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Novel personalized and humidified air supply for airliner passengers
Building and Environment 45 (2010) 2345e2353
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Novel personalized and humidified air supply for airliner passengers P. Zítek a, *, T. Vyhlídal a, *, G. Simeunovi c a, L. Nováková b, J. Cí zek b a
Centre for Applied Cybernetics and Department of Instrumentation and Control Engineering, Faculty of Mechanical Eng., Czech Technical University in Prague, Technická 4, 160 00 Prague, Czech Republic b Department of Fluid Dynamics and Power Engineering, Faculty of Mechanical Eng., Czech Technical University in Prague, Technická 4, 160 00 Prague, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 November 2009 Received in revised form 9 April 2010 Accepted 13 April 2010
The micro-environment control in an airliner cabin presented here consists in supplying each of the passengers with her/his own supply of fresh, humidified air in order to prevent possible airborne health problems and to provide local compensation for the humidity deficit. Unlike the environment control systems widely used in commercial aircraft, each of the seats in the cabin will be supplied individually with a separate airflow, which is also separately exhausted. This arrangement forms a personalized microclimate in the seat area. Essentially, focusing a personal air supply into the breathing area of the passenger works on the principle of individual seat ventilation, with the air supply and exhaust nozzles built into the back of the seat ahead. The system design, originally based on Computational Fluid Dynamics (CFD) models, has been verified by means of laboratory experiments. The results presented here have been achieved within the framework of FP6 EU Project AST5-CT-2006-030958, under the acronym SEAT (www.seat-project.org). Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Aircraft cabin Air distribution Relative humidity control Computational Fluid Dynamics Personalized air distribution system Particle Image Velocimetry
1. Introduction The typical cruise altitude of commercial aircraft is around 11,000 m, and the internal environment in an aircraft cabin needs to be provided artificially by means of an Environmental Control System (ECS), which compresses and cools the outside air to an indoor temperature and pressure that is suitable for passengers’ comfort. The ambient air at a temperature of below 50 C, and at atmospheric pressure five times lower than that at sea level, is compressed (and therefore heated) by the engine turbo-compressors and is then adjusted by the ECS to the state desired for the airconditioned interior. As the moisture content of the ambient air is near to zero, the initial relative humidity of the cabin air may drop considerably during the initial stage of flight to values below 10% at cruising altitude [1]. On-board measurements have shown surprisingly severe drops in humidity e from at least 47% to 11% within just 30 min of flight [2]. The longer the flight lasts, the more serious the humidity deficit becomes. Hence, long haul flights with drops in humidity to 5% and even lower are a matter of particular concern. According to the report of BRE [3], low humidity can have direct effects on passengers’ health and comfort, and the range of these effects is dependent on the humidity deficit, duration of exposure and other factors (e.g. temperature, water ingestion, wearing * Corresponding authors. E-mail addresses: [email protected] (P. Zítek), [email protected] (T. Vyhlídal). 0360-1323/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2010.04.005
contact lenses, use of a moisturizer, etc.) The main potential problems are: drying of the body surface (mucous membranes and skin), dehydration, a worse perception of air quality at a low level of RH, effects on thermal comfort (passengers feel cooler due to low RH) [4,5]. At the same time, a non-negligible source of cabin air humidity at cruising altitude flight is passengers’ exhalations. The respiration of each passenger is a humidity source for the cabin, because the expired air is saturated with water vapour. The mean flow rate of this expiration is usually considered as 0.14 l/s. However, this source of humidity originating from respiration cannot cover more than about 20% of the needed amount of water vapour [6]. This is why the cabin air humidity deficit is as great as was mentioned above. Another major difficulty in providing a suitable internal cabin environment, in addition to the humidity deficit, is due to the required intensity of the air exchange. The density of passengers in the cabin is extraordinarily high, higher than in other means of transport, and the air exchange rate therefore needs to be exceptionally intensive. Intensive ventilation is essential above all due to the levels of carbon dioxide (CO2), the main contaminant generated in the cabin [4]. To meet the ASHRAE Standard on carbon dioxide limits [1], it is necessary to exchange from 8 to 10 l of air per airliner passenger per second. The cabin air volume has to be totally exchanged approximately 20e25 times per hour, an intensity that is incomparable with any air-conditioning requirement in buildings. It is therefore a very difficult and very challenging task to design a comfortable and healthy cabin environment for commercial aircraft. The discrepancy between the necessary oxygen supply
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and the required rate of exhausting the carbon dioxide will be an important consideration. At the same time, it should be noted that the more intensive the cabin ventilation is in order to decrease the content of contaminants, the worse the humidity deficit will be [3]. Obviously the required high intensity of air exchange results in a need for high ventilation velocity and considerable air mixing rates. This leads to an ever-increasing risk of draughts, which is very difficult to overcome in huge present-day aircraft. Thus, the environment control systems commonly used in commercial airliners lead inevitably to contaminant transfer among the passengers, as has been confirmed by numerous measurements and studies [7e10]. Due to the mixing principle of air distribution in commonly-used ECS design, the spread of infectious airborne diseases, such as influenza, SARS, H1N1 flu, etc., from an infected passenger to another passenger may hardly be avoided. In order to maintain acceptable power consumption in the ECS the cabin air is re-circulated, i.e. the exhaust air is filtered and mixed back with the fresh air. The rate of recirculation is mostly about 50% of the whole supply [1]. The HEPA filters can remove around 99.7% of fine particles (0.3 micron in diameter) but cannot remove CO2 and other gaseous contaminants. In general, as far as medical considerations are concerned, the highest possible percentage of fresh air is recommended [1]. However, cost and power-saving considerations make it unacceptable to exclude recirculation altogether. This paper deals with ways of providing a personalized fresh air supply to each passenger, separated from the general cabin ventilation, and is organized as follows. Recent investigations of the cabin environment and modifications to air circulation are reviewed in Section 2. Section 3 presents a novel proposal for seat-focused air supply based on the principle of a local closed-flow pattern. The theoretical background of Computational Fluid Dynamics (CFD) modelling for the cabin environment is presented in Section 4. Both CFD and experimental verification of the proposed personalized ventilation system are presented in Section 5. In Section 6, the mutual influence of personalized and central ventilation in a cabin is studied using CFD. Emphasis is laid on the distribution of relative humidity in the cabin. Section 7 concludes the paper and summarizes the results obtained within the research presented here. 2. Recent personalized aircraft cabin air distribution and humidification systems The environmental control systems widely used in commercial airliners do not apply any humidification of the cabin air. The usual arguments against humidification of the cabin air are as follows: the weight penalty associated with the extra water that would have to be carried, the biological growths that are often associated with humidifiers, and the maintenance requirements of humidification systems. Safety concerns also need to be taken into consideration, mainly the risk of water condensation, dripping and freezing of moisture on the inside of the aircraft shell, which can lead to various safety problems [1]. Nevertheless, the problem of humidifying cabin air has recently received a great deal of attention. For example, Lindgren et al. [11] studied ways of humidifying the cabin air on eight intercontinental flights with the use of ceramic evaporation humidifiers. The measurements showed that the humidification system increased the level of RH by 10% without any measurable increase in micro-organisms in the cabin air. As will be shown, several theoretical studies include air humidification in a personalized ventilation system. Numerous studies and measurements of the cabin environment show that one way to overcome undesirable mixing of contaminated air among the passengers is via a more personalized air supply system. Personalization means that the cabin air-conditioning system should be designed to achieve as far as possible separate air
flow towards each passenger. Several arrangements for implementing the idea of a personalized air supply are known from airconditioning systems in buildings, and these have recently been considered for application in airliner cabins. Zhang and Chen [12] compare three options for air circulation. The usual scheme for air distribution in an airliner cabin (from the ceiling inlet to bottom exhaust) is compared with the application of so-called displacement air distribution, where clean, cool air is supplied from the floor and contaminated air is exhausted from the ceiling level. A third option investigated by the authors is the possibility of supplying the passenger’s air ration directly through a nozzle built into the back of the seat ahead. Consequently, Zhang et al. [13] proposed a novel under-aisle distribution system with the same objective. However, the main aim of that novel system is to humidify the cabin air by under-aisle air nozzles. CFD-based validation of the proposed method showed that the relative humidity was increased by 10% without causing moisture condensation in the interior of the cabin. The water consumption rate in air humidification is only around 0.05 kg/h per person. The micro-environment around a human body with and without a personalized air supply was studied by Gao and Niu [6], using an air supply pipe located in the breathing area. Their results consider the satisfactory range of air flow up to 3 l/s, and unlike the real situation in an airplane cabin, the person is considered as seated in a solitary working place. Gao and Niu [14] used a CFD model to investigate whether this type of personal ventilation can be used to improve the air quality in an aircraft passenger’s breathing area. In the proposed ventilation set-up, 2.5 l/s per person of fresh air humidified to a level of 40% RH is supplied directly to the breathing area of the passenger by an adjustable air supply nozzle, while 7.5 l/s per person of air is supplied by the conventional mixing central ventilation. It has been shown that personalized ventilation can shield the breathing area from contaminants and improve the removal of expiratory aerosols from the passenger, see also Ref. [15]. In addition, it has been shown that this set-up can increase the level of RH in the breathing area of the passenger, while the overall RH in the cabin is kept below 15%. Jacobs and de Gids [16] developed a new concept of aircraft cabin air-conditioning, in which the main indoor air quality and temperature control system for the passengers is built into the seat. Clean, humidified air from the outside is supplied directly to the passenger’s breathing area through the headrest of the seat. The exhaust imbedded into the seat enables the passenger to control the seat temperature by adjusting the exhaust ventilation rate through the seat. According to the CFD model, this seat-based ventilation system provides efficient shielding against contaminants, humidifies the air to a comfortable level, and controls the temperature in the seat area. The very challenging idea of providing a personalized environment was presented by Penot and Mayer [17], who investigated a pair of concentric nozzles for the driven supply and exhaust of air for a single person. Unfortunately, the arrangement presented by Penot and Mayer is not applicable to aircraft cabins, since the personalized air supply is considered as local heating from the ceiling. Although the effect of closing the streamlines between the supply and the exhaust is dependent on a temperature difference, and on the airflow velocities in the supply and in the exhaust, the obtained virtual separation of a personal microclimate zone from the surroundings is almost perfect. In an airliner cabin, the air supply has a slight cooling role, rather than a heating role, and it was therefore necessary to find a flow arrangement by analogy with the Penot nozzle, but with the opposite thermal influence. The best basis for building in the nozzles appears to be to place them in the back of the seat ahead, which can be used to create a microclimate zone with closed streamlines similar to the Penot arrangement.
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3. Seat-personalized supply of fresh and humidified air As far as the personalized environment problem is concerned, the following major conclusions can be drawn from the available references 1. If the usual present-day ECS system using a mixing principle of ventilation is employed, the passengers cannot be protected from considerable mutual exchange of contaminated air. 2. Basically, re-circulated air cannot be considered as entirely safe, due to the danger of increased levels of CO2 and other gaseous contaminants. 3. The outside air at cruising altitude is free of organic contaminants, but is very dry. A steady supply of outside air is much more than sufficient to flush the human-generated moisture from the cabin, and it reduces the interior air humidity to levels below 10% on long haul flights. However, full humidification of the cabin air supply is risky, due to the threat of dew condensation inside and above the cabin shell. 4. On the other hand, only about one third of the cabin air supply is needed to cover the oxygen requirements of the passengers. The rest of the total supply is necessary only for diluting the concentration of carbon dioxide and other pollutants in the cabin [1] and for pressurizing the cabin. To sum up, in order to protect passengers from intensive sharing of exhaled air with their neighbours, it is necessary to adapt a scheme of cabin ventilation that supplies each passenger with her/his own air ration. As has already been proposed by Zhang and Chen [12], Gao and Niu [14,15], Jacobs and de Gids [16], the personal air ration should be supplied directly into the breathing area of each passenger. The appropriate amount of air has to cover the physiological oxygen demand of the passenger and also the requirement for flushing away gaseous contaminants. 3.1. The novel concept of personalized air supply Similarly as in Ref. [14], the modification of the cabin ventilation system described in this paper and shown in Fig. 1 uses about 2.5e3 l/s/passenger of outside fresh air humidified to a level of 40% RH for the personalized ventilation (PV) supply. The rest of the required air exchange, i.e. 6.5e9 l/s/passenger is supplied by the mixing-based central ventilation (CV). Unlike the arrangement in Ref. [14], the personal ventilation (PV) air supply nozzle is built into the back of the seat in the front of the passenger, as shown in Fig. 1.
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Our system is in this way similar to that of Zhang and Chen [12]. However, the system that we propose does not require such extensive modification of the cabin air ventilation system as the Zhang and Chen [12] system. Moreover, our proposed system has an air exhaust nozzle, located just below the air supply nozzle in the back of the seat ahead (see Fig. 1). The nozzle removes the contaminated air from the seat space by analogy with the arrangement presented by Penot and Mayer [17]. In this respect, our proposed system resembles that of Jacobs and de Gids [16], though the design of the set-up is different. The design of nozzles that will supply and exhaust the personalized air supply, however, is not a simple task due to specific temperature and draught limitations connected with passenger comfort requirements. Firstly, a closed-flow pattern similar to that presented in Ref. [17] can be achieved only if the temperature in the supply nozzle is at least 2e5 C lower than the temperature in the exhaust. Fortunately, this demand is in good agreement with the typical situation in an aircraft cabin, where the supply air temperature is between 16 and 19 C, while the cabin temperature is usually between 21 and 24 C [1]. On the one hand, the personalized air supply has to be directed towards the breathing area, i.e. towards the passenger’s chin. On the other hand, however, it is necessary to avoid the supply flow causing an inconvenient draught for the passenger (bear in mind, for example, the high flow velocity from the widely-used gaspers over the passenger’s head). The usual requirement is that the air flow velocity around the face should not exceed approximately 0.2 m/s [6]. However, perceptions of warmth and draught are very individual, and according to Gao and Niu the limit of 0.2 m/s holds if a 14% dissatisfaction rate [23] is considered acceptable. Due to this limitation and the required air flow value, the cross area of the supply nozzle needs be relatively large, around 60 cm2. Its design should enable the air ration to be brought towards the lower part of the passenger’s face without exceeding the draught limits. In Ref. [19], the two nozzles are designed using CFD-based optimization, and their performance is verified by laboratory tests. In any case, the personalized air supply is considered to be an optional service that can be attenuated or even switched off by the passenger, similarly as in the case of overhead gaspers. While personalized air circulation predominantly affects the breathing area of the passengers, the role of central air distribution by ECS may remain almost unchanged. The central system uses the mixing principle, provides most of the ventilation rate, and pressurizes the cabin air in the usual way, see Fig. 1. For economic reasons, a substantial part of the air processed and supplied by this part of ECS is re-circulated and filtered. Around 35% of the
Fig. 1. The proposed modification of the cabin ventilation system; central ventilation (CV) flow streams e mixture of outside and re-circulated air provided by the central ventilation (CV) system via the classical mixing principle, personal ventilation (PV) flow streams e fresh and humidified outside air provided by personalized ventilation (PV) to the local area of each seat.
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exhausted air from the cabin both through the local nozzles and through the central exhaust nozzles is taken back and re-circulated. In the scheme, the central air supply nozzles are considered to be in the middle part of the cabin ceiling, and the central air exhaust is in the cabin floor corners. To sum up, the personalized ventilation system in Fig. 1 has the following features: i) each passenger is supplied with fresh, humidified outside air via the PV system through a pair of nozzles built into each of the cabin seats, ii) the PV air supply is only approximately one third of the total ventilation volume, and the rest of the cabin air is still provided as a mixture of outside and re-circulated air, which is supplied by the CV system via the classical mixing-based principle. CV air is not humidified. In what follows, we first concentrate on CFD-based verification of the proposed single seat personalized ventilation system. The aim is to verify the possibility of forming a microclimate zone of circulating fresh air in front of each passenger by adjusting the flow rates and temperature differences between the supply and exhaust nozzles. Subsequently, the results of the CFD models are verified via laboratory experiments. The results of the experiments also validate the CFD methodology. Thus, the CFD model of the whole cabin sector is presented as the final result. Obviously, as shown in Fig. 1, the flow patterns of the PV and CV systems are perpendicular to each other, i.e. while the streamlines of CV are transversal, the streamlines of PV are longitudinal to the cabin tube. The aim of the cabin CFD model is thus to study the mutual influence of these streams. The distribution of the relative humidity in the cabin will also be investigated. Simplified CFD models of the set-up, together with the first experimental results, were presented in Refs. [18,19]. The simultaneous investigation of air velocity, temperature and humidity in the CFD models brings an advantage in tracing the motion of the air supplied by the PV. Due to humidification of the personalized air supply, the increased moisture content can serve as a tracer gas visualizing the circulation of the added air between the inlet and exhaust nozzles. As the temperature differences are only slight, the moisture content distribution corresponds approximately to the distribution of the relative humidity. From the RH distribution we can thus track the trajectory of the personalized air stream. 4. CFD model characteristics of the cabin air ventilation set-up The numerical investigation of the air circulation inside the cabin involves solving a set of partial differential Reynolds-Averaged NaviereStokes (RANS) equations within appropriate boundary conditions. These equations describe the mutual dependence of the momentum, energy, moisture content, turbulent kinetic energy and its dissipation rate in the cabin space. The model is the standard ke3 model, using numerous approximations, and a certain validation of the model by experimental measurements is therefore strongly recommended [20]. The partial differential equations were discretized into algebraic equations using the finite volume method with a second order up-wind scheme. The GAMBIT program was used to generate the grid of cells. A special task in this case was the air humidification and the resulting buoyancy effects. The simulations were implemented in the FLUENT environment [21], where the algebraic equations were solved over all the cells. Due to the nonlinearity of the equations, iterations need to be applied to achieve a convergent solution. The flow velocity is solved from the momentum equations, and is then corrected through the continuity equation in the SIMPLE algorithm. In addition, the continuity
equation is repeatedly solved to settle the steady state values of both pressure and velocity. The cabin interior fluid is considered as a mixture of two components, air and water vapour. The mass conservation equation of the mixture, consisting of two species, is given by the following equation
vðrYi Þ þ VðrvYi Þ ¼ VJi ; vt
i ¼ 1; 2
(1)
where Yi is the local mass fraction of each component, v is the velocity vector, Ji is the diffusion flux of the i-th component, and r is air density. The turbulence model parameters, namely the kinetic energy rate k and the dissipation rate 3, are calculated using the following transport equations
vðrkÞ vðrkui Þ v ¼ þ vt vxi vxi vðr3Þ vðr3ui Þ v ¼ þ vt vxi vxj
"
"
C23
#
m vk mþ t þ Gk þ Gb r3 YM ; sk vxj
(2)
#
m v3 3 mþ t þ C13 ðGk þ C33 Gb Þ sk vxj k
r32 k
(3)
where Gk expresses the generation of turbulence kinetic energy due to the mean velocity gradients, and mt is the turbulent viscosity, YM determines the effect of compressibility on the turbulence, and the model constants C13, C23 and C33 are given in Refs. [20,22]. Both the cabin interior and the passengers’ bodies have an extremely complex geometry. For this reason, an unstructured tetrahedral grid was used in building up the CFD model, although this kind of grid necessarily provides lower precision than a structured grid. The primary objective of the research presented here is to investigate the distribution of humidity, temperature and flow velocity near the passengers’ bodies, with special reference to the supply and exhaust nozzles of PV. Later, in Section 6, the CFD model also incorporates the intake of humidity originating from the passengers’ respiration. With regard to these priorities, a denser grid is desirable in these parts of the cabin space than in the other parts. Namely around the supply and exhaust nozzles the size of a grid cell is taken as 10 mm, on the passenger’s surface the size is 30 mm, and towards the rest of the space these values increase continuously up to 50 mm. 5. Verification of the proposed single seat ventilation set-up and CFD model validation In this section, we proceed to the use of laboratory experiments to verify the possibility of forming a local microclimate zone in the seat area. To implement the personalized air supply principle in the
Fig. 2. Scheme and geometric model of the PV set-up of the seat.
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Fig. 3. Laboratory set-up: Left e seat model with personalized ventilation system and with displacement of temperature sensors. Right e detail of the pair of nozzles built in the laboratory seat set-up.
Table 1 Conditions of the experimental verification of PV system. Personalized supply Supply airflow rate [l/s per person] Exhaust airflow rate [l/s per person] Supply airflow temperature [ C] External air temperature
2.5 2.5 20.9 23.4
laboratory, a seat mock-up with a manikin and a pair of supply and exhaust nozzles, as in Fig. 2, was built up. Due to technical limitations in the laboratory, the effect of central ventilation was not considered in the experiments. However, as will be shown later in the CFD models of the cabin sector, the influence of central ventilation on the PV airflow patterns is relatively weak. The results of the experiments will also be used to validate the CFD model. 5.1. Experimental verification of the single seat ventilation set-up In order to test the potential of the PV system to form a virtually closed micro-environment zone in the seat area and to validate the CFD model, an experimental set-up was built, see Fig. 3. The simplified model of the seat with a seated passenger and the back of the seat ahead was constructed from aluminium profiles, acrylic sheets and polystyrene foam. The supply and exhaust nozzles were both fixed into the back of the seat ahead, and were connected by pipelines with ventilators. The supply and exhaust flow rates can both be adjusted independently by means of the ventilator control. To stabilize the airflow pattern in the seat area, the supply nozzle air is cooled by about 3 C in comparison with the temperature of the ambient air to obtain the necessary temperature difference. The flow patterns were visualized by two Particle Image Velocimetry (PIV) methods, namely helium bubbles and SAFEX fog. The results
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Fig. 5. Velocity vectors and contours of velocity magnitude measured close to the manikin surface using PIV (helium bubbles). The grid in the measured area is used to determine the values to be compared with the CFD model in Fig. 7 e left, see Table 4.
of the experiments, with the conditions given in Table 1, are shown in Figs. 4e5. Fig. 4 shows the fog visualization of the steady state airflow distribution in the seat area. Both the side view and the front view show clearly that micro-environment zones are constituted in the seat area. The passenger’s breathing area is efficiently covered by the discharged air from the supply nozzle. It can be estimated from the SAFEX fog pattern that the supply nozzle airflow is also able to provide efficient shielding of the passenger’s breathing zone from the air of the CV system. The results of the air velocity measurements close to the surface of the manikin are given in Fig. 5. The velocity distribution was measured using the PIV method with helium bubbles. The results shown in the figure were obtained by two measurements in different rectangular areas, which partly overlapped each other. As can be seen, the air velocity in the vicinity of the surface of the manikin does not exceed the recommended comfort limit of 0.2 m/s. Finally, the results of the temperature measurements at points shown in Fig. 3 are given in Table 2. The ambient air temperature was measured as T1, three thermometers were fixed on the surface of the manikin, T2 (chest), T3 (neck), T4 (face), and the temperature of the air leaving the supply nozzle was measured as T5. The settled temperature values after 15 min of measurement, given in Table 2, confirm that the temperature of the air touching the body surface is at least 2 C higher than the temperature of the air supply. To sum up, the experiments prove the possibility of forming a local environment zone in the passenger’s area. 5.2. CFD model to be verified The CFD model of a single seat in Fig. 2 consists of about one million cells meshed with an unstructured tetrahedral grid. As
Fig. 4. Airflow distribution in the seat area visualized by SAFEX fog.
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Table 2 Comparison of temperature measurements at points in Fig. 3 with the CFD model values and their relative errors e ¼ jTm TsjT1 m 100%, Tm e measured, Ts e CFD model temperature. Temperature [ C]
Experiment
CFD model
e
T1 T2 T3 T4 T5
23.2 23.1 22.9 23.0 20.8
23.1 22.5 22.4 22.8 20
0.4% 2.6% 2.2% 0.9% 3.8%
Table 3 Boundary conditions for the CFD model of a single seat microclimate. Personalized supply Supply airflow rate [l/s per person] Exhaust airflow rate [l/s per person] Supply airflow temperature [ C] Supply airflow relative humidity [%] Turbulence intensity [%] Aircraft air and seat temperature [ C] Floor temperature [ C] Temperature of the manikin surface Tm [ C] Dimension of modelled volume height width length [m] Temperature of the modelled volume boundary [ C]
2.5 2.5 19 40 10 23 22 30.3 (23) 1.6 1.1 1.2 22
mentioned in section 4, the FLUENT program is used for CFD simulation, and the air circulation, heat transfer and humidity distribution are modelled by the standard ke3 model, using the boundary conditions listed in Table 3. The seat with the manikin was placed in the closed modelled volume 1.6 m in height, 1.1 m in width and 1.2 m in length with a boundary temperature of 22 C. A specific verification problem emerged concerning the surface temperature of the manikin. The CFD model temperature should be as close to reality as possible, in order to predict the contribution of the buoyancy force from the thermal plume. In modelling the cabin environment, the average surface temperature of a dressed passenger was considered as 30.3 C, according to Zhang and Chen [12]. However, for the purposes of validating the modelling method by laboratory experiments, it was more correct to consider this temperature as equal to the real temperature of the manikin during the experiments. Hence, for validation purposes only, the surface temperature was set at 23 C, i.e. approximately equal to the ambient air temperature in these model experiments. Besides, no expiration from the manikin’s mouth is considered. The results of the CFD models for comparison with the experimental results are shown in Fig. 6. Table 4 presents the relative
Table 4 Air flow velocity relative errors between the measurement vm and CFD model results vs in the grid points in Fig. 5, e ¼ jvm vsjv1 m 100%. A B C D E F
9.2 5.2 4.6 12.7 4.4 2.0 1
9.1 1.6 7.1 11.8 9.9 8.0 2
2.3 11.8 10.2 10.3 2.7 8.1 3
1.3 21.8 10.4 11.4 5.4 4.9 4
5.4 17.9 12.1 16.4 5.8 4.2 5
5.1 20 13 19.6 8.9 2.7 6
3.9 22 13.6 17.8 11.2 2.1 7
3.4 17.8 12.1 18.7 12.2 5.4 8
deviations of air velocity magnitude between the measurement data (Fig. 5) and the CFD simulation results (Fig. 6, left). The measured velocity magnitude values are evaluated using PIV at the grid points shown in Fig. 5. These values are compared with the CFD model results in the same grid points of the model results shown in Fig. 6-left. As shown in Table 4, the relative error does not exceed 20%, and its average value for all the grid points is 10%. Thus, taking into consideration the complexity of the measured velocity field, the agreement between the model and the measurements are relatively good. Table 2 presents the modelled temperature values at the measurement points shown in Fig. 3. In addition, the relative error between the measurement data and the CFD simulation results (Fig. 6 right) is presented in the last column of the table. As can be seen, the maximum relative error is 3.8%, and the average value taken over all the points is 2%. Thus, the agreement between the temperature measurement and the model results is very good. Based on general matching of CFD and experimental results, it can be concluded that our CFD methodology provides credible results and corresponds sufficiently with the measurements on the real set-up. On the basis of this experience, a more realistic CFD model will be used in Section 6 to verify the single seat ventilation set-up. Later, the model will be extended to model the air circulation in a whole sector of the cabin. 5.3. Limitations of temperature control in PV In this section we draw attention to the limits of air temperature control via personalized ventilation. This is due to the need for a temperature difference in the principle of the Penot nozzle [17], i. e. the temperature in the supply nozzle needs to be 2e5 C lower than the temperature of the air being exhausted from the seat space. This is demonstrated in a 2D CFD model in Fig. 7. If the required temperature difference is achieved, a local micro-environment is formed. However, as soon as this difference disappears, or if it is even reversed, the effect of the closed air flow pattern
Fig. 6. CFD model results for comparison with the experiments e the body temperature Tm ¼ 23 C, no expiration from the manikin’s mouth is considered. Left e contours of velocity magnitude of air, right e static air temperature of air.
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Fig. 7. 2D models of airflow in the seat local zone with respect to the temperature difference at the supply and exhaust nozzle.
between the supply and exhaust nozzles is destroyed. This is because the natural upward flow of warmer air overcomes the influence of the exhaust nozzle. This phenomenon was also confirmed by the laboratory experiments.
6. CFD model analysis of a cabin sector environment Let us now return to the issue of humidification. The task is to verify how the proposed personalized air supply can provide a suitable humidity distribution in the seat space and in the cabin as a whole. As above, the air humidity also serves as a tracer helping to observe the mixing or separation of the flows. Unlike the models of the circulation in this stage of CFD modelling, the passengers’ respiratory contributions to the air humidity are considered as local sources of air saturated by water vapour. These expiration flows are assumed as constant, and are set as 0.14 l/s per passenger [6]. However, a more detailed representation of the humidity sources necessitates a simpler specification of the seat and manikin shape. This simplification is justified by experience with various versions of cabin interior specification where, in practical terms, such changes in detail had a negligible effect on the temperature, humidity and velocity distributions. The boundary conditions for the model experiments presented in this section are given in Table 5. The boundary temperatures were taken from Zhang and Chen [12]. As usual, the aircraft shell, the windows, the cabin floor and ceiling are considered in a steady
thermal state. The CFD model experiments were performed for both environmental versions, i.e. for standard ECS operation and for the proposed personalized air supply. The results presented below are mainly for the humidity and temperature distribution, and also for the interaction between the central and personalized ventilations. In comparing the results of these two versions, attention is mainly paid to the influence of personalized humidification and also the humidity contribution of passengers’ breathing. The humidity effect of passenger’s respiration is shown in Fig. 8, where the whole amount of cabin air is supplied by the conventional ECS. The small spots of different intensity show the range of humidity increase due to passengers’ breathing. Although the expiration air is in a state of saturation, the impact of breathing on the humidity distribution is very limited inside a cabin with standard ventilation, without humidification. The size of the increased humidity spots is constrained to the close vicinity of the passengers’ faces, so that the expiration humidity is dispersed very quickly. The humidity increase due to the passengers’ respiration is noticeable only in the closest proximity of their mouths. In fact, with regard to the intensive air exchange, this result is not surprising. On the other hand, the impact of personalized
Table 5 Boundary condition for the CFD model of the cabin sector.
Supply airflow rate [l/s per person] Supply airflow temperature [ C] Supply airflow relative humidity [%] Turbulence intensity [%] Aircraft skin temperature [ C] Aircraft seat temperature [ C] Window temperature [ C] Floor temperature [ C] Temperature of the passenger clothes [ C] a
Personalized ventilation (outside air)
Central ventilation (mixture of outside and re-circulated air)
3 19 40
10 (7)a 19 5
10 22 23 16 22 30.3
5
10 l/s is considered if only CV is considered, while 7 l/s is considered in CV in combination with PV.
Fig. 8. Relative humidity distribution resulting from the CFD model for the set-up, where humidification of the whole cabin volume via central air distribution system is considered.
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Fig. 9. Distribution of cabin air relative humidity, ECS with the personalized ventilation set-up. Fig. 11. Air velocity patterns in the cabin, ECS with the personalized ventilation set-up.
ventilation is shown in Figs. 9e12. Fig. 9 shows how the inlet flow of fresh and humidified air is dispersed and mixed together with the respiration air. Nevertheless, it should be noted that real breathing does in fact produce an alternating flow, which probably will lead to even more favourable mixing conditions than the fixed flow in the simulation. In order to make the impact of PV more visible, the first row in Fig. 9 has been considered as without personal air supply. The difference between the first row and the next rows can be seen clearly. As regards temperature distribution, the main result is shown in Fig. 10, which displays a longitudinal distribution section. Again, unlike the other rows, the first row is considered as without PV. The simulation shows a slightly greater cooling effect of PV than for the seat where this local circulation is removed. In order to supplement the results presented here by the pattern of air flow circulation, the velocity distribution in a longitudinal section is displayed in Fig. 11. To compare the humidity distribution of standard CV arrangement in Fig. 8 with the distribution of personalized air supply, a cross section of this distribution is displayed in Fig. 12. A comparison of these two results shows that the humidity impact of breathing is approximately five times lower
than that of the personalized air supply. Fig. 12 depicts the most significant effects of the proposed PV: it not only supplies fresh air but, above all, it forms seat-fixed environment zones relatively separated from each other. In the displayed cross section laid through the breathing areas, we can see distinct zones of increased humidity above each seat. The humidity distribution in Figs. 9 and 12 confirms the conclusions arrived at with the single seat investigations. It is proved again that the proposed PV design is able not only to supply the passenger with fresh and satisfactorily humidified air, but also to establish a separate environment zone over the seat. Figs. 9 and 12 show that zones with increased relative humidity to 20% are maintained in the breathing areas of the passengers. At the same time, Fig. 12 also shows that the main central ventilation flows only slightly disturb the personalized air flow within the mid seat space. The low mixing rate of the PV and CV air flows is noticeable from the low humidity levels outside the seat zones. In this way, the proposed ventilation system can be considered relatively safe with respect to the risk of humidity condensation in the cabin shell. As
Fig. 10. Distribution of the cabin air temperature, ECS with the personalized ventilation set-up.
Fig. 12. Distribution of relative humidity in the cabin sector with combination of PV and CV system, where the humidification is via PV only.
P. Zítek et al. / Building and Environment 45 (2010) 2345e2353
regards water consumption, this ventilation set-up requires 0.08 kg/h per passenger. 7. Conclusions The presented arrangement of personalized air distribution and humidification in an airliner cabin has been proposed and verified by means of the FLUENT CFD program using the standard ke3 model, and by laboratory mock-up experiments. The FLUENT models were developed not only for a single seat but also for cabin sectors with twenty-four seats to compare the single seat patterns of air flow and humidity and temperature distributions with those in the whole cabin interior section. The CFD models have revealed the possibility of designing personalized air distribution towards the breathing area for each passenger. Although the passengers are supplied with conditioned fresh air via personalized ventilation, around 65% of the cabin air is still provided by central ventilation. Using the pair of supply and exhaust nozzles built into the back of the seat ahead, the personalized air supply protects each seated passenger from undesirable sharing of the breathed air with her/his neighbours. Personalized ventilation thus helps to protect the passenger against the possible spread of airborne diseases during the flight. In addition, due to the use of personalized ventilation the increased level of humidity is concentrated in the breathing zones of the passengers, while the level of RH in the remaining cabin space is kept low. Primarily, a way of separating a single seat environment from another single seat environment has been designed and verified by CFD models of cabin air circulation, and then the feasibility of forming a closed air stream effect has also been verified on a laboratory mock-up of the seat ventilation. Unfortunately, implementation of the proposed personalized air supply necessitates a relatively major e and therefore expensive e redesign of the cabin ventilation system. However, as our CFD experiments proved, the personalized air inlet and exhaust can hardly be simplified, since the desired closed circuit needs the two nozzles to be not too far apart. They also need to be fitted close enough to the passenger’s face. To build both of them into the back of the seat ahead therefore seems to be a favourable option. Acknowledgment The research results presented here have been achieved in the framework of the SEAT Project (Smart technologies for stress free air travel), sponsored by the European Commission DG H.3 Research, Aeronautics Unit, within the 6th Framework Programme, under contract number AST5-CT-2006-030958. It has also been supported by the Ministry of Education of the Czech Republic, under Project 1M0567.
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References [1] Committee on Air Quality in Passenger Cabins of Commercial Aircraft, Board on Environmental Studies and Toxicology and National Research Council. The airliner cabin environment and the health of passengers and crew. Washington D.C: National Academy Press; 2002. [2] Eng W, Harada L, Jagerman L. The wearing of hydrophilic contact lenses aboard a commercial jet aircraft. Aviation Space and Environmental Medicine 1982;53(3):235e8. [3] BRE (British Research Establishment). Study of possible effects on health of aircraft cabin environments e stage 2. Garston, Watford, UK: British Research Establishment, Environment Division; 2001. [4] Hinninghofen H, Enck P. Passenger well-being in airplanes. Autonomic Neuroscience 2006;129(1):80e5. [5] Thibeault C. Cabin air quality. Aviation, Space and Environmental Medicine 1997;68(1 Jan):80e2. [6] Gao NP, Niu JL. CFD study on micro-environment around human body and personalized ventilation. Building and Environment 2004;39(7):795e805. [7] Olander L, Westlin A. Airflow in aircraft cabin. Staub-Reinhaltung der Luft 2002;51(7 & 8):283e8. [8] Mizuno T, Warfield M. Development of three-dimensional thermal airflow analysis computer program and verification test. ASHRAE Transactions 1992;98(2):329e38. [9] Lin C, Horstman R, Ahlers M, Sedgwick L, Dunn K, Topmiller J, et al. Numerical simulation of airflow and airborne pathogen transport in aircraft cabinsdpart 1: numerical simulation of the flow field. ASHRAE Transactions 2005;111 (1):755e63. [10] Lin C, Horstman R, Ahlers M, Sedgwick L, Dunn K, Topmiller J, et al. Numerical simulation of airflow and airborne pathogen transport in aircraft cabinsdpart 2: numerical simulation of airborne pathogen transport. ASHRAE Transactions 2005;111(1):764e8. [11] Lindgren T, Norbäck D, Wieslander G. Perception of cabin air quality in airline crew related to air humidification, on intercontinental flights. Indoor Air 2007 Jun;17(3):204e10. [12] Zhang T, Chen QY. Novel air distribution systems for commercial aircraft cabins. Building and Environment 2007;42:1675e84. [13] Zhang T, Yin S, Wang S. An under-aisle air distribution system facilitating humidification of commercial aircraft cabins. Building and Environment 2010;45(4):907e15. [14] Gao NP, Niu JL. Improvement of facial humidity and reduction of pollutant inhalation in commercial aircraft cabins via application of personalized ventilation. ASHRAE Transactions 2008;114(2):37e44. [15] Gao NP, Niu JL. Personalized ventilation for commercial aircraft cabins. Journal of Aircraft 2008;45(2):208e12. [16] Jacobs P, de Gids WF. Individual and collective climate control in aircraft cabins. International Journal of Vehicle Design 2006;42(1e2):57e66. [17] Penot F, Meyer P. Thermal and dynamic study of a new concept of HVAC system using an annular air jet. In: 7th international conference on air distribution in rooms: air distribution in rooms; 2000. [18] Zítek P, Simeunovi c G, Vyhlídal T. Personalized air distribution system for a seated passenger in aircraft cabin. In: Proceedings of the 11th international ROOMVENT conference. Seoul: Seoul National University; 2009. p. 987e94. [19] Zítek P, Vyhlídal T, Simeunovi c G, Nováková L, Cí zek J. CFD model aided design of personalized air supply for aircraft cabin seat. In: Proceedings of the 6th Vienna conference on mathematical modelling. Vienna: ARGESIM; 2009. p. 2236e45. [20] Launder BE, Spalding DB. Lectures in mathematical models of turbulence. London, England: Academic Press; 1972. [21] Fluent 6.2 user’s guide. Fluent Inc; 2005. [22] Henkes RAWM, van der Flugt FF, Hoogendoorn CJ. Natural convection flow in a square cavity calculated with low-Reynolds-number turbulence models. International Journal of Heat and Mass Transfer 1991;34:1543e57. [23] Fanger PO. Thermal comfort, analysis and applications in environmental engineering. McGraw-Hill; 1970.
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Building and Environment journal homepage: www.elsevier.com/locate/buildenv
Novel personalized and humidified air supply for airliner passengers P. Zítek a, *, T. Vyhlídal a, *, G. Simeunovi c a, L. Nováková b, J. Cí zek b a
Centre for Applied Cybernetics and Department of Instrumentation and Control Engineering, Faculty of Mechanical Eng., Czech Technical University in Prague, Technická 4, 160 00 Prague, Czech Republic b Department of Fluid Dynamics and Power Engineering, Faculty of Mechanical Eng., Czech Technical University in Prague, Technická 4, 160 00 Prague, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 November 2009 Received in revised form 9 April 2010 Accepted 13 April 2010
The micro-environment control in an airliner cabin presented here consists in supplying each of the passengers with her/his own supply of fresh, humidified air in order to prevent possible airborne health problems and to provide local compensation for the humidity deficit. Unlike the environment control systems widely used in commercial aircraft, each of the seats in the cabin will be supplied individually with a separate airflow, which is also separately exhausted. This arrangement forms a personalized microclimate in the seat area. Essentially, focusing a personal air supply into the breathing area of the passenger works on the principle of individual seat ventilation, with the air supply and exhaust nozzles built into the back of the seat ahead. The system design, originally based on Computational Fluid Dynamics (CFD) models, has been verified by means of laboratory experiments. The results presented here have been achieved within the framework of FP6 EU Project AST5-CT-2006-030958, under the acronym SEAT (www.seat-project.org). Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Aircraft cabin Air distribution Relative humidity control Computational Fluid Dynamics Personalized air distribution system Particle Image Velocimetry
1. Introduction The typical cruise altitude of commercial aircraft is around 11,000 m, and the internal environment in an aircraft cabin needs to be provided artificially by means of an Environmental Control System (ECS), which compresses and cools the outside air to an indoor temperature and pressure that is suitable for passengers’ comfort. The ambient air at a temperature of below 50 C, and at atmospheric pressure five times lower than that at sea level, is compressed (and therefore heated) by the engine turbo-compressors and is then adjusted by the ECS to the state desired for the airconditioned interior. As the moisture content of the ambient air is near to zero, the initial relative humidity of the cabin air may drop considerably during the initial stage of flight to values below 10% at cruising altitude [1]. On-board measurements have shown surprisingly severe drops in humidity e from at least 47% to 11% within just 30 min of flight [2]. The longer the flight lasts, the more serious the humidity deficit becomes. Hence, long haul flights with drops in humidity to 5% and even lower are a matter of particular concern. According to the report of BRE [3], low humidity can have direct effects on passengers’ health and comfort, and the range of these effects is dependent on the humidity deficit, duration of exposure and other factors (e.g. temperature, water ingestion, wearing * Corresponding authors. E-mail addresses: [email protected] (P. Zítek), [email protected] (T. Vyhlídal). 0360-1323/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2010.04.005
contact lenses, use of a moisturizer, etc.) The main potential problems are: drying of the body surface (mucous membranes and skin), dehydration, a worse perception of air quality at a low level of RH, effects on thermal comfort (passengers feel cooler due to low RH) [4,5]. At the same time, a non-negligible source of cabin air humidity at cruising altitude flight is passengers’ exhalations. The respiration of each passenger is a humidity source for the cabin, because the expired air is saturated with water vapour. The mean flow rate of this expiration is usually considered as 0.14 l/s. However, this source of humidity originating from respiration cannot cover more than about 20% of the needed amount of water vapour [6]. This is why the cabin air humidity deficit is as great as was mentioned above. Another major difficulty in providing a suitable internal cabin environment, in addition to the humidity deficit, is due to the required intensity of the air exchange. The density of passengers in the cabin is extraordinarily high, higher than in other means of transport, and the air exchange rate therefore needs to be exceptionally intensive. Intensive ventilation is essential above all due to the levels of carbon dioxide (CO2), the main contaminant generated in the cabin [4]. To meet the ASHRAE Standard on carbon dioxide limits [1], it is necessary to exchange from 8 to 10 l of air per airliner passenger per second. The cabin air volume has to be totally exchanged approximately 20e25 times per hour, an intensity that is incomparable with any air-conditioning requirement in buildings. It is therefore a very difficult and very challenging task to design a comfortable and healthy cabin environment for commercial aircraft. The discrepancy between the necessary oxygen supply
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and the required rate of exhausting the carbon dioxide will be an important consideration. At the same time, it should be noted that the more intensive the cabin ventilation is in order to decrease the content of contaminants, the worse the humidity deficit will be [3]. Obviously the required high intensity of air exchange results in a need for high ventilation velocity and considerable air mixing rates. This leads to an ever-increasing risk of draughts, which is very difficult to overcome in huge present-day aircraft. Thus, the environment control systems commonly used in commercial airliners lead inevitably to contaminant transfer among the passengers, as has been confirmed by numerous measurements and studies [7e10]. Due to the mixing principle of air distribution in commonly-used ECS design, the spread of infectious airborne diseases, such as influenza, SARS, H1N1 flu, etc., from an infected passenger to another passenger may hardly be avoided. In order to maintain acceptable power consumption in the ECS the cabin air is re-circulated, i.e. the exhaust air is filtered and mixed back with the fresh air. The rate of recirculation is mostly about 50% of the whole supply [1]. The HEPA filters can remove around 99.7% of fine particles (0.3 micron in diameter) but cannot remove CO2 and other gaseous contaminants. In general, as far as medical considerations are concerned, the highest possible percentage of fresh air is recommended [1]. However, cost and power-saving considerations make it unacceptable to exclude recirculation altogether. This paper deals with ways of providing a personalized fresh air supply to each passenger, separated from the general cabin ventilation, and is organized as follows. Recent investigations of the cabin environment and modifications to air circulation are reviewed in Section 2. Section 3 presents a novel proposal for seat-focused air supply based on the principle of a local closed-flow pattern. The theoretical background of Computational Fluid Dynamics (CFD) modelling for the cabin environment is presented in Section 4. Both CFD and experimental verification of the proposed personalized ventilation system are presented in Section 5. In Section 6, the mutual influence of personalized and central ventilation in a cabin is studied using CFD. Emphasis is laid on the distribution of relative humidity in the cabin. Section 7 concludes the paper and summarizes the results obtained within the research presented here. 2. Recent personalized aircraft cabin air distribution and humidification systems The environmental control systems widely used in commercial airliners do not apply any humidification of the cabin air. The usual arguments against humidification of the cabin air are as follows: the weight penalty associated with the extra water that would have to be carried, the biological growths that are often associated with humidifiers, and the maintenance requirements of humidification systems. Safety concerns also need to be taken into consideration, mainly the risk of water condensation, dripping and freezing of moisture on the inside of the aircraft shell, which can lead to various safety problems [1]. Nevertheless, the problem of humidifying cabin air has recently received a great deal of attention. For example, Lindgren et al. [11] studied ways of humidifying the cabin air on eight intercontinental flights with the use of ceramic evaporation humidifiers. The measurements showed that the humidification system increased the level of RH by 10% without any measurable increase in micro-organisms in the cabin air. As will be shown, several theoretical studies include air humidification in a personalized ventilation system. Numerous studies and measurements of the cabin environment show that one way to overcome undesirable mixing of contaminated air among the passengers is via a more personalized air supply system. Personalization means that the cabin air-conditioning system should be designed to achieve as far as possible separate air
flow towards each passenger. Several arrangements for implementing the idea of a personalized air supply are known from airconditioning systems in buildings, and these have recently been considered for application in airliner cabins. Zhang and Chen [12] compare three options for air circulation. The usual scheme for air distribution in an airliner cabin (from the ceiling inlet to bottom exhaust) is compared with the application of so-called displacement air distribution, where clean, cool air is supplied from the floor and contaminated air is exhausted from the ceiling level. A third option investigated by the authors is the possibility of supplying the passenger’s air ration directly through a nozzle built into the back of the seat ahead. Consequently, Zhang et al. [13] proposed a novel under-aisle distribution system with the same objective. However, the main aim of that novel system is to humidify the cabin air by under-aisle air nozzles. CFD-based validation of the proposed method showed that the relative humidity was increased by 10% without causing moisture condensation in the interior of the cabin. The water consumption rate in air humidification is only around 0.05 kg/h per person. The micro-environment around a human body with and without a personalized air supply was studied by Gao and Niu [6], using an air supply pipe located in the breathing area. Their results consider the satisfactory range of air flow up to 3 l/s, and unlike the real situation in an airplane cabin, the person is considered as seated in a solitary working place. Gao and Niu [14] used a CFD model to investigate whether this type of personal ventilation can be used to improve the air quality in an aircraft passenger’s breathing area. In the proposed ventilation set-up, 2.5 l/s per person of fresh air humidified to a level of 40% RH is supplied directly to the breathing area of the passenger by an adjustable air supply nozzle, while 7.5 l/s per person of air is supplied by the conventional mixing central ventilation. It has been shown that personalized ventilation can shield the breathing area from contaminants and improve the removal of expiratory aerosols from the passenger, see also Ref. [15]. In addition, it has been shown that this set-up can increase the level of RH in the breathing area of the passenger, while the overall RH in the cabin is kept below 15%. Jacobs and de Gids [16] developed a new concept of aircraft cabin air-conditioning, in which the main indoor air quality and temperature control system for the passengers is built into the seat. Clean, humidified air from the outside is supplied directly to the passenger’s breathing area through the headrest of the seat. The exhaust imbedded into the seat enables the passenger to control the seat temperature by adjusting the exhaust ventilation rate through the seat. According to the CFD model, this seat-based ventilation system provides efficient shielding against contaminants, humidifies the air to a comfortable level, and controls the temperature in the seat area. The very challenging idea of providing a personalized environment was presented by Penot and Mayer [17], who investigated a pair of concentric nozzles for the driven supply and exhaust of air for a single person. Unfortunately, the arrangement presented by Penot and Mayer is not applicable to aircraft cabins, since the personalized air supply is considered as local heating from the ceiling. Although the effect of closing the streamlines between the supply and the exhaust is dependent on a temperature difference, and on the airflow velocities in the supply and in the exhaust, the obtained virtual separation of a personal microclimate zone from the surroundings is almost perfect. In an airliner cabin, the air supply has a slight cooling role, rather than a heating role, and it was therefore necessary to find a flow arrangement by analogy with the Penot nozzle, but with the opposite thermal influence. The best basis for building in the nozzles appears to be to place them in the back of the seat ahead, which can be used to create a microclimate zone with closed streamlines similar to the Penot arrangement.
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3. Seat-personalized supply of fresh and humidified air As far as the personalized environment problem is concerned, the following major conclusions can be drawn from the available references 1. If the usual present-day ECS system using a mixing principle of ventilation is employed, the passengers cannot be protected from considerable mutual exchange of contaminated air. 2. Basically, re-circulated air cannot be considered as entirely safe, due to the danger of increased levels of CO2 and other gaseous contaminants. 3. The outside air at cruising altitude is free of organic contaminants, but is very dry. A steady supply of outside air is much more than sufficient to flush the human-generated moisture from the cabin, and it reduces the interior air humidity to levels below 10% on long haul flights. However, full humidification of the cabin air supply is risky, due to the threat of dew condensation inside and above the cabin shell. 4. On the other hand, only about one third of the cabin air supply is needed to cover the oxygen requirements of the passengers. The rest of the total supply is necessary only for diluting the concentration of carbon dioxide and other pollutants in the cabin [1] and for pressurizing the cabin. To sum up, in order to protect passengers from intensive sharing of exhaled air with their neighbours, it is necessary to adapt a scheme of cabin ventilation that supplies each passenger with her/his own air ration. As has already been proposed by Zhang and Chen [12], Gao and Niu [14,15], Jacobs and de Gids [16], the personal air ration should be supplied directly into the breathing area of each passenger. The appropriate amount of air has to cover the physiological oxygen demand of the passenger and also the requirement for flushing away gaseous contaminants. 3.1. The novel concept of personalized air supply Similarly as in Ref. [14], the modification of the cabin ventilation system described in this paper and shown in Fig. 1 uses about 2.5e3 l/s/passenger of outside fresh air humidified to a level of 40% RH for the personalized ventilation (PV) supply. The rest of the required air exchange, i.e. 6.5e9 l/s/passenger is supplied by the mixing-based central ventilation (CV). Unlike the arrangement in Ref. [14], the personal ventilation (PV) air supply nozzle is built into the back of the seat in the front of the passenger, as shown in Fig. 1.
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Our system is in this way similar to that of Zhang and Chen [12]. However, the system that we propose does not require such extensive modification of the cabin air ventilation system as the Zhang and Chen [12] system. Moreover, our proposed system has an air exhaust nozzle, located just below the air supply nozzle in the back of the seat ahead (see Fig. 1). The nozzle removes the contaminated air from the seat space by analogy with the arrangement presented by Penot and Mayer [17]. In this respect, our proposed system resembles that of Jacobs and de Gids [16], though the design of the set-up is different. The design of nozzles that will supply and exhaust the personalized air supply, however, is not a simple task due to specific temperature and draught limitations connected with passenger comfort requirements. Firstly, a closed-flow pattern similar to that presented in Ref. [17] can be achieved only if the temperature in the supply nozzle is at least 2e5 C lower than the temperature in the exhaust. Fortunately, this demand is in good agreement with the typical situation in an aircraft cabin, where the supply air temperature is between 16 and 19 C, while the cabin temperature is usually between 21 and 24 C [1]. On the one hand, the personalized air supply has to be directed towards the breathing area, i.e. towards the passenger’s chin. On the other hand, however, it is necessary to avoid the supply flow causing an inconvenient draught for the passenger (bear in mind, for example, the high flow velocity from the widely-used gaspers over the passenger’s head). The usual requirement is that the air flow velocity around the face should not exceed approximately 0.2 m/s [6]. However, perceptions of warmth and draught are very individual, and according to Gao and Niu the limit of 0.2 m/s holds if a 14% dissatisfaction rate [23] is considered acceptable. Due to this limitation and the required air flow value, the cross area of the supply nozzle needs be relatively large, around 60 cm2. Its design should enable the air ration to be brought towards the lower part of the passenger’s face without exceeding the draught limits. In Ref. [19], the two nozzles are designed using CFD-based optimization, and their performance is verified by laboratory tests. In any case, the personalized air supply is considered to be an optional service that can be attenuated or even switched off by the passenger, similarly as in the case of overhead gaspers. While personalized air circulation predominantly affects the breathing area of the passengers, the role of central air distribution by ECS may remain almost unchanged. The central system uses the mixing principle, provides most of the ventilation rate, and pressurizes the cabin air in the usual way, see Fig. 1. For economic reasons, a substantial part of the air processed and supplied by this part of ECS is re-circulated and filtered. Around 35% of the
Fig. 1. The proposed modification of the cabin ventilation system; central ventilation (CV) flow streams e mixture of outside and re-circulated air provided by the central ventilation (CV) system via the classical mixing principle, personal ventilation (PV) flow streams e fresh and humidified outside air provided by personalized ventilation (PV) to the local area of each seat.
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exhausted air from the cabin both through the local nozzles and through the central exhaust nozzles is taken back and re-circulated. In the scheme, the central air supply nozzles are considered to be in the middle part of the cabin ceiling, and the central air exhaust is in the cabin floor corners. To sum up, the personalized ventilation system in Fig. 1 has the following features: i) each passenger is supplied with fresh, humidified outside air via the PV system through a pair of nozzles built into each of the cabin seats, ii) the PV air supply is only approximately one third of the total ventilation volume, and the rest of the cabin air is still provided as a mixture of outside and re-circulated air, which is supplied by the CV system via the classical mixing-based principle. CV air is not humidified. In what follows, we first concentrate on CFD-based verification of the proposed single seat personalized ventilation system. The aim is to verify the possibility of forming a microclimate zone of circulating fresh air in front of each passenger by adjusting the flow rates and temperature differences between the supply and exhaust nozzles. Subsequently, the results of the CFD models are verified via laboratory experiments. The results of the experiments also validate the CFD methodology. Thus, the CFD model of the whole cabin sector is presented as the final result. Obviously, as shown in Fig. 1, the flow patterns of the PV and CV systems are perpendicular to each other, i.e. while the streamlines of CV are transversal, the streamlines of PV are longitudinal to the cabin tube. The aim of the cabin CFD model is thus to study the mutual influence of these streams. The distribution of the relative humidity in the cabin will also be investigated. Simplified CFD models of the set-up, together with the first experimental results, were presented in Refs. [18,19]. The simultaneous investigation of air velocity, temperature and humidity in the CFD models brings an advantage in tracing the motion of the air supplied by the PV. Due to humidification of the personalized air supply, the increased moisture content can serve as a tracer gas visualizing the circulation of the added air between the inlet and exhaust nozzles. As the temperature differences are only slight, the moisture content distribution corresponds approximately to the distribution of the relative humidity. From the RH distribution we can thus track the trajectory of the personalized air stream. 4. CFD model characteristics of the cabin air ventilation set-up The numerical investigation of the air circulation inside the cabin involves solving a set of partial differential Reynolds-Averaged NaviereStokes (RANS) equations within appropriate boundary conditions. These equations describe the mutual dependence of the momentum, energy, moisture content, turbulent kinetic energy and its dissipation rate in the cabin space. The model is the standard ke3 model, using numerous approximations, and a certain validation of the model by experimental measurements is therefore strongly recommended [20]. The partial differential equations were discretized into algebraic equations using the finite volume method with a second order up-wind scheme. The GAMBIT program was used to generate the grid of cells. A special task in this case was the air humidification and the resulting buoyancy effects. The simulations were implemented in the FLUENT environment [21], where the algebraic equations were solved over all the cells. Due to the nonlinearity of the equations, iterations need to be applied to achieve a convergent solution. The flow velocity is solved from the momentum equations, and is then corrected through the continuity equation in the SIMPLE algorithm. In addition, the continuity
equation is repeatedly solved to settle the steady state values of both pressure and velocity. The cabin interior fluid is considered as a mixture of two components, air and water vapour. The mass conservation equation of the mixture, consisting of two species, is given by the following equation
vðrYi Þ þ VðrvYi Þ ¼ VJi ; vt
i ¼ 1; 2
(1)
where Yi is the local mass fraction of each component, v is the velocity vector, Ji is the diffusion flux of the i-th component, and r is air density. The turbulence model parameters, namely the kinetic energy rate k and the dissipation rate 3, are calculated using the following transport equations
vðrkÞ vðrkui Þ v ¼ þ vt vxi vxi vðr3Þ vðr3ui Þ v ¼ þ vt vxi vxj
"
"
C23
#
m vk mþ t þ Gk þ Gb r3 YM ; sk vxj
(2)
#
m v3 3 mþ t þ C13 ðGk þ C33 Gb Þ sk vxj k
r32 k
(3)
where Gk expresses the generation of turbulence kinetic energy due to the mean velocity gradients, and mt is the turbulent viscosity, YM determines the effect of compressibility on the turbulence, and the model constants C13, C23 and C33 are given in Refs. [20,22]. Both the cabin interior and the passengers’ bodies have an extremely complex geometry. For this reason, an unstructured tetrahedral grid was used in building up the CFD model, although this kind of grid necessarily provides lower precision than a structured grid. The primary objective of the research presented here is to investigate the distribution of humidity, temperature and flow velocity near the passengers’ bodies, with special reference to the supply and exhaust nozzles of PV. Later, in Section 6, the CFD model also incorporates the intake of humidity originating from the passengers’ respiration. With regard to these priorities, a denser grid is desirable in these parts of the cabin space than in the other parts. Namely around the supply and exhaust nozzles the size of a grid cell is taken as 10 mm, on the passenger’s surface the size is 30 mm, and towards the rest of the space these values increase continuously up to 50 mm. 5. Verification of the proposed single seat ventilation set-up and CFD model validation In this section, we proceed to the use of laboratory experiments to verify the possibility of forming a local microclimate zone in the seat area. To implement the personalized air supply principle in the
Fig. 2. Scheme and geometric model of the PV set-up of the seat.
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Fig. 3. Laboratory set-up: Left e seat model with personalized ventilation system and with displacement of temperature sensors. Right e detail of the pair of nozzles built in the laboratory seat set-up.
Table 1 Conditions of the experimental verification of PV system. Personalized supply Supply airflow rate [l/s per person] Exhaust airflow rate [l/s per person] Supply airflow temperature [ C] External air temperature
2.5 2.5 20.9 23.4
laboratory, a seat mock-up with a manikin and a pair of supply and exhaust nozzles, as in Fig. 2, was built up. Due to technical limitations in the laboratory, the effect of central ventilation was not considered in the experiments. However, as will be shown later in the CFD models of the cabin sector, the influence of central ventilation on the PV airflow patterns is relatively weak. The results of the experiments will also be used to validate the CFD model. 5.1. Experimental verification of the single seat ventilation set-up In order to test the potential of the PV system to form a virtually closed micro-environment zone in the seat area and to validate the CFD model, an experimental set-up was built, see Fig. 3. The simplified model of the seat with a seated passenger and the back of the seat ahead was constructed from aluminium profiles, acrylic sheets and polystyrene foam. The supply and exhaust nozzles were both fixed into the back of the seat ahead, and were connected by pipelines with ventilators. The supply and exhaust flow rates can both be adjusted independently by means of the ventilator control. To stabilize the airflow pattern in the seat area, the supply nozzle air is cooled by about 3 C in comparison with the temperature of the ambient air to obtain the necessary temperature difference. The flow patterns were visualized by two Particle Image Velocimetry (PIV) methods, namely helium bubbles and SAFEX fog. The results
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Fig. 5. Velocity vectors and contours of velocity magnitude measured close to the manikin surface using PIV (helium bubbles). The grid in the measured area is used to determine the values to be compared with the CFD model in Fig. 7 e left, see Table 4.
of the experiments, with the conditions given in Table 1, are shown in Figs. 4e5. Fig. 4 shows the fog visualization of the steady state airflow distribution in the seat area. Both the side view and the front view show clearly that micro-environment zones are constituted in the seat area. The passenger’s breathing area is efficiently covered by the discharged air from the supply nozzle. It can be estimated from the SAFEX fog pattern that the supply nozzle airflow is also able to provide efficient shielding of the passenger’s breathing zone from the air of the CV system. The results of the air velocity measurements close to the surface of the manikin are given in Fig. 5. The velocity distribution was measured using the PIV method with helium bubbles. The results shown in the figure were obtained by two measurements in different rectangular areas, which partly overlapped each other. As can be seen, the air velocity in the vicinity of the surface of the manikin does not exceed the recommended comfort limit of 0.2 m/s. Finally, the results of the temperature measurements at points shown in Fig. 3 are given in Table 2. The ambient air temperature was measured as T1, three thermometers were fixed on the surface of the manikin, T2 (chest), T3 (neck), T4 (face), and the temperature of the air leaving the supply nozzle was measured as T5. The settled temperature values after 15 min of measurement, given in Table 2, confirm that the temperature of the air touching the body surface is at least 2 C higher than the temperature of the air supply. To sum up, the experiments prove the possibility of forming a local environment zone in the passenger’s area. 5.2. CFD model to be verified The CFD model of a single seat in Fig. 2 consists of about one million cells meshed with an unstructured tetrahedral grid. As
Fig. 4. Airflow distribution in the seat area visualized by SAFEX fog.
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Table 2 Comparison of temperature measurements at points in Fig. 3 with the CFD model values and their relative errors e ¼ jTm TsjT1 m 100%, Tm e measured, Ts e CFD model temperature. Temperature [ C]
Experiment
CFD model
e
T1 T2 T3 T4 T5
23.2 23.1 22.9 23.0 20.8
23.1 22.5 22.4 22.8 20
0.4% 2.6% 2.2% 0.9% 3.8%
Table 3 Boundary conditions for the CFD model of a single seat microclimate. Personalized supply Supply airflow rate [l/s per person] Exhaust airflow rate [l/s per person] Supply airflow temperature [ C] Supply airflow relative humidity [%] Turbulence intensity [%] Aircraft air and seat temperature [ C] Floor temperature [ C] Temperature of the manikin surface Tm [ C] Dimension of modelled volume height width length [m] Temperature of the modelled volume boundary [ C]
2.5 2.5 19 40 10 23 22 30.3 (23) 1.6 1.1 1.2 22
mentioned in section 4, the FLUENT program is used for CFD simulation, and the air circulation, heat transfer and humidity distribution are modelled by the standard ke3 model, using the boundary conditions listed in Table 3. The seat with the manikin was placed in the closed modelled volume 1.6 m in height, 1.1 m in width and 1.2 m in length with a boundary temperature of 22 C. A specific verification problem emerged concerning the surface temperature of the manikin. The CFD model temperature should be as close to reality as possible, in order to predict the contribution of the buoyancy force from the thermal plume. In modelling the cabin environment, the average surface temperature of a dressed passenger was considered as 30.3 C, according to Zhang and Chen [12]. However, for the purposes of validating the modelling method by laboratory experiments, it was more correct to consider this temperature as equal to the real temperature of the manikin during the experiments. Hence, for validation purposes only, the surface temperature was set at 23 C, i.e. approximately equal to the ambient air temperature in these model experiments. Besides, no expiration from the manikin’s mouth is considered. The results of the CFD models for comparison with the experimental results are shown in Fig. 6. Table 4 presents the relative
Table 4 Air flow velocity relative errors between the measurement vm and CFD model results vs in the grid points in Fig. 5, e ¼ jvm vsjv1 m 100%. A B C D E F
9.2 5.2 4.6 12.7 4.4 2.0 1
9.1 1.6 7.1 11.8 9.9 8.0 2
2.3 11.8 10.2 10.3 2.7 8.1 3
1.3 21.8 10.4 11.4 5.4 4.9 4
5.4 17.9 12.1 16.4 5.8 4.2 5
5.1 20 13 19.6 8.9 2.7 6
3.9 22 13.6 17.8 11.2 2.1 7
3.4 17.8 12.1 18.7 12.2 5.4 8
deviations of air velocity magnitude between the measurement data (Fig. 5) and the CFD simulation results (Fig. 6, left). The measured velocity magnitude values are evaluated using PIV at the grid points shown in Fig. 5. These values are compared with the CFD model results in the same grid points of the model results shown in Fig. 6-left. As shown in Table 4, the relative error does not exceed 20%, and its average value for all the grid points is 10%. Thus, taking into consideration the complexity of the measured velocity field, the agreement between the model and the measurements are relatively good. Table 2 presents the modelled temperature values at the measurement points shown in Fig. 3. In addition, the relative error between the measurement data and the CFD simulation results (Fig. 6 right) is presented in the last column of the table. As can be seen, the maximum relative error is 3.8%, and the average value taken over all the points is 2%. Thus, the agreement between the temperature measurement and the model results is very good. Based on general matching of CFD and experimental results, it can be concluded that our CFD methodology provides credible results and corresponds sufficiently with the measurements on the real set-up. On the basis of this experience, a more realistic CFD model will be used in Section 6 to verify the single seat ventilation set-up. Later, the model will be extended to model the air circulation in a whole sector of the cabin. 5.3. Limitations of temperature control in PV In this section we draw attention to the limits of air temperature control via personalized ventilation. This is due to the need for a temperature difference in the principle of the Penot nozzle [17], i. e. the temperature in the supply nozzle needs to be 2e5 C lower than the temperature of the air being exhausted from the seat space. This is demonstrated in a 2D CFD model in Fig. 7. If the required temperature difference is achieved, a local micro-environment is formed. However, as soon as this difference disappears, or if it is even reversed, the effect of the closed air flow pattern
Fig. 6. CFD model results for comparison with the experiments e the body temperature Tm ¼ 23 C, no expiration from the manikin’s mouth is considered. Left e contours of velocity magnitude of air, right e static air temperature of air.
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Fig. 7. 2D models of airflow in the seat local zone with respect to the temperature difference at the supply and exhaust nozzle.
between the supply and exhaust nozzles is destroyed. This is because the natural upward flow of warmer air overcomes the influence of the exhaust nozzle. This phenomenon was also confirmed by the laboratory experiments.
6. CFD model analysis of a cabin sector environment Let us now return to the issue of humidification. The task is to verify how the proposed personalized air supply can provide a suitable humidity distribution in the seat space and in the cabin as a whole. As above, the air humidity also serves as a tracer helping to observe the mixing or separation of the flows. Unlike the models of the circulation in this stage of CFD modelling, the passengers’ respiratory contributions to the air humidity are considered as local sources of air saturated by water vapour. These expiration flows are assumed as constant, and are set as 0.14 l/s per passenger [6]. However, a more detailed representation of the humidity sources necessitates a simpler specification of the seat and manikin shape. This simplification is justified by experience with various versions of cabin interior specification where, in practical terms, such changes in detail had a negligible effect on the temperature, humidity and velocity distributions. The boundary conditions for the model experiments presented in this section are given in Table 5. The boundary temperatures were taken from Zhang and Chen [12]. As usual, the aircraft shell, the windows, the cabin floor and ceiling are considered in a steady
thermal state. The CFD model experiments were performed for both environmental versions, i.e. for standard ECS operation and for the proposed personalized air supply. The results presented below are mainly for the humidity and temperature distribution, and also for the interaction between the central and personalized ventilations. In comparing the results of these two versions, attention is mainly paid to the influence of personalized humidification and also the humidity contribution of passengers’ breathing. The humidity effect of passenger’s respiration is shown in Fig. 8, where the whole amount of cabin air is supplied by the conventional ECS. The small spots of different intensity show the range of humidity increase due to passengers’ breathing. Although the expiration air is in a state of saturation, the impact of breathing on the humidity distribution is very limited inside a cabin with standard ventilation, without humidification. The size of the increased humidity spots is constrained to the close vicinity of the passengers’ faces, so that the expiration humidity is dispersed very quickly. The humidity increase due to the passengers’ respiration is noticeable only in the closest proximity of their mouths. In fact, with regard to the intensive air exchange, this result is not surprising. On the other hand, the impact of personalized
Table 5 Boundary condition for the CFD model of the cabin sector.
Supply airflow rate [l/s per person] Supply airflow temperature [ C] Supply airflow relative humidity [%] Turbulence intensity [%] Aircraft skin temperature [ C] Aircraft seat temperature [ C] Window temperature [ C] Floor temperature [ C] Temperature of the passenger clothes [ C] a
Personalized ventilation (outside air)
Central ventilation (mixture of outside and re-circulated air)
3 19 40
10 (7)a 19 5
10 22 23 16 22 30.3
5
10 l/s is considered if only CV is considered, while 7 l/s is considered in CV in combination with PV.
Fig. 8. Relative humidity distribution resulting from the CFD model for the set-up, where humidification of the whole cabin volume via central air distribution system is considered.
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Fig. 9. Distribution of cabin air relative humidity, ECS with the personalized ventilation set-up. Fig. 11. Air velocity patterns in the cabin, ECS with the personalized ventilation set-up.
ventilation is shown in Figs. 9e12. Fig. 9 shows how the inlet flow of fresh and humidified air is dispersed and mixed together with the respiration air. Nevertheless, it should be noted that real breathing does in fact produce an alternating flow, which probably will lead to even more favourable mixing conditions than the fixed flow in the simulation. In order to make the impact of PV more visible, the first row in Fig. 9 has been considered as without personal air supply. The difference between the first row and the next rows can be seen clearly. As regards temperature distribution, the main result is shown in Fig. 10, which displays a longitudinal distribution section. Again, unlike the other rows, the first row is considered as without PV. The simulation shows a slightly greater cooling effect of PV than for the seat where this local circulation is removed. In order to supplement the results presented here by the pattern of air flow circulation, the velocity distribution in a longitudinal section is displayed in Fig. 11. To compare the humidity distribution of standard CV arrangement in Fig. 8 with the distribution of personalized air supply, a cross section of this distribution is displayed in Fig. 12. A comparison of these two results shows that the humidity impact of breathing is approximately five times lower
than that of the personalized air supply. Fig. 12 depicts the most significant effects of the proposed PV: it not only supplies fresh air but, above all, it forms seat-fixed environment zones relatively separated from each other. In the displayed cross section laid through the breathing areas, we can see distinct zones of increased humidity above each seat. The humidity distribution in Figs. 9 and 12 confirms the conclusions arrived at with the single seat investigations. It is proved again that the proposed PV design is able not only to supply the passenger with fresh and satisfactorily humidified air, but also to establish a separate environment zone over the seat. Figs. 9 and 12 show that zones with increased relative humidity to 20% are maintained in the breathing areas of the passengers. At the same time, Fig. 12 also shows that the main central ventilation flows only slightly disturb the personalized air flow within the mid seat space. The low mixing rate of the PV and CV air flows is noticeable from the low humidity levels outside the seat zones. In this way, the proposed ventilation system can be considered relatively safe with respect to the risk of humidity condensation in the cabin shell. As
Fig. 10. Distribution of the cabin air temperature, ECS with the personalized ventilation set-up.
Fig. 12. Distribution of relative humidity in the cabin sector with combination of PV and CV system, where the humidification is via PV only.
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regards water consumption, this ventilation set-up requires 0.08 kg/h per passenger. 7. Conclusions The presented arrangement of personalized air distribution and humidification in an airliner cabin has been proposed and verified by means of the FLUENT CFD program using the standard ke3 model, and by laboratory mock-up experiments. The FLUENT models were developed not only for a single seat but also for cabin sectors with twenty-four seats to compare the single seat patterns of air flow and humidity and temperature distributions with those in the whole cabin interior section. The CFD models have revealed the possibility of designing personalized air distribution towards the breathing area for each passenger. Although the passengers are supplied with conditioned fresh air via personalized ventilation, around 65% of the cabin air is still provided by central ventilation. Using the pair of supply and exhaust nozzles built into the back of the seat ahead, the personalized air supply protects each seated passenger from undesirable sharing of the breathed air with her/his neighbours. Personalized ventilation thus helps to protect the passenger against the possible spread of airborne diseases during the flight. In addition, due to the use of personalized ventilation the increased level of humidity is concentrated in the breathing zones of the passengers, while the level of RH in the remaining cabin space is kept low. Primarily, a way of separating a single seat environment from another single seat environment has been designed and verified by CFD models of cabin air circulation, and then the feasibility of forming a closed air stream effect has also been verified on a laboratory mock-up of the seat ventilation. Unfortunately, implementation of the proposed personalized air supply necessitates a relatively major e and therefore expensive e redesign of the cabin ventilation system. However, as our CFD experiments proved, the personalized air inlet and exhaust can hardly be simplified, since the desired closed circuit needs the two nozzles to be not too far apart. They also need to be fitted close enough to the passenger’s face. To build both of them into the back of the seat ahead therefore seems to be a favourable option. Acknowledgment The research results presented here have been achieved in the framework of the SEAT Project (Smart technologies for stress free air travel), sponsored by the European Commission DG H.3 Research, Aeronautics Unit, within the 6th Framework Programme, under contract number AST5-CT-2006-030958. It has also been supported by the Ministry of Education of the Czech Republic, under Project 1M0567.
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