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ELSEVIER

PII: SO950-4230 96)00021-6

J. Loss /‘rev. Process Ind. Vol. 9. No. 4. pp. 285-293, 1996

Copyright 0 19 96 Elsevier Science Ltd

Printed in Great Britain. All rights reserved

0950-4230/96 15.00 + 0.00

Blowdown of carbon dioxide from initially

supercritical conditions

Bernhard Gebbeken and Rudolf Eggers

Technische UniversiCit Hamburg-Harburg, Arbeitsbereich Verfahrenstechnik II,

Eissendorfer Strasse 38, 21073 Hamburg, Germany

Received 1 October 1995

The paper presents experimental investigations of the thermohydraulic phenomena of top

vented blowdown processes of initially supercritical carbon dioxide. The initial fluid conditions

were chosen such that flashing occurred after saturation conditions were reached. The investi-

gations were focused on pressure and temperature transients. It was observed that the pressure

at which flashing occurred first depends mainly on the initial fluid conditions due to the almost

isentropic change of state during the supercritical/subcooled blowdown. Furthermore, void frac-

tion profiles along the axis of the vessel were measured by means of a gamma densitometer.

The void fraction profile is influenced strongly by phase separation effects. Various stages of

characteristic void profile were observed. Copyright 0 1996 Elsevier Science Ltd

Keywords: vessel blowdown, flashing, phase distribution, supercritical carbon dioxide

The blowdown phenomenon, amongst other transient

release processes, is a subject of particular interest to the

chemical, o il/gas, and power industries. Accurate p redic-

tion of the releasing process is importan t in determin ing

the consequ ences of an accident, in particular the spill-

age of material and the safety of the equipmen t. If the

maximum operating pressure of the processing equip-

ment is exceeded due to an exothermic chemical run-

away reaction, or exterior or interior heat sources, a

pressure release is unavoidable. During a blowdown pro-

cess production plants are exposed to fast pressure an d

temperature transients as well as to phase transitions.

Thus hazardous situations can occur due to blowdown-

induced dynamic loads, to the very low temperatures

generated within the fluid during depressurization, and

to the formation of hydrates which blo ck the venting line

if free water is present. In order to provide accurate pre-

diction of the blowd own process the transient p ressure,

temperature, and void fraction profile have to be calcu-

lated. Intolerable uncertainties are caused if flashing or

desorption occurs during blowdo wn so that a two-phase

flow develops. Appropriate models m ust be applied to

consider the non-eq uilibrium effects between the

coexisting phases and the dynamic behaviour of multi-

phase flow. Thus to guarantee a controlled blowdown

without any unexpected operating conditions, blowdo wn

scenarios have to be investigated to supply d ata for the

appropriate design of chemical processing equipment.

During the last two decades research on chemical

processes using supercritical fluids has increased con-

siderably. New production processes such as extraction

with supercritical solvents and chemical reactions in

supercritical fluid environm ents (polymerization, free

radical halogenatio ns) have been developed. In this field

of application supercritical carbon dioxide is frequently

used due to its solvent pow er for non-polar substances.

Processes using supercritical fluids are usually carried

out under high pressure conditions and often batchw ise.

Hence blowdown predictions must be provided for

safety considerations.

So far no experimental blowdown data are available

for pressure release of typical sup ercritical solvents.

Therefore, experimental investigatio ns on top vented

blow dow n processe s of initially supercritical carbon

dioxide have been carried out. Initial conditions were

chosen such that flashing occurred during the pressure

release. The aim of this paper is to present detailed infor-

mation about the two-phase thermohydraulic phenom-

ena, particularly the transient pressure beh aviour and

phase distribu tion during p ressure release from a 50 litre

vertical pressure vessel. Various initial fluid conditions

have been applied. Furthermore, the phase distribution

obtained from the experimen ts is compa red to those

obtained from blowdown processes from initially satu-

rated two-phase conditions.

Blowdown phenomena

Blowdown of pressure vessels and gas pipelines are of

great importance to the chemical and oil/gas industries

as pressure release can occur during a hazard, for main-

tenance reasons, or even as a part of the process cycle.

Haque

et al.’

investigated ex perimentally the tempera-

ture distribution within large pressure vessels of various

geometry during pressure release using nitrogen and

285



286

Blowdown of carbon dioxide: B. Gebbeken and R. Eggers

mixtures of nitrogen and carbon dioxide. Initial con-

ditions were varied to be subcritical as well as supercriti-

cal (PO=

15 MPa), the fluid conditions during the blow-

down w ere gaseous an d two-phase due to condensing.

Full scale depressurization of a section of a riser plat-

form using natural gas was conducted by Evanger

et al.*

in order to measu re th e outer surface temperature of the

pipeline. E xperimen ts of initial fluid pressures up to

pa = 8.8 MPa were carried out. Two-phase phenomena

during the pressure release were not reported. Theoreti-

cal work in this area was carried out by Chen et ~1 ~

A hyperbolic finite difference method u sing separated

momentum equations was proposed to calculate the two-

phase blowdown of single- and multi-component mix-

tures from pipelines.

In the nuclear pow er industries the hypothetical loss

of coolant accident (LOCA) is one of the most signifi-

cant aspects of the design and test of the emergency

cooling system of a reactor. H ence blow dow n of the

cooling circuit of pressurized water reactors has been

extensively investigated 4. Extremely rap id depressuriz-

ation of long horizontal pipes containing initially su b-

cooled water was investigated by Edwards and O’Brien’,

and later by Alamgir

et ~1 ~ New

theories have been

proposed to explain the pressure disturbances and the

violent phase change effects. Theoretical investigation s

on blowdown processes of long pipes have been

accomplished by Krause’. A one-dimensional mixture

model was proposed to be solved u sing the method of

characteristics. Applying the pipe problem of Edwards

and O’Brien the model was verified.

Further inve stigations were carried out for blow-

down processes from initially saturated conditions. This

kind of initial condition is indicated by State A in

Figure 1

The vessel contains saturated liquid with satu-

rated vapour on top. After the top vented blowdown is

started only vapour is being b lown off so that the press-

ure inside the vessel decreases. Subsequ ently, the boiling

process within the saturated liquid starts to develop. The

liquid-vapo ur interface rises due to the growth of vapour

bubbles. Two-phase flow venting occurs when the

liquid-vapo ur interface reaches the top of the vessel.

T = TCn

I

I

I

A ,J’

p < kit

A

-73

’

S'

EC

I

apor

,’

I’

Specific entropy

Figure 1 Temperature-specific entropy diagram, various initial

fluid conditions for pressure release

Fundamental research in this area has been done by

Mayinger’. Most of the published papers concentrated

on the thermohy draulics in the depressurizatio n pipe.

Investigations on the system behaviour inside the press-

ure vessel were done by Friede19 who focused on the

influence of physical properties on the boiling delay.

Transient wall heat transfer in a pressure vessel during

pressure release of carbon dioxide from initially satu-

rated conditions has been investigated experimentally by

Eggers and Green

.

O

Theoretical and experimen tal inves-

tigations of blow dow n processes from vertical vessels

using saturated w ater were performed by Friz”, and by

Prasser’*. Both authors focused on the transient void

fraction profile alon g the axis of the vessel an d the level

swell dynamics. Similar experiments were conducted by

Friedel and Purp~‘~ using the refrigerant R12 as the test

fluid. Thies and Mewesi4 investigated the axial phase

distribution for blowdown of a water/carbon dioxide gas-

desorbin g system in a vertical vessel. It wa s discovered

that the axial void fraction profile is strongly influenced

by phase separation processes. Research on the separ-

ation of the phas es due to different phase velocities dur-

ing blowdown was carried out by Viecenz15. To consider

phase se paration effects in two-ph ase flow the drift flux

model was proposed by Zuber and Findlay16. The volu-

metric flux density of the mixture j is defined by

j = &V% (1 - &)Vl.

(1)

Introducing a radial distribution parameter Co the

weighted mean gas velocity can be expressed as

is=

Coj

+

V,,,

(2)

where ig and idrift are the mean value w eighted by the

volume concentration of the gas velocity and the drift

velocity, respectively. The value of the radial distri-

bution parameter Co is usually close to 1. However, Co

and id,+, can be expressed by empirical correlations such

that they depend only on the physical properties of the

fluid and on the two-p hase flow pattern. For vertical bub -

bly flow which is expected for blowd own of pressure

vessels the following general correlation for the drift

velocity is proposed 16

-

-* U P

drift =

vdrift

[-I

:

i i, =

1.18 - 1.53. (3)

From th is equation it becomes evident that the drift velo-

city strongly d epends on the difference of the phase den -

sities and on the surface tension. The surface tension in

turn strongly affects the size of the bubbles.

Con sidering depressurizatio n of initially supercriti-

cal fluids various p rocess characteristics can be observed

depend ing on the initial conditions. Possible initial con-

ditions are indicated by State B and State C in

Figure 1

Cons idering State B the pressure release is started from

the supercritical condition on the left-hand-side of the

critical point (sB<S ,,i~), wherea s for State C the process

is started from the right-han d-side of the critical point

(S&S,&. Assum ing an isentropic change of state until

saturation cond itions are reached, condensatio n occurs if

the initial state is located on the right-hand -side wherea s

flashing will occur if the process is initiated from the

left-hand-side of the critical point. The latter case

denoted by State B in

Figure 1

was investigated by the




287

experiments presented in this paper. During the pressure

large temperature changes occur during the blowdown

release two

phases can be

observed:

the

process. The results of previous experimen ts proved that

“supercriticaYsubcooled blowdown” and the “saturated

the differential pressure between the top and bottom of

blowdown”.

The supercritical/subcooled blowdown

the vessel after the onset of flashing is negligible relative

phase terminates when the system pressure drops below

to the accuracy of the transduc er. Hence for the exper-

the saturation pressure and flashing occurs. The saturated

iments discussed in this paper the axial pressure gradient

blowdown phase lasts until ambient pressure is reached. is neglected.

Experimental

The experimen tal investigation s consisted of pressure

release experimen ts from initially subcooled and

supercritical conditions using pure carbon dioxide (CO,).

The critical data of carbon dioxide are pcrit = 7.38 MP a

and Tcrit= 304.2 K17.

Six thermocou ples (NiCr-N i) of outer diameter

D = 0.5 mm are installed in the centre of the vessel along

the axis. To seal the signal wires of the temperature sen-

sors between the pressure chamber and the ambient the

capsulated signal wires were brazed into a plug.

In Figure 2 a sketch of the experimental plant is

show n. The volume of the cylindrical high pressure test

vessel is V= 0.05 m3. A system pressure up to

P

= 30 MPa can be achieved. The inner diameter of

thmeapressure essel is

d = 0.242

m, the height to diameter

ratio is h/d = 4.5. A venting p ipe of cross-sectional area

A = 50 mm* is connected to the top. An exchangeable

orifice for varying the cross-sectional area of the venting

line is installed. A quick-opening ball valve is used to

assure the controlled initiation of the blowdown. Amb i-

ent pressure of approximately pa = 0.1 MPa can be

assumed at the end of the venting line. The test fluid

was filled into the vessel from the top by using a dia-

phragm pump.

A gamm a densitometer is used to measure the fluid

density averag ed across the cross-sectional area of the

pressure vessel. The gamma source (“7Cs) and the scin-

tillation counter are installed on a support system which

can be moved along the axis of the vessel. By repeating

the experiments at the identical conditions and measur-

ing the fluid density for various axial levels the axial

density distribution can be obtained. T he phase densities

and the respective intensities with the channe l full of gas

and liquid change considerably during a pressure release

such that the void fraction cannot be obtained directly

from the counted num ber of pulses . Therefore, the void

fraction wa s calculated by

Various measuring sensors are used to obtain infor-

mation about the transient process parameters. A piezo-

resistive pressure transduce r is attached to the bottom of

the vessel. It proved to be impo rtant to use a sensor with

low signal deviation due to temperature change since

P=e ‘+(l -&)P’

(4)

using the saturation densities of the liquid phase and the

gaseous phase

P’ = P’(P) (5)

p” = p”(p).

(6)

The liquid to vapor phase density ratio during the exper-

iments performed varied from p Ipv = 3 to p /pv = 20.

Approx imately 70% of the cross-sectional area of

the pressure vessel was irradiated by a broad beam as

can be seen in Figure 3. Thus an accurate cross-sectional

averaged magnitude for the density can be assured.

Howev er, the dynam ic error due to a chang ing void frac-

tion during a time interval was proved to be negligible

due to the large cross-sectional area irradiated and the

low phase velocities. The error due to void orientation

effects is negligible due to the fact that ‘“7 Cs is a hard

source for which the absorption is weak an d can be

approximated by a linear relationship. This was proved

stor ge

tonk

CCD-Comer0

I

LrzFF-1

7’

T”

I

pressure

T r--

vessel 0

I

3

h

c?

R

I------

00

R WR

67 mm

counter,

photomultiplier

Figure 2 Experimental set up

Figure 3 Gamma densitometer



288


by calculations of the intensity profile alon g the axis of

the bar detector for varied radial vo id profiles. The mea s-

uring signal was low-pass filtered using a limiting fre-

quency off= 1 s-i and smoo thed with respect to time

afterwards. Experim ents were repeated several times for

the identical initial conditions. The measu red void frac-

tion profiles an d pressure transients proved to be repro-

ducible.

The experimental plant is placed on a scale. Thus

the weight of the fluid in the vessel during the process

can be measured. The outgoing mass flow rate can be

obtained by calculating the derivative of the mas s signa l

with respect to time.

i t) = l(t)

The weight sensors are operated by strain gauge tech-

nique. The measuring signal is low-pass filtered u sing a

limiting frequency off= 10 s-l.

All measuring signals are processed by an A/D con-

verter and a personal computer. The pneumatic ball

valve wh ich is used to open the venting line is also con-

trolled by the PC. A meas uring frequency off= 20 s-l

per sensor proved to be convenient for the experiments

performed such that no information was lost and the

amount of data could still be managed.

Besides measuring the process parameters an

impo rtant is sue for describing the thermohy draulic

phenomena is a visual observation. In high pressure

technology, observation window s of various designs are

used. The visible diameter of these windows is small

because of thermal and mecha nical stress. Strict safety

requirements must be met when designing window s

exposed to dynamic stress.

To observe the transient therm ohydrau lics in the

vessel during the depressurizatio n of initially sup ercriti-

cal carbon dioxide a pressure capsula ted b orescope com-

bined with a CCD camera is used. Usually borescopes

are designe d for low pressure ranges. Th erefore, a press-

ure capsule for the borescope had to be designed for this

application. The capsulated borescope is put through the

cover of the vessel. T hus a view from the top into the

vessel is achieved. The CCD camera is connected to the

borescope outside of the vessel.

The pressure capsule is mou nted on the cover of

the pressure vessel. The borescope is capsulate d by a

pressure pipe. The pressure pipe is supported and sealed

in a bore of the cover of the vessel. U sing an electrical

drive the borescope can be moved vertically from the

lower end of the cover to a depth of 500 mm inside the

vessel. At the tip of the pressure pipe a sapphire glass

is located. The window is sealed such that mechanical

stress within the window cannot develop even for high

temperature gradient condition s. T he thickne ss of the

window is 8 mm. A n anti-reflex coating is provided for

both s ides of the window to avoid reflections of the light

which is transported through the borescope into the ves-

sel to illumin ate the object.

During the pressure release the pictures obtained by

the CCD camera are taped. The video pictures are pro-

vided w ith a time code. T he timing of the video pictures

and the measuring signals are synchronised. Thus the

height of the flashing liquid layer can be obtained.

Results and discussion

Experim ents were accom plished for initial conditions

that varied in temperature, pressure, and minimum diam-

eter of the venting line (orifice). The initial pressures

were within the range of p,, = 15-30 MPa, whereas the

initial temperatures varied from T, = 298 to 323 K. This

implied initial densities in the range from p,, = 700-

970 kg mm3 . Thus the initial state of the fluid was

supercritical for most cases. Only for initial temperatures

less than the critical tem perature was the state of the

fluid subcooled.

Pressure transients

To illustrate the phenomena observed during a

depressu rization process from initially supercritical con-

ditions (J+,= 20 MPa, To = 3 13 K) the phase distribution

along the pressure transient obtained by borescopic

observation is shown in Figure 4 A steep pressure

decrease as well as a temperature decrease can be

observed during the supercriticaYsubcooled blowdown .

The change of state of the fluid during the

supercritical/subcooled blowdown was almost isen-

tropic. Thus dissipative effects and wall heat transfer

must be low during this period of time. Comp aring the

borescopic observation s to the pressure and temperature

signals obtained, the flashing process starts at the vessel

wall before the bulk fluid has reached saturation con-

ditions. The fluid temperature within the bounda ry layer

is higher than the bulk temperature due to heat flow from

the wall. Thus saturation conditions are reached first at

the vessel w all, Subseq uently, flashing also occurs in the

bulk region. For several seconds the pressure does not

change significantly. This is caused by the low void

quantity and the low mixture en thalpy, respectively, of

the vented fluid in this period of time as well as by the

high evaporation rate. Subsequ ently, the void quantity

20

15

; i i

; ‘0

3

8

h

5

0

Initial condi t ions:

pO =

20 MPa

T,=313K

I---+

min = 17 mm 2

/

50

100

150

Time (s)

Figure 4 Phase distribution during blowdown from initially

supercritical conditions




289

of the vented fluid rises as the vapour concen tration in

the

top of the vessel increases and the process pressure

decreases with respect to time. After about t = 30 s a

vapour dome deve lops on top of the boiling liquid. First

the volume of the vapour layer is sma ll but it increases

while the boiling liquid layer drops further down. Simi-

lar pheno mena have been observed for all depressuriz-

ation processes which proceed through the liquid region

and subsequently reach the two-phase region across the

boiling curve.

The pressure transient strongly depends on the

initial conditions of the fluid. In Figure 5 pressure transi-

ents as a function of time are show n for varied initial

temperature. The initial pressure w as chosen to be

p0 = 15 MPa for these

experiments. During the

supercriticalkubcooled blowdown the pressure decreases

faster with respect to time in the case of lower initial

temperatu res. The process pressure at the onset of flash-

ing-indicated by the bend in the pressure curve-was

observed to decrease for lower initial temperatu res due

to lower initial specific entropy

Figure I).

Considering

an almost isentropic change of state during the

supercriticalkibcooled blowdown saturated conditions

are reached at lower process pressure s. Hence the pro-

cess pressure at the onset of flashing varied from

P/Pcrit = 0.7 to 0.95. The phenomenon of the sharp bend

in the pressure curve due to the onset of flashing was

observed for all experimen ts performed. This phenom -

enon is due to an increase in the compressibility caused

by formation of bubb les. T he effect of varied initial

pressure on the pressure transient is shown in

Figure

6.

The initial temperature was held constant at T,, = 3 13 K.

It can be observed that the process pressure at the onset

of flashing was higher for lower initial pressure due to

higher initial specific entropy. For these cases the pro-

cess pressure decreased faster after the flashing was

Initial conditions:

p0 = 15 MPa

A,i, = 50 mm2

----T,=298K

-- T,=313K

25

50

75

Time (s)

Figure 5 Pressure transients during blowdown, varied initial

fluid temperature

Initial conditions:

T,=313K

Amin = 50 mm2

2

0

0 25 50 75

Time (s)

Figure 6 Pressure transients during blowdown, varied initial

pressure

initiated due to lower m ass inventory at the onset of

flashing.

By enlarging the relief cross-sectional area the out-

going m ass flow rate from the vessel is increased. In

Figure

7 pressure transients are show n for different relief

cross-sectional area. The initial fluid conditions were

pO =

15 MPa and T, = 3 13 K. It can be seen that the onset

of flashing o ccurs earlier for a larger orifice diameter but

at the identical process pressure. For both cases the pro-

15

10

Initial conditions:

PO

15MPa,T,=313K

--

A,i, = 17 mm2

- A,i, =

50

mm2

I

I

\

. .

\

\

.

.

25 50

75

Time (s)

Figure 7

Pressure transients during blowdown, varied venting

line cross-sectional area



290


cess pressure remains almost constant for a few seconds.

Subseq uently, the process pressu re is observed to

decrease faster for larger orifice diameters.

Ax i a l t emper a t u r e p r o j l e

An enormous decrease of the fluid temperature was

observed during the blowdown experiments. The meas-

ured temperatures with respect to time are shown in

Figure 8

for various ax ial levels. The temperature sen-

sors have been installed at various axial positions in the

center of the pressure vessel as indicated by the sketch

in

Figure 8.

Sensor Tl is located at h = 0.1 m below the

cover of the vessel, w hereas sensor T6 is attached at

h = 0.05 m above the bottom of the vessel. The initial

fluid conditions for this experiment were

p0 =

15 MPa

and T,, = 313 K. The inner diameter of the orifice was

A m i n 17 mm’.

During the supercriticaYsubcooled blowdown a

steep decrease in fluid temperature can be observed. The

temperature decrease is a result of the change of the ther-

modynamic equilibrium state according to fluid density

and pressure. At the onset of flashing the change of tem-

perature with respect to time decreases sudden ly as it

was found out for the pressure transients. Saturation con-

ditions are reached. No evident superh eating of the fluid

was observed at the time when the flashing is initiated.

Accord ing to the process pressure the fluid temperature

further decreases. Tem peratures of less than

T = 200

K

are reached at the bottom of the vessel after phase tran-

sition to dry ice occurred. How ever, in the vessel wall

a large temperature gradient and temperature induced

stress, respectively, occur as reported by Egg ers and

Green”.

During the blowdow n process no axial temperature

gradients were measured until after approximately

t =

100 s the fluid temperature in the upper region of the

313

288

238

213

Figure 8

In i t ia l condi t ions:

pO = 15 MPa

T,=313K

7

Amin= 17mm2

I I

4-T8

0

50 100

150

200

Time (s)

Temperature transients during blowdown from

initially supercritical conditions

vessel increases relative to the fluid temperature in the

lower region. This is caused by the heat flow from the

vessel wall which heats up the gaseous phase after all

entrained droplets are vaporized.

A x i a l v o i d pr o j l e

The axial density profile was measured using a

gamma densitometer.

An averaged magnitude with

respect to the cross-section of the vessel is obtained. The

experimen ts were repeated several time to take measu re-

ments at various axial positions. The void fraction was

calculated from the saturation densities which change

significantly during the pressure release.

The measured void fraction profile along the axis

of the vessel is shown in Figure 9. The initial fluid con-

ditions were p . = 15 MPa and To = 3 13 K. The inner

cross-sectional area of the orifice was Am in= 17 mm2.

After the flashing process is initiated a fast increase of

the void fraction can be observed for axial levels near

the top of the vessel. For lower levels the void fraction

decreases after a maximum is reached. T he values tend

close to E = 1 as soon as the boiling liquid layer drops

below the measu ring level. Furthermo re, for the identical

initial conditions the dimensionless height inside the ves-

sel as a function of the void fraction is show n in

Figure 9.

The void fractions could not be measured at

the bottom of the vessel and below the cover such that

the profile curves a re dashe d in these regions. The void

fraction at the bottom is assumed to be E = 0. This

assumption is valid if the thin vapour layer caused by

bubble nucleation at the wall is neglected.

After the onset of flashing no obvious void fraction

gradient is obtained. At approximately

t =

20 s a gradient

in void fraction caused by phase separation can be

observed, particularly in the bottom and top regions. The

relative velocity betw een vap our and liquid phase

increases. This is caused by an increasing difference

between the saturation densities and an increase in the

surface tension as the blowdown process proceeds. The

drift velocity id,, increases accord ing to Equatio n (3).

Hence a gradient in void fraction along the axis of the

vessel develops. The void fraction gradient in the top

region further increases as the overall vo id fraction level

rises. A vapour dome has developed in the top of the

vessel after

t =

30 s which was found out by borescopic

observation s. Subseq uently, the void fraction tends

towards E = 1 for the upper region. As the change in

pressure with respect to time decreases, the void fraction

at the bottom goes dow n due to the lower evaporation

rate.

In Figure 10 a void fraction profile is shown for

raised initial pressure

p. =

25 MPa. As before, the initial

temperature was To = 3 13 K and the inner cross-sectional

area of the orifice was

A m i n

17 mm*. Compared to the

experiment starting from initial pressure p . = 15 MPa

Figure 9) axial void gradients can be observed right

from the beginning of the saturated blowdow n. This was

expected because the onset of flashing occu rs at a lower

process pressure for the case of a higher initial pressure

Figure 6).

Thus the density difference between the two

phases and the surface tension are larger which implies

a higher drift velocity according to equation (3) such that

phase separation takes effect. It can be seen that a void



Blowdown of carbon dioxide:

B. Gebbeken and R. Eggers

291

1.0

1.0

0 . 8

0 . 8

‘c

Initial Condiiins

0 . 2

0 . 2

T,=313K,p0=15MPa

=lOs A t=2os

I ~

t=30s + t=4os

0 . 0

0 . 0

0

2 5

5 0 7 5

100

0.0

0 . 2 0 . 4

0 . 6

0 . 8 1 . 0

Time (s)

Void fraction e

Figure 9

Void fraction profile for blowdown from initially supercritical fluid conditions, initial pressure p0 = 15 MPa, orifice cross-

sectional area Am i = 17 mm2

I I I

Initial Conditions:

To=313K,po=25MPa

*min =

17

mm*

t=1os

A t=20s

0.0 0.2

0.4

0.6 0.8

1.0

Void fraction E

Figure

10 Void fraction profile for blowdown from initially

supercritical fluid conditions, initial pressure p0 = 25 MPa, orifice

cross-sectional area A = 17 mm*

profile of nearly constan t gradient develops along the

axis of the vessel before the void fraction profile turns

into two regions of different void profile. Furthermore,

in the case of experimen ts starting from high er initial

pressure the increase in void fraction with respect to time

was observed to be slower and the maximum values of

the void fraction at any level are lower. T his effect is

due to the lower decrease of pressure with respect to

time and the resulting lower evaporation

ingly, the liquid layer wa s observed to

later times.

rates. Accord-

drop down at

The effect of varied cros s-sectional area of the vent-

ing pipe on the void fraction profile is shown in

Figure 11

No orifice was installed in this case such that

the minimum cross-sectional area of the venting line was

A,i” = 50

mm *. The initial fluid conditions were

1.0

0.8

0.2

0.0

Initial Conditions

- T,=313K,p0=15MPa

&,,, = 50 mm’

W t=1os A t=20s

0 t=3os t=4Os

0.0 0.2 0.4

0.6

0.8

1.0

Void fraction E

Figure

11 Void fraction profile for blowdown from initially

supercritical fluid conditions, initial pressure p,, = 15 MPa, orifice

cross-sectional area &, = 50 mm2



292


p. = 15 MPa and To = 3 13 K (compare to

Figure 9).

The

onset of flashing w as observed to be earlier in the case

of larger cross-sectional area of the venting line due to

the faster change in pressure. The faster change in press-

ure and the respective higher evaporation rate also

implies a faster overall ch ange of the void fraction with

respect to time. Furthermore, the void fraction level was

observed to be higher at any level.

The transient phase distribution during blowdown

from initially supercritical conditions can be subdiv ided

into four typical phases. P hase 1 coincides with the

supercriticaYsubcooled blowdown. Until the onset of

flashing the void fraction along the axis is defined to be

E = 0. During Phase 2 which starts at the beginning of

the saturated blowdow n a constant void profile develops

as long as phase separation is not taking effect, which

depends m ainly on the thermodynamic fluid conditions.

In any case void gradients along the axis of the vessel

become ob vious. Two regions of different vo id gradient

are observed, one at the bottom of the vessel where the

void fraction is increasing considerably and the second

of almost constant void fraction on top. Due to the

accumu lation of vapour phase in the top of the vessel

during Phase 3 the void fraction in the upper region

increases almost linearly such that the profile can be sub-

divided into three regions along the vessel axis. In the

lower region as well as in the upper region of the vessel

a considerable void fraction gradient is obtained . F or the

middle region the gradient in void fraction is lower. As

a vapour dome is establish ed on top of the liquid layer,

Phase 4 is initiated. A constant void fraction gradient

through out the liquid layer an d a vapour layer of void

fraction E = 1 in the top of the vessel can be o bserved.

A transition layer of large void gradient has developed

inbetween. The typical phase distributions during these

four phases are sketched in Figure 12.

Further investigatio ns of the void distribution dur-

ing pressure release were accomplished by Friedel and

Pulps

.

3

They measured the axial void fraction profile

within a cylindrical, vertical pressu re ves sel (0.106 m3,

h /d = 2 ) by mean s of local capacitive sensin g probes.

Although their blowdown experiments using the

refrigerant R12 were initiated from saturated two-p hase

Phase 1

7

hase 3

E

Figure 12 Characteristic phases

of void fraction profile

h

LJ-

hase 2

h

k

hase 4

E

conditions their void profiles obtained for high initial

liquid level are similar in quality to the distributio ns of

Phases 3 and 4 of the results presented in this paper.

Thies and Mewes14 investigated blowdown processes

of a gas-desorbing system from saturated (two-phase)

initial conditions instead of a vaporizing system which

was considered in the experiments presented in this

paper an d by Friedel and Purp~‘~. Thies and M ewes used

differential pressure tran sducers in order to measu re the

axial void fraction profile within a cylindrical, vertical

pressure vessel (0.035 m3,

h / d =

1.8). They reported void

distributions which were, in the case of high initial liquid

level, s imilar to those describe d in Phase s 3 and 4 of

the results obtained in these investigations. Considering

Phase 3 they explain the large void fraction gradient in

the upper region by backflow of vapour phase along the

wall of the vessel due to instabilities in the region near

the inlet of the venting pipe. The constant void fraction

in the middle region is traced back to the change of the

two-phase flow pattern from homogeneous to hetero-

geneous bubbly flow. After a heterogeneous bubbly flow

is established, Phase 4 is initiated such that the middle

region of nearly con stant void fraction is eliminated .

Similarities in quality of the void profiles can be

observed for blow down experiments from initial

supercritical or subcooled conditions (flashing system)

and those from saturated two-phase conditions (flashing

or gas-desorbing system), although the blowdown pro-

ceeds differently. In the case of a blow dow n from

initially two -phase condition s, a free surface is present

where the gaseous phase can separate from the liquid

layer until the free surface has reached the top of the

vessel due to level sw ell. In the case of a blowd own from

supercritical conditions there is no free surface until the

liquid layer collapses.

Conclusions

Experimental investigations of blowdown processes for

initially supercritical carbon dioxide are presented. The

pressure release was started from the left-hand-side of

the critical point (so < s,,it) such th at flashing occurred

after saturation conditions were reached. Thermohyd-

raulic phenomena were discussed, particularly the press-

ure transients , the axial temperature profile, and the axial

void fraction profiles.

After blowdown is initiated the fluid remains sin-

gle-phased until saturation conditions have been reached.

A two-ph ase mixture is blown off after the onset of

flashing. A vapour dome develops on top of the liquid

layer. Subseq uently, the vapour phase including

entrained liquid drops is blown off.

For all experimen ts performed a sharp bend of the

pressure curve w as observed at the onset of flashing. The

pressure at which flashing occurred first depends mainly

on the initial cond itions due to the almost isentropic

change of state during the supercritical/subcooled blow-

down.

The void fraction profile was measured using a

gam ma densitometry. Significant g radients in void frac-

tion can be observed during the blowdown process due

to phase separation effects. Phase separation effects

depend on the difference between the saturation densities

of both phase s and the surface ten sion. T herefore, the




293

influence of phas e separatio n on the axial void fraction

profile is more obviou s at lower process pressures . It is

proposed to subdivide the blowdown process into four

phas es of characteristic void profile. Similar void distri-

butions were obtained a s discovered by other authors

investigating the blowdown of a flashing system from

initially saturated conditions and a gas-desorbing system

from saturated two-phase conditions.

cknowledgements

The authors thank the Deutsche Forschungsgemeinschaft

for financial support of this research under grant No.

Eg 7212.

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Nomenclature

A

Cross-sectional area (m’)

C0

Radial distribution parameter

d

Inner diameter (m)

D

Outer diameter (m)

f

Frequency (s-l)

h

Vessel height (m)

j

Volume flux density (m s-‘)

M Mass (kg)

ni

Mass Rowrate (kg ss’)

P

Pressure (Pa)

.Y Specific entropy (J kg-’ Km’)

t Time (s)

T

Temperature (K)

I Velocity (m ss’)

vdrlft

Drift velocity (m ss’)

V

Volume (ml)

Greek symbols

E

Void fraction

P

Density (kg m-‘)

cT Surface tension (N m-l)

Subscripts

a

Ambient

crit

Critical

g Gas

I Liquid

max Maximum

0 Initial

Saturated liquid

n

Saturated vapor

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Blowdown of Carbon Dioxide From Initially Supercritical Conditions - Copy