Estudo numérico dos efeitos de microbolhas para redução de atrito em canais com escoamento ascendente

Abstract

Our environment is surrounded by turbulence, and turbulent flows are often encountered in a variety of engineering applications, such as in compressors, pumps and pipelines. In most applications, friction is largely responsible for energy transformation and is considerably larger in turbulent flows than in laminar flows. Thus, reducing the drag force in turbulent flows is an issue that is constantly evolving and several techniques have been developed for this purpose. The use of microbubbles as a means of reducing friction draws attention to the reduction achieved with the technique and its environmental advantages. A better understanding of the physics that entails the reduction of microbubble drag can generate significant financial benefits in different types of transport. With this objective, numerical simulations were performed using the DNS methodology in flow with microbubbles in an ascending turbulent channel flow. For the simulations, the UNSCYFL3D code, developed in the MFlab, was used in which the Navier-Stokes equations in the incompressible formulation are solved numerically through the finite volume method in unstructured meshes. The Euler-Lagrange approach was used, where the continuous phase is treated on an Eulerian reference and the microbubbles by a Lagrangian reference, with the movement of the bubbles governed by Newton's second law, and a two-way coupling between the phases. The influence of two values for the volume fraction of the gas (0, 1% e 0, 5%) and three different diameters for the microbubbles (100 µm, 200 µm e 500 µm) was analyzed. The addition of the microbubbles in the flow caused that for the same pressure gradient the velocity profiles of the cases with microbubbles presented an increase in relation to the profile of the single phase flow, implying in the reduction of friction. The presence of microbubbles in the flow significantly impacted the flow in the viscous sublayer. By means of Fanning's coefficient of friction it was possible to quantify the overall reduction of friction in this study for the six possible combinations of cases, and the maximum reduction achieved was around 20%. This percentage was computed for the volume fraction of 0, 5% and the smaller diameter, 100 µm. The results confirm the direct influence of the volumetric fraction on the reduction of friction, besides showing that the size of the bubbles also plays a role in reducing friction, even if with less influence.