Articulo 6 Transferencia de Calor

7/27/2019 Articulo 6 Transferencia de Calor

1/13

Insulation of commercial aircraft with an air stream barrier along fuselage

Tengfei (Tim) Zhang a, Linlin Tian a, Chao-Hsin Lin b, Shugang Wang a,*

a School of Civil Engineering, Dalian University of Technology (DUT), 2 Linggong Road, Dalian 116024, Liaoning, Chinab Boeing Commercial Airplanes, Environmental Control System, Seattle, WA, USA

a r t i c l e i n f o

Article history:

Received 7 February 2012

Received in revised form

2 April 2012

Accepted 20 April 2012

Keywords:

Aircraft

Insulation

Air channel

Asymmetric heat transfer

Experiment

CFD

a b s t r a c t

Modern commercial airplanes cruise at a high altitude where it is extremely cold. To withstand the cold

atmosphere, the airplane

s inner skin is covered by a layer of

berglass insulation. However, this porousinsulation material can entrap a large quantity of moisture after just a few months of an airplane s

operation, resulting in weight increase, insulation degradation and various corrosions. This paper

proposes to insulate an aircraft by an air stream barrier running through an annular channel along the

cross section of the fuselage. Hot air is supplied to the channel entry at the lower lobe of the aircraft to

heat the aircraft before it is nally delivered into the passenger cabin. As both channel surfaces are

neither in uniform temperature nor uniform heat ux, the existent correlation formulas cannot be

applied to fulll the insulation design. This investigation has applied a computational uid dynamics

(CFD) program to model a two-dimensional aircraft section insulated by such an air channel. A partial

aircraft cabin mockup is constructed and put to a psychrometric chamber that is conditioned to 19Cfor experimental test. The results reveal the air channel is effective to insulate an airplane. The highly

asymmetric temperature proles across the channel also lead to asymmetric velocity proles. In the

near-window region of the passenger cabin, temperature is much elevated due to the channel, and thus

cold sidewall and draft that have been repetitiously experienced by passengers seated near windows can

be much alleviated.

2012 Elsevier Ltd. All rights reserved.

1. Introduction

Modern commercial airplanes cruise at a typical altitude of

11,000 m where the outside air temperature can be as lowas 65 C[1]. To withstand the cold atmosphere, the aircraft fuselage is

covered by insulation blankets in thickness of a couple of inches. As

insulation blankets are commonly made ofberglass, this porous

material can entrap a large quantity of moisture especially when

airplanes operate in humid regions. It is reported that the moisture

gain in the insulation blankets of a conventional twinjet aircraft

amounts a maximum weight of 680 kg, while trijet aircrafts may

reach a maximum weight of 1089 kg[2]. A Boeing 747 airplane cangain thousands of pounds of extra weight after just a few months of

revenue ights[3]. The acquired moisture soaks blankets, induces

aircraft shell corrosion, degrades thermal insulation and noise

reduction performance, etc. The increased expense is not just in

extra fuel consumption, but also in corrosion repair and additional

routine maintenance.

Many efforts have been dedicated to improve aircraft insulation

design. The insulation panel is proposed to be manufactured with

better air tightness or just to supply some heated air directly into

the insulation panel to vaporize the moisture [4]. Condensate

drainage and water collection ducts or containment drainage bags

[5]are also proposed for condensed water removal or storage. The

above conceptual design maysomehow lessen the moisture gain by

the insulation but cannot extensively eliminate it. To thoroughly

prevent moisture gain, dry air should be continuously delivered to

displace the moisture out of the insulation element.

As early as 1940s, a US engineer proposed the so-called insu-

lating and ventilating air channel for rooms, compartments, aircraftcabins and the like for heat insulation and condensate moisture

removal[6]. The hot dry air is ventilated through a double walled

channel around the enclosures envelope. Later near the end of the

last century, some engineers[2]designed the envelope ventilation

structure for commercial airplanes. Hot air is supplied into the

channel formed by the fuselage surface and the lining of the

passenger cabin, to the upper collection duct or the overhead

plenum. They claimed that no insulation blanket is needed since

the air channel behaves as a heat insulation barrier. However, the

above designs are just concepts disclosed in patent publications.

There is no published literature which has reported the* Corresponding author. Tel.:86 411 8470 6407.

E-mail address: [email protected](S. Wang).

Contents lists available atSciVerse ScienceDirect

Building and Environment

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c om / l o c a t e / b u i l d e n v

0360-1323/$ e see front matter 2012 Elsevier Ltd. All rights reserved.

doi:10.1016/j.buildenv.2012.04.013

Building and Environment 57 (2012) 97e109
mailto:[email protected]://www.sciencedirect.com/science/journal/03601323http://www.elsevier.com/locate/buildenvhttp://dx.doi.org/10.1016/j.buildenv.2012.04.013http://dx.doi.org/10.1016/j.buildenv.2012.04.013http://dx.doi.org/10.1016/j.buildenv.2012.04.013http://dx.doi.org/10.1016/j.buildenv.2012.04.013http://dx.doi.org/10.1016/j.buildenv.2012.04.013http://dx.doi.org/10.1016/j.buildenv.2012.04.013http://www.elsevier.com/locate/buildenvhttp://www.sciencedirect.com/science/journal/03601323mailto:[email protected]


2/13

investigation of such air channels from the theoretical perspective.

Our recent investigation [7] proposed to supply the outdoor air into

a channel passage that is embedded within the cabin walls to the

passenger cabin, without focusing on thermo uid performance of

the air channel but instead on the created air distribution inside the

cabin. Therefore there is still lack of fundamental analysis of heat

transfer and ow inside the air channels that can aid to fulll the

insulation design.

To design an air channel for airplane insulation, some researches

on ow and heat transfer within parallel plate channels can be

referred. Depending on ow characteristics and surface boundary

conditions, analytical, experimental and numerical solutions are

carried out to obtain the correlations of convective heat transfer

and ow resistance with the parallel plate channels. If the ow is

laminar in the fully-developed regime, some correlations are

provided in the textbook [8] for constant heat ux or surface

temperature conditions. The analytical solution was shown being

able to obtain the correlations when the laminar channel ow is

exposed to asymmetric temperature and heat ux conditions, but

the temperature or heat ux on each surface should be constant

and uniform[9,10]. If the ow falls in the transitional to turbulent

regime, a numerical solution or experimental test has to be

implemented. The numerical modeling of transitional ow andturbulent ow has been carried out in [11,12], respectively. When

subject to asymmetric wall temperature or heat uxes, the exper-

imental studies of turbulent channel ow have also been reported

[13,14]. In the above mentioned literatures, no research considers

thermoow in the entry region where it is still in the developing

regime except[14]. In addition, all channel surfaces are maintained

at either uniform heat ux or uniform surface temperature.

However, such uniformity does notexist in a channel passage along

the aircraft fuselage. This is because during cruise the outdoor

temperature is relatively xed, but the air temperature within the

channel decreases along the streamwise direction. The temperature

difference between the air stream and the channel walls varies

along the ow path. This makes it hard to maintain either uniform

heat ux or uniform temperature on both channel surface walls.The above review reveals there is a big potential to improve

aircraft insulation by an air channel. However, the current available

thermo-ow correlations are still incomplete to appropriately

design such an insulation system. The channel ow to form an air

barrier can be laminar,transitional or turbulent dependingon speed

of the supply air. Thermal boundary conditions on both channel

surfacesare neither uniform heatux nor uniform temperature.The

thermo owin the channel entry is in the developing regime that is

highly different from that in the fully-developed regime. This

implies that either a numerical solution or an experimental test

must be applied to investigate such complicated channel ow. This

paper presents an investigation of the thermo ow in a channel

passage along the cross section of fuselage by computational uid

dynamics (CFD) and experimental test. An appropriate design of theair channel to insulate a single-aisle commercial aircraft is outlined.

As a series of study to reduce moisture accumulation on airplanes,

the thermo-ow performance of the air channel is reported rst in

this paper, followed by moisture transfer modeling in our subse-

quent publications.

2. Modeling of an air channel along the cross section of

a fuselage

2.1. Design and modeling of a crusing aircraft with an air channel

insulation

In order to insulate an airplane and minimize moisture accu-

mulation in the porous blankets, a channel passage along a single-

aisle aircrafts envelope is designed as shown in Fig. 1(a). The air

channel is formed by the insulation panel surface in the outer side

and the cabin lining surface in the inner side. The mixed air from

the manifold is discharged into the channel passage from the

longitudinal air supply ducts located at the lower lobe of the

aircraft. After sweeping through the insulation panels, the cooled

air is delivered into the passenger cabin via the overhead linear slot

diffusers. The air circulates in the cabin and hence dilutes the

contamination therein and removes heat dissipated by passengers

and electric appliances. There are air exhaust grilles on the deck

near the window side, which extract air from the cabin to the cargo

hold below the oor. In this study, the discharged air also circulates

in the cargo compartment and keeps it in a warm status. Finally, the

air is collected by the longitudinal air return duct for further

recycling use or being dumped overboard.

For simplicity, a half two-dimensional aircraft section as shown

inFig. 1(b)is selected for numerical modeling. In the middle of the

aircraft the symmetric boundary condition is assumed to be valid.

There are three passengers seated in each side of the cabin repre-

sented by simplied thermal manikins. The manikin surface

temperature is set to 30 C, supposing they are exposed toa comfortable air temperature of around 24 C. The cargocompartment is empty. To evaluate the role ofberglass insulationto the thermal status of the aircraft, three cases as shown in Table 1

are designed by varying insulation thickness and air supply

temperature. The insulation thickness is 5 cm in Case 1, which

corresponds to the typical situation on an actual airplane. The

Fig. 1. An air channel along the cross section of fuselage to insulate an aircraft: (a)

schematics of system design, (b) a half geometric model for CFD simulation.

T. Zhang et al. / Building and Environment 57 (2012) 97e10998


3/13

insulation is reduced to 1 cm in Case 2, until it is completely

eliminated in Case 3. Suppose each passenger is provided a mixture

of outdoor air and cabin recirculated air at 10 l/s, which corre-

sponds to a supply air speed of 0.94 m/s at the channel entry, if the

pitch of passenger seats is 0.8 m. To maintain a comfortable

condition in the passenger cabin, the supply air temperature at the

channel entry shall be increased gradually when thinning insu-

lation thickness. The supply air temperature provided in Table 1

was inversely determined to acquire the targeted air temperature

of 24e25C in the occupied zone of the cabin.At a typical cruise altitude of nine to 12 km above sea level, the

static outdoor air temperature can be as low as65C. The heatloss from an aircraft includes convection heat transfer with the free

air and radiation heat transfer to the sky. To estimate the external

convective heat transfer coefcient during ight, the correlation

formula recommended by the ASHRAE handbook [15]is adopted in

this study, which is obtained from the analogy with a at plate.

hx rcpu 0:185log10Rex2:584Pr2=3 when 107


4/13

passenger cabin and cargo hold. The thermo-ow governing

equations can be casted into the general scalar format as,

v

v

trf

v

v

xjrujf

v

v

xjGf;eff

vf

v

xj!

Sf (7)

wherer is air density, f is a scalar variable,tis time,ujis a velocity

component in three directions (xj, j1,2,3) of a Cartesian coordi-nate system, Gf;eff is the effective diffusion coefcient, Sf is the

source term. By varying f, the above equation can represent the

continuity, momentum, energy and turbulence governing equa-

tions, respectively.

Inside the aircraft cabin, ows are generally turbulent. The

Reynolds number in the air channel is also greater than the critical

threshold of 2300. An economical approach to model ow turbu-

lence is by solving the Reynolds-averaged NaviereStokes (RANS)

equations. Many eddy-viscosity turbulence models are available to

enclose the RANS equations, which can compromise between

accuracy and expense. The enhanced wall treatment is activated to

adopt the two-layer turbulence models, in which the uid domainis divided into a fully turbulent region and a viscosity-affected

region (Reyhryffiffiffi

kp

=m< 200). In the fully turbulent region, the

RNG ke 3 model proposed by Yakhot et al. [16] is adopted for

turbulence effect approximation. However in the viscosity-affected

region, the one-equation model[17]is employed. At least 10 CFD

cells within the viscosity-affected near-wall region must be

deployed to be able to resolve the mean velocity and turbulent

quantitychange, and therst grid cell should fall within the viscous

sublayer (y


5/13

the similar temperature condition on airplane. A fan is placed on

the oor to promote mixing of the inside air.

During test the whole cabin mockup is put into a psychrometric

chamber (Fig. 4) that is conditioned to around 19C. As shown inFig. 4(b), there is a set of fans in the ceiling corner of the chamber

that blow cold air to sweep the outer shell surface of the cabin

mockup. The driven air speed can reach a maximum of 12 m/s, soair inside the chamber is well mixed. The cross-owfan attached to

the mockup extracts the cold air from the chamber directly into the

air channel after being heated to appropriate temperature. Other

envelope surfaces of the mockup are covered by insulation foam to

reduce heat loss to the chamber.

The velocity and temperature proles within the air channel and

discrete temperatures on the outer shell surface and lining walls

are measured. The instrument to measure velocity proles is

a portable thermo anemometer (type VT50; Kimo, France), which

has two measuring ranges: 0e3 m/s and 3e30 m/s. In the range of

0e3 m/s that is used in the test, the resolution is 0.01 m/s with an

uncertainty of(3% reading0.06) m/s. The sensing element fortemperature measurement is Pt 100 resistance probe wires, whose

voltages are read by a data logger (model 2700; Keithley Instru-

ment, USA). Before conducting measurements, the temperature

sensing device is calibrated by a standard mercury thermometer in

a water bath. The resolution of the Pt 100 measurement device is

0.01 C and the accuracy is shown ranging from0.55 C to 0.26 C.

3.2. CFD modeling of the validation case

After the experimental test, the air movement within the

channel and heat transfer in the shell, insulation and air channel is

modeled by CFD. Fig. 5 illustrates the created geometry model. Only

the air channel, insulation and shell are included in the solution

domain. Except for the bottom horizontal part of the channel which

spans 8 cm, the width of the rest channel is 4 cm everywhere. The

surfaces of the horizontal channel sections are treated as adiabatic

for simplicity because they are all covered by insulation foam in

a thickness of 2 cm. Nevertheless, the horizontal channel sectionpossesses only a small part as compared to the whole channel ow

path. The third kind of boundary condition is specied to the outer

shell skin and the inner lining surfaces. In addition, the thermal

radiation is activated on the outer shell surface, so the combined

convection and radiation heat transfer boundary conditions are

specied to the outer shell surface.

Tables 3 and 4 summarize the thermal boundary conditions

adopted on the outer shell surface and inner lining surface,

respectively. Because non-uniform thermo-ow conditions appear

on the outer shell and inner lining surfaces, both surfaces are

divided into nearly identical six sections to reect such difference.

In the chamber, the cold air from the ceiling fans impinges the shell

surface between She2andShe3 asshownin Fig. 5, which results in

a maximum convective heat transfer coefcient on these two

surfaces. The convective heat transfer coefcient is gradually

reduced toward to either the top or bottom sections. These

convective heat transfer coefcients are estimated by the correla-

tion formula for laminar ow when sweeping a at plate[8]. The

background mean radiant temperatures are also specied to the

outer shell surface, which are estimated based on the measured

inner wall surface temperatures of the psychrometric chamber and

the corresponding view angles with the shell. Inside the compart-

ment, a fan is placed on the oor to mix the air therein. Along the

Fig. 4. The psychrometric chambers for experimental test: (a) outside view; (b) interior view.

Fig. 5. Geometry model of an air channel used in the numerical modeling validation and a part of the CFD grid distribution.

T. Zhang et al. / Building and Environment 57 (2012) 97e109 101


6/13

generated air stream on the inner lining surface, convective heat

transfer coefcients decrease with height (Table 4).

As the same to those described in the previous section, the RNGke 3turbulence model with the enhanced wall treatment is adop-

ted. The generated CFD grid shape and size (Fig. 5) in the shell,

insulation, channel and lining are similar to those outlined in

Table 2. The total grid number is 96 k.

3.3. Result comparison between CFD simulation and measurement

Fig. 6 presents comparison of air speed proles within the

vertical channel at different heights. AtH 0.02 m, which is at the

corner between the horizontal and vertical sections of the channel,

the ow is highly non-uniform. This is because vortexes are

generated in the corner and most of the air is thrown toward to the

outer channel surface when the ow changes its direction. With

increase of height, ow proles become more and more at and

thus ow is more uniform across the channel width. The CFD under

predicts the maximum velocity at H 0.02 m. However, theagreement is generally quite good at other positions, which shows

the CFD has captured good velocity proles in the channel passage.

Fig. 7 compares temperatures between the CFD and measure-

ment at many points inside the cabin mockup. On the outer shell,

measured surface temperatures are generally higher than the

ambient air by 1.5e5 C. The surface temperature is highly non-uniform along the height. The higher temperature at the bottom

part is because of a smaller convective heat transfer coefcient

when impinged air approaches the ground. The surface tempera-

ture of the chamberoor is also higher than those on vertical walls

and ceiling during the test, which exchanges more heat to the

bottom part of the mockup by radiation. The CFD over predicts

temperatures at the top part but under predicts temperature at the

bottom part. The over prediction is because all Pt 100 resistance

probes are just attached to the solid shell surface without being

implanted into the shell body, which provides lower surfacetemperature than the actual situation. Due to overexposure of the

probes into the ambient air, the sensed temperature indicates the

combined temperature between the shell surface and the

surrounding colder air. The temperature under prediction at the

bottom part is because there is heat loss in the -X direction (see

Fig. 3(b)) by the structural frame from the cabin inside to the outer

shell, which is not taken into account by the numerical modeling.

Inside the air channel, the air stream is slightly cooled down

after passing the vertical channel. This shows the berglass layer

Table 3

Thermal boundary conditions for the outer shell surface of the cabin mockup.

Section She1 She2 She3 She4 She5 She6

External convective heat

transfer coefcient

(W/m2 K)

7.03 16.96 16.96 7.03 5.39 4.55

External main stream

air temperature (C)19.1 18.98 18.92 18.84 18.87 18.87

External mean radianttemperature (C) 16.7 16.84 16.93 16.98 17.1 17.1

Table 4

Thermal boundary condition for the inner lining surface of the cabin mockup.

Section LL e1 LLe2 LLe3 LLe4 LLe5 LLe6

External convective heat

transfer coefcient

(W/m2 K)

1.52 1.94 2.29 2.91 4.06 10.37

External main stream

air temperature (C)24

Fig. 6. Comparison of air speed proles within the vertical channel at different heights, symbols for measurement and lines for CFD: (a) at H0.02 m; (b) at H0.54 m; (c) at

H

0.8 m; (d) at H

1.06 m.



7/13

provides good thermal insulation. The CFDsimulation obtains more

or less the same temperatures with those provided by the

measurement. At the bottom part along the lining layer, air stream

temperature inside the channel is a little hotter than the cabininside, so there is a small amount of heat transferring to the cabin

inside. However with air cooled down at the top part, the cabin

inside is warmer than the air stream within the channel, and hence

heat transfer direction is reversed. The general comparison shows

that the CFD model has provided results that are comparable to the

measurement.

4. Parametric study results and discussion

In this section, ow and temperature distribution inside the

passengercabin and cargo hold are discussed rst, and then focus is

put to the air channel for analysis of convection heat transfer

therein. These results are used to evaluate the parametric design as

shown inTable 1for an air channel insulation system.

4.1. Temperature distribution inside the passenger cabin and cargo

hold

Regardless of any insulation design, it shall assure comfortable

temperature conditions in the passenger cabin and also shall be

suitable for luggage storage in the cargo compartment. Fig. 8

presents the temperature distribution for each case when insu-

lation thickness varies. The designed average temperature in the

occupied zone is around 24e25 C. Temperature distribution isgenerally quite uniform except for the region near the central

overhead ceiling. The emerged low temperature below the central

ceiling is due to cool air supplied from the diffusers. There is also

a vortex generated in this region as shown inFig. 9, which prevents

Fig. 7. Comparison of discrete temperatures (C) between the measurement and CFD modeling inside the partial cabin mockup.

Fig. 8. Temperature distribution on the airplane: (a) case 1 for a berglass insulation

thickness of 5 cm; (b) case 2 for a thickness of 1 cm; (c) in the passenger cabin for case

3 without

berglass insulation; (d) in the cargo hold for case 3.



8/13

hot air circulating into this part. Though Table 5 provides the

averaged air temperature at the diffuser, the jet streams layer by

layer discharged out from the diffuser do not hold uniform

temperature. This is because asymmetric boundary conditions are

formed on the parallel channel surfaces. The outer channel surface

is colder than the inner surface due to heat loss to the cold ambient

outside. The asymmetric status is even severe for Case 3 in which

no berglass insulation is installed. More details on convective heat

transfer within the channel are to be discussed in the next section.

The heat balance inside the passenger cabin and cargo hold is

summarized inTable 5. Note inTable 5, different heat generation

rates by thermal manikins are computed in the three cases. This is

because identical manikin surface temperature is specied but air

supply temperature from the diffusers varies case by case. In Case 1,

evenwith5 cmberglass insulation, there is a small amount of heat

loss from the passenger cabin to the channel as the channel is

maintained at relatively low temperature. With increase of

supplied air temperature at the channel entry in Case 2 and Case 3,

hot air within the channel heats the aircraft. This shows the airchannel is extremely helpful for keeping the passenger cabin in

a comfortable status, especially for the near-window region. On

current airplanes, cold sidewall and draft have long been repeti-

tiously experienced [7,19,20] especially by the passengers seated

neighboring to windows. However with the proposed air channel,

temperature in the near-window region can be much elevated and

thus cold sidewall and draft complaints can be minimized.Temperatures in the cargo hold as shown in Fig. 8 are quite

different in these three cases. It can be found that the averaged

temperature in the cargo hold increases with the air supply

temperature at the channel entry. Though temperatures of dis-

charged air from the passenger cabin to the cargo compartment are

nearly identical, the heat transfer rate from the channel passage to

the cargo hold (as shown inTable 6) increases sharply with rise of

air supply temperature at the channel entry. In Case 3, the averaged

temperature in the cargo hold is higher than those in Case 1 and

Case 2 by nearly 10 C. An interesting phenomenon in Case 3 is thatthe ow pattern in the cargo hold is reversed when the channel

passage is maintained at high temperature, which is unlike those in

Cases 1 and 2. The ow in Case 3 goes clockwise due to thermal

buoyancy generated by the hot lining surface. The above resultsconclude it is possible to insulate an aircraft with an air stream

barrier. However, if no berglass insulation is installed to the shell

side, hot air must be supplied at the channel entry, which may lead

overheating in the cargo hold (like Case 3). In such consequence,

the cargo hold may have to be insulated by a berglass layer to

prevent high temperature therein.

Fig. 9. Flow distribution in the aircraft: (a) case 1 (berglass insulation thickness: 5 cm); (b) case 2 (berglass insulation thickness: 1 cm); (c) case 3 (without berglass insulation).

Table 5

Heat balance of the aircraft.

Case 1 Case 2 Case 3

Averaged air supply temperature at

the diffuser (C)21.26 20.84 17.62

Heat release by passengers (W) 129.75 125.36 143.95Heat release by the inner lining

surface to the passenger cabin (W)

5.81 17.21 98.10

Heat transfer from the cargo hold to the

passenger cabin through the deck (W)

0.32 1.73 8.14

Average discharged air temperature

to the cargo hold (C)24.62 24.81 24.35

Heat release by the inner lining surface

to the cargo hold (W)

17. 52 108. 48 439.83

Air return temperature in the cargo hold (C) 25.09 27.73 36.43

Table 6

Heat balance of the air channel.

Case 1 Case 2 Case 3

Air supply temperature at the

channel entry (C)30 57 167.85

Heat loss to the interior aircraft

through the lining surface (W)

11.67 125.69 535.29

Heat loss to the outside through

shell (W)

293.21 1170.67 4867.47

Discharged air temperature at

the diffuser (C)21.26 20.84 17.62

Fig. 10. Schematics of different positions to show velocity and temperature proles

within the air channel.



9/13

4.2. Heat transfer analysis of the air channel

Lets focus on heat transfer in the channel passage. Table 6

provides the overall energy balance in the air channel. With

decrease of insulation thickness and increase of air supply

temperature at the channel entry, the heat loss to both the interior

aircraft and the outdoor space increases remarkably. From Case 1 to

Case 3, the averaged temperature at the channel exit decreases.

Since airplanes cruise in the cold atmosphere, the channel passage

is effective to cool down the hot supplied air, especially when there

is no berglass insulation installed to the shell side. The air channel

can thus be applied to cool down a part of engine bleed air that is

nally delivered to the passenger cabin. Consequently, the designed

air temperature leaving the manifold can be much higher than that

found on current airplanes, but the air can still be effectively cooled

down by the channel passage. The energy that was originally

consumed to cool down the engine bleed air can be saved, so the

ECS can work more energy-efciently.

To illustrate velocity and temperature proles within the air

channel, we select ve different positions along the channel

passage as shown inFig.10. P1 is just behind the channel entry and

P5 is close to the channel exit. Fig. 11shows both the velocity and

temperature proles at these ve positions for Case 1 (berglass

insulation thickness: 5 cm). The horizontal coordinate is the posi-

tion along the radial direction across the channel width, in which

the origin of the coordinate is located at the outer shell surface. Air

speed within the channel can be read from the left vertical coor-

dinate, and only the speed within the channel is presented. To well

represent temperature change across the airplanes envelope,

temperature proles are plotted from the outer shell surface, across

Fig. 11. Velocity and temperature proles across the air channel for case 1 (berglass insulation thickness: 5 cm), left vertical coordinate for air speed, right vertical coordinate for

temperature: (a) at P1; (b) at P2; (c) at P3; (d) at P4; (e) at P5 (acronym, ST: shell thickness, IT: insulation thickness, ACW: air channel width, LT: lining thickness).



10/13

the berglass insulation layer and air channel until the near-

boundary region inside the aircraft cabin. The temperature values

can be read from the right vertical coordinate. Since P1 is just

behind the channel entry, both velocity and temperature proles

are quite at except for the near-wall region within the channel.

The velocity proles in these ve positions are nearly symmetric

across the channel. At P1, P2 and P3, air temperature within the

channel is slightly higher than the inside cabin, so there is a small

amount of heat transferred to the cabin. However at P4 and P5, heat

transfer direction reverses with air cooled down in the channel. But

the heat loss rate to the channel is quite small, so there is no

problem to keep the passenger cabin in a comfortable condition.

If the insulation thickness reduces to 1 cm (Case 2), the shape of

velocity proles as shown in Fig.12 is somewhat different from that

shown in Case 1. The velocity proles within the channel at P2 to P5

are no more perfectly symmetric. The position where peak velocity

appears, shifts to the inner side (right side in the gure) of the

channel because of highly different temperature presented on

channel surfaces. As the air near the outer channel surface is at

much lower temperature, the air density increases. The main air

stream must overcome the denser air near the outer channel

surface, so the uid motion near the outer surface slows down

quickly. However, there is no big difference for temperature

between the air stream and the inner channel surface. Therefore it

is not surprising that the peak velocity shifts to the inner side. Since

P4, the outer channel surface temperature already drops below the

freezing point, but the inner channel surface is still maintained at

high temperature. On beneting from the hot air stream barrier

Fig.12. Velocity and temperature proles across the air channel for case 2 (berglass insulation thickness: 1 cm), left vertical coordinate for air speed, right vertical coordinate for

temperature: (a) at P1; (b) at P2; (c) at P3; (d) at P4; (e) at P5 (acronym, ST: shell thickness, IT: insulation thickness, ACW: air channel width, LT: lining thickness).



11/13

near the lining surface, the inside aircraft cabin can be kept at

a warm status. This shows the channel passage is effective to

insulate the aircraft.

With no berglass insulation applied in Case 3, the air within

the channel sweeps the shell surface directly.Fig.13 shows velocity

and temperature proles. Due to large difference of temperature

between the air streamand the outerchannel surface (i.e.,the inner

shell surface), velocity proles since P2 are highly asymmetric. The

peak velocity quickly shifts to the inner channel surface (to the

right in the gures). However, at P4 and P5 the position where peak

velocity appears shifts a little back to the cold outer channel

surface. The underlying reason is that the temperature difference

between the surface and main air stream decreases with air cooled

down along the streamwise direction. After P4, the ow also

changes direction in the curved passage fromXdirection to X

direction. The presented highlyasymmetric air temperatures across

the channel vary layer by layer. The whole outer channel surface is

far below the freezing point. Fortunately, the air stream layer

neighboring to the inner channel surface is kept hot, so it can still

heat the aircraft.

Fig. 14presents the convective heat transfer coefcients on the

cold outer channel surface along the streamwise direction. The

formula to calculate the convective heat transfer coefcient is,

hx qxTb;x Tw;x

(8)

where qx is the local surface heat ux by convection heat transfer,

Tb,x is the bulkuid temperature within the channel, Tw,x is the local

channel surface temperature. From the gure it can be seen the

local convective heat transfer coefcient decreases sharply in the

Fig. 13. Velocity and temperature proles across the air channel for case 3 (no berglass insulation attached to the shell), left vertical coordinate for air speed, right vertical

coordinate for temperature: (a) at P1; (b) at P2; (c) at P3; (d) at P4; (e) at P5 (acronym, ST: shell thickness, IT: insulation thickness, ACW: air channel width, LT: lining thickness).



12/13

entry region. This is because of the quick increase of boundary layer

thickness. Since S

0.3 m, the varying trends of convective heat

transfer coefcients with streamwise distance are quite different.For Case 1 whose temperature difference between the both channel

surfaces is the smallest, the convective heat transfer coefcient

decreases gradually. This is due to the increase of boundary layer

thickness when ow advancing. It seems the completely fully-

developed condition has not reached. When the insulation thick-

ness is reduced to 1 cm in Case 2, convection heat transfer is

enhanced due to turbulence effect. The cold surface induces larger

air density nearby, which suppresses the upward motion ofuid.

The ow tends to be easier to shift into turbulence when the uid

motion is suppressed near the cold surface within the channel. The

larger temperature difference between the both channel surfaces,

the more turbulent ow is found inside the channel, and thus more

active convection heat transfer is presented especially in Case 3.

Such is consistent with the ndings in the parallel plate ows[14],in which the buoyancy-opposed ow is concluded promoting

turbulence.

It shall point out many factors co-contribute to the convective

heat transfer inside the channel. At the channel entry, turbulent

ow is supplied in at an intensity of 10%. However, the both

surfaces are just separated in 4 cm, so the uid motion is highly

susceptible to the damp effect of the no-slip solid boundary. If there

is large temperature difference between both opposing surfaces,

the uid near the cold surface becomes denser and the gravity

hinders the upward ow motion. The ow turbulence tends to be

promoted in the buoyancy-opposed ow. However, the buoyancy-

opposed effect reduces if surface temperature difference decreases

when air is cooled down in the end ofow path. In addition, the

ow changes direction in the curved channel passage. BelowY 2 m(Fig.10), the ow goes toXdirection, but it changes to Xdirection when Y> 2 m. The above multiple factors are coupled

together, which make the varying trend of convective heat transfer

coefcient very complicated. There is no way to obtain plentiful

heat transfer information inside the channel other than CFD

modeling. Therefore, it is recommended to apply CFD modeling to

study the channel ow when there is large difference in surface

temperatures, especially in case non-uniform thermal boundary

conditions exist on both channel surfaces.

5. Conclusions

This paper proposes to insulate an aircraft by an air stream

barrier within a channel passage running along the cross section of

the fuselage. The air channel behaves as a heat exchanger to cool

down the hot supply air that is nally delivered into the passenger

cabin and also heats the cabin. After a careful analysis of the system

by experimental test and CFD modeling, major ndings and

conclusions are:

1. The experimental test in a partial aircraft cabin mockup that is

exposed to

19C in a psychrometric chamber shows that theCFD modeling is able to accurately predict both velocity and

temperature proles across the air stream barrier. It is valid to

specify combined convection and radiation heat transfer

boundary condition to the outer shell surface of an aircraft in

modeling.

2. The simulation of a cruising aircraft section in the nighttime

shows the air channel is effective to insulate an airplane. In the

near-window region of the passenger cabin, temperature is

much elevated due to the channel passage. The cold sidewall

and draft complaints especially for passengers seated neigh-

boring to windows can be much alleviated using this system.

3. When thinning berglass insulation to the shell side, air supply

temperature at the channel entry must be increased to assure

air delivered into the passenger cabin with suitable tempera-

ture. If there is no berglass insulation to the shell side, veryhot air must be supplied at the channel entry. However, this

may lead overheating of the cargo hold. Consequently, the

cargo compartment may have to be insulated by a berglass

layer to prevent from being overheated.

4. The highly asymmetric temperature proles across the channel

also lead to asymmetric velocity proles. Near the cold channel

surface air density increases remarkably, which suppresses

upward motion of the air stream and results to shifting of peak

velocity toward to the high-temperature surface.

5. Many factors co-contribute to the convective heat transfer

within the channel. If there is large difference of temperature

between both channel surfaces, the turbulence tends to be

promoted and hence the convective heat transfer is enhanced.

However, the varying trend of convective heat transfer with thestreamwise distance is very hard to predict. One may have to

apply CFD to assist the air channel design to insulate an

airplane.

Acknowledgments

The work presented in this paper is in part to fulll the research

project of the National Key Basic Research and Development

Program of China (the 973 Program) through grant No.

2012CB720100.

References

[1] Boeing Commercial Airplanes. The airplane cabin environment-the air thatyou breathe. Seattle: Boeing; 2005.

[2] Specht PR, Asa JE, Buzza TG, von Flotow A. Air curtain insulating system foraircraft cabin. US Patent No. 5,897,079; 1999.

[3] Brunt C. Speech interference levels in aircraft interior noise measurement:their use and interpretation [accessed in Feb. 2012], http://www.armchair.com/sci/brunt1.html ; 2000.

[4] Calamvokis HE. Aircraft fuselage heating. US Patent No. 2008/0302910 A1;2008.

[5] Sloan FP. Containment systems for insulation, and insulation elementsemploying such system. US Patent No. 5,577,688; 1996.

[6] Arnhym AA. Insulating and ventilating structure. US Patent 2,427,698; 1944.[7] Zhang T, Yin S, Wang S. An under-aisle air distribution system facilitating

humidication of commercial aircraft cabins. Build Environ 2010;45(4):907e15.

[8] Incropera FP, DeWitt DP. Fundamentals of heat and mass transfer. 5th ed. John

Wiley & Sons; 2002.

Fig. 14. Calculated convective heat transfer coefcient on the outer channel surface

versus the streamwise distance from the channel entry (acronym, IT: insulation

thickness).

http://www.armchair.com/sci/brunt1.htmlhttp://www.armchair.com/sci/brunt1.htmlhttp://www.armchair.com/sci/brunt1.htmlhttp://www.armchair.com/sci/brunt1.html


13/13

[9] Barletta A. Heat transfer by fully developed ow and viscous heating ina vertical channel with prescribed wall heat uxes. Int J Heat Mass Transf1999;42:3873e85.

[10] Nield DA. Forced convection in a parallel plate channel with asymmetricheating. Int J Heat Mass Transf 2004;47:5609e12.

[11] Abraham JP, Sparrow EM, Minkowycz WJ. Internal-ow Nusselt numbers forthe low-Reynolds-number end of the laminar-to-turbulent transition regime.Int J Heat Mass Transf 2011;54:584e8.

[12] Mokni A, Mhiri H, Palec GL, Bournot P. Mixed convection in a vertical heatedchannel: inuence of the aspect ratio. Int J Eng Appl Sci 2009;5(1):60e6.

[13] ZhangX, Sutta S. Heattransfer analysis of buoyancy-assisted mixed convectionwith asymmetric heating conditions. Int J Heat Mass Transf 1998;41:3255e64.

[14] Wang J, Li J, Jackson JD. A study of the inuence of buoyancy on turbulentowin a vertical plane passage. Int J Heat Fluid Flow 2004;25:420e30.

[15] ASHRAE. ASHRAE handbook HVAC applications. Atlanta: American Society ofHeating, Refrigerating and Air-Conditioning Engineers; 2011.

[16] Yakhot V, Orzag S, Thangam S, Gatski T. Development of turbulence modelsfor shear ows by a double expansion technique. Phys Fluid A 1992;4(7):1510e20.

[17] Wolfstein M. The velocity and temperature distribution of one-dimensionalow with turbulence augmentation and pressure gradient. Int J Heat MassTransf 1969;12:301e18.

[18] FLUENT. Fluent 6.3 documentation. Lebanon, NH: Fluent Inc.; 2006.[19] Zhang T, Chen Q. Novel air distribution systems for commercial aircraft cabins.

Build Environ 2007;42(4):1675e

84.[20] Zhang T, Li P, Wang S. A personal air distribution system with air terminals

embedded in chair armrests on commercial airplanes. Build Environ 2012;47(1):89e99.


Articulo 6 Transferencia de Calor

Documents

Transcript of Articulo 6 Transferencia de Calor