A Correlative CFD Study Between Recirculation Area and FPM in VHC Design

Abstract

A major part of asthma treatment is made by the use of preventive inhalation drugs. The Pressurized Metered-Dose Inhalator (pMDI) has been the backbone device for this treatment, due to its simplicity, portability and widely acceptance. But no device comes without its limitations, and pMDI is hard to handle properly by elders and children < 5 years old, resulting in reduced amount of drug to the patient lungs. Add-on devices (e.g. spacers) were developed to mitigate the need for coordination and reduce the oral/throat deposition, namely the Valved Holding Chambers (VHC). These devices are incorporated with a one-way valve and a chamber that allows the spray droplets to rapidly reduce their size upon release on a stagnated and confined flow. The VHC main ability, in terms of efficiency, is to reduce the coarse fraction (i.e. particles with diameter > 4.7µm) of the plume by impaction and allow the fine fraction to be inhaled by the patient. The VHC geometry will play a very importance role in the entrapment of small drug particles (i.e. fine fraction). The hypothesis proposed by this study is that a small particle has more probability to be trapped in geometries with higher recirculation areas (and stagnation zones). These macro vortices will cause a particle with small Stokes number to be entrapped; to assess this hypothesis a numerical study was modelled. The numerical study was carried out on an idealized geometry of a VHC device, using a 2D axisymmetric approach. Different coordinates for a “corner” point, were tested. FLUENT® was used to obtain the unsteady numerical solution, meshes were generated using Meshing® software from ANSYS®. Once the flow field is stabilized (around 0.6s), a pMDI spray was injected into the domain during 0.1 seconds and the simulation continued until perform 4 seconds. The simulation takes into account the vaporization of the HFA-134a propellant present in the droplets of spray and a User Defined Function (UDF) for modeling the particle –wall interaction. The post-processing of the results included the calculation of the recirculation area and the Fine Particle Mass (FPM) that exits the domain. Results show that the percentage of recirculation area decreases linearly with the increase of axial position of the corner point, and rapidly increases with the radial displacement. FPM results are not so linear; nevertheless they show opposite behavior to the recirculation area. Additionally, results show that high recirculation area reduces the amount of FPM emitted. Data can be correlated through a power function (FPM = 101.805*Area-0.244; R2=0.460). Results are more strongly correlated for lower values of radial displacement. The results seem to corroborate the hypothesis that smaller particles tend to be entrapped by recirculation areas.