A numerical study of flow and particle deposition in bifurcating tubes

Abstract

Asthma drugs are administered through the mouth and the drug particles enter the upper airways. Most of these particles do not go further into the central airways, because they get deposited in the airway walls. The aim of the project is to simulate the flow Held within a human airway segment and identify the proportion of particles that get deposited. A laminar steady flow model was used after making some relevant approximations on the flow behaviour due to inspiratory flow in central airways. A segment of airway in central region of the airway tree was chosen, since the flow simulation in whole airways computationally expensive and very time consuming. The geometrical parameters of a typical airway branch was investigated and double airway bifurcation that lie in the 5^ to 7 generation of the Weibel model A airway model was chosen for flow simulation and particle tracking. The Computational Fluid Dynamical (CFD) model in Fluent (commercial software) was chosen to well represent the physical flow model, maintain high degree of numerical stability, and faster convergence of residuals below at least four orders of magnitude less than the initial residual in first couple of iterations. A structured grid generation scheme was chosen for its accuracy and speed of solution. A strategy for creating multi-block grid is explained and some two- and three-dimensional multi-block designs were created that gave the best possible grid quality. Due to the length of time taken for particle tracking and flow simulation two-dimensional simulation was conducted in detail, instead of the three- dimensional simulation. Grid independent results were used to analyse some interesting flow features for various Reynolds numbers and various branch outlet static pressure variations. Particle equations of motion were chosen out of most probable forces that particle could encounter when it moves with the fluid in laminar flow field. Time advance of particle equations of motion was done by 4^^ order Runge-Kutta scheme and the interpolation of fluid velocity using the discrete field (in CFD) at particle position was done by Shape Function interpolation scheme. Vortical flow Geld and potential flow were used as test cases and the combination of Runge-Kutta time advance with Shape Function interpolation gave adequate accuracy for particle tracking. Particle tracking program was written in C++ language, which uses the Fluent solver dump data in "Tecplot" format, and is also able to do multi-particle runs on a structured grid. A data structure was created for the particle for tracking in structured grid, which took into consideration the different data structures present in commercial flow solvers. The particle deposition sites were found and particle deposition efGciency was calculated. It was found that when particles were released away from the centre of the parent branch they had more chances of reaching the outlet.

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