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Design and Characterization of Additively Manufactured NGVs Operated in a Small Industrial Gas Turbine
Abstract
The use of 3D-printing methods, for example Selective Laser Melting (SLM), is poised to spark a revolution in the way high-temperature components for gas turbines are designed. SLM enables engineers to use cooling configurations that were hitherto either too costly or downright impossible to manufacture. Yet, although the potential SLM carries can hardly be overstated, a number of grave uncertainties remain. These lie mainly with the materials sciences, but some questions with regard to manufacturing and operating SLM-parts for the harsh environments encountered by hot gas path components and the associated demands on the tolerances of the cooling features remain as well. Because of the uncertainties mentioned above, the risk associated with the application of SLM to high-temperature parts in general and rotating parts in particular still is very high. Therefore, in a first step, MAN decided to use SLM to manufacture Nozzle Guide Vanes (NGV’s) with a geometry that would normally be investment-cast and perform a back-to-back comparison of vanes from the two manufacturing processes. This procedure also provides valuable input for future “Design for Additive Manufacturing”, which will probably lead to changes even in traditional features like pin-fin matrices. The vanes were made from MAR M-509, the inserts from Inconel 718. The SLM-manufacturing process of these NGV’s including the inserts will be described. In particular the use of MAR M-509, which is the default material of the NGV’s but seldom used for SLM, will be discussed in detail. The NGV’s were subsequently operated for approximately 100 hours at high part-load and full-load conditions in a highly-instrumented test engine of the MGT6000-1s type on MAN’s test bed at its Oberhausen plant. The temperatures on the hot gas path walls of the additively manufactured (AM) and investment-cast NGV’s were measured using Thermal History Paints (THP’s). A comparison between the two sets of vanes will be made. In addition, the NGV’s were characterized with 3D-scans of the outer geometry before and after operation. The pin-fin matrix of the SLM-vanes was also scanned. The material of the NGV’s manufactured by SLM was inspected before and after operation. The results of all of these investigations and a comparison between the additively manufactured and the investment-cast NGV’s will be presented.
The requirement for reduced emissions and the growing demand on gas turbine efficiency are in part met through increasing firing temperatures. However, development budgets leave only limited time for dedicated thermal testing. Consequently, manufacturers are seeking novel temperature measurement technologies to validate new engine designs. This paper will demonstrate how a new temperature mapping technology can be utilized for non-dedicated (multi-cycling) testing while still delivering high-resolution temperature data in a non-dedicated test on a combustor of an industrial gas turbine.
Typically, thermocouples are used to monitor the temperature during tests, but they only provide one data point. Colour changing thermal paints are used to deliver measurements over complete surfaces, but they require dedicated testing with short-duration exposure, necessitating dismantling and re-assembling the engine for further testing.
Thermal History Coatings (THC) present an alternative solution to providing high-density temperature information. This coating permanently changes consistent with the maximum temperature of exposure during test. A laser-based instrumentation technique is then used to obtain temperatures. The maximum temperature profile of the surface can be determined through a customized calibration. Given the complex cooling system of a combustor, the high temperatures and the long-time exposure, this case offers a unique possibility for the testing of the coating under real engine conditions. The coated region covered the external surface of the can. Highly significant is the number of measurement points in excess of 7,000 (2 × 2 mm resolution, which enables advanced analysis. This provides insight into the impact of local features, e.g. the region adjacent to a cooling hole.
The temperature profile is compared to a CFD-CHT model and thermocouple measurements for the calibration of cooling pre-design methods.
In the near future, when designing and using Double Wall Airfoils, which will be manufactured by 3D printers, the positional relationship between the impingement cooling nozzle and the heat transfer enhancement ribs on the target plate naturally becomes more accurate. Taking these circumstances into account, an experimental study was conducted to enhance the heat transfer of the wall jet region of a round impingement jet cooling system. This was done by installing circular ribs or vortex generators (VGs) in the impingement cooling wall jet region. The local heat transfer coefficient was measured using the naphthalene sublimation method, which utilizes the analogy between heat and mass transfer. As a result, it was clarified that, within the ranges of geometries and Reynolds numbers at which the experiments were conducted, it is possible to improve the averaged Nusselt number Nu up to 21% for circular ribs and up to 51% for VGs.
3D printing results in anisotropy in the microstructure and mechanical properties. The focus of this study is to investigate the structure, texture and phase evolution of the as-printed and heat treated IN718 superalloy. Cylindrical specimens, printed by powder-bed additive manufacturing technique, were subjected to two post-treatments: homogenization (1100 °C, 1 h, furnace cooling) and hot isostatic pressing (HIP) (1160 °C, 100 MPa, 4 h, furnace cooling). The Selective laser melting (SLM) printed microstructure exhibited a columnar architecture, parallel to the building direction, due to the heat flow towards negative z-direction. Whereas, a unique structural morphology was observed in the x-y plane due to different cooling rates resulting from laser beam overlapping. Post-processing treatments reorganized the columnar structure of a strong {002} texture into fine columnar and/or equiaxed grains of random orientations. Equiaxed structure of about 150 µm average grain size, was achieved after homogenization and HIP treatments. Both δ-phase and MC-type brittle carbides, having rough morphologies, were formed at the grain boundaries. Delta-phase formed due to γ″-phase dissolution in the γ matrix, while MC-type carbides nucleates grew by diffusion of solute atoms. The presence of (Nb0.78Ti0.22)C carbide phase, with an fcc structure having a lattice parameter a = 4.43 Å, was revealed using Energy dispersive spectrometer (EDS) and X-ray diffractometer (XRD) analysis. The solidification behavior of IN718 alloy was described to elucidate the evolution of different phases during selective laser melting and post-processing heat treatments of IN718.
Turbine cooling is a prime application for additive manufacturing because it enables quick development and implementation of innovative designs optimized for efficient heat removal, especially at the micro-scale. At the micro-scale, however, the surface finish plays a significant role in the heat transfer and pressure loss of any cooling design. Previous research on additively manufactured cooling channels has shown the surface roughness increases both heat transfer and pressure loss to similar levels as highly-engineered turbine cooling schemes. What has not been shown, however, is whether opportunities exist to tailor additively manufactured surfaces through control of the process parameters to further enhance the desired heat transfer and pressure loss characteristics.
The results presented in this paper uniquely show the potential of manipulating the parameters within the additive manufacturing process to control the surface morphology, directly influencing turbine cooling. To determine the effect of parameters on cooling performance, coupons were additively manufactured for common internal and external cooling methods using different laser powers, scan speeds, and scanning strategies. Internal and external cooling tests were performed at engine relevant conditions to measure appropriate metrics of performance. Results showed the process parameters have a significant impact on the surface morphology leading to differences in cooling performance. Specifically, internal and external cooling geometries react differently to changes in parameters, highlighting the opportunity to consider process parameters when implementing additive manufacturing for turbine cooling applications.
Additive manufacturing (AM), also known as 3D printing, is a rapidly developing technology with tremendous potential in both developmental and production applications. Solar Turbines Incorporated is committed to AM technology for gas turbine applications. The ability to metal 3D print novel designs of turbine blades capable of actual turbine engine operation would effectively reduce design validation cycle time, and allow acquisition of key performance data early in a design campaign. In support of Solar’s advanced manufacturing development and ongoing engine efficiency improvement goals, Solar initiated a project to print a full set of Mercury™ 50 stage 2 turbine blades to be run in a development engine.
Solar leveraged years of experience with design and serial production of AM components in support of this project. A significant challenge faced when printing turbine blades is producing metal with mechanical properties sufficient to withstand the rigors of engine operation. As a rotating component within the hot section of the engine, turbine blades experience high centrifugal and pressure loads at elevated temperatures. After investigation of possible alloys capable of meeting the requirements of the Mercury™ 50 design envelope, the gamma prime (γ’) strengthened nickel superalloy Inconel™ 738LC was selected to provide the best opportunity for successful development engine testing.
Solar partnered with Oak Ridge National Laboratory (ORNL) to produce the Inconel™ 738LC blades with Electron Beam Melting (EBM) powder bed fusion process. Once a rough blade shape was printed, the fir-tree attachment, blade tip shroud, and air flow path surfaces were finished using both conventional and non-conventional machining processes. In-process monitoring, metallurgical evaluations, mechanical testing, and non-destructive inspection techniques were used to validate the printed blade material integrity and conformance to geometric design intent. Planned future activities include assembly of the AM blades onto a disk for spin pit testing to validate the mechanical integrity and design margin of the blades. The final phase of the project will be to install the bladed disk assembly into a Mercury™ 50 engine at Solar Turbines to conduct a series of hot-fired engine performance tests.
This study is concerned with the film cooling effectiveness of the flow issuing from the gap between the NGV and the transition duct on the NGV endwall, i.e. the purge slot. Different slot widths, positions and injection angles were examined in order to represent changes due to thermal expansion as well as design modifications. Apart from these geometric variations, different blowing ratios (BR) and density ratios (DR) were realized to investigate the effects of the interaction between secondary flow and film cooling effectiveness. The experimental tests were performed in a linear scale-1 cascade equipped with four highly loaded turbine vanes at the Institute of Fluid Mechanics and Fluid Machinery of the University of Kaiserslautern. The mainstream flow parameters were, with a Reynolds number of 300,000 and a Mach number (outlet) of 0.6, set to meet real engine conditions. By using various flow conditioners, periodic flow was obtained in the region of interest (ROI). The adiabatic film cooling effectiveness was determined by using the Pressure Sensitive Paint (PSP) technique. In this context, nitrogen and carbon dioxide were used as tracer gases realizing two different density ratios DR = 1.0 and 1.6. The investigation was conducted for a broad range of blowing ratios with 0.25 ≤ BR ≤ 1.50. In combination with 10 geometry variations and the aforementioned blowing and density ratio variations 100 single operating points were investigated. For a better understanding of the coolant distribution, the secondary flows on the endwall were visualized by oil dye. The measurement results will be discussed based on the areal distribution of film cooling effectiveness, its lateral spanwise as well as its area average. The results will provide a better insight into various parametric effects of gap variations on turbine vane endwall film cooling performance — notably under realistic engine conditions.
A double-wall cooling scheme combined with effusion cooling offers a practical approximation to transpiration cooling which in turn presents the potential for very high cooling effectiveness. The use of the conventional conjugate CFD for the double-wall blade can be computationally expensive and this approach is therefore less than ideal in cases where only the preliminary results are required.
This paper presents a computationally efficient numerical approach for analysing a double-wall effusion cooled gas turbine blade. An existing correlation from the literature was modified and used to represent the two-dimensional distribution of film cooling effectiveness. The internal heat transfer coefficient was calculated from a validated conjugate analysis of a wall element representing an element of the aerofoil wall and the conduction through the blade solved using a finite element code in ANSYS*.
The numerical procedure developed has permitted a rapid evaluation of the critical parameters including; film cooling effectiveness, blade temperature distribution (and hence metal effectiveness) as well as coolant mass flow consumption. Good agreement was found between the results from this study and that from literature. This paper shows that a straightforward numerical approach that combines an existing correlation for film cooling from the literature with a conjugate analysis of a small wall element can be used to quickly predict the blade temperature distribution and other crucial blade performance parameters.
Innovative designs for turbines can be achieved by advances in nickel-based superalloys and manufacturing methods, including the adoption of additive manufacturing. In this regard, selective electron beam melting (SEBM) and selective laser melting (SLM) of nickel-based superalloys do provide distinct advantages. Furthermore, the direct energy deposition (DED) processes can be used for repair and reclamation of nickel alloy components. The current paper explores opportunities for innovation and qualification challenges with respect to deployment of AM as a disruptive manufacturing technology. In the first part of the paper, fundamental correlations of processing parameters to defect tendency and microstructure evolution will be explored using DED process. In the second part of the paper, opportunities for innovation in terms of site-specific control of microstructure during processing will be discussed. In the third part of the paper, challenges in qualification of AM parts for service will be discussed and potential methods to alleviate these issues through in situ process monitoring, and big data analytics are proposed.
Selective laser melting (SLM) is an emerging metal additive manufacturing (AM) technology. It has been employed in many applications including propulsion components which are made of nickel-based superalloys, such as Inconel 718. In this study, the effect of the build height on the mechanical properties and microstructure of SLM-processed Inconel 718 parts was investigated. The samples were cut from the Inconel 718 build part after stress relief. They were prepared for microstructure observations and nanoindentation testing. The results are summarized as following. Young's modulus and hardness obtained from the nanoindentation tests are comparable with or superior to that from traditional manufacturing methods. In addition, there do not appear significant differences in mechanical properties along the build height of the parts, though the columnar grains of the side-surface specimens are narrower at the bottom layers of the part. Moreover, the texture analysis results imply no significant anisotropic characteristics for the mechanical properties between the scanning surface and the side surface of the build parts.
The demand for higher efficiency is ever-present in the gas turbine field and can be achieved through many different approaches. While additively manufactured parts have only recently been introduced into the hot section of a gas turbine engine, the manufacturing technology shows promise for more widespread implementation since the process allows a designer to push the limits on capabilities of traditional machining and potentially impact turbine efficiencies.
Pin fins are conventionally used in turbine airfoils to remove heat from locations in which high thermal and mechanical stresses are present. This study employs the benefits of additive manufacturing to make uniquely shaped pin fins, with the goal of increased performance over conventional cylindrical pin fin arrays. Triangular, star, and spherical shaped pin fins placed in microchannel test coupons were manufactured using Direct Metal Laser Sintering. These coupons were experimentally investigated for pressure loss and heat transfer at a range of Reynolds numbers. Spacing, number of pin fins in the array, and pin fin geometry were variables that changed pressure loss and heat transfer in this study. Results indicate that the additively manufactured triangles and cylinders outperform conventional pin fin arrays, while stars and dimpled spheres did not.
Turbine cooling is a battle between the desire for greater hot section component life and the techno-economic demands of the marketplace. Surprisingly little separates the haves from the have nots. The evolution of turbine cooling is loosely analogous to that of the Darwinian theory of evolution for animals, starting from highly simplistic forms and progressing to increasingly more complex designs having greater capabilities. Yet even with the several generations of design advances, limitations are becoming apparent as complexity sometimes leads to less robust outcomes in operation. Furthermore, the changing environment for operation and servicing of cooled components, both the natural and the imposed environments, are resulting in new failure modes, higher sensitivities, and more variability in life. The present paper treats the evolution of turbine cooling in three broad aspects including the background development, the current state-of-the-art, and the prospects for the future. Unlike the Darwinian theory of evolution however, it is not feasible to implement thousands of small incremental design changes, random or not, to determine the fittest for survival and advancement. Instead, innovation and experience are utilized to direct the evolution.
Over the last approximately 50 years, advances have led to an overall increase in component cooling effectiveness from 0.1 to 0.7. Innovation and invention aside, the performance of the engine has always dictated which technologies advance and which do not. Cooling technologies have been aided by complimentary and substantial advancements in materials and manufacturing. The state-of-the-art now contains dozens of internal component cooling methods with their many variations, yet still relies mainly on only a handful of basic film cooling forms that have been known for 40 years. Even so, large decreases in coolant usage, up to 50%, have been realized over time in the face of increasing turbine firing temperatures. The primary areas of greatest impact for the future of turbine cooling are discussed, these being new engine operating environments, component and systems integration effects, revolutionary turbine cooling, revolutionary manufacturing, and the quantification of unknowns. One key will be the marriage of design and manufacturing to bring about the concurrent use of engineered micro cooling or transpiration, with the ability of additive manufacturing. If successful, this combination could see a further 50% reduction in coolant usage for turbines. The other key element concerns the quantification of unknowns, which directly impacts validation and verification of current state-of-the-art and future turbine cooling. Addressing the entire scope of the challenges will require future turbine cooling to be of robust simplicity and stability, with freeform design, much as observed in the “designs” of nature.
The ever-increasing requirements on gas turbine efficiency and the simultaneous demand for reduced emissions, necessitate much more accurate calculations of the combustion process and combustor wall temperatures. Thermal History Paints (THPs) is an innovative alternative to established measurement techniques, but so far only a limited number of tests has been conducted under real engine conditions. A typical THP comprises of oxide ceramic pigments and a water based binder. The ceramic is synthesized to be amorphous and when heated it crystallizes, permanently changing the microstructure. The ceramic is doped with lanthanide ions to make it phosphorescent and as the structure of the material changes, so do the phosphorescent properties of the material. By measuring the phosphorescence the maximum temperature of exposure can be determined, enabling post operation measurements at ambient conditions. This paper describes a test in which THP was applied to an impingement-cooled front panel from a combustor of an industrial gas turbine. The panel was instrumented with a thermocouple and thermal paint was applied to the cold side of the impingement plate. THP was applied to the hot-gas side of this plate for validation against the other measurement techniques and to evaluate its resilience against the reacting hot gas environment. The durability and temperature results of the three different measurement techniques are discussed. It is shown that the THP exhibited greater durability compared to the conventional thermal paint. Furthermore, the new technology provided detailed measurements indicating local temperature variations and global variations over the complete component.
Additive manufacturing (AM) with metal powder has made possible the fabrication of gas turbine components with small and complex flow paths that cannot be achieved with any other manufacturing technology presently available. The increased design space of AM allows turbine designers to develop advanced cooling schemes in high-temperature components to increase cooling efficiency. Inherent in AM with metals is the large surface roughness that cannot be removed from small internal geometries. Such roughness has been shown in previous studies to significantly augment pressure loss and heat transfer of small channels. However, the roughness on these channels or other surfaces made from AM with metal powder has not been thoroughly characterized for scaling pressure loss and heat transfer data. This study examines the roughness of the surfaces of channels of various hydraulic length scales made with direct metal laser sintering (DMLS). Statistical roughness parameters are presented along with other parameters that others have found to correlate with flow and heat transfer. The pressure loss and heat transfer previously reported for the DMLS channels studied in this work are compared to the physical roughness measurements. Results show that the relative arithmetic mean roughness correlates well with the relative equivalent sand grain roughness. A correlation is presented to predict the Nusselt number of flow through AM channels, which gives better predictions of heat transfer than correlations currently available.
A generic impingement cooling system for turbomachinery application is modeled experimentally and numerically to investigate heat transfer and pressure loss characteristics. The experimental setup consists of an array of 9 by 9 jets impinging on a target plate with cubic micro pin fins. The cubic micro pin fins have an edge length of 0.22 D and enlarge the target area by 150%. Experimentally heat transfer is measured by the transient liquid crystal (TLC) method. The transient method used requires a heated jet impinging on a cold target plate. As reference temperature for the heat transfer coefficient we use the total jet inlet temperature which is measured via thermocouples in the jet center. The CFD model was realized within the software package ANSYS CFX. This model uses a steady state - 3D - RANS approach and the shear stress transport (SST) turbulence model. Boundary conditions are chosen to mimic the experiments as close as possible. The effects of different jet-to-plate spacing (H/D = 3–5), crossflow schemes and jet Reynolds number (15,000–35,000) are investigated experimentally and numerically. The results include local Nusselt numbers as well as area and line averaged values. Numerical simulations allow a detailed insight into the fluid mechanics of the problem and complement experimental measurements. A good overall agreement of experimental and numerical behavior for all investigated cases could be reached. Depending on the crossflow scheme the cubic micro pin fin setup increases the heat flux to about 134%–142% compared to a flat target plate. At the same time the Nusselt number slightly decreases. The micro pin fins increase the pressure loss by not more than 14%. The results show that the numerical model predicts the heat transfer characteristics of the cubic micro pin fins in a satisfactory way.
Selective laser melting, a quite new layer-wise manufacturing process for metals, is used for processing the nickel-based superalloy IN718. The objective of this work is to compare the microstructure and the mechanical properties of the produced specimens, directly after the manufacturing process and additionally after two diverse heat treatments subsequent to the manufacturing process. As the resulting microstructure and properties for specimens manufactured by selective laser melting are directional, all investigations are made for specimens oriented vertically and horizontally. Optical, scanning, and transmission electron microscopy are carried out in order to characterize the microstructure explicitly. For investigating the texture of the material, additional EBSD measurements are undertaken. Mechanical tests include tensile testing at room temperature and at elevated temperatures and hardness measurements. The investigations reveal a very good quality of the SLM-produced specimens. Nonetheless, differences in the grain sizes, the orientation, and especially in the precipitation behavior could be found.
Temperature profiling of components in gas turbines is of increasing importance as engineers drive to increase firing temperatures and optimize component’s cooling requirements in order to increase efficiency and lower CO2 emissions. However, on-line temperature measurements and, particularly, temperature profiling are difficult, sometimes impossible, to perform due to inaccessibility of the components. A desirable alternative would be to record the exposure temperature in such a way that it can be determined later, off-line. The commercially available thermal paints are toxic in nature and come with a range of technical disadvantages such as subjective readout and limited durability. This paper proposes a novel alternative measurement technique which the authors call thermal history paints and thermal history coatings. These can be particularly useful in the design process, but further could provide benefits in the maintenance area where hotspots which occurred during operation can be detected during maintenance intervals when the engine is at ambient temperature. This novel temperature profiling technique uses optical active ions in a ceramic host material. When these ions are excited by light they start to phosphoresce. The host material undergoes irreversible changes when exposed to elevated temperatures and since these changes are on the atomic level they influence the phosphorescent properties such as the life time decay of the phosphorescence. The changes in phosphorescence can be related to temperature through calibration such that in situ analysis will return the temperature experienced by the coating. A major benefit of this technique is in the automated interpretation of the coatings. An electronic instrument is used to measure the phosphorescence signal eliminating the need for a specialist interpreter, and thus increasing readout speed. This paper reviews results from temperature measurements made with a water-based paint for the temperature range 100–800 °C in controlled conditions. Repeatability of the tests and errors are discussed. Further, some measurements are carried out using an electronic hand-held interrogation device which can scan a component surface and provide a spatial resolution of below 3 mm. The instrument enables mobile measurements outside of laboratory conditions. Further, a robust thermal history coating is introduced demonstrating the capability of the coating to withstand long term exposures. The coating is based on thermal barrier coating (TBC) architecture with a high temperature bondcoat and deposited using an air plasma spray process to manufacture a reliable long lasting coating. Such a coating could be employed over the life of the component to provide critical temperature information at regular maintenance intervals for example indicating hot spots on engine parts.
MHI recently developed a 1600°C class J-type gas turbine, utilizing some of the technologies developed in the National Project to promote the development of component technology for the next generation 1700°C class gas turbine.
This new frame is expected to achieve higher combined cycle efficiency and will contribute to reduce CO2 emissions.
The target combined cycle efficiency of the J type gas turbine will be above 61.5% (gross, ISO standard condition, LHV) and the 1on1 combined cycle output will reach 460MW for 60Hz engine and 670MW for 50Hz engine.
This new engine incorporates:
1) A high pressure ratio compressor based on the advanced M501H compressor, which was verified during the M501H development in 1999 and 2001.
2) Steam cooled combustor, which has accumulated extensive experience in the MHI G engine (> 1,356,000 actual operating hours).
3) State-of-art turbine designs developed through the 1700°C gas turbine component technology development program in Japanese National Project for high temperature components.
This paper discusses the technical features and the updated status of the J-type gas turbine, especially the operating condition of the J-type gas turbine in the MHI demonstration plant, T-Point.
The trial operation of the first M501J gas turbine was started at T-point in February 2011 on schedule, and major milestones of the trial operation have been met. After the trial operation, the first commercial operation has taken place as scheduled under a predominantly Daily-Start-and-Stop (DSS) mode. Afterward, MHI performed the major inspection in October 2011 in order to check the mechanical condition, and confirmed that the hot parts and other parts were in sound condition.
Double wall cooling is a very effective technique for increasing heat transfer in hot gas path components utilizing a narrow channel near the surface of the component. Multiple techniques exist to increase the heat transfer within the narrow channel, including the use of impingement jets, turbulators and microchannels. A preliminary study has been performed using computational fluid dynamics (CFD) to determine the heat transfer benefits of double wall cooling technology when compared to a smooth wall square channel and a ribbed wall square channel. Conjugate CFD simulations of flow through an aluminum channel were performed to include the effects of conduction through the solid and convection within the main channel. The design for the preliminary study consists of a square main channel and a narrow impingement channel connected by a series of holes creating impingement jets on the outer surface of the impingement channel. The study examines multiple parameters to increase heat transfer without increasing the pumping power required. The parameters studied include diameter of impingement jets, jet-to-jet spacing, number of impingement jets, and jet-to-wall spacing. Results show that the impingement channel height-to-diameter ratio has a strong impact on heat transfer effectiveness. This study also provides a new optimization methodology for improving cooling designs with specific targets.
A system for and method of determining thermal history of components which are subjected to high-temperature environments, such as in boilers, fuel cells, furnaces, engines and gas turbines, and temperature-monitoring materials and coatings for use in such a system and method.
Selective laser melting (SLM) technology based on powder bed has been used to manufacture IN718 samples. The starting material, manufacturing processes, heat treatment and characterization procedures of mechanical properties are presented. It is found that the microstructure is crucial for the mechanical properties of IN718. A regular microstructure with good metallurgical bonding, minimal defects and fine dendritic grains is formed by SLM. After heat treatment, the regular dendritic structure disappears and a needle-like δ phase precipitates at grain boundaries when γ′ and γ″ phases dissolve in the matrix. The microhardness of all samples shows directional independent. The tensile strengths and ductility of SLM + HTed IN718 at room temperature are comparative with those of the wrought IN718.
An befeuchteten, waagerechten Oberflächen, die vom Luftsßtrahl aus einer runden Einzeldüse senkrecht getroffen werden, ist der Stoffübergang gemessen und zusammen mit den Ergebnissen von Wärmeübergangsmessungen anderer Autoren einheitlich dargestellt worden. Besonderheiten im Verlauf der Wärme- und Stoffübergangszahlen längs der Austauschfläche sowie ihr Verhalten unmittelbar im Staupunkt werden durch Turbulenzgradmessungen im Freistrahl und in der Plattengrenzschicht verständlich. Zur Auslegung von Düsentrocknern werden zwei Diagramme mit den zugehörigen Gleichungen angegeben, die – je nach den zulässigen Einschränkungen – alternativ benutzt werden können.
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