We propose a methodology for the rheological characterization of a semisolid metal slurry using experimental squeeze-flow data. The slurry is modeled as a structural thixotropic viscoplastic material, obeying the regularized Herschel–Bulkley constitutive equation. All rheological parameters are assumed to vary with the structure parameter that is governed by first-order kinetics accounting for the material structure breakdown and build-up. The squeeze flow is simulated using finite elements in a Lagrangian framework. The evolution of the sample height has been studied for wide ranges of the Bingham and Reynolds numbers, the power-law exponent as well as the kinetics parameters of the structure parameter. Systematic comparisons have been carried out with available experimental data on a semisolid aluminum alloy (A356), where the sample is compressed from its top side under a specified strain of 80% at a temperature of 582 °C, while the bottom side remains fixed. Excellent agreement with the experimental data could be achieved provided that at the initial instances (up to 0.01 s) of the experiment, the applied load is much higher than the nominal experimental load and that the yield stress and the power-law exponent vary linearly with the structure parameter. The first assumption implies that a different model, such as an elastoviscoplastic one, needs to be employed during the initial stages of the experiment. As for the second one, the evolution of the sample height can be reproduced allowing the yield stress to vary from 0 (no structure) to a maximum nominal value (full structure) and the power-law exponent from 0.2 to 1.4, i.e., from the shear-thinning to the shear-thickening regime. These variations are consistent with the internal microstructure variation pattern known to be exhibited by semisolid slurries.

We propose a methodology for the rheological characterization of semisolid metal slurries using experimental squeeze flow data. The material is modelled as a structural thixotropic viscoplastic material, obeying the regularized Herschel-Bulkley constitutive equation. The yield stress and the power-law exponent are assumed to vary with the structural parameter that is governed by a first-order kinetics. The squeeze flow is simulated using finite elements in a Lagrangian framework. The evolution of the sample height has been studied for all the ranges of the Bingham and Reynolds numbers, the power-law exponent as well as the kinetics parameters of the structural parameter. Systematic comparisons have been carried out with experimental data on a semisolid aluminium alloy (A356) sample, compressed from its topside at a temperature of 582 °C under a specified load, which eventually becomes constant. Excellent agreement with the experimental data could be achieved provided that at the initial instances (up to 0.01s) of the experiment the applied load is much higher than the nominal experimental load and that the yield stress and the power-law exponent vary linearly with the structural parameter. The first requirement implies that a different model should be employed during the initial stages of the experiment. As for the second one, the evolution of the sample height can be reproduced allowing the yield stress to vary from 0 (no structure) to a maximum nominal value (full structure) and the power-law exponent from 0.2 to 1.4.

Semisolid castings are usually produced using primary Al alloys to ensure significant mechanical performances. However, the need to increase the use of secondary alloys is becoming more and more urgent to reduce the carbon footprint of the manufacturing process. In fact, it is well known that the production of primary alloys is far more demanding in terms of energy and emissions than the recycling route. To extend the use of secondary alloys to semisolid processing, it is necessary to thoroughly understand their properties and how they can influence a material with peculiar properties as the semisolid one. Besides microstructural and mechanical features, the rheological behaviour also plays a major role when dealing with processing metals in the semisolid state. Therefore, in the present study, a rheological characterization of secondary AlSi7Mg commercial alloy was carried out and compared to that of the conventional primary alloy. In details, a different content of Fe, Cu and Mn was considered, as these impurities easily form primary intermetallic particles, which can remain dispersed in the liquid matrix of the semisolid metal. The aim of this work is to understand if this can affect the rheological properties of the considered semisolid alloy. Flow curves and yield stresses were obtained from the experimental results to compare the behaviour of the different alloys.

Oscillation and creep experiments have been performed with Semi-Solid Material (SSM) AlSi7 with 35% solid fraction to investigate the early visco-elastic properties after shearing of the material in a Searle Rheometer. The preparation of the SSM has been done in situ using a standard procedure to guarantee for all experiments the same initial properties of the material. First, oscillation experiments at low amplitude allowed to study the evolution of material structure with time. Subsequently, creep experiments have been performed changing the resting period based on previous results. Creep experiments are characterized by exposing the material to a sudden increase of shear stress. The resolution in time has been 0.01 seconds, which allows observing the dynamics of the development of visco-elastic properties.The material exhibits viscoelastic properties that are becoming more pronounced with longer resting time. This is in accordance with previous experiments where the ratio between elastic and viscous properties increases with increasing resting time. The development of the elastic properties follows the increase of the yield stress due to the creation of an internal structure of the material, which starts immediately after stopping shearing. The investigation of the short-term response of SSM can be particularly relevant for industrial practice, where material deformation during die filling is very fast and the material flow does not take place in steady-state condition.

An oxygen transport membrane module capable of total 596 Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) membranes was constructed and tested under various capacities. The module is designed to operate with up to 20 bar feed pressure and a vacuum on the permeate side. The separation performance was tested with of up to 96 membranes tubes. A maximum oxygen purity of 98.9% was obtained and a maximum oxygen flux of 2.8 ml cm−2 min−1 was measured under a feed pressure of 10 bar. The maximum permeate oxygen flow rate achieved was 22 l min−1. The performance is lower than expected, for which an inhomogeneous module temperature and suboptimal flow distribution can be accounted for, as CFD simulations show. A lot of membrane breakages occurred during operation. Some reasons for that could be identified and the module could be optimized throughout the 2 years of testing. Within that time, three long term tests were performed and a maximum continuous operational time of 1800 h was achieved.

Dynamic Mechanical Analysis (DMA) of semisolid aluminum alloys was performed in a rheometer of Searle type. DMA was applied on a binary AlSi7 alloy to demonstrate the advantages of the method for the investigation of the behavior of alloys in semi-solid state and it was compared to classical shear experiments. Frequency sweeps, amplitude sweeps as well as constant condition experiments (CCE) were performed. It became obvious that elastic properties are getting more dominant with increasing resting time without shearing. The shift from a more viscous to more elastic nature of the material can be quantified. Interestingly, it was found that the semi-empirically based Cox-Merz rule, usually applied for polymers, holds for the semisolid material as well. This allows investigating the shear viscosity under different relevant conditions - important to improve material models for die-filling simulation of semisolid alloys.

In the present review, the main findings on the rheological characterization of semi-solid metals (SSM) are presented. Experimental results are a fundamental basis for the development of comprehensive and accurate mathematics used to design the process effectively. For this reason, the main experimental procedures for the rheological characterization of SSM are given, together with the models most widely used to fit experimental data. Subsequently, the material behavior under steady state condition is summarized. Also, non-viscous properties and transient conditions are discussed since they are especially relevant for the industrial semi-solid processing.

In many industrial units such as packing columns, falling film reactors, etc., the liquid phase is designed as a falling film. It is well known that the mass and heat transfer in laminar wavy film flows is significantly enhanced compared to flat films. The kinetic phenomena underlying the increase in mass and heat transfer are, however, still not fully understood. For an efficient design of falling film units, computational models that account for these enhanced transport mechanisms are of key importance. In this article, we present a reduced modeling approach based on a long‐wave approximation to the fluid dynamics of the film. Furthermore, we introduce a new two‐dimensional (2D) high‐resolution laser‐induced luminescence measurement technique. Both in the numerical simulation results and in the high‐resolution 2D‐concentration measurements obtained in the experiments we observe similar patterns of high concentrations locally, especially in the areas close to the wave hump. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2265–2276, 2018

We analyse the circular Couette flow of Herschel-Bulkley fluids to investigate the validity of the assumption that the rate of strain distributions across the gap share a common point. It is demonstrated that this is true only with fully-yielded Bingham-plastic flow. In other cases, e.g., in partially-yielded Bingham-plastic flow or fully-yielded Herschel-Bulkley flow, the common point for the fully-yielded Bingham case provides a good approximation for determining material constants only if the gap is sufficiently small. We also revisit the important issue of determining material properties of viscoplastic fluids by using "true values" for the rate of strain and demonstrate that the material properties can be very different from those obtained using "apparent' values for the rate of strain.

In this work, the effect of stirring conditions on agglomeration and coalescence-coarsening in isothermal, globulitic, semisolid AlCu10%wt was investigated. It is shown that for the investigated system, a shear rate regime exists which promotes crystallographic alignment of the agglomerated grains, leading to a rapid coalescence process, thereby the formation of rosettes. It is also experimentally validated that for lower shear rates, the size and porosity of aggregates are increased compared to that at higher level of shear.

Screw press processing of biomass can be considered as a suitable mechanically based pretreatment for biofuel production since it disrupts the structure of lignocellulosic biomass with high shear and pressure forces. The combination with chemical treatment has been suggested to increase the conversion of lignocellulosic biomass to fermentable sugars. Within the study, the synergetic effect of alkaline (sodium hydroxide, NaOH) soaking and screw press pretreatment on wheat straw was evaluated based on, e.g., sugar recovery and energy efficiency. After alkaline soaking (at 0.1 M for 30 min) and sequential screw press pretreatment with various screw press configurations and modified screw barrel, the lignin content of pretreated wheat straw was quantified. In addition, the structure of pretreated wheat straw was investigated by scanning electron microscopy and measurement of specific surface area. It could be shown that removal of lignin is more important than increase of surface area of the biomass to reach a high sugar recovery. The rate constant of the enzymatic hydrolysis increased from 1.1 × 10−3 1/h for the non-treated material over 2.3 × 10−3 1/h for the alkaline-soaked material to 26.9 × 10−3 1/h for alkaline-assisted screw press pretreated material, indicating a nearly 25-fold improvement of the digestibility by the combined chemo-mechanical pretreatment. Finally, the screw configuration was found to be an important factor for improving the sugar recovery and for reducing the specific energy consumption of the screw press pretreatment.

The application of CCS technology involves considerable efficiency losses and significant additional investments. The aim is therefore to reduce these efficiency losses and to cut costs. Against this background, membrane-based carbon capture routes for the post-combustion, oxyfuel and pre-combustion technology lines will be analyzed in the following for hard-coal-fired power plants. To the best knowledge of the authors, this paper is the first one comparing membrane based capture routes on common technical and economic boundary conditions. The post-combustion process involves a cascade arrangement of polymer membranes. In the optimum case, the efficiency losses for this concept amount to 9.6 percentage points. In comparison, efficiency losses for the other two membrane-based concepts, i.e. oxyfuel (oxygen transport membrane (OTM) with vacuum pump) and pre-combustion (water-gas shift reactor-WGSMR), are considerably lower (5.3/5.5 percentage points). The main goal of this paper is to assess levelized cost of electricity (LCOE) for the process routes under consideration and their sensitivity on CO2 allowance costs, yearly operating hours, membrane costs and membrane lifetime. The specific investment costs for the capture plants are 2410(sic)/kWh (oxyfuel), 2572(sic)/kW h (post-combustion) and 2660(sic)/kWh (pre-combustion). This is 66% (post-combustion), 55% (oxyfuel) and 33% (pre-combustion) above the specific investment costs for the corresponding reference case without carbon capture. Allowance prices in a range from (sic)20 (pre-combustion) to (sic)39 (post-combustion) per tonne of CO2 would be necessary to compensate for the additional investments. Since it can be assumed that the membranes have a limited lifetime, the influence on electricity generation costs was calculated for different lifetimes. The results show that a technical service life of more than 3 years does not have a significant impact on generation costs. This applies to all the technological concepts investigated. In terms of LCOE and CO2 avoidance costs ((sic)/t(co2)) it turns out that oxyfuel and pre-combustion based membrane power plants are favorable compared to the post-combustion route. However, it has to be kept in mind that the uncertainty in membrane costs are higher for the oxyfuel membranes (ceramic oxygen transport membranes) and the pre-combustion membranes (microporous ceramic membranes) compared to the polymeric post-combustion membranes which already have achieved a commercial level. (C) 2015 Published by Elsevier Ltd.

Rheological properties of liquid metals are difficult to investigate experimentally because of the extreme border conditions to consider. One difficulty is related to the low viscosity of liquid metals. Surface tension effects can cause forces that can be considerably higher than the viscous forces in the liquid metals. Evaluating the experimental data without considering these effects leads to an apparent shear thinning behavior of the material. In the present study, experiments were performed by means of a Searle rheometer changing the dimension of the measuring system with metals of high surface tension, as mercury and tin. It became evident that surface tension plays a significant role in the effects that falsify measurements at low shear rate. Conclusions can be drawn to what extent measurements of semi-solid metals are affected.

In this work we revisit the issue of obtaining true material constants for semisolid slurries. Therefore, we consider the circular Couette flow of Herschel-Bulkley fluids. We first show how true constants can be obtained using an iterative procedure from experimental data to theory and vice versa. The validity of the assumption that the rate-of-strain distributions across the gap share a common point is also investigated. It is demonstrated that this is true only for fully-yielded Bingham plastics. In other cases, e.g., for partially-yielded Bingham plastics or fully-yielded Herschel-Bulkley materials, the common point for the fully-yielded Bingham case provides a good approximation for determining the material constants only if the gap is sufficiently small. It can thus be used to simplify the iterative procedure in determining the material constants.

Rheological properties, crystallization and ripening phenomena have been investigated for three different steel grades in the semi-solid state. For each steel grade detailed investigation has been performed via differential scanning calorimetry to quantify the solid fraction as a function of temperature. The experimental procedure for all samples has been as follows: the completely liquid samples were cooled with a constant rate under constant shearing to temperatures corresponding to solid fractions of 30 and 40%. Tracking the viscosity during this procedure allowed analyzing the crystallization kinetics and final ripening process, which differ for the different steel grades. Evaluating the viscosity data on the base of a combination of convection dominated ripening model and a modified Krieger-Dougherty equation gives valuable information of the microstructural evolution. This is validated by additionally quenching samples.

Nowadays, in steel industries, spray quenching has been used as a cooling method for the microstructure optimization after a forging process. In comparison with other cooling systems, there are many process parameters involved in spray quenching, which make it versatile, effective and able to adapt the quenching upon different cross sections of heavy parts. Simulation can represent a useful tool in properly designing the spray process for a specific forging and its reliability depends on the proper definition of input data, in particular of the heat transfer coefficient between the water and the part. Volumetric spray flux has been proposed as one of the preeminent factors on determining the heat transfer coefficient during spray quenching. In industrial processes, the volumetric spray flux is affected by the overlap of sprays on the forging surface due to the use of multi-nozzles systems. The present paper is aimed at developing a method able to take into account, by using a defined amplification facto; the effect of overlapping of sprays on the heat transfer coefficient to be applied in simulation of quenching processes.

Recently, an innovative technique called Hybrid Aluminium Forging (HAF) has been developed as a combination of casting and forging processes. In the present paper the characterization of a new Al-Si-Mg alloy with Cr addition is reported; it was used in a preliminary study to evaluate its suitability for the production of wheels by HAF process. At first, calorimetric analysis was performed to identify solid fraction change with temperature for process optimization. The alloy after heat treatment shows good strength and hardness, but poor ductility, while Cr modifies the morphology of Fe-containing intermetallics. Finally, rheological measurements show a shear thinning behaviour of metal in semi-solid condition.