4.1 Introduction
The N95 FFR is widely used for personal protection from exposure to contaminated particles.Respirator comfort is a significant factor for wearers.When an FFR is wore,the main reasons for discomfort include:a high temperature in the FFR cavity leading to thermal discomfort [1-6],the accumulation of CO2 and decreased oxygen which may cause headaches while wearing the N95 FFR [2,7-8],and mechanical factors such as increasing pressure and wall shear stress under the FFR cover [2,4,9].Important studies have been carried out in terms of the experimental approaches for respirator comfort.Many researchers have utilized an automated breathing and metabolic simulator [8-9].Experiments with human subjects have also been conducted [4,6];however,these experimental approaches are time-consuming and expensive.Moreover,little experimental work has simultaneously considered the previously mentioned three factors due to the complexity and high-cost of the experiments.Computational Fluid Dynamics provides an alternative low-cost but effective means to study this type of problem.CFD uses numerical methods to solve fluid problems and obtains information about velocity,pressure,and temperature.Previous research simulated the air flow in the FFR cavity using the CFD method,and some have even used the CFD method to determine the position of face seal leaks while wearing an FFR [10].
Moreover,the digital model of headform and FFR during contact is not easy to obtain due to the flexible properties of both the FFR and facial muscles.For example,most of the National Institute for Occupational Safety and Health headforms are rigid.NIOSH headforms are widely used in many fields,but cannot be directly used in CFD simulation that considers FFRs.The current way to obtain a geometric model for CFD regarding FFR wear is to simulate the FFR wearing process firstly [11-12].After the deformation of the FFR and headform are calculated,the results of contact simulation are then converted into CFD-solvable models [10];this method is complex and time-consuming.Moreover,the contact simulation result may have errors compared to actual deformation.As a result,a different way of modeling based on direct scanning has been developed.
In addition,the most recent research using CFD only focused on the outer flow field around human objects [10,13-17],or the inner flow field of the nasal cavity [18-20],whereas the real breathing process is a continuous flow between the inside and outside.To our knowledge,no simulation of this continuous breathing process has been conducted.
Reverse modeling is an approach of the computer-aided design model reconstructed from a physical object.In this chapter,a headform wearing an FFR was built based on CT scanning using a reverse modeling approach.The model included the upper respiratory airway,FFR cavity,FFR medium and ambient air.Using the CFD simulation method,we analyzed the flow characteristics of velocity,CO2 volume fraction and temperature distribution inside the FFR cavity during full breathing cycles.In addition,the distribution characteristics of pressure and wall shear stress inside the upper respiratory airway were investigated.