The interfacial turbulence in falling film absorption: effects of additives

hid

ELSEVIER

Int J. Refrig. Vol. 19, No. 5, pp. 322 330, 1996 Copyright © 1996 Published by Elsevier Science Ltd and IIR Printed in Great Britain. All rights reserved PII: S0140-7007(96)00025-4 0140-7007/96/$15.00 + 00

The interfacial turbulence in falling film absorption: effects of additives Kwang J. Kimt, Neil S. Berman and Byard D. Wood Center for Energy Systems Research (CESR), College of Engineering and Applied Science, Arizona State University, Tempe, A Z 85287, USA The results of vertical falling film experiments on the absorption of water vapor to aqueous lithium bromide solutions with an additive, 2-ethyl-l-hexanol, are reported. During the absorption, the film becomes highly turbulent. Consequently, the heat and mass transfer is significantly enhanced by turbulent mixing. In addition, the instability mechanisms are detailed. In the vicinity of water absorption, surface-tension gradients due to the lower LiBr concentration, the lower additive concentration, and the higher temperature at the interface, can favor instability of the falling film. Copyright © 1996 Published by Elsevier Science Ltd and IIR (Keywords: absorption system; lithium bromide; water; absorber; falling film; turbulent flow; surface tension; additive)

Turbulence d'interface pendant l'absorption en films tombants" effet d'additifs On rapporte les rOsultats d'expdriences effectudes sur l'absorption de vapeur d'eau par des solutions aqueuses de bromure de lithium, en films tombants, avec un additif" le 2-~thyl-l-hexanol. Au cours de l'absorption, &film devient trbs turbulent. Par consOquent, le transfert de chaleur et de masse est fortement augmentd par la turbulence. On examine dgalement les m~canismes d'instabilitd. Dans le voisinage des zones d'absorption de l'eau, les variations de tension superficielle dues it une concentration plus faible en bromure de lithium, it la

concentration plus faible de l'additif et it la temp~rature plus ~levOe it l'interface peuvent favoriser l'instabilit~ du film tombant. Copyright © 1996 Published by Elsevier Science Ltd and IIR (Mots clrs: systrme ~ absorption; bromure de lithium, eau; absorbeur; film tombant; regime turbulent; tension superficielle; additif)

Interfacial turbulence, often called the surface convection or 'Marangoni effect' is an important mechanism I in various transport processes such as absorption, extraction and crystal growth. It is well known that this surface motion is caused by local variations in the interfacial tension. Thus, the interfacial turbulence is ultimately related to the concentration and temperature dependence of interfacial tension. It is well known that an interfacial tension gradient causes the violent convection flows and leads to a substantial increase in heat and mass transfer rate 2. For the purpose of intensification of heat and mass transfer in absorption cooling systems, interfacial turbulence is an effective way3-~ to overcome the limitation that diffusion has without applying externally driven forced convection. In a typical absorption refrigeration cycle, water is absorbed into aqueous lithium bromide solution and

water is subsequently evaporated. The heat- and masstransfer rates at the absorber are crucial to achieve the maximum cooling performance. It has been observed that small amounts of heat-transfer additives, such as 2-ethyl-l-hexanol, can promote interfacial turbulence. Then, improved mixing in the liquid phase can increase the heat- and mass-transfer rate dramatically. Kashiwagi9 has reported that heat transfer was increased by a factor of approximately four during water absorption into aqueous lithium bromide solution in the presence of an additive. Furthermore, interfacial turbulence in the film absorption process was visualized by a laser holographic interferometry technique. Hihara and Saito 10 found similar increases in a vertical falling film. Also, Hozawa et al. tl observed interfacial turbulence during water absorption into an aqueous LiBr solution and carried out a simple numerical simulation of interfacial turbulence for a static film. The object of this paper is to describe instability mechanisms and to obtain film heat-transfer coefficients that can be used in the design of a falling film absorber for a LiBr/water absorption system.

t Author to whom all correspondence should be addressed. Present address: Airconditioning and Heating Laboratory, Mechanical Engineering Department, University of New Mexico, Albuquerque, NM 87131, USA

322

Additives in falling film absorption

323

Nomenclature Area, (m 2) Concentration, (wt%) Diffusivity, (m z s-l) Diameter, (m) Gravitational acceleration, (9.8 m s-2) Heat-transfer coefficient, (kW m -2 K - l) Enthalpy, (kJ kg- 1) H k Thermal conductivity, (kWm -l K -]) L Length, (m) LMPD Log mean pressure difference, (kPa) LMTD Log mean temperature difference, (K) Mass flow rate, (kg s -1) M Nusselt number Nu Pressure, (kPa) P Total heat transfer to the cooling water, O (kW) Re Reynolds numbcr Sherwood number Sh Temperature, (K) T Overall heat-transfer coefficient, U (kW m -2 K -l) Average velocity, (m s- 1) Thickness, (m) x

Difference Dynamic viscosity, (kg m- 1 s- t) Density, (kg cm -~) Surface tension, (N m -1) Kinematic viscosity, (m 2 s- t)

A #

A C D d g h

p cr u

Subscripts abs Absorption add Additive ch Characteristic

cw cwin cwout f h i

in LiBr lm m o out P sin sout

Greek letters

v

P

Flowrate per unit perimeter, (kg m -l s -t)

~--~--2222£27-1-75:?zT77:z7-zzzzztzzzii

wl

T

Cooling water Cooling water inlet Cooling water outlet Film Heat transfer Inside Inlet Lithium Bromide Log mean Mass transfer Outside Outlet Pipe Solution inlet Solution outlet Vapor Water in lithium bromide

AB

Absorber

CT

CollectingTank

EV

Evaporator

ST

SolutionTank

WT

Water Tank

[~h

HeatingElement

£" .

"

.

.

.

.

.

.

. . . .

. . . . . .

ImmersedHeater

. . . . . . .

Vacuum Pump

< i, J

.....Q-----m

Ion Pump

~------~X} .................... - ................................. i : i'. .....

!!i

O

D~ _

i: •

........... Q.._.....~

.......... i

I~ --]

&-

SolutionPump MassSpectroscopy Cold Trap

kA

(~

SamplingPort

O

Mass FIowmeter Rotameter

._Q Figure 1 A schematic of the experimental system Figure 1 Schdma du systbme experimental

M

SolutionValve Vacuum Valve

K.J. Kim et al.

324 Cooling Water Out

Table 1 Controlled test conditions Tableau 1 Conditions des assais

A Tcout

LiBr Solution Inlet T / Tsin, Csin

Solution I Distributor........

LiBr Solution

Stainless Steel Tube ....... "'-..°.... for Cooling Water

Solution concentration, Csi n Solution temperature, Tsin Absorber pressure, P.bs Cooling water temperature, Tcwin Absorber length, Leff Solution flowrate, Ref Cooling water flowrate, Recw Additive concentration, Cadd

60 wt% lithium bromide by mass 40°C 7.6 mmHg 30°C 0.85 and 0.4m 6O 104 100 ppm by mass

!

Glass Tube ..............

Solution Collector .......... ....

,41

H20 Vapor , from Evaporator

Tsout, Csout Weak Solution

Tcwin Cooling Water In

Figure 2 An absorber unit Figure 2 Absorbeur

added to adjust the pH to approximately 10 to inhibit corrosion. The flowrate of the cooling water is given by a rotameter. The flowrate of the aqueous lithium bromide solution is determined by a direct mass flow meter with a maximum relative error of +0.27%. The inlet and outlet solutions are sampled by the pre-evacuated sampling bottles. Precise measurement of the aqueous LiBr solution is performed by a 2.5 × 10-Sm 3 (25ml) pycnometer with a constant temperature bath. A curve fit to the density of the aqueous LiBr, presented in Washburn 13 as a function of concentration at 30°C, is used to determine the aqueous LiBr concentrations within a relative error of ±0.06%. Type K thermocouples having an accuracy of ±0.1°C are located at various locations, such as absorber inlet/outlet and the cooling water inlet/outlet. The absorber pressure is monitored by a Balzers APG010 total pressure gauge with a range of 0 - 7 6 0 m m H g (103mbar). This Balzers pressure transducer is installed at the top of the absorber. Deviations in terms of mass-transfer driving potential was ensured within 1% for the appropriate pressure range of the experiments.

Test conditions

Experimental method Experimental apparatus and instrumentation A schematic of the experimental system is shown in Figure 1. The system operates as a batch system. The absorber unit has two concentric tubes, an inner stainless-steel tube and an outer Pyrex tube, as illustrated in Figure 2. The aqueous LiBr solution flows down the outside of the inner stainless-steel tube. The outer Pyrex tube serves to facilitate flow observation. The water absorption takes place at the outer wetted surface of the inner tube. The upward flow of cooling water inside the inner tube removes the heat of absorption. The inner tube has an outside diameter of 3.81 × 10-2m and a length of 1.83 m. The system between the tubes is under vacuum and non-absorbable concentration is controlled as described by Kim et al. 12. The aqueous LiBr solution is introduced through the solution distributor, stays more than 20s in the solution distributor so as to eliminate possible disturbances, and flows into the annular space between the head and the absorber tube. The weak solution (after absorption) is collected at the bottom of the absorber by a funnel and flows into a collecting tank by gravity. The maximum effective absorber length available is 0.85m. The cooling water temperature is maintained at 30 ±0.1°C. A small amount of lithium hydroxide (less than 0.01 wt%) was

Controlled test conditions are described in Table 1. The most widely used heat-transfer additive in the LiBr/water absorption cycle, 2-ethyl-l-hexanol, was of particular interest. The additive concentrations of 2-ethyl-1-hexanol were successively varied to 100ppm because the solubility 14'15 of 2-ethyl-l-hexanol appears to be near 50 ppm by mass. Cooling water temperature was set at 30°C although the nominal wet-bulb temperature is in the range of 25-35°C during the cooling season. A 60wt% LiBr with a temperature of 40°C was used for the inlet condition to match closely the conventional water-cooled LiBr/water machine, where the absorber pressure was maintained at 7.6mmHg corresponding to7°C of the refrigerant (water) temperature. The cooling water flowrate was set to 2.52 × 10-4m 3 s -1 (4.0gpm). This flowrate corresponds to a Reynolds number (Recw as defined by di('cwPcw/#cw) of 104. It can provide turbulent cooling water flow so as to greatly reduce the heat-transfer resistance inside the tube.

Data reduction The absorption rate, Mabs, is calculated from a species balance. Then, the total heat transfer to the cooling water, Q, is determined by an energy balance assuming no radiation heat transfer inside the absorber: Mab s : Msi n (Csi n / Csout - 1 )

(i)

Additives in falling film absorption 5~

10 I

! i

4 1

. .............

8

3

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~E v

325

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xz

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0

4

~

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,

....

.............

........

...........

(a)

,-

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40

2O

o

g

N

4 ~ ............. (a)

.......................

80

60

0

100

20

40

60

80

100

C°dd (ppm)

Cad d (ppm)

1.0

!

7

0.8 !

6

=-

0.6

8

0.4

o

8

8

g~5

08

~'

0.2 ~ ) (b)

8

0

0 ..............................

4

0

•

....

3

0.0 ' ~ ~ - ~ o 20

(b) 40

60

80

2

7

,

I

20

lOO

C~,1,~(ppm)

40

'

60

I

80

100

Cadd (ppm) (a) hf a n d (b) Nuf Pabs -- 7 . 6 m m H g .

Figure 4 A d d i t i v e c o n c e n t r a t i o n effect on (a) h~n a n d (b) Sh (CLmr = 6 0 w t % , Tsin : 4 0 . 0 ° C , Tcwin : 30.0°C, P a b s = 7 . 6 m m H g , Ref = 69.3, L = 85 cm)

F i g u r e 3 Effet de la concentration de l'additif sur (a) hf et (b) Nuf (Ctmr - 6 0 w t % , T~,. - 40,0°C, T,.wi. - 30, O°C, P~t,~ - 7 , 6 m m H g , Ret 69,3. L - 8 5 c m )

F i g u r e 4 Effet de la concentration de l'additif sur (a) h,, et (b) Sh (Ctmr = 6 0 w t % , T~i~ = 40,0 °C, T,.win = 30, O°C, Pahs= 7 , 6 m m H g . Ref = 69,3, L - 85 cm)

Figure 3

Additive concentration

(CLiar = 6 0 w t % ,

Ref - 69.3. L

Q

Tsi n -- 40.0°C,

effect o n Tcwin -

30.0°C,

85cm)

= MsinHsi n -

MsoutHsout + MabsHv

(2)

Csin, Csout , M s i n , M s o u t , n s i n , n s o u t a n d Hv a r e t h e inlet and outlet solution concentration, inlet and outlet solution flowrate, inlet and outlet solution enthalpies at the corresponding temperature and concentration, and the vapor enthalpy, respectively. The overall heattransfer coefficient, U, and film heat-transfer coefficient for flow in a circular tube, hf, are obtained from where

Q = UAhATIm

(3)

hf = [1/U - (xplkp)(do/dlm) - do/(dihi)] -1

(4)

ATlm = (ATin - ATout)lln(AXTinlATout)

(5)

where Ah, hi, Xp, kp, dim, do, di, ATin and ATout are the transfer area, the cooling water heat-transfer coefficient, pipe thickness, thermal conductivity of pipe, log mean diameter, outside diameter, inside diameter, inlet and outlet side temperature differences (mean solution temperature minus cooling water temperature), respectively. McAdam s correlation ~6 for the heat-transfer coefficient at the solid wall in turbulent flow was used for h i . The mass-transfer coefficient, hm, can be found from Mab s = hmAmAflm

(6)

In general, the log mean pressure difference 17 (LMPD)

can be used for the driving potential, L M P D = (APin

--

AClm , as

APout)/ln(APin/APout)

(7)

where APin and APout are the vapor pressure differences between gas and liquid at the inlet and outside, respectively. If the inlet and outlet solution temperature and concentrations are known, the corresponding vapor pressures can be obtained. Knowing the value of LMPD, the mass-transfer coefficient, hm, can be determined. From the measured quantities, the following dimensionless parameters that control the absorption process are determined. The gravity-driven falling film is characterized by the film Reynolds number, Ref: Ref = 4r/~

(8)

where F and # are the mass flowrate of film per unit perimeter and the dynamic viscosity, respectively. Though the actual film thickness is the definite length scale in falling film geometry, in practice it is difficult to determine or measure the film thickness. Therefore, the characteristic length, Lch, for a vertical film is used as Lc h = (2/g)1/3

(9)

where u is the kinematic viscosity of the film. Using this characteristic length for a vertical film, the film Nusselt number, Nuf, and the Sherwood number, Sh, are

K.J. Kim et al.

326

where kLiBr and Dwl a r e the thermal conductivity of LiBr and water diffusivity in LiBr, respectively. A complete listing of the sources of these data is listed in Kim 18.

,..-'" LiBr Film Flow Direction

Results and discussion ..-

Wave Inception

Additive concentration effect Figure 3 shows the additive concentration effect on the film

tO 0

<

¢ o ~

...........

-r"

Film Thickness

Figure 5 Wavy film absorption Figure 5

Absorption par film ondulO

determined by

Nuf = hfLch/kLiBr

(10)

Sh = hmLch/ Dwl

(11)

heat-transfer coefficients, hr, as well as the film Nusselt number, Nuf. The additive concentration was varied from 1 to 100ppm by mass while all other test conditions remained unchanged as shown in Table 1. Heat transfer sharply increases below 10ppm and reaches a maximum value near 20-30ppm. This indicates that there is no further improvement in absorber heat transfer no matter how much heat-transfer additive is added to the aqueous LiBr solution. Under nominal operating conditions, the film heat-transfer coefficients were accelerated by a factor of as much as three compared with the case without additives. Similar effects on the mass-transfer coefficient, hm, and Sherwood number, Sh, are also observed in Figure 4. As the additive concentration increased, both the masstransfer coefficient and the Sherwood number were accelerated by a factor of as much as 1.8. Knowing that the solubility of 2-ethyl-l-hexanol in 60wt% LiBr is approximately 50ppm, the results of this work indicate the existence of a separate additive phase, as suggested by Kashiwagi 9, is not necessary for liquid mixing. In other words, enhancement starts far below the solubility limit of additives in the LiBr solution.

Top of the Absorber 10-35 crn below Top of the Absorber 0.10 cm E

t~ to ¢o II

0.15 cm

c-

0.20 cm O

E

<

0.25 cm

8_ 0.30 cm

0.35 cm Absorber Figure 6 A photograph of falling film absorption with additives (Cadd = 100 ppm,

CLiBr ~

60 wt%,

Tsi n =

40.1 °C,

Tcwin =

30.0°C, Pabs = 7.6 m m H g ,

Ref = 60.1, L = 85 cm)

Figure 6 Photographic de l'absorption par film tombant avec des additifs (Caaa = lOOppm, CLiBr = 60Wt%, Tsi, = 40,1°C, T,.win = 30,0°C. Pabs = 7 , 6 m m H g , Ref = 60,1, L = 8 5 c m )

Additives in falling film absorption Flow observation In the case without additives, the film flow shows a typical wavy flow pattern that wave inception starts 0.14m from the top of the absorber for this particular flow rate (Rer ~ 60). From the hydrodynamic standpoint, even small deformities can trigger the onset o f surface waves due to the system instability of a falling film flow. Roll waves and push waves were observed. Occasionally, wetting became a problem during the absorption process. This may be the absorption effects that alter the interfacial boundary conditions associated with surface tension variations. The slight difference in flow patterns between the failing film and the falling film under absorption seems to be caused by the decreased wave frequency and the decreased wave amplitude. It may be due to a stabilizing effect caused by water absorption. This effect will be discussed later. This vertical falling film flow is illustrated in Figure 5. The addition of 2-ethyl-1-hexanol improved the wetting

327

condition of the tubular surface where absorption takes place, because the interfacial tension at the liquid/solid is lowered due to the adsorbed 2-ethyl-l-hexanol on the solid surface. When absorption took place, the film flow became fully unstructured and turbulent. This film flow showed a 'ropy' or 'rivulet' nature 4, moving back and forth around the tube while falling down the tube surface. The photograph taken shows this effect reasonably well in Figure 6. However, the intensity of turbulent mixing caused by additives decayed at the lower portion of the absorber. In other words, the initial turbulent mixing died out shortly after its birth. The observed lifetime of turbulence was merely on the order of 10°s (2 3s) which appears to be the same order as the characteristic time scale 23, which is the time requirement for one circulation in the aqueous liquid LiBr film. A similar type of turbulence mixing with decay has been reported in the study of interfacial turbulence induced by absorption with vertical wetted wall by Imaishi and Fuginawa

Instability mechanism ............................. ~ i Cl < C2 i ST1 < ST2 1 .......................................

\

---

rich

J

Surface: Low ST

t LiBr rich

(2)

(1)

0 Bulk: High ST

TI > T2 ST1 < ST2

(a)

Caddl < Cadd2 C~ < C2 ST1 > STz Surface: High ST

\

-=-

rich

/ LiBrrich

(1)

(2)

©

~:U'; f2....... ST1 >

Bulk: Low ST

ST2

(b) Figure 7 Lithium bromide/water solution: (a) no additives, (left) a surface flow, (right) a bulk flow; (b) with additives, (left) a surface flow, (right) a bulk flow (Notation C, Cadd, T and ST represent the LiBr concentration, additive concentration, temperature and surface tension of water-rich (1) and LiBr-rich (2) portions) Figure 7 Solution bromure de lithium/eau, (a) sans additifs (gauche) en dcoulement superficiel (droite) en ~coulement de masse (b) avec additifs (gauche) en ~coulement superficiel (droite) en ~coulement de masse (Notations C, Caaa, T et S T repr~sentent la concentration en bromure de lithium, la concentration de l'additif, la tempOrature et la tension superficielle dans les zones riche en eau (1) et riche en bromure de lithium (2))

Absorption without additives. First, consider the absorption of water without an additive, recognizing that water has a lower surface tension and lower density than aqueous LiBr solution. As shown in Figure 7a, the water absorbs on the surface and if there is an initial surface wave instability, the water will be collected at the crest of the wave and aqueous LiBr solution will be at the trough. Since the surface tension of water is less than that of the aqueous LiBr solution (note that the surface tension of aqueous LiBr decreases as temperature increases), there will be a surface flow from the water-rich portion (crest) to the aqueous LiBr solution (trough). This flow will spread out the crest and stabilize the surface wave 2°. If some of the bulk solution finds its way to the surface surrounded by water-enriched solution with lower surface tension, there will be a flow from the water-rich surface layer to the bulk solution, that will return the bulk solution back from where it came. Therefore, absorption of water into aqueous LiBr solution without heat-transfer additives results in a stable flow. Absorption with additives. When an additive is added to the aqueous LiBr solution, the surface tension drops. During absorption, the surface flow is from the trough to the crest and the instability is enhanced (see Figure 7b) since only a small amount of heat-transfer additive is needed to reduce the surface tension to a value to initiate turbulence. It is also necessary to consider the bulk fluid which will be brought to the surface by the surface motion 21. If the surface tension of the surface fluid is much less than the surface tension of the bulk fluid, there will be a tendency to suppress the movement of the bulk fluid to the surface. This can occur if there is an excess of additive on the surface and the movement of the additives on the surface is more rapid compared with the diffusion from the bulk fluid to the surface. In addition, water absorption into aqueous LiBr solution simultaneously includes considerable amounts of heat of absorption. Therefore, the temperature effect on surface tension cannot be ignored. The measured surface tension data by Kim et al.] 5 indicates that surface tension actually increases as temperature increases.

K.d. Kim et al.

328

10

10

10

,

60wt% LiBr

T=24oc

T=24°C. 60wt% LiBr

do > 0 o'~Lm~

0_o<0

o--___, o OT

~o - - < 0

Oo > 0

6~

6

%

z %

X

X

z

13

4

<0

' ''"J'l

'

' ...... I

10

'

x u

oJI

'''"~'

100

1000

dT

6

%

2- 1 4

'

E

30

o

0 ppm

[]

10 ppm

&

20 ppm

I

I

I

I

40

50

60

70

Cadd (ppm)

CLiBr

4

0

80

0 [] A

0 ppm 10 ppm 30 ppm

V

100 ppm

I

I

I

20

40

60

80

T (°C)

(Wt%)

Figure 8 Surface tensions of LiBr with 2-ethyl-l-hexanoP Figure 8 Tensions superficielles du bromure de lithium avec du 2-dthyl-l-hexanol ~5

Therefore, in the presence of heat-transfer additives, the surface flow would be from the LiBr solution to the water-rich solution and instability would be also enhanced.

Onset of instability. The above discussion related to interfacial turbulence leads to the conclusion that surfacetension-induced flow at the interface with make-up flow from the bulk seems to be responsible for mixing and that the considerable changes in surface tension by LiBr concentration, heat-transfer additive concentration and temperature gradients can affect the interfacial boundary condition at which absorption occurs. It is difficult to determine the true surface-tension gradient for these experiments because an equilibrium surface is never present. Surface-tension effects in absorption with heat-transfer additives are summarized in Table 2 based upon the equilibrium surface tension data 15 shown in Figure 8. The combined effect of LiBr concentration, heat-transfer additive concentration and temperature on surface tension appears to be complicated. Therefore, it is necessary to look at the complex behavior of aqueous LiBr with additives where the sign of each

gradient changes. In the system with a dissolved heattransfer additive less than 10 ppm, both LiBr concentration and temperature gradients show the stabilizing effects, but an additive gradient shows the destabilizing effect. In contrast, with higher additive concentration, all three gradients can favor instability of the falling film, where surface flow is toward the crests, because of the lower

5

60.5 0 []

With additive No additive

O [~]

59.5

~.

59.0

Q 0 o

3

58.5

0

0

0-0.1m 0 1-0.2m

0 2-0.3m

2

57.5 0.4-0.5m

0.7-0.85m

,/

Table 2 Surface tension effects in absorption with additives Tableau 2 Effets de la tension superficielle dans le processus d'absorption avec des additifs

Gradient LiBr concentration

low (less than

10 p p m )

Ocr/OCLiBr

high

+ -

stable unstable

Additive concentration

low (less than 10 p p m )

OqtT/OCadd

high

-

unstable unstable

Temperature ~r/0T

low (less than high

10 p p m )

-

+

Stability

stable unstable

58.0 v

0

0.343:

Sign of Additive concentration gradient

60.0

0.O

0.2

I

56.5

56.0

55.5

0.6

0.4

57.0

0.8

1.0

Le, (m)

Figure 9 Mass-transfer enhancement, Era, and concentration profile along the absorber length (Cadd = 1 0 0 p p m , Csi n = 6 0 . 0 w t % , Tsin = 39.8°C, Tcwin = 30-2°C, Pabs = 7 . 6 m m H g , Ref = 57.5)

Figure 9 Augmentation du transfert de masse, E m, et profil de la concentration le long de l'absorbeur ( Cadd = lOOppm, Csi, = 60,0 wt%, Tsin = 39,8°C, T,,wi, = 30,2°C, Pabs = 7,6mmHg, Re/-= 57,5)

Additives in falling film absorption LiBr concentration, the lower additive concentration and the higher temperature at the interface in the vicinity of water absorption.

© 5

329

No additive With additive

Absorption length effect Visual observation indicated that the axial absorption length is an important parameter. The effect of the effective absorber length on heat and mass transfer was thoroughly investigated for a contact length in the range of 10-85cm. The absorber length was varied by changing the level of the solution collector. Inlet LiBr concentration and the additive concentration were maintained at 60.0 wt% and 100 ppm, respectively. The results are presented in Figure 9, where the masstransfer enhancement, Era, is defined by the ratio of the mass-transfer rate with additives to the mass-transfer rate without additives. The results show large enhancement at the top portion of the absorber relative to the lower portion of the absorber. Similar decay of mass transfer has been reported in chemical absorption l° and liquid liquid extraction 22. Several reasons for this effect have been proposed 8'23'24. The concentration profiles along the absorber length are also overlaid in Figure 9. The profile with additives does not behave as a straight line which occurs in vertical falling film absorption without additives • for the Dven ftowrate""~5 (10 < Ref < 2 x 102).

The effect of mass-transfer driving potential on mass transfer The experimental investigation of the mass-transfer driving potential effect on water vapor absorption into aqueous LiBr with additives was accomplished. First of all, it should be noted that the effective absorber length was shortened by 0.45 m (the total absorber length was reduced to 0.4m) in order to investigate the effect of mass-transfer driving potential under the mixing region. Second, the additive concentration was set at 100 ppm. Therefore, additive concentration dependence

) L]

No additive With additive

[,

%

[]//~

"US

J

F~

0

"

00

,

Oo

1 --7

0.2

~---~-

0.4

I

0.6

I--

0.8

]

10

AP,~ (kPa)

Figure 10 Effect of mass-transfer driving potential on mass-transfer coefficient Figure 10 masse

Effet du transfert de masse sur le eoefficient de transfert de

/

/41

3i 3~2 I E]

El

-0

dc~

I !

oi-

'

0.0

0.2

-f

~-

0.4 APlm

I

-i ~L ~

0.6

-

-

F

0.8

=

"

1.0

(kPa)

11 Effect of mass-transfer driving potential on heat-transfer coefficient

Figure

Figure 1 I ehaleur

Effet du transfert de masse sur le eoeO~cient de tran~fert de

of surface tension, O0"/OCadd,w a s kept constant during absorption. Fick's law dictates that the mass-transfer resistance is independent of the magnitude of the driving force, but it is not valid when there are changes in the interfacial properties. As the mass-transfer driving potential increases, the surface of the liquid will contain larger local concentration gradients and more areas with these gradients. Consequently, the mass-transfer phenomena deviates from Fick's law due to the changed interfacial properties. The mass transfer results for the cases without additives and with additives are presented in Figure 10. Although the vapor pressure of LiBr varies with the absorber length, L M P D was adopted as an appropriate expression of mass-transfer driving potential for both cases. The values of LMPD were calculated by measuring the mean LiBr concentrations and temperatures for the inlet and outlet. The changes in the mass-transfer driving potential was obtained by controlling the absorber pressures. Figure 10 shows that mass-transfer coefficient, without additives, is essentially constant with increasing mass-transfer driving potential. In other words, Fick's law governs the mass-transfer phenomena when there are no additives in the LiBr. But, with additives, it is obviously observed that the mass-transfer coefficient is increased as the mass-transfer driving potential increases. It was found that the masstransfer coefficient is increased by factors of 1.5-3.0 as the driving potential increases from 0.12 to 0.55 kPa as compared with the case without additives. We can reach a conclusion that Fick's law is no longer valid in the case with additives, since there are substantial changes in mass-transfer coefficients as the mass-transfer driving potential increases. In Figure 11, a similar enhancement effect on the film heat-transfer coefficient, hf, was also found. The film heat-transfer coefficient with additives is sharply increased mainly due to the improved liquid mixing with enhanced absorption that brings the vapor enthalpy

K.J. Kim et a l.

330

into the aqueous LiBr solution. As the mass-transfer driving potential increases from 0.12 to 0.55kPa, the heat transfer, in terms of heat-transfer coefficient, increases by as much as 250% compared with the case without additives.

7

8

Concluding remarks 1.

2.

3.

The additive experiments were designed to investigate the onset of enhancement over the additive concentration of 1-100 ppm. Interfacial turbulence starts at 3-6 ppm and reaches a maximum effect on both film heat and mass transfer near 30 ppm, The results of this work indicate that the existence of a separate phase is not necessary for a liquid mixing. It has been explained that interfacial turbulence in water absorption into aqueous LiBr solution is from surface-tension gradients caused by LiBr concentration, heat-transfer additive concentration and temperature. Observations from falling film absorption experiments indicate the primary evidence that the improved mixing of the aqueous LiBr solution is responsible for heat- and mass-transfer enhancement because of the presence of additives in the LiBr solution.

9 10

11

12 13 14 15 16

Acknowledgement

18

The authors thank the support of the Gas Research Institute (GRI) under contract number S089-260-1874. Dr Timothy Ameel provided fruitful suggestions for successful experiments.

19

I 2 3 4 5

6

Seriven, L. E., Sternling, C. V. On cellular convection driven by surface tension gradients: effects of mean surface tension and surface viscosity J Fluid Mech (1964) 19 321-339 Golovin, A. A. Mass transfer under interfacial turbulence: kinetic regularities Chem Engng Sci (1992) 47 2069-2080 Zawaeid, T., Leipziger, S., Weil, S. A. Inducement of convective motion in static absorbers Fourth Joint Chemical Engineering ConfAIChE-CSChE Vancouver, Canada (1973) Cosenza, F., Viiet, G. C. Absorption in falling water/LiBr films on horizontal tubes A S H R A E Trans (1990) 96 Pt 1 693-701 Ji, W., Bjurstorm, H., Setterwall, F. A study of the mechanism for the effect of heat transfer additives in an absorption system J Colloid InterfSci (1993) 160 127 140 Iyer, G. R., Setoguchi, T., Perez-Blanco, H. A model of

Hozawa, M., Inoue, M., Sato, J., Tsukada, T., Imaishi, N. Marangoni convection during steam absorption into aqueous LiBr solution with surfactant J Chem Engrs .lap (1991) 24 209-214

17

References

enhancement of vapor absorption in a liquid pool with mass transfer additives Enhanced Heat TransJer (1993) 1 87 97 Kim, K. J., Belanan, N. S., Wood, B. D. Experimental investigation of enhanced heat and mass transfer mechanisms using additives for vertical failing film absorber Int Absorption Heat Pump ConfNew Orleans, LA (1994) 31 41-47 Kim, K. J., Berman, N. S., Wood, B. D. Effect of 2-ethyl- l-hexanol on the absorption of water vapor into lithium bromide solutions AICHE J (1996) 42(3) 884-889 Kashiwagi, T. The activity of surfactant in high-performance absorber and absorption enhancement Refrigeration (1985) 60 72-79 Hihara, E., Saito, T. Effect of surfactant on falling film absorption In J Refrig (1993) 16 339-346

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