A flow boiling critical heat flux correlation for water and Freon-12 at low mass fluxes

Nuclear Engineering and Design 72 (1982) 381-389 North-Holland Publishing Company

381

A FLOW BOILING CRITICAL HEAT FLUX CORRELATION FREON-12 AT LOW MASS FLUXES

FOR WATER AND

W.J. G R E E N AAEC Research Establishment, Lucas Heights Research Laboratories, Private Mail Bag, Sutherland, NSW, 2232, Australia

Received 18 May 1982

An empirical correlation has been developed for calculating critical heat flux (CHF) at low mass fluxes for vertical upflow in uniformly heated tubes. The correlation is based upon dimensionless groups. It compares favourably with experimental CHF data for both Freon-12 and pressurized water. When solved iteratively in conjunction with the heat balance equation, an overall mean ratio of predicted to experimental CHF of 0.986 was obtained with a root means square (r.m.s.) error of 7.0%, for the 233 low flow rate data sets examined. The boundary between the high flow rate correlation developed in earlier work and the proposed low flow rate correlation can be specified by a dimensionless factor 8 I. For values of 8~ greater than 0.07, the low flow correlation is valid whereas for values less than 0.07 the high flow correlation applies. Development of this correlation and a means of defining its range of validity enables the prediction of CHF levels to be made over an increased range of coolant flow conditions. This is important in the analysis of postulated loss-of-coolant accidents in water-cooled nuclear reactors.

1. Introduction Over the past few years, the Australian Atomic Energy Commission has supported a program of experimental and theoretical work which has been designed to enable better understanding and predictions of the fluid and thermal responses associated with a postulated lossof-coolant accident (LOCA) in a water-cooled nuclear reactor. One facet of this program has been that of investigating the heat transfer characteristics associated with the coolant changing from pre-dryout to postdryout conditions [1]. As a part of these investigations, effort has been directed toward developing a more general, accurate critical heat flux (CHF) correlation which would be suitable for inclusion in computer codes for thermal transient calculations. An empirical formula has been reported [2,3] which is capable of accurately correlating extensive data for both pressurized Freon-12 and water in uniformly heated vertical tubes. However, for the limited amount of data in which mass fluxes were less than approximately 300 kg s - l m - 2 , the correlation was found to be less accurate. It was considered, therefore, that a more intensive study should be made of low flow rate data, using techniques similar to that already reported [2]. 0029-5493/82/0000-0000/$02.75

In this investigation, consideration has been given to the concept, first postulated by Macbeth [4], that C H F data may be divided into two flow regimes (low flow rate and high flow rate), similar to the laminar and turbulent regimes of single phase flow. The establishment of two such distinct regimes should then lead to the definition of a suitable criterion for identifying the boundary between these flow regimes.

2. Development of correlation Low flow experimental C H F data from various sources [5,6,7] were analysed at different stages of the development of the low flow C H F correlation. The same calculational techniques (i.e. computer programs) were used to evaluate the non-dimensional groups defined in earlier work [2]. The data were then examined to determine, for as many groups as possible, relationships between C H F number and the dimensionless groups, each group being considered in turn at conditions where all other dimensionless groups were constant. However, to minimise such extraneous influences as rig to rig variations, data from one test facility were examined independent of data from other sources, whenever feasible.

© 1982 N o r t h - H o l l a n d

382

W.J. Green / A flow boiling critical heat flux correlation

2.1. Reynolds number

Under experimental conditions in which - the coolant pressure is maintained constant, the tube is uniformly heated, and only the flow rate and inlet enthalpy are independently varied, where it is assumed that fluid physical properties are at saturation conditions, C H F data sets can be expressed as four dimensionless groups, namely -

-

/x~ ;

~-J

v;

2.2. Saturation boiling length ratio

L / D ; and L s / D . '

The data of Stevens, Elliott and Wood [5] for Freon12 at a coolant pressure of 1.07 MPa cover reasonable ranges of low mass flux, inlet subcooling, tube diameter and heated length. Using these data, it was possible to investigate the relationship between C H F number and vapour Reynolds n u m b e r when all other dimensionless groups were effectively constant. Fig. 1 illustrates the results of plotting C H F number against vapour Reynolds number for a range of saturation boiling length ratios ( L s / D ) and tube bores. From this figure, and the ,

DLS/D I00C

--

i

,

,

,

= 60

=8-5,11.5,16.1

~IB -u~

/

Ls/D =142 Ls/D =173 D= 8 S~ I / / D = 1 6 1

Ls/D=123 /

co

results of similar analyses of water data obtained at a pressure of approximately 3.4 MPa [6], it is clear that (i) the C H F number is directly related to the vapour Reynolds number for the tube bores and saturation boiling length ratios considered, i.e. 0N (the C H F number) cc Rev, and (ii) tube bore, as an independent entity, does not influence the relationship between critical heat flux number and vapour Reynolds number at a particular saturation boiling length ratio.

The data of Stevens, Elliott and Wood were obtained at one pressure, so the influence of density ratio and Prandtl n u m b e r could not be ascertained. Since tube bore appears to have no influence on the 0N versus Re v relationship at a constant saturation boiling length ratio, it was deduced that there was most probably no direct significant effect of aspect ratio (i.e. L / D ) or surface tension number (o N is proportional to L / D at constant pressure). Consequently, the relationship between critical heat flux number and saturation boiling length was examined. Fig. 2 shows that for both Freon-12 and water, the C H F number is inversely proportional to the saturation boiling length. Because both Freon-12 and water data appear to be correlated by exactly the same equation, it would appear that neither density ratio nor Prandtl number has any significance in the correlation. The C H F number can therefore be correlated by the equation ON = A R e v D / L s ,

/

where A, determined from fig. 2, is 0.248. Further, this equation can be reduced to

d .< s

U 1OC

= Az/Ls/D= 307

)ffJ [3/ ~ -

50

It is interesting to note that Macbeth [6] correlated low flow data by the equation

Coolant Freon -12 Source of experimental data Stevens, Ettiott z Wood [8]" Tube bores 8-5.115,16 1 mm CootQnt Pressure 1,O7 MPQ

I

I

I

I

I

I

10 5

I

I

0 = 0 . 2 3 2 ~ - + 2 - 1 5 ( ~ L ~ \ 2 / 3 ) Ah,

(2)

where parameters are in Imperial units. For conditions in which the inlet subcooling is zero, Macbeth's equation reduces to

10 6 VAPOUR REYNOLDS NUMBER ( Rev}

Fig. 1. Relationship between CHF number and vapour Reynolds number.

0 = 0.232XG/L~.

(3)

In discussing the implications of eq. (3), Macbeth combined it with the heat balance equation (i.e. O =

W.J. Green / A flow boiling critical heat flux correlation 1000

'

~ '

'

'

'

'

'

'

I

'

'

383

'

ds Number : 10 5

G cr

z d kt. LO 7_J < L) F-

•

Freonm12at 1.O7 MPa [5]

•

Water at ~ 3-4 MPa [6]

•

Water at ~ 7 . 0 MPa [7]

I~ %

1OC

£9

50

I

I

I

I

I

I

I

I

I

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I

1OO

10

I

500

SATURATION BOILING LENGTH RATIO (Ls/D)

Fig. 2. Relationship between CHF number and saturation boiling length ratio.

GDXX/4L s) and derived the surprising result that "irrespective of geometry, the mass velocity or the pressure, if greater than 200 psia, burnout, when Ah = 0, will occur only when the steam quality reaches approximately 93%" [6]. In reality, eqs. (1) and (3) show that for these low flow conditions, the use of dimensional analysis techniques results basically in the derivation of the heat balance equation. Such a correlation is of little use when CHF values have to be accurately predicted from a knowledge of mass flow rate, tube geometry, coolant inlet pressure and inlet subcooling only. As an aside, it should be noted that in the case of the high flow correlation previously developed [2], this problem did not occur because the CHF number was not a direct function of vapour Reynolds number. 2.3. Inclusion of a defect factor The problem that arises is how to formulate a precise low flow correlation which is similar in form to the heat balance equation and yet capable of being used in conjunction with the heat balance equation to predict CHF values accurately. To overcome this difficulty, eq. (1) was used as the basis for a correlation, since its ability to correlate experimental data reasonably well had been established,

but it required two modifications: the constant 0.248 was set at 0.25, to match the constant in the heat balance equation, and - a positive 'defect' factor was included in the following manner -

D 1 q~N =0.25 Re,,Ls (1 + 8 ) "

(4)

The significance of making these modifications is that when eq. (4) is solved in conjunction with the heat balance equation (namely ~ = GD?~X/4Ls), it can be shown that the critical quality X is equal to 1/(1 + 8 × P~/Pv), an expression which signifies that X must lie between limits of 0 and 1. The next objective was to determine the relationship between the defect factor and the various dimensionless groups. This was achieved by first calculating values of the defect factor 8 from eq. (4) using experimental values of q~. After that the procedure was to examine the relationship of 8 with other dimensionless groups either by considering each dimensionless group in turn at conditions where all other dimensionless groups were constant, or by normalizing the defect factor with regard to one or more dimensionless groups in order to determine the -

-

W.J. Green / A flow boiling critical heat Jlux correlation

384

dependence of the normalized group on each of the remaining dimensionless groups.

2.3.1. Influence of low flow number on defect factor Because in the earlier work [2] it had been found that

.

' D :

3~

7

L~

,q

low flow effects were related to the dimensionless group

Go/(~pdh),

the relationship between this group and

L:

g

the defect factor was examined first. This was done by

taking the results of Stevens, Elliott and Wood [5] and examining data in which all dimensionless groups excepting Go/(~kpl/.tl), (henceforth denoted as low flow

10

×

3

number (LFN)) were constant. Although only a limited

NormoLis

amount of data fulfilled these requirements, fig. 3 shows

fact°r

°c ( L s / D )

P65 ~

X

'

L~/D : 1}8

that the defect factor is proportional to the low flow

number in the following manner: 8 cc exp(0.25 X 105 L F N ) . 104

I

,

I

J

i i

i~l

10

2.3.2. Influence of aspect ratio and saturation boiling length ratio on defect factor Having estimated the influence of the low flow number on the defect factor 8, the next dimensionless groups to be considered were those of aspect ratio and saturation boiling length ratio. To determine any relationship

between these groups, it was first necessary to normalize the calculated defect factor by dividing it by exp(0.25 × 10 5 LFN) and then to examine the relationship of this normalized group with saturation boiling length ratio ( L I D ) at various constant aspect ratios ( L / D ) . Again the data of Stevens, Elliott and Wood were used. Fig. 4

I

1

I

i

CooLant Freon -12 Pressure : 1 0 7 M P a Aspect Ratio :178 S a t u r a t i o n Boding L e n g t h

i

i

i

100 SATURATION

BOILING

LENGTH

RATIO

I I I i 100(

(Ls/D)

Fig. 4. Relationship between defect factor and saturation boiling length ratio.

shows that, bearing in mind the paucity of the data and the magnitude of the 8 values, to a fair approximation 6 / e x p ( 2 5 0 0 0 L F N ) cc [ L J D ]

--I.65

By determining, from fig. 4, the normalized defect factor values for each aspect ratio at a value of L , / D of

i

'0; Z

R a t i o : 173 Ln r~ ©

uo

ff a:" 0 " 0 1

,5 1<.}: u

S ,p, O'OO1

4

I

I

5

6 LOW

I

7

I

8

FLOW N U M B E R I G O ' / P e ~ ,

I

9

1Ox 10-5

), )

Fig. 3. Relationship between defect factor and low flow number.

5Z 10 -L

I 10

I

I

i

i I I i I 10,0

ASPECT

RATIO

I

I

I

J l I..L..,L-

L/D)

Fig. 5. Relationship between defect factor and aspect ratio.

W.J. Green / A flow boiling critical heat flux correlation 100 and replotting these data as shown in fig. 5, it was feasible to estimate the relationship between normalized defect factor and aspect ratio. As can be seen in fig. 5 to a fair approximation, this relationship is

2.4. Proposed correlation In summary, the proposed low flow correlation can be expressed as D 1 , ~ =0.25 RevL s (1 + 8 ) '

8/exp(25000 LFN) E ( L / D ) ' " 2 . It should be emphasised that the indices of both L s / D and L / D can be considered only as estimates and may require reoptimization when more experimental data are available.

where ~ = 0.046(~_)1'12(

2.3.3. Influence of density ratio on defect factor The influence of the density ratio on the defect factor is particularly difficult to identify since density ratio is pressure dependent and changes in pressure affect all of the relevant properties of the coolant, thereby altering many of the dimensionless groups. Furthermore, CHF data from three different sources had to be considered to find this relationship: - Stevens, Elliott and Wood [5]--Freon-12 data at - 1.07 MPa, - Macbeth [6]--Water data at - 3.4 MPa, - Thompson and Macbeth [7]--Water data at - 10.3 MPa. Despite these difficulties, fig. 6 shows that the relationship between the defect factor and density ratio can be approximated by 8 cx e x p ( - 0.038pl/pv). Furthermore, if the effect of Prandtl number is assumed to be negligibly small, and the influence of all dimensionless groups on 8 has been considered, fig. 6 could be used to derive the equation 8 = 0.046 exp( - O.038Pl/O v ) [ L \l.12[ Ls

×exp''°°° O"1

,

i

I ~--~

• ~ ~/

1

="

~

tq

~- Z tD ta.~

,

i

i

I

1.65

i

,

i

,

• Data of Thompson ~ Macbeth [7] X D o t o o f Stcvcns, Elliott &Wood [51 • Data of Macbeth [6]

u_~

N or x ~

001

~ff

--

o~z O.OO1 O

1 • I• q[ P f f - ' ~

X x X

I 10

I 20

g

I 30

N. . . .

385

~ised 6 = O O 4 6

I I I I I 40 50 60 70 80 DENSITY RATIO (Pf/l~

/pv )

I ~ I 9 0 1OO 120

v )

Fig. 6. Relationship between defect factor and density ratio.

130

D ) 1'65

~Lss

exp(0.25 x 105 LFN

-O.O38p,/pv) and LFN -

Go piP.i) k "

3. Comparison

of calculated

and experimental

CHF

val-

ues

Having derived a correlation based upon selected data from the experimental results given by Stevens, Elliott and Wood [5], Macbeth [6], and Thompson and Macbeth [7], to assess the correlation's validity, it was necessary to compare calculated and experimental CHF values for all the low flow data given in these references. Two calculational approaches were used: - Values of CHF number were calculated with the correlation given in eq. (4) but based upon values of saturation boiling length and mass fraction corresponding to the experimentally determined values of CHF. The calculated CHF values are compared with experimental CHF data in table 1. - CHF numbers were predicted from a knowledge of mass flow rate, tube geometry, inlet subcooling and inlet coolant pressure by iteratively solving eq. (4) and the heat balance equation. Pressure losses along the tube were estimated using a homogeneous model and included head, acceleration and frictional components. Comparison of values of CHF number determined by this procedure with experimental data are given in table 2. Also given in table 2 are comparisons of experimental data with values of CHF number derived from two low flow correlations proposed by Macbeth [6,8]. Table 2 shows that although eq. (2) is marginally more accurate than the present correlation for the data from which Macbeth developed his correlation [6], it is slightly less accurate for other water data and represents Freon-12

W.J. Green / A flow boiling critical heat flux correlation

386

Table 1 Comparison of calculated and experimental C H F values Source of data

Coolant

Stevens, Elliot t & Wood [5]

Freon-12

Macbeth [6]

Water

Thompson & Macbeth [7] (table 8) Thompson & Macbeth [7] (table 6)

Pressure (MPa)

Exit quality

Tube bore (mm)

Heated length (mm)

Mass flux (kg s "l m -2)

Number of data analyzed

Mean ratio of calculated to exptl. CHF

r.m.s. error value 1%)

0.38 to 1.00

8.5; 11.5; 16.1

650; 890; 1300; 2860

200 to 400

120

0.998

0.61

3.1 to 4.0

0.85 to 1.00

7.8 and 9.9

1040 to 3120

140 to 450

26

0.995

0.13

Water

10.3

0.77 to 1.22

4.6

240

40 to 45

12

1.000

1.08

Water

6.9

0.77 to 1.05

4.6

240

40 to 100

18

0.999

0.80

1.07

Table 2 Comparison of predicted and experimental C H F values Source of data

Number of data analyzed

Mean ratio of predicted to exptl. C H F

r.m.s, error value %

Comments

Present work

Using eq. (2)

Using Macbeth [81

1.03

8.8

20.3

11.2

Present work

Using eq. (2)

Using Macbeth [81

Stevens, Elliott & Wood [5]

120

0.97

1.08

Macbeth [6]

26

0.99

0.98

1.00

4.5

3.1

1.4

0.98

7.3

7.4

6.3

Excluding data where X>I.0

0.99

8.3

9.3

7.7

Excluding data where X>I.0

Thompson & Macbeth [7] table 8

11

1.03

1.05

Thompson & Macbeth [7] table 6

14

1.04

1.04

Bailey [9] 6.9 MPa

31

0.99

2.3

Bailey [9] 4.1 MPa

31

1.00

2.4

233

0.986

7.0

Overall mean values

W.J. Green/ A flow boiling critical heatflux correlation data poorly. Macbeth [8] also proposed another correlation of the form: q,=

G(X + Ah) 0.135~- + 4 L / D '

where again the parameters are in Imperial units. As can be seen in table 2, comparison of predictions using this latter correlation with experimental data indicates that it is more accurate than the present correlation for all water data but less accurate for Freon-12 data. Scrutiny of the performance of both Macbeth correlations, however, shows that their accuracy decreases rapidly as the critical quality decreases. This observation is important since, as can be seen in table 2, experimental data have been obtained at nominally low flow rates in which the critical quality is as low as 0.77 for water and 0.38 for Freon-12. Examination of the remaining information given in tables 1 and 2 shows that for the present correlation, the r.m.s, error in predicting CHF values is approximately an order of magnitude greater than for CHF values calculated using mass fractions and saturation boiling lengths derived from experimental data. This observation illustrates that since the proposed correlation is very similar to the heat balance equation, it is the accuracy to which the difference between these equations can be correlated which is important. Precise prediction of the defect factor 8 is essential and any inaccuracy in the correlation of this parameter significantly affects the r.m.s, error of the predicted CHF values.

to extend the high flow correlation down to mass flux values of - 300 kg s-1 m-2. This dimensionless group (81) was 0.75 exp{ - 1 3 0 . 5 ( L ) ( p ~

)exp(5 exp( - 0 - 0 2 L ) ) }

When the ratio of calculated to experimental CHF was plotted against the group 81 , it was proved that the low flow correlation was valid for data where 81 was greater than 0.07. For smaller values of 81 , corresponding to higher mass fluxes, the correlation became increasingly inaccurate as can be seen in fig. 7; the magnitude of the inaccuracy was dependent upon flow quality and saturation boiling length. Earlier work [2,3] indicated that the high flow correlation was valid for values of ~l of between 0 and 0.2. It is therefore reasonable to deduce that a suitable limiting criterion for 81 is 0.07; for values below this, the high flow correlation is recommended and for those above the low flow correlation should be used.

5. Comparison of experimental and predicted CHF values for water data which span the transition from low to high flow regimes

Bailey [9] has obtained CHF data for pressurized water at flow rates which range from 50 to 1400 kg s - 1 m -2. Such data are particularly suitable for making comparisons between predicted and experimental CHF values since they involve mass flow rates which:

I

4. Boundary between high and low flow CHF correlations

As Macbeth [4] implied, separate and quite different C H F correlations apply for high and low mass fluxes. Scrutiny of water and Freon-12 data indicates that the transition between these regimes occurs at mass fluxes of approximately 300 kg s -1 m -2. However, this boundary also appears to be influenced to a lesser extent, by the geometry of the system, the type of coolant and the coolant pressure. Attempts were made using the full range of mass flux data investigated by Stevens, Elliott and Wood [5] (200 to 4070 kg s - 1 m - 2) to determine some means of distinguishing between the high and low flow regimes. This was done by plotting the ratio of the experimental CHF to the value calculated from eq. (4) against various dimensionless groups. The most appropriate group to describe the transition from high to low flow was one used in earlier work [2]

387

I

,

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C o o t o n t ffreon 12 Pressure : 1 0 7 MPo Saturation E~oilincj Length 100 ~150

I

Ratio:

0 I0 -I LJ Z ©

•

j

.\] IO-2

, 088

""T I 090

"'--I I ~7"~I092 094 096

I

[ 098

I

[ I

RAT[O OF CALCULATED C H F / EXPERIMENTAL C H F

Fig. 7. Determination of transition from low to high flow.

388

W.J. Green / ,4 flow boiling crittcal heat flux correlation

x

i Experimental

•

Predicted

1.046 and the corresponding r.m.s. 9.0%. Unlike the comparisons made with data obtained at 6.9 MPa, the high flow correlation consistently overpredicts experimental data in the high flow regime for these lower pressure data. This may be attributable to the assumption of homogeneity becoming less valid as water pressure decreases. (The assumption of homogeneity is an integral part of the high flow correlation.) The comparisons between predicted and experimental CHF values for those data accredited to the low flow regime have been included in table 2.

results

results

Coolant Pressure 6 9 M P o Source of Experimental D a t a N BaKey [9] A E E W R 1 0 6 8 10

I Catcutated Trans~Uon ix From High to Low Ftow CorreLation

08

:~

\ \ \ aebeth [8]

8

\

£) P

6. Conclusions

o 06

rm s error of PreOcted / ExperimentaL CHFs 3 7°/o Mean VaLue of Ratio o f Predicted t o Experimental CHF 1 O O 9

X X ~

~ \

04 O

I 500

I 1OOO

15OO

Based upon dimensionless groups and the concept of a defect factor, an empirical CHF correlation has been developed which is consistent with experimental data for uniformly heated tubes internally cooled by either Freon-12 or water at low mass fluxes (less than - 300 kg s + m-2). The proposed correlation is

G (kg / m 2 / s )

Fig. 8. Comparison of predicted and experimental critical qualities•

OD

(D)

where (i) come within both of the postulated high and low flow regimes, and (ii) systematically span the change from one regime to the other, thus enabling the verification that, for water, the boundary between the two regimes is consistent with 61 being equal to approximately 0.O7. Fig. 8 shows a comparison of predicted and experimental critical qualities for the range of mass fluxes investigated at 6.9 MPa. Predictions arising from the Macbeth correlation [8] are also included. There is reasonably good agreement between predicted and experimental critical quality values when the present correlation is used. Moreover, the calculated mass flux at which transition from low to high flow regimes occurs is much lower than that predicted by Macbeth but appears to be in better agreement with the experimental data. A similar comparison with experimental data given by Bailey [9] for a pressure of 4.1 MPa indicates that predicted transition from low to high flow regimes occurs at approximately 285 kg s - l m-2. This is again considerably lower than the value indicated by Macbeth but is in better agreement with the experimental data. For the whole of the data obtained at 4.1 MPa the mean value of the ratio of predicted to experimental CHF is

8 = 0 " 0 4 6 ( L ) ' " 2 ('Lss D ) TM

X exp(0.25 X 105 LFN - 0.038 0~ ) and Go

LFN = - -

pi/~lX

•

This correlation has been used in conjunction with a heat balance equation to predict CHFs for coolant conditions at which experimental data are available, namely: - Freon-12 at 1.07 MPa or water at - 3.4, 4.1, 6.9 or 10.3 MPa; and - a limited range of tube geometries and coolant exit qualities. The mean ratio of predicted to experimental CHF for 233 low flow data examined is 0.986 and the r.m.s, error 7.0%. It was also found that the boundary between the high and low flow regimes corresponds to the geometrical and coolant conditions which apply when the complex dimensionless factor 6~ is equal to 0.07. That is, for

W.J. Green / A flow boiling critical heat flux correlation values of 81 greater t h a n 0.07 (corresponding to low flows), the low flow correlation p r o p o s e d here applies.

389

Subscripts v 1

vapour liquid

Nomenclature References A CHF D G Ah k L Ls Pr Re v V X 8 81 P U tY N

X

C o n s t a n t defined in text specific heat critical heat flux tube bore mass flux inlet subcooling thermal conductivity total heated length saturation boiling length Prandtl n u m b e r = Cp/x/k vapour Reynolds number = DVvpv/pv = D V p v / # v (for slip ratio of l) velocity mass fraction (quality) defect factor dimensionless factor associated with low mass fluxes density viscosity surface tension surface tension n u m b e r = O/pvDX latent heat of vaporization critical heat flux critical heat flux n u m b e r = q~D//tvX

[1] W.J. Green and K.R. Lawther, An investigation of transient heat transfer in the region of flow boiling dryout with Freon-12 in a heated tube, Nucl. Engrg. Des. 55 (1979) 131-144. [2] W.J. Green and K.R. Lawther, A flow boiling burnout correlation for water and Freon-12, Nucl. Engrg. Des. 67 (1981) 13-25. [3] W.J. Green and K.R. Lawther, Application of a general critical heat flux correlation for coolant flows in uniformly heated tubes to high pressure water and liquid nitrogen, 7th Int. Heat Transfer Conference, Munich 1982. [4] R.V. Macbeth, Burnout analysis, Part 2: The basic burnout curve, AEEW R167 (1963). [5] G.F. Stevens, D.F. Elliott and R.W. Wood, An experimental investigation into forced convection burnout in freon, with reference to burnout in water, AEEW R321 (1964). [6] R.V. Macbeth, Burnout analysis, part 3: The low velocity burnout regime, AEEW R222 (1963). [7] B. Thompson and R.V. Macbeth, Boiling water heat transfer burnout in uniformly heated round tubes: a compilation of world data with accurate correlations, AEEW R356 (1964). [8] R.V. Macbeth, Burnout analysis, part 4: Application of a local conditions hypothesis to world data for uniformly heated round tubes and rectangular channels, AEEW R267 (1963). [9] N. Bailey, Dryout and post dryout heat transfer at low flow in a single tube test section, AEEW R1068 (1977).