Effect of surface roughness & polymeric additive on nucleate pool boiling at subatmospheric pressures

INT. O0MM. HEAT MASS TRANSFER 0735-1933/86 $3.00 + .00 Vol. 13, pp. 503-514, 1986 ©Pergamon Journals Ltd. Printed in the United States

EFFECT OF SURFACE ROUGHNESS & POLYRERIC ADDITIVE ON NUCLEATE POOL BOILING AT SUBATMOSPHERIC PRESSURES

P.K. Tswari, R.K. Verma, m.P.S. Ramani Desalination Division Bhabha Atomic Research Centre Bombay - 400 085 (India) S.P. Rahajan Department of Chemical Engineering Indian Institute of Technology Bombay - 400 076 (India)

(CuLui~unicated by D.B. Spalding)

ABSTRACT This investigation pertains to boiling heat transfer from a submerged flat surface at subetmospheric and atmospheric pressures in the presence of hydroxy ethyl cellulose (HEC) as a polymeric additive in small doses. 8oiling was carried out in presence of the additive on smooth end rough aluminium surfaces havin 9 effective cavity size uithin the range as predicted by Hau model and the pressure was kept i n the range o f 8 - 100 KN/sq.m ( a b e ) . Effects of surface roughness, s a t u r a t i o n p r e s s u r e and polymer c o n c e n t r a t i o n on boiling heat transfer were studied and the results were compared with Rohsenow's correlation.

Introduction Rany heat transfer processes involve boilin9 on one side of the surface end condensation on the other.

They have attracted grouin9

interest owing to their use in heat pumps, OTEC, electronic cooling, desalination etc.

The part played by these heat exchangers in the

overall irrevereibility of the system is very significant and there is an obvious interest in the improvement of their transfer ability. Heat exchange during boilin 9 and condensation are not yet completely understood.

Efforts are continuin 9 to transfer a given heat load

at the lowest possible temperature driving force for achieving highest performance ratio and reducing the cost. 503

504

P.K. Tewari, et al.

Vol. 13, No. 5

The role of surface active solutes was explored by Morgan at el. ~

3ontz & Myers [2] noted an increase in heat transfer rate and

predicted that it resulted due to reduced surface tension.

Kotcha-

Phakdee and William [3~ observed that nucleate boiling tats of water increased significantly

at atmospheric pressure on s horizontal

steam-heated chromeplstsd surface when it wee dosed with s small amount of soluble polymers.

Experimental Studles The experimental setup f o r the present i n v e s t i g a t i o n include a power input system, a boiling vessel and s condenser assembly ~ 4 ] .

® (~ ¢i~In~Icalimul ® sW. SQ,,

~ ) I~tmetm (~) Dimmerstat ~) U t~e m~ometer

~j) In~letl~

~) c ~ , N ~ ~,~t (~ Coolingwter IN~ To Ve~___-__=m pump

~ Cm4mMr Molmwltch

® FIG. I POOl boiling heat transfer setup Fig. I Is s schsmatlc drawing of the test apparatus,

The b o i l i n g

vessel i s a c y l i n d r i c a l carbon s t e e l vessel of 150 mm diameter and 300 mm length.

A c o i l type condenser of 245 mm dlameter and 300 mm

height i s connected to the b o i l i n g vessel to condense the water vapours generated in boiling vessel.

Cooling water is circulated

through the coil and the coolant flow tats is measured from time to

Vol. 13, NO. 5

time,

NiI~,~ATEIO3LBOILINGAT~PHERICPREKSURES

The condensate i s

m a i n t a i n the s o l u t i o n

r e c y c l e d back to the b o i l i n g

c o n c e n t r a t i o n and l i q u i d

pipelines

a r e 15 NB p i p e s .

insulated

w i t h 50 mm t h i c k

505

vessel to

level.

All

The p i p e l i n e s and t h e b o i l i n g

connecting v e s s e l ar e

glass wool,

Temperatures a r e measured by

using copper-conetantan thermocouples.

Accuracy o f t h e e x p e r i m e n t a l

d a t a on h e a t t r a n s m i s s i o n depends upon t h e e x a c t t e m p e r a t u r e measurement of the surface and boiling liquid, which in turn depends how the junction is attached to the surface. i n a narrow s l o t electrical

The thermocouples were imbedded

i n the s u r f a c e w i t h good t h e r m a l c o n t a c t and

insulation.

The j u n c t i o n s a r e s o l d e r e d i n p l a c e as

proposed by C o l b u r n and Hougen [ 5 ]

for accurate surface temperature

measurement. A water ring vacuum pump is used to create and maintain desired vacuum in the boiling vessel and a U-shaped mercury manometer is used to measure t he same.

The e r r o r i n v o l v e d f o r

the measurement o f

a b s o l u t e p r e s s u r e by U-tube manometer i s l i m i t e d The t e s t l i q u i d

to 2 p e r c e n t .

was v i g o r o u s l y b o i l e d f o r an hour to degas i t

b e f o r e each run and e x p e r i m e n t s were c a r r i e d o u t by d e c r e a s i n g t h e h e a t f l u x monotonically.

The system was permitted to s t a b i l i e e f o r

10 to 15 minutes at each flux setting.

2.5 lltres of liquid was

b o i l e d over a c i r c u l a r aluminium p l a t e of 150 mm diameter and 3 mm thickness for each test run using a one kilo-watt electrical heater c o m p r i s i n g a 24 gauge nichrome wi~e c o i l

( 8 0 / 2 0 ) connected t o a

dimmeretat to vary the voltage and thus the heat input. was s u p p o r t e d on an i n s u l a t e d i r o n placed over

the

The heater

frame and a c o p p e r p l a t e was

h e a t e r in o r d e r t o obtain a uniform d i s t r i b u t i o n o f The thermocouplee were

heat due to its high thermal conductivity.

e a t a t the a n g u l a r p o s i t i o n s o f O°j 120 ° and 240 ° a l o n g t h e o u t s i d e circumference of the aluminium p l a t e .

All

the t her mo c ouplee used

were i n d i v i d u a l l y c a l i b r a t e d before assembly in the t e s t section, against a platinum resistance thermometer on the workln9 temperature range o f t h e e x p e r i m e n t s . thermocouplo p o s i t i o n

Any s y s t e m a t i c e r r o r a s s o c i a t e d w i t h a

was d e t e r m i n e d by comparing the o u t p u t o f a l l

thermocouplee during unheated rune at various system t e m p e r a t u r e s . Systematic

errors

were,

in most of

the

cases,

less

than

0.2°C.

Three

thermocouples located on the aluminium plate a t various positions

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P.K. Tewari, et al.

Vol. 13, No. 5

disclosed that the temperature distribution was uniform within accuracy of the measurement.

The heat loss from the test section was evaluated

by the heat balance using the heat input, condensate flouters, cooling water requirement, ies.

coolant temperature rise and fluid thermal propert-

The difference between heat input and heat rejected from the

condenser was the heat loss from the test section. found to be approximately

The heat loss was

2 percent of the maximum heat input.

The

heat input was measured in terms of electrical power input with voltmeter and ammeter of accuracy 1V and 0 . 1 A

respectively.

The range of operating conditions studied is given in Table I.

TABLE 1 Range of Operating Conditions

Heat flux

10,000 - 70,000 --W/m2

Boiling liquid

Distilled water 100 ppm HEC solution 300 ppm HEC solution 500 ppm HEC solution

Pressurs

8 to 100 KN/m2 (abe)

Surface material

Aluminium

Surface roughness A1 1A

Smooth

A1 2A

Rough ( C a v i t y

s i z e range

30 - 75 m i c r o n )

A sample o f HEC was a n a l y s e d i n t h e l a b o r a t o r y V o l 21 D2364-74.

as p e r 1980 ASTM

The s u r f a c e roughness o f t h e a l u m i n i u m p l a t e s was

measured by "Surtronic" roughnessmeter.

It displays surface roughness

in rme value throughout a prescribed sampling length.

The experiment-

al plate was first cleaned so that it was free from abrasive material, grease etc.

Range r e q u i r e d

was s e l e c t e d .

complete, rms roughness was displayed.

When the t r a v e r s e was

Magnified photographs of the

surface was taken by scanning electron microscope mouth opening.

to get the cavities

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NIr~.WATEPOOLBOILINGAT

~PHERIC

~

507

Heu Rode1

An engineering surface i s c h a r a c t e r i e s d nucleation

sits

nor by m u l t i p l e

neitha= by a s i n g l e

s i t e s of same s i z e but r a t h e r by a

distribution of sites or cavities of various sizes and geometries. Experimental observations have indicated that for a 9ivan wall superheat,

there exists a size range of active cavities.

Hsu ~6] proposed a mathematical model in order to define the size range of active cavities considering the heat transfer from superheated liquid into the bubble as transient conduction process.

A cavity is considered effective only if the waiting period is finite. This criterian 9ires the limiting sizes of effective cavities.

rcmax = (~/2C 1) (1 - esat/@s + ((1 - s e a t / a s ) 2 - 4Ac3/&es) 0"5) rcmin = (~/2CI)

(I - esat/e s - ((1 -

es=t/ee)2

- 4AC3/~es) 0"5)

(21 (3)

Mean film thermal layer thickness, for s linear temperature distribution as suggested by Zuber, ie calculated from! % = K ((8 s - 8eat)/q )

(4)

For liquids st saturation temperature Tsat = T~ i.e.

@eat = 0 Therefore,

equation (2) (3) (4) can be w r i t t e n

~cmax = (~/2C1) (1 * (1 - 4AC3/~SS) 0°5)

(5)

(tlzc 1) (1 - (I - 4AC31tee) °'5)

(6)

rcmin =

% = K/h

uhere A = ( 2 ¢ - T s a t ) / A If

as:

(7)

~V

h e i g h t of the bubble nucleus i s . e q u a l C1 = 2 C3 = 1.5

to c a v i t y mouth diameter then

508

P.K. Tewari, et al.

Vol. 13, No. 5

Results & Discussions The heat transfer rates to the boiling liquid were evaluated indirectly

by measuring the power supplied to the heating element,

assuming that heat losses after insulation could be neglected. temperature of the experimental plate use computed~7~

Surface

using Fourier's

law of conduction. HEC displayed fast dispersion without lumping when added to water. Fig. 2 shows the effect of HEC concentration on nucleate boiling curve at 47 KN/aq.m (abe) pressure.

It shows that the general effect of

Alumlnlum surface (AItA) oJ

50~

/ I . 5OOp ~ HEC

Z

I0 5

i I0

I 20

( T s " T sot)

"K

I 50

I00

FIG. 2. Effect of HEC concentration on nucleate boiling curve at 47KN/sq.m.(abs)pressure

increasing the concentration of polymeric additives (HEC) increases the heat flux.

Further the boiling curves also shift to the left by

increasing the amount of additive implying lower wall superheat requirement for the same heat flux.

The boiling bahaviour of pure water

changes due to addition of a small amount of HEC, due to smaller and faster bubble generation.

It is e surfactant leading to enhancement

in heat transfer due to reduction in surface tension.

Greater number

of bubbles and faster bubble generation is due to the fact that polymers are lass soluble than small molecules.

They adsorb on

surface 91ring large number of potential nucleation sites.

The

Vol. 13, No. 5

NIY~.~ATEPOOLBOILINGAT~C~

509

aubetentlel change i n b o i l i n g behavlour of water due to addition of HEC

yes documented i n photographs by Yang and Raa [ 8 ~ .

They hovever d i d

not f l n d any s i g n i f i c a n t effect of H(C additives on the heat transfer rate.

But considerable improvement i n heat transfer c o e f f l c i e n t by

adding the small amount o f sodium l a u r y l benzene eulfonate (SLBS) and

sodium l a u r y l

sulfate

(5LS) s u r f a c t a n t s was noted by them ~9~.

Reduced

bubble size i s a consequence of increased l i q u i d viscosity too.

Bubble

This tendency is

grouth rate relates with increase in viscosity.

further enhanced by elastic stresses in the liquidp a phenomena unique to polymeric systems [3~*

H[C solution viscosity increases uith

decrease in saturation temperature and p r e s s u r e .

In addition,

evaporation or the water into a bubble serves to conentrate the solute locally and the viscous and elastic liquid properties are knoun to be extremely sensitive to s l i g h t changes i n polymer concentration.

It

seems solution viscosity plays s major role in the enhancement of heat transfer coefficient.

It i e a

function or HEC concentration,

operating

temperature and pressure. Nucleate boiling curves for 500 ppm HEC solution on alumlnium

s u r f a c e have been shoun i n F i g . 3 a t s a t u r a t i o n p r e s s u r e s v a r y i n g Prom

lOOt

5o~

2O e I0 ~-

j 4

5

Alumlnlum surface ~231(AI . 1A)

IOOKNaoj~(®s)

•74 KN/scl~n.(al~) .47 KN/sqJ~(abs) . 34KN/~nLlals) 21 KN/eq.m,(abs] 6~ 8 KNA~m.(abs)

i

i 20

i 50

I00

( T s - Tsat)*K --~ FIG. 3 . Nucleate boiling curve for 5OOppm HEC solution of different saturation pressures

510

P.K. Tewari, et al.

8 to 100 KN/sq.m (abe). results

Vol. 13, No. 5

They show that lower saturation pressure

in reduction of h e a t flux for the same surface superheat and

roughness.

It implies that boiling heat transfer coefficient decreases

with reduction in saturation pressure. Fig. 4 gives the boiling heat transfer coefficient vs. HEC c o n c e n t r a t i o n a t 17=K w a l l s u p e r h e a t and B KN/sq.m (abs) p r e s s u r e f o r

t

5000 A T = 17*K

ROUGH AI. S U R F A C E ~

3000 2000

E o

u

/

SURFACE (AItA)

!000

p.

E

4.-

1=

500

0

I-

' I00

' :500

I 500

HEC Concentroflon ( p p m ) - - ~

FIG. 4 . Effect of surface roughness and concentration of HEC aqueous solution on the boiling heat transfer coefficient smooth and rough aluminium surfaces.

Rough surface has the cavity

size 30 - 75 microns which is within the effective size range as predicted by Hsu-model.

It shows that boiling heat transfer

coefficient is more for surface having effective cavity size range as par Hsu-model compared to the smooth aluminium surface.

Also At

increases with the HEC concentration if added in ppm quantity.

Using equations (5), (5) and (7), i t is noted that effective cavity size range narrows down with increase in polymeric additive (HEC) concentration.

The effective minimum cavity size increases with

the increase in polymeric additive.

Mean thermal film thickness

Vol. 13, No. 5

NLr~.FATEPOOLBOILINGATS~PHERICPRF-,SSURES

511

assuming l i n e a r temperature d i s t r i b u t i o n appears to be of the same order of magnitude a s the effective maximum cavity diameter. Experimental results were compared with Rohsenow's correlation

Cp L (T s _ T s a t ) / ~ =

Csf((q/'kLX)

(e-/(g(f L - ~V)))O'5)0"33prL m

where m = 1 for water end 1.7 for other l i q u i d s .

(8)

At constant p r e s s u r e ,

the correlation can be reduced to following form

(T s - Teat) = ~ T Exponent ~ i s

(9)

=Y(q)P 0.33 as pet Roheenow's c o = r e l a t i o n .

The r e s u l t s

of

t h i s study show that r varies with the HEC conoentretlon~ saturation temperature end pressure.

I t wee noted that the exponent (~) i s 0.3 for

d i s t i l l e d water boiling on smooth alumlnlum surface at 4? KN/eq.m (abe). ~ l e more than the value of 0.33 for HEC solution et d i f f e r e n t saturation pressures.

I t Is 0.5 for 300 ppm end 0.67 for 500 ppm HEC solution

b o i l i n g at 47 KN/eq.m (abe) pressure.

This implies that the slops of the

experimental boiling curves (q vs ~ T ) i s lower than for HEC solutions at 47 KN/eq.m (abe).

0.04

Fig. 5 gives the Rohsenow's parameter (Csf) at

ALUMINIUM SURFACE { SMOOTH ) BOILING SOLUTION : 5OOppm HEC

t I

_

u 0.02

/

f O

0

I

I

I

J

20

40

60

80

Absolute

pret=ure

I0(

-~

(KN/SQ.M)

FIG. 5. The effect of subatmospheric pressure on the Rohsenow's parameter (Csf)

512

P.K. Tewari, et al.

Vol. 13, No. 5

p r e s s u r e s v a r y i n g from 8 t o 100 KN/sq.m (abs) f o r 500 ppm HEC s o l u t i o n boiling on smooth aluminium surface. from equation (8).

Csf values have been calculated

It varies from 0.013 at 8 KN/eq.m

(abs) to 0.021

a t 100 KN/sq.m (abs) p r e s s u r e . Conclusions The c o n c l u s i o n s may be summarised as f o l l o w s : 1.

An enhancement i n p o o l b o i l i n g

heat transfer

i s noted a t

a t m o s p h e r i c and e u b a t m o s p h s r i c p r e s s u r e s on a d d i t i o n

o f a few

hundred ppm of p o l y m e r . 2.

The p r e s s u r e o f t h e system i n f l u e n c e s

curve,

such t h a t

the w a l l s u p e r h e a t a t a g i v e n h e a t f l u x

w i t h the decrease i n s a t u r a t i o n 3.

the n u c l e a t e p o o l b o i l i n g increases

pressure.

A higher b o i l i n g heat transfer c o e f f i c i e n t Is noted for b o i l i n g

on the e l u m i n i u m s u r f a c e ( A I - 2 A ) w i t h a c a v i t y

s i z e i n the ¢ange o f

30 to ?5 microns as compared to the smooth one.

T h i s agrees w e l l w i t h

the Hsu-model as the AI-2A cavity size l l e s wall w i t h i n the l i m i t i n g sizes of effective

cavities

as p r e d i c t e d by Hsu. Acknouledoement

The a u t h o r s a r e e x t r e m e l y g r a t e f u l Chemical E n g i n e e r i n g Group, BARC f o r

t o S h r i R.K. Garg, D i r e c t o r ,

the v a l u a b l e guidance r e c e i v e d

from t i m e to time d u r i n g the r e s e a r c h work. Ncmgnclatuz @ A

Parameter r e p r e s e n t i n g

C1 , C3

Constants

CpL Csf g

Specific

( 2 ~ ' T s a t / ~ ~v )

h e a t of l i q u i d

phase

Dimensionless coefficient Acceleration

due to g r a v i t y

h

Boiling

K

Thermal c o n d u c t i v i t y

Pr L

P r a n d t l number f o r l i q u i d

q

Heat f l u x

r

cmax

of Rohsenow's e q u a t i o n

heat transfer

Maximum c a v i t y

radius

coefficient phase

Vol. 13, NO. 5

rcmln t

Nir~.WATEPOQLBOrr.TNGAT~~C

PRESSURES

Minimum c a v i t y radius Time Temperature

T T

S

Surface temperature

Teat

Saturation temperature

~T

Temperature d i f f e r e n c e Greek L e t t e r s

Thermal d l f f u e i v l t y

P

Exponent term in eq. (9) Parameter at constant pressure in eq. (9) Thermal l a y e r thickness Temperature minus bulk temperature, (T - T~)

e G e

Geat

k tL

Surface temperature minus bulk temperature, (T s - T~)

Saturation temperature minus bulk temperature, (Teat - T~) Latent heat of vapourlsetlon Liquid viscosity Liquid density Vepour density Surface t e n s i o n References

1o

A.To Morgan, L.A. Bromley and C.R. Wilko, Ind. Engng.Chem., 41, 2767 (1949).

2°

P.D. 3ontz and 3.E. Myers, A . I . C h . E . 31, ~, 34 (1960).

3.

P. Kotchephakdee and M.C. Williams, 13, 833 (1970).

4.

P.K. Tewari, R.K. Verma, M.P.5. Ramani end $.P. Mahsjan, D e s a l i n a t i o n , 52, 335 (1985).

5.

A.P. Colburn and O.A. Hougen, Ind. En9ng. Chem., 22, 5, 522 (1930).

6e

?o

Int.

3° Heat Mass T r a n s f e r ,

Y.Y. Hsu, Trans. ASME-OournaZ of Heat Transfer, 20? (1952). P.K. Tewart, R.K. Verma, R°P.S. Ramani, A. Chattopadhyaya and 5°P. Mahejan, E f f e c t of surface roughne=s on nucleate b o i l i n g of sodium c h l o r i d e s o l u t i o n s at atmospheric and eubatmospheric pressures, Seventh National Heat and Mesa T r a n s f e r Conf., Kharagpur, I n d i a (1983).

513

514

P.K. Tewari, et al.

Vol. 13, No. 5

8.

Y.M. Yang and J.R. Maa, Letters in Heat Mass Transfer, ~, 237 (1982).

9.

Y.M. Yang and 3.R. Maa, Trans. ASME-Journal of Heat Transfer,

Io5, 19o (1983).