Effects of Grain Morphology on Flow Behavior of Semi-Solid Slurries

The flow behavior of semi-solid slurry determines the quality of the castings produced by the semi-solid forming process. Many studies have done to investigate the flow behavior of slurry under different conditions, and results show that the rheological behavior of slurry with dendritic structure is inappropriate for semi-solid forming. In this study, slurries with varying morphologies of grain for the same alloy with the same fraction solid have tested using a partial filling method. The SEED process was employed, and the pouring temperature adjusted to prepare semi-solid slurries with different grain morphologies. The flow pattern, entrapped air during the filling process, and also microstructure of the samples were examined to characterize the macro and micro flow behavior. The results show that a turbulent macro-flow, leading to entrapped air, and severe segregation appeared in the sample using slurry of Tpour ≥ 660 °C . For the slurry of Tpour < 660 °C, none of the three phenomena found in the sample. This investigation further showed that the detriment of dendrite on the semi-solid forming process, and implied that large size dendrite in semi-solid slurry must avoided.

Iatrogenic subcutaneous emphysema – A case report

Subcutaneous tissue emphysema in general terms is defined as an abnormal presence of air under pressure, along or between fascial planes. A sudden blast of air, during an ongoing endodontic treatment or an endodontic surgery can sometimes cause movement of this air through the loose connective tissue layers to distant areas. In case of occurrence of emphysema, the condition should be carefully examined as the entire diagnosis is based merely on clinical examination. The clinician should be aware of the common causes, characteristic diagnostic features, possible complications and the management of the condition. This entrapped air in some cases is self healing and in some cases can lead to serious complications and death. The case report described is a successful management of unintentional and unfortunate tissue emphysema. The case report also emphasis on the importance of timely and accurate diagnosis of the condition.

2D CFD Modeling of Rapid Water Filling with Air Valves Using OpenFOAM

The rapid filling process in pressurized pipelines has been extensively studied using mathematical models. On the other hand, the application of computational fluid dynamics models has emerged during the last decade, which considers the development of CFD models that simulate the filling of pipes with entrapped air, and without air expulsion. Currently, studies of CFD models representing rapid filling in pipes with entrapped air and with air expulsion are scarce in the literature. In this paper, a two-dimensional model is developed using OpenFOAM software to evaluate the hydraulic performance of the rapid filling process in a hydraulic installation with an air valve, considering different air pocket sizes and pressure impulsion by means of a hydro-pneumatic tank. The two-dimensional CFD model captures the pressure evolution in the air pocket very well with respect to experimental and mathematical model results, and produces improved results with respect to existing mathematical models.

Air entrapment and bubble formation during droplet impact onto a single cubic pillar

We study the vertical impact of a droplet onto a cubic pillar of comparable size placed on a flat surface, by means of numerical simulations and experiments. Strikingly, during the impact a large volume of air is trapped around the pillar side faces. Impingement upon different positions of the pillar top surface strongly influences the size and the position of the entrapped air. By comparing the droplet morphological changes during the impact from both computations and experiments, we show that the direct numerical simulations, based on the Volume of Fluid method, provide additional and new insight into the droplet dynamics. We elucidate, with the computational results, the three-dimensional air entrapment process as well as the evolution of the entrapped air into bubbles.

Effect of Air-entrained Agent for the Mortar of Roller Compacted Concrete

Roller compacted concrete (RCC) is the zero slump concrete produced from the same materials used in conventional concrete. The mortar used in RCC plays a significant role for the workability, strength and durability of the concrete. The air content in the mortar is the key factor for durability, especially to resist the freezing and thawing action. The main target is to produce the microscopic air cells inside the mortar using air-entrained agent and minimize the entrapped air as little as possible. Air content may range from 4~7% depending upon the type of concrete. The effect of the content of AE 303A type air-entrained agent was studied for the fresh and hardened properties of the RCC mortar. The result showed that it has an almost negligible effect on the workability of mortar, but highly effective for the density and compressive strength of hardened mortar. The use of 0.02% (by weight of cement) increased the air content about 4.5 times of the base mortar (without the use of the agent), from 2% to 9.1%. However, the density was decreased by about 10 % (from 2.18 gm/cm3 to 1.96 gm/cm3) and the 28 days compressive strength by about 49% (from 21.90 MPa to 14.73 MPa). The model, developed for the mortar of the dam concrete, has also been well satisfied with the experimental results for the case of RCC mortar.

Absorption and Strength Properties of Short Carbon Fiber Reinforced Mortar Composite

This paper presents the water absorption and strength properties of short carbon fiber reinforced mortar (CFRM) composite. Four CFRM composites with 1%, 2%, 3%, and 4% short pitch-based carbon fibers were produced in this study. Normal Portland cement mortar (NCPM) was also prepared for use as the control mortar. The freshly mixed mortar composites were tested for workability, wet density, and entrapped air content. In addition, the hardened mortar composites were examined for compressive strength, splitting tensile strength, flexural strength, and water absorption at the ages of 7 and 28 days. The effects of different carbon fiber contents on the tested properties were observed. Test results showed that the incorporation of carbon fibers decreased the workability and wet density, but increased the entrapped air content in mortar composite. Most interestingly, the compressive strength of CFRM composite increased up to 3% carbon fiber content and then it declined significantly for 4% fiber content, depending on the workability and compaction of the mortar. In contrast, the splitting tensile strength and flexural strength of the CFRM composite increased for all fiber contents due to the greater cracking resistance and improved bond strength of the carbon fibers in the mortar. The presence of short pitch-based carbon fibers significantly strengthened the mortar by bridging the microcracks, resisting the propagation of these minute cracks, and impeding the growth of macrocracks. Furthermore, the water absorption of CFRM composite decreased up to 3% carbon fiber content and then it increased substantially for 4% fiber content, depending on the entrapped air content of the mortar. The overall test results suggest that the mortar with 3% carbon fibers is the optimum CFRM composite based on the tested properties.

Dynamic analysis of vertical stormwater storage systems

A below-grade vertical stormwater storage system is one of the solutions to reduce the volume of sewer overflows released into the environment. The system is submerged most of the time during filling, which can result in hydraulic problems. This research intent to provide some insight on potential hydraulic problems that can occur in a vertical storage system during intense rain events. An experimental study was conducted using a physical scale model that consists of two vertical storage shafts, a horizontal tunnel and an inflow drop shaft. The results showed that both entrapped air in the system and mass flow oscillation in the system can cause a rapid rise of water level, or a geyser, at the drop shaft. The predictions of a modified version of HAMMER compared well with the experimental result while the InfoWorks CS model was unable to simulate vertical momentum in the drop shaft.

Dynamic analysis of vertical stormwater storage systems

A below-grade vertical stormwater storage system is one of the solutions to reduce the volume of sewer overflows released into the environment. The system is submerged most of the time during filling, which can result in hydraulic problems. This research intent to provide some insight on potential hydraulic problems that can occur in a vertical storage system during intense rain events. An experimental study was conducted using a physical scale model that consists of two vertical storage shafts, a horizontal tunnel and an inflow drop shaft. The results showed that both entrapped air in the system and mass flow oscillation in the system can cause a rapid rise of water level, or a geyser, at the drop shaft. The predictions of a modified version of HAMMER compared well with the experimental result while the InfoWorks CS model was unable to simulate vertical momentum in the drop shaft.