Contrastive analysis of air supply pattern in aircraft cabin considering bleedingair contam inant

Zhen-bin WANG

，Gang XU1，Xi-yuan CHEN2

（1 Aviation Engineering Institute，Civil Aviation Flight University of China，Guanghan 618307，China）

（2 Tianjin Key Laboratory of Civil Aircraft Airworthiness and Maintenance，Civil Aviation University of China，Tianjin 300300，China）

Abstract：In order to compare the effect of both contaminant removal and ventilation inside the aircraft cabin under differentair supplymodes considering the issue ofbleedingair contaminant transmission，an experimentalmeasurementwas performed in a cabin mockup which contains five rows of seats.The concentration diffusion of both instantaneous and continuous bleeding air contaminantweremeasured under air supplymodes of ceiling and mixture type respectively insidethe cabin mockup，and the corresponding air qualities in the cabin were evaluated.The results of experiment indicated that bleeding air contaminant arrived earlier at the seat near the window of cabin than other places under the air supplymode of ceiling while arrivedearlier at themiddle seat undermixture air supply.Mixture air supply ismore likely toaccelerate the transmission of contaminant butwith a lower concentration and a higher rate of contaminant removal.More effective ventilation and better air quality exist under the mixture air supplymode while the ventilation volume is equal.

Key words：Bleed air contaminant，Cabin environment，Air quality，Age of air

Air supply in the cabin ofmodern commercial passenger aircraft is amixture air of cabin recirculated air and outboard engine bleeding air.Once the engine oil leaks into the bleeding air，itwill cause various chemical pollutants to enter the aircraft cabin，which is seriously threatening the health of both passengers and crew members［1］，this type of pollution is called bleeding air contaminant.With the development of the aviation industry，an increasing high level of the air quality in aircraft cabins has been required.Studies have shown that about3.9 times of bleed air contaminant events occur amid per 1 000 flights［2］.Therefore，how to control and reduce bleeding air contaminant and thereby create good cabin air quality is essential to the healthy development of the civil aviation industry.

Researches related to the air quality inside aircraft cabins have been conducted domestically and abroad，including the study onimpactof contaminant resource locations inside the cabin on diffusion rules of the contaminant［3-5］，of ventilation volume on the rate of contaminant removal inside the cabin［6-7］，of different air flow field on the spreading of contaminants［8-10］and so on.The research illustrates thatthe airflow field inside the cabin might cause low Reynolds and high turbulence.The way of airflow movement and contaminant transmission vary with different air supply boundary conditions and the geometry of the cabin.Jia-yu li，et al［11］research on the gasper jets interacting with themain ventilation in the aircraft cabin.However，the researchesabove primarily focused on the diffusion of pollution sourced inside the aircraft cabins，without considering the bleeding air contaminant，which is one of the significant cabin pollution sources.Recently，as the coronavirus epidemic outbreak around the world，how to control the bleeding air contaminants and create a better，safer cabin environment deservesmuch more attention.The ordinary way to reduce the concentration of bleed air pollutants via increasing the amount of air supply which is contrary to reducing the economic cost of the aircraft，so it’s unappreciated to reduce the bleed air contaminant by this way.Therefore，this article investigates the bleed air pollutants transmission and the air quality inside the aircraft cabin under two different air supply patterns.This actually has the guiding significance for both comprehensive contrastive analysis of the cabin air supplymodes and the energy saving of the aircraft.

The researchmethods are generally divided into experimentalmeasurements and simulation calculations.Experimentalmeasurements can simulate the working conditionsmore realistically and obtain more reliable results［12］.Although the cost of simulation calculations is low，the calculation results are not necessarily accurate without experimental verification.Therefore，in order to carry outmore accurate research，this paper conducts experimentalmeasurements in an aircraft cabinmock-up which contains five rows of seats.Sulfur hexafluoride（SF6）is used as a tracer gas，the two types of air supply，ceiling and combined air supply（ceiling+side wall），is operated respectively to investigate the distribution of bleed air pollutants and eventually provides a theoretical basis and reference for effective control of bleed air contaminantand air supply pattern contrastive analysisin the aircraft cabin.

1 Experimental set up

1.1Experimantal platform

Fig.1 shows a full-scale cabin mockup of a Boeing 737-200 aircraft contains five rows of seats.The interior of the cabinmockup is symmetrically arranged，the geometric dimensions are 4.8 meters×3.8 meters×2.1 meters（length×width×height），the aisle width is 0.5meter，and there are 3 seats on each side of the aisle per row.Thecabinmockup is entirely built in an air-conditioned laboratory，inside which the temperature can be maintained at a certain constantvalue by adjusting the air conditioner.There is an air-conditioning unit outside the cabin，which can adopt both the supply air temperature and volume for the cabin withthe ranges of 15～35℃and 450～5 000 m3/h respectively.In this study，the supply air temperature is stabilized at22±0.5℃ and the supply air volume is 1 020±5 m3/h，which is corresponding to 9.4 L/（person·s）according to the ASHRAE standard［13］.Both ceiling and combined air supply are adopted for research.

Fig.1 Five row cabin mockup

1.2 Measurement system

An infrared photoacoustic spectrum gas detector，model INNOVA 1412i，is used asmain equipment to measure the gas concentration during the experiment，cooperated with the collector，model INNOVA 1409.The facility is shown in Fig.2，and the measurement range is 0.01～20 000 mg/m3，the measurement accuracy is 10-4mg/m3.

Fig.2 In frared photoacoustic gas m onitor

1.3 Experimental arrangement

In the experiment，sulfur hexafluoride（SF6）was used as the tracer gas（the mixture of 1%SF6and 99%N2，hereinafter referred to as SF6gas），which is chemically stable，non-toxic and odorless，easy tomonitor，and the density of themixture gas is close that of air，which effectively avoids the influence of gas buoyancy，and has been used as a tracer gas in many experiments.The schematic of the experimental setup is shown in Fig.3.Open the gas source valve to release the tracer gas，adjust the opening degree of the flow valve tomake the gas source release at a constant flow rate，and the gas enters the cabin through the air intake duct of the air conditioning unit.The length of the duct is around 15meters，which allows the tracer gas and air fully mix in the duct.Taking the amount of contaminant in the real situation into account，the two states，the“instantaneous”and“continuous”bleed air contaminant were simulated in the experiment.In order to make the measured values reflect a significant difference，the release flow rate is taken as a larger value due to the short release time of“instantaneous”contaminant，which is continuously released at flow rate of10 L/min for 40 seconds.The“continuous”pollutant release time is longer and the flow value is smaller，is3 L/min，and the gas is released until the concentration value of themonitoring location in the cabin reach a stable value.Considering the symmetry ofboth cabin structure and the supply air，the measure pointswere setat the left side of the third row in the cabin（position D，E，and F in Fig.3）.The breathing zone and the corresponding height position in the aisle weremeasured.

Fig.3 Experim en t set up

2 Experim ental resu lt

2.1 Instantaneous contaminant

Fig.4 shows the concentration change forinstantaneous contaminant at fourmeasurementlocations under two air supply conditions.In Fig.4（a），at40thsecond the contaminant concentration at location F near the window is higher than others under ceiling air supply.This is primarily due to the initial stage of contaminant，the release amount was relatively limited and could not fill the entire cabin.When the ceiling air supply is taken，the airflow first reached the position close to the window and then reached the center position of the cabin.At the 80thsecond，the concentration of tracer gas at each measurement point reached their peak，the contaminant basically filled the whole cabin，and the peak value of concentrationat each point was slightly different.However，the difference in contaminant concentration at each point in the initial stage is much smallerunder mixed air supply，which indicates that the contaminant arrive each point is almost atthe same time undermixture air supply.

Fig.4 Distribu tion of instan taneous con tam inan t concentration

2.2 Continueous contaminant

As shown in Fig.5（a），the time during which thecontinuouscontaminantconcentration rises to the peak value under the ceiling air supply varieswith different positions.The time for concentration reaching the peak value at position D and E is shorter，and the concentration at position F is rising fastest，while the concentration at the aisle is rising slower than other positions.Under the ceiling air supply，the contami-nantis driven by the airflow and firstly reach the seatsnear the window and at themiddle position，and then reach the aisle position slower than other positions，which is consistentwith the trend of instantaneous contaminant transport.During the process of contaminant removal，the speed of removal at the middle seat is fastest，and the overall time forremoval is approximately the same for all positions.This ismainly because of the decreasing for contaminant concentration at the later stage of the removal，and the concentration exclusion rate declined aswell.

Fig.5 Distribution of continuous contam inant concentration

It can be seen in Fig.5（b）that the time for continuous contaminant to reach the peak value at each point is basically the same，which is maintained around 500thsecond，is about 100 secondshorter than that is under the ceiling air supply.There is no apparent difference among the times of contaminant removal，but differences of contaminant concentration peak value did exist among the measure points.The position F had the highest concentration peak as 3.95 mg/m3while the aisle position had the lowest concentration peak as 3.75 mg/m3.This shows that the bleed air contaminant can bemore easily to reach the position E and F near the cabin windowunder mixed air supply.The contaminant concentration atpositions E and F decrease most rapidly when the release of contaminant source is quit，whereas the concentration at position D near the aisle decreases the slowest.This shows that position E and F are greater affected by the airflow，a larger amount of contaminant will be more likely to reach the location if bleed air pollution occursunder themixed air supply.Finally，for the continuous contaminant removes to the concentration level of 0.5 mg/mt，it takes about 100 seconds shorter under mixed air supply than it’s under ceiling air supply，the forward one can eliminate contaminant with a higher speed.

3 Evaluation for air quality

The tracer gas can be used notonly as a representative of pollutants but also as a sign of fresh air to measure space ventilation effect during experimental measurements.Considering indicators such as air age and air exchangeratewere used to quantitatively evaluate the ventilation effect inside buildings or spaces.This paper proposes a reference indicator based on traditional ones，the time for the concentration of tracer gas rises to its peak value.The ventilation conditions in the aircraft cabin under two different air supply modes were evaluated，and the results are shown in Fig.6 below.

Fig.6 Eva Iuation index of air qua Iity

Fig.6（a）shows the time for tracer gas concentration reaches its peak value at each measurement point under the two air supplymodes，marked with different colors.Each point corresponding to the valuemarked on the vertical axis represents how long the time wastaken to make the concentration reach its peak at each corresponding location.And the value dashed line representsis the average of the time it takes for the tracer gas concentration to reach the peak value at each point under the two air supply modes.It can be seen in the figure that it takes a longer time for the tracer gas concentration to reach the peak value under the ceiling air supply mode，120 seconds longer for average time than that under the mixed air supply.Thatmeans the mixed air supply makes the fresh air reach the positionsmuch faster in the cabin.

It is worth noting that the time for tracer gas reaching concentration peakateach location is basically the sameunder the mixed supply air，whereas the differencesare larger under the ceiling supply air，the longest time ataisle position and the shortest time atmiddle position.This phenomenon may be related closely to airflow field characteristics inside the cabin.

Age of air is a common air quality evaluation index，which refers to the time after air enters the space，and it can quantitatively indicate how fast fresh air replaces the original air in the space.The smaller the air age value is，the faster replacement is，hence the better the ventilation effect.The local average air age at each measurement point in the cabin can be calculated by equation（1）［14］.

There inCp（t）is continuous record for contaminant concentration values atcollection points；Cp（0）is the initial concentration of contaminant at the collection point.

Air exchange rate，η，can be used to evaluate the limit value for ventilation，it is usually regarded plenty ventilation while the value of air age is smaller than it，which can be calculated by equation（2）［15］

There inVcabinas the volume inside the cabin；vventilationas the air quality of ventilation.

Fig.6（b）shows the air age value at each measurement point and it can be seen that the air age varies with different locations.It can be generally seen that the air age at each location is lower under the mixed air supply，which means that the air supply is easier to reach the various locations and the ventilation effect is better under the mixed air supply condition.The dashed line in the figure indicates the air exchange rate in the cabin.The air age at the two locations near the window under mixed air supply is lower than the air exchange rate，which indicates that the locations are adequately ventilated，and the air age in each location under the ceiling air supply is higher than the cabin air exchange rate and itmeans inadequate ventilation.Therefore，under the same air volume，the air quality of themixed supply air is better，which is consistentwith the evaluation resultof thetime for concentration reach peak.Mixed air supply can provide passengerswith amore comfortable and healthier air supply environment regardless of contaminant.

4 Conclusion

The concentration diffusion ofbleeding air contaminantinside the aircraft cabin under both the ceiling and mixed air supply modes were experimentally measured，and the air quality in the cabin was evaluated，the following conclusionswere reached：

（1）Different air supply modes directly affect the transmission of bleeding air contaminant and the ventilation effect in the cabin.

（2）The bleeding air contaminant will firstly reach the position close to thewindow under ceiling air supply condition whilereach the middle seat firstly in mixed supply air.

（3）Mixture air supply ismore likely to accelerate the transmission of contaminantwitha lower concentration but has a higher rate of contaminant removal.Better ventilation effect and air quality can be given by themixture air supplywhilethe ventilation volume is equal.