NBSIR 79-1607

PRODUCING SLUSH OXYGEN WITH AN AUGER AND MEASURING

THE STORAGE CHARACTERISTICS OF SLUSH HYDROGEN

R. 0. Voth

P.R. Ludtke

Thermophysical Properties Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

March 1979

Prepared for.

National Aeronautics and Space Administration

Kennedy Space Center, Florida 32899

(Contract No. CC-6253QA)

NBSIR 79-1607

PRODUCING SLUSH OXYGEN WITH AN AUGER AND MEASURING

THE STORAGE CHARACTERISTICS OF SLUSH HYDROGEN

R. 0. Voth

P.R. Ludtke

Thermophysical Properties Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

Prepared for:

National Aeronautics and Space Administration

Kennedy Space Center, Florida 32899

(Contract No. CC-6253QA)

March 1979

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

CONTENTS

Page

Part 1. PRODUCING SLUSH OXYGEN WITH AN AUGER 1

1.1 INTRODUCTION 1

1.2 DESCRIPTION OF THE AUGER. 2

1.3 RESULTS OF TESTS PRODUCING SLUSH OXYGEN WITH THE AUGER 2

1.4 CONCLUSIONS FROM THE SLUSH OXYGEN PRODUCTION TESTS 13

Part 2. LONG TERM STORAGE OF SLUSH HYDROGEN 14

2.1 INTRODUCTION 14

2.2 DESCRIPTION OF SLUSH STORAGE APPARATUS 14

2.3 RESULTS OF THE HYDROGEN STORAGE TESTS 19

2.4 THE RADIO FREQUENCY MASS GAUGE 30

2.5 CONCLUSIONS FROM THE STORAGE TESTS 35

3.0 REFERENCES 36

iii
PRODUCING SLUSH OXYGEN WITH AN AUGER

AND MEASURING THE STORAGE CHARACTERISTICS OF SLUSH HYDROGEN

by

R. 0. VOTH and P. R. LUDTKE

Part 1. PRODUCING SLUSH OXYGEN WITH AN AUGER

An auger rotating inside a brass tube refrigerated with liquid helium

was used to produce liquid-solid (slush) mixtures of oxygen. The auger

produced small particles of solid oxygen so that the resulting mixture could

be transferred and stored. The auger could produce slush continuously in an appropriate system, and it could produce slush at pressures higher than the

triple point pressure of the oxygen.

Part 2. STORAGE CHARACTERISTICS OF SLUSH HYDROGEN

Three long term storage tests were conducted on an Apollo hydrogen tank.

The tank was filled to 88 percent with normal boiling point liquid hydrogen

then the pressure rise rate to 17.6 bar and the venting rate at 17.6 bar were

measured. The two other tests were similar except the vessel was filled with

slush hydrogen. In one of these tests, the slush was mixed to eliminate

thermal stratification. Filling with slush instead of liquid hydrogen

increased the storage time before venting by 1.08 to 1.17 times and increased

the mass loaded by 1.11 to 1.13 times.

Key words: Cryogenic; hydrogen; oxygen; scraping auger; slush; slush

production; slush storage.

iv
Part 1. PRODUCING SLUSH OXYGEN WITH AN AUGER

1.1 INTRODUCTION

Liquid-solid mixtures (slushes) of oxygen exhibit a higher density and

heat capacity than the normal boiling point liquid. These characteristics

are advantageous in space applications where longer storage times and higher

storage densities are important. But to effectively use slush oxygen, we must be able to produce it economically, to store and transfer it successfully and to measure its stored mass and mass flow rates.

In this study, an auger previously used to produce slush hydrogen [1] was used to produce slush oxygen. The auger system can continuously produce

slush, and since the auger system can be immersed in liquid, slush can be

produced at pressures above triple point pressure. The increased pressure

can be produced pneumatically or by generating a temperature stratification

near the surface of the liquid. The auger system produced oxygen particles

smaller than the particles produced in slush hydrogen, so the auger produced

slush oxygen should be capable of being transferred and stored in a conventional cryogenic system.

The auger scraped frozen oxygen from the inside of a liquid helium

refrigerated tube. The safety of such a device could be questioned because

high forces and therefore high energies could be present in the scraping

process. The general ground rules for an oxygen system are that the

temperature generated by energy input to a system should be less than the

ignition temperature of the fuels and oxygen present. In the auger system,

the auger and wall are metal with high ignition temperatures and the total

input energy to rotate the auger is less than 5 watts. Therefore, the

precautions taken with this system were the same as those taken for liquid

oxygen systems in general. The system was cleaned to remove hydrocarbons

with low ignition temperatures and to remove particulate matter where

energies could be concentrated, resulting in high temperatures. No evidence

of rapid oxidation occurred during the production of slush oxygen.

Producing slush oxygen by the thoroughly investigated freeze-thaw

method is difficult because of the low triple point pressure of oxygen [2].

The freeze-thaw method uses the latent heat of vaporization to freeze the

cryogen. By removing vapor at the optimum rate from a container of triple

point liquid, a porous solid layer is formed on the liquid surface.

Stopping the vapor removal and mixing this solid layer into the remaining

liquid produces a liquid-solid mixture of the cryogen. To obtain high solid

fractions, the pumping-mixing or freeze-thaw cycle must be repeated numerous

times. While with hydrogen, the freeze-thaw production method successfully

produces slush with small solid particles, oxygen has a much lower triple

point pressure than hydrogen (.0015 bar versus .07 bar for hydrogen), thus

requiring a large vapor removal tube. The large tube must be sized to allow
large volumes of vapor to flow at a low pressure drop. Because of this

requirement, slush oxygen would be difficult to produce using the

freeze-thaw method —the production of slush oxygen has not been reported in

the literature before the current study. The direct freeze method in the

auger overcomes the problem of removing vapor at the low triple point

pressure of oxygen, and it produces slush with sufficiently small particles

so that the slush can be transferred and stored in conventional cryogenic

systems

.

1.2 DESCRIPTION OF THE AUGER

A cross section of the auger and heat exchanger assembly used to

produce slush oxygen is shown on figure 1. A photograph of the partially

disassembled unit is shown on figure 2. The 400 series stainless steel

auger with an outside diameter of 4.676 cm (1.841 in) is supported within a close fitting brass tube by a ball bearing at each end. The radial

clearance between the auger and brass tube is 0.0178 cm (0.007 in) at

ambient temperature. Because of the difference in thermal expansion between

the stainless steel auger and brass tube, the radial clearance decreases to

0.0127 cm (0.005 in) at the operating temperature. Since solid oxygen has a low thermal conductivity, this annular clearance should be small to allow a high heat transfer rate.

A ribbon packed, vacuum insulated heat exchanger surrounds the brass

tube. The ribbon packing increases the refrigerant heat transfer area while

the vacuum insulation minimizes heat transfer to the exterior surface.

Vacuum insulated refrigerant lines are provided at the top and bottom of the

heat exchanger.

The auger assembly is suspended in the vertical position from a top

plate by a stainless steel tube. Located concentrically within the support

tube is the stainless steel torque tube used to rotate the auger. A shaft

rotating in a magnetic fluid for a no-leak seal connected the torque tube to

the ambient temperature drive mechanism. The drive motor is an air motor

with numerous gear speed reducers. By changing the number of gear reducers

and air supply pressure to the motor ,

\

the auger rotational speed could be

varied from about 1 rad/s to 10 rad/s.

The instrumentation used with the auger system measured the torque and

rotational speed of the auger drive, and the refrigerant flow rate.

Refrigerant pressure and slush container pressure were also displayed on convenient gauges. Strip silvered glass dewars were used to contain the

auger system so that the slush oxygen could be viewed and photographed.

1.3 RESULTS OF TESTS PRODUCING SLUSH OXYGEN WITH THE AUGER

Primarily, the auger tests were conducted to determine if the auger

could produce sufficiently fine frozen particles so that the resulting slush

oxygen could be stored and transferred easily. Secondarily, the auger

2
Figure 1. Cross Section of the Auger and Heat Exchanger Assembly.

3
4

NBS,

Boulder
power required at various rotational speeds and refrigeration supplied to

the auger were measured. Also, the slush was held in the settled condition

for 56.5 hours to determine how the solid oxygen particles changed with

time.

Figure 3 shows freshly produced solid particles floating past a 2mm

square grid (the grid and the shovel used to move the slush are shown in

figure 4). The fresh slush particles are random in size varying from

approximately 0.1 mm to 5 mm in the largest dimension. This is smaller than

the slush hydrogen particles produced by either the auger or the freeze-thaw

production method [2]. Although the oxygen particles produced by the auger

appear smaller, a much more extensive measuring program than conducted

during these tests is necessary to determine the actual size distribution.

Aged auger produced slush particles, figures 5, 6, 7, and 8 appear to have

about the same size distribution as the fresh particles although they become

more rounded and more transparent. The photographs of the aged particles

were made by using the shovel (Figure 4) to reach into the settled slush and

to lift the particles above the grid where they were slowly dumped allowing

them to settle past the grid. The settling particles were photographed with

a 16mm movie camera. The depression left by the shovel in the settled slush

was filled immediately after the shovel was removed by the remaining slush

particles. Especially in the aged condition, the particles were so spherical that they simply rolled into the depression, and the surface of the

settled slush remained nearly horizontal throughout the test. This rolling

action indicates that aged slush oxygen has a low angle of repose and it

should transfer easily from a storage tank.

The total power required to turn the auger is the sum of the power

required to turn the auger when no oxygen is freezing (tare power) and the

power required to scrape the frozen oxygen from the brass tube (scraping

power). The tare power was proportional to the auger rational speed or

P t = .0359R

where

P

t = tare power, W (1)

R = auger rotational speed, rad/s

Figure 9 shows the scraping power as a function of the refrigeration

supplied to the auger assembly. The points shown are the experimental

points taken at various rotational speeds. The straight line is a hand fit

of the data and is expressed by

P s = 0.0392 (P r ), (2)

5
6

Figure

3.

Fresh

Oxygen

Particles,

Less

than

1/2

Hour

Old,

Grid

Size

is

2mm

square.
7
8

Figure

5.

Aged

Slush

Oxygen

Particles,

Aged

12

Hours,

Grid

Size

is

2mm

square.
9

Figure

6.

Aged

Slush

Oxygen

Particles,

Aged

24

Hours,

Grid

Size

is

2mm

square.
10

Figure

7.

Aged

Slush

Oxygen

Particles,

Aged

27.5

Hours,

Grid

Size

is

2mm

square.
11

Figure

8.

Aged

Slush

Oxygen

Particles,

Aged

56.5

Hours,

Grid

Size

is

2mm

square.
REFRIGERATION

SUPPLIED

TO

AUGER

SYSTEM,

W

Figure 9

The Auger Scraping Power Requirement when Producing

Slush Oxygen.
where P

s is the scraping power in Watts, and P r is the refrigeration

supplied to the auger assembly in Watts.

The total power required to rotate the auger during slush oxygen

production is expressed by:

P a = 0.0359R + 0 . 0 3 9 2 P r (3)

where

P a

= total auger power requirement, W

1.4 CONCLUSIONS FROM THE SLUSH OXYGEN PRODUCTION TESTS

The auger successfully produces a slush oyxgen with sufficiently small

particles, so that the slush can be stored and transferred in cryogenic

systems. Aged slush, in particular, contained spherical particles that

easily rolled into any depression formed in the settled slush surface.

Thus, the slush oxygen can probably be transferred from the bottom of a storage vessel with less agitation than that required to transfer a high

mass fraction of slush hydrogen.

The auger system produces slush by direct freezing so that the low

triple point pressure of oxygen has no effect on the production process.

The slush can be produced by the auger system at pressures above the triple

point pressure by pressurizing the liquid pneumatically or by utilizing a thermal stratification near the surface of the liquid oxygen and immersing

the auger well below the surface.

Although producing slush oxygen required about 2.6 times the scraping

power required to produce slush hydrogen, the oxygen scraping power is still

only four percent of the total refrigeration power supplied to the auger.

13
Part »2 . LONG TERM STORAGE OF SLUSH HYDROGEN

2.1 INTRODUCTION

Slush cryogens offer significant advantages when long term storage is

required. The latent heat of fusion of the solid particles in the slush

increases the amount of heat the fluid can absorb before venting must occur.

In order to take advantage of the latent heat, an ullage volume in the

storage vessel is required to allow for expansion during the melting

process. Even though the required ullage volume decreases the stored fluid

volume, the higher density of the slush maintains or increases the amount of mass stored in a vessel above that stored by an equal volume vessel

completely filled with normal boiling point liquid. While these advantages

can be shown theoretically, a practical system requires knowledge of a filling system, a filling procedure and a measuring system to achieve the

correct ullage and slush mass fraction at loading. In order to determine

areas where additional experimental work is required and to demonstrate the

longer storage capabilities of slush, an Apollo flight weight tank was filled three times, and the pressure rise and venting characteristics were measured for an extended period of time for each filling. The first filling

was with normal boiling point liquid hydrogen to provide data to judge the

performance of the subsequent tests with slush hydrogen. During one slush

test, the fluid was allowed to thermally stratify, and during the second

slush test, the fluid was mixed using the in-tank electric fans. A radio

frequency mass gauge was used during the tests to determine if its

An existing Apollo hydrogen tank was chosen for the experimental fills

because no funds were available to custom design and build a better

experimental container and because the results would apply to an existing

flight weight tank similar to the fuel cell reactant tanks where slush could

be used in the Space Shuttle flights. The use of the Apollo tank did,

however, limit the accuracy of the experimental results because there was no slush compatible instrumentation in the tank. Since the radio frequency

gauge is still being developed, it could not be used to determine mass in

the tank. Therefore, the amount of hydrogen in the tank was determined by

weighing the tank before and after the fill, and the ullage volume was determined by tilting the tank to locate the vent outlet at the required

liquid level to provide the chosen ullage.

The Apollo tank is constructed entirely of titanium making alterations

to the tank difficult. The only changes were to the fill and vent lines.

An epoxy joint to the titanium fill line was made using a high nickel alloy

insert having thermal expansion characteristics similar to titanium. This

joint was vacuum insulated to reduce heat leak. A similar epoxy joint was

ics could be used to measure mass in a slush container

2.2 DESCRIPTION OF SLUSH STORAGE APPARATUS

14
used on the uninsulated vent line. Valves were installed on both lines to

isolate the tank after filling.

The radio frequency gauge was connected to an existing electrical

connection to the interior of the tank. Because of the poor coupling

obtained through this connection and because extraneous material existed in

the tank (capacitance probe, mixing motors and electrical heater) the

signals obtained from the resonant frequencies in the tank were weak.

Figure 10 shows a schematic of the system used to fill the Apollo tank.

The fill line was short to limit the heat transfer to the fluid during the

transfer. Two valves were located in the slush fill line to allow liquid

hydrogen to enter either the slush generator or the Apollo tank. The Apollo

tank could be vented either directly out a vent to the atmosphere or through

a steam heat exchanger to a 34m3/min (1200 cfm) vacuum pump. The Apollo

tank was mounted at an angle to achieve a 12 percent ullage volume. A

photograph of the Apollo tank being filled is shown on figure 11.

The filling procedure included a fill to about half full with normal

boiling point liquid at least 24 hours before the slush fill to temper the

insulation. The boiloff gas generated during the 24 hour hold was vented

through the vapor cooled shield to completely cool the insulation. Twenty

four hours later when the slush hydrogen was being prepared by the

freeze-thaw process, the remaining liquid in the Apollo tank was pumped to

triple point pressure.

About 400 liters of slush were prepared, and after the vacuum insulated

loading line was cooled with liquid hydrogen from the storage vessel, the

slush transfer was started. The excess amount of slush prepared (the Apollo

tank only held 170 liters) was used to provide some upgrading during

transfer by transferring in slush and removing triple point liquid at the

vent line. Also some solid melted during the pressurization of the slush

generator during the slush transfer. The fill tube inside the Apollo tank

is uninsulated and the ullage vapor condensed on the tube during the fill to

maintain triple point pressure in the ullage. To insure that the slush

level was below the vent line penetration, the venting pressure was maintained near triple point pressure during the fill. To establish the

correct slush level, pumping on the vent line was continued a short time

after the transfer was stopped. Since the vent line was uninsulated, large

volumes of gas were removed during the transfer process, and the transfer

took from 30 to 45 minutes depending on how much liquid was in the Apollo

tank at the start of the transfer.

Heat leak to the Apollo vessel was established prior to the long term

storage tests. The heat leak was measured by filling the vessel with normal

boiling point liquid hydrogen and measuring the venting rate for an extended

(2-3 day) period. Heat leak measurements were made for both a cooled shield

15
Connect

To

Gas

Flow

Meter

CD o3

_£Z CD I— OT CT' C C

-M O »- ro O I— CL_d

16

!

Figure

10.

Schematic

of

the

Liquid

and

Slush

System

Used

to

Fill

the

Apollo

Vessel.
17

Figure

11.

Apollo

Tank

Mounted

at

an

Angle

and

Being

Filled

with

Liquid

Hydrogen.
and noncooled shield condition. The measured heat leak with a noncooled

shield was 4.92 watts (16.79 Btu/h) and the heat leak with a cooled shield

was 2.44 watts (8.33 Btu/h). Other parameters of the Apollo tank used in

the theortical calculations for the tests are shown on table 1. Also shown

is the estimated error in these parameters during the fill test.

TABLE 1 Important Parameters of the Apo llo Tank

1

.

Volume*

;

Full

1 at ambient Pressurebar, 20.3 K .19170 to .19190m 3 (.1918m 3 used in calculations)

2 . Volume*

;

Full at 15.5 bars, 23.1 K .19245 to

(

. 19252m 3 19259m 3 used in calculations)

3 . Volume*

;

Full at 17.9 bars, 23.1 K .1927 to

(

. 19264m 3 .192720m 3 used in calculations)

4 . Heat Leak

With Cooled Shield 2.44 W + . 02W

With Noncooled Shield 5.92 W + .02W

5 . Tare Weight

NBP Storage Test

Unmixed Slush Test (changed

electrical connector)

Mixed Slush Test (changed

c onne c tor)

6. Full Weight

NBP St orage Test

Unmixed Slush Test

Mixed Slush Test

7 . Loaded Mass

NBP Storage Test

Unmixed Slush Test

Mixed Slush Test

67.81 kg + .04kg

67.82 kg + • 0 4kg

67.95 kg + .04kg

79.61 kg + .0 4kg

81.11 kg + .04kg

81.07 kg + • 04kg

11.80 kg + • 0 8kg

13.2 9 kg + .08kg

13.12 kg + .08kg

^Volume figures obtained from Manual Number M-13532 of

September 6, 1968

18
2.3 RESULTS OF THE HYDROGEN STORAGE TESTS

Three long term storage tests were conducted. The first test was with

a normal boiling point liquid hydrogen fill. The second test was with a slush hydrogen fill and was conducted without mixing. The third test used a slush hydrogen fill and it was mixed at various times during the test.

After obtaining the weight of the empty Apollo tank (tare weight), the

three tests were conducted following the general procedure outlined below.

1. The vessel was filled to the proper ullage volume.

2. The vessel was valved off.

3. The full weight was measured.

4. The pressure rise rate was measured to about 17.2 bar (250 psia)

5. When the pressure reached 17.2 bar the vessel was vented at nearly

a constant pressure through the shield and a wet test gas meter.

6. After about 1 week, the venting valves were changed to vent without

cooling the shield.

7. When the venting stopped, the Apollo tank was evacuated and purged

and reweighed to check the tare weight.

The pressure rise rate for the three storage tests are shown in figures

12, 13, and 14. The NBP liquid test and the unmixed slush test were not

mixed except for some movement as the vessel was weighed and moved to a remote site. This movement occurred during the first 2 hours after the

fill. The third or mixed slush test was mixed using the internal fans at

variable intervals as shown on table 2. The mixers were run until the

pressure stopped decreasing for each interval. The heat generated by the

fan motors was added to the measured heat leak of the vessel to calculate

the theoretical pressure rise rate.

TABLE 2 Mixing Times During Pressure Rise Period

Total Time Since Filling Mixing Time Pressure, bar

h

19.25

24.75

40.73

47.00

59.25 (venting)

63 .

7

30

202 .

2

150

90

Before Mixing

1.360

1 .667

4.630

9.664

17 .800

After Mixing

.607

.748

1.184

9.243

17.800

In every test, the measured pressure rise time was shorter than the

theoretical time for a system in thermal equilibrium. For the unmixed case,

the difference in time can be due to thermal stratification, but in the

mixed test thermal stratification would not exist. The difference in

19
^eq ‘3bnSS3tid »N\/1 OTlOdV

i

20

Figure

12.

The

Theoretical

and

Actual

Pressure

Rise

for

the

Apollo

Tank

Filled

with

Normal

Boiling

Point

Liquid

Hydrogen.
o

‘3dnSS3dd 13SS3A

21

Figure

13.

The

Theoretical

and

Actual

Pressure

Rise

for

the

Apollo

Tank

Filled

with

Slus~i

I-ydrogen.

The

Slush

was

not

Mixed

During

the

Test.
^eq ‘3bnSS3Hd 33SS3A

22

figure

14.

The

Theoretical

and

Actual

Pressure

Rise

for

the

Apollo

Tank

Filled

with

Slush

Hydrogen.

The

Slush

was

Mixed

at

the

Points

Shown.
theoretical versus measured pressure rise time could be due to a combination

of the following potential factors:

1. A higher heat leak than that measured prior to the storage tests.

2. The mixing fans stopped working near the end of the test (very

little pressure reduction due to mixing occurred at 47 hours into

the test —See table 2).

3. The Apollo vessel had a different volume than that used to

calculate pressure rise time.

4. The inaccuracy in the weight measurement used to determine loaded

mass was sufficient to cause the disagreement.

5. The experimental ullage volume was different than the ullage

volume used in the theoretical calculations.

Analyzing the effect of heat leak on the pressure rise time shows that

the calculated pressure rise time is inversely proportional to the heat leak

to the tank. The heat leak could be increased by the changing pressure in

the mixed slush test. Decreasing the vessel pressure by mixing will tend to

allow warm gas from the vent and fill lines to be mixed into the cold fluid

in the tank. This mixing will increase the heat input. Calculations,

however, show this heat input to be insignificant when compared to the heat

gain through the insulation. Since heat gain must be in error by nearly 20

percent to explain the shorter measured pressure rise time, an error in heat

gain alone does not explain the discrepancy.

We have no reason to believe the mixing fans did not work during the

test (they obviously worked in the early part of the test), and the measured

pressure rise rate nearly duplicates the theoretical pressure rise rate

calculated after the tank becomes full showing that the tank was probably

mixed properly.

An error in the Apollo vessel volume used in the calculations leads to an approximate proportional error in pressure rise time. Figure 15 shows

the effect of vessel volume on the total calculated pressure rise time for a tank filled with 13.29 kg of triple point liquid. The slope of the curve on this figure shows that a 1 percent error in volume yields a 1 percent error

in pressure rise time. An error of 20 percent of the total volume is

required to explain the difference between the measured and calculated

pressure rise times. Therefore an error in the vessel volume does not

explain the discrepancy.

A similar calculation can be made on the error involved in weighing the

tank. The total weight of the tank was about 80 kg and measuring this

weight to 0.1 percent (errors could accumulate in the tare measurement and

the full measurement) yields an error of 0.08 kg or 0.6 percent in the

weight of the hydrogen loaded. Figure 16 shows the change in calculated

total pressure rise time versus mass loaded. An error in the loaded mass of

23
/

o

OsJ

o CS)

M ‘3rtU 3S I a 3dflSS3dd

24

Figure

15.

The

Theoretical

Pressure

Rise

Time

Based

on

a

Variable

Apollo

Tank

Volume.
CO

CO

<£) co

CO

CD

QC

o

LT>

C/5 <c

C\J CO

o

CO

X>

CO

ra

a

05

itf CQ

CDB

<D05 •H

Cd

0)3

05

05a

05 r-H 05a

u S'. •H

-u ^

05 C

u d

O H

0) rn c H rH

pH

Q) O X C- H <

a

Jmd

CO

4 ‘3wu 3S i y 3anss3dd

25
13.29 kg results in very small changes in the calculated pressure rise

times. Again the error in mass measurement cannot explain the difference

between the measured and calculated pressure rise time.

Without instruments in the Apollo tank, it is impossible to tell the

exact ullage volume in the filled container. Since the inlet tube to the

vessel passes through the ullage volume, the ullage volume will always be at or near triple point pressure. Unless the venting pressure is below the

triple point pressure, the ullage volume can fill, reducing its volume below

that calculated by tilting the tank. We tried to insure a correct ullage

volume by continuing to pump on the vent line after the transfer was complete. But even with all the precautions, the assurance of a correct

ullage volume in the tank can not be proven because of the lack of instrumentation. Figure 17 shows the calculated pressure rise time versus ullage

volume and mass fraction of the loaded slush for the Apollo vessel. An error of 1.5 cm in the fluid surface height yields a change of 10 hours in

the pressure rise time. Thus, the difference between the measured and

calculated pressure rise time is most likely due to an error in the loaded

ullage volume. This result indicates that besides a mass meter, an accurate

fluid level gauge would be required in the Apollo vessel to insure the

correct loaded ullage volume to yield the desired pressure rise time.

Even though the results of the experiment differ from the theoretical

calculation, the loaded slush mass was from 1.11 to 1.13 times the mass in

the normal boiling point liquid fill and the slush pressure rise time was 1.08 to 1.17 times the pressure rise time of normal boiling point liquid

hydrogen.

The venting rates at constant pressure are compared to calculated

venting rates on figures 18 and 19. A fairly high scatter is apparent at

the early part of the experiment with high remaining mass. This scatter

results from the unknown heat leak during the early part of the experiment.

When venting starts, the shield is warm, and heat leak decreases by a factor

of 2 in about 24 hours as the shield is cooled. In order to decrease the

time of the storage test, we later vented directly without cooling the

shield so the heat leak increased by a factor of two over numerous hours.

Also, the pressure in the Apollo vessel varied about 1 bar in a 24 hour

period during the test so that instantaneous venting rates taken during a pressure rise were lower than average and venting rates taken during a pressure fall were higher than average. The experimental data appears to

fit the calculated venting rates quite well establishing a further confirmation of the heat leak used for the theoretical calculation. Because of

the good fit of the calculated and measured venting rates, slush was probably not present in the tank when venting started and thermal

stratification was not effecting the data.

26
Mass

Fraction

Slush=0.7

O

CD<C

CO ro

O'- E jo rO jQ CO 'vJ- CD t—

O C\J CVJ cr> f\ C\J cn »—

o CO r^ *—

o o r~H ,

CD

CO i

CD

CO CD S- CD CO Z3 CD > CO CO S- CD CO 13 c CD CO • r— CO M— Q_ CD c rO O SCD CD i

Q_

CT) _J (D ro O E •i— i

4-> =3 4-> rO “O ro i

• i— C

>> CD

OC •i— zc m > •—

Li_

CJ> c

ljcc: ZDCOoOc

Q_OO

ozoc

I—

00Q

m ‘ 3 w 1 1 3S i y 3anss3dd

27

Figure

17.

The

Theoretical

Pressure

Rise

Time

with

Various

Ullage

Volumes

in

the

Apollo

Vessel.
HEAT

REQUIRED

TO

EXPELL

HYDROGEN,

J/g

Figure 18. Venting Characteristics for the Apollo Vessel

at a pressure near 15.65 bar.

28
HEAT

REQUIRED

TO

EXPELL

HYDROGEN,

J/g

Figure 19. Venting Characteristics for the Apollo Vessel

at a pressure near 17.2 bar.

29
2.4 THE RADIO FREQUENCY MASS GAUGE

The difficulty in determining the conditions in a loaded Apollo tank

even when it is weighed confirm the need for a mass gauge for the slush

storage vessel. (Also an accurate liquid level gauge to determine ullage

volume is required.) The radio frequency gauge was connected to the tank

through an existing wire to the capacitance gauge already in the tank. This

connection provided a weak coupling to the tank interior and the weak

coupling had two major effects on the results. First, the signal strength

obtained was low making it difficult to differentiate between the resonance

signal of the tank and the noise in the cables and measuring system. A

block diagram of the system used to measure the tank resonant frequencies is

shown in figure 20. Figures 21a, 21b and 21c compare the signals obtained

from the Apollo tank to the signals from a clean (no internal capacitor,

heater or mixers) sphere with a good internal antenna. The signal from the

Apollo tank requires amplification, so the background noise becomes

competitive with the resonance signal. The resonant signal from the clean

sphere is much greater in strength, so less amplification is required and

the background noise becomes insignificant. Figures 21b and 21c show the

primary resonant frequency from the Apollo vessel, and figure 21a shows both

the primary and secondary reasonances of the clean sphere.

The second effect of the weak coupling was apparent when the resonant

frequencies of the unmixed slush test were compared to the resonant

frequencies of the mixed test. In the second test, the mixing motors had

been connected to the power supply, and this connection changed the empty

resonant frequency of the vessel. Because of this effect it is impossible

to compare the resonant frequency versus remaining mass for the two tests.

Figures 22 and 23 show the primary resonant frequency as determined

from two peaks versus remaining mass for the two slush tests. Even with the

difficulty of determining the resonant frequency, these results are encouraging. The frequency counter can be read to 5 significant digits with

the fifth digit being questionable. With a four significant digit readout,

the mass can be determined to within 1 percent if stratification and weak

coupling do not affect the resonant frequency (from the slope of the curves

on figures 22 and 23). In the mixed slush test, RF readings were taken

before and after mixing. The resonant frequency readings changed by about

+.05 percent leading to a +1 percent error in mass reading due to

stratification. The overall error in contained mass should be well within

+2 percent for a properly designed radio frequency system.

One major unexplained frequency change occurred in the tests. The

resonant frequency increased as the pressure increased in the nonventing

pressure rise portion of the test. Since both the pressure and temperature

of the vessel were increasing, the vessel volume was also increasing. The

30

!
O I—

I— CDCO C CO O CD U O

(V r— C f—

c: o o o. o <£

31

Figure

20.

Block

Diagram

of

the

Instrumentation

Used

to

Measure

the

Resonant

Frequencies

of

the

Apollo

Vessel.
32

The

Primary

Resonant

Frequency

and

Background

Noise

Signals

from

the

Apollo

Vessel.
PRIMARY

RESONANT

FREQUENCY,

MHz

MASS OF HYDROGEN IN THE VESSEL, kg

Figure 22 . Resonant Frequency for Several Peaks Near the Primary

Resonant Frequency of the Apollo Vessel Versus

Remaining Hydrogen Mass.

33
PRIMARY

RESONANT

FREQUENCY,

MHz

Figure 23. Resonant Frequency for Several Peaks Near the Primary

Resonant Frequency of the Apollo Vessel Versus

Remaining Mass.

34
resonant frequency is inversely proportional to the tank radius [3] so the

larger volume should result in a lower resonant frequency. Mixing during

this period had little effect on the resonant frequency rise and the only

other explanation for the increasing frequency would be the poor coupling.

2.5 CONCLUSIONS FROM THE STORAGE TESTS

Even though the accuracy of the experimental results was limited by the

lack of instrumentation in the Apollo tank, significant results were obtained. The slush pressure rise time was 1.08 to 1.17 times the pressure

rise time for normal boiling point liquid hydrogen. The mass of slush

hydrogen loaded was 1.11 to 1.13 times the normal boiling point liquid

loaded in the vessel with the same ullage volume. Both the loaded mass and

the tank volume had insignificant effects on the calculated pressure rise

time while the ullage volume (the most difficult to measure) had a significant effect.

To use slush hydrogen in a practical application, the loading procedure

and loading system must be carefully conceived. The latent heat of the

solid hydrogen in slush with a mass fraction of 0.5 is comparable to the

latent heat of vaporization for helium. Therefore, the loading system must

be of helium transfer line quality. Not only must the heat leak be low, but

since slush hydrogen exhibits nearly the same tendency to thermal oscillate

as helium, the line penetrations should be similar to those used in a helium

transfer system. The thermal oscillation develops at penetrations to the

cold line that lead to ambient temperature conditions. These penetrations

include gauge lines, pressure relief lines, and even valves used in the

system. Secondly, the loading line inside the storage vessel should either

enter the bottom of the vessel or be vacuum insulated as it passes through

the ullage space. Otherwise, thermal stratification cannot develop on the

fluid surface during the fill leaving the ullage volume at triple point

pressure. With the ullage at triple point pressure, it is possible to

overfill the tank unless the venting pressure is reduced to below triple

point pressure at the completion of the fill. Finally, the venting system

should be vacuum insulated to eliminate the higher pressures associated with

flashing in the vent line and to conserve liquid hydrogen. Also an appropriately designed screen over the vent line penetration inside the tank

would allow the slush to be upgraded in the vessel as it is being filled.

The radio frequency gauge does demonstrate a change in frequency with

changing mass in the vessel. However, a vessel with a good antenna inside

to achieve good coupling is necessary to yield good accuracy. Even without

good coupling the R.F. gauge appears to be accurate to within 2 percent in mass loaded

.
3,0 REFERENCES [1]

Voth, R. 0., Producing Liquid-Solid Mixtures of Hydrogen Using an Auger, Nat. Bur. Stand. (U.S.), NBSIR 78-875, 21 pages (Feb. 1978).

[ 2 ] Sindt, C. F „ , A summary of the characterization study of slush

hydrogen, Orogenies 10 , No. 5, 372-380 (Oct. 1970).

[ 3 ] Collier, R. S.; Eller bruch, D.; Cruz, J. E . ; Stokes, R. W.; Luft, P. E. Peterson, R. G.; Hiester, A. E., Mass Quantity Gauging By RF Mode

Analysis, Nat. Bur. Stand. (U.S.), NBSIR 73-318, 184 pages (June 1973)

36
46S-114A (REV. 9-76)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 79-1607

3. ftecitstenf & Accession No,

4. TITLE AND SUBTITLE

PRODUCING SLUSH OXYGEN WITH AN AUGER AND MEASURING

THE STORAGE CHARACTERISTICS OF SLUSH HYDROGEN

5. Publication Date March 1979

7. AUTHOR(S)

Roland 0. Voth and Paul R. Ludtke

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No. CC-6253QA

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, state, zip) 13. Type of Report & Period Covered

ASA

ennedy Space Center, Florida 32899

15. SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

6, ABSTRACT (A 200-word or less (actual summary of most significant information. If document includes a significant bibliography or

literature surveys mention it here.) Part 1. PRODUCING SLUSH OXYGEN WITH AN AUGER

An auger rotating inside a brass tube refrigerated with liquid helium was used

to produce liquid-solid (slush) mixtures of oxygen. The auger produced small particles

f solid oxygen so that the resulting mixture could be transferred and stored. The

uger could produce slush continuously in an appropriate system, and it could produce

islush at pressures higher than the triple point pressure of the oxygen.

Part 2. STORAGE CHARACTERISTICS OF SLUSH HYDROGEN

! Three long term storage tests were conducted on an Apollo hydrogen vessel. The

vessel was filled to 88 percent with normal boiling point liquid hydrogen then the >ressure rise rate to 17.6 bar and the venting rate at 17.6 bar were measured. The two

nther tests were similar except the vessel was filled with slush hydrogen. In one of

these tests, the slush was mixed to eliminate thermal stratification. Filling with

slush instead of liquid hydrogen increased the storage time before venting by 1.08 to

1.17 times and increased the mass loaded by 1.11 to 1.13 times.

17. KEY WORDS ("six to twelve entries; alphabetical order; capitalize only the first letter ot the first key word unless a proper name;

separated by semicolons)

Cryogenic; hydrogen; oxygen; scraping auger; slush; slush production; slush storage

18. AVAILABILITY QT} Unlimited

I | For Official Distribution. Do Not Release to NTIS

| | Order From Sup. of Doc., U.S. Government Printing Office, Washington, DC

20402, SD Stock No. SN003-003- Ol Order From National Technical Information Service (NTIS), Springfield, VA. 22161

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF

PRINTED PAGES

42

22. Price

$ 4 . 00

J YfU.S. GOVERNMENT PRINTING OFFICE: 19 7 9

- 0

- 677 - 072/1284 u SCOMM- DC