Multiphase CFD Investigation on Convective Heat Transfer Enhancement for Turbulent Flow of Water-Al2O3 Nanofluid

Because of extraordinary heat transfer capability, nanofluids have become a potential interest in engineering sectors. Despite being a multiphase fluid, nanofluids were treated as single phase fluids in many previous studies and comparison between single and two phase models was drawn. Examining nanofluids capability to augment heat transfer is one of the keys to utilize them properly in the field of thermofluids. However, the optimal multiphase model to simulate nanofluids heat transfer enhancement is yet to be found out. In this study, the method of computational fluid dynamics has been used to simulate flow of water-Al2O3 nanofluid in a circular pipe in the purpose of identifying the best multiphase model to simulate heat transfer enhancement of nanofluids. Two multiphase models have been taken into account: Volume of Fluid and Mixture model. Three different volume fractions of nanoparticles in nanofluid have been tested for each of these models such as 1%,4% and 6% for highly turbulent flows where Reynolds number was ranged between 20000 to 80000. The standard k-ɛ turbulence model has been employed to model the flow of nanofluid with the mentioned multiphase models in the present study. The results have been carried out in forms of correlation between Re and Nu and have been compared with existing experimental results. The results showed that the heat transfer enhancement of nanofluid is mostly dominated by concentration of nanoparticles present in the fluid and suggested that Mixture model is suitable for predicting convective heat transfer enhancement of nanofluid for cases with high particle concentration though the necessity of further experimental study in some scopes has been detected.

Optimization of hydrocyclon for phosphatic rock separation using CFD

Hydrocyclones are equipment for the separation of solid-liquid and liquid-liquid mixtures through the centrifugal flow. The phosphate rock is an essential raw material to the industry of phosphate fertilizers. The mineral needs to be concentrated in its processing, and this can be done through hydrocyclones, considering its robustness and low operation costs. This work aimed to use the computational fluid dynamics to study different multiphase models to represent the hydrocyclones, as well as modifications to its geometry to increase its efficiency. Three multiphase models were studied in order to analyze their efficiency in simulating the separation through hydrocyclones: Eulerian-Lagrangian, Eulerian-Eulerian, and Mixture Model. In order to optimize the separation process and reduce operating costs, 11 modifications were proposed in the geometry of HC11, called B1, B2, B3, C1, C2, C3, D1, D2, E1, E2 and E3. The first 8 proposals involved changes in the vortex finder and the last 3 proposals added a wall in the air core formation region. Geometry and mesh were generated in the GAMBIT® software and the simulation was made in the FLUENT® 19.2 software. In order to compare the multiphase models, the individual and overall efficiency were used along with the experimental results. The Mixture model had the smallest relative error and was used for the subsequent simulations. The parameters evaluated to measure the optimization of HC11 were the pressure drop (?P), the liquid ratio (RL) and the overall efficiency (?). The results obtained for each of the proposals were compared with the value found in the HC11 simulations to evaluate the possible optimization. With that, it was possible to verify that modifications B2, B3, and D1 improved all the parameters evaluated, optimizing the separation process and reducing energy costs involved in the operation.

The Effect of Longitudinal Rails on an Air Cavity Stepped Planing Hull

The use of ventilated hulls is rapidly expanding. However, experimental and numerical analyses are still very limited, particularly for high-speed vessels and for stepped planing hulls. In this work, the authors present a comparison between towing tank tests and CFD analyses carried out on a single-stepped planing hull provided with forced ventilation on the bottom. The boat has identical geometries to those presented by the authors in other works, but with the addition of longitudinal rails. In particular, the study addresses the effect of the rails on the bottom of the hull, in terms of drag, and the wetted surface assessment. The computational methodology is based on URANS equation with multiphase models for high-resolution interface capture between air and water. The tests have been performed varying seven velocities and six airflow rates and the no-air injection condition. Compared to flat-bottomed hulls, a higher incidence of numerical ventilation and air–water mixing effects was observed. At the same time, no major differences were noted in terms of the ability to drag the flow aft at low speeds. Results in terms of drag reduction, wetted surface, and its shape are discussed.

Assessment of surface roughness and blood rheology on local coronary haemodynamics: a multi-scale computational fluid dynamics study

The surface roughness of the coronary artery is associated with the onset of atherosclerosis. The study applies, for the first time, the micro-scale variation of the artery surface to a 3D coronary model, investigating the impact on haemodynamic parameters which are indicators for atherosclerosis. The surface roughness of porcine coronary arteries have been detailed based on optical microscopy and implemented into a cylindrical section of coronary artery. Several approaches to rheology are compared to determine the benefits/limitations of both single and multiphase models for multi-scale geometry. Haemodynamic parameters averaged over the rough/smooth sections are similar; however, the rough surface experiences a much wider range, with maximum wall shear stress greater than 6 Pa compared to the approximately 3 Pa on the smooth segment. This suggests the smooth-walled assumption may neglect important near-wall haemodynamics. While rheological models lack sufficient definition to truly encompass the micro-scale effects occurring over the rough surface, single-phase models (Newtonian and non-Newtonian) provide numerically stable and comparable results to other coronary simulations. Multiphase models allow for phase interactions between plasma and red blood cells which is more suited to such multi-scale models. These models require additional physical laws to govern advection/aggregation of particulates in the near-wall region.