Expiratory aerosol particle escape from surgical masks due to imperfect sealing

Wearing surgical masks or other similar face coverings can reduce the emission of expiratory particles produced via breathing, talking, coughing, or sneezing. Although it is well established that some fraction of the expiratory airflow leaks around the edges of the mask, it is unclear how these leakage airflows affect the overall efficiency with which masks block emission of expiratory aerosol particles. Here, we show experimentally that the aerosol particle concentrations in the leakage airflows around a surgical mask are reduced compared to no mask wearing, with the magnitude of reduction dependent on the direction of escape (out the top, the sides, or the bottom). Because the actual leakage flowrate in each direction is difficult to measure, we use a Monte Carlo approach to estimate flow-corrected particle emission rates for particles having diameters in the range 0.5–20 μm. in all orientations. From these, we derive a flow-weighted overall number-based particle removal efficiency for the mask. The overall mask efficiency, accounting both for air that passes through the mask and for leakage flows, is reduced compared to the through-mask filtration efficiency, from 93 to 70% for talking, but from only 94–90% for coughing. These results demonstrate that leakage flows due to imperfect sealing do decrease mask efficiencies for reducing emission of expiratory particles, but even with such leakage surgical masks provide substantial control.

Influence of Stator Hub Cavities on the Forced Response Behaviour of an Embedded Compressor Rotor

This paper focuses on the impact of hub labyrinth seal leakage flows on the aeromechanical behavior of an embedded compressor rotor. End wall flows are critical in determining the performance of gas turbine engine compressors, particularly the hub leakage flows that can contribute to a significant reduction in performance due to the loss in efficiency induced by the leakage. While the current literature does contribute extensively to the understanding of the influence of this leakage flow on the steady compressor performance, no attention has been given to its impact on the multi-row unsteady aeromechanical influence. The authors of this paper have talked about the multi-row influence at various modes and operating condition using models without the hub cavities included [11–12;33–34]. The embedded compressor rotor utilized for this study is a part of a 3.5 stage subsonic rig located at the Zucrow Laboratory at Purdue University. The current paper first addresses the steady aerodynamics of a multi-row compressor with hub cavities and talks in detail about the effect of cavities on the performance at both the torsional mode and a higher order mode. Next the influence on the forcing function utilizing both 3-row (S1/R2/S2) and 4-row (S1/R2/S2/R3) simulations at both the Peak Efficiency (PE) and the High Loading (HL) operating conditions is determined. To reduce the computational domain significantly, the time transformation (TT) method was utilized within ANSYS CFX. The first part of the paper describes the multi-row influence of two neighboring stators having the same vane count, which excites the embedded rotor at the same resonant frequency; the second part shows the influence of having physical waves reflecting from a rotating row downstream (R3). The results show the significance of modelling the stator hub cavities and the drastic improvement in the modal force prediction with the cavities included. However, the authors observed that the impact tends to be more significant when the computational domain is small, i.e., fewer rows are included. As the number of rows are increased the influence of hub cavities diminish. Some of the conclusions drawn from this study are: 1) The presence of hub cavities changes the angle of incidence to the stators thereby reducing flow separation at the hub. The influence of these propagate throughout the domain i.e., a change in the angle of incidence in the first stage has an effect even at a downstream row. 2) The modal force prediction improved by ∼10% for the 3-row case and 1% for the 4-row case and the values moved closer to the experimental values in both cases. 3) The influence of hub cavities is more significant at torsional modes compared to higher order modes.

Parametric analysis of energy recovery ventilation performance in the high rise residential sector: a Toronto case study

The performance of energy recovery ventilation (ERV) units is analyzed for high rise residential buildings in the Toronto climate. ERV units are scrutinized by their ability to recover heating and cooling energy throughout the year. The rated effectiveness of ERV units is well documented through applicable Standards. Several factors are identified which have the potential to influence the impact of ERVs under actual operation conditions, including infiltration rate, ERV leakage flows, temperature set points, and operation schemes. EnergyPlus is used to represent a typical residential suite, through which the impacts of the parameters are studied. Performance of the ERV model is validated through comparison to collected temperature and humidity data from an operating ERV unit in the GTA. Results indicate that leakage flows within the ERV represent the highest potential contributor to ERV performance. Economizer control strategies are determined as a viable option for improving performance during warmer seasons. The performance of energy recovery ventilation (ERV) units is analyzed for high rise residential buildings in the Toronto climate. ERV units are scrutinized by their ability to recover heating and cooling energy throughout the year. The rated effectiveness of ERV units is well documented through applicable Standards. Several factors are identified which have the potential to influence the impact of ERVs under actual operation conditions, including infiltration rate, ERV leakage flows, temperature set points, and operation schemes. EnergyPlus is used to represent a typical residential suite, through which the impacts of the parameters are studied. Performance of the ERV model is validated through comparison to collected temperature and humidity data from an operating ERV unit in the GTA. Results indicate that leakage flows within the ERV represent the highest potential contributor to ERV performance. Economizer control strategies are determined as a viable option for improving performance during warmer seasons.

Parametric analysis of energy recovery ventilation performance in the high rise residential sector: a Toronto case study

The performance of energy recovery ventilation (ERV) units is analyzed for high rise residential buildings in the Toronto climate. ERV units are scrutinized by their ability to recover heating and cooling energy throughout the year. The rated effectiveness of ERV units is well documented through applicable Standards. Several factors are identified which have the potential to influence the impact of ERVs under actual operation conditions, including infiltration rate, ERV leakage flows, temperature set points, and operation schemes. EnergyPlus is used to represent a typical residential suite, through which the impacts of the parameters are studied. Performance of the ERV model is validated through comparison to collected temperature and humidity data from an operating ERV unit in the GTA. Results indicate that leakage flows within the ERV represent the highest potential contributor to ERV performance. Economizer control strategies are determined as a viable option for improving performance during warmer seasons. The performance of energy recovery ventilation (ERV) units is analyzed for high rise residential buildings in the Toronto climate. ERV units are scrutinized by their ability to recover heating and cooling energy throughout the year. The rated effectiveness of ERV units is well documented through applicable Standards. Several factors are identified which have the potential to influence the impact of ERVs under actual operation conditions, including infiltration rate, ERV leakage flows, temperature set points, and operation schemes. EnergyPlus is used to represent a typical residential suite, through which the impacts of the parameters are studied. Performance of the ERV model is validated through comparison to collected temperature and humidity data from an operating ERV unit in the GTA. Results indicate that leakage flows within the ERV represent the highest potential contributor to ERV performance. Economizer control strategies are determined as a viable option for improving performance during warmer seasons.

PHANTOM COOLING EFFECTS ON ROTOR BLADE SURFACE HEAT FLUX IN A TRANSONIC FULL SCALE 1+1/2 STAGE ROTATING TURBINE

An experimental and numerical investigation of phantom cooling effects on cooled and uncooled rotating high pressure turbine blades in a full scale 1+1/2 stage turbine test is carried out. Objectives set to capture, separate, and quantify the effects of upstream vane film-cooling and leakage flows on the downstream rotor blade surface heat flux. Multiple series of tests were carried out in the Air Force Research Laboratory, Turbine Research Facility, at Wright-Patterson Air Force Base, Ohio. A non-proprietary research turbine test article is uniquely instrumented with high frequency double-sided thin film heat flux gauges custom made at AFRL. High bandwidth, time resolved surface heat flux is measured on multiple film-cooled and non-film-cooled HPT rotor blades downstream of both film-cooled and non-film-cooled vane sectors. Upstream wake passing and heat flux is characterized on both rotor pressure and suction side surfaces, along with quantifying rotor phantom cooling effects from non-uniform 1st stage vane film cooling and leakage flows. Fast response heat flux measurements quantify how rotor phantom cooling impacts the blade pressure side greatest; increasing along the pressure side towards the trailing edge. It is discovered upstream vane film-cooling alone can account for 50% of the rotor blade cooling effect, and even outweigh the rotor blade film cooling effect far from the blade showerhead holes. Added unsteady numerical simulation demonstrates how variations in inlet total temperature and incidence angle can also contribute to circumferentially non-uniform rotor heat flux.

Effect of in-service burnout effect on the transonic leakage flows over cavity tip model

Increasing the gas temperature at the inlet to the high pressure turbine of gas turbine engines is known as a proven method to increase the efficiency of these engines. However, this will expose the blades’ surface to very high heat load and thermal damages. In the case of the un-shrouded turbine blades, the blade tip will be exposed to a significant thermal load due to the developed leakage flows in the tip gap, this leads to in-service burnout which degrades the blade tip and shortens its operational life. This paper studies the in-service burnout effect of the transonic tip flows over a cavity tip which is a configuration commonly used to reduce the tip leakage flows. This investigation is carried out experimentally within a transonic wind tunnel and computationally using steady and unsteady Reynolds Averaged Navier Stokes approaches. Various flow measurements are established and different flow behaviour including separation bubbles, shockwave development and distinct flow interactions are captured and discussed. It is found that when the tip is exposed to the in-service burnout, leakage flow behaves in a significantly different way. In addition, the effective tip gap becomes much larger and allows higher leakage mass flow rate in comparison to the sharp-edge tip (i.e. a tip at the beginning of its operational life). The tip leakage losses are found much higher for the round-edge cavity tip (i.e. a tip exposed to burn-out effect). Experimental and computational flow visualisations, surface pressure measurements and discharge coefficient variation are given and analysed for several pressure ratios across the tip gap.