Search Results for spray nozzles

The rapid pace of advancement in thermal spraying is in large part due to the availability of diagnostic tools that are used to measure process variables such as particle temperature, particle trajectories, and plume geometry. The work presented here deals with the adaptation of an imaging method that can be used to visualize, quantify, and distinguish between laminar, transitional, and turbulent flow regimes. The method is based on schlieren photography and an image processing procedure that extracts the density field of a flow relative to a background image. In this study, the promising new method known as background-oriented schlieren (BOS) is used to evaluate different nozzle designs for twin-wire arc spraying. The results are presented and compared with CFD simulations, conventional schlieren images, and color photographs of the spray plume.

The present study numerically investigates the effectiveness of co-flowing nozzles for cold spray applications. A convergent-divergent axi-symmetric nozzle system was simulated with high-pressure nitrogen flow. The particle acceleration is modelled by a two-way Lagrangian approach and validated with reference to experimental values reported in the literature. An annular co-flowing nozzle with circular central nozzle was simulated for nitrogen gas flow. The momentum preservation for central nozzle flow was observed, which results in higher particle speed for longer axial distance after nozzle exit. It is envisioned from the outcome that utilization of co-flow can lead to reduction in the divergent section length of cold spray central nozzles, which may ultimately help to address clogging issues for continuous operation. Co-flow operating at 3 MPa, same as with a central nozzle, can increase supersonic core length up to 23.8%.

The advantages of the solid state deposition process Cold Spray (CS) over conventional spray technologies go hand in hand with the requirement of high and well-predictable particle velocities. The acceleration of particles primarily takes place within the CS-nozzle while measurements of their velocity are conducted downstream of its exit. Despite their essential value, these observations are limited, in that only the result of the acceleration can be evaluated, not the actual driving mechanisms themselves. Previous work has indicated that there is no conclusive understanding of these mechanisms, especially in cases of increasing particle loading. This study therefore presents a transparent rectangular CS-nozzle design (made out of quartz) for a low stagnation pressure regime. A novelty to the field of thermal spray is the first report of particle in-flight measurements within the CS-nozzle using Particle Tracking Velocimetry (PTV) at varying particle loadings and pressure levels. It is found that particle velocities in the jet decrease with increasing particulate loading as the momentum exchange of the gas is enhanced, while in the subsonic flow region, the average velocity level increases due to particle-particle interactions with shallower axial velocity profiles. This effect is aggravated for higher working pressures, as energetic collisions cause increasing losses, depending on the number density of particles. This study forms the basis for a comprehensive nozzle-internal analysis.

In this paper, a commercial AZ91D magnesium alloy powder and its mixture with 30 vol.% SiC powder were used to deposit coatings by cold spraying. Two types of converging-diverging nozzles with different cross-sectional shapes were employed. The velocity and temperature of in-flight particles under different operating conditions were simulated using the FLUENT software. The simulated results show that the particle velocity through the rectangular cross-section nozzle is the same with that through the circular one. However, the coating observation shows that the AZ91D coating and its composite could only be deposited using the rectangular cross-section nozzle. The increase of gas temperature has little effect on the coating microstructure, porosity and microhardness. Furthermore, the observation of the composite coating produced under the gas temperature of 600°C shows that the SiC content in the composite is about 23 vol.%. The microhardness of the composite is improved to about 140 HV 0.3 due to the enhancement of SiC particles, compared to that of about 100 HV 0.3 for the AZ91D coating.

In this study, a downstream injection cold spray nozzle is modeled numerically under various loadings. Instead of micron-sized particles, liquid feedstock as a carrier of nanoparticle suspension is fed into the nozzle through a port located 6 mm downstream of the nozzle throat at the diverging section. Water is used as the liquid carrier with a droplet size distribution of 5-100 µm and liquid-to-gas ratio ranging from 5 to 15%. The radial injection of droplets is simulated by Lagrangian particle tracking which includes the effects of heating and evaporation. The effect of the feedstock on downstream flow is analyzed and the optimum solid-to-liquid fraction in the suspension is determined.

The development of efficient ice mitigation systems for surfaces exposed to atmospheric ice has been in progress for decades. The need for passive anti-icing systems is essential as current ice mitigation systems require a substantial amount of energy and their implementation involves complex manufacturing considerations. Fluorinated polymer coatings are among the candidates for passive anti-icing systems. While many processes have been investigated to produce them, these methods can be costly, time consuming and can cause thermal damage to the substrate. The current work aims to explore a green and cheap alternative approach by using cold spray. Furthermore, the cold spray process offers advantages such as being a portable easy to perform solid-state coating process for eventual repairs. This work uses computational and experimental approaches to design and test a new dedicated nozzle for the efficient deposition of adhesive perfluoroalkoxy alkane. Computational results reveal that for the same operating conditions, the use of the new nozzle design increases particle impact temperature, improving the deposition of the feedstock material, as confirmed experimentally. The wetting behaviour, ice nucleation time and ice adhesion strength were compared for 6 different surface types, including bare aluminum, various polymer materials and the cold spray perfluoroalkoxy alkane coating on aluminium substrate. Results indicate that the as-sprayed coating performs as both a superhydrophobic and icephobic surface.

This study investigates the effect of water injection in the high pressure chamber of a cold spray nozzle. A de Laval nozzle geometry with constant back pressure and temperature is modeled numerically using Reynolds Stress Model coupled equations. Water spray with a droplet size of 10 – 100 μm is modeled using both uniform and Rosin-Rammler size distributions. The two-phase flow of gas-liquid is modeled using an unsteady discrete phase mass source with two-way coupling with the main gas flow. Upon injection, the droplets in the water spray evaporate while travelling through the nozzle due to momentum and energy exchange with the gas flow. The evaporation behavior in presence of water content is modeled and a correlation between the initial diameter and the diameter just before the throat is obtained. As a result, the proper droplet size distribution with a fully evaporative spray can be used as a carrier of nano-particles in cold spray nozzles. Having the results, guide us to substitute the un-evaporated part of the droplet with an equal diameter agglomerate of nano particles and find a minimum fraction of nano particles suspended in the liquid which guarantees fully evaporative liquid spray injection.

The coatings of zinc and its alloys are broadly used to prevent the rusting of substrate surfaces such as steel. Cold gas dynamic spray (CGDS) is an innovative coating technique in which the deposition of solid powder particles depends upon the kinetic energy of the particles rather than thermal energy. Therefore, application of cold spray is to provide superior rust resistance by depositing more materials, formation of passivation layer, and cathodic protection. In this study, numerical investigations on zinc micro and nano size particles in CGDS were carried out. The height of the injector, the expansion ratio and the diameter of the inlet of the de-Laval nozzle was varied systematically by optimizing the stand-off distance using the two-dimensional axisymmetric models of CGDS, to study their effects on the velocity and the distribution of the particles. Prediction of the deposition efficiency was carried out using the various critical and erosion velocity models.

This study investigated the accelerating behavior of spray particles during cold spraying (CS) by employing a computational fluid dynamics program, FLUENT. Optimization of the dimensions of CS nozzle was conducted to maximize particle velocity. The results show that the expansion ratio, divergent length, particle density and size, operating temperature significantly influence particle acceleration. It is found that the spray particles in nozzles with long divergent length can obtain a relatively higher impact velocity, but too long divergent length will reduce the particle velocity. Besides, the particle impact velocity shows a downward trend with increasing the particle size or density. Hence, the optimal divergent length should increase with the increase of particle mass. Moreover, higher gas temperature leads to a higher particle velocity, but it has no influence on the optimal divergent length.

Wear of plasma spray nozzles is a documented source of variation in the plasma spray process. Current wear detection relies upon observed gun voltage drops during operation. Operating voltage or hours of operation are rarely recorded when nozzles are changed over resulting in the loss of nozzle wear state knowledge. An offline means of determining wear state of GH type nozzles for OM 9MB plasma spray guns has been developed. By recording and analyzing the acoustic signal of a controlled gas flow through each nozzle, 28 nozzles were accurately categorized by their degree of wear. This test also allowed for the distinction between three manufacturers of new GH nozzles. Degree of wear was determined using bore profilometry and verified by voltage drop. This method allows for improved quality assurance and enhanced useful nozzle life.

Based on the principles of cold spray, Cold Gas Dynamic Manufacturing (CGDM) is a high-velocity metal spraying process capable of the high-rate deposition of dissimilar materials under cold conditions. However, unlike many cold-spraying techniques that have focussed primarily on surface coatings, the CGDM process has been tailored for solid free-form fabrication. Central to the needs of the process is the ability to accelerate the feedstock powder – through entrainment into a high-velocity gas flow – to speeds in excess of its critical deposition velocity. The critical factor here is not the gas velocity but rather the ultimate particle velocity; and it is the drag force experienced by the individual powder particles that affect their velocity. This paper documents the methods used to predict, model and validate nozzle performance – in terms of particle acceleration and ultimate velocity – via Computational Fluid Dynamics (CFD) and Particle Image Velocimetry (PIV). Results are presented and discussed pertaining to the effects of nozzle type (de Laval, MLN etc.) and geometry, as well as gas type (Helium and Nitrogen), composition and temperature. Abstract only; no full-text paper available.

A convergent-barrel (CB) cold spray nozzle was designed through numerical simulation. It was found that the main factors influencing significantly the particle velocity and temperature include the length and diameter of the barrel section, the nature of the accelerating gas and the operating gas pressure and temperature, and the particle size. Particles can achieve a relatively low velocity but a high temperature under the same gas pressure using a CB nozzle compared to a convergent-divergent (CD) nozzle. The experiment results with Cu powder using the designed CB nozzle confirmed that the deposition can be realized under a lower gas pressure with a CB nozzle.

The generation of a high velocity carrier gas flow for cold metal particle applications is addressed, with specific focus on titanium cold spraying. The high hardness of this material makes cold spraying titanium difficult to achieve by industry standard nozzles. The redesign of a commercial conical convergent-divergent cold spray nozzle is achieved by the application of aerospace design codes, based on the Method of Characteristics, towards producing a more isentropic expansion by contouring the nozzle walls. Steady three-dimensional RANS SST k-ω simulations of nitrogen are coupled two-way to particle parcel tracking in the Lagrangian frame of reference. The new contoured nozzle is found to produce higher particle velocities with greater radial spread, when operated at the same conditions/cost of operation as the commercial nozzle. These numerical results have shown the potential for extending cold spray to high density and low ductility particles by relatively minor rig modifications, through an effective synergy between gas dynamics and material science.

This study compares the performance of circular and rectangular cold spray nozzles based on numerical simulations and experimental results. It shows how nozzle geometry and gas pressure affect the velocity and temperature of copper particles at various points in their travel. The goal of the investigation is to establish a nozzle design that achieves a uniform spray pattern with suitable particle impact velocity for the materials and temperatures involved. It was found that a rectangular nozzle with an expansion ratio in the range of 11-12 can provide a more uniform particle velocity with high deposition efficiency at a reasonable gas pressure.

In the cold spray process, cross-sectional shape of the nozzle has a significant effect on spray pattern of coatings. There is a rectangular and a circular cross-sectional shapes on the cold spray nozzle. It has been reported that the rectangular nozzle provide a more uniform particle velocity across the exit of the nozzle. The circular exit nozzle is parabolic in shape. So spray pattern with the rectangular nozzle is wider than that with the circular spray nozzle. The goal of this investigation is to establish a design for the cold spray gun nozzle in order to gain more uniform spray profile of coatings. We have investigated the influence of expansion ratio, nozzle total length and the ratio of nozzle length of divergent section and parallel section of rectangular nozzle on behaviours of gas and particle by the computational fluid dynamics (CFD) in high pressure cold spraying. In this study, the spray pattern and microstructure of copper coatings with the rectangular cross-section nozzle optimized by CFD analysis in high-pressure cold spraying are reported. The optimized rectangular nozzle provide a more uniform spray profile of coatings. Relatively finer powder tended to deposit on the end of the coating.

In the cold spray process, cross-sectional shape of the nozzle has a significant effect on spray pattern of coatings. The circular exit nozzle is parabolic in shape. So, spray pattern with the rectangular nozzle is wider than that with the circular spray nozzle. The goal of this investigation is to establish a design for the cold spray gun nozzle to gain more uniform spray profile of coatings. We have investigated the influence of expansion ratio, nozzle total length and the ratio of nozzle length of divergent section and parallel section of rectangular nozzle on behaviors of gas and particle by the computational fluid dynamics (CFD) in high pressure cold spraying. We have studied copper particles so far. In this study, we will examine aluminum particles. First, we investigate the influence of the size and shape of the rectangular section nozzle on the velocity, temperature, and particle distribution of aluminum particles by CFD. After that, the rectangular section nozzles were fabricated and coating formation experiments were conducted, spray patterns and coating cross-sectional structures were observed, and coating adhesion was also evaluated. The nozzle material was polybenzimidazole resin, which is difficult for aluminum particles to attach to nozzle walls.

The goal of this work is to establish a design for a cold spray nozzle that produces a flatter spray pattern. To that end, experiments and CFD calculations are carried out to investigate the influence of nozzle geometry and expansion ratio on the behavior of copper particles and gas flows during high-pressure cold spraying. It is found that there is an optimal ratio for the lengths of the diverging and parallel sections in a rectangular nozzle in regard to particle velocity and deposition efficiency. The results are also compared with circular nozzle designs.

In our laboratory, we have developed a method to simultaneously inject different powders from the central axis direction and radial direction of the cold spray nozzle and are producing a composite coating by this method. In the previous research of our laboratory, an Al-12Si alloy coating with excellent wear resistance was produced by micro-forging assisted cold spray using the simultaneous nozzle injection method of powder in the axial and radial directions. Here, Al- 12Si alloy, which has excellent wear resistance, was used for the coating-formed particles, and stainless steel was used for the micro-forging particles. However, because the micro-forging particles were hollow, they remained in the coating. In this paper, we evaluated the influence of increasing the mixing ratio of micro-forging particles instead of solid (no holes) micro-forging particles on the coating structure. At the same time, the behaviors of particles by computational fluid dynamics are also investigated.

Cold spray nozzles can have a significant impact on particle behavior and coating quality depending on their shape and size. This study investigates the influence of nozzle geometry on the cold spray process. Simulations show that particle velocities are highest at the outlet of de Laval type nozzles and that convergence pipe nozzles achieve the highest particle temperatures. Paper includes a German-language abstract.

The clogging, a frequent gas passage deformation phenomenon because of powder accumulation on inner nozzle wall, is a major issue in long duration Cold Spray (CS) operations and a major challenge for Cold spray technology to be adopted for additive manufacturing. This study aims to design and integrate new nozzle design in Cold Spray operations for addressing the clogging issues in traditional circular convergent-divergent (CD) nozzles. The concept of the Aerospike nozzle is proposed for that purpose and is investigated using numerical simulation methods in this paper. An aerospike nozzle allows gases to accelerate externally bounded by environment on one-side and contoured spike wall on other side. After accelerating along the spike wall, aerospike nozzle can generate a longer supersonic gas stream. The spike region can be truncated near the tip to provide a flat face for powder injection. This proposed strategy will allow powder particles to accelerate through a longer supersonic core region, without interacting with nozzle wall. With appropriate operating parameters, an aerospike nozzle can reduce or eliminate the clogging issue completely. The efficiency and operation of aerospike nozzle is compared with same Mach number C-D nozzle using numerical simulations at stagnation pressure of 30 bar and temperature of 623K, where the aluminium powder particles are injected at 30 g/min in the centerline of both nozzles and are accelerated to similar velocities. The powder particles are accelerated in supersonic core region of aerospike nozzle without interacting with nozzle wall, it is concluded that the aerospike nozzle can be a promising nozzle design to provide clogging free long duration CS operations.