The drag effect of air bubbles on triple junction migration of pure ice

The migration of a grain triple junction was studied on ice pure samples with bubbles at -2°C for almost 3 h. This work studies the interaction between Grain Boundary (GB) and bubbles. The evolution of the triple junction was recorded from successive photographs obtained from a LEICA® optical microscope. Simultaneously, numerical simulations of grain triple junction with mobile bubbles were carried out using Monte Carlo method with the following conditions: The bubbles in the bulk were kept immobile and those in the GB were allowed to move. In addition, mobile bubbles were forced to stay inside the GB. The simulations show that bubbles slow down the movement of the GB and of the triple junction. What’s more, the simulated triple junction obtained fits very well the experimental triple junction geometry, and the GB diffusivity values obtained coincide with those measured experimentally at the same temperature and reported by other authors. Finally, the drag effect of the mobile bubbles on the GB migration was verified.

3D Unsteady RANS Computation of the Mixing on a T-junction

Turbulent mixing is a very common phenomenon in industrial processes. It is well know that the turbulence model has a massive impact on the accuracy of a turbulent flow, principally when it is used in processes of turbulent mixing. For this reason, this paper aims to investigate the impact of two specific turbulence models on calculating a mixture of gas-gas, using a 3D T- junction geometry. The differences between the calculation with two RANS based model, the kw-SST and SAS are investigated here. A mixture of Air and N2 is performed. The sensibility of the refinement of the mesh of calculation is assessed to calculate the discretization error. A comparison of results obtained with the distinct models of turbulence is made with available experimental data. In this comparison it is showed that the SAS model, due to its capability of capturing some vortexes that SST couldn’t, offers a better accuracy, with an error maximum bellow the 7%, in comparison to the experimental data. Keywords: T-juntion, Turbulent mixing, RANS, CFD

CFD Analysis of Flow Structures in a Mixing Chamber

Special mixing chambers are usually used to perform scientific experiments or for routine industrial production processes. This is the case, typically, of fan mixers in a baffled tank. Mixing chambers comprise, among other alternative elements, two counter-rotating fans at the bottom and top. These will eventually allow a mixing effect on the chamber with an adequate level of uniformity. Herein a computational flow simulation is performed for the mixing conditions of air and SO2 inside the chamber used in the CLOUD experiment, by studying in detail the flow structures and uniformity inside the chamber. This Unsteady Navier-Stokes computation is performed using the kω-SST and SAS turbulence models. A first validation step is performed by using an experimental test case, comprising a T-junction geometry, that performs the mixing of air and N2. Following this validation step a detailed analysis of the flow structures inside the 3D chamber is conducted, and specific insights are given regarding the flow uniformity. A detailed analysis of the computed mixing flow structures for the SST and SAS turbulence models is also described. It is shown that the SAS model captures with more detail the macro and meso-mmixing process with an accuracy of, at most, 6%. This value can be further reduced to values around 2% by resorting to high density meshes, with the associated computational burden.

A Comprehensive Study of Asymmetric Micro-Droplet Splitting in T-Junction

Splitting a single droplet into two unequal portions using a microfluidic T-junction has been an important functional feature of many modern lab-on-a-chip devices. A recent study introduced a general criterion for asymmetric droplet break-up in the range of intermediate Capillary numbers. The current work attempts to analyze, in more details, the different underlying mechanisms governing the asymmetric break-up process. In particular, this work focuses on the relationship between the break-up mechanism versus the splitting ratio of the daughter droplets. CFD simulation is used to closely monitor the effect of different fluid properties on the evolution of droplet break-up process. The splitting ratio under different flow conditions is characterized. Four mechanisms for primary droplet break-up are defined as follows: break-up with permanent obstruction, unstable break-up, breakup with tunnels and non-breakup. In particular, the main focus of this study is on the unstable break-up mechanisms where is very likely results to a much-deviated splitting ratio. Typically, yet unexpectedly, the resulting splitting ratio is often larger than the pressure gradient ratio in the T-junction. However, the two ratios are approximately equals to each other under a limited set of flow conditions. It has been observed that the splitting ratio could be more than double the pressure gradient ratio of the T-junction. The break-up is observed to be in the permanent obstruction mode if the splitting ratio is about the same magnitude as the pressure gradient ratio. The effects of the T-junction geometry on the break-up will also be examined.

Numerical assessment of stress concentration in Francis runner blade — crown/band junctions

The paper presents the results of numerical investigation for stress distribution near runner blade-crown/band junctions. Starting with the geometrical investigation of the geometries for a real turbine, different junction geometries were identified. Eight different junction geometries were numerically investigated in order to determine the stress distribution. The applied load for structural analysis was the pressure distribution, obtained from a Computational Fluid Dynamics analysis of the runner blade. The results of the stress distribution will allow to quantify the effect of different junction geometry on stress concentration and integrity of the runner.

Investigation of Flow Characteristic on Ram Pump in Different of Pipe Junction Geometry

Ram Pump is a mechanical device that uses phenomenon of difference pressure to distribute the water toward high state level. The advantages of ram pump are low cost, construction of reliable and without electrical energy. Kinetic and potential energy are main factor to flow the water in driver pipe. This research purposed to determine flow characteristic in different geometries of ram pump based on experiment data. Ram pump operated on different pressure of inlet area which was get from water level. To get the flow characteristic of rum pump, it was simulated using computational fluid dynamic. The result of simulation was found that junction type of U shape appeared wake in driver pipe area; thus, this phenomenon indicated any turbulent flow because decreasing of pressure. In other junction pipe, wake phenomenon appeared in driver and delivery pipe area.