Water Droplet Erosion Research Articles

Water-droplet erosion behavior of high-velocity oxygen-fuel-sprayed coatings for steam turbine blades

Abstract The water-droplet erosion of low-pressure steam turbine blades under wet steam environments can alter the vibration characteristics of the blade, and lead to its premature failure. Using high-velocity oxygen-fuel (HVOF) sprayed water-droplet erosion resistant coating is beneficial in preventing the erosion failure, while the erosion behavior of such coatings is still not revealed so far. Here, we examined the water-droplet erosion resistance of Cr3C2–25NiCr and WC–10Co–4Cr HVOF sprayed coatings using a pulsed water jet device with different impingement angles. Combined with microscopic characterization, indentation, and adhesion tests, we found that: (1) both of the coatings exhibited a similar three-stage erosion behavior, from the formation of discrete erosion surface cavities and continuous grooves to the broadening and deepening of the groove, (2) the erosion rate accelerates with the increasing impingement angle of the water jet; besides, the impingement angle had a nonlinear effect on the cumulative mass loss, and 30° sample exhibited the smallest mass loss per unit area (3) an improvement in the interfacial adhesion strength, fracture toughness, and hardness of the coating enhanced the water-droplet erosion resistance. These results provide guidance pertaining to the engineering application of water erosion protective coatings on steam turbine blades.

The water droplet erosion resistance of Ni-based composite coating through laser cladding

Water droplet erosion (WDE) usually occurs in TC4 turbine blades under actual working conditions, which seriously endangers the safe and stable operation of the turbine. To solve the problem of WDE in TC4 turbine blades under high-speed solid-liquid impact, CeO2/Ni60A composite coatings with different laser powers were successfully developed on the surface of TC4 by using laser cladding technology. The working environment of the turbine blade under high- speed solid-liquid impact was simulated by water jet technique, and the effect of laser power on the coating resistance to water droplet erosion was analyzed. It can be seen from the results that the coating was composed of α-Ti, Ti2Ni, TiB2 and TiC, and laser power had a significant influence on the growth of grain structure. At 6000 W laser power, the average microhardness of the coating reached 1105 HV0.3, i.e., 3.25 times of TC4 substrate. However, too high laser power made the hard phases of TiC and TiB2 brittle. In the water droplet erosion experiment, the high power resulted in the cracks in the coating. At the laser power of 5000 W, the grain refinement effect was the best. The width and depth of water erosion were 0.908 mm and 0.148 mm, respectively, which were lower than TC4 substrate by 27.5% and 41.5% times. The preparation of CeO2/Ni60A coatings on the surface of TC4 alloy was effective to solve the problem of WDE in TC4 turbine blades and prolong the service life of TC4 blades. Moreover, the finer the grain structure in the coating, the better the WDE resistance.

Water Droplet Erosion Life Prediction Method for Steam Turbine Blade Materials Based on Image Recognition and Machine Learning

Abstract In this paper, a method for predicting remaining useful life (RUL) of turbine blade under water droplet erosion (WDE) based on image recognition and machine learning is presented. Using the experimental rig for testing the WDE characteristics of materials, the morphology pictures of specimen surface at different times in the process of WDE are collected. According to the data processing method of ASTM-G73 and the cumulative erosion-time curves, the WDE stages of materials is quantitatively divided and the WDE life coefficient (ζ) is defined. The life coefficient (ζ) could be used to calculate the RUL of turbine blades. One convolutional neural network model and three machine learning models are adopted to train and predict the image dataset. Then the training process and feature maps of the Resnet model are studied in detail. It is found that the highest prediction accuracy of the method proposed in this paper can be 0.949, which is considered acceptable to provide reference for turbine overhaul period and blade replacement time.

Evaluation of water washing efficiency and erosion risk in an axial compressor for different water injection conditions

Gas turbines performance losses are mainly due to the deposition of dirt on the compressor blades that needs to be periodically removed. This is the reason motivating the presence of water washing systems (WWS) in most of the compressor gas turbines. Water washing is generally achieved by installing a number of nozzles on the compressor casing and spraying water that clean the dirty surfaces of the compressor. The side effect of such a technique is the rising risk of erosion due to the impact of water droplets on the compressor blades which is even more pronounced when dealing with online water washing systems that is done while the unit is at normal load. The design of these systems must balance benefits and disadvantages associated to the process itself. The benefits can be measured in terms of water washing efficiency that is a quantity not uniquely defined. In previous works, the authors introduced some indices useful to evaluate the spatial cleaning coverage (the wet to the total surface) and the quantity of water mass actually impacting the dirty surfaces (the impacted to injected mass). On the other hand, water washing erosion is a complex phenomenon depending on several parameters, such as the mechanical properties of the blade material, the impact velocity and angle and the droplet diameter. For this reason, the WWS are strongly influenced by the adopted nozzles and by the injection conditions. The present paper aims at assessing water washing for six different injection conditions in the first stage of a real axial compressor. Two-phase CFD simulations are carried out with Ansys Fluent where a User Defined Function implemented by the authors is used to properly model water droplet erosion mechanism and to obtain all the quantities needed to evaluate the washing quality. Results confirm the strong influence of the injection conditions on the main features of the washing system. The study is part of an ongoing partnership between Baker Hughes and Sapienza University of Rome aiming at maximizing the washing of the compressor blades while maintaining the erosion under specific thresholds.

Experimental investigation on the water droplet erosion characteristics of blade materials for steam turbine

In this paper, the water droplet erosion ( WDE) characteristics of four blade materials under different high-speed liquid-solid impingement conditions are tested with the experimental steps and data processing methods in ASTM-G73. On the basis of cumulative erosion curves, the maximum erosion rate ( ERmax) is obtained to analyze the WDE resistance of the testing materials quantitatively. The effects of target surface roughness, impact angle and impact velocity on the WDE characteristics of blade materials under specific working conditions are studied. The velocity coefficients ( n) of the testing materials under high-speed liquid-solid impact are analyzed. In order to investigate the mechanism of WDE, the material failure characteristics and shallow layer hardness ( SLH) at three characteristic moments corresponding to different WDE periods, are investigated with the SEM morphology of erosion section and hardness distribution along the depth direction at the impact position. The research results can provide reference data and technical support for the structure design and material selection of steam turbine blades.

Results of steel 20Kh13 samples with combined hardened-structured surface erosion tests

The work presents the results of erosion tests and metallographic studies of samples from blade steel 20Kh13 with diffusion coating obtained by ion nitriding and samples from nitrided steel 20Kh13 with a relief previously formed on the surface. Studies of the samples resistance to high-speed water droplet erosion were carried out using the unique hydraulic impact rig «Erosion-M» of the NRU «MPEI». It was established that the use of nitriding and nitriding with preliminary surface structuring allows increasing the relative erosion resistance in the area with a maximum wear rate of at least 2 times, and in the area with a steady wear rate of at least 3 times. It was also found that structuring the surface layer before the nitriding process allows reducing the wear rate in the area with a maximum erosion rate compared to the nitrided surface by at least 10%.

Experimental Research on Water Droplet Erosion Resistance Characteristics of Turbine Blade Substrate and Strengthened Layers Materials.

In this paper, the water droplet erosion (WDE) performance of typical martensitic precipitation substrate 0Cr17Ni4Cu4Nb in steam turbine final stage, laser solid solution strengthened sample, laser cladding sample and brazed stellite alloy samples have been studied based on a high-speed rotating waterjet test system. The WDE resistance of several materials from strong to weak is in sequence: Brazed stellite alloy > laser cladding sample > laser solid solution sample > martensitic substrate. Furthermore, the WDE resistance mechanism and the failure mode of brazed stellite alloy have been revealed. It is found that the hard carbide in the stellite alloy is the starting point of crack formation and propagation. Under the continuous droplet impact, cracks grow and connect into networks, resulting in the removal of carbide precipitates and WDE damage. It is proved that the properties of the Co-based material itself is the reason for its excellent WDE resistance. And the carbides have almost no positive contribution to its anti-erodibility. These new findings are of great significance to process methods and parameter selection of steam turbine blade materials and surface strengthened layers.

Experimental investigation on effect of surface strengthening process and roughness on water droplet erosion behavior in turbomachinery

In this paper, the water droplet erosion (WDE) performance of a typical martensitic blade material 17-4 PH (0Cr17Ni4Cu4Nb), surface strengthened laser cladding stellite and brazed stellite alloy is studied based on high-speed water jet test system. The WDE resistances from high to low are the brazed stellite alloy, laser cladding stellite alloy and 17-4 PH. For the same material, the greater the initial roughness of the target surface, the more severe is the WDE damage. Improving the surface finish of the target is conducive to the diversion and expansion of the liquid flow, prolonging the incubation period, and thus enhancing the WDE resistance.

Cavitation erosion performance of CVD W/WC coatings

In the aviation industry, water droplet erosion (WDE) takes place when an aircraft takes off on as wet runway or flies through rain or clouds. The leading edge of turbofan blades suffers from high-speed (300–400 m/s) impingements of water droplets, resulting in material removal that subsequently changes the leading edge profile and surface roughness. This affects the aerodynamic performance of turbofan blades, which eventually leads to efficiency drop of the aircraft engine and the need to replace and/or recondition the blades. A coating solution is targeted, such that, not only that it resists the high impact pressure but also inhibits stress wave reinforcements at the coating-substrate or interlayers interfaces. Past studies indicate similar damage mechanisms to WDE are generated by cavitation erosion (CE) during the early stages (incubation). Hence, CE is introduced in this study to predict the WDE performance. The coatings studied were nanostructured CVD tungsten/tungsten carbide coatings, either hierarchical or monotonic in design, on Ti6Al4V alloy grade 5 substrates. In-depth understanding on the coating damage mechanisms are established by correlating the coating performance with microstructure, crystallographic texture, interface design, coating deposition conditions and mechanical properties for the first time. A particular crystalline texture was found that gives optimum performance. The effect of the initial coating topography on the CE performance is effectively characterised by the surface parameters Ssk and Sku. The damage was found to initiate at the grain boundaries of the exposed surfaces. The hierarchical coating microstructure demonstrated enhancement in CE performance compared to a monotonic columnar grain structure. Additionally, it is found that the coating performance under dynamic compressive loadings could not be predicted by a simple H/E approaches. However, combining the H/E ratios with the factors of microstructure and crystal orientations might further facilitate the understanding of coating performances along with better understanding of the role of compressive residual stresses and stress waves propagation or reinforcement through coating depth and at the top surface of the coating.

Water Droplet Erosion of Wind Turbine Blades: Mechanics, Testing, Modeling and Future Perspectives

The problem of erosion due to water droplet impact has been a major concern for several industries for a very long time and it keeps reinventing itself wherever a component rotates or moves at high speed in a hydrometer environment. Recently, and as larger wind turbine blades are used, erosion of the leading edge due to rain droplets impact has become a serious issue. Leading-edge erosion causes a significant loss in aerodynamics efficiency of turbine blades leading to a considerable reduction in annual energy production. This paper reviews the topic of water droplet impact erosion as it emerges in wind turbine blades. A brief background on water droplet erosion and its industrial applications is first presented. Leading-edge erosion of wind turbine is briefly described in terms of materials involved and erosion conditions encountered in the blade. Emphases are then placed on the status quo of understanding the mechanics of water droplet erosion, experimental testing, and erosion prediction models. The main conclusions of this review are as follow. So far, experimental testing efforts have led to establishing a useful but incomplete understanding of the water droplet erosion phenomenon, the effect of different erosion parameters, and a general ranking of materials based on their ability to resist erosion. Techniques for experimentally measuring an objective erosion resistance (or erosion strength) of materials have, however, not yet been developed. In terms of modelling, speculations about the physical processes underlying water droplet erosion and consequently treating the problem from first principles have never reached a state of maturity. Efforts have, therefore, focused on formulating erosion prediction equations depending on a statistical analysis of large erosion tests data and often with a combination of presumed erosion mechanisms such as fatigue. Such prediction models have not reached the stage of generalization. Experimental testing and erosion prediction efforts need to be improved such that a coherent water droplet erosion theory can be established. The need for standardized testing and data representation practices as well as correlations between test data and real in-service erosion also remains urgent.

Modeling of Water Droplets Erosion on a Subsonic Compressor Cascade

In this paper an analysis of the erosion mechanism of water droplets impacting a compressor blade is performed. Three different models are compared: the Springer et al. model, developed in 1974 for predicting rain erosion on wind turbine blades; a new model developed by the authors to predict water droplet erosion on stainless steel surfaces and Tabakoff et al. (1979) model for solid particle erosion. This last model was included in the comparison because in commercial CFD codes a specific model for water erosion is commonly not implemented, and one may be tempted to consider this phenomenon similar to that of solid particle erosion. 50 μm droplets/particles are injected at the inlet of a compressor blade, and erosion predictions are then compared. The results show that while all the tested models provide correct information about the impact sites and accumulated energy, they differ greatly when talking about eroded material and only the new model here proposed returns result in agreement with the industrial evidences. In such kind of application, Springer et al. and Tabakoff et al. models can be only used for qualitative considerations, but their quantitative predictions are quite unrealistic, greatly overpredicting the eroded thickness.

Experimental study on water droplet erosion resistance of coatings (Ni60 and WC-17Co) sprayed by APS and HVOF

In order to simulate the actual operating environment of the last stage blades of steam turbines，especially under the condition of high-speed liquid-solid impact (over 600 m/s), a new experimental rig for testing the water droplet erosion (WDE) characteristics is built, and three kinds of specimen structure adapted to different impact angles are designed. The WDE resistance of the four kinds of thermal spraying coating, which are prepared using atmospheric plasma spraying (APS) and high velocity oxygen fuel spraying (HVOF) with two kinds of alloy powder (Ni60 and WC-17Co) on the substrate material (17-4PH stainless steel), are tested using the experimental rig. In this paper, three-dimensional optical microscopy is used to obtain the morphology of WDE grooves on the target surface of different coating specimens. During the measurement process, three characteristic sections are selected, and the average profile of erosion grooves at different times are given respectively. The average area loss of the erosion section is obtained with integrating method. The cumulative loss curves are analyzed and the WDE resistance of the present coatings are compared. It is found that the HVOF-WC17Co coating is of the best WDE resistance.

An Experimentally Derived Model to Predict the Water Film in a Compressor Cascade With Droplet Laden Flow

In recent years, overspray fogging has become a powerful means for power augmentation of industrial gas turbines. Despite the positive thermodynamic effect on the cycle, droplets entering the compressor increase the risk of water droplet erosion and deposition of water on the blades leading to an increase of required torque and profile loss. Due to this, detailed information about the structure and the amount of water on the surface is key for compressor performance. Experiments were conducted with a droplet laden flow in a transonic compressor cascade focusing on the film formed by the deposited water. Two approaches were taken. In the first approach, the film thickness on the blade was directly measured using white light interferometry. Due to significant distortion of the flow caused by the measurement system, a transfer of the measured film thickness to the undisturbed case is not possible. Therefore, a film model is adapted to describe the film flow in terms of height averaged film parameters. In the second approach, experiments were conducted in an undisturbed cascade setup and the water film pattern was measured using a nonintrusive quantitative image processing tool. Utilizing the measured flow pattern in combination with findings from the literature, the rivulet flow structure is resolved. From continuity of the water flow, a film thickness is derived showing good agreement with the previously calculated results. Using both approaches, a three-dimensional (3D) reconstruction of the water film pattern is created giving first experimental results of the film forming on stationary compressor blades under overspray fogging conditions.

The results of water droplet erosion tests of ion-plasma coatings formed on titanium Ti-6Al-4V alloy samples manufactured by using 3D-printing and traditional technological process

The paper presents the results of erosion tests and metallographic studies of titanium alloy samples manufactured by using 3D printing and traditional technological methods without coatings and with various types of ion-plasma coatings. As a result of the tests, the kinetic curves of erosion wear of the studied alloys and coatings were obtained, showing that the best type among those considered is a DLC (Diamond Like Carbon) coating, which increases the relative erosion resistance of uncoated samples in incubation period duration of the water droplet erosion process by not less than in 1.8 times.

Cavitation erosion performance of CrAlYN/CrN nanoscale multilayer coatings deposited on Ti6Al4V by HIPIMS

Water droplet erosion (WDE) protection of Ti6Al4V turbofan blades is of paramount importance to the aviation industry. A novel CrAlYN/CrN nanoscale multilayer coating deposited by the HIPIMS technique was evaluated as a potential candidate for this application. Literature suggests a strong correlation in performance ranking under WDE and cavitation erosion (CE) tests. Hence, the WDE performance of the CrAlYN/CrN coating on Ti6Al4V was investigated with an ultrasonic cavitation device.The results show excellent adhesion and superior erosion resistance of the CrAlYN/CrN coating (erosion rate lower by a factor of 14 compared to the bare Ti6Al4V substrate) and compared to coatings reported in the literature with spallation as their main erosion mechanism. Cross-sectional FIB studies revealed formation of substrate cracks underneath the coating when CE generated stresses exceeded the fatigue strength of the Ti6Al4V alloy. The interfaces of the nanoscale multilayers protected the substrate by forming an effective barrier against shock waves, delayed fatigue crack formation, deflected and arrested any cracks formed impeding the overall coating damage. The research shows that the CE resistance is influenced by the coating's texture and elastic properties, (Young's modulus).The paper discusses the erosion mechanisms of the coating and the excellent CE protection it offers.

The prospects of using textured relief on the modified blade steel surface as a passive protection against water droplet erosion

The paper proposes a concept for solving the problem of increasing the service life of the critical elements of the turbine plants flow section by creating a protective structured relief using pulse-laser surface treatment in combination with a protective ion-plasma coating or modification (diffusion saturation with elements of nitrogen, chromium, etc.) surfaces formed before and/or after the appearance of the relief. The prospects of application are shown and various variants of the proposed methods of protection are proposed.

Metallographic studies results of 20kH13 steel samples with textured relief, modified surface and protective coating

One of the possible ways to improve passive methods of the steam turbine blade material protection from water droplet erosion is to create “smart”-surface that combines the properties and characteristics that could not be implemented simultaneously when using one or another proven method of protection. To solve the problem of developing such surface, this article presents the metallographic researches results of the blade steel 20kH13 samples with various already used and promising passive methods of protection, consisting in the use of ion-plasma and polyurethane coatings, surface modification by means of diffusion saturation with nitrogen ions, creation of the structured surface relief by laser ablation. The results of the carried out researches have allowed to reveal both morphology of a possible “smart”-surface, and influence on a microstructure and characteristics of a near-surface layer of samples after treatments considered.

Anti-icing performance and durability of suspension plasma sprayed TiO2 coatings

Superhydrophobic coatings are a potential solution for mitigating the in-flight icing problem for aircraft. However, to develop a superhydrophobic coating which can be practically used for aircraft and that possesses sufficient durability is an ongoing challenge. In this work, superhydrophobic coatings are developed using suspension plasma spraying (SPS) as a flexible, versatile and scalable coating technique. The anti-icing and deicing performances of these SPS coatings are studied in icing wind tunnel experiments. Furthermore, the durability of these SPS coatings is tested in dry particle and cloud-sized water droplet erosion and icing/deicing cyclic tests. The capability of SPS superhydrophobic coatings to reduce ice accretion is comparable to that of commercial superhydrophobic coatings but perform better in deicing tests using heating. Additionally, compared to commercial superhydrophobic coatings, the SPS coatings demonstrate significantly better performance in dry particle and icing/deicing cyclic tests while showing comparable durability in cloud-sized water droplet erosion tests. It is also shown that in case of high intensity water erosion, when the superhydrophobicity of SPS coatings deteriorates, it can be restored using a simple and quick retreatment process due to the robustness of the hierarchical micro-textured TiO2 base coatings.

Importance of Surface Curvature in Modeling Droplet Impingement on Fan Blades

When modeling a droplet impingement, it is reasonable to assume a surface is flat when the radius of curvature of the surface is significantly larger than the droplet radius. In other contexts where water droplet erosion (WDE) has been investigated, the typical droplet size has either been sufficiently small, or the radius of curvature of the surface sufficiently large, that it has been sensible to make this assumption. The equations describing the kinematics of an impinging water droplet on a flat surface were reformulated for a curved surface. The results suggest the relatively similar radii of curvature, of the leading-edge of a fan blade and the impinging water droplet, will significantly affect the application of the initial high-pressures, along with the onset of lateral outflow jetting. Jetting is predicted to commence substantially sooner and not in unison along the contact periphery, leading to an asymmetric flow stage. This is likely to have significant implications for the WDE that occurs, and thus, the engineering approaches to minimize the WDE of fan blades.

Comparative erosion tests results of titanium alloy Ti-6Al-4V samples obtained by using 3D - printing technology and manufactured by the traditional technological method

Rotating blades of steam turbines and compressors operate in conditions of various damaging factors, including shock-dynamic impacts of liquid droplets contained in steam or in air. One of the most promising ways to reduce damage from water droplet erosion can be the application of the 3D - printing technology that will allow to replace damaged parts of structural elements, as well as cut repair costs and prolonging the service life of the equipment.The aim of present work was to determine the possibility of using the 3D - printing technology of the turbine blades for use in power engineering. The main task of the work was to check samples of the material obtained by this method for water droplet erosion resistance, which is one of the regulated tests type at the turbine blades design stage.This paper presented the results of water droplet erosion process experimental studies of the titanium alloy Ti-6Al-4V samples prepared by the method 3D - printing and by the traditional technological process at a collision velocity Cimp = 300 m/s with a monodisperse flow of liquid droplets with a diameter dd = 1000 μm. Erosion tests were carried out by using a set of research equipment URI «Hydroshock rig «Erosion-M» NRU «MPEI». It has been established that the erosion resistance for the incubation period duration of the Ti-6Al-4V titanium alloy samples manufactured by the 3D - printing method is 1.4 times less than for the samples manufactured by the traditional technological process. The data obtained can be used in the future in the development of complex passive methods for protection steam turbines and compressors last stages rotating blades: creation of replaceable erosion-resistant inserts by 3D - printing with a protective coating.