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Knowledge of respiratory flow behaviour is important in many respiratory medical fields. The usefulness of numerical models in providing a better understanding of flow phenomena has made the Computational Fluid Dynamics (CFD) an indispensable research tool due to the difficulty of measuring in vivo data. In this research, the extrathoracic airways and the upper tracheobronchial region, trachea and main bronchus bifurcation were modelled. Oral and nasal breathing routes have been considered under steady and cyclic unsteady conditions. A realistic far boundary condition was imposed as the flow inlet. Different ventilation levels and frequencies were simulated. The model presented has been validated successfully by two parts: nasal and oral models. The airflow distributions through oral and nasal routes were determined, analysed and compared under different breathing conditions. The flow behaviour and respiratory effort during inhalation and exhalation phases change from rest to high activity; the flow can increase 40% with the same respiratory effort, opening the mouth during the inspiration. Significant differences in turbulent intensity contours in steady and unsteady cases have been observed. This study demonstrated the relevance of considering different breathing patterns and more realistic unsteady conditions.

This article examines the notion of critical wall shear stress as the key control parameter of the local fouling removal process. In this study, an experimental setup was developed for measuring fouling on enhanced surfaces. Specifically, the experimental configuration consists of a forced convection plate heat exchanger containing a one-pass rectangular channel with two ribbed plates arranged in a symmetrically staggered manner. The exhaust gases flow by the rib-roughened sides of the plates, and the flat sides can be cooled with water from the independent external coolant circuit.As a result of soot particle deposition from exhaust gases, a layer of fouling is deposited over the ribs.After asymptotic conditions were reached during the tests, detailed fouling thickness measurements were conducted. The dimensionless particle relaxation time during these tests was determined to be in the range of 0.3-10. The measurements were then complemented with a numerical analysis. In particular, the local wall shear stress was calculated using a commercial computational fluid dynamics (CFD) software package.The fouling thickness profiles deposited over the ribs and the local critical shear stress values were compared and discussed for two different geometries. The results obtained clearly support the idea that critical wall shear stress is an appropriate criterion for facilitating the understanding of the local behaviour of fouling deposits.

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.On the effect of surface roughness and material on the subcooled flow boiling of water: Experimental study and global correlation. AbstractIn this paper, a new correlation based on experimental results for subcooled flow boiling of water at low pressure is proposed, preceded by a brief review on how the solid-fluid interaction has been dealt with in past correlations. The experimental sample comprises seven heating surfaces of different material (copper, aluminium and stainless steel) and roughness. The experimental facility is presented in detail and the surface morphology of each test specimen is analysed by means of an optical interferometer. The correlation is based on the assumption that the effect of material and roughness can be captured by means of modifiers of a general expression. The surfaces chosen in this work were selected to capture a broad range of industrial applications, and, though the correlation found fits well in the range of commercial and relatively high values of R a (up to 7 µm in the case of copper), further study is needed for larger values, as a discontinuity in the effect was observed, which has been previously determined by some authors. Thus, the proposed global correlation permits the calculation of the boiling heat flux taking into account, in addition to the more classical parameters such as pressure and bulk temperature, the effect of both the roughness and material of the wall heater, allowing its general use in low pressure applications such as those commonly found in the automotive industry.