An Aisle Displacement Ventilation System for Twin-Aisle Commercial Airliner Cabin

. The environmental control system in most commercial airliner cabins supplies air from shoulder and ceiling level and exhausts air at floor level on both sides of the cabin walls. The ventilation system mixes air in the cabins to create a relative uniform air temperature distribution which is great for passengers’ thermal comfort. However, the mixing ventilation also enhances airborne contaminant transfer. Many displacement ventilation methods have been proposed to use in aircraft cabins, but the disadvantages of the ventilation approach are usually creating draft on the passengers’ ankles and high air temperature stratification between passengers’ heads and feet. This investigation developed an aisle displacement ventilation (ADV) system which can reduce air temperature stratification effectively without occupying the legroom space under the seats, so it is good for passengers and crew members’ comfort and friendly for luggage storage, in addition, it can also be easily installed in aircraft cabins. By installing the system in a five-row, twin-aisle cabin mockup, our study found that the ADV system can create a low air velocity distribution in the cabin and can maintain an acceptable air temperature stratification without draft. The system created an uprising airflow which can effectively remove airborne contaminant which was generated from index passengers’ respiratory activities. The experimental data were used to validate a computational-fluid-dynamics (CFD) program. The validated CFD program was used to compare the ADV with under-seat displacement ventilation (USDV) and underfloor air distribution (UFAD) system along the aisles. The comparison results show that the ADV had obvious better thermal comfort than the other two systems, and the cabin air quality of the three ventilation systems was similar, all far better than the “perfectly-mixed” ventilated condition.


Introduction
Environmental control systems (ECS) in commercial airliners are used to pressurize and condition air cabins to provide a suitable internal environment to passengers and crew [1] Mixing ventilation methods which supply air from shoulder and ceiling levels and exhausts air at floor level on both side walls were widely used in the current aviation industry. Fan and Zhou [2] reviewed that contaminant under the mixing ventilation system cannot be effectively eliminated. In addition, airborne infectious disease transmissions through commercial airliners have long been a major concern. The transmissions of SARS in 2003 [3] , H1N1 influenza in 2009 [4] , and COVID-19 in 2020 [5] are just a few examples of the transmissions in a few flights. Airborne infectious diseases come and go in a few years and their damages to the economy were very serious [6] . The existing mixing ventilation systems used in air cabins might even enhance the virus spreading of airborne infectious diseases. Therefore, many scholars proposed displacement and personalized ventilation systems intended to improve cabin air quality. For example, an underfloor air distribution or displacement ventilation system along aisles [7] , and a personalized ventilation system from the lower part of the seat of the front row [8] were developed. Chanfiou et al. [9] compared six personalized air supply and exhaust systems and concluded some could provide comfortable environment. Kong et al. [10] installed personalized exhaust diffusers on the back of passenger seats. The efforts can prove the displacement and personalized system can provide better cabin air quality than the mixing ventilation system. However, for the displacement ventilation method, high air velocity and low air temperature near the floor can cause draft to the passengers' ankles and large temperature difference between passengers' heads and feet. As for the personalized ventilation system, there were too many supply pipes, ducts and diffusers which not only occupy the valuable cabin space but also may be a safety concern in case of cabin evacuation. To remedy the above mentioned problems, an aisle displacement ventilation (ADV) system was proposed in this article, which would not use the legroom space under the seats nor supply the cold air directly to passengers' feet.

Experimental measurements
This investigation used a twin-aisle cabin mockup that simulated a five-row, economy-class Boeing 767 fuselage as shown in Fig. 1. The cabin dimension was 5.92 m long, 4.60 m wide, and 2.10 m high, with a cabin volume of 30.85 m 3 . There was a door at the front of the cabin section for easy entry and exit for the testers and equipment. The air was supplied by 20 diffusers along the two aisles on the side of passenger seats. Each diffuser was 420 mm wide and 90 mm high. The supply airflow rate was 9.4 L/s per passenger and the supply air temperature was 22±0.5 o C. Each diffuser was equipped with a flow equalizer, which was composed of a fiber layer of 10 mm and a honeycomb core of 5 mm thick. The fiber layer was used to reduce the velocity and make the distribution evenly, while the honeycomb core maintained the air supply direction. Hence, the air was supplied in the direction normal to the diffuser surface. The exhaust outlets were at the ceiling level above the aisles. The leaked air from the door gap accounted for 5% of the total air volume due to the positive pressure inside the cabin. This investigation used ultrasonic anemometers (Model DA650-3TV & TR-92T) to measure three-dimensional air velocity and temperature in cabin air. The anemometers can measure air velocity in the range of 0-10 m/s with an accuracy of 0.006 m/s. The temperature resolution was 0.025 K with 1% error. At each measuring location, the flow was first stabled for 5 minutes and then the measurements took another 3 minutes with a frequency of 20 Hz. This investigation injected 1% of SF6 and 99% N2 gas mixture into the breathing zone of the passenger as illustrated by the two red points in Fig. 2, which depicts a cross section and a longitudinal section where the air velocity, air temperature, and tracer-gas concentration were measured.

Performance evaluation of different ventilation systems
To compare the ADV system with other systems, such as USDV system as shown in Fig. 3 (a) [11] and UFAD system in Fig. 3(b) [7] , this investigation used the same exhausts and the same interior thermal conditions measured from the above experiment for the three systems. The only difference was the air supply inlet location and size. The supply air temperature and airflow rate were also the same for the three systems. Different from the measurement, only one pollutant source was considered for simplicity. The pollutant source was released from passenger 3A, 3B, 3C and 3D, respectively, instead of two simultaneous releases as that in the measurement case. numerically the governing equation for all the variables. A simulation was considered as convergence when the absolute residuals for all the flow parameters were less than 10 -3 .  k-ε model agree qualitatively with the experimental data. Even though there were discrepancies between the computed and measurement results, the airflow in the cabin was very complex and the difference can be explained reasonably. The CFD program can predict the distributions of air velocity, air temperature, and tracer-gas (SF6) concentration in the cabin mockup with reasonable accuracy.    5 shows the vertical air temperature profiles for passengers 3A, 3B, 3C, and 3D. The temperature was stratified for all the three systems but the temperature difference between passenger head and foot levels was less than 3 K. The ankle region was defined as ranging from 0.05 m to 0.10 m above the floor, and the head region from 1.05 to 1.28 m above the floor. The ASHRAE standard [12] sets a temperature gradient less than 2 K/m to be acceptable. Therefore, the temperature difference would not cause discomfort. The ADV system had the smallest overall temperature difference than the other two systems so it would be the best. The USDV and UFAD systems had similar temperature difference. Figure 5. Vertical air temperature profiles of the three ventilation systems for passengers 3A, 3B, 3C, and 3D.

Cabin air quality
To evaluate the air quality performance of the three ventilation systems, this investigation assumed that an airborne contaminant was released due to breathing activities of the passengers. This study used the dimensionless contaminant concentration, C*: where C dimensional contaminant concentration (m 3 /m 3 ) in any location of the cabin at in the breathing level, ̇ source strength released from an index patient due to respiratory activity (m 3 /s), and ̇ flow rate for the fiverow cabin mockup (m 3 /s). In most cases, airborne diseases could be transferred two rows before and two rows behind an index patient [13] . Thus, the average contaminant concentration in the five rows could be a good measure for evaluating cabin air quality due to respiratory activities. When C* is lower than 1, the air quality is better than that under perfectly-mixed ventilation.   for the three ventilation systems. Except at the contaminant source and its proximity, the contaminant concentration in the cabin air was very low (<<1). The contaminant moved slightly forwards so the passenger in front of the source had high contaminant concentration. Fig. 7 compares quantitatively the dimensionless contaminant concentration at the breathing level when the contaminant source was released from the breathing level of passenger 3A. The results shown in Table 1 were identical to those in Fig.6. Due to the symmetrical airflow pattern in the cross section, the contaminant from passenger 3A could not be transported to the other side of the cabin. The contaminant concentration in seats C, D, E, F, and G was very low (C*<0.1) so they would not be at risk even if the contaminant is COVID-19, compared with that of perfectly-mixed condition (C* = 1).

Conclusions
This investigation tried to improve thermal comfort and air quality in a twin-aisle cabin mockup through experimental measurement and CFD simulation. Performance of the ADV, USDV, and UFAD systems were examined and compared. Based on the obtained results, the following conclusions are drawn: The RNG k-ε model can predict the distributions of air velocity, air temperature, and tracer-gas (SF6) concentration in the cabin mockup with reasonable accuracy.
The three ventilation systems maintain a low air velocity less than 0.2 m/s around the passengers in the cabin. The temperature difference is less than 3 K between the head and foot levels of the passengers. The ADV system has the lowest air temperature difference, so it is slightly better than the other two systems in terms of thermal comfort.
The three systems provide similar cabin air quality and prevail over the mixing ventilation system. Contaminant is well confined and only dispersed to the passengers who are seated next to the source passenger in the three systems.