Diffusive Airflow Research Articles

Simplified models of aerosol collision and deposition for disease transmission

Fluid-mechanics research has focused primarily on droplets/aerosols being expelled from infected individuals and transmission of well-mixed aerosols indoors. However, aerosol collisions with susceptible hosts earlier in the spread, as well as aerosol deposition in the nasal cavity, have been relatively overlooked. In this paper, two simple fluid models are presented to gain a better understanding of the collision and deposition between a human and aerosols. The first model is based on the impact of turbulent diffusion coefficients and air flow in a room on the collisions between aerosols and humans. Infection rates can be determined based on factors such as air circulation and geometry as an infection zone expands from an infected host. The second model clarifies how aerosols of different sizes adhere to different parts of the respiratory tract. Based on the inhalation rate and the nasal cavity shape, the critical particle size and the deposition location can be determined. Our study offers simple fluid models to understand the effects of geometric factors and air flows on the aerosol transmission and deposition.

Numerical simulation of the spatial and temporal distributions of the downwash airflow and spray field of a co-axial eight-rotor plant protection UAV in hover

The downwash airflow of a multi-rotor unmanned aerial vehicle (UAV) in hover can improve the penetration rate of droplets in the canopy of fruit trees, but the uncertain distribution of the downwash airflow can also lead to the generation of uneven distribution of droplets. With a co-axial eight-rotor plant protection UAV as the research object, the computational fluid dynamics (CFD) method is adopted in this study to establish the coupled flow field model of the downwash airflow and the two-phase flow model of airflow-droplets in hover. The spatial and temporal distribution law of the downwash airflow is analyzed, and the impact of the motion law of the droplets in the spray field is investigated. The results show that the downwash airflow shifts from an unsteady state to a steady state over time. Various wind vortices such as wake vortex, ground diffusion airflow, and vortex ring are generated during the evolution process, with a “negative velocity channel” of downwash airflow velocity identified in the central region under the rotors. The farther the airflow is from the rotor, the larger the area field of wake vortex airflow. Consequently, there is a gradual coupling of the left and right adjacent wake vortices from mutual independence, with the “spiral effect” of the downwash wind field offset by the mutual coupling of the upper and lower rotors of the UAV. When the installation position of the nozzle falls within the below-rotor region, the distribution of droplet deposition becomes more penetrative than the central axis installation position of the nozzle. It indicates that the below-rotor installation position of the nozzle can prevent droplets from entering into the coupling region of the trailing vortex, which reduces droplet drift. According to the field test, the measurement and simulation results of the downwash air velocity are consistent, with the minimum and maximum relative error reaching 4% and 29%.

Complex Restrictive Lung Disease Patterns Affecting Survival and Outcomes in Patients Undergoing Allogeneic Stem Cell Transplant: A Single-Institution Retrospective Analysis

Background: Allogeneic stem cell transplant is the only curative therapy for some patients with hematologic malignancies. While pulmonary function impairment related to diffusion capacity and air flow obstruction is part of the predictive Hematopoietic Cell Transplant-Comorbidity Index (HCT-CI), restrictive changes are poorly reflected. A new entity known as complex restrictive lung disease (CRLD) refers to a discrepancy between Forced Vital Capacity percent predicted (FVCpp) and Total Lung Capacity percent predicted (TLCpp). FVCpp being disproportionately lower than TLCpp occurs due to impaired lung emptying from neuromuscular disease, obesity, bronchiectasis, occult obstruction, pulmonary hypertension and chest wall limitations. In simple restrictive lung disease (SRLD) such as pulmonary fibrosis, FVCpp is proportionately decreased compared to TLCpp. Methods: Over three years, 124 patients received myeloablative (MA) or reduced intensity conditioning (RIC) followed by matched unrelated donor (MUD) or matched related donor (MRD) grafts. Patients had complete pulmonary function tests (PFT) within 60 days prior to transplant and 6 months post-transplant. Simple Restriction was defined as TLCpp - FVCpp <10%. Complex Restriction was defined as TLCpp - FVCpp >10%. Our primary outcome in this study was Median Overall Survival (mOS). Median follow up was 14 months. Results: For the 124 enrolled patients, demographics included median age of 55 years (range: 18-70), 60% male, MUD status (73%) with male donor (75%); half received RIC (52%) with fludarabine melphalan (62%) vs fludarabine/busulfan (FLU/BU2) (32%) compared to MA (40%) with FLU/BU4 conditioning (77%). The demographics of diagnosis include Acute Myeloid Leukemia (40%), Lymphoma (17%) and Acute Lymphoblastic Lymphoma (16%) and Myelodysplastic Syndrome (16%). The median HCT-CI/Age score was 4 (0-11). Half (62) developed acute graft-versus-host disease (aGVHD) with 80% as Grade 1-2. One patient was lost to follow up, 65 patients were still alive and 22 had relapse of their initial disease with 17 deaths related to relapse. Out of 58 deaths in this study, 41 deaths (71%) were attributed to non-relapse mortality (NRM) with NRM at 33% and relapse mortality rate at 14%. Out of 124 patients, 10 patients had complex restriction prior to transplant and 9 patients developed complex restriction at 6 months post-transplant. The mOS was 7 months for patients with CRLD prior to transplant (1-14 months) compared to 13 months for patients without CRLD (1- 36 months) (p=0.0026). Eight patients with CRLD before transplant were deceased at follow-up of 14 months, with four (50%) out of those eight deaths related to acute respiratory distress syndrome (25%) and hypoxic respiratory failure (25%). The non-relapse mortality rate for this population was 60% and relapse mortality rate was 10%. For all patients with aGVHD, mOS was greater than 30 months for aGVHD Grades 0-1, 16 months for Grade 2, 6 months for Grade 3 and 5 months for Grade 4 (p=0.0048). Six out of six patients with GVHD and CRLD prior to transplant died from acute respiratory distress syndrome (ARDS) hypoxia, or other cause. Out of the 4 patients with CRLD without GVHD, one patient developed respiratory failure as cause of death. There was no statistical difference in mOS due to BMI (p=0.2895). Nonsmokers and past smokers had a m0S of 22 months, with current smokers at 10 months m0S (p=0.13). We found no statistical difference in survival for the 9 patients who developed CRLD after transplant (p=0.45), the 10 patients with simple restriction prior to transplant (p=0.55), the 21 patients who developed Bronchiolitis Obliterans Syndrome (B0S) after transplant (p=0.25), or the 26 patients with Chronic Obstructive Pulmonary Disease (COPD) prior to transplant compared to normal PFT (p=0.14). The mOS for these groups was 12 months. Conclusion: Patients with complex restrictive lung disease prior to allogeneic stem cell transplant are at high risk for poor outcomes from respiratory and non-respiratory disease when compared to the overall population. As complex restriction is not included in the current HCT-CI, patients with this lung disease should have closer monitoring and scrutiny prior to allogeneic stem cell transplantation. Based on the results of this study, addition of complex restrictive lung disease to the HCT-CI to improve predictability of HCT outcomes warrants further investigation. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Air leakage in seals with application to syringes

We study the leakage of air in syringes with Teflon coated rubber stopper and glass barrel. The leakrate depends on the interfacial surface roughness, the viscoelastic properties of the rubber and on the elastoplastic properties of the Teflon coating. The measured leakage rates are compared to the predictions of a simple theory for gas flow, which takes into account both the diffusive and ballistic air flow, and the elastoplastic multiscale contact mechanics which determines the probability distribution of interfacial separations. The theory shows that the interfacial air flow (leakage) channels are so narrow that the gas flow is mainly ballistic (the so called Knudsen limit). The implications for container closure integrity is discussed.

Modeling of supply airflow from slot line diffuser on ceiling for CFD of thermal environment in perimeter zone

The slot line diffuser is widely used in Japan near the perimeter windows because of its blocking effect on convective heat transfer. However, research on this diffuser has mainly focused on the effects of the turbulence model on CFD simulations and longitudinal airflow distribution, while studies on its detailed outlet characteristics in three-dimensional have been limited. In order to obtain and summarize this diffuser's outlet characteristics more accurately, this paper set a slot line diffuser with an adjustable outlet area in free space. The detailed velocity and turbulence statistics distribution on the diffuser's outlet surface were obtained by a hot-wire anemometer, and the diffuser's airflow distribution in the free space was measured by an ultrasonic anemometer. Then the outlet characteristics of this diffuser were summarized using these measured data in order to make a detailed supply airflow model. At the same time, the difference of the airflow of full-size supply and that of half-size supply is characterized. Moreover, a detailed CFD simulation of the diffuser's outlet airflow was performed using the characteristics model to define the boundary conditions. The detailed CFD result with outlet characteristics model were validated by comparing with experimental data. Additionally, the prescribed velocity (P.V.) method and simple boundary simulation were also attempted to reproduce the diffuser's outlet airflow, and compared with the experiment and detailed CFD results. This study clarified this diffuser's outlet characteristics in three-dimensional, and it is considered that the outlet characteristics model can help improve the simulation accuracy of the kind of diffuser.

CHARACTERIZING AIRFLOW IN THE ANNULAR COMBUSTOR DIFFUSER OF AN AVIATION GAS TURBINE ENGINE

In the combustor of aviation gas turbine engines, the diffuser’s role is to reduce the airflow velocity (generated from the compressor) to restrain the pressure loss. Characterizing the airflow in the diffuser is essential for further studies on the processes occurring in the combustor such as fuel atomization, mixing, and combustion. This paper has successfully developed a 3D simulation model using Star CCM+ to characterize airflow in an annular combustor diffuser equipped in a gas turbine engine. The results obtained from this study show that this diffuser reduces airflow velocity by 65%, increases static pressure by 2%, whereas the total pressure slightly reduces (the maximal reduction is 0.8% in the emergency regime) in comparison with the conditions at the compressor outlet. The amount of air directly supplying into the burner attains about 20%, in which the amount of air passing through the swirler makes up 9% of the total amount under the nominal regime.

Assessment of the dust pollution level in the workspaces by the methods of mathematical modeling

Coal dust leakage from coal stacks leads to intensive contamination of the working areas. Therefore, determining level of air pollution near the coal piles for various meteorological situations, when new coal piles are arranged, is an important applied problem. For practice, it is necessary to have quick-calculating mathematical models whiсh take into account the most significant physical processes impacting on formation of contamination zones near the coal piles. Today, to predict the level of air pollution at dust leaking from the coal stacks, the normative technique OND-86 or the Gauss model is used. These forecasting methods make it possible to quickly calculate the contamination zones, but they do not take into account geometric shape of the coal stack and unevenness of the velocity field near it. The subject of this research was creation of a mathematical model for predicting the level of air pollution near coal piles with taking into account the aerodynamics of air flows around them. The purpose of the work was to develop a set of numerical models for quick calculation of the level of air pollution at coal dust leaking from the coal piles with taking into account the uneven emission of dust from different areas of the stack surface. The modelling equations are the three-dimensional mass transfer equation (Marchuk's model) and the three-dimensional equation for the velocity potential. The Marchuk's model allows taking into account rate of the coal dust particles settling, parameters of turbulent diffusion and uneven air flow rate near the coal pile. The Dirac delta function is used for modelling coal dust leaking from various sections of the coal pile. For the numerical solution of the mass transfer equation, a difference splitting scheme is used. On the basis of the three-dimensional equation for the velocity potential, an uneven velocity field near the coal pile is determined. The Richardson method is used for numerical solution of the three-dimensional equation for the velocity potential. An algorithm for solving the problem of calculating a level of contamination of working areas near a coal pile is considered. The description of the structure of the created complex of computer programs is given.

Numerical investigation of a swirl diffuser with a novel design using large eddy simulations

In the present study, a swirl diffuser with a novel design is proposed, in which three geometrical parameters, the angle of the guide vanes φ, the height of the convergent chamber h, and the radius of the diffuser outlet r, can be varied independently. Large eddy simulations were conducted to examine the airflow characteristics downstream of the swirl diffuser. Grid independence was obtained for the grid number 4000,000, and this result was validated by comparison with an experiment. The vorticities, the instantaneous and averaged velocities, and the evolution of the age of air were investigated. The turbulent kinetic energy budget and the momentum balance in the time-averaged Navier–Stokes equations were then studied in an attempt to clarify the mechanism of the swirl diffuser's high diffusing performance. With the increasing swirling strength, the pressure gradient force could not balance the centrifugal force in the radial direction. As a result, the vortex core expanded, generating turbulence. A non-dimensional swirl number parameter was suggested which can be used for determining the structure of the swirl diffuser airflow. To validate this non-dimensional parameter, different combinations of φ, h, and r were examined. It was found that only if the swirl number is the same, will the airflow structure be similar. The results of our study are likely to be useful for obtaining deeper insights about the swirl diffuser airflow, and for better understanding of the effect of the controlling parameter on the swirl diffuser's performance.

Effects of Airflow Deflection Angle in Diffuser on Forced Response Caused by Impeller-diffuser Interaction in Centrifugal Compressors

Aeroelasticity has always been a significant issue of compressors. Impeller-diffuser interaction (IDI) produces periodic aerodynamic excitation which forces the blades to vibrate in centrifugal compressors, and eventually causes the damage of blades. Therefore, it is of great importance to investigate the IDI in centrifugal compressors and the effects on the vibration of blades. In this paper, it is discussed in detail that the deflection angle of the airflow in diffuser affects the IDI of the centrifugal compressor and the vibration of blades. The results show that if deflection angle is positive, the interaction of the diffuser on impeller and the vibration of main blades are decreased. However, on condition that the deflection angle is excessive, the interaction of the diffuser on the flow field of impeller will be intensified, and the vibration of the main blades will be stronger. Therefore, it is necessary to reasonably control the range of the deflection angle so that centrifugal compressors are able to work in a safer environment.

複雑な形状の吹出し口を有する室内気流のCFD解析に関する研究 (その8)噴流理論を用いたアネモ型ディフューザー冷房設定時の吹出し気流性状把握

複雑な形状の吹出し口を有する室内気流のCFD解析に関する研究 (その8)噴流理論を用いたアネモ型ディフューザー冷房設定時の吹出し気流性状把握

The study of the velocity field of the air flowing the swirl diffusers using PIV method.

The geometry of swirl diffusers plays a primary role in the effective airflow distribution. In this work, the airflow of swirl diffusers has been studied with the use of the Particle Image Velocimetry (PIV) method in order to analyse the mean swirling air flow and the geometry influence on the results. The presented laboratory model was a simplification of the real system and was made in the geometric scale 1:10. Based on the measurements, the velocity flow field was evaluated and the range of the effective operation of the diffuser in the axis of the flow was determined. This will enable a better definition of the conditions of ventilation in buildings and will help in the design of more efficient diffusers. Also, it will help in a better understanding of the swirl diffuser airflow and the physical effects of the key geometric features.

3D Moth-inspired chemical plume tracking and adaptive step control strategy

This paper presents an adaptive-step-control (ASC) strategy for autonomous chemical plume tracking (CPT) in both 2D and 3D spaces. The aim of CPT is tracking a target chemical flow back to its source and declaring its location. While the majority of related works are based on wheeled robots in 2D, the rapid development of rotorcrafts makes 3D plume tracking possible. Hence, in this paper, we first discuss the generalized form of moth-inspired CPT work flow, then extend the orthodox moth-inspired CPT from 2D to 3D. More importantly, we also propose an ASC strategy to deal with the “passing-by” problem and to gain better control performance. Simulation results are given to demonstrate that ASC can solve the inflexibility caused by “hard” step length in diffusive airflow, because it can autonomously adjust the step lengths of the robot according to the status of the environment, enabling our robot to stride confidently in the low chemical concentration areas and focus on highly suspicious areas.

CFD Simulation of Turbulent Air Flow in Diffuser and it's Effect on Fluid Properties

CFD Simulation of Turbulent Air Flow in Diffuser and it's Effect on Fluid Properties

Effects of diffuser airflow minima on occupant comfort, air mixing, and building energy use (RP-1515)

There is great energy-saving potential in reducing variable air volume box minimum airflow set-points to about 10% of maximum. Typical savings are on the order of 10%–30% of total HVAC energy, remarkable for an inexpensive controls set-point change that properly maintains outside air ventilation. However, there has long been concern whether comfort and room air mixing are maintained under low flows through diffusers, and this concern has prompted variable air volume minima to be typically set at 20%–50% of maximum. RP 1515 evaluated occupants’ thermal comfort and air quality satisfaction in operating buildings under both conventional and reduced minimum variable air volume flow set-points, and measured the air diffusion performance index and air change effectiveness for typical diffuser types in the laboratory. The hypotheses were that lowered flow operation would not significantly reduce comfort or air quality and that HVAC energy savings would be substantial. The hypotheses were almost entirely confirmed for both warm and cool seasons. But beyond this, the reduction of excess airflow during low-load periods caused occupants’ cold discomfort in the warm season to be halved, a surprising improvement. It appears that today's widespread overcooling of buildings can be corrected without risk of discomfort by lowering conventional variable air volume minimum flow set-points.

Quantification of the defect size of ultrafiltration membrane system using mathematical model

The estimation of the damaged part size could be conducted by using a relationship between the value of pressure decay rate and the size of the damaged part, since pressure decay rate performs as a good indicator for the molar flow of air leakage or diffusion airflow. This study presents the development of a predictive model for estimating the air leakage through a defect and its contribution to pressure decay, and develops a prediction model of the size of membrane damage to evaluate the size of the defect. The results obtained from nitrogen flow rate measurement and pressure decay rate (PDR) allowed for the consistent determination of membrane defect size. The results also indicated that nitrogen flow rate and PDR are relatively dependent on holdup volume and independent on membrane area for a specific membrane under certain margin of applied pressure with the same water temperature. The experimental results demonstrated that the mathematical model could estimate the defect size as a function of applied pressure and pressure decay rate.

Autonomous tracking of chemical plumes developed in both diffusive and turbulent airflow environments using Petri nets

This paper presents a Petri net model for autonomous tracking of chemical plumes developed in both diffusive and turbulent airflow environments. It has been challenging to develop a generalized algorithm to effectively trace both types of chemical plumes due to the significant differences of their kinetic and dynamic properties. Our idea is to utilize a Petri net to model the change relationships of chemical concentrations acquired by two sensors mounted on the both sides of a DaNI robot during a tracing process. Because the relationships imply the effects of flow variation on chemical puffs, a flow sensor is eliminated. To express and maintain the knowledge of chemical concentration changes using the Petri net, we design a mapping algorithm for generating the Petri net from production rules. The Petri net model is implemented on the robot using LabVIEW. The chemical plume tracing experiments achieve 93.8% and 87.5% of source localization rates under both turbulent and diffusive airflow environments, respectively.

Effect of Diffuser Angle on the Reattachment Point of an Incompressible Airflow

Numerical Simulation of the two dimensional incompressible airflow in an asymmetric diffuser using one commercially available Computational Fluid Dynamics (CFD) code are reported. The subject is to analyze the effect of diffuser angle on the reattachment point of air flow. Results are presented varying diffuser angle from 2 degree to 20 degree. The mathematical model equations (mass conservation and momentum) are solved using finite volume methods (FVM) and a pressure based approach. To get the grid independent, intensive refinement studies were carried out. Results obtained were compared to experimental data presenting a good agreement. The numerical results of this work show that as the diffuser angle elevates, the flow recirculation region also increases. When there are flow recirculation regions, the value of turbulent kinetic energy also increases. Hence, it is possible identify a need for improvement the design of diffuser or other alternatives to reduce the size of recirculation region, reducing the power necessary at the flow machine to realize the work at the diffuser.

Numerical investigation of the swirling air diffuser: Parametric study and optimization

During the recent decade, high induction diffusers have become more appealing in applications which require relatively high ventilation airflow rates, such as clean rooms. In this research, the effect of geometric parameters on the performance of a specific type of swirling air diffuser is investigated numerically. The results show that although the diffuser slots geometry, namely their angle and aspect ratio, is impressive on the diffuser performance, it is not as important as the swirling blade angle and the performance is almost constant in a wide range of slots specifications. The results also demonstrate that the diffuser performance and the resultant indoor airflow distribution highly depends on the blades angle and their sensitivity is surprisingly significant when the angle is between 30° and 35°. It is also worth mentioning that the optimum angle of 32° is almost independent of the diffuser airflow rate. Finally, the diffuser jet indexes, namely jet decay coefficient and entrainment ratio, are evaluated for the optimum diffuser and are compared with other common types of diffusers. This research and its results are useful in gaining a better understanding of the swirling diffuser airflow and the effect of diffuser geometry features on its performance.

Membrane filtration for cold regions – impact of cold water on membrane integrity monitoring tests

Low-pressure water treatment membranes, microfiltration (MF) and ultrafiltration (UF) have enjoyed unprecedented growth during the past decade. These systems are particularly suited for cold region communities because of their operational simplicity, small footprint, and consistent performance despite the fluctuations in raw water characteristics. Effective operation of MF and UF membrane systems in cold regions, however, must account for the impact of low water temperatures on membrane operation. Of particular interest is the impact of cold water on direct membrane integrity monitoring tests. Most regulatory bodies require that regular integrity monitoring tests be performed for low-pressure membranes to ensure the integrity of the membrane systems. Currently, the most widely-used integrity monitoring test is the pressure decay test. Our studies, however, indicated that the pressure-based direct integrity monitoring tests are affected by variations in water temperature, especially in the range of 0 to 5 °C. The considerable drop in diffusive air flow rates and consequent decrease in pressure decay rate for an intact membrane as water temperatures approach 0 may mask the impact of a defect and should be accounted for. It is suggested that the criteria for membrane integrity tests in cold regions shift downward to account for the effect of temperature. This paper presents the results obtained from studies on the effect of water temperature on the pressure-based direct integrity monitoring tests. Key words: membrane integrity monitoring, pressure decay test, diffusive air flow test, low-pressure membranes, microfiltration, ultrafiltration, water treatment, pathogen removal.

Investigation of far infrared radiation heating as an alternative technique for surface decontamination of strawberry

Postharvest heat treatments have recently received attention as a means to prevent fungal spoilage of strawberry during shelf life. Far infrared radiation (FIR) heating technology may be an alternative to conventional methods because it can achieve rapid and contactless heating. Monte Carlo FIR radiation simulations combined with convection–diffusion air flow and heat transfer simulations were performed in the CFD code ANSYS CFX5.7 to investigate the suitability of the method for surface decontamination. Computations were validated against measurements with a thermographic camera. FIR heating achieved more uniform surface heating than air convection heating, with a maximum temperature well below the critical limit of about 50 °C at the same average temperature. In this configuration, the resulting surface FIR heating rate was, however, smaller or only equal to the air convection heating (at 0.2 m s −1), depending on the heater temperature used. A better configuration consisted of FIR heaters on four sides combined with a cyclic heating operation.