CALIBRATION OF A CFD METHODOLOGY FOR THE SIMULATION OF ROUGHNESS EFFECTS ON FRICTION AND HEAT TRANSFER IN ADDITIVE MANUFACTURED COMPONENTS

It is well-known from the literature that surface roughness significantly affects friction and heat transfer. This is even more evident for additive manufactured (AM) components, which are taking an increasingly important role in the gas turbine field. However, the exploitation of numerical approaches to improve their design is hindered by the lack of dedicated correlations and CFD models developed for such high roughness conditions. Usually the additive manufactured components are simulated considering the surfaces as smooth or applying an equivalent sand-grain roughness (ks) that results in a velocity shift in the boundary layer. However, determining a priori the most appropriate value of ks is challenging, as dozens of correlations are available, returning scattered and uncertain results. A previous work proved how the CFD prediction of friction and heat transfer returns significant deviations, even exploiting the ks values obtained from experimental tests on the very same test case. That work also allowed identification of a promising CFD methodology based on friction and thermal corrections proposed by Aupoix from ONERA. The aim of this work is to further the assessment and calibration activity of the model, by analyzing additional experimental data of friction factor and Nusselt number from new test cases considering different geometries and flow conditions. The new coupons consisted of straight circular channels and wavy channels. This work represents a further step in the generation of a more validated and general methodology for the high-fidelity CFD analysis of additive-manufactured components.

Analysis of thermo-hydraulic and entropy generation characteristics for flow through ribbed-wavy channel

Purpose The purpose of this study is to analyze the thermal, hydraulic and entropy generation characteristics for laminar flow of water through a ribbed-wavy channel with the top wall as wavy and bottom wall as flat with ribs of three different geometries, namely, triangular, rectangular and semi-circular. Design/methodology/approach The finite element method-based numerical solver has been adopted to solve the governing transport equations. Findings A critical value of Reynolds number (Recri) is found beyond which, the average Nusselt number for the wavy or ribbed-wavy channel is more than that for a parallel plate channel and the value of Recri decreases with the increase in a number of ribs and for any given number of ribs, it is minimum for rectangular ribs. The performance factor (PF) sharply decreases with Reynolds number (Re) up to Re = 50 for all types of ribbed-wavy channels. For Re > 50, the change in PF with Re is gradual and decreases for all the ribbed cases and for the sinusoidal channel, it increases beyond Re = 100. The magnitude of PF strongly depends on the shape and number of ribs and Re. The relative magnitude of total entropy generation for different ribbed channels varies with Re and the number of ribs. Practical implications The findings of the present study are useful to design the economic heat exchanging devices. Originality/value The effects of shape and the number of ribs on the heat transfer performance and entropy generation have been investigated for the first time for the laminar flow regime. Also, the effects of shape and number of ribs on the flow and temperature fields and entropy generation have been investigated in detail.

Calibration of a CFD Methodology for the Simulation of Roughness Effects on Friction and Heat Transfer in Additive Manufactured Components

It is well-known from the literature that surface roughness significantly affects friction and heat transfer. This is even more evident for additive manufactured (AM) components, which are taking an increasingly important role in the gas turbine field. However, the exploitation of numerical approaches to improve their design is hindered by the lack of dedicated correlations and CFD models developed for such high roughness conditions. Usually the additive manufactured components are simulated considering the surfaces as smooth or applying an equivalent sand-grain roughness (ks) that results in a velocity shift in the boundary layer. However, determining a priori the most appropriate value of ks is challenging, as dozens of correlations are available, returning scattered and uncertain results. A previous work proved how the CFD prediction of friction and heat transfer returns significant deviations, even exploiting the ks values obtained from experimental tests on the very same test case. That work also allowed identification of a promising CFD methodology based on friction and thermal corrections proposed by Aupoix from ONERA. The aim of this work is to further the assessment and calibration activity of the model, by analyzing additional experimental data of friction factor and Nusselt number from new test cases considering different geometries and flow conditions. The new coupons consisted of straight circular channels and wavy channels. This work represents a further step in the generation of a more validated and general methodology for the high-fidelity CFD analysis of additive-manufactured components.

Heat And Mass Transfer With Radiation In A Convective Flow Between Vertical Wavy Channels Due To Travelling Thermal Waves

The objective of this paper is to investigate analytically the convective flow of heat and mass transfer in vertical wavy channels due to travelling thermal waves. Effect of radiation, temperature dependent heat source/sink, concentration dependent mass source are taken into account. To tackle the highly complex non-linear problem, the perturbation technique is applied with long wave approximation.

Anilox Cell Geometries For Printable Electronics and Flexible Packaging

Flexography is a major high-volume printing process used extensively for flexible packaging. The heart of flexographic press is the anilox roll, which meters the flow of ink to the image carrier (plate) by virtue of the engraved cells on the surface. The anilox was original engraved mechanically using a stylus to peck at the surface. This limited the size and profile of the engraved cells. However, laser engraving has enabled much more control with a variety of shapes and aspect ratios. Much has been claimed by the manufacturers for these new designs – improved ink transfer, higher volumes of ink transfer and better half tone reproduction – on the basis of industrial field trials. The objective of the research reported in this thesis has been to quantify the ink release from the anilox to the plate for both traditional cell profiles and the open channel designs.Previously, the ink release was mostly determined by examining the optical density of the print products. The optical density is a qualitative indicator of the ink release from anilox cells. These studies were limited to closed anilox cells with a low ink viscosity, as typically used for graphic prints. This study explores an extended range of anilox cell shapes, including open channel geometries, and the ink viscosities. The ink released from the anilox cells has been be directly measuring and quantified.Experiments were performed printing directly to glass and on flexible packaging at a commercial printers to establish the current industry position. A laboratory scale printability tester was then used to study ink release using three inks: UV Cyan, Carbon and Silver. These represented a link to the graphic experiments in previous published studies, while the Carbon and Silver were highly viscoelastic functional inks used in printed electronics. Four cell geometries were used: laser engraved closed cells, extended hexagonal and wavy channels together with mechanically engraved conventional closed pyramid cells. The laser engraved anilox afforded the opportunity to vary key parameters of cell width, depth, profile and volume. A brief exploration of print speed was also undertaken with exemplar anilox of each cell type. The main study considered ink transfer to a 100% solid plate, as this would allow the ink release to be studied without influence of the plate distorting into the cells thereby extracting more ink. A limited study was then undertaken with a half tone plate to establish the impacted on ink transfer.The amount of ink transferred was highly dependent on the absolute volume of cells, i.e. the amount of ink available on the anilox. The anilox cells with wider, shallower and smaller depth-to-width ratio released a higher proportion of the ink. The ink’s physical characteristics of viscoelasticity and extensional viscosity also determine the proportion of ink transferred.The anilox hexagonal closed cells (typically used in the flexographic printing process) performed best with the low viscosity ink. The information gaining from this study would aid in the design of anilox cell geometries and development of ink characteristics to enhance its capability for functional print applications such as printable electronics. The anilox wavy channels released the greatest proportion of the ink with high viscosity, elastic modulus, and filament breakup time. The anilox wavy channel has the potential to be used for the functional print as it increased the release of paste-like ink. Additionally, it improved the ink lay-down. The anilox engraving technique affected the ink release. The anilox cells, which were engraved by the laser technique, gave greater ink release comparing to the anilox cells, which were engraved by the mechanically engraving technique. The increase of the dot coverage increased the ink release out of the anilox cells because of the increase of the receiving area. However, the increase of the ink release plateaued after the dot coverage of 50% for UV Silver because of its large filament breakup time.The characteristics of ink influenced the ink release out of the anilox cells. Unlike previous work which examined only the ink viscosity, this studied included ink elastic modulus and filament breakup. The ink with high viscosity and elastic modulus, but small filament breakup time gave greatest ink release for all anilox shapes.When the printing speed increased, it decreased the ink release due to two factors; reduction of engagement time between the anilox cells and the plate (reducing time for ink to transfer) and enlargement of the filament extension rate (reducing the amount of ink transfer). The decrease of ink release was affected by the ink characteristics and the anilox cells shapes. The decrease of ink release was significant when UV Cyan ink (small viscosity and elastic modulus) was used with the anilox open cells and wavy channels. Contrarily, the decrease of ink release was insignificant when UV Carbon and Silver inks (large viscosity and elastic modulus) was used with the anilox open cells and wavy channels.The experimental data was analysed and the critical parameters in releasing the ink of the anilox cells were identified. The depth of anilox cell was the most critical parameter; the shallower cell depth released a higher proportion of the ink. The ratio of depth-to-width was the second most important parameter in determining the ink release. The smaller depth-to-width ratio released more ink. The width of anilox cell could not be used as a parameter predicting the ink release because the wider anilox cell did not always release the higher proportion of the ink.