction problem has been solved using the measured black face temperature. For both the water and nanofluid droplets, the convective heat transfer coefficient reduces with the decrease in surface wettability. It is found that the nanofluid produce a significantly higher convective heat transfer coefficient during droplet impingement than water, with the enhancement increasing with increasing wettability.

Fabricating high-resolution offset color-filter black matrix by integrating heterostructured substrate with inkjet printing

This paper presents a self-aligning ink by integrating an inkjet printing technique and heterostructures to fabricate a black matrix with a micrometer-scale tunable thickness. The black matrix is a grid-like structure used in color filters. Traditionally, a black matrix has been fabricated using photolithography techniques, the disadvantages of which are high material consumption, less fabrication flexibility, complex processing procedures, and high chemical pollution. Inkjet printing technology has garnered attention because of its low material costs, high fabrication flexibility, and reduced processing procedures and pollution. In this study, a fabricating process combining an inkjet printing technique with heterostructures to form stripe-arranged and delta-arranged thickness-tunable black matrices has been demonstrated. The deformation and self-aligning process of ink droplet impingement onto gutters are driven by designed heterogeneous surface properties. The minimum track width attained is 10 µm, which is competitive for color filter resolutions for thin-film transistor liquid crystal displays. The developed technology surmounts the bottlenecks of inkjet printing resolution, and saves more than 75% black material than modern photolithography.

A LES Study on Passive Mixing in Supersonic Shear Layer Flows Considering Effects of Baffle Configuration

Under the background of dual combustor ramjet (DCR), a numerical investigation of supersonic mixing layer was launched, focused on the mixing enhancement method of applying baffles with different geometric configurations. Large eddy simulation with high order schemes, containing a fifth-order hybrid WENO compact scheme for the convective flux and sixth-order compact one for the viscous flux, was utilized to numerically study the development of the supersonic mixing layer. The supersonic cavity flow was simulated and the cavity configuration could influence the mixing characteristics, since the impingement process of large scale structures formed inside the cavity could raise the vorticity and promote the mixing. The effect of baffle's configurations on the mixing process was analyzed by comparing the flow properties, mixing efficiency, and total pressure loss. The baffle could induce large scale vortexes, promote the mixing layer to lose its stability easily, and then lead to the mixing efficiency enhancement. However, the baffle could increase the total pressure loss. The present investigation could provide guidance for applying new passive mixing enhancement methods for the supersonic mixing.

Stress analysis of a polyethylene acetabular component in the extreme flexion position—a finite element analysis

Background: Many factors cause impingement of the femoral neck and polyethylene liners. Impingementsdefinitely increase the rate of polyethylene wear. This effect has been a major cause of revision hip surgery.Squatting and sitting cross-legged are specific sitting positions typically used by Asian people, including Thai.These types of positions may cause impingements and abnormal stress distributions, resulting in massivedestruction of polyethylene liners. Objective: To analyze the effects of squatting and sitting in a cross-legged position in total hip replacementpatients using finite element analysis. Methods: A three-dimensional finite element model was developed to study the effects of squatting and sittingin a cross-legged position. The study was divided into two parts. First, mean hip ranges of motion of sittingpositions were analyzed. Second, hip ranges of motion at one standard deviation were also analyzed. Locationsof the impingement area and stress distribution were demonstrated using finite element software. Results: Squatting and sitting cross-legged at one standard deviation obviously cause significant impingementat the superior part of the polyethylene liner. Maximum principal stresses are 103 MPa and 24.5 MPa in squattingand sitting cross-legged, respectively. There is no impingement when the mean hip ranges of motion are used.The distance between the neck and cup are 4.05 mm and 4.15 mm in squatting and sitting cross-legged,respectively. Conclusion: Squatting and sitting cross-legged can cause significant impingement in commonly used total hipreplacements. Massive destruction of the prostheses can develop by the process of the impingement. Keywords: Finite element analysis, impingement, polyethylene, stress, total hip replacement