NBSIR 73-344

HEAT TRANSFER AND MIXING OF SLUSH HYDROGEN

. F. Sindt

P. R. Ludtke

Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

November 1973

Final Report

Prepared for:

NASA Headquarters

Washington, D. C.

NBSIR 73-344

HEAT TRANSFER AND MIXING OF SLUSH HYDROGEN

C. F. Sindt

P. R. Ludtke

Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

November 1973

Final Report

Prepared for:

NASA Headquarters

Washington, D. C.

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS. Richard W Roberts Director

CONTENTS

Page

1. Introduction 1

2. Experimental Apparatus 2

2. 1 Heat Transfer System 3

2. 2 The Mixer 7

3. Instrumentation 11

3, 1 Heat Transfer Instrumentation 11

3. 2 Mixer Instrumentation 12

4. Experimental Procedure 15

4. 1 Heat Transfer Test Procedure 15

4. 2 Mixer Test Procedure 16

5. Test Results and Discussion 16

5. 1 Heat Transfer Test Results and Discussion .... 16

5. 2 Mixer Test Results and Discussion 25

6. Conclusions and Recommendations 29

7. References 33

8. Appendix 34

8. 1 Bibliography of Publications by NBS Cryogenics

Division on Slush Hydrogen 34

8. 2 Unpublished Reports 36

iii
LIST OF FIGURES

Page

Figure 1. Schematic of Experimental System 4

Figure 2. Experimental Dewar Schematic 5

Figure 3. Heat Transfer Rate From the Radiation Panel . . 8

Figure 4. Slush Mixer 10

Figure 5. Bullseye Capacitance Densitometer 14

Figure 6. Vertical Temperature Profile in Liquid Over

Settled Slush . 18

Figure 7. Liquid Temperature Versus Heat Transfer Rates

in Liquid Over Settled Slush ........ 20

Figure 8. Self-Pressurization of the Ullage Over Settled

Slush 21

Figure 9. Self-Pressurization with Intermittent Mixing. . . 22

Figure 10. Self-Pressurization Versus Mixing Cycle .... 24

Figure 11. Mixing of Fresh Slush With a Turbine Mixer. . . 27

Figure 12. Mixing of Aged Slush With a Turbine Mixer ... 28

Figure 13. Mixing Slush With a Paddle Mixer 30

iv
Heat Transfer and Mixing of Slush Hydrogen'''

C. F. Sindt and P. R. Ludtke

Heat transfer to slush hydrogen and mixing were investigated in a 1 rQ-^ cylindrical vessel. The effects of heat transfer rates on thermal stratification and on self-pressurization

were measured. Temperature profiles in thermal stratification were found to be more dependent on slush level and slush

settling rates than on liquid level. Solids in the slush appear

to be involved in the heat transfer mechanism as slush level

affected the amount of warm liquid reaching the top of the

dewar and therefore affected the self-pressurization rates.

Mixing effectiveness and power requirements to mix

slush hydrogen were determined for two configurations of turbine mixers and one paddle mixer. Mixing power requirements

were found to be sensitive to the mixer location and configuration.

Key words: Heat transfer; liquid hydrogen; mixing; mixing

power; paddle mixers; slush hydrogen; turbine mixers.

1. Introduction

Liquid and slush hydrogen are the selected propellants for

current and future space exploration rockets and more recently have

become of great interest as fuel for the aircraft industry because of

impending shortages of hydrocarbon fuels. Slush hydrogen has the

advantage over liquid hydrogen of additional heat capacity and greater

density. Slush hydrogen characterization studies have been conducted

at the Cryogenics Division of NBS over the past decade. These studies

have included preparation, flow, pumping, aging, solid particle configuration, and instrumentation of slush hydrogen.

Characteristics that have not been extensively investigated but are

required to get maximum benefits from the increased heat capacity are

heat transfer and mixing. Heat transfer and mixing investigations to date

have been very limited in scope. Basic heat transfer to slush has been

previously measured in a small scale laboratory apparatus as phase I of

this program and was reported by C. Sindt [1972, 1973]. The objective

of this phase of the program was to determine larger scale heat transfer

characteristics, thermal stratification, and mixing energy requirements.

This research was performed under NASA Contract No. W1 3300.
2. Experimental Apparatus

The experimental apparatus consisted of two vacuum insulated

3

vessels. One vessel of 0. 4 m capacity was used as a slush generator.

3 The second vessel of approximately 1 m capacity was the experimental

vessel. This vessel was cylindrical and was 0. 76 m in diameter and

2. 34 m deep. It was vacuum insulated at the bottom and side walls.

The inner wall of this dewar was 0. 9 mm thick stainless steel. These

thin stainless steel walls were well suited to allow thermal stratification in the liquid. The top cover was a 25. 4 mm thick stainless steel

flat plate. The top plate had two 1 1 0 -mm-diamete r windows--one for

lighting, the other for visual observation. Also included in the top plate

was a 230-mm-diameter access hole. The access hole cover plate was

used as a mounting base for an experimental mixer drive. The mixer

drive and the mixer instrumentation were mounted externally on the

access cover. The mixer was suspended on a driving shaft.

Thermal insulation for the top plate of the experimental vessel

consisted of a 7 5-mm-thick layer of high density polystyrene foam, which

was suspended below the top plate on three 1/4-inch bolts, and a radiation

shield suspended at 25 mm below the foam on three separate 1/4-inch

bolts. The radiation shield was chromium plated stainless steel.

Incorporated in the shield were two hinged, closable doors to cover the

window areas. The radiation shield had 13 mm radial clearance. With

this arrangement the vent gas passed around the outside edge of the shield

and flowed across the shield to the vent which was located near the center

of the top plate. The vent gas, therefore, helped to cool the

shield.

Liquid hydrogen for the experiments was provided from a portable

dewar. This dewar was connected to the generator and the experimental

2
dewar with a vacuum insulated transfer line. The transfer line was also

used to transfer slush from the generator to the experimental dewar.

In the process of filling the experimental dewar with slush,

several batches of slush were made in the generator., These were

transferred to the experimental dewar; then the excess liquid over the

settled slush was back transferred to the generator to be used in prepara

tion of the next batch. This process required two transfer lines and

valves into the experimental dewar, one for transferring slush into the

dewar and one for back transferring liquid. Both lines connected to the

main transfer line outside the dewar; both extended to near the dewar

bottom and were vacuum insulated to near the end. The liquid return

line was enlarged at the end and covered with a 30 mesh''^ screen to

prevent back transfer of the solid hydrogen particles. Upgrading techniques ajid slush production reported by Sindt and Ludtke [1970] were

developed in this system which is shown in figure 1.

2. 1 Heat Trajisfer System

Temperature controlled thermal radiation heaters were used to

get uniform heat transfer to the thin stainless steel walls of the experimental dewar. The heaters consisted of a cylinder and a dished head

to cover the dewar bottom and were Z5 mm in diameter larger than the

experimental vessel. The two sections were thermally insulated from

each other, as the lower head was suspended by four 16 mm by 3. 2 mm

tetrafluoroethylene straps. The radiation heaters were both 1 mm thick

copper plate. They were heated to the desired temperature electrically.

The heaters are shown in figure 2.

A 30 mesh screen has 0. 59 mm square openings.

3

TORQUE a SPEED

TRANSDUCERS

FOAM BLOCK

INSULATOR

RADIATION

SHIELD

CYLINDRICAL

RADIANT HEATER

RACK FOR VERTICAL

ADJUSTMENT OF

MIXER

BULLSEYE

DENSITOMETER

(Typical of 6)

DISHED HEAD

RADIANT HEATER

MIXING MOTOR

MIXER ADJUSTING SHAFT

(Pull to disengage mixer)

WINDOWS

ELECTRICAL HEATERS

(18 on cylindrical heater

and 2 on dished head)

-MIXER DRIVE SHAFT

MIXER BAFFLE

(Typical of 3

spaced at 120*)

MIXER

LOWER MIXER

BEARING

Figure 2. Experimental dewar schematic.
Electrical resistance units were applied to the heaters using a

filled epoxy. They were spaced approximately 130 mm apart and were

wrapped around the cylinder and laid in a spiral on the dished head.

The electrical connections were made so that the cylindrical heater was

heated by three separate circuits and the dished head by one circuit.

Type T thermocouples were mounted on the copper between heating units

and in the epoxy at the units. These thermocouples, which were referenced

to an ice bath, were used for temperature control of the radiation heaters.

To reduce heat transfer from the outer shell to the radiation heaters,

the heaters were wrapped on the outer surface with one sheet of aluminum

foil after they were assembled and instrumented. To increase heat transfer from the radiation heaters to the experimental dewar walls, the outer

surface of the dewar wall and the inner surface of the radiation heaters

were coated with a "velvet textured" black paint which is commercially

prepared for improving radiant heat transfer [ ASTM, 1973].

The actual heat transfer by the radiation heaters to the inner vessel

was determined in a series of tests conducted prior to the heat transfer

tests. The method used to measure total heat transfer to the inner vessel

was to measure the amount of liquid hydrogen boiled away at a constant

pressure while the temperature of the radiation heaters was controlled

at different levels. The temperature of the heaters was easily controlled,

and the difference in temperature indicated by thermocouples on the copper

walls and thermocouples in the mounting epoxy of the electrical units was

less than two K at the maximum heating rate. Therefore, the radiation

heaters were assumed to be of constant temperature throughout when all

of the thermocouples mounted on the copper gave the same reading.

The mass of liquid that boiled away and the heat of vaporization of

the liquid hydrogen at the dewar pressure were used to calculate the total

6
heat input. The mass was calculated from measurements of the gas

volume with a dry gas meter and with a calibrated gas holder. Pressure

and temperature of the gas were measured with a manometer and mercury

filled thermometer respectively. The manometer accuracy was estimated

to be ± 12 Pa. The temperature was estimated to be accurate to ± 0. 2 K.

The pressure in the vessel was controlled by the back pressure of the

gas holder, which was susceptible to ambient pressure changes. Therefore, the barometer was read frequently, and if a change larger than

20 Pa occurred, the data prior to the change was excluded.

Some heat entered the dewar from the top plate. Calculations of

the heat transferred to the liquid from the top revealed that the heat flux

was less than one percent of the total at the lowest heater temperatures

and was therefore considered negligible.

The radiant heaters were calibrated using liquid hydrogen in the

experimental vessel. They floated at a temperature of about 110 K with

the experimental vessel full of liquid and no electrical energy input.

Data from the calibration of the radiation heaters were reduced to

the form of heat transfer per unit area versus the radiation heater temperature. Two sets of data were taken, one with the bottom heater on,

the other with the bottom heater off. These data are in agreement and

are shown in figure 3 with the calibration curve that was used for all of

the slush heat transfer data reduction.

2. 2 The Mixer

The mixer was supported and driven from a shaft through the

access plate as previously noted. The mixer was driven with an air

motor capable of delivering 2050 watts at 304 rad/s (2900 rpm). A

gear reduction unit was installed on the motor for the last group of tests.

The mcLximum power of 2050 watts was then produced at 75 rad/s (720 rpm).

7
110 130 150 170 190 210 230 250

RADIATION PANEL TEMPERATURE, K

Figure 3. Heat transfer rate from the radiation panel.

8
A torque meter and speed sensor were mounted in the shaft above

a hermetic seal that was mounted in the access plate. The hermetic

seal was a commercially available unit that seals with magneticallycontrolled fluid suspension of ferrous particles held between the rotating

element and the stationary element.

The mixer shaft extended to the bottom of the dewar where it was

supported radially by a bottom mounted bearing. A concentric tube

extended down from the access plate around the shaft for 1. 52 m and

had two bearings to provide more radial support and prevent shaft whip.

The mixer was mounted on the shaft in such a way as to allow vertical

adjustment using a rack and pinion gear. The adjustment was from

0. 22 m to 0, 82 m off the dewar bottom. The mixing head also contained

gearing to allow adjustment of the mixer blade angle through 180 degrees

of rotation. The two adjustments were made with a rod supported from

the top plate, and adjustments could be made at any time that the mixer

was stopped and the slush level was below the mixer such that the operation could be observed through the view port. Figure 4 shows the mixer

with the blades set at 45°.

The blades selected were 100 mm long by 76 mm wide and rectangular. The center hub was 84 mm square and 64 mm thick. The mixer

was 305 mm in overall diameter and had four blades. The diameter is

larger than recommended for turbine mixing of low viscosity fluids

[Uhl 1966]; however, the hub of the mixer is large because of the adjusting

features. The large hub resulted in shorter blades for the same diameter,

and considering the actual blade length, the blade diameter to vessel

diameter ratio was 0. 26 which is about that recom.mended. With the

blades set at an angle of 45°, the mixer is an axial flow turbine, and

with the blades set at 90°, the mixer is a paddle or a radial flow turbine.

From the review of mixing as reported by McConnell[ 1971] , these mixers

are two of the more preferred mixers for mixing slush hydrogen in a one "g" environment.

^ 9
10
Mixing in low viscosity fluids requires baffling. Three equallyspaced baffles 76 mm wide were mounted 15 mm from the dewar walls.

The baffles extended the full length of the used dewar volume. The

width of 76 mm satisfied the recommended ratio of one-to-ten for baffle

width to vessel diameter in low viscosity fluids [LFhl, 1966] .

3. Instrumentation

All of the data for the heat transfer and the mixing experiments

were recorded using an automatic data acquisition system (ADAS) with

a 56 channel input multiplexer. The ADAS output wa,s recorded on

magnetic tape which was then processed in a large computer for final

data reduction.

3. 1 Heat Transfer Instrumentation

One vertical and three radial thermocouple rakes were mounted

in the experimental dewar. The radial rakes had eight thermocouples

spaced at 1, 3, 6, 10, 40, 100, 200, and 380 mm from the dewar wall.

They were located at 0. 55, 1. 27, and 1.73 m from the dewar bottom.

The first of 10 thermocouples on the vertical rake was 1. 38 m from the

bottom. The 10 thermocouples were spaced 50 mm apart and were

200 mm from the dewar wall. All of the thermocouples were copper

vs gold (0. 07 atomic percent iron) and were referenced to the thermocouple located at 0. 55 m from the bottom and 100 mm from the dewar

wall. This reference thermocouple was attached to a calibrated carbon

resistance thermometer. The outputs from the thermocouples were

amplified 1000 times, then fed to the ADAS. The resistance thermometer

used 20 )_L a constant current, and the voltage across the thermometer

was measured directly by the ADAS.

It was not necessary to use the output of the resistance thermometer

for the tests since the reference thermocouple was always surrounded by

slush and therefore was always at triple -point temperature, 13. 80 K.

11
Thermocouples were also mounted on the outside wall of the dewar

inner vessel. Because the ADAS would not accept a grounded input, the

decision was made to electrically isolate these thermocouples from the

dewar. They subsequently failed and dewar wall temperatures were not

available for data reduction.

The estimated error of the thermocouple measurement based on

the output of thermocouples always surrounded by slush hydrogen is

± 0. 25 K accuracy and ± 0. 1 K precision with the largest single contribution to uncertainty in precision being the drift in the amplifiers. The

drift of the amplifier in two hours was equivalent to ± 0. 05 K; however,

the day-to-day drift was twice as large. Since the reference thermocouple and the resistance thermometer were always in slush, these

temperatures were always assumed to be 13. 803 K[Roder, 1965].

Based on this assumption, the resistance thermometer and the associated

electronics, which did not include an amplifier, indicated a maximum

uncertainty of ± 0. 05 K over the entire test program of more than a month duration.

Pressure in the experimental vessel was measured with a variable

reluctance type of pressure transducer which was calibrated using a mercury manometer. The transducer was referenced to vacuum. The

mercury manometer used for calibration was readable to ± 40 Pa,

and the pressure transducer was linear and repeatable to ± 0, 25 percent

of the used range, or ± 250 Pa. Therefore, the pressure measurements

are estimated to be accurate and precise to ± 290 Pa.

3. 2 Mixer Instrumentation

The mixer torque, speed, position and blade angle were measured.

The torque was measured with a torque meter which used a torsion

12
member with a photoelectric sensing device for determining the torsion

member displacement. The meter was calibrated prior to the test

program. The estimated accuracy of the torque meter from the manufacturers specification is ± 0. 06 Nm.

The mixer speed was measured with an inductance pick up which

was converted to volts dc using a frequency to voltage converter.

Frequent calibration of this system indicated a maximum drift of ± 1 percent at frequencies near those produced by the pick up at mixing

speeds. This translates into uncertainty in power measurement of 5%

at low power (30 watts) to 2% at powers near 600 watts.

The mixer position was measured with a steel tape measure prior

to running the tests. Blade angle was determined from the number of

turns made on the adjusting gear train. Neither of these measurements

was considered critical, so no precise method of measurement was

devised.

Six slush densitometers were installed in the dewar to assist in

determining the effectiveness of the mixer. These densitometers used

capacitance as the measurement method and were bullseye in shape. A

typical ca.pacitance measurement densitometer is shown in figure 5. The densitometers were located at 0. 36, 0. 43, 0. 71, 1. 12, 1. 47, and

1. 78 meters off the bottom and at 0. 1 to 0. 18 m from the dewar wall.

Four were perpendicular to the wall, one was parallel to the wall, and

the top one was parallel to the liquid surface.

The densitometer signal conditioning units were capacitance

bridges with circuits to convert capacitance unbalance to a dc voltage.

This voltage was fed to the ADAS. Stability of the circuits was a problem, and short term drift limited the duration of accurate measure

ments to less than one hour. This problem precluded their use as

13
14
absolute density measuring instruments. However, they proved

effective in determining when mixing of the slush was thorough. When

the densitometer readings remained relatively stable with increased

mixer speed, and visual observation substantiated that mixing was

apparent, the mixing was assumed complete. Marginal mixing was

evident by large fluctuations in the densitometer signal and was also

verified by visual observation of the fluid near the surface and around

the top bullseye densitometer.

4. Experimental Procedure

4. 1 Heat Transfer Test Procedure

Three types of heat transfer experiments were conducted. They

were 1) heat transfer to slush with the dewar pressure starting at

triple-point and self-pressurizing to approximately one atmosphere

pressure, 2) heat transfer to slush with the dewar pressurized to one

atmosphere with warm helium gas immediately after the final slush

solid fraction upgrading cycle, and 3) heat transfer to slush with mixing

to drop the dewar pressure when it reached 26. 7 k Pa. In each experiment slush was made in the generator and transferred to the experimental dewar with warm helium gas as the pressurant. Liquid was

returned to the generator through the 30 mesh screen until the liquid

level was 100 mm above the settled slush level. Batches of slush

were made in the generator and transferred into the experimental dewar

until the slush and liquid level was very near the top thermocouple in

the vertical rake. When the experimental dewar was about half filled,

the radiation heaters were energized so that by the time the dewar was

full, the heaters were at the desired temperature.

For the self-pressurization experiments, the experimental dewar

was pumped to triple -point pressure after filling and the vent was closed.

The dewar was allowed to self-pressurize, and data were taken automatically at intervals of 20 to 60 s.

15
The experiments where the slush was mixed when the pressure

reached 26. 7 k Pa were identical except that the mixer was run when

the pressure reached 26. 7 k Pa, and it continued to run until the pressure dropped below 13. 3 k Pa. Mixing was then stopped, but the

pressure frequently continued to fall to near triple-point pressure,

7. 042 k Pa [Roder, 1965]. This continuing loss of pressure was

attributed to residual turbulence after the mixer was stopped.

The experiments where the dewar was pressurized with helium

gas were conducted in the same way as those described above except

that warm helium gas was introduced immediately after the last slush

transfer. Helium gas was introduced until the vent relief valve opened

at about 93 k Pa pressure. Helium gas was continuously introduced

until it was no longer required to maintain the pressure at vent pressure.

Data were taken during the pressurization and for the duration of the

test at 20 to 60 s intervals. The test duration was one hour or until

the slush level was well below the second radial thermocouple rake.

4. 2 Mixer Test Procedure

Two different types of mixing tests were run with three mixer

configurations and two mixer positions. Mixing tests were performed

in freshly prepared slush prior to heat transfer tests, and in aged slush

after the heat transfer tests. The mixer configurations were mixing

with the blades at 45° pushing down, mixing with the blades at 90°, and

mixing with the blades at 45° pushing up. The two mixer locations

were at 0. 25 m and 0. 71 m above the bottom of the dewar.

5. Test Results and Discussion

5. 1 Heat Transfer Test Results and Discussion

The objectives of the heat transfer experiments were: 1) measure

the thermal stratification rates in the liquid over settled slush and

16
correlate stratification rates with heat transfer rates; 2) measure self

pressurization rates in a vessel filled with slush hydrogen and determine

the effects of heat transfer rates on pressurization; 3) determine if the

dependence of heat transfer rate on wall temperature was the same as reported for the small scale heat transfer experiments [Sindt, 1973].

The long range objective of the heat transfer experiments was to obtain

enough data to enable development of an analytical model to predict pressure rise and thermal stratification for various heat transfer rates in

vessels used for slush hydrogen storage.

Seven tests were conducted in which the pressure in the experimental vessel was raised to near one atmosphere with helium gas

immediately after the last slush transfer. These tests were conducted

to determine thermal stratification rates in the dewar at one "g" and

at one atmosphere pressure. This series of tests covered heat transfer

2

rates from 0. 001 to 0. 0175 W/cm . Several significant characteristics

were evident during the series of tests. First, the thermal stratification

profiles that developed were dependent on initial slush level and the age

of the slush in the dewar. The age of the slush affects stratification

because it determines the settling rate of slush level as a function of

time. The instrumentation did not include a liquid level or slush level

indicator, therefore, these levels were estimated by visual observation.

Initial slush levels could be observed to ± 5 cm; however, the inaccuracy

as well as the infrequency of visual observation as slush level dropped

resulted in increased dispersion of this data from test to test.

The second significant characteristic observed was that all of the

liquid over the settled slush increased in temperature with time. The

thermal stratification followed the slush level as it dropped due to

melting and settling. Figure 6 shows the typical temperature profiles

at even time intervals as indicated by the vertical temperature rake.

17

Graphs similar to those shown in figure 6 were prepared for each

heat transfer rate. Then, to correlate the effects of heat transfer rate,

a cross plot was made of temperature versus heat transfer rate for

selected levels and times. The best correlation resulted when the levels

were referred to the initial slush level and not the liquid level. Figure 7

shows a plot of temperature for different levels below the initial slush

level at times of 10, 30, and 50 minutes after pressurizing with helium

gas. From these data it appears that the relation between heat flux and

stratification temperature at a prescribed location is linear over most

of the range investigated. The data for levels between 0 and 0. 1 m

were very inconsistent. This inconsistency is probably caused by the

previously mentioned slush settling rate, especially in the top . 05 m of

slush. Also, the initial heating in the liquid over settled slush is dependent on the manner of pres surization with the warm helium gas. Ideally,

it would be desirable to start with the liquid and slush at the same levels

after pressurizing with cold helium gas. Liquid and slush level indicators

and a helium precooler should be used in future experiments.

Three tests were conducted to determine self-pressurization rates.

2

These were at 0. 001, 0. 0045 and 0. 0067 W/cm heat rates. The data

are shown in figure 8. A trend in increased pressure rise rate versus

increased heat flux is evident, but the data are not sufficient to define

this trend well enough to establish a correlation. The similarity between

2

the pressure rise rate for the 0. 0045 and 0. 0067 W/cm heat rate may

be attributable to the variance of the initial slush levels.

The significance of liquid and slush levels on pressurization rate

is more closely shown by the test where the slush was mixed when the

pressure reached 26. 7 k Pa. Pressure rise versus time for this test

are shown in figure 9. Pressure over-shoot at both high and low pressure

19
0.020 1

O-40cm beiow initial slush level

- 30 cm

O - 20 cm

A - I 0 cm

30 min

, f 50 min after pressurization

f/H

////

0

14 15 16 17 18 19

LIQUID TEMPERATURE, K

20 21

Figure 7. Liquid temperature versus heat transfer rates in liquid over settled slush.

20
100

10 20 30 40 50 60 70

TIME, min

Figure 8. Self-pres surization of the ullage over settled slush.

21
o

o «n o lo o o

ro cvj cj — —

22
resulted because the mixing operation was not controlled by a pressure

sensing device but was controlled manually from visual observation of

pressure. To eliminate the effects of this overshoot in the data, a second curve was drawn showing the time required for a given pressure

rise of 13.4k Pa from 1 3. 3 to 26. 7 K Pa . This is shown in figure 10.

The significance of these data are twofold. First, mixing is an effective

means of holding pressure in a slush container within a desired range;

and second, pressure rise over settled slush due to self-pressurization

is dependent on both the liquid and slush levels. The fact that the selfpressurization rate increases with lower slush levels has significant

implications about the heat transfer mechanism to slush. In a vessel

containing liquids, the heat flows from the wall to the liquid, and the

liquid then flows up the wall in an ever increasing thickness boundary

layer. This warmed liquid then flows across the top of the liquid. The

resulting model is a thin layer of warmed liquid flowing up the wall and

counter -flowing down the center, ["Schwind and Uhet, 1964; Evans and

Reed, 1968]. If this were the case in slush, the time to repressurize

after each mixing cycle should be about constant since the liquid starts

each cycle at about the same temperature. However, what appears to

be occurring in the slush system is that the heat that flows in below

the slush level is actually being absorbed by the slush at the expense of

solid melting. If this were the case, the amount of heat flowing up the

wall above the slush would increase as the slush level decreased and,

therefore, the top layer of liquid would heat faster, resulting in an

increased pres surization rate. This increase in pressurization rate

was observed in the experiments. The implication is that the solid

particles near the dewar do participate in the heat transfer mechanism.

23
25

c

*E 20

LiJ

Q/A = 0.001 W/cm^

IN

cr

cn

UJ

a:

Ql

UJ

cr.

10

12 3 4 5 6

MIXING CYCLE

Figure 10. Self-pres surization versus mixing cycle.

8

24
This premise is further substantiated by thermal gradients that develop

over settled slush. This gradient at any one time looks like the gradient

in a vessel of liquid with the bottom displaced to the slush level. More

data and a more detailed analysis of this observation seems warranted

to confirm this observation before attempts are made to develop an

analytical model of heat transfer to vessels containing slush hydrogen.

Another significant result from the heat transfer tests was that

no radial temperature gradient was ever measured in any of the tests.

This was the case whether the radial thermocouples were in slush or in

liquid over settled slush. Apparently the warm liquid flowing up the wall

does not develop a stream as thick as 1 mm, which was the location of

the closest thermocouple to the wall.

5. 2 Mixer Test Results and Discussion

The mixing tests were conducted in conjunction with the heat transfer tests. A mixer test was conducted immediately before starting each

heat transfer test. These were considered to be tests in mixing of fresh

slush. Other tests were conducted after completion of a heat transfer

test. These were considered mixing tests in aged slush. The fresh slush

was really a mixture of solids that were from 15 minutes to 2 hours old, as this was the time required to fill the experimental dewar with settled

slush. The aged slush was one to two hours older than the fresh slush.

Although this is a relatively short aging period significant aging effects

did occur as was evident by the drop in slush level in excess of that

caused by melting.

Very early in the experimental program it was discovered that

slush which had been upgraded in the experimental vessel without mixing

during transfer tended to pack to very high solid fraction in the bottom

of the vessel. The slush densitometer that was located in the plane of

25
the mixer indicated solid fractions as high as 0. 8 during one attempt

to mix after upgrading was completed. This is the highest solid fraction

observed in slush to date. The increase in density at the mixer plane

was not observed in the slush that was mixed during transfer.

A very significant development that accompanied the high solid

fraction was that on several occasions transfer of slush from the generator to the experimental dewar was blocked. This occurred when the

experimental dewar was nearly full of settled slush and only if the slush

had not been mixed during any of the previous transfers. The slush

transfers were made using pressure differences with a total available

pressure difference of 100 k Pa. The apparent blockage could be relieved

immediately by running the mixer if and only if the mixer mixed the

entire dewar of slush.

A second very significant observation was that the power required

to mix a dewar nearly filled with previously unmixed slush was much

greater than was required to mix a dewar of slush previously mixed.

The data for fresh slush mixing with the mixer blades at 45° are

shown in figure 11. The curve is from the mixing data and gives the

power required to run the mixer in all cases except when the slush was

not mixed during transfer. The symbols indicate the point where total

mixing started. The one point at 575 watts power was for slush not

premixed. As is indicated, this point is not on the curve for all other

mixing data. The second high power point shown is for slush that was

mixed for all but the last two transfers during upgrading. As is indicated,

the power to initiate mixing is considerably greater than that for premixed

slush, but in this case it does fall on the same power curve as the other

mixing data.

The data for mixing of aged slush with the blades at 45° is shown

in figure 12. The power versus speed relationship was found to be the

26
FRESH SLUSH

OFF BOTTOM

DISTANCE

O - 45*» Up at 0.71 m

A - 45* Down at 0.71 m

- 45** Up at 0.25 m

Slush not

pre-mixed

Partially

pre-mixed

100 200 300 400 500 600 700

MIXER SPEED, rpm

rad/s = rpm • 0.10472

Figure 11. Mixing of fresh slush with a turbine mixer.

800

27
AGED SLUSH

600

500

rM . OFF BOTTOM

DISTANCE

O - 45*' Up at 0.71 m

A - 45** Down at 0.71 m

- 45** Up at 0.25 m

V - 45* Down at 0.25 m

^

400

GC

UJ

O 300

oc

UJ

X 200

00

Shaded point is partial mixing.

200 300 400 500 600

MIXER SPEED, rpm

rad/s = rpm -0.10472

800

Figure IZ. Mixing of aged slush with a turbine mixer.

28
same as for fresh slush. The speed required to start mixing is in

general less for the aged slush than for fresh slush. This inference,

that aged slush mixes with less speed and power than fresh slush, maybe false because the aged slush settled solid level was always less than

fresh slush, since settling and solid melting had occurred. To get an

accurate comparison, the levels and solid fraction of the aged and fresh

slush should be the same; however, no such equal comparison was made

primarily because neither an accurate slush level device nor accurate

densitometers were included in the instrumentation.

Figure 13 shows the mixing characteristics for the blades at 90° . For this configuration the mixing power was sensitive to the distance of

the mixer from the bottom. As for the other figures, the symbol indicates

when total mixing is developed, except for the point at 650 watts. This

point was the maximum speed the mixer would run, but total mixing did

not develop. This was in slush that had not been previously mixed.

From the mixing experiments it is apparent that the least power

and speed are required for the mixer blades at 45° pushing up and with

the mixer at 0. 7 m off the bottom. Because of the difficulty in transferring slush without mixing, this configuration was not evaluated with

unmixed slush.

6. Conclusions and Recommendations

Several significamt results were obtained from the heat transfer

and mixing experiments. These results lead to the following conclusions:

Heat Transfer

1. Thermal stratification develops in liquid over settled slush,

and the stratification characteristics are similar to those in a vessel of

liquid except that the stratification develops starting at the slush level

instead of the bottom of the vessel.

29
700

600

500

400

UJ

I^

300

ocUJ

X

S 200

00

OFF BOTTOM

DISTANCE

This is maximum point run. No mixing occurred here.

Slush was not mixed during transfer. — 0 -^

0.71 m

" 0.25 m J

^ Fresh Slush

V -

0.71 m

'

0.25 m .

> Aged Slush /

^0.25 m

v/

/—0.1\ m

100 200 300 400 500 600

MIXER SPEED, rpm

rad/s = rpm • 0.10472

Figure 13. Mixing slush with a paddle mixer.

700 800

30
2. Heat entering below the slush level is apparently absorbed at

the expense of melting solid and is not carried up the vessel wall in the

boundary layer as in liquid.

3. Temperatures at specific locations in the stratified liquid

are linearly dependent on heat transfer rates at the dewar walls„

4. Shapes of temperature profiles in the liquid over settling

slush are more dependent on the initial slush level and on slush settling

rate than on liquid level.

Mixing

1. Mixing of slush is an effective way to reduce pressures to

near triple -point. However, pressure control with mixing at some level

above triple -point pressure may be difficult because of the tendency for

the slush, once mixing starts, to mix immediately and therefore condense

ullage gas until the pressure is very near triple-point pressure.

2. Mixing power required to initiate mixing is greatly increased

if slush solid fraction is upgraded without mixing.

3. Slush solid fraction upgrading as reported by Sindt and

Ludtke [1970] may require effective mixing to prevent flow stoppage at

the end of transfer lines, especially in freshly prepared slush.

4. Mixing power required to initiate inixing is very dependent on

mixer configuration and location. Of the three configurations and two

locations tested, a turbine mixer with blades at 45° pushing up and

located at about 1/4 the depth of the slush mixed the best with the least

power. The power required for mixing fresh slush with this configura3

tion was 45 W/m .

5. Mixer and baffle design used in commercial applications for

low viscosity fluids are effective for slush hydrogen. Designs other

than those recommended fMcConnell, 1971] were not evaluated to

determine if other configurations may perform better.

31
The heat transfer experiments woiild have been more productive

had the temperature measurements at the dewar been successful.

Because of the failure of these thermocouples, data from these tests

could not be compared to the basic heat transfer data reported in phase I

[Sindt, 1973],

From these experiments several recommendations are evident.

Instrumentation techniques to obtain accurate wall temperature in

vacuum environments needs verification before more tests of this type

are conducted. More testing is needed to verify and understand the

apparent different heat transfer mechanism in slush than in liquid. More

testing is needed to determine the relationship of pressure rise rate,

slush levels, and heat transfer rates at the dewar walls. The need for

more testing is also indicated before any reliable analytical models can

be verified for cylindrical tank configurations.

32
7. References

McConnell, P. M. , and Sindt, C. F. , Slush hydrogen mixing:

preliminary study, Unpublished NBS Report (1971).

Sindt, C. F. , Part A. Hydrocarbon suspension in slush hydrogen.

Part B, Heat transfer to slush hydrogen. Unpublished NBS

Report (1972).

Sindt, C. F. , Heat transfer to slush hydrogen, to be published in

Advances in Cryogenic Engineering 19 .

Sindt, C. F. , and Ludtke, P, R. , Slush hydrogen flow characterization

and solid fraction upgrading. Book, Advances in Cryogenic

Engineering Ed. K. D. Timmerhaus, pp. 382-390 (Plenum

Press, New York, N. Y. , 1970).

Roder, H. M. , Weber, L. A. , and Goodwin, R. D. , Thermodynamic

and related properties of parahydrogen from the triple point to

100° K at pressures to 340 atmospheres, Nat, Bur. Stand. (U. S. ) Monogr. 94 (1965).

Standard method of test for heat flux through evacuated insulations using

a guarded flat plate boiloff calorimeter, to be published by the

ASTM

Uhl, V. W. , Mixing Theory and Practice 1_ (Academic Press,

New York, N. Y. , 1966).

33
8. Appendix

8. 1 Bibliography of Publications by NBS Cryogenics Division on

Slush Hydrogen

Mann, D. B. , Ludtke, P. R. , Sindt, C. F. , and Chelton, D. B. , Liquid- solid mixtures of hydrogen near the triple point. Book,

Advances in Cryogenic Engineering IJ^, Ed. K. D. Timmerhaus,

pp. 207-217 (Plenum Press, New York, N. Y. , 1966).

Mann, D. B. , Sindt, C. F. , Ludtke, P. R. , and Chelton, D. B. , Slush hydrogen characteristics, Proc. of the Conference on

Long Term Cryo-Propellant Storage in Space, NASA, Marshall

Space Flight Center, Huntsville, Alabama (1966).

Sindt, C. F. , and Mann, D. B. , Temperature-entropy diagram

for parahydrogen triple -point region, Nat. Bur. Stand. (U. S. ) Tech. Note 343 (1966).

Knight, B. L. , Timmerhaus, K. D. , and Flynn, T. M. , A

superconducting liquid-level sensor for slush hydrogen use. Book,

Advajices in Cryogenic Engineering _1_1_, Ed. K. D. Timmerhaus,

pp. 218-222 (Plenum Press, New York, N. Y. , 1966).

Daney, D. E. , cind Mann, D. B. , Quality determination of liquidsolid hydrogen mixtures , Cryogenics 7^ , No. 5 (Oct. 1967).

Daney, D. E. , Ludtke, P. R. , Chelton, D. B. , and Sindt, C. F. , Slush hydrogen pumping characteristics, Nat. Bur. Stand. (U.S.),

Tech. Note 364 (1968).

Weitzel, D. H. , Sindt, C. F. , and Daney, D. E. , Hydrogen slush

density reference system. Book, Advances in Cryogenic

Engineering J_3 , Ed. K. D. Timmerhaus, pp. 523-533,

(Plenum Press, New York, N. Y. , 1968).

34
8. Sindt, C. F. , Ludtke, P. R. , and Daney, D. E. , Slush hydrogen

fluid characterization and instrumentation, Nat. Bur. Stand.

(U.S.) Tech. Note 377 (1969).

9. Rapial, A. S. , and Daney, D. E, , Preparation and characterization

of slush hydrogen and nitrogen gels, Nat. Bur. Stand. (U. So ) , Tech. Note 378 (1969).

10. Daney, D. E. , Ludtke, P. R. , and Sindt, C. F. , Slush hydrogen

pumping characteristics using a centrifugal type pump. Book,

Advances in Cryogenic Engineering 14_ , Ed. K. D. Timmerhaus,

pp. 438-445 (Plenum Press, New York, N. Y. , 1969).

11. Sindt, C. F. , and Ludtke, P. R. , Slush hydrogen flow characterization and solid fraction upgrading. Book, Advances in Cryogenic

Engineering 2_5, Ed. K. D. Timmerhaus, pp. 382-390

(Plenum Press, New York, N. Y. , 1970).

12. Daney, D. E. , and Rapial, A, S. , Preparation and characterization

of slush hydrogen and nitrogen gels. Book, Advances in Cryogenic

Engineering J_5, Ed. K. D. Timmerhaus, pp. 467-475

(Plenum Press, New York, N. Y. , 1970).

13. Sindt, C. F. , A summary of the characterization study of slush

hydrogen. Cryogenics J_0 , No. 5, 372-80 (Oct. 1970).

14. Sindt, C. F. , Heat transfer to slush hydrogen. To be published

in the Advances in Cryogenic Engineering 19 .

15. Weitzel, D. H. , Cruz, J. E. , Lowe, L. T. , Richards, R, J. , and Mann, D. B. , Instrumentation for storage and transfer of

hydrogen slush. Book, Advances in Cryogenic Engineering 16,

Ed. K. D. Timmerhaus, pp. 230-240 (Plenum Press , New York,

N. Y. , 1971).

16. Ellerbruch, D. A. , Microwave methods for cryogenic liquid and

slush instrumentation, Sook, Advances in Cryogenic t-ngineering 1

, Ed. K. D. Timmerhaus, pp. 241-250 (Plenum Press , New York,

N. Y. , 1971).

35
8. 2 Unpublished Reports

1. Mann, D. B. , Ludtke, P. R. , Sindt, C. F. , Chelton, D. B. , Daney, D. E. , and Pollack, G. L. , Characteristics of Liquidsolid mixtures of hydrogen at the triple -point, NBS Report

(Oct. 1965).

2. Mann, D. B. , Sindt, C. F. , Ludtke, P. R. , and Chelton, D. B. , Slush hydrogen fluid characterization and instrumentation

analysis, NBS Report (1966).

3.. Daney, D. E. , and Mann, D. B. , Quality determination of

liquid- solid hydrogen mixtures, NBS Report (1966).

4. Alspach, W. J., Flynn, T. M. , Richards, R. J., Slush hydrogen

instrumentation study, NBS Report (1966).

5. Daney, D. E. , Ludtke, P. R. , Sindt, C. F, , and Chelton, D. B. ,

> Slush hydrogen fluid characterization and instrumentation

analysis, NBS Report ( 1967).

6. Sindt, C. F, , and Ludtke, P. R. , Slush hydrogen fluid characterization and instrumentation, NBS Report (1969).

7. Ludtke, P. R. , Slush hydrogen flow facility, NBS Report (1970).

8. Weitzel, D. H. , Collier, R, S. , Ellerbruch, D. A. , Cruz, J. E. , Low, L. T. , Instrumentation for hydrogen slush storage

containers, NBS Report (1971).

9. Weitzel, D. H. , Low, L. T. , Ellerbruch, D. A. , Cruz, J. E. , and Sindt, C, F. , Hydrogen slush density reference systemFinal report, NBS Report (1971).

10. Sindt, C. F. , Part A. Hydrocarbon suspension in slush hydrogen.

Part B. , Heat Transfer to slush hydrogen, NBS Report (1972),

11. Collier, R. S. , Thermally induced pressure oscillations in

cryogenic systems, NBS Report (1972).

36
FORM NBS-114A 11-71) u^. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. 2. Gov't Accession BIBLIOGRAPHIC DATA NBSIR 73-344

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

HEAT TRANSFER AND MIXING OF SLUSH HYDROGEN

5. Publication Date November 1973

6. Performing Organization Code

7. AUTHOR(S)

G. F, Sindt and P. R. Ludtke

8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS , Boulder Labs.

DEPARTMENT OF COMMERCE

Boulder, Colorado 80302

10. Project/Task/Work Unit No. 2750154

11. Contract/Grant No. NASA Order No.

W13, -300

12. Sponsoring Organization Name and Address

iN xxtDxx ne aQC[u.arte r s Washington, D. C.

13. Type of Report & Period Covered p-^^^j Report

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) 3 Heat transfer to slush hydrogen and mixing were investigated in a 1 m

cylindrical vessel. The effects of heat transfer rates on thermal stratification

and on self-pressurization were measured. Temperature profiles in thermal

stratification were found to be more dependent on slush level and slush settling

rates than on liquid level. Solids in the slush appear to be involved in the heat

transfer mechanism as slush level affected the amotint of warm liquid reaching

the top of the dewar and therefore affected the self-pressurization rates.

Mixing effectiveness and power requirements to mix slush hydrogen

were determined for two configurations of turbine mixers and one paddle mixer.

Mixing power requirements were found to be sensitive to the mixer location

and configuration.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Heat transfer; liquid hydrogen; mixing; mixing power; paddle mixerg;

slush hydrogen; turbine mixers.

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