Antonio F. Miguel

Constructal theory of flow architecture of the lungs

Here we explain the reasons why we have a bronchial tree with 23 levels ofbifurcation. Based on Bejans Constructal Principle we found that the best oxygen access to the alveolar tissues as well as carbon dioxide removal is provided by a flow structure composed of ducts with 23 levels of bifurcation (bronchial tree) that ends with spaces (alveolar sacs) from where oxygen diffuses into the tissues. The model delivers the dimensions of the alveolar sac, the total length of the airways, the total alveolar surface area, and the total resistance to oxygen transport inthe respiratory tree. A constructal law also emerges: the length defined by the ratio of the square of the airway diameter to its length is constant for all individuals of the same species and is related to the characteristics of the space allocated to the respiratory process and determines univocally thebranching level of the respiratory tree.

Airflow through porous screens: From theory to practical considerations

Reliable prediction of airflow through porous screens is essential to the selection of such devices, as well as to the manufacture of new screening materials. In the attainment of a satisfactory description of the airflow through porous screens, the principles of fluid mechanics provided a simple and accurate methodology for the identification of airflow characteristics. In order to evaluate the impact of screen usage on airflow reduction, the presence or absence of screens (when covering window or ventilator openings) was simulated. Finally, airflow characteristics of a number of porous screens with different uses were determined based on experiments. In view of these findings, some suggestions are made regarding screen design, aiming, for example, at the optimisation of ventilating systems management.

Effect of air humidity on the evolution of permeability and performance of a fibrous filter during loading with hygroscopic and non-hygroscopic particles

Abstract A model was developed to predict the permeability and performance (i.e., the product of the permeability by the number of aerosol particles caught per unit surface) of a fibrous filter in dynamic regime. Besides the mass loading of the aerosol particles, this model took into account the structural characteristics of the filter, the size of the loading particles and the filtration velocity. An experimental filtration study was carried out to obtain the necessary input data for a comprehensive estimation of some relevant parameters of the model and to verify it. This study was performed with sodium chloride and alumina aerosols in the size range of 0.5– 1.3 μm and 0.8– 6 μm , respectively, and filtration velocities of 2, 5 and 15 cm/s . The relative air humidity was controlled in the range of 32–90%. It was observed that the permeability and the performance of tested filter were not only influenced by the size of aerosol particle and mass loading of particles but also by the particle hygroscopicity and air humidity, through the interdependence of this quantities with cake particle density. Furthermore, the filter was found to be more efficient when it operated at high humidities, except for sodium chloride particles at humidities above the deliquescent point.

Constructal theory of particle agglomeration and design of air-cleaning devices

In this paper we propose a constructal approach to understanding and predicting the morphology of agglomerates of aerosol particles and also to the design of air-cleaning devices. We show that particle agglomeration begins in spherical shape and switches to needle-shaped agglomeration, which performs best as a particle collector at long times. The shape of the needle depends on the dipolar moment of the particles, while the critical number of particles prior to switching to needle shape does not depend on the particle properties.We also show how constructal theory leads to the design of air-cleaning devices that achieve maximum performance per unit volume, under the imposed global constrains. The optimal geometry (internal spacing) of devices composed of parallel-plate channels or tubes and porous filters is shown to depend on known non-dimensional numbers that characterize particle deposition. The work reported in this paper adds to many studies that show how constructal theory provides a deterministic basis for the generation of flow configuration everywhere.

On the experimental evaluation of permeability in porous media using a gas flow method

Several studies indicate that experimental determination of porous medium permeability by a gas flow method may be influenced by gas properties and ambient temperature. In order to explain these influences, a very simple model is developed based on the idea that the flow in the porous medium may results from both pressure-driven and free molecular flows. Measurements of the permeability of sand and a woven fibrous material performed with air and helium flows seem to corroborate this view. Based on the approach developed, we also study the phenomenon known as slip, and a methodology to quantify the slip length is presented.

The emergence of design in pedestrian dynamics: Locomotion, self-organization, walking paths and constructal law

Gait is inherent to human life and hence its importance is often overlooked. But walking remains the most basic form of transportation and almost all journeys begin and end with a walk, regardless of the modes used in-between. Gaining a good understanding of pedestrians dynamics is thus a crucial step in meeting the mobility and accessibility needs of people by providing safe and quick walking flows. This paper presents a critical and integrative review of research on pedestrians dynamics and associated topics. The review focuses on comprehensive theories and models, with an emphasis on the advances made possible by the application of the constructal law. Constructal law points out that the emergence and evolution of design in pedestrian dynamics is analogous to that of animate flow systems. Most importantly, it also highlights that the basic features of pedestrian dynamics and supportive walking infrastructures can be optimally envisaged with the help of a few fundamental physics laws.

Indoor Deposition and Forced Re-suspension of Respirable Particles

The rate at which particles deposit and re-suspend from indoor surfaces is an essential parameter in determining human exposure to aerosol particles. In this study, we investigate experimentally particle deposition and forced re-suspension in a cubical enclosure for three particle sizes (0.7, 1.5 and 4 m). For particle deposition, the results are presented in terms of particle deposition velocity. It was observed that the particle deposition velocity is particularly significant for the floor surface. Deposition on vertical and ceiling surfaces becomes important only when these surfaces are electrically charged. Air humidity did not influence the particle deposition apart from when it was close to saturation. Re-suspension of particles from the floor surface due to household activities was also studied. It was observed that small particles are more difficult to detach from the floor than bigger ones. Besides, there is a pressure above which re-suspension is very likely to occur (minimum re-suspension pressure). This minimum pressure is dependent on the particle size.

Solar irradiation inside a single span greenhouse

A model is offered through which the solar irradiation can be computed inside a single span hemicylindrical tunnel greenhouse. The model is mathematically exact. From the physical point of view it is founded on the assumption that the cladding surface is fully diffusive and that the radiation diffused through the atmosphere and by the ground is fully isotropic. It is found that the solar irradiation inside the greenhouse depends upon its orientation. A greenhouse with its longitudinal axis aligned along the north-south direction collects more radiation in summer and less in winter as compared with a greenhouse with its axis aligned along the east-west direction. On the other hand the east-west oriented greenhouse is more efficient in collecting solar radiation in winter than in summer, while the north-south oriented greenhouse exhibits a constant efficiency the year round. Finally, it is shown that the collected radiation depends upon the optical properties of the cladding surface being mainly determined by its transmittance.

Porous materials to control climate behaviour of enclosures: an application to the study of screened greenhouses

The aim of this paper was to study the influence of porous materials (screens, shelters, filters, etc.) on the climate behaviour of enclosures. First, a theoretical approach, describing the climate behaviour of a multi-zone enclosure, was proposed, being afterwards validated against experimental data. Having obtained a satisfactory agreement between the theoretical predictions and the experimental data, applications related with the use of porous screens in greenhouses were researched. This research concerned screened greenhouses, but other aspects, such as indoor climate control in buildings with ventilation filters, can greatly benefit from the approach presented in this paper.

Wind-induced airflow through permeable materials, Part I: the motion equation

An approach to describe the air infiltration in enclosed spaces is proposed. The approach is obtained by developing the momentum equation in terms of the method of volume averaging. The physical meaning of each equation term is presented and the range of validity and applicability are analysed.