Heat transfer at pool boiling of mixtures with R22 and R114

Heat transfer at pool boiling of mixtures with R22 and Rl14* D. Gorenflo, P. Blein, G. Herres, W. Rott, H. Schiirnann and P. Sokol Laboratorium fiir W/irme- und Kiiltetechnik, Universitfit--GH--Paderborn, D-4790 Paderborn, Pohlweg 55, F R G

The phase equilibrium and the heat transfer coefficient, ~t, at pool boiling were measured with eight mixtures and the pure components of the system R22(CHFzCI)-Rl14(C2F4C12). The saturation pressure varied between the critical pressure of each mixture and ~ 20~ of this pressure. The critical line of the system deviates less than 1 K from the molar average of the critical temperatures of the pure components but reaches pressures up to ~- 2 bar higher than the molar average of the critical pressures. The difference between the compositions ofvapour and liquid is found to be greatest at ~ 35 mol ~ of R22 in the liquid and at the lowest saturation pressure investigated. The difference vanishes at the critical line. As is known from the literature about pool boiling of mixtures, the heat transfer coefficient, ~, decreases below the corresponding molar average of the heat transfer coefficients for the pure components, the effect becoming more and more pronounced when the saturation pressure approaches the critical pressure. Furthermore, the dependence of ~t on heat flux and saturation pressure is less with the mixtures than with the pure components. A comparison with correlating techniques for ct in the literature shows a comparatively good agreement with a correlating method proposed by Schliinder.

(Keywords: refrigerants; R22; Rl14; refrigerantmixtures; heat transfer; pool boiling)

Transfert de chaleur lors de l'bbullition libre des mblanges de R22 et de Rl14 ?

On a mesure l'bquilibre de phase et le coefficient de transfert de chaleur ct lors de l'bbullition libre pour 8 mblanges et les composants purs du systkme R22 (CHF2Cl)-R114 (C2F4C12). La pression de saturation variait entre la pression critique de chaque mklange et environ 20 %ode cette pression. La courbe critique du systbme s'bcarte de moins de 1 K de la moyenne molaire des temperatures critiques des composants purs, mais atteint des pressions supbrieures d'environ 2 bars ~ la moyenne molaire des pressions critiques. On trouve que la difference entre les compositions de la vapeur et du liquide est la plus grande [~environ 35 moles % de R22 dans le liquide et ~t la pression de saturation la plus basse ktudibe. Cette difference disparah & la courbe critique. Comme on le sait d'apr~s la littbrature sur l'bbullition libre des mblanges, le coefficient de transfert de chaleur ct diminue au-dessous de la moyenne molaire correspondante des coefficients de transfert de chaleur pour les composants purs, l"effet devient de plus en plus net lorsque la pression de saturation approche de la pression critique. De plus, l'influence du flux thermique et de la pression de saturation surct est moindre avec les m~langes qu'avec les composants purs. Une comparaison avec les techniques de correlation de ct d'aprks la littbrature fait appara~tre une assez bonne concordance avec une mbthode de corrblation proposbe par Schliinder. (Mots ties: frigorig~nes; R22; R114; mblanges de frigorigbnes;transfert de chaleur; ebullition libre)

Thermodynamic properties of the binary system R22(CHF2C1)-Rl14(C2F4CI2) were investigated by various authors because of a possible application of this system in vapour compression cycles I a. Although it is not very likely that cycles using RI14 will be realized in future, R22/R 114 could serve as a reference system for the more promising binary refrigerant mixtures without fully halogenated hydrocarbons as components. When comparing the C O P of cycles working with mixtures or pure substances, respectively, heat transfer with boiling and condensation should be taken into account, too, because the heat transfer coefficients can be much smaller for the mixtures than for the pure components,

In the Paper, measurements of the phase equilibrium and of the heat transfer coefficient ~ at pool boiling of R22/R 114 in a wide pressure range are discussed. The measuring techniques and the data processing are equivalent to those in References 9-11. Eight mixtures with different compositions and the pure components were investigated. The saturation pressure varied between ~ 6 bar and 50 bar (between the critical pressure of each mixture and ~ 2 0 ~ of this pressure); the corresponding saturation temperatures are 19 and 146°C. Five normalized saturation pressures between 90~o and 20 ~o of the critical pressure were chosen for the heat transfer measurements. The experimental results are

* Based on a paper to be presented at Progress in the Design and Construction of Refrigeration Systems (Meeting of IIR Commissions

compared with correlating techniques for the critical points of mixtures 12 and for the heat transfer coefficient ~3'~4

B1, B2, E1 and E2), Purdue, Indiana, USA, 18-21 July 1988 0140-7007/88/040257-07503.00 © 1988 Butterworth & Co (Publishers) Ltd and IIR

Rev. Int. Froid 1988 Vol 1 1 Juillet

257

Pool boiling of mixtures with R22 and R 114: D. Gorenflo et al. r

Test apparatus and experimental procedure

1--11_Therm°statted

= :

.

~iiii~rre ~

1 I i11-~ ,c,hamber

The heat transfer measurements were carried out with a natural circulation loop mounted in a thermostatted chamber, the air temperature of which is adjusted to the saturation temperature of the boiling liquid, thus minimizing heat transfer with the surroundings (Figure 1) 9'15. The liquid evaporates on the outside of a horizontal copper tube (8 mm diameter, 200 mm heated length) internally heated by a coaxial d.c. heater. The heat is removed at the condenser with a temperaturecontrolled coolant. During the measurements, temperature or pressure fluctuations are less than + 5 mK or + 5 mbar, respectively. The heat transfer coefficient ~ is given by

' L~"~vap°ur[ I /Evaporator

"~!

[

~

~ . _ _ ~ ~

Heated tube supply

Power

~

" Jl

To vacuum pump

L..J " - f ° r heated

tube

.~Thermocouples

.~.......~ ] : ~ A m p l i f i e r

To gas

chromatograph

|\

Preheater

I_.....1"--Digital voltmeter

FigMre 1 Schematic diagram of the test apparatus for the heat transfer measurements Figure 1 Schema de l'apparei! de mesure du transfert de chaleur

== q/aT= q/(Tw- 7",)

(I)

with q=heat flux, measured by separately measuring voltage and current, AT= excess temperature of the tube

,,o ,

,o

~,a

b . . . . . . . .

"%...%

130

"q

"c "~,

120

o

-~

110

,i

40 ¢•;/

O%

100

~4k

90

'

1.5 c '

'

"

4k

ra '

30 25

"r~

,

,

,

,

,

d

,

o

2C 1.0

~. ~ ' - - ' - - Q . ~ /"

o~"

0.5

/ ' " . i&

"

"~.A

a.

15

~...

~.~ A

.

,

~

.

.

.

.

X.\

-I .5 0 RI 14

El

5

~

u '

x, ,.. i/" 0.4

0.6 XC

0.8

A

A

13 -&" D

o_

0.2

•

A

... .

-o.s

-~ .o

13

10

.0 R22

u

4-

0Oi~--~

*O--i

A

~; ~'

•

•

a 4$=

•

-5

-lo

,t

-15" 0 RI 14

"

&

0.2

li&

, 0.q

, Xc

, 0.6

, 0.8

I .0 R22

Figure 2 (a) Critical temperature, To and (b) critical pressure, Pc, as functions of the mole fraction of R22. (c) Deviation of Tc from the molar average To,not and (d) relative difference between Pc and the interpolating Equation (2). ~ , Data from Kabata et al. (taken from diagrams in Reference 6); I-3, data from H/gash/et al. (taken from table in Reference 3); &, this work, heat transfer evaporator; 0 , this work, circulation bubble point cell; O, this work,static vapour pressure cell; - - -, molar average of Tc and Pc; , Equation (2) for Pc; . . . . . , calculation method of Kabata et al.6; . . . . , calculation method of Chueh and Prausnitz~z, zij fitted to O Figure 2 (a) Temperature critique, To, et (b ) press/on critique, Pc, en fonction de lafraction molaire de R22. (c) Ecart de T c de la moyenne molaire Tc.ml; et (d) dif~rence relative entre Pc et l'bquation d'interpolation (2)./k, Rbsultats de Kabata et al. (d'aprbs les diagrammes de la rbfbrence 6); r-l, rbsultats d'Higashi et al. (d'apr~s le tableau de la r~J~rence 3); 8 , cette btude bvaporateur de transfert de chaleur; q), cette btude, cellule ;1 point de bulles de circulation; (3, cette ~tude, cellule de press/on de vapeur statique ; - - - , mo yenne molaire de Tc et Pc; , bquation (2) pour Pc; . . . . . , mi~thode de calcul de Kabata et al.6; . . . . . , mbthode de calcul de Chueh et Prausnitz 12, ~o ajust~ d 0

258

Int. J. Refrig. 1 9 8 8 Vol 11 J u l y

Pool boiling of mixtures with R22 and R 114: D. Gorenflo et al. wall, directly measured by thermocouples soldered within the tube wall, the reference junctions of which are placed in the boiling liquid, ~ 40 mm below the tube. The overall experimental limit of error for • varies between + 2 % at low pressures, high heat fluxes and intermediate concentrations, and + 15 % at the highest pressure and lowest flux investigated, and at concentrations near the pure components, Immediately before starting the heat transfer tests, the test tube was polished with emery paper according to a R22 1.0

'

....

' ~--

' ×

~

Y

~-'~"~

0.927

0.0

0.6 ~ . . . . . . . . . ~ × "~"

~ Y ""~'~e~

0.q

0.366

------~-o-- x ---e

=

~

method applied in former investigations~6, resulting in a mean roughness of the surface near 1 #m. The following roughness parameters according to DIN 4762/DIS and DIN 4768 were measured at various positions on a measuring length of 0.42 m m : R a = 0.3 #m, Rp= 1.2 #m, /~ = 1.8 #m, Rh = 2.7 #m. The heat transfer measurements were always started at the highest pressure and heat flux to avoid hysteresis at initial nucleation. Then the heat flux was cautiously lowered at constant pressure without disturbing the state of the liquid within the evaporator. The phase equilibrium was determined by measurements of temperature, pressure, and composition of liquid and vapour, either in the evaporator for the heat transfer tests (Figure 1) or in the bubble chamber of a smaller circulation loop similar to the one in Figure 1, or in a static vapour pressure cell with agitation of the liquid by a micropump. The static cell was especially used to determine the critical state. A comparison of the data received with the different phase equilibrium cells should reveal systematic errors caused by the test equipment. The composition of vapour and liquid was analysed in a gas chromatograph, with a maximum error of +0.1 mol%.

0.2 ,e~

R11q

"[

0.2

.

. O.q . .

x ...--e

. 0. . 6 .

Y ~

¢

~

0.8

0.066

~

Phase equilibrium The critical line of R22/RlI4 is shown in Figure 2a,

1.0

c o m p a r e d w i t h d a t a f r o m literature. I t is seen that the

P* = P/Pc Figure3 Typicalpressuredependenceofthemolefractionintheliquid (x) and vapour (y) at intermediate compositions (middle) and at compositions near the pure components (highest and lowest curves) Figure 3 Influence typique de la fraction molaire sur la pression dans le liquide (x) et la vapeur 0I) a des compositions intermbdiaires (milieu) e t a des compositions voisines des composants purs (courbes supbrieure et inf~rieure)

critical temperature T~ deviates only slightly from the molar average of the critical temperatures of the pure components, while the critical pressure P~ reaches values u p t o ~ 2 bar higher than the molar average, in accordance with the results of Kabata e t al. 6 and Higashi The difference between T~ and the molar average T¢~aot,as well as the relative difference between Pc and the

e t al. 3.

8

"C t/O

o

, .~

¢0

'So

, -~ J" ~ l ~ I " ~~ ~ L j ~

I I I

-'So

Figure 4 (a) Vapour pressure as a function of temperature and composition of the liquid. O, Heat transfer evaporator; A , circulation bubble point cell; V , static vapour pressure cell. Temperature range, T < 40°C, fitted to data of Kiiver and Kruse 7. (b) Bubble point curves and dew point curves of the system R22/R114 at constant temperature, pressure and composition, respectively Figure 4 (a) Pression de vapeur en fonction de la temperature et de la composition du liquide. O , Evaporateur de transfert de chaleur ; A , cellule iz point de bulles a circulation; V , cellule de pression de vapeur statique. Domaine de temperature, T < 40°C, ajust~ aux r~sultats de Kiiver et Kruse 7. (b) Courbes du point de bulles et courbes du point de ros~e du syst~me R22/Rl14 ~ temperature, pression et composition constantes, respectivement

Rev. Int. Froid 1 9 8 8 V o l 11 J u i l l e t

259

Pool boiling of mixtures with R22 and R114. D. Gorenflo et al.

105

.

.

.

.

.

.

.

.

.i_*?~ 1

~"/,~ 5 2 q

.//~/ =

. ] ~

/...o.a

104

s

//~>/~

'~

1

~/ P* "

10000 W m-2

~

J

,..//~

~

//

2

104 . s

/~, _

/./ 5

o~ s

°o.2

, -

...~Z°.

102]J/'°,~

,

t0' 2

2 k, 103 7e ~=

.,

/_~

, ~1

equimolar composition (x = 45 mol ~o R22) are shown in Figure 5 for two normalized saturation pressures P * = P/Pc. The main qualitative results, already known from the literature about nucleate pool boiling of mixtures in the same pressure range 9-1., are as follows:

I

./ ,

,,

5 ,

.,

s 10~ 2

,

,

s 104 2

,

-eJ

mixture.

102

Each of the three effects is less pronounced when the mixture contains only a small amount of one of the components, as can be seen by comparing the systematic measurements for XR22 5 mol ~o (a) and 45 mol ~o (b) in

s 105

q [W rn-2)

fonetion du flux thermique, q, a deux pressions de saturation normalisbes differentes pour les composants purs et un melanye presque bquimolaire, A , R22," D, RII4; Q, R22/R114 (45mo1% R22)

preliminary interpolating equation for Pc Pc= 32.504+ 26.219x- 5.622x 2

are given in Figure 2b as functions of the mole fraction of R22. A correlating method of Chueh and Prausnitz ~2 for Tc represents the experimental data of this work quite well, if the fitting parameter zq is slightly dependent on the composition. It seems that the experimental scatter of the results found with the small static cell is less than that of the two circulation loops. Some of the data from References 3 and 6 coincide with the experimental values of this work, but most deviate systematically. Typical results of the composition in the liquid (x) and vapour (y) are plotted as functions of the normalized saturation pressure P* = P/Pc in Figure 3. The difference A(x' Y) vanishes at the critical p°ints and is greatest at the lowest pressures and intermediate compositions. The whole area of the measurements is shown in Figure 4a. It is extended to - 2 0 ° C and at temperatures between -20°C and +40°C, experimental data of Kiiver and KrusC are used for fitting the equation the plot is based on. Further calculations are currently under way to improve the equation, especially near the critical line. Figure 4b demonstrates the gap between the bubble point surface and dew point surface of R22/R114. It is seen that for x,y = constant and T= constant the gap is greatest at intermediate compositions and temperatures, while for P = constant, it is increasingly widening with decreasing pressure.

Heat transfer Typical heat transfer measurements with the pure components R22, Rl14 and a mixture with almost

Int. J. Refrig. 1 9 8 8 Vol 11 J u l y

straightlines in Figure 5 are smaller in the case of the

2

Figure 5 Typical results of the heat transfer coefficient, ~t, as a function of the heat flux, q, at two different n o r m a l i z e d s a t u r a t i o n pressures for the pure c o m p o n e n t s a n d a nearly e q u i m o l a r mixture. ~ , R22; 5 , Rl14;O, R22/R114(45mol~oR22) F i g u r e 5 Resultats typiques du coefficient de transfert de chaleur :t en

260

1. the absolute values of ~ are much smaller for the mixture than for the pure components at the same heat flux q and normalized pressures P*; 2. as with pure substances, the heat transfer coefficients at constant pressure may be interpolated fairly well by straight lines in a double logarithmic plot of a versus q; 3. the heat transfer coefficient for the mixture increases less with heat flux and saturation pressure than for the pure components: the slope and the distance of the

=

F i g u r e 6.

At low pressures and small heat fluxes, the domain of natural convection without bubble formation is reached, while at the highest pressure and at heat fluxes of ~ 30 kW m - 2, nucleation is intensive and n e a r t o burn out. Values for Figure 7a are obtained a t c o n s t a n t heat 2 a .

.

.

.

.

.

.

.

'° 4

.

__o..//~'

. . / ~/~--g'fff//:~/

~ o / / / / / 2 l°3 5

~

!

2 o 102~-" ,

~, ? E 104 b . ~ 5 ~ 21

~

. .

.

R22/R

.

.

.

~

. .

~

. .

114 5molto R22 Pc= 33.8 bar . .

. .

.

~

-~/~" ~..v.-~"..z__//A-~'~"

10 ~, / ~ ' ~ y 2~ ~ / o ~ ~"~-"" 102 | , . s 102 2

R22/Rl14 ,

s

10, 3

,

2

s,

45mO1~o R22 Pc= 42.5 bar . , ,

104 2

s

10s

q (w m-2) Figure 6 H e a t transfer coefficient, ct, as a function of the heat flux, q, for 5 m o 1 % R22 (a) and 45mo1~o R22 (b). P a r a m e t e r : n o r m a l i z e d s a t u r a t i o n pressure, P*. V , P * = 0 . 9 ; O, P * = 0 . 8 ; O , P * = 0 . 6 ; [3, P* = 0.4; A , P* = 0.2 F i g u r e 6 Coefficient de transfert de chaleur, ct, en fonction du flux thermique, q, pour 5mo1% de R22 (a) et 45mol% de R22 (b). Parambtre :

pressiondesaturationnormalis~e,P*.V,P*=O.9;©,P*=O.8;O,P*=

0.6;~,P*=O.4;A,P*=O.2

Pool boiling of mixtures with R22 and R114: D. Gorenflo et al. a

0.8

'

1..

0.6

0 7

b o.8

.

/

p*

25~54 mol ~o R22 0.4

0.6

' 5

'

q = 10000 W m -2

0-

0.4

• I

0...

0.8

I ~

~

0.9

0.4

'

~

•

~

'

'

'

'

'

q = 10000 W m -2

-

5

"

,

104

g/.

o.0

25 64 m o l ~ R22 103 10 - I

i 2

i 5 P*

=

103 100

0 RI 14

•

, 0.2

, 0.4

, 0.6

0.8

1.0 R22

P/Pc

x

Figure 7 Influence of pressure (a) and composition (b) on heat transfer coefficient at constant heat flux (10 000 W m - 2) and on the exponent, n, of the proportionality ctocq". (a): i , R22; 0 , R114; O, 5 m o l t o R22; [~, 92mo1~o R22; (R22) and - - . - - (R114), VDI-W~irmeatlas ~ Figure ? Influence de la pression (a) et de la composition (b) sur le coefficient de transfert de chaleur a flux thermique constant (10000 W/m 2) et sur l'exposant, n, de la proportionalit~ ctozq". (a): R , R22; Q , Rl14; ~ , 5 tool% R22; Q, 92mo1% R 2 2 ; - (R22) and - - . - - (RI ]4), VDI-Wdrmeatlas I T

flux(lO4Wm-Z)ffomFig ure 6 for the different mixtures investigated, to consider the pressure dependence of the heat transfer coefficient, a. At intermediate compositions, from 25 to 64 mol ~o R22, the relative increase of~ with P* is approximately five times less than for the pure components, with the values for compositions near the pure components (5 and 92 mol ~o) being in between. For the pure components smaller c~ values than the experimental results of this work are calculated by a design procedure explained in Reference 17 (dashed lines in Figure 7a), resulting in a rather conservative design of evaporators for higher pressures, The pressure dependence of the exponent n in the proportionality c~oc q" is also given in Figure 7a. The exponent decreases slightly with increasing normalized pressure for all substances, the values for the mixtures with intermediate compositions being smallest. Figure 7b shows the dependence of a and n on the composition of the boiling liquid. At all pressures

investigated, a flat minimum occurs for a and n at compositions near 50 mol ~o, which is markedly more pronounced at 90 ~ of the critical pressure than at 20 ~oA similar result has been found for the system SF6/R12(CFzCIE), with about the same distance for the critical temperatures of the pure components as in the case of R22/R 1141 o. The deterioration of pool boiling heat transfer to mixtures, compared with the pure substances, is often explained in literature by the fact that the vapour within the bubbles is enriched in the lower boiling component, and the liquid in the boundary layer near the heated wall is depleted of that component. According to Stephan and K6rner 18, the concentration difference between vapour and liquid, y - x , can therefore be combined with the excess temperature AT of the heated wall by

AT=q=AT~a+ATE

Rev. Int. Froid 1988 Vol 11 Juillet

(3)

261

Pool boiling of mixtures with R22 and R114: D. Gorenflo et al.

0.4

with Ah~, ).~, Pl, Cp= specific heat of vaporization, heat conductivity, density, and specific heat of the liquid, and D, ill, Bo=liq uid mass diffusivity, mass transfer coefficient, and fitting parameter. With both correlations, the coefficients for the mass transport, D and Bo/flt, respectively, were used as fitting parameters and were fitted to the experimental data at P* = 0.2, q = 10 000 W m - 2 (Figure 9), resulting in DR22= 4 x 1 0 - 1 ° m 2 s - 1 , DR114=4X10-~° m2s -1, and Bo/ 3t =0.7 x 10 +4 s m -1. It is seen that the correlating method of Schliinder (Figure 9) agrees comparatively well with the experimental values, within the whole pressure range investigated and for a considerable variation of the heat flux. Introducing a slight dependency of B o/ill on heat flux and pressure, the deviations would not probably be much greater than the experimental limits of error. The correlation of Thome is much more sensitive to variations of pressure and heat flux, thus the value D of the mixture

I

p,

/ZX....-A~,

0.2 r~.,.._m_4..._m~ 0.4 / _--o ~ o_-""'Q;,A 0.6/o~ov~_o~,_~'~ 0.8 i v v ~v ~ y c - - . ~ _ f f - ~ . , 0.9 v~..~

,- 0.2 ~" 0 1.0

0"8!2) ~ 0.6 p* 0.2

~

0.4

~.

,,

//

50° -o

o2 \v\( ' ~0 . 8 •

/a ~

o---.o

/

o~-------o

o.9_---0~o

/ P

o/o/, /7 / 7

--o--'~2/

,

. . . . . . . . .

2

p*

q = 1000 W m - 2 , 0

,

0.2

,

A

~

~

0.4

,

0.6

J

0.8

,

R114

104

.0 R22

////]]'lJ

,

Figure 8 Deterioration of ct and concentration difference y - x , as functions of the R22 mole fraction in the liquid Figure 8 D&krioration de ctet difference de concentration y x, en

5

fonction de la fraction molaire de R22 dans le liquide

2

, x

/ ~ ~ . . . . .

~"'/

~ ~ O - - - ' - ' ~ "

°

~.\""-

with

x4.\

I

~

or

,

--

=

-

0~id ~.~//R22

+

(4)

Rl14

The resulting, qualitative connection of the deterioration of ~ to the concentration difference A(x, y) is shown in Figure 8 for the ~-values at 10 000 W m - 2. It is seen that a comparatively close connection between the maxima of A(x, y) and the minima of C(/~idexists, but approaching the critical pressure' the deteri°rati°n °re increases' while the concentration difference decreases. Finally, the experimental results are compared with correlating methods of Thome ~3 and Schliinder ~4, considering the mass transport in the superheated liquid boundary layer near the heated wall:

5 ~t~D-"~

~=

~ e5

5

~id

-- 1

/{

l

(Yl--X1)~Dp~/

( C3~xTl) -

j

(5)

(Y1 - - X l ) ~ q

[

262

1-exp

(

-B 0

Int. J. Re/rig. 1988 Vol 11 July

~1~91~)1}

(6)

[

l

'T - ~ t - - ~ ~

, ,

~ ~ ~"

L "" ~ "- . . . . . . t 04 ~ . . sI i ~

0 "2

,0.9

/

,,

.

.

j

:

o

~ 0.2

0

R114

~--1 1-Ctid / L Schliinder

//

q = 10000 W m-2

2

10

Thome

// //1

/ . // /

Xo\ .

t 03

ATid=(xAT)R22dc(xAT)RI14

I 0.9

/

0.2

0.4

0.6 x

0.8

1.0 R22

Figure 9 Comparison of experimental results with the correlation Equations (5) and (6) for two heat fluxes and the highest and lowest saturation pressure investigated. - - - , Data from Thome, Equation (5)13; - .-, data from Schliinder, Equation ( 6 ) 1 4 ; - - , experimental results, this work Figure 9 Comparaison des rbsultats expbrimentaux, avec les bquations de corrblation (5) et (6) pour deux flux thermiques et les pressions de saturation laplus~levkeetlaplusbassebtudi~es.--,R~sultatsde Thome, bquation (5)12,' . . . . , rbsultats de Schliinder, bquation (6)1'*," ,rbsultats expbrimentaux de la pr~sente btude

Pool boiling of mixtures with R22 and R114: D. Gorenflo et al. s h o u l d n o t o n l y d e p e n d o n DR22 a n d DRll4 , b u t s h o u l d also c o n t a i n a s t r o n g d e p e n d e n c e o n t h e h e a t flux a n d

7

pressure.

8

Acknowledgements Thanks are due to Deutsche Forschungsgemeinschaft,

9

Bad G o d e s b e r g , for f i n a n c i a l s u p p o r t o f p a r t of this i n v e s t i g a t i o n a n d t o H o e c h s t , Ag., F r a n k f u r t , a n d K a l i

Chemie Ag., Hannover, for supplying the test fluids. The

10

a u t h o r s a r e i n d e b t e d to D i p l . - I n g . F . - J . H e s s e for his h e l p

in the final preparation of the paper.

!1

References 1

2 3 4

5

6

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