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Due to the large degree of freedom in terms of design and installation, flexible ventilation ducts are commonly used in ventilation systems. However, excessive use of flexible ducts may lead to greater pressure drop and higher energy consumption. This study conducted experimental measurements to characterize the pressure drop in flexible ventilation ducts with different compression ratios and bending angles. This investigation first measured the pressure drop in straight flexible ducts with four compression ratios under various airflow rates. The calculated friction factor for the straight flexible ducts was negatively associated with the compression ratio. Next, the pressure drops in single-bend flexible ducts with various bending angles from 30° to 150° were measured under various airflow rates. The calculated loss coefficient of the bend increased with the bending angle for single-bend flexible ducts. Finally, the influence of the intermediate duct length on the pressure drop across two bends was experimentally investigated. When the length of the intermediate duct was greater than eight times the inner diameter, the pressure drop across a double-bend flexible duct could be calculated from the friction factors and loss coefficients with a relative error less than 1%. The data obtained in this study can be used to calculate the total pressure loss in flexible ventilation ducting systems in buildings.

Aerosol generated from a human cough can be a potential major indoor health risk due to the possible transmission of infectious respiratory diseases to surrounding individuals within the same room and even could spread out via air-ventilation/conditioning systems. This study aims to investigate the transport characteristics and trajectory of coughed aerosols under the influence of conditioned air ventilation as well as near-by human breathing zone using computational fluid dynamics (CFD). An experimental system consisting of air-conditioned space with multiple inlets and outlets, a cough simulator and a receiver was built to validate the CFD predictions. The comparison is in good agreement. The CFD model was established as a transient three-dimensional multiphase multicomponent Eulerian–Lagrangian model and numerically solved using commercial software ANSYS Fluent. Both gas and liquid phases were modelled as multicomponent mixtures. With this CFD model, the indoor transport and trajectory of coughed aerosols can be accounted for the distributions of portions inhaled by each manikin, deposited on surfaces of manikins and chamber walls, as well as recirculated back into the ventilation system. Results reveal that the aerosol source location and the ambient air movement can be crucial factors of aerosol trajectory in terms of direct and indirect influence.

In the present study, the effect of bend angle on pressure drop and flow behavior in a small-diameter corrugated duct is numerically investigated and experimentally validated under a fully developed flow condition. The large eddy simulation, together with the proper orthogonal decomposition (POD) method, is employed to study the pressure drop, mean flow pattern, and unsteady flow evolution for a corrugated duct with various bend angles. The results show that the pressure drop exhibits a monotonic increase with increasing bend angle. Specifically, as the bend angle increases from $$0^{\circ }$$ to $$90^{\circ }$$ , the pressure drop of the corrugated duct experiences a striking increase of about 43%. Accordingly, a larger bend angle is found to induce the occurrence of stronger Dean cells or larger swirl intensity downstream the duct bend. Meanwhile, as for larger bend angles, the main turbulent properties of the Dean cells could be, to some extent, governed by the first few POD modes, which appear to be featured with one or a few large-scale vortices. Generally, the larger bend angle causes stronger swirl intensity and wave-like structures, thus rendering severer pressure drop or larger pressure loss coefficient in the corrugated duct.

The air distribution system in an airliner plays a key role in maintaining a comfortable and healthy environment in the aircraft cabin. To evaluate the performance of a novel displacement ventilation (DV) system and a traditional mixing ventilation (MV) system in an airliner cabin, this study conducted experiments and simulations in a seven-row cabin mockup. This investigation used ultrasonic anemometers and T-thermocouples to measure the air velocity, temperature and distribution of 1 μm and 5 μm particles. Simulation verifications were performed for these operating conditions, and additional scenarios with different occurrence source locations were also simulated. This study combined the Wells-Riley equation with a real case based on a COVID-19 outbreak among passengers on a long-distance bus to obtain the COVID-19 quanta value. Through an evaluation of the airflow organization, thermal comfort, and risk of COVID-19 infection, the two ventilation systems were compared. This investigation found that polydisperse particles should be used to calculate the risk of infection in airliner cabins. In addition, at the beginning of the pandemic, the infection risk with DV was lower than that with MV. In the middle and late stages of the epidemic, the infection risk with MV can be reduced when passengers wear masks, leading to an infection risk approximately equal to that of DV.

Volatile organic compounds (VOCs) as a non-negligible aircraft cabin air quality (CAQ) factor influence the health and comfort of passengers and crew members. On-board measurements of carbonyls (short-chain (C1 -C6 )) and other volatile organic compounds (VOCs, long-chain (C6 -C16 )) with a total of 350 samples were conducted in 56 commercial airliner cabins covering 8 aircraft models in this study. The mean concentration for each individual carbonyl compound was between 0.3 and 8.3 μg/m3 (except for acrolein & acetone, average = 20.7 μg/m3 ) similar to the mean concentrations of other highly detected VOCs (long-chain (C6 -C16 ), 97% of which ranged in 0-10 μg/m3 ) in aircraft cabins. Formaldehyde concentrations in flights were significantly lower than in residential buildings, where construction materials are known formaldehyde sources. Acetone is a VOC emitted by humans, and its concentration in flights was similar to that in other high-occupant density transportation vehicles. The variation of VOC concentrations in different flight phases of long-haul flights was the same as that of CO2 concentration except for the meal phase, which indicates the importance of cabin ventilation in diluting the gaseous contaminants, while the sustained and slow growth of the VOC concentrations during the cruising phase in short-haul flights indicated that the ventilation could not adequately dilute the emission of VOCs. For the different categories of VOCs, the mean concentration during the cruising phase of benzene series, aldehydes, alkanes, other VOCs (detection rate > 50%), and carbonyls in long-haul flights was 44.2 µg/m3 , 17.9 µg/m3 , 18.6 µg/m3 , 31.5 µg/m3 , and 20.4 µg/m3 lower than those in short-haul flights, respectively. Carbonyls and d-limonene showed a significant correlation with meal service (p

A condensation-induced depressurization and steam purging technology has been recently developed to drive a continuous gas flow at a sub-atmospheric pressure. The technology is based on a continuous regeneration of depressurization by a system of two or multiple alternatively-operated chambers with steam condensation and refilling. Comparing with traditional methods, this new vacuum generation technology has a great potential of high energy conversion and utilization for an open-flow system. A complete operation cycle of each chamber undergoes three stages in sequence: vacuum generation by condensing pre-refilled steam, vacuuming gas from application system, and purging gas out of chamber while refilling steam. This paper presents a mechanistic model of process characteristics and parametric analysis for the thermodynamic cyclic operations. Each process stage has been separately modelled, and the complete cycle characteristics are then integrated by connecting these stages in series. The mechanistic model provides a parametric analysis capability for the optimized operation of the developed technology to vacuum driving continuous gas flows at sub-atmospheric pressures. The model result is compared with experiment data with good match. The parametric study of the steam purging process indicates that increasing the steam flowrate and pre-heating of chamber can effectively reduce the purging time required by 20–40%.

As most airplanes do not have HEPA filters for filtering outside air, particulate matter in the outdoor air can deposit on the environmental control systems (ECS) of the airplanes. The particles that accumulate on the various surfaces of the ECS components can affect their thermal performance and may lead to component failures. This study experimentally and numerically investigated the particle deposition on a heat exchanger and a turbocharger, which are key components of ECS with complex geometry. A test rig was built to obtain the monodisperse particle deposition fractions by measuring the particle concentration upstream and downstream of the components with the weighing method. The tested particles ranged from 1 to 8 μm in diameter. Different Reynolds-averaged Navier-Stokes (RANS) models, together with a modified Lagrangian method, were used to predict the total particle deposition fractions in the tested components. The computed particle deposition was compared with the experimental data. The results showed that the RNG k-e model with near-wall correction provided the most accurate prediction of the particle deposition fraction on the heat exchanger and turbocharger. The particle deposition fraction increased significantly with the particle size. CFD simulation provided detailed information about the particle deposition distribution inside the heat exchanger and turbocharger. The location and number of deposited particles depended mainly on the particle size and air velocity. This investigation identified a suitable tool for studying particle deposition in the ECS of commercial airplanes.

Enhanced soiling around multi-slot air diffusers due to particle deposition is frequently observed in commercial airplanes. The dirty black soiling is very unsightly and influences the passengers’ perception of cabin air quality. This study conducted experimental measurements and large eddy simulations with Lagrangian tracking for the distribution of particle deposition around a multi-slot diffuser. This investigation first used a relatively simple case of indoor particle deposition to compare the LES-Lagrangian model with the RANS-Lagrangian model with near-wall turbulence kinetic energy correction. The comparison shows that the LES-Lagrangian model was more robust than the RANS-Lagrangian model in predicting particle deposition indoors. The superior LES-Lagrangian model was then applied in predicting the particle deposition distribution around a multi-slot diffuser. This investigation also conducted detailed measurements of the distribution of particle deposition around the multi-slot diffuser in a laboratory chamber using a wiping method on a resolution of 3 × 20 mm2. The measurement accuracy of the wiping method was within 20%. The particle deposition distribution predicted by the LES-Lagrangian model was compared with the experimental data to validate the model. The results indicated that the LES-Lagrangian model correctly predicted the order of magnitude of the particle deposition velocity distribution around the multi-slot diffuser with an average relative error of 63.2%.

Porous insulation materials may acquire moisture, and the moisture content must be accurately measured for appropriate maintenance of the materials. Current thermal methods for measuring moisture are applicable only to an infinite test domain. This paper used computational fluid dynamics (CFD) to investigate the effects of a finite test domain and the associated boundary heat loss on moisture measurement. Insulation material of various thicknesses, bounded by two solid plates, was modeled. The convective heat loss to the air from both solid plates was considered. The over-measured moisture content due to a finite geometric domain was regressed into formulas, by means of which the excessive amount could subsequently be deduced. The corrected moisture content was then compared with that obtained by gravimetric weighing using a digital precision balance. The results show that significant over-measurement of moisture content can occur for the tested insulation material with a thickness less than 4.5 cm. The remedial strategy is quite effective and can provide moisture content results that are close to the gravimetric values.

This study develops a control system to automate the operation of a condensation-induced depressurization technology, which is used to achieve sub-atmospheric pressure in an open-flow system on ground. The continuous depressurization is maintained via an integrated series of chambers inside which vacuum is regenerated by condensing and refilling of saturated steam. The low pressure generated inside the chambers is then used to alternatively extract the air out of a flow system for maintaining its sub-atmospheric pressure. The thermodynamic cycle in such a vacuum chamber consists of three sub-processes: air purging to ambient by steam refilling, depressurization by steam condensation, and air-extraction from a flow application. As one chamber undergoing these consecutive processes, another chamber operates in a coordinated different phase to seamlessly maintain a continuous air-extraction operation. This new system provides a quiet and efficient way of using low-grade energy to generate hypobaric environment for needed applications. A cascade arrangement of a proposed multiple-chamber operation is also illustrated. A control system is designed and implemented to realize the automatic and coordinated operation in a dual-chamber, laboratory-scaled system. Exemplified results on process characteristics such as chamber depressurization and air purging are also provided.

Identifying the exact transmission route(s) of infectious diseases in indoor environments is a crucial step in developing effective intervention strategies. In this study, we proposed a comparative analysis approach and built a model to simulate outbreaks of 3 different in‐flight infections in a similar cabin environment, that is, influenza A H1N1, severe acute respiratory syndrome (SARS) coronavirus (CoV), and norovirus. The simulation results seemed to suggest that the close contact route was probably the most significant route (contributes 70%, 95% confidence interval [CI]: 67%‐72%) in the in‐flight transmission of influenza A H1N1 transmission; as a result, passengers within 2 rows of the index case had a significantly higher infection risk than others in the outbreak (relative risk [RR]: 13.4, 95% CI: 1.5‐121.2, P = .019). For SARS CoV, the airborne, close contact, and fomite routes contributed 21% (95% CI: 19%‐23%), 29% (95% CI: 27%‐31%), and 50% (95% CI: 48%‐53%), respectively. For norovirus, the simulation results suggested that the fomite route played the dominant role (contributes 85%, 95% CI: 83%‐87%) in most cases; as a result, passengers in aisle seats had a significantly higher infection risk than others (RR: 9.5, 95% CI: 1.2‐77.4, P = .022). This work highlighted a method for using observed outbreak data to analyze the roles of different infection transmission routes.

Enhanced soiling on the surfaces around air supply nozzles due to particle deposition is frequently observed in commercial airliners. The problem is worsened by severe outdoor air pollution and flight delays in China. The particles in an aircraft cabin originate from both outdoor and in-cabin sources. This study conducted measurements on multiple commercial flights to obtain particle emission rates from in-cabin sources. Additional experiments on a retired MD-82 airplane provided justification of the in-flight measurements. The in-cabin sources emitted more particles during boarding/deplaning than during meal servicing and sitting. The average PM2.5 emission rates were 7.2, 2.6, 1.9, and 1.8 (μg/min per person), respectively, during the boarding/deplaning, sitting on the ground, sitting in the air, and meal servicing. The corresponding PM10 emission rates were 15.4, 6.1, 5.3, and 5.4 (μg/min per person), respectively, for these four periods. The average particle emission rate from in-cabin sources varied seasonally and was the highest in winter. With the measured data, this investigation used a CFD model to predict the accumulation of particles deposited around the nozzles of an airplane, taking into account the flight routes and the outdoor particle concentrations at the airports where the airplanes were parked. For the most polluted airplane in China, the dirty spots/areas around the nozzles inside the airplane became visible after 6 months. The method proposed in this study can be used for any commercial airplane to predict the accumulation of particles deposited around the air supply nozzles.

In airliner cabins, mixing ventilation systems with gaspers are not efficient in controlling contaminant transport. To improve the cabin environment, this investigation proposed an innovative ventilation system that would reduce contaminant transport and maintain thermal comfort. We manufactured and installed the proposed ventilation system in an occupied seven-row, single-aisle aircraft cabin mockup. Air velocity, air temperature, and contaminant distribution in the cabin mockup were obtained by experimental measurements. The investigation used the experimental data to validate the results of CFD simulation. The validated CFD program was then used to study the impact of the locations and number of exhausts on contaminant removal and thermal comfort in a one-row section of a fully occupied Boeing-737 cabin. Although the diffusers in the proposed system were close to the passengers' legs, the air velocity magnitude was acceptable in the lower part of the cabin and the leg area. The proposed system provided an acceptable thermal environment in the cabin, although passengers could feel cold when placing their legs directly in front of the diffusers. The four-exhaust configuration of the new ventilation system was the best, and it decreased the average exposure in the cabin by 57% and 53%, respectively, when compared with the mixing and displacement ventilation systems., Highlights • An innovative ventilation system for airliner cabins was proposed. • The system was manufactured and installed in an aircraft cabin mockup. • The system was optimized and assessed.

Aircraft can acquire large amounts of moisture during routine flights. Once moisture is trapped in aircraft walls, the resulting expenses are not just in extra fuel consumption, but also in corrosion repair and additional maintenance. To date, however, there is a lack of concrete data on temporal moisture accumulation and its distribution within aircraft walls. This investigation constructed reduced-scale mockups of an aircraft section for measurement of accumulated moisture during flights. The mockup models were placed in a psychrometric altitude chamber to mimic short-haul, medium-haul, and long-haul flights. For simulation of different flight conditions, the mockups varied in terms of shell material (aluminum alloy or composite plastic), insulation blankets (wrapped in water-resistant bags or not), and operating season (summer or winter). The moisture accumulation within insulation layers and on the inner skin of the shell after each flight was weighed on a digital precision balance. The results show t...

Our understanding of the fomite transmission route of diseases remains at an empirical level. There are no data on how surface contamination is propagated by human touching. We designed a novel and effective benchtop experiment to investigate the dynamic transmission of contaminants on multiple environmental surfaces due to touching. The benchtop experiment setting design was based on an inflight norovirus outbreak. Hundreds of representative environmental surfaces in the plane were scaled down, and fluorescent particles were used as surrogate indicators of virus-laden aerosols. The fluorescent particles were initially carried by six index “patients” and then transmitted to other surfaces through the touching behavior of one hundred and twenty-four “passengers.” The distributions of fluorescent particles were photographed by cameras when exposed to UV light and the acquired photos were processed using fluorescence imaging techniques to quantify fluorescent particles on each surface. The temporal diffusion of contaminated surfaces was found to follow an S-shaped logistic curve. The aisle seats were found to be more contaminated, which was consistent with the reported higher attack rates in passengers seating along the aisle in the outbreak. This study confirmed the findings of the logistic growth from the multi-agent simulations, and provided a possible mechanism for the role played by environmental surfaces in the fomite route of diseases.

Identifying the exact transmission route(s) of infectious diseases in indoor environments is a crucial step in developing effective intervention strategies. In this study, we proposed a comparative analysis approach and built a model to simulate outbreaks of 3 different in-flight infections in a similar cabin environment, that is, influenza A H1N1, severe acute respiratory syndrome (SARS) coronavirus (CoV), and norovirus. The simulation results seemed to suggest that the close contact route was probably the most significant route (contributes 70%, 95% confidence interval [CI]: 67%-72%) in the in-flight transmission of influenza A H1N1 transmission; as a result, passengers within 2 rows of the index case had a significantly higher infection risk than others in the outbreak (relative risk [RR]: 13.4, 95% CI: 1.5-121.2, P = .019). For SARS CoV, the airborne, close contact, and fomite routes contributed 21% (95% CI: 19%-23%), 29% (95% CI: 27%-31%), and 50% (95% CI: 48%-53%), respectively. For norovirus, the simulation results suggested that the fomite route played the dominant role (contributes 85%, 95% CI: 83%-87%) in most cases; as a result, passengers in aisle seats had a significantly higher infection risk than others (RR: 9.5, 95% CI: 1.2-77.4, P = .022). This work highlighted a method for using observed outbreak data to analyze the roles of different infection transmission routes.

Severe air pollution and low on-time performance of commercial flights in China could increase particle deposition in the environmental control systems (ECSs) of commercial airliners. The particles deposited in the ECSs could negatively affect the performance of the airplanes. In addition, particles that penetrate into the aircraft cabin could adversely impact the health of passengers and crew members. This investigation conducted simultaneous measurements of particle mass concentration and size distribution inside and outside the cabin during 64 commercial flights of Boeing 737 and Airbus 320 aircraft departing from or arriving at Tianjin Airport in China. The results showed that the PM2.5 mass concentration deposition in the ECSs of these airplanes ranged from 50% to 90%, which was much higher than that measured in an airplane with a ground air-conditioning unit. The average deposition rates of particles with diameters of 0.5–1 μm, 1–2 μm, 2–5 μm, 5–10 μm, and >10 μm were 89 ± 8%, 85 ± 13%, 80 ± 13%, 73 ± 15%, and 80 ± 14%, respectively. The in-flight measurement results indicated that the particle concentration in the breathing zone was higher than that in the air-supply zone, which implies a significant contribution by particles in the interior of the cabin. Such particles come from human emissions or particle resuspension from interior surfaces.

It is not clear whether turning on the gaspers in the cabins of commercial airliners actually improves the air quality. To answer this question, this study first developed a hybrid turbulence model which was suitable for predicting the air distribution in an aircraft cabin with gaspers turned on. Next, the investigation validated the model using two sets of experimental data from a cabin mockup and an actual airplane. This study then used the validated model to systematically investigate the impact of gaspers on cabin air quality in a seven-row section of the fully-occupied, economy-class cabin of Boeing 767 and 737 airplanes. The CFD calculations formed a database consisting of 9660 data points that provide information about SARS infection risk. It was found that the distribution of opened gaspers can influence the infection risk for passengers. Even though the gasper supplies clean air, it is possible for it to have a negative impact on the passengers' health. Statistically speaking, the overall effect of turning on the gaspers on the mean infection risk for the general population was neutral., Highlights • A hybrid turbulence model for an aircraft cabin with gaspers on was proposed. • The model's ability to predict the air and contaminant distribution was validated. • The impact of gaspers on cabin air quality was investigated.

The objective of this work is to examine the effect of dynamic pressure on the behavior of a horizontally burning flame based on experimental approach. The experiments were conducted in a large-scale altitude chamber of size 2 m × 3 m × 4.65 m under three different dynamic pressures with various depressurization rates. Two round steel fuel pans of 20 or 30 cm in diameter were chosen for the pool fire tests. The fuel pan was filled with 99% pure liquid n-heptane. Cold water was added beneath the fuel layer to cool the pan and minimize the temperature rise in the fuel. The experimental result that the average burning rate for the 30-cm-diameter pan is approximately triple to that for the 20-cm-diameter pan implies the significant influence of pan size on mass burning rate, while the depressurization rate shows minor influence on the mass burning rate. Mass burning rate $$\dot{m}$$ during quasi-steady burning stage increases with pressure P as a power exponent function $$P^{\alpha }$$ , and the power exponent factor α increases with the pool size and is lower under higher depressurization rate. The peak radiative heat flux in 30-cm-diameter cases is approximately 1.5 times higher than that in 20-cm-diameter cases. But there is little impact on the peak radiative heat flux from the depressurization rate. Additionally, the flame temperature at the flame root in 20-cm-diameter pan fire tests is more stable and higher than that in 30-cm-diameter pan fire tests. But at flame front, the flame temperature in 20-cm-diameter pan fire tests falls rapidly and is apparently lower than that in the 30-cm-diameter pan fire tests.

Ventilation systems for commercial airliner cabins are important in reducing contaminant transport and maintaining thermal comfort. To evaluate the performance of a personalized displacement ventilation system, a conventional displacement ventilation system, and a mixing ventilation system, this study first used the Wells-Riley equation integrated with CFD to obtain the SARS quanta value based on a specific SARS outbreak on a flight. This investigation then compared the three ventilation systems in a seven-row section of a fully occupied, economy-class cabin in Boeing 737 and Boeing 767 airplanes. The SARS quanta generation rate obtained for the index patient could be used in future studies. For all the assumed source locations, the passengers' infection risk by air in the two planes was the highest with the mixing ventilation system, while the conventional displacement ventilation system produced the lowest risk. The personalized ventilation system performed the best in maintaining cabin thermal comfort and can also reduce the infection risk. This system is recommended for airplane cabins.

The commercial airliner cabin environment is very important for passengers and crew, and airflow and contaminant concentration distributions in cabins play a key role in creating a comfortable and healthy cabin environment. To study the airflow and the contaminant concentration distributions in airliner cabins, this investigation used a seven-row aircraft cabin mockup with well-controlled thermo-fluid boundary conditions. This study measured air temperature and velocity distributions with displacement ventilation by using T-type thermocouples and ultrasonic anemometers (UA), and contaminant distributions by using tracer gas and particle from a generator to simulate contaminant breathed out by a passenger. This paper discusses our effort to control the thermo-fluid boundary conditions with high accuracy. The measured results show that the air temperature was stratified and the air velocity was low. The contaminant was not transported to the other side of the cabin by the displacement ventilation system and was removed effectively through the ceiling exhaust. The Richardson number (Ri) indicated that the airflow was dominated by natural convection. However, the vertical temperature stratification may create discomfort for passengers.

Enhanced soiling is frequently observed near the multi-slot cabin supply air nozzles in twin-aisle commercial airplanes, which would increase the cleaning and maintenance costs. This study proposed a computer-aided design approach to develop a new cabin supply air nozzle for reducing particle deposition in commercial airplanes. In the new nozzle, the slot dividers were removed to reduce particle deposition, while the shape of the plenum was re-designed using computational fluid dynamics (CFD) to ensure supply air uniformity. The new nozzles were fabricated using a 3D printing technique. The performance of the new cabin supply air nozzles was compared with that of the original multi-slot nozzles by both experimental measurements and numerical simulations, in terms of supply air velocity distribution, particle deposition, pressure drop, noise level, and weight. The results show that the new cabin supply air nozzles can significantly reduce particle deposition on the target surface in comparison to the original multi-slot nozzles, while providing similar airflow and temperature distributions in the aircraft cabin. Furthermore, the new nozzles exhibited better supply air uniformity, lower pressure drop, lower noise level, and were lighter in weight than the original multi-slot nozzles.

Enhanced soiling on the wall/ceiling around a diffuser due to particle deposition is very unsightly and reduces our quality of life. This study aimed to model the particle deposition on the surfaces around multi-slot diffusers, which are widely used in transportation vehicles. An SST k-ω model with a modified Lagrangian method was proposed and validated with experimental data on particle deposition rate from the literature. This investigation then conducted chamber tests to qualitatively validate model's ability to predict the deposition distribution around a multi-slot diffuser. Using the validated model, this study numerically investigated the effects of slot setting, supply air angle, and temperature differential on particle deposition around a multi-slot diffuser. The results indicated that, with the same supply airflow rate, increasing the area ratio of openings to bars in a multi-slot diffuser can reduce the particle deposition. When the angle between the supply air jet and the wall was increased to more than 45°, the particle deposition was significantly reduced. Furthermore, the impact of thermophoresis on particle deposition around a multi-slot diffuser was negligible.

The environmental control system (ECS) of a commercial airplane supplies air to the cabin in order to maintain a safe, comfortable, and healthy environment for passengers and crew members. Because about half of the air supplied to the cabin is outside air, atmosphere particles could deposit in the ECS before entering the cabin. This investigation developed a model to calculate the particle deposition rates in the ECS for different particle sizes on the basis of a set of empirical equations from the literature. The model was used to predict particle deposition in five types of commercial airplanes (a regional jet, Boeing 737-800, Airbus 319, Airbus 320, and MD-82). The predicted results were compared with data measured in-flight or during operation on the ground and agreed well with the measured data. Both the simulated and measured results showed that almost all the large particles (d p ≥ 5.0 μm) and 75% of small particles (d p = 0.3–5.0 μm) were deposited in the ECS. Most of the particle deposition occurred near the entrance to the ECS where the geometry was the most complex.

This study investigated the air distribution and contaminant transport in aircraft cabins with gaspers by using computational fluid dynamics (CFD). If the detailed gasper geometry were used in the CFD simulations, the grid number would be unacceptably high. To reduce the grid number, this investigation proposed a method for simplifying the gasper geometry. The method was then validated by two sets of experimental data obtained from a cabin mockup and a real aircraft cabin. It was found that for the cabin mockup, the CFD simulation with the simplified gasper model reduced the grid number from 1.58 to 0.3 million and the computing cost from 2 days to 1 hour without compromising the accuracy. In the five-row economy-class cabin of the MD-82 airplane, the CFD simulation with the simplified gasper model was acceptable in predicting the distribution of air velocity, air temperature, and contaminant concentration.

Modern commercial airplanes operate in an extreme atmospheric environment, where they can easily acquire a large amount of moisture within the porous insulation layers. The accumulated moisture may increase the aircraft’s weight, induce corrosion, and degrade thermal and sound insulation performance. This investigation proposes a numerical model to predict moisture and heat transfer within porous insulation blankets. To capture natural convection flows of gases within the porous material, the volumetric-based Navier–Stokes equations were solved. The moisture in three phases was considered and the phase change rates between any two phases were predicted. The latent heat of absorption or release during phase changes is balanced with the heat conduction in the mixture and the heat convection of the gaseous substances. To validate the model, the moisture gain in a reduced-scale aircraft mockup was measured by placing the mockup inside an altitude psychrometric chamber. Then the validated model was applied to predict the moisture accumulation in a rectangular insulation block from a Boeing 777 airplane. The results reveal that the flight altitude and cabin humidity are the two key parameters determining the moisture accumulation rates. A high outer shell surface temperature is helpful for reducing moisture gain in the insulation blankets.

Commercial aircraft use environmental control systems (ECSs) to control the thermal environment in cabins and thus ensure passengers’ safety, health, and comfort. This study investigated the interaction between ECS operation and cabin thermal environment. Simplified models were developed for the thermodynamic processes of the key ECS components in a commercial software program, ANSYS Simplorer. A computational fluid dynamics (CFD) program, ANSYS Fluent, was employed to simulate the thermal environment in a cabin. Through the coupling of Simplorer and Fluent, a PID control method was applied to the aircraft ECS in Simplorer to achieve dynamic control of the temperature of the supply air to the cabin, which was used as a Fluent input. The calculated supply air temperature agreed with the corresponding experimental data obtained from an MD-82 aircraft on the ground. The coupled model was then used to simulate a complete flight for the purpose of studying the interaction between ECS operation and the cabin thermal environment. The results show that the PID controller in the ECS can maintain the cabin air temperature within ±0.6 K of the set point, with an acceptable air temperature distribution. The coupled models can be used for the design and analysis of the ECS and cabin thermal environment for commercial airplanes.

Serious air pollution and low on-time performance of commercial flights in China could result in more particles being deposited in the environmental control systems (ECS) of the commercial airliners and ground air-conditioning carts (GAC). The particle deposited in the ECS and GAC could cause performance issues of the airplanes and GAC. In addition, particles penetrated to the aircraft cabin could cause adverse health impact on the passengers and crew. This investigation measured the PM 2.5 particle concentrations and the quantities of particles of different sizes at the inlet and outlet of the GAC and ECS in an MD-82 airplane parked next to Tianjin Airport under different air quality levels. The results showed that the deposition rate of the PM 2.5 mass in the GAC and ECS was 40–50%, with most (30–40%) of the deposition occurring in the ECS. For particles with a diameter of 5 μm or larger, the deposition rate was greater than 90%. For particles with a diameter of 0.5 μm or less, the deposition rate was less than 25% so they entered into the aircraft cabin. In addition, particle mass and number concentration was measured on commercial flights. The results indicated that particle concentrations were high compared with that during the cruising when the airplanes were on the ground at the Chinese airports where ambient particle concentrations were also high.

Overhead gaspers are prevalently installed in aircraft cabins as a personalized ventilation system. The air distribution in cabins with gaspers on is crucial for creating a thermally comfortable and healthy cabin environment. However, very few studies have investigated the suitable turbulence model to simulation air distribution in cabins with gaspers turned on. This study first conducted experimental measurements of airflow distribution in a mock-up of half of a full-scale, one-row, single-aisle aircraft cabin with a gasper on. Particle image velocimetry was used to measure the complex airflow field above a human simulator. This investigation then used the measured data to evaluate the performance of computational fluid dynamics with the re-normalization group (RNG) k–e model and the shear stress transport (SST) k–ω model. The results showed that the SST k–ω model was more accurate than the RNG k–e model for predicting the airflow distribution in gasper-induced jet dominant region in an aircraft cabin.

Gaspers installed in commercial airliner cabins are used to improve passengers' thermal comfort. To understand the impact of gasper airflow on the air quality in a cabin, this investigation measured the distributions of air velocity, air temperature, and gaseous contaminant concentration in five rows of the economy-class section of an MD-82 commercial aircraft. The gaseous contaminant was simulated using SF6 as a tracer gas with the source located at the mouth of a seated manikin close to the aisle. Two-fifths of the gaspers next to the aisle were turned on in the cabin, and each of them supplied air at a flow rate of 0.66 l/s. The airflow rate in the economy-class cabin was controlled at 10 l/s per passenger. Data obtained in a previous study of the cabin with all gaspers turned off were used for comparison. The results show that the jets from the gaspers had a substantial impact on the air velocity and contaminant transport in the cabin. The air velocity in the cabin was higher, and the air temperature slightly more uniform, when the gaspers were on than when they were off, but turning on the gaspers may not have improved the air quality.

The bacteria and fungi captured on heat insulating materials or air filter media in the ventilation system in civil flights may reentrant into the indoor air which poses a threat to passengers’ health. The E. coli and B. subtilis bacteria survival rates on the heat insulating blankets typically used as padding materials inside the aircraft walls under the mimicked environmental condition were studied. Then the antibacterial performance of positively charged electret and chitosan modified filter media which can be used in the ventilation systems against E. coli bacteria was investigated. Our results showed that the E. coli survival rate deceased to less than 0.2% and that of the B. subtilis decreased to less than 2.1% of the control samples on the hydrophilic and hydrophobic blankets under the mimic conditions after one flight cycle. The survival rate of E. coli on the positively charged electret filter media decreased to less than 30% compared to the uncharged filter media after six hours due to the disruption of their metabolic balance by the positive charges on the fibers surface. Moreover, the E. coli survival rate on the pure nylon, nylon/chitosan and chitosan dipped nylon-6 nanofibrous filter media decreased to 8.4%, 7.1% and 2.8% after 120 min, respectively. This study sheds light on fabrication of the eco-friendly antibacterial filter materials for the ventilation systems., Building and Environment, 180, ISSN:0360-1323

Enhanced soiling due to particle deposition is often observed around multi-slot cabin supply air nozzles in commercial airplanes. This study aimed to investigate the influence of surface roughness on the particle deposition distribution around multi-slot cabin supply air nozzles of commercial airplanes. This investigation constructed a half-row cabin mockup installed with three 3D-printed supply air nozzles of a twin-aisle commercial airplane. A cutting method was proposed to measure the detailed particle deposition velocity distribution on the target surface of the nozzles covered by different grades of sandpaper with different roughness heights. This study also conducted numerical calculations for the particle deposition velocity distribution using an Eulerian particle deposition model. Both the experimental and modeling results show that strong particle deposition occurred on the cut samples near the slot dividers. When the surface roughness height was greater than or equal to 6 μm, the particle deposition velocity increased significantly with the surface roughness height. However, when the surface roughness height was less than or equal to 3 μm, the particle deposition velocity was relatively insensitive to the surface roughness. The analysis indicated that polishing the surfaces of cabin supply air nozzles may not effectively solve the problem of enhanced soiling in the aircraft cabin.

The current thermal environments in airliner cabins may not provide satisfactory comfort levels, and the design of these environments should be improved. This study aimed to design a desirable thermal environment for a single-aisle airliner cabin and used the CFD-based adjoint method to find the optimal design variables of air supply locations, size, and parameters. The design variables are used as the boundary conditions for solving the Navier–Stokes equations. By setting the occupant region as the design domain with a minimal predicted mean vote for thermal comfort, this study aimed to determine the corresponding air supply conditions for mixing and displacement ventilation systems under summer and winter conditions. The results show that it is possible to find the optimal air supply conditions in fewer than 10 design cycles if the initial conditions for design variables are provided within a reasonable range. This design method has a high computing efficiency as it takes one hour for a design cycle using a 16-core cluster. In addition, the results show that a displacement ventilation system provides a better thermal comfort level than a mixing ventilation system.

Correctly predicting transient particle transport in indoor environments is crucial to improving the design of ventilation systems and reducing the risk of acquiring airborne infectious diseases. Recently, a new model was developed on the basis of Markov chain frame for quickly predicting transient particle transport indoors. To evaluate this Markov chain model, this study compared it with the traditional Eulerian and Lagrangian models in terms of performance, computing cost, and robustness. Four cases of particle transport, three of which included experimental data, were used for this comparison. The Markov chain model was able to predict transient particle transport indoors with similar accuracy to the Eulerian and Lagrangian models. Furthermore, when the same time step size (Courant number ≤1) and grid number were used for all three models, the Markov chain model had the highest calculation speed. The Eulerian model was faster than the Lagrangian model unless a super-fine grid was used. This investigat...

Obtaining information about particle dispersion in a room is crucial in reducing the risk of infectious disease transmission among occupants. This study developed a Markov chain model for quickly obtaining the information on the basis of a steady-state flow field calculated by computational fluid dynamics. When solving the particle transport equations, the Markov chain model does not require iterations in each time step, and thus it can significantly reduce the computing cost. This study used two sets of experimental data for transient particle transport to validate the model. In general, the trends in the particle concentration distributions predicted by the Markov chain model agreed reasonably well with the experimental data. This investigation also applied the model to the calculation of person-to-person particle transport in a ventilated room. The Markov chain model produced similar results to those of the Lagrangian and Eulerian models, while the speed of calculation increased by 8.0 and 6.3 times, respectively, in comparison to the latter two models., Highlights • A Markov chain model was developed for predicting transient particle transport. • The Markov chain model does not require iterations in each time step. • The Markov chain model can significantly reduce the computing cost. • The Markov chain model can predict particle transport with reasonable accuracy.

Computational fluid dynamics (CFD) with the Lagrangian method has been widely used in predicting transient particle transport in indoor environments. The Lagrangian method calculates the trajectories of individual particles on the basis of Newton's law. Statistically speaking, a large number of particles are needed in the calculations in order to ensure accuracy. Traditionally, modelers have conducted an independence test in order to find a reasonable value for this particle number. However, the unguided process of an independence test can be highly time-consuming when no simple method is available for estimating the necessary particle number. Therefore, this investigation developed a method for estimating the necessary particle number in the Lagrangian method. Furthermore, the computing cost of the Lagrangian method is positively associated with the particle number. If this number is too large, the computing cost may not be affordable. Thus, this study proposed the superimposition and time-averaging meth...

Indoor air distributions, such as the distributions of air temperature, air velocity, and contaminant concentrations, are very important to occupants' health and comfort in enclosed spaces. When point data is collected for interpolation to form field distributions, the sampling locations (the locations of the point sensors) have a significant effect on time invested, labor costs and measuring accuracy on field interpolation. This investigation compared two different sampling methods: the grid method and the gradient-based method, for determining sampling locations. The two methods were applied to obtain point air parameter data in an office room and in a section of an economy-class aircraft cabin. The point data obtained was then interpolated to form field distributions by the ordinary Kriging method. Our error analysis shows that the gradient-based sampling method has 32.6% smaller error of interpolation than the grid sampling method. We acquired the function between the interpolation errors and the sampling size (the number of sampling points). According to the function, the sampling size has an optimal value and the maximum sampling size can be determined by the sensor and system errors. This study recommends the gradient-based sampling method for measuring indoor air distributions.

Porous insulation materials may easily acquire a large amount of moisture when they are exposed to high-humidity conditions. The acquired moisture can cause degradation of insulation performance, micro-organism growth, corrosion, etc. Currently, there is lack of an accurate, simple and inexpensive method for in-situ, nondestructive and rapid measurement of the moisture content in porous materials. This investigation proposes measuring the moisture content by means of the material's volumetric heat capacity using a hot wire. The moisture content is inferred from the change of the volumetric heat capacity before and after the moisture acquirement, which is the ratio of the thermal conductivity to the thermal diffusivity. To evaluate the measurement performance, the method was applied to determine both liquid water and ice content in sponge blocks. The results indicate that the discrepancies in the measured moisture mass content are within ±4% (w/w) between our proposed hot-wire method and the benchmark gravimetric method using a precision balance. Unlike the measurement of the thermal conductivity or the thermal diffusivity, the hot-wire method is insensitive to the contact resistance when measuring the volumetric heat capacity. This method may even be extended to detect foreign substances in porous materials if there is a meaningful difference in the measured volumetric heat capacity before and after the substance intrusion.

Surfaces and objects surround us, and touching them is integral to everyday life. Pathogen contaminated surfaces (fomites) are known to transmit diseases. However, little is known about the ways and speed at which surfaces become contaminated. We found that under certain conditions, the number of contaminated surfaces grows logistically, corresponding to possible rapid transmission of infection. In such a surface network, pathogen can be transmitted great distances quickly—as far as people move. We found that the surface contamination network in aircraft cabins exhibits a community structure, with small communities connected by the aisle seatback surfaces and toilets, which are high-touch surfaces. In less than two to three hours, most high-touch surfaces in the cabin are contaminated, and within five to six hours nearly all touchable surfaces are contaminated. During short haul flight, aisle passengers have higher fomite exposure. This closely matches the spatial infection pattern of one reported inflight norovirus outbreaks. Our model is generally applicable to other crowded settings. The commonly repeated advice to “wash hands frequently” may be replaced in future by more strategic advice such as “clean surfaces right now”, or advice based on who should wash their hands, and when.

Expiratory droplets from human coughing have always been considered as potential carriers of pathogens, responsible for respiratory infectious disease transmission. To study the transmission of disease by human coughing, a transient repeatable cough simulator has been designed and built. Cough droplets are generated by different mechanisms, such as the breaking of mucus, condensation and high-speed atomization from different depths of the respiratory tract. These mechanisms in coughing produce droplets of different sizes, represented by a bimodal distribution of 'fine' and 'coarse' droplets. A cough simulator is hence designed to generate transient sprays with such bimodal characteristics. It consists of a pressurized gas tank, a nebulizer and an ejector, connected in series, which are controlled by computerized solenoid valves. The bimodal droplet size distribution is characterized for the coarse droplets and fine droplets, by fibrous collection and laser diffraction, respectively. The measured size distributions of coarse and fine droplets are reasonably represented by the Rosin-Rammler and log-normal distributions in probability density function, which leads to a bimodal distribution. To assess the hydrodynamic consequences of coughing including droplet vaporization and polydispersion, a Lagrangian model of droplet trajectories is established, with its ambient flow field predetermined from a computational fluid dynamics simulation.

Volatile organic compounds are indoor air pollutants with many adverse health effects for humans. Ozone reactions with human surfaces (skin, hair, and clothing) are an important source of volatile organic compounds in the indoor air, especially in aircraft cabins because of their typically high ozone concentrations and occupant densities. Therefore, it is important to study the ozone-initiated volatile organic compound emissions from ozone reactions with passengers in an aircraft cabin and assess their resulting exposure. This investigation developed empirical models for computing the emissions of several major volatile organic compounds, including acetone, 4-oxopentanal, nonanal, and decanal, from ozone reactions with human-worn clothing. The empirical models were used to compute the contributions of human surfaces to these volatile organic compounds in an aircraft cabin mockup under different environmental conditions. The computed results were then compared with the corresponding experimental data obtai...

It has long been realized that condensation in a chamber prefilled with condensable vapor leads to chamber depressurization, and the condensation rate can be cooling controlled. While the final state can be reasonably estimated based on the thermodynamic equilibrium, the dynamic process of depressurization is not only transient but also in thermal non-equilibrium. This transient and non-equilibrium characteristics, mainly governed by the rate of condensation, has not been satisfactorily modeled, which is due to the complicated coupling mechanisms of heat and mass transfer during the condensation, the presence of non-condensable gas (NCG) within vapor, as well as the complex geometry and properties of chamber and cooling device involved. In this paper, we have conducted an experimental study on depressurization by steam condensation onto an internal cooling coil in a steam-prefilled closed chamber. To reveal various parametric effects on the depressurization process, a parametric model consisting of a set of coupled ordinary differential equations has been established, with some simplified assumptions including lumped heat capacity sub-models for chamber walls, cooling coils and the gas phase. To further explore the thermal non-equilibrium characteristics during the process, a full-field and transient simulation of computational fluid dynamics (CFD) is also conducted using FLUENT with user-defined function (UDF) on boundary of condensation. Both parametric and CFD models consider the existence of NCG that is pre-mixed with the vapor as impurity. By comparison with the experimental measurements, both models reasonably predict the dynamic and asymptotic characteristics of depressurization with time. The CFD simulation indicates an almost instant equilibrium in pressure within the chamber and yet non-equilibrium in temperature with noticeable temperature gradients over the gas phase. The simplified parametric model gives quick predictions of some major parametric effects (e.g., vapor purity, coolant flow rate, and vessel volume) on the rate of depressurization. The detailed mechanistic understanding, gained from proposed models, provides insights essential to the optimized design and operation of the depressurization by cooling-controlled condensation.

Airborne particles are an important type of air pollutants in aircraft cabin. Finding sources of particles is conducive to taking appropriate measures to remove them. In this study, measurements of concentration and size distribution of particles larger than 0.3 μm (PM>0.3) were made on nine short haul flights from September 2012 to March 2013. Particle counts in supply air and breathing zone air were both obtained. Results indicate that the number concentrations of particles ranged from 3.6 × 102 counts L−1 to 1.2 × 105 counts L−1 in supply air and breathing zone air, and they first decreased and then increased in general during the flight duration. Peaks of particle concentration were found at climbing, descending, and cruising phases in several flights. Percentages of particle concentration in breathing zone contributed by the bleed air (originated from outside) and cabin interior sources were calculated. The bleed air ratios, outside airflow rates and total airflow rates were calculated by using carbon dioxide as a ventilation tracer in five of the nine flights. The calculated results indicate that PM>0.3 in breathing zone mainly came from unfiltered bleed air, especially for particle sizes from 0.3 to 2.0 μm. And for particles larger than 2.0 μm, contributions from the bleed air and cabin interior were both important. The results would be useful for developing better cabin air quality control strategies.

To better understand the overall VOC levels and their key influencing factors in aircraft cabin, we conducted in-flight measurements in 51 randomly selected flights by quantitative methods that were previously introduced in part I of this research program. Of those 51 flights, several potential influencing factors, including flight phases ( N = 51), meal services ( N = 22) and air supply ( N = 13) were analyzed by statistical and contrastive methods. Based on the statistical results, two detailed case studies on peak values and air supply during flights further indicate the source fluctuation characters of different VOCs. Key conclusions derived from this study include: 1) Combined with qualitative results on some specific VOC species with high detection rate in part I, a list of 29 compounds were proposed as target VOCs in cabin air. 2) Most target VOCs had significant differences ( p p

UNLABELLED Covering a cough can be useful in reducing the transmission of airborne infectious diseases. However, no simple method is available in the literature for predicting the exhaled airflow from a cough with the mouth covered. This investigation used smoke to visualize the airflow exhaled by 16 human subjects. Their mouths were covered by a tissue, a cupped hand, a fist, and an elbow with and without a sleeve. This study then developed simplified models for predicting the airflow on the basis of the smoke visualization data. In addition, this investigation performed numerical simulations to assess the influence of mouth coverings on the receptor's exposure to exhaled particles. It was found that covering a cough with a tissue, a cupped hand, or an elbow can significantly reduce the horizontal velocity and cause the particles to move upward with the thermal plumes generated by a human body. In contrast with an uncovered cough, a covered cough or a cough with the head turned away may prevent direct exposure. PRACTICAL IMPLICATIONS This study developed simplified models for predicting the exhaled airflow from a cough with the mouth covered. The proposed models can easily be used to investigate the risk of transmission of airborne infectious diseases in enclosed environments.

To better understand the overall VOC species and levels and their key influencing factors in aircraft cabin, we conducted in-flight measurements in 107 commercial flights from August 2010 to August 2012. These flights were randomly selected and a sampling method using syringe was developed to obtain overall information of VOCs in actual cabins. On average 59 VOCs in each flight were detected within a total of 346 VOCs in the 107 flights, with the percentage of 41% for alkanes and alkenes, 15% for esters and alcohols, 11% for ketones and aldehydes, 6% for halides, 20% for aromatics and 6% for other VOCs (e.g. nitrogen-containing compounds). Main VOCs with high detection rate were compared at three different flight phases and at meal services for further analysis, respectively. Influences of several potential factors including air route, aircraft model and seasonal variation on the detection rates of cabin VOC species were preliminarily analyzed by chi-square test and logistic regression methods. The statistical results indicate that certain level of correlation between o-xylene, tetrachloroethene, benzene, 1,4-dichloro-, formamide, N,N-dimethyl-, ethane, 1,2-dichloro- and nonane (p