Effects of curved inlet tubes on air flow and particle deposition in bifurcating lung models

https://doi.org/10.1016/S0021-9290(00)00233-5Get rights and content

Abstract

In vivo bifurcating airways are complex and the airway segments leading to the bifurcations are not always straight, but curved to various degrees. How do such curved inlet tubes influence the motion as well as local deposition and hence the biological responses of inhaled particulate matter in lung airways? In this paper steady laminar dilute suspension flows of micron-particles are simulated in realistic double bifurcations with curved inlet tubes, i.e., 0°⩽θ⩽90°, using a commercial finite-volume code with user-enhanced programs. The resulting air-flow patterns as well as particle transport and wall depositions were analyzed for different flow inlet conditions, i.e., uniform and parabolic velocity profiles, and geometric configurations. The curved inlet segments have quite pronounced effects on air-flow, particle motion and wall deposition in the downstream bifurcating airways. In contrast to straight double bifurcations, those with bent parent tubes also exhibit irregular variations in particle deposition efficiencies as a function of Stokes number and Reynolds number. There are fewer particles deposited at mildly curved inlet segments, but the particle deposition efficiencies at the downstream sequential bifurcations vary much when compared to those with straight inlets. Under certain flow conditions in sharply curved lung airways, relatively high, localized particle depositions may take place. The findings provide necessary information for toxicologic or therapeutic impact assessments and for global lung dosimetry models of inhaled particulate matter.

Introduction

The human respiratory tract is a system where particulate matter can be inhaled via the mouth/nose and may deposit at focal points in the airways. Toxicologic or therapeutic health effects of inhaled particulate matter depend largely on the particle deposition patterns and concentrations at critical sites in the respiratory airways. Excessive retention of inhaled particles has been known to be directly related to a number of occupational diseases, such as coal worker's pneumoconiosis, silicosis and asbestosis (Douglas, 1989). Deposition of inhaled allergens causes acute bronchoconstriction for asthma sufferers (Gonda, 1997), and elevated depositions of some particulate pollutant cause tissue injuries and lung diseases such as bronchial carcinoma (Schlesinger and Lippmann, 1978) and even lung cancer (Spencer, 1985). In contrast, inhalation of drug aerosols deposited directly to lung target areas results in rapid absorption across bronchopulmonary mucosal membranes and reduction of the adverse reactions in the therapy of asthma and other respiratory disorders (Smith and Bernstein, 1996). However, in the inhalation route for drug delivery, the technique to ensure that the medication reaches its intended sites of action in the lung and the dose-prediction of the medication in the specific sites of airways are most critical (Ganderton, 1999). Therefore, the understanding of aerosol transport and local deposition characteristics is important for dose-effect analyses as well as drug aerosol therapies.

The in vivo airway geometries are very complicated and the in vivo measurements of air-flow profiles and detailed particle deposition patterns are very difficult. Computational fluid dynamics (CFD) simulation is a useful and non-invasive tool of obtaining detailed particle deposition patterns. The respiratory airways in the lung can be approximated as a network of repeatedly bifurcating tubes with progressively decreasing dimensions (cf. Weibel, 1963). Supra-micron particles tend to deposit in the large airways (cf. Gerrity et al., 1979; Kim et al., 1983; Martonen, 1986), especially around the carinal ridge mainly due to impaction (cf.Balásházy and Hofmann, 1993; Balásházy et al., 1999; Kim and Iglesias, 1989; Gatlin et al., 1997). These studies assumed that airway branches were straight except in the transition region between the parent tube and daughter branches. However, real airway segments are not always straight, but somewhat curved to various degrees (Ikeda, 1974; Hammersley and Olson, 1992; Fox, 1999). With a curved inlet, one can expect skewed velocity profiles which may have profound effects on the airflow fields and particle deposition patterns in downstream branches and junction areas. These effects have been demonstrated to a certain extent in the double bifurcation simulation studies with a straight tube inlet (Lee et al., 1996; Comer et al., 2000). However, the role of curved airway segments leading to the bifurcations has not been studied systematically. How do the curved inlet tubes influence the airflow, particle motion as well as the local deposition patterns in the downstream bifurcations? How do specific inlet velocity profiles and different inlet tube angles, 0°⩽θ⩽90°, influence micron-particle deposition? The present study focuses on these questions.

Section snippets

Materials and methods

A configuration of the double bifurcation of interest, representing airway generations from 3 to 5 after Weibel (1963) is depicted in Fig. 1. The effect of curved inlet tubes is analyzed for selected values in the parameter ranges 0°⩽θ⩽90°, 0.9cm⩽R⩽3cm, 500⩽Re⩽2000, 0.02⩽St⩽0.12, and equal outlet pressures (cf. Fig. 1 and Table 1), where θ and R are the tube angle and curvature radius of the inlet, respectively; Re is the Reynolds number, Re=2ρUa/μ, ρ is the fluid density, U is the mean inlet

Results

In order to employ the flow simulation model as a predictive tool for airway deposition, the present model has been successfully validated with several measured velocity profiles and particle deposition data in double bifurcations with straight parent tubes (cf. Comer et al., 2000). Good agreement was also obtained when the calculated results were compared to the experimental data points of Austin (1971) for developing axial velocity profiles in a 90°-bend (cf. Figs. 2a and b). Particle

Discussion

The bifurcating airway system in vivo is complex and varies greatly in terms of branching angle, branching asymmetry, and carinal ridge shape (Oho and Amemiya, 1980; Phillips and Kaye, 1997). The airway segments leading to the bifurcation zones are not always straight, but curved to various degrees. Such curved inlets to the bifurcations may cause locally high or low depositions of inhaled toxic particulate matter or therapeutic aerosols at some specific sites (“hot spots”) which is very

Disclaimer

Although the research described in this article has been supported by the United States Environmental Protection Agency, it has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.

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