Papers by Keyword: Air Entrainment

Abstract: High-Pressure Die Casting (HPDC) processes are often affected by complex thermo-fluid-dynamic phenomena that lead to casting defects and premature die degradation. In this study, an approach based on the Response Surface Methodology (RSM) is proposed to improve the quality of the cast part (aluminum window brackets) and extend the dies’ service life by introducing limited modification to the geometry of the die cavities.A multi-physics numerical model was initially built up to reproduce the filling and thermal behavior of the process. Infrared thermography, used to validate the numerical results, confirmed the accuracy of the model, with an average temperature error of approximately 2%. The analysis revealed that the baseline configuration (i.e. the dies’ geometry currently adopted in the industrial process) was characterized by non-negligible thermal imbalances (temperature gradients of about 50 °C and localized hot spots associated with high melt velocities), which reflected in the occurrence of flashes, metallization, and impression pad damage.New die geometries with the aim of improving the thermal uniformity while reducing the temperature gradients where investigated by varying the geometrical properties of the gating system according to a DoE-based approach. The numerical results, collected in terms of total amount of porosity in the casting critical areas, were used to train accurate metamodels that, in turns, were adopted as the starting base for a multi-objective optimization. Results from the optimization allowed to identify different scenario, each characterized by a specific geometry of the gating system able to remarkably reduce the occurrence of porosity in the cast part (up to 42% less than the current condition). The results demonstrate that the proposed methodology enables effective and sustainable optimization of HPDC processes without costly trial-and-error approaches.

Abstract: Dross is one of the most challenging quality issues in high integrity ductile iron castings. The formation of dross is linked to the applied metallurgy, metal treatment, process control and gating/rigging design. One of the primary concerns in this regard is entrainment of air into the melt during filling of the casting. Today, avoiding and controlling dross relies on the practical experience and process know-how of foundry experts. A quantitative understanding of the formation mechanisms or the prediction of final amounts and locations of dross in castings is not available. In this paper a computational model is developed for predicting the formation, motion and final location of dross inclusions during pouring of ductile iron castings. The focus is on the important mechanism of dross generation due to air entrainment during filling of the mold. The model predicts the local air entrainment rate as a function of the turbulent kinetic energy of the liquid metal and the normal velocity gradient of the metal at the liquid metal-air interface. The dross inclusions resulting from the exposure to the entrained air are transported with the melt flow under the combined influences of drag and buoyancy, and captured by the solidifying casting structure. The model is implemented in a commercial casting simulation software code. Ductile iron casting experiments are conducted to validate the model using castings having a variety of gating systems, section thicknesses, and surface orientations. Dross is measured by serial sectioning of the solidified castings. Good agreement between measured and predicted dross amounts and locations is obtained. The results reveal how gating system design affects dross formation. Clean ductile iron castings can be obtained by minimizing air entrainment during pouring. The present model allows foundries to evaluate different gating designs and pouring parameters before the first casting is poured. The paper also discusses open issues and practical challenges in quantifying the amounts of dross as a function of the entire processing route of a casting.

Abstract: The basic feature characterising the concrete’s quality and its usefulness for a structure is compressive strength. However, other features, such as frost resistance, absorbability, tightness, abrasiveness, decide on the durability of concrete in the structure as well. The paper presents the results of tests conducted on an ecological air entraining agent applied as a concrete mix admixture. The air entrainment of a concrete mix is aimed at entraining additional air pores with appropriate shape, size, and distribution into the concrete structure. The air entrainment is one of the main technological processes aimed at improving the concrete’s resistance to destructive effect of frost [1,2,3,4].

Abstract: Aligned with the world’s tendency of searching for sustainable technologies, the waste addition into materials of the civil construction is an interesting option for wastes that do not have proper destination. This work has the objective of studying the feasibility of incorporating paint waste from furniture painting booth, which currently does not have any correct ecological destination, into covering mortars and investigate if this addition brings any improvement in performance. For this purpose, it was prepared six compositions of covering mortars, with a cement-sand volume proportion of 1:6 and a Flow-table consistency of 260 mm +- 10mm, varying the waste percentage added among the proportions of 0%, 2%, 4%, 6%, 8% and 10% on the cement weight. It was determined, for each composition, the specific gravity and the air-entrained content in the fresh stage. For physical tests, it was moulded nine prismatic samples and one mortar substrate for the bond tensile strength test per mortar composition. It was determined the flexural and compressive strength in the hardened stage, the specific gravity, the water absorption coefficient due to capillary action and the bond tensile strength. Leaching tests were performed for environmental analysis of the mortars produced with waste addition. It was observed that all the mortars with waste received a similar classification to the mortar without waste, according to NBR 13281 [], and the leaching tests showed that mortars with waste were not ecologically dangerous, demonstrating the feasibility of the incorporation. In addition, mortars produced with waste presented an increase in the entrained air percentage, which might be a beneficial action of the paint waste.

Abstract: During fire, one or two faces of structural members experience higher temperatures than other faces and the deterioration on these faces may continue after fire. High temperature exposure above 400 °C causes deterioration in strength, modulus of elasticity and durability of concrete. Inclusion of fibers and air entraining agents in concrete mixes may reduce the destructive effects of high temperatures on concrete. Therefore, 8 groups of 0.45 w/c ratio of concrete were designed by using polypropylene fibers as low melting point fibers and hooked end steel fibers as high melting point fibers and air entraining admixture as a chemical additive. 15 cm cubic concrete specimens were produced and the five sides of the cubes were insulated with gypsum boards to maintain one face heating. An electrical furnace was used to heat concrete to 1000 °C and K-type thermocouples were placed in specimens to monitor temperature distribution in concrete. Moreover, two different re-curing methods, air and water, were applied after heating to see the change in mechanical properties and crack occurrences on the heated surface of concrete specimens. SEM and XRD investigations were conducted on the samples taken from the heated surfaces and the inner parts of the concrete in order to understand the morphological changes due to heating and re-curing. Results showed that deterioration on the surfaces due to high temperature exposure continued during air re-curing process and compressive strength and modulus of elasticity values of these specimens also diminished. On the other hand, compressive strength of water re-cured concrete stayed constant after heating and partial recovery of modulus of elasticity were obtained and the positive effect of water re-curing were observed on polypropylene fiber reinforced concrete prominently.

Abstract: This paper describes freeze-thaw resistance with sodium chloride solution of concrete mixtures based on alkali-activated fly ash. There are shown relationships between freeze-thaw resistance with sodium chloride solution and air content in fresh alkali-activated concrete and relationship between freeze-thaw resistance with sodium chloride solution and spacing factor of alkali-activated concrete. Also there is described relationship between air content in fresh mixture and compressive strength of alkali-activated concrete.

Abstract: The spillway system allows stabilization of the water free level and avoids variation in free level of water along the flow passage as a function of flow rate. The main problem in the spillway is the profiling of weir crest. The criteria that need to be satisfied are there should be no flow separation from the crest and there should be uniform circumferential flow to avoid flow asymmetry in the flow passage. Separation of flow leads to large impact velocity of the falling water, which would lead to the large-scale entrainment of air in water. This paper describes the effect of spillway materials by coating over the weir and changing the profile on air entrainment characteristics in the downstream.

Abstract: Oxygen transfer through the air entrainment over the stepped weir by the flow types was presented through the laboratory experiments. In the nappe flow, dominant flow features included an enclosed air pocket, a free-falling nappe impact and subsequent hydraulic jump on the downstream step. Most air was entrained through a free-falling nappe impact and a hydraulic jump. In the skimming flow, air entrainment occurs from the step edges. Downstream of the inception point, the flow is highly aerated at each and every step with very significant splashing. The average values of the oxygen transfer efficiency in the region of the nappe flow and of the skimming flow are about 0.40 and 0.28, respectively. The stepped type of the weir was found to be efficient for water treatment associated with substantial air entrainment.

Abstract: Investigated coeffect of superplasticizer S-3 and air-entraining agents LHD (wood chemical additive) on workability, cohesion and air entrainment of concrete mix. Found that under introducing an additive LHD into the concrete mix comprising supeplastifikator S-3, the mobility of the concrete mix is reduced despite the fact that the additive LHD itself is a plasticizer. Decreased mobility from the introduction 0.1 % LHD makes: slump Abrams - 5-6 %, slump flow - 18-22 %. At the same time LHD additive contributes significantly air entrainment and reduce separating water and mortar. Effect of increasing the connectivity of the concrete mix associated with air entrainment. Regardless of the LHD additive S-3 additive does not reduces water separating, and does not have air-entraining action, and virtually no effect on air entrainment caused by the addition of LHD. With increasing content in the concrete mix additives LHD regardless of dosage S-3 amount of entrained air increases from 3 to 7%, and water separation is reduced to almost 0. Thus, synergy additives S-3 and LHD against plasticize the concrete mix was negative, but a positive effect has been observed in respect of connectivity concrete mix.

Abstract: The objective of this study is to investigate the air entrainment and oxygen transfer characteristics of central-driven ejector with various ejector design parameters. The ejector design parameters are primary nozzle diameter, mixing tube length and diffuser angle. The entrainment ratio decreased with the primary nozzle diameter and diffuser angle, while the ratio increased with the mixing tube length. The trend of dissolved oxygen concentration with the diffuser angle and mixing tube length is equal to the result of entrainment ratio, however, the trend with the primary nozzle diameter is different to the result of entrainment ratio.