Concepción Paz

Development of a Predictive CFD Fouling Model for Diesel Engine Exhaust Gas Systems

This article presents a numerical simulation procedure for studying soot particle deposition in diesel exhaust systems, with a particular focus on fouling layer thickness evolution. In the proposed algorithm, particle transport toward the wall, adhesion, and reentrainment of particles from the surface have been modeled, including Brownian motion and turbulent diffusion, thermophoresis, adhesion, and removal. This model has been implemented in ANSYS Fluent, which makes the inclusion of local effects possible. A cross-flow device, with a tube positioned transverse to the flow, has been simulated and tested. A comparison of the predicted fouling layer at several angular positions with the experimental observation shows acceptable agreement. This model makes it possible to predict the real depth of the fouling layer and its effects on the hydrodynamics of the flow. This model represents a valuable tool for the prediction of the main aspects of the performance of heat exchangers exposed to fouling.

CFD transient simulation of the cough clearance process using an Eulerian wall film model

Abstract In this study, a cough cycle is reproduced using a computational methodology. The Eulerian wall film approach is proposed to simulate airway mucus flow during a cough. The reproduced airway domain is based on realistic geometry from the literature and captures the deformation of flexible tissue. To quantify the overall performance of this complex phenomenon, cough efficiency (CE) was calculated, which provided an easily reproducible measurement parameter for the cough clearance process. Moreover, the effect of mucus layer thickness was examined. The relationship between the CE and the mucus viscosity was quantified using reductions from 20 to 80%. Finally, predictions of CE values based on healthy person inputs were compared with values obtained from patients with different respiratory diseases, including chronic obstructive pulmonary disease (COPD) and respiratory muscle weakness (RMW). It was observed that CE was reduced by 50% in patients with COPD compared with that of a healthy person. On average, CE was reduced in patients with RMW to 10% of the average value of a healthy person.

Development of a computational fluid dynamics model for predicting fouling process using dynamic mesh model

Abstract This article presents a comprehensive computational model capable of simulating fouling layer thickness evolution using dynamic mesh model. This computational methodology has been developed to reproduce the deposit generation during fouling process with an innovated work method. Dynamic mesh model, from Ansys Fluent software, and external routines have been used to implement this advanced numerical model which allows to move the boundaries of a region relative to other boundaries of the zone. The displacement of the nodes of the mesh is the mechanism that this model uses to adjust the geometry according to the fouling layer evolution. During the simulation process, the geometry under investigation is modified to reproduce the emergence and gradual change of the fouling layer. Different rules of deposition and removal of the fouling process can be implemented in the proposed algorithm. The direct interaction between fouling expressions and governing equations of the main flow is used to predict deposits formation and growth. In this article, numerical simulations of soot fouling layer formation have been presented. Deposit evolution has been calculated inside different heat exchanger technologies used in exhaust gas recirculation systems to analyze fouling process and to verify the advantages of this new computational strategy.

On the Application of Image Processing Methods for Bubble Recognition to the Study of Subcooled Flow Boiling of Water in Rectangular Channels

This work introduces the use of machine vision in the massive bubble recognition process, which supports the validation of boiling models involving bubble dynamics, as well as nucleation frequency, active site density and size of the bubbles. The two algorithms presented are meant to be run employing quite standard images of the bubbling process, recorded in general-purpose boiling facilities. The recognition routines are easily adaptable to other facilities if a minimum number of precautions are taken in the setup and in the treatment of the information. Both the side and front projections of subcooled flow-boiling phenomenon over a plain plate are covered. Once all of the intended bubbles have been located in space and time, the proper post-process of the recorded data become capable of tracking each of the recognized bubbles, sketching their trajectories and size evolution, locating the nucleation sites, computing their diameters, and so on. After validating the algorithm’s output against the human eye and data from other researchers, machine vision systems have been demonstrated to be a very valuable option to successfully perform the recognition process, even though the optical analysis of bubbles has not been set as the main goal of the experimental facility.

CFD Simulation of the Oral-Nasal Flow Partitioning During a Breathing Cycle Based on the Soft Palate Movement

The latest developments in the computational field have promoted the application of the Computational Fluid Dynamics (CFD) to the study of human health. CFD has become a new tool for in-depth investigations of the human cardiovascular and respiratory system. Therefore, this new technique provides a better understanding of the respiratory airflow, enabling the reproduction of the complex geometries of the breathing airways. In this research, a full extrathoracic model of the human airways is built from TC of an adult healthy subject. Some specific regions, as the oral cavity and the oro-pharynx, has been reconstructed from previous articles. Moreover, the soft palate has been modelled. This tissue is of main importance when considering the study of airflow patterns and the oronasal partitioning. At low ranges of physical activity, the soft palate is in anterior position allowing only nasal breathing. However, when a person begins to realize any activity, their breathing demand increases and the soft palate is displaced to a posterior position widening the oral route. The oronasal airflow partitioning was characterized on this research with five different positions of the soft palate. The breathing pattern obtained was compared with experimental data from other studies. The pressure drop and the velocity contours are analysed, to get a more detailed understanding of the breathing process.

Glottis effects on the cough clearance process simulated with a CFD dynamic mesh and Eulerian wall film model

Abstract In this study, we have reproduced the cough clearance process with an Eulerian wall film model. The simulated domain is based on realistic geometry from the literature, which has been improved by adding the glottis and epiglottis. The vocal fold movement has been included due to the dynamic mesh method, considering different abduction and adduction angles and velocities. The proposed methodology captures the deformation of the flexible tissue, considers non-Newtonian properties for the mucus, and enables us to reproduce a single cough or a cough epoch. The cough efficiency (CE) has been used to quantify the overall performance of the cough, considering many different boundary conditions, for the analysis of the glottis effect. It was observed that a viscous shear force is the main mechanism in the cough clearance process, while the glottis closure time and the epiglottis position do not have a significant effect on the CE. The cough assistance devices improve the CE, and the enhancement rate grows logarithmically with the operating pressure. The cough can achieve an effective mucus clearance process, even with a fixed glottis. Nevertheless, the glottis closure substantially improves the CE results.