Heat Convective Effects on Turbulence and Airflow Inside An

fluids

Article Heat Convective Eﬀects on Turbulence and Airﬂow inside an B767 Aircraft Cabin

Maher Shehadi

School of Engineering Technology, Purdue University, West Lafayette, IN 47907, USA; [email protected]; Tel.: +1-765-455-9219 Received: 15 June 2019; Accepted: 5 September 2019; Published: 8 September 2019

Abstract: Thermal plumes generated by human bodies can affect the temperature and humidity of the surrounding environment. An experimental study investigated the effects of thermal plumes formed by aircraft passengers on airflow and turbulence characteristics inside aircraft-cabins. An 11-row, wide-body B767 cabin mockup was used with actual seats, air diffusers and cabin profile. Thermal manikins were used simulating passengers in the cabin. Tracer gas and air speed inside the cabin were measured while the heat from the manikins was turned on and off to help understand the effects of the thermal heat released by the manikins. Results showed that tracer gas distribution were more uniformly and equally distributed around the release source and the air speed fluctuation were lower under cooler environments when the thermal manikins were turned off. Heated environments increased the values of turbulence kinetic energy and the turbulence intensity levels. However, the effects on the turbulence intensity were less significant compared to the turbulence kinetic energy. On the other hand, the dissipation rates were higher for unheated cases in the front and back sections of the mockup cabin. The relative uncertainty for tracer gas sampling ranged between 5–14% for ± heated manikins versus 8–17% for unheated manikins. Higher uncertainty levels accompanied the ± turbulence measurements due to the highly chaotic nature of the flow inside the cabin.

Keywords: tracer gas; aircraft cabin environment; air quality; turbulence characteristic; turbulence kinetic energy; dissipation rate; thermal plume eﬀects

1. Introduction Several studies have been conducted to understand air quality, airflow characteristics and human thermal comfort inside aircraft cabins. Investigations used various experimental and analytical techniques such as particle image velocimetry (PIV), particles dispersion, computational fluid dynamics (CFD), and tracer gas. The effect of air nozzle sizes and direction on the airflow inside aircraft cabin was investigated in 2006 by Lebbin et al. [1] inside a generic room using stereoscopic PIV techniques. The generic room was described by Lin et al. [2]. Reynolds number was held constant at the inlet slot of the room with a value of approximately 2226. It was noted that the center of rotation of the overall airflow significantly changed with a change in the size of the air inlet slot size, whereas the turbulence levels in the room was not affected significantly since Reynolds number was not changed [1]. Large Eddy Simulation (LES) and Reynolds Averaged Navier–Stokes (RANS) methods were used by Ebrahimi et al. [3] to understand the airflow and turbulence characteristics inside an 11-row B767 cabin mockup. The simulation results were validated against available experimental data. The studies showed that LES with Werner–Wengle wall function could predict unsteady airflow velocity fields inside the cabin mockup with high accuracy. On the other hand, when air circulation was present, the RGN k-ε model with a non-equilibrium wall function model predicted the airflow velocities with good agreements against experimental results. In a later study by Ebrahimi et al. [4], tracer gas and

Fluids 2019, 4, 167; doi:10.3390/fluids4030167 www.mdpi.com/journal/fluids Fluids 2019, 4, 167 2 of 18 particle dispersion inside the same 11-row B767 cabin mockup was predicted. The initial airflow velocities exiting the supply nozzles were experimentally measured using omni-probes. Three different grid sizes were examined for grid uncertainty. The effect of gaspers on the airflow inside a Boeing 767 cabin was investigated using tracer gas. The tracer gas was released and sampled around the nasal area of a seated passenger a thermal manikin represented with a thermal manikin. The personal supply gaspers were found to have some effect on the contaminant plume inside the cabin impacting the local exposure around the release zone. It was also found that there was significant reduction in close-range, person-to-person exposure, while in other cases there was negligible or even negative impacts [5]. Local ventilation effectiveness inside an 11-row B767 cabin mockup and in a 5-row section of a salvaged B737 passenger cabin was investigated by [6] and [7], respectively. Tracer gas was used as the tracing agent during the experiments in both cabins. For the 11-row B767 cabin mockup, the tracer gas, which was mainly composed of CO2, was sampled in all seats. The overall ventilation rate was found approximately at 27 air changes per hour (ACH) based on total supply air flow. The ventilation effectiveness ranged from 0.86 to 1.02 with a mean value of 0.94. These ventilation effectiveness values were higher than what typically is found in other indoor environments. This gain in effectiveness is likely due to the relatively high airspeeds that can improve mixing rates. On the other hand, experiments inside the 5-row sectional B737 were carried with similar thermal manikins as was used in the B767 cabin mockup. Local ventilation effectiveness was found to be uniform throughout the B737 cabin regardless of the location inside the cabin. For gaseous transport, similar conclusions were found in both cabins, B767 and B737, where gaseous transport was significantly transported in the transverse and longitudinal directional of the cabins. Outbreaks of Severe Acute Respiratory Syndrome (SARS) and H1N1-swine flu can cause serious hazardous and threats to humans due to the large number of passengers using airplanes. In 2002, it was believed that a person infected with SARS led to 22 other passengers being infected during a flight from Hong Kong to Beijing. Furthermore, concerns of possible terrorist attacks onboard commercial flights have significantly risen due to the use of the nerve agent to attack the Tokyo subway in 1995 and the anthrax cases in Florida and Washington, D.C. in 2001. Consequently, it became considerably important to understand particulates dispersion behavior inside aircraft cabins, develop means for detecting undesirable components inside aircraft cabins and to find methods for preventing the aircraft from being used for intentional contaminant deployment [8]. The potential of contaminating aircraft cabins with oil and hydraulic fluids was recognized by the Committee on Aviation Toxicology (CAT) of the Aero Medical Association acknowledged shortly after the introduction of bleed air system [9]. The concentration of airborne contaminants inside aircraft cabins is expected to vary depending on aircraft type, specific airline maintenance practices, and bleed air source already too old [10]. Frequency of bleed air contamination incidents were estimated between 0.09 to 3.88 incidents per a thousand flight cycles, according to [11], which could mean 2–3 bleed air incidents per day. Another study by [8] indicated that there might be 0.2–0.8 incidents per thousand flights according to incidents reported by the US Federal Aviation Administration (FAA), Department of Transportation, NASA and flight attendant association databases [12]. Thus, it is important to understand the airflow characteristics inside aircraft cabins to help in preventing and minimizing bacterial and viral spreads or intentional nerve attacks onboard air flights. Previous studies have investigated bacterial, particulate and gaseous dispersion inside passenger aircraft flights under normal operating conditions mostly assuming fully occupied cabins. However, since airplanes are not always fully occupied and since heat released by the passengers can affect the indoor airflow and air quality as they generate thermal plumes, it was important to investigate the effect of such variables on the airflow characteristics inside the cabin. Thermal plumes can significantly influence indoor airflow distribution as well as indoor air quality. Aircraft passenger cabins are occupied by humans whose body released heat can rise both temperature and humidity. This rise can affect the velocity and the airflow distribution inside aircraft cabins. A change in the number of passengers, such as in a partially full flight, can affect the convective Fluids 2019, 4, 167 3 of 18 transport mechanism inside the aircraft cabin. For these reasons, this study analyzed the effects of heat generated by passengers on the airflow and turbulence characteristics inside a B767 aircraft cabin mockup using thermally heated manikins. The study used tracer gas, mainly composed from carbon dioxide, to track the airflow inside the cabin. Air speed transducers were used to measure the speed at various locations inside the cabin. Tracer gas and air speed were collected when the cabin mockup was assumed to be fully equipped or empty by running electric current in thermal wires wrapped around the thermal manikins on and off for each case. The study will provide experimental data for researchers to better understand the airflow characteristic inside aircraft cabins as well as airline manufacturers who might have to modify their designs. The study also provides some data necessary for computational fluid dynamics work to help in developing “user-defined functions” (UDF) and to validate some simulations on the same aircraft model and type.

2. Background To analyze turbulence characteristics of a fluid, the k–ε model is one of the most frequently used models utilizing two diffusive equations. The first one modeling the turbulent kinetic energy “k” and the second one modeling the dissipate rate “ε” as shown in Equations (1) and (2), respectively: ! ∂ ∂ (k) ∂k (k) (ρk) + ρkui Γ = ρ P ε , (1) ∂t ∂xi − ∂xi − ! ∂ ∂ (ε) ∂ε (k) ε (ρε) + ρεui Γ = ρ Cε1P εCε2 , (2) ∂t ∂xi − ∂xi − k

Rate of Change Advection Diﬀusion Source

µT µT where Γ(k) = µ + ; Γ(ε) = µ + . µT is the turbulence dynamic viscosity, P(k) is the turbulence σk σε production rate, ui is the velocity vector, and ρ is the density. Values for the different empirical constants were proposed by Launder and Spalding in 1974 [13] and were given as Cε1 = 1.44; Cε2 = 1.92; σ = 1.3; T 2 (k) ∂u µ ∂u ∂ui σk = 1; P = u0v0 = or u0iu0 j , where u0 and v0 are the fluctuating components of − ∂y ρ ∂y − ∂xj the velocities in two-dimensional flow. These values were based on extensive examination of free turbulent flows, but they cannot be used for wall flows. Thus, this model is applicable only to flows or flow regions with high turbulence rates and cannot be applied near walls, where viscous effects become dominant. For such situations, a near wall function approach is usually used. Assuming that the turbulence was isotropic, the turbulence kinetic energy “k” in its tensor format in Equation (3) reduces to Equation (4), where v’ is the isotropic speed fluctuation part:

1 k = u u , (3) 2 i i which reduces to 3 2 k = (v0) (4) 2 Kolmogorov formulated the hypothesis of local isotropy based on deﬁnitions of local homogeneity and isotropy. This hypothesis postulates that at large Reynolds numbers all the symmetries of the Navier–Stokes equations are restored in the statistical sense [14]. The local dissipation rate as a function of the local strain created by the ﬂow is then given in Equation (5) [14]: Fluids 2019, 4, 167 4 of 18

ε = γSijSij, (5) where Sij is the strain deﬁned in Equation (6). Thus, the local dissipation rate “ε” becomes according to Taylor dissipation rate for isotropic ﬂow as given in Equation (7), where v is the isotropic local speed and A is an empirical constant (approximately = 15) [14]: ! 1 ∂Ui ∂Uj Sij = + , (6) 2 ∂xj ∂xi

!2 ∂v ε = Aγ . (7) ∂x

3. Experimental Setup

3.1. Testing Facility A full-scale aircraft mockup with dimensions of 9.6 m in length and 4.7 m in width was used to conduct the testing. The aircraft cabin mockup was one of the largest available research mockup cabins in its class and it modelled a B767, 11-row aircraft cabin. Each row consisted of seven seats in the transverse direction. The seats, air supply ducts, and diffusers were all utilized from salvaged B767 aircrafts. To simulate heat gains by a seated passenger, 10 m wire heater elements were wrapped and distributed around manikins that were seated in each seat inside the cabin mockup. The heat gain by each manikin was approximately 100 Watts similar to that released by a seated human [15]. The heat for each manikin could be turned on or off by cutting the electrical supply to each manikin. Fresh air was conditioned (cooled or heated) and filtered inside a set of HEPA (high efficiency particulate air) filters before was supplied to the cabin. The diffusers supplying air to the cabin were linear and had a decreasing supply duct upstream that balanced the air pressure across the diffusers. The exterior and interior of the cabin mockup along with the cooling and heating loops are shown in Figure1. The air supplied into the cabin was according to [16] standards supplying 8.57 L/s (18 ft3/min or CFM) per passenger seat. This made the total supplied air to the cabin 660 L/s (1400 CFM). The mockup cabin was supplied with 100% outside air, conditioned to 15.6 ◦C (60 F) at the upstream of the cabin main supply duct shown in Figure1a (Air Inlet Supply Duct point). The outdoor air temperature was conditioned to the desired set point using an electric hear and water-glycol loops. The temperature at the main cabin supply duct was measured using an Omega 3-wire RTD model PR-10-2-100-1/4-6-E sensor (Norwalk, CT, USA). The humidity of the air was treated in a Munter dehumidifier (Amesbury, MA, USA). The air temperature was conditioned to 15.6 ◦C (60 ◦F) at the duct entering the cabin, as shown in Figure1a. The temperature inside the cabin was monitored, with the manikin heat on, using twenty-four thermocouples along the cabin interior surfaces, distributed evenly between the left and right side, and along the vertical height using 12 more thermocouples. The average temperature of all thermocouples ranged between 19–24 ◦C. The temperature ranges came into agreement with O’Donnell et al. [17] who reported that temperature onboard passenger aircraft cabins ranges between 19–23 ◦C. Before conducting any testing, it was necessary to ensure the uniformity of the air exiting the two linear diffusers shown in Figure1c. The air exited the cabin through two exit openings along the bottom side of the walls on each side of the cabin mockup. Fluids 2019, 4, 167 5 of 18

Fluids 2019, 4, x 5 of 18

FigureFigure 1.1.Cabin Cabin mockup mockup (a )(a exterior;) exterior; (b) heating(b) heating and coolingand cooling loops; loops; and (c )and interior (c) interior of the cabin of the showing cabin theshowing thermally the thermally heated manikins. heated manikins. 3.2. Tracer Gas Testing Setup and Methodology 3.2. Tracer Gas Testing Setup and Methodology Tracer gas mixture made up of 62.4% CO2 and 37.6% helium was used as the tracer gas agent in the Tracer gas mixture made up of 62.4% CO2 and 37.6% helium was used as the tracer gas agent in experiments. The tracer gas was released in different seats inside the cabin though a 25.4 mm diameter the experiments. The tracer gas was released in different seats inside the cabin though a 25.4 mm copper tube. The mass flow of each of the two gases was controlled using two mass flow controllers diameter copper tube. The mass flow of each of the two gases was controlled using two mass flow (model MKS 1559A-200L-SV-S for CO2 and MKS 2179A-00114-CS-18V for He). The flow controllers controllers (model MKS 1559A-200L-SV-S for CO2 and MKS 2179A-00114-CS-18V for He). The flow were controlled by an Agilent DAQ system (model 34970A) (Santa Clara, CA, USA), MKS PR4000 controllers were controlled by an Agilent DAQ system (model 34970A) (Santa Clara, CA, USA), MKS power supply and RS-232 interface unit. The tracer gas was sampled using PP-Systems CO2-analyzers PR4000 power supply and RS-232 interface unit. The tracer gas was sampled using PP-Systems CO2- (Amesbury, MA, USA). The analyzers contained non-dispersive infrared sensors. To increase the analyzers (Amesbury, MA, USA). The analyzers contained non-dispersive infrared sensors. To simultaneous collection frequency during each test, a collection tree was connected to the inlet of the increase the simultaneous collection frequency during each test, a collection tree was connected to tracer gas analyzer. The tree had four ports and were distanced from each other so each port would be the inlet of the tracer gas analyzer. The tree had four ports and were distanced from each other so in the same seat in consecutive rows. This enabled all four ports of the sampling tree to be utilized in each port would be in the same seat in consecutive rows. This enabled all four ports of the sampling four separate seat locations (in consecutive rows) during a single experiment. LabVIEW (LabView6.1) tree to be utilized in four separate seat locations (in consecutive rows) during a single experiment. was used to operate the Agilent DAQ, the interface unit and the mass flow controllers. It was designed LabVIEW (LabView6.1) was used to operate the Agilent DAQ, the interface unit and the mass flow to allow sequential collection from the first port until the fourth one. The tracer gas release tube and controllers. It was designed to allow sequential collection from the first port until the fourth one. The the collection tree are shown in Figure2. The tip of the collection ports was fixed at a height of 1.23 m tracer gas release tube and the collection tree are shown in Figure 2. The tip of the collection ports from the cabin floor. Tracer gas was released at very low speeds (approximately 0.53 0.02 m/s) to was fixed at a height of 1.23 m from the cabin floor. Tracer gas was released at very± low speeds minimize the disturbance to the airflow inside the cabin caused by the gas injection. To normalize the (approximately 0.53 ± 0.02 m/s) to minimize the disturbance to the airflow inside the cabin caused by tracer gas sampled inside the cabin, two more CO2-analyzers were used. One was installed to sample the gas injection. To normalize the tracer gas sampled inside the cabin, two more CO2-analyzers were used. One was installed to sample the inlet CO2 at with the air coming through supply duct and Fluids 2019, 4, 167 6 of 18

Fluids 2019, 4, x 6 of 18 the inlet CO at with the air coming through supply duct and another one analyzing the tracer gas another one 2analyzing the tracer gas leaving the cabin through the exhaust fans that are shown in leaving the cabin through the exhaust fans that are shown in Figure1a. Figure 1a. Previous smoke visualization testing inside the cabin was done by [10] to decide on the best Previous smoke visualization testing inside the cabin was done by [10] to decide on the best locations where tracer gas needed to be released. The airflow inside the cabin was shown to be very locations where tracer gas needed to be released. The airflow inside the cabin was shown to be very chaotic and thus the testing and visualization scenarios were repeated multiple times to decide on chaotic and thus the testing and visualization scenarios were repeated multiple times to decide on best local flow patterns. The smoke tracks along with the used tracer gas release locations are shown best local flow patterns. The smoke tracks along with the used tracer gas release locations are shown in Figure3. The tracer gas was generally collected in the same row of release, three rows to the back in Figure 3. The tracer gas was generally collected in the same row of release, three rows to the back and three rows to the front of the release row. The decision to cover three rows in each direction and three rows to the front of the release row. The decision to cover three rows in each direction away away from the release row was based on results from [18] who concluded that particles and tracer from the release row was based on results from [18] who concluded that particles and tracer gas gas dispersion are significantly reduced beyond 3-rows. The released particles or tracer gas would be dispersion are significantly reduced beyond 3-rows. The released particles or tracer gas would be present beyond the 3-row limit, but in very small concentrations. In this study, tracer gas was injected present beyond the 3-row limit, but in very small concentrations. In this study, tracer gas was injected continuously during each test for a total duration of 20-min. Samples were collected in each port of continuously during each test for a total duration of 20-min. Samples were collected in each port of the sampling tree every 5 s resulting in 240 samples for each port or seat. Each 20-min sampling test the sampling tree every 5 s resulting in 240 samples for each port or seat. Each 20-min sampling test was repeated in each seat three times for statistical consistency. With the 8-release locations were was repeated in each seat three times for statistical consistency. With the 8-release locations were distributed around the cabin and with sampling done three to four rows to the front and three to distributed around the cabin and with sampling done three to four rows to the front and three to four four rows to the back, in mostly every seat in the transverse direction of each row, the data collected rows to the back, in mostly every seat in the transverse direction of each row, the data collected provided strong indication for the tendency of the tracer gas movement inside the cabin. provided strong indication for the tendency of the tracer gas movement inside the cabin. To ensure steady state conditions inside the cabin prior to any testing, it was estimated that 5 min To ensure steady state conditions inside the cabin prior to any testing, it was estimated that 5 were enough to exhaust any available CO2 from the background of the cabin remaining previous tests, min were enough to exhaust any available CO2 from the background of the cabin remaining previous from any person entering into the cabin to modify the sampling tree or entering through the cabin tests, from any person entering into the cabin to modify the sampling tree or entering through the doors. Thus, after 5 min, the tracer gas was released sequentially in each of the eight seats shown in cabin doors. Thus, after 5 min, the tracer gas was released sequentially in each of the eight seats Figure3. shown in Figure 3. To investigate the effects of heat generated by each manikin inside the cabin, the same tests were To investigate the effects of heat generated by each manikin inside the cabin, the same tests were run but with no heat released by the manikins by cutting the electrical source into them. run but with no heat released by the manikins by cutting the electrical source into them.

(a)

Figure 2. Cont. Fluids 2019, 4, 167 7 of 18

Fluids 2019, 4, x 7 of 18 Fluids 2019, 4, x 7 of 18

(b) (b) Figure 2. ((aa)) tracer tracer gas gas release release tube; tube; ( (b)) tracer tracer gas gas sampling sampling tree. Figure 2. (a) tracer gas release tube; (b) tracer gas sampling tree.

Figure 3. Airflow prediction based on smoke visualization testing [13] and tracer gas release locations used. Figure 3. Airflow prediction based on smoke visualization testing [13] and tracer gas release locations Figure 3. Airflow prediction based on smoke visualization testing [13] and tracer gas release locations used. 3.3. Turbulenceused. Characteristics Investigation Setup 3.3. TurbulenceA spherical Characteristics omni-directional Investigation TSI velocity Setup transducer (8475 series model) was used inside the cabin to3.3. measure Turbulence and Characteristics record the speed Investigation of the air. Setup This would help in evaluating the turbulence intensity A spherical omni-directional TSI velocity transducer (8475 series model) was used inside the (TI) andA spherical turbulence omni-directional kinetic energy (k). TSI The velocity speed transducer transducer (8475 had a 5series s response model) time was and used its averagedinside the cabin to measure and record the speed of the air. This would help in evaluating the turbulence uncertaintycabin to measure was estimated and record to be the1%. speed The sphericalof the air. transducer This would was help fixed in at evaluating a height of the 1.23 turbulence m above intensity (TI) and turbulence kinetic± energy (k). The speed transducer had a 5 s response time and its theintensity cabinfloor (TI) and in each turbulence seat across kinetic the cabinenergy similar (k). Th toe wherespeed transducer the tracer gas had was a 5 sampleds response in. time The and probe its averaged uncertainty was estimated to be ±1%. The spherical transducer was fixed at a height of 1.23 isaveraged shown in uncertainty Figure4. Similar was estimated to the tracer to be gas ±1%. testing, The spherical a 5-min waitingtransducer period was wasfixed used at a priorheight to of any 1.23 m above the cabin floor in each seat across the cabin similar to where the tracer gas was sampled in. datam above collection. the cabin The floor measured in each speed seat wasacross used the to cabi estimaten similar the turbulenceto where the intensity tracer gas and was the sampled turbulence in. The probe is shown in Figure 4. Similar to the tracer gas testing, a 5-min waiting period was used kineticThe probe energy. is shown Speed testsin Figure were done4. Similar with heatedto the tracer and unheated gas testing, manikins a 5-min to checkwaiting on period the effects was of used the prior to any data collection. The measured speed was used to estimate the turbulence intensity and thermalprior to plumes any data caused collection. by the The heated measured manikins speed on thewas turbulence used to estimate characteristics the turbulence inside theintensity cabin. and the turbulence kinetic energy. Speed tests were done with heated and unheated manikins to check the turbulence kinetic energy. Speed tests were done with heated and unheated manikins to check on the effects of the thermal plumes caused by the heated manikins on the turbulence characteristics on the effects of the thermal plumes caused by the heated manikins on the turbulence characteristics inside the cabin. inside the cabin. Fluids 2019, 4, 167 8 of 18

Fluids Assuming2019, 4, x isotropic ﬂow, the TI and k were calculated as shown in Equations (4) and8 of (8), 18 respectively, where v0 is the ﬂuctuating part of the speed and V is the average speed: Assuming isotropic flow, the TI and k were calculated as shown in Equations (4) and (8), q respectively, where 𝑣′ is the fluctuating part of the speed2 and 𝑉 is the average speed: v0 TI = . (8) 𝑇𝐼 = V . (8)

Figure 4. 8475 series TSI speed transducer. Figure 4. 8475 series TSI speed transducer.

The heat dissipation rate was numerically evaluated by discretizing Equation (7) and by implementing three three omni-transducers, omni-transducers, as shown as shown in Equation in Equation (9). The (9). middle The transducer middle transducer was represented was asrepresented “i” and those as “ ini” frontand those and backin front were and represented back were as represented “i + 1” and as “i “i 1”,+ 1” respectively: and “i − 1”, respectively: − 𝜀 = 15𝛾 2. (9) vi+1 ∆vi 1 ε = 15γ − − . (9) 2∆x The separation distance “∆𝑥” was the distance between two consecutive transducers and 𝛾 was the kinematic viscosity of the air inside the cabin. The optimal separation distance ∆𝑥 was investigated to maintain minimal numerical errors. Five separation distances were considered between 5–25 cm. It was found that a separation distance of 12.7 cm was the best option providing the least numerical errors. Fluids 2019, 4, 167 9 of 18

The separation distance “∆x” was the distance between two consecutive transducers and γ was the kinematic viscosity of the air inside the cabin. The optimal separation distance ∆x was investigated to maintain minimal numerical errors. Five separation distances were considered between 5–25 cm. It was found that a separation distance of 12.7 cm was the best option providing the least numerical errors.

4. Results and Discussion

FluidsTracer 2019 gas, 4, x testing and speed measurements were done and repeated while having the9 electricalof 18 source supplying the thermal wires around the manikins turned on and off. With heated manikins, 4. Results and Discussion the temperature inside the cabin ranged between 19–23 ◦C, whereas, with unheated manikins, this air temperatureTracer dropped gas testing to 14–15 and speed◦C. The measurements sampled CO were2 through done and the repeated analyzers while in the having cabin the was electrical normalized source supplying the thermal wires around the manikins turned on and off. With heated manikins, against the inlet and exit CO2 concentrations. This normalization is shown in Equation (10), where C is the temperature inside the cabin ranged between 19–23 °C, whereas, with unheated manikins, this the sampled concentration of CO2, and the subscript n stands for normalized value: air temperature dropped to 14–15 °C. The sampled CO2 through the analyzers in the cabin was normalized against the inlet and exit CO2 concentrations.C C This normalization is shown in Equation C = cabin inlet (10), where C is the sampled concentrationn of CO2, and− the subscript. n stands for normalized value: (10) Cexit Cinlet − 𝐶 = . (10) Results for the normalized tracer gas and turbulence characteristics inside the B767 cabin mockup were publishedResults withfor the more normalized analysis andtracer details gas and in [19turb].ulence This paper characteristics focuses on inside similar the resultsB767 cabin but when no heatmockup was dissipatedwere published from with the more manikins analysis and and compares details in the [19]. results This paper to the focuses heated on cases. similar Any results time the termbut “heated” when no was heat used, was it dissipated meant that from the manikinsthe manikins were and heated compares or the the thermal results wiresto the wereheated switched-on cases. and similarlyAny time the when term “unheated” “heated” was term used, was it meant used thenthat the that manikins meant thewere thermal heated wiresor the thermal were switched wires off and cuttingwere switched-on any heat dissipationand similarly by when the manikins.“unheated” term was used then that meant the thermal wires were switched off and cutting any heat dissipation by the manikins. The heated cabin testing resulted in multiple circulations inside the cabin [19]. The 2D circulations The heated cabin testing resulted in multiple circulations inside the cabin [19]. The 2D at a height of 1.23 m from the cabin floor are shown in Figure5. circulations at a height of 1.23 m from the cabin floor are shown in Figure 5.

FigureFigure 5. 5.Results Results for for airﬂow airflow in the cabin cabin with with heated heated manikins manikins [19]. [ 19].

NormalizedNormalized tracer tracer gas gas distribution distribution results results for he heatedated and and unheated unheated cases cases when when releasing releasing in seats in seats 2D and2D 7Dand are 7D presented are presented in Figure in Figures6a,b and6a,b Figureand 7a,b),7a,b), respectively. respectively. AllAll individual individual test testresults results for the for the heatedheated manikins manikins were were published published in in [19 [19].]. ForFor either case, case, heated heated or or unheated, unheated, the theresults results of more of more than than one releaseone release location location were were combined combined together together toto conclude the the behavior behavior of ofthe the flow flow in specific in specific sections sections insideinside the cabinthe cabin mockup. mockup. For For heated heated cases, cases, basedbased on on results results for for release release in seats in seats 2D (Figure 2D (Figure 6a) and6a) 5B, and 5B, a major drift in the front section of the cabin from east to west side was observed. On the other side, a major drift in the front section of the cabin from east to west side was observed. On the other side, tracer gas analysis for release in seat 5D and seat 4F showed a major drift in the front-west side tracertowards gas analysis the east for side. release Moving in seat further 5D and to the seat back 4F showed section aof major the cabin drift and in theconsidering front-west results side towardsfor the eastrelease side. in seat Moving 7D (Figure further 7a), to another the back drift section from th ofe west the to cabin the east and side considering of the cabin results was concluded. for release in seat 7DThe (Figure final combined7a), another analysis drift of from the results the west was todiscussed the east in side detail of thein [19] cabin and was showed concluded. the behavior The final combinedshown analysisin Figure of 5. the The results smoke was visualization discussed resu in detaillts shown in [ 19in] Figure and showed 3 (red thelines), behavior which were shown in conducted with heated manikins, and the tracer gas results shown in Figure 5 both concluded the existence of multiple circulations inside the cabin mockup. Based on tracer gas results and analysis, the length of these circulations was shown to depend on the integral length scale of the cabin in each region. With a cabin length of 9.6 m, width 4.7 m and height of approximately 2 m, isotropic Fluids 2019, 4, 167 10 of 18

Figure5. The smoke visualization results shown in Figure3 (red lines), which were conducted with heated manikins, and the tracer gas results shown in Figure5 both concluded the existence of multiple circulations inside the cabin mockup. Based on tracer gas results and analysis, the length of these circulations was shown to depend on the integral length scale of the cabin in each region. With a cabin length of 9.6 m, width 4.7 m and height of approximately 2 m, isotropic circulations with dimensions Fluids 2019, 4, x 10 of 18 between 2 to 4 m were concluded. The dimensions of the circulations at a height of 1.23 m above the ﬂoorcirculations are shown with in Figure dimensions5. between 2 to 4 m were concluded. The dimensions of the circulations at Thea height unheated of 1.23 tracerm above gas the results, floor are which shown were in conductedFigure 5. with temperatures 6 ◦C less than the heated cases, showedThe unheated more uniformly tracer gas and results, symmetrically which were distributedconducted with tracer temperatures gas around 6 °C the less release than location,the especiallyheated when cases, itshowed was released more uniformly in the centerlineand symmetrically seats of distributed the cabin tracer as shown gas around in Figures the release6b and 7b. In alllocation, of the release especially cases, when the it normalized was released tracer in the gascenterline samples seats did of notthe favorcabin as any shown side in of Figures the cabin 6b over the otherand 7b. as wasIn all contrarily of the release observed cases, the with normalized heated cases tracer such gas assamples in Figures did not6a andfavor7a. any side of the cabin over the other as was contrarily observed with heated cases such as in Figures 6a and 7a.

(a)

(b)

Figure 6. (a) normalized CO2 results for release in seat 2D (1 being at the front side of the cabin)— Figure 6. (a) normalized CO2 results for release in seat 2D (1 being at the front side of the cabin)—Heated Heated case; (b) normalized CO2 results for release in seat 2D (1 being at the front side of the cabin)— case; (b) normalized CO2 results for release in seat 2D (1 being at the front side of the cabin)—Unheated case. Unheated case. Fluids 2019, 4, 167 11 of 18 Fluids 2019, 4, x 11 of 18

(a)

(b)

FigureFigure 7. (a 7.) normalized(a) normalized CO CO2 results2 results for for release release in in seat 7D—Heated7D—Heated case; case; (b )(b normalized) normalized CO 2COresults2 results for releasefor release in seat in seat 7D—Unheated 7D—Unheated case. case.

All of the above analysis checked the dispersion at a height of 1.23 m; it was important to check All of the above analysis checked the dispersion at a height of 1.23 m; it was important to check any significant differences at different heights inside the cabin. Tracer gas vertical exposure was done any significantin the east and differences west aisles at different in the front heights and middle inside sections the cabin. of the Tracer cabin gas mockup. vertical For exposure the front was part, done in thethe east tracer and gas west was aisles released in the in row front 2 andand collected middle sections in the same of the row cabin but in mockup. the east andFor westthe front aisles, part, the tracerwhereas, gas for was the released middle part, in row the tracer2 and gas collected was released in the in same row5 row and collectedbut in the in east theeast and and west west aisles, whereas,aisles for of rowsthe middle 4 and 6. part, The resultsthe tracer for bothgas heatedwas rele andased unheated in row results5 and arecollected shown in in the Table east1. Moreand west aislesuniform of rows results 4 and or 6. smaller The results variability for wereboth seenheated with and unheated unheated than results with heated are shown environments, in Table except 1. More uniformin the results lower regionsor smaller near thevariability floor. The were non-uniform seen with distribution unheated in than the lower with sectionsheated ofenvironments, the cabin exceptnear in the the floor lower was regions expected near due the to manyfloor. disturbancesThe non-uniform such as distribution seats and manikins. in the lower sections of the cabin near the floor was expected due to many disturbances such as seats and manikins.

Fluids 2019, 4, 167 12 of 18

Table 1. Vertical normalized tracer gas comparison with heated and unheated cases. Fluids 2019, 4, x 12 of 18 Location East Aisle West Aisle Table 1. Vertical normalized tracer gas comparison with heated and unheated cases. Above Floor Heated Unheated Heated Unheated Location East Aisle West Aisle 1792 mm 16.5% 41% 51% 34% Above Floor Heated Unheated Heated Unheated Release in 1344 mm 17% 36% 70.0% 35.5% Row 2 Seat 2D 1792896 mm 16.5% 16% 41% 30%51% 91%34% 52% 1344448 mm 17% 23% 36% 39% 70.0% 99% 35.5% 74% 896 mm 16% 30% 91% 52% Row 2 Seat 2D Seat 2D

Release in Release 1792448 mm mm 23% 29% 39% 20%99% 13%74% 19% 1344 mm 33.5% 26% 16% 19% Row 4 1792 mm 29% 20% 13% 19% 896 mm 32% 36% 16% 19% 1344 mm 33.5% 26% 16% 19% Release in 448 mm 37% 46% 20% 29% 896 mm 32% 36% 16% 19% Seat 5D Row 4 1792448 mm mm 37% 22% 46% 24%20% 26%29% 26% 1344 mm 21.4% 30% 31% 33% Row 6 1792 mm 22% 24% 26% 26% 1344896 mm 21.4% 23% 30% 35%31% 33%33% 30% 448 mm 26% 40% 41% 29%

Release in Seat 5D in Seat Release 896 mm 23% 35% 33% 30% Row 6 448 mm 26% 40% 41% 29% Turbulence Characteristics Results and Analysis 4.1. Turbulence Characteristics Results and Analysis Sample results for the airflow speed for both cases, heated and unheated, are presented in Figure8Samplea–c. The results local for speeds the airflow in the speed east, for center both andcases, west heated sides and are unheated, also presented are presented along in with Figure 95% confidence8a–c. The intervals local speeds in Figure in the9 east,. With center heated and manikins, west sides theare middlealso presented rows in along the east with side 95% had confidence relatively intervals in Figure 9. With heated manikins, the middle rows in the east side had relatively higher air higher air speeds than in the front rows, whereas, in the west side, the air experienced higher speeds speeds than in the front rows, whereas, in the west side, the air experienced higher speeds in the back in the back section of the cabin. These observations were changed with unheated manikins, where section of the cabin. These observations were changed with unheated manikins, where the speed was the speed was more uniform across the cabin except near the front and back walls. Thus, the heated more uniform across the cabin except near the front and back walls. Thus, the heated environment environment showed higher fluctuations and higher speeds, which was shown in Figure8a–c and in showed higher fluctuations and higher speeds, which was shown in Figure 8a–c and in most seats in mostFigure seats 9. inThis Figure was9 also. This reflected was also on reflected the values on of the the values turbulence of the kinetic turbulence energy kinetic “k” and energy turbulence “k” and turbulenceintensities intensities “TI” at the “TI” different at the locations different inside locations the cabin. inside The the average cabin. values The average based on values Equations based (4) on Equationsand (8) for (4) k and and (8)TI, forrespectively, k and TI, respectively,are shown in Figure are shown 10 for in k Figureand in 10Figure for k11 and for inTI. Figure The values 11 for in TI. TheFigures values 9 inthrough Figures 119– also11 also included included 95% 95%confidence confidence intervals. intervals.

FigureFigure 8. 8.Comparison Comparison of of thethe ﬂuctuatingfluctuating speed in seat ( (aa)) 9D; 9D; ( (bb) )7D; 7D; and and (c ()c )6D 6D with with heated heated and and unheatedunheated manikins. manikins. Fluids 2019, 4, 167 13 of 18

Fluids 2019,, 4,, xx 13 of 18

FigureFigure 9. 9.Comparison Comparison of of the the speed speed with with heatedheated andand unheatedunheated manikins manikins in in east, east, centerline, centerline, and and west west sideside seats. seats.

Figure 10. Turbulence kinetic energy comparison for heated and unheated cases in the east, center FigureFigure 10. 10.Turbulence Turbulence kinetic kinetic energy energy comparison comparison for for heated heated and and unheated unheated cases cases in in the the east, east, center center and and west sides of the cabin. westand sides west of sides the cabin.of the cabin. Fluids 2019, 4, 167 14 of 18 Fluids 2019, 4, x 14 of 18

Figure 11.11. TurbulenceTurbulence intensity intensity comparisons comparisons for for heated heated and and unheated unheated cases cases in the in east,the east, center center and westand sideswest sides of the of cabin. the cabin.

The relative change between heated and unheated results for the turbulence kinetic energy and turbulence intensityintensity were were evaluated evaluated using using Equation Equa (11)tion and (11) plotted and inplotted Figures in 12 Figures and 13, respectively:12 and 13, respectively: Heated Unheated Relative_Change = . (11) 𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒_𝐶ℎ𝑎𝑛𝑔𝑒 = − . Heated (11) The relative change would serveserve asas a quick indicatorindicator toto which casecase had higher values.values. A positive value means that the the heated heated results results were were higher higher than than the the unheated unheated ones ones and and vice vice versa. versa. Out Out of 33 of 33comparison comparison locations locations in inthe the east, east, center center and and west west sides sides of of the the cabin, cabin, the the heated heated cases showed a majority ofof higherhigher valuesvalues (27 (27 points points higher higher in in both both k andk and TI). TI). There There were were two two negative negative peaks peaks for kfor and k fourand four negative negative peaks peaks in TI in comparisons TI comparisons that had that relative had relative change change lower thanlower 0.5. than Relative 0.5. Relative change change values forvalues k showed for k showed that 22 seatsthat (66%22 seats of the(66% seats) of the had seats) values had between values 0.5–1. between This 0.5–1. indicated This theindicated significant the esignificantffect of thermal effect plumesof thermal or the plumes heat generated or the heat by thegenerated manikins by onthe the manikins kinetic energyon the behaviorkinetic energy inside thebehavior cabin. inside On the the other cabin. side, On forthe turbulenceother side, for intensity, turbulence most intensity, of the values most wereof the between values were 0–0.5, between which indicated0–0.5, which warmer indicated environments warmer environments increased the increase turbulenced the intensityturbulence levels, intensity but thelevels, effect but was the noteffect as muchwas not as as for much turbulent as for kinetic turbulent energies. kinetic In energies. general, theIn general, TI in the the east TI andin the west east sides and ofwest the sides cabin of were the lesscabin sensitive were less to sensitive heat than to the heat turbulence than the kineticturbulence energies kinetic were. energies were. Fluids 2019, 4, 167 15 of 18

Fluids 2019, 4, x 15 of 18 Fluids 2019, 4, x 15 of 18

Fluids 2019, 4, x East T.K.E. change West T.K.E. change Center T.K.E. change 15 of 18 East T.K.E. change West T.K.E. change Center T.K.E. change 1.5 1.5 East T.K.E. change West T.K.E. change Center T.K.E. change 1.0 1.01.5 0.5 0.51.0 0.0 0.0 01234567891011

TKE "K" 0.5 01234567891011 TKE "K" -0.5 -0.50.0 -1.0 01234567891011 TKE "K"

RELATIVE CHANGE IN CHANGE RELATIVE -1.0-0.5 RELATIVE CHANGE IN CHANGE RELATIVE -1.5 -1.5 -1.0 LONGITUDINAL ROW INDEX #

RELATIVE CHANGE IN CHANGE RELATIVE LONGITUDINAL ROW INDEX #

-1.5 Figure 12. Relative change in k between heated and unheated manikins. FigureFigure 12. 12.Relative Relative change changeLONGITUDINAL in in kk betweenbetween heated heatedROW INDEXand and unhe unheated # ated manikins. manikins.

FigureEast 12. T.I. Relative change change in k Centerbetween T.I. heated change and unheatedWest manikins. T.I. change 1.5 East T.I. change Center T.I. change West T.I. change 1.5 1.0 East T.I. change Center T.I. change West T.I. change 1.01.5 0.5 0.51.0 0.0 0.00.5 01234567891011 01234567891011 -0.5 -0.50.0 -1.0 01234567891011 -1.0-0.5

RELATIVE CHANGE IN "TI" -1.5 RELATIVE CHANGE IN "TI" -1.5-1.0 LONGITUDINAL ROW INDEX # LONGITUDINAL ROW INDEX #

RELATIVE CHANGE IN "TI" -1.5 Figure 13. Relative change in TI between heated and unheated manikins. Figure 13. Relative changeLONGITUDINAL in TI between ROW heated INDEX and #unheated manikins. Figure 13. Relative change in TI between heated and unheated manikins. The change in k is related to the heat dissipated in each location. The discretized heat dissipation The change inFigure k is related 13. Relative to the change heat dissipatedin TI between in heatedeach location. and unhe Theated discretized manikins. heat dissipation Thefor heated change and in unheated k is related cabins to the were heat evaluated dissipated using in eachthree location.transducers The separated discretized by aheat distance dissipation of for heated and unheated cabins were evaluated using three transducers separated by a distance of for heated12.7 cm and and unheated utilizing cabinsEquation were (9). evaluatedThe relative using change three in transducersthe dissipation separated rates with by unheated a distance of 12.7 Thecm changeand utilizing in k is relatedEquation to the(9). heat The dissipated relative change in each location.in the dissipation The discretized rates heatwith dissipation unheated 12.7 cmmanikins and utilizing over heated Equation cases (9).were The calculated relative using change Equation in the (11) dissipation and the results rates with are shown unheated in Figure manikins manikinsfor heated over and heatedunheated cases cabins were were calculated evaluated using using Equation three (11) transducers and the results separated are shownby a distance in Figure of 14. Negative values would favor unheated results over heated ones or would mean the unheated over heated14.12.7 Negative cm cases and values wereutilizing calculatedwould Equation favor using (9).unheated The Equation relative results (11) changeover and heated the in resultsthe ones dissipation areor would shown rates mean in Figurewith the unheated 14. Negative cabin dissipated more energy than the heated ones and vice versa for positive values. As can be seen valuescabinmanikins would dissipated over favor heated unheatedmore casesenergy resultswere than calculated the over heated heated using ones ones Equation and orvice would versa(11) and meanfor thepositive theresults unheated values. are shown As cabin can in be Figure dissipated seen in Figure 14, the dissipation rates were higher for the unheated case in the front-west and back-east morein14. energy Figure Negative than14, thevalues the dissipation heated would onesfavorrates andwereunheated vicehigher versaresults for the forover unheated positive heated caseones values. in or the would As front-west can mean be seenandthe unheatedback-east in Figure 14, sides of the cabin. This indicated that the velocities and k were lower in these regions. the dissipationsidescabin ofdissipated the ratescabin. more wereThis energy indicated higher than for that the the the heated unheated velocities ones anand cased vicek inwere theversa lower front-west for positivein these and regions.values. back-east As can sidesbe seen of the in Figure 14, the dissipation rates were higher for the unheated case in the front-west and back-east cabin. This indicated that the velocities and k were lower in these regions. sides of the cabin. This indicated that theEast velocitiesCenter and k were lowerWest in these regions. 1.5 East Center West 1.5 1.0 East Center West 1.01.5 0.5 0.51.0 0.0 0.00.5 01234567891011 01234567891011 -0.5 -0.50.0

RELATIVE CHANGE IN 01234567891011

RELATIVE CHANGE IN -1.0 -1.0-0.5 TAYLORS' DISSIPATION RATE TAYLORS' DISSIPATION RATE

RELATIVE CHANGE IN -1.5 -1.5-1.0 LONGITUDINAL ROW INDEX # LONGITUDINAL ROW INDEX # TAYLORS' DISSIPATION RATE -1.5 Figure 14. Dissipation rateLONGITUDINAL relative change ROW between INDEX heated # and unheated cases. Figure 14. Dissipation rate relative change between heated and unheated cases.

FigureFigure 14. 14.Dissipation Dissipation rate rate relative relative changechange be betweentween heated heated and and unheated unheated cases. cases. Fluids 2019, 4, 167 16 of 18

5. Uncertainty Analysis For either the tracer gas measurements, TI, k or ε, the relative uncertainty contained two parts. The ﬁrst one was the uncertainty due to measurement error “urandom” and the second one was due to the bias in the analyzer itself “ubias”. The total uncertainty was estimated using Equation (12): q 2 2 utotal = (ubias) + (urandom) . (12)

The bias and random uncertainties for tracer gas measurements are given in Equations (13) and (14), respectively, where N is the number of samples collected, t95% is the 95% conﬁdence interval constant and was estimated approximately 1.96, σ is the standard deviation of the samples collected, Cn is the average normalized CO2 concentration in each location as indicated, and ucabin, uinlet, uexit are the relative uncertainty obtained by each analyzer. The uncertainty for each analyzer also included the uncertainty of the tracer gas used, repeatability of each analyzer, linearity of the measurements and uncertainty of the DAQ system used:

" #2 " #2 " #2 2 C Cexit(Ccabin Cinlet) Cinlet(Ccabin Cexit) cabin − − ugas,bias = ucabin + 2 uexit + 2 uinlet , (13) Cn(Cexit Cinlet) Cn(C C ) Cn(C C ) − exit− inlet exit− inlet

t95%.σn ugas,random = . (14) √N 1.Cn − Similarly,the random and bias uncertainties for k, TI and ε were calculated as given in Equations (15) through (20), where Uv,bias is the total bias uncertainty for the omni-directional TSI transducer (approximately 1%V), < > is the average of the reading (k, TI, or ε), (v, V , v0) are the instantaneous, averaged and ﬂuctuating components of the measured speed, and σ is the standard deviation of the variable: t95%.σTKE uk,random = , (15) √N 1. k, readings − !2 2 Uv,bias uk,bias = 8 , (16) v0

t95%.σTI uTI,random = , (17) √N 1 TI, readings − !2 !2 2 Uv,bias  v  u =  +  TI,bias 1  , (18) v0 V

t95%.σε uε,random = , (19) √N 1. ε − h i uε,bias = √2.utransducer. (20) The relative uncertainty for all tracer gas sampling procedures ranged between 5–14% for ± heated manikins versus 8–17% for unheated manikins. For turbulence measurements, the relative ± uncertainties for heated and unheated environments, respectively, were 14–39% and 14–28% for the ± ± turbulence kinetic energy “k” and between 11–34% and 13–40% for dissipation rate analysis. For TI, ± ± the relative uncertainty was almost the same for both heated and unheated cases and ranged between 7–11%. Thus, heated cases had higher uncertainties associated with k, whereas, with unheated cases, ± the normalized tracer gas concentrations and the dissipation rates were accompanied with higher uncertainties. This was expected due to the highly chaotic nature of the airﬂow inside the cabin. Fluids 2019, 4, 167 17 of 18

6. Conclusions An experimental study was conducted to check on the dispersion of tracer gas inside a Boeing aircraft model B767 cabin mockup made up of 11 rows with seven seats in the transverse direction of each row. The sampled tracer gas in the cabin was normalized against the inlet and outlet concentrations into and out of the cabin. Results for both manikins’ statuses, heated and unheated, were compared against each other. Testing with heated minikins showed multiple air circulations, which agreed with smoke visualization testing done by [19]. The multiple circulations were controlled by the minimum allowed distance inside the cabin, which was the height from the floor to the ceiling. The identified circulations in the rear section of the cabin were of comparatively smaller size than in other sections. The unheated tracer gas results showed more uniform tracer gas distribution around the release point. Speed measurement showed smaller fluctuations with lower temperature environments indie the cabin. Hence, the turbulence kinetic energy values were higher with heated manikins testing than with unheated ones. This was the case as well with turbulence intensity, except that the difference between heated and unheated results were smaller than for turbulence kinetic energy. Regions inside the cabin that had lower turbulence kinetic energy level were accompanied with higher dissipation rates, which indicated that kinetic energy has been significantly dissipated as compared to heated regions. Thus, heated cases had higher uncertainties when evaluating “k”, whereas, with unheated cases, the normalized tracer gas concentrations and the dissipation rate estimations were accompanied with higher uncertainties. This was expected due to the highly chaotic nature of the airflow inside the cabin. In conclusion, the air temperature inside the cabin can play an important role in affecting the air flow distribution and turbulence levels due to changes in convective heat and transport phenomena of the air.

Funding: The results presented are from research funded, in part, by the U.S. Federal Aviation Administration (FAA) Office of Aerospace Medicine through the National Air Transportation Center of Excellence for Research in the Intermodal Transport Environment under Cooperative Agreement 07-C-RITE-KSU. Although the FAA supported this project, it neither endorses nor rejects the findings of this research. The presentation of this information is in the interest of invoking technical community comment on results and conclusions of the research. Conflicts of Interest: The author declares no conflict of interest.

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