NBSIR 76-834

AFCRL-TR-76-0012

.« ' < II" 1 1

j

A SYSTEM FOR INFLATING A BALLOON

USING HELIUM STORED IN THE LIQUID PHASE

C. F. Sindt

W. R. Parrish

BATE.

Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

January 1976

Prepared for : Air Force Cambridge Research Laboratories

Air Force Systems Command

United States Air Force

Hanscom AFB, Massachusetts 01731

NBSIR 76-834

AFCRLTR-76-0012

A SYSTEM FOR INFLATING A BALLOON

USING HELIUM STORED IN THE LIQUID PHASE

C. F. Sindt

W. R. Parrish

Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

January 1976

Prepared for : Air Force Cambridge Research Laboratories

Air Force Systems Command

United States Air Force

Hanscom AF3, Massachusetts 01731

U.S. DEPARTMENT OF COMMERCE, Rogers C. B. Morton, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson , Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director
SECURITY CL ASS'FIOATION OF THIS D A G E (W**n Data Entered) REPORT DOCUMENTATION PAGE RE A D INSTRUCTIONS

BEFORE COMPLETING FORM

AFCRL-TR-76-0012

3»• PECl°' fc- V» 1 Fk* fcPIT'^) \<P ar\ T1 aW : dfi \J \J miiurfb Pi \J fr\ O n

4. TITLE (and Subtitle? A SYSTEM FOR INFLATING A BALLOON

USING HELIUM STORED IN THE LIQUID

PHASE

5. TYPE OF REPORT 6. PERIOD COVERED

Scientific. Interim.

6. PERFORMING ORG. REPORT NUMBER

NBSIR 76-834

7. Au THORfaj C.F.Sindt

W. R. Parrish

8. CONTRACT OR GRANT NUMBERfs)

TVT / A

JN / A

9. PERFORMING ORGANIZATION NAME AND ADDRESS

Cryogenics Division

iilDLllULC IKJ L JJaOl^ O LallUai Li o National Bureau of Standards

Boulder, Colorado 80302

10. PROGRAM *iL EM EN T, PROJECT, TASK

A PET A * WORK UNIT NUMBERS

11. CONTROLLING OFFICE NAME AND ADDRESS

Air Force Cambridge Research Laboratories

Hanscom AFB, Massachusetts 01731

Monitor/ Andrew S. Carten, Jr. /LCH

12. REPORT DATE

January 1976

13. NUMBER OF PAGES

37

14. MONITORING AGENCY NAME & ADDRESSff/ different from Controlling Office) 15. SECURITY CLASS, (of this report) Unclassified

15«. DECLASSI F| CATION/ DOWN GRADING

SCHEDULE

16. DISTRIBUTION STATEMENT (of thta Report)

Approved for public release; distribution unlimited.

17. DISTRIBUTION STATEMENT (of the abstract entered in Block 20, If different from Report) 1

18. SUPPLEMENTARY NOTES

19. KEY WORDS (Continue on reverse aide if necessary and identify by block number)

Balloon

Cryogenic balloon inflation

Heat transfer

Liquid helium

Packed bed heat exchangers

20. ABSTRACT (Continue on reverse side If necessary and Identify by block member) | This report covers the design and development of the prototype of a \ system to be used to fill a balloon that has been launched from an aircraft. : The described system uses a hot packed bed heat exchanger to gasify 45.4 kg

of liquid helium and warm the gas to 260 K for filling the balloon. The prototyp

system was successfully demonstrated on the ground by filling an ambient

pressure balloon with 300 m3 of helium gas at an average temperature of

260K in six minutes and 45 seconds.

DD , ^NRM

73 1 473 EDITION OF 1 NOV 65 IS OBSOLETE Unclassified
CONTENTS

1. « INTRODUCTION • • • 1

2. AIR-LAUNCHED BALLOON SYSTEM DESIGN OBJECTIVES ..... 2

3. EVALUATION OF PROPOSED SYSTEMS 3

A. REDUCED SCALE TESTS AND MATHEMATICAL MODELS . , 4 4.1 Helium Dewar Performance . 4 4.2 Removing Helium from the Dewar ...... 5 4.3 Thermal Shock Test of the Bed Media 8 4.4 Modeling and Experimental Test of the Small Scale Heat Exchanger 9 4.4.1 Modeling the Packed Bed Heat Exchanger 9 4.4.1.1 The Mathematical Model . 9 4.4.1.2 Determination of Heat Transfer Coefficients 10

4.4.1.3 Nomenclature 11 4.4.2 Test Program for the Small Scale Heat Exchanger 11

5. DESIGN OF THE FULL SCALE SYSTEM 14

6. INSTRUMENTATION 18

7. FULL SCALE TEST PROGRAM 19 7.1 Startup Transient and Valve Timing Tests 19 7.2 Testing of the Packed Bed and the Gas Mixer 20

8. FILLING THE BALLOON 24

9. DISCUSSION OF THE EXPERIMENTAL SYSTEM 24

9.1 Insulation in the Hot Bed Discharge Tube 24 9.2 ALBS System Weight .24

10. CONCLUSIONS ..... 31

11. REFERENCES 32

LIST OF FIGURES

Figure 1. Air-Launched Balloon System (ALBS) ... 2 Figure 2. Blow-down characteristics of a dewar of saturated liquid helium 6 Figure 3. The mass discharged by the blow-down of a dewar of supercritical and

saturated liquid helium 7 Figure 4. Schematic of an ALBS using dewar blow-down for liquid removal 8 Figure 5. Schematic of an ALBS using warm gas pressure for liquid removal ....... 9 Figure 6. Aluminum oxide spheres used for the hot bed media 12 Figure 7. Schematic of reduced scale hot bed test apparatus 13

Figure 8. Bed discharge temperature vs. time, predicted and test data 14

Figure 9. Hot bed inner vessel 16

Figure 10. Hot bed inner vessel with the insulation blanket 17 Figure 11. Cross section of the heat exchanger assembly 18

Figure 12. Typical data printout from the test program 19

Figure 13. Louvered gas mixer 21

Figure 14. Schematic of the ALBS experimental unit 22 Figure 15. ALBS heat exchanger and plumbing 23

iii
Figure 16. ALBS before the demonstration filling of the balloon 25

Figure 17. The cold gas plumbing during the filling of the balloon 26

Figure 18. The filled halloon 27 Figure 19. Discharge tube insulation using powder on a butterfly valve 28

Figure 20. Discharge tuba insulation using contained powder on a burst disc 29 Figure 21. Discharge tube insulation using a moving fiber plug 30

LIST OF TABLES

Table 1. Selected Properties of Aluminum Oxide and Beryllium Oxide 4 Table 2. ALBS Weights 31

iv
A SYSTEM FOR INFLATING A BALLOON USING HELIUM STORED IN THE LIQUID PHASEt

C. F. Sindt and W. R. Parrish

Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

Abstract

This report covers the design and development of the prototype of a system to be used to fill a balloon that has been launched from an aircraft. The described system uses a hot packed bed heat exchanger to gasify 45.4 kg of liquid helium and warm the gas to 260 K for filling the

balloon. The prototype system was successfully demonstrated on the

3 ground by filling an ambient pressure balloon with 300 m of heliura gas

at an average temperature of 260 K in six minutes and 45 seconds.

Key words: Balloon; cryogenic balloon inflation; heat transfer;

liquid helium; packed bed heat exchangers

.

1. INTRODUCTION

Balloons have been used by man since late in the 18th century and are still used extensively for military applications, for atmospheric observations and for recreati l. Launchin

a balloon has historically required low ground winds, hours of preparation and a favorable

launching site. Placing a balloon and its payload at a desired location at a specific time

requires considerable planning combined with satisfactory atmospheric conditions. Some of

these restraints were overcome by Payne [1] in the late 1950' s and early 1960's when he

successfully launched a small balloon from a flying aircraft; however, the growth of his

balloon system was restricted because of the very heavy compressed gas storage tanks used

for the inflating gas. In the last few years the Air Force has been investigating the feasibility of launching

large balloons from aircraft because of a potential application in supporting ccmmunication

relay platforms. The Air Force has established a project to advance the technology needed

to develop a successful system. A report on this project by Carten [2] describes the system

concept and covers several methods of inflating and deploying balloons from aircraft and

rockets. The aircraft launching system described by Carten is depicted in figure 1. Carten' s investigation indicated the most desirable method of providing inflating gas

for the Air-Launched Balloon System (ALBS) is to use either liquid helium or liquid hydrogen

to convert it to gas and to heat it to a temperature acceptable to the balloon. This method

takes advantage of the low volume and pressure of the liquid phase, but it does require a heat source and a large heat transfer rate between the inflating gas and the heat source. The Cryogenics Division of the National Bureau of Standards was called upon to develop

an experimental system that would convert liquid hydrogen or helium to gas in a short time

and to demonstrate that the system could eventually be used to fill a balloon in mid-air.

This report covers the work at the Cryogenics Division from the preliminary evaluation to the first demonstration filling of a balloon on the ground.

tThis work was jointly sponsored by the Air Force Cambridge Research laboratories and the

Electronics Systems Division (AFSC) , Hanscom Air Force Base, Bedford, Massachusetts 01731,

under Air Force Project Order Numbers Y74-898, ¥75-808,-841, Y76-800.
FLOAT ALTITUDE

(2k. kW) 80,000 FT OPERATE SENSORS

DROP ALBS FROM AIR TRANSPORT

(7.62KM) 25,000 FT

INFLATE BALLOON DURING PARACHUTE DESCENT

MORNING FLOAT ALTITUDE ARELEASE I NFLATANT

(BALLOON DESCENDS)

ASCENT OF INFLATED BALLOON WITH PAYLOAD

DROP CHUTE AND

INFLATION HARDWARE

RELEASE PAYLOAD

ON CHUTE

RECOVERY OF PAYLOAD

I

Figure 1. Air-Launched Balloon System (ALBS)

2. AIR-LAUNCHED BALLOON SYSTEM DESIGN OBJECTIVES

At first liquid hydrogen and liquid helium were both considered as a possible source

for the gas to inflate the balloon. Liquid hydrogen was excluded early in the program

because of the logistics of handling a combustible liquid and because the combustible liquid

and venting gas needed to be in close proximity to a high temperature device required for

liquid conversion. After the decision was made to use helium, the program effort was then

directed toward selection of a system for liquid helium storage and for conversion of the

liquid to gas. The criteria established for the program were:

1. To convert 45.4 kg of liquid helium to gas;

2. To produce gas at a temperature between 218 and 323 K, with an average gas

temperature of 220 to 260 K;

2
3. To complete the entire process (vaporization, gas warming, balloon inflation) in

five minutes;

4. To package the system in a cylinder 91 cm in diameter by 166 cm long and have it

weigh 160 kg.

Using these criteria the heat required to convert the liquid to gas and heat it to 260 K was between 5.3 X lO'' and 6 X lo'' J and the heat transfer rate had to be between 177 and 200 kW.

From these general criteria, the following design objectives were established:

1. The system must store 45.4 kg of helium in the minimum space with minimum weight for

a period of four to seven days without refilling or other service.

2. The system must be instantly ready to supply helium to the balloon at a continuous

flow rate of 0.16 kg per second and at an average temperature of 220 to 260 K.

3. For the first 30 seconds, the flow rate is to be approximately one half of the

average rate.

4. The system must be compatible with the environment in a nonpressurized aircraft

and must operate in the ambient conditions between sea level and 7.6 km altitude.

5. It must withstand the shock loads incurred in cargo aircraft operations, in the

opening of a parachute, and in the landing by parachute.

6. It must be refurbishable after the parachute landing.

The size and weight specifications given by Carten [2] were established as design

objectives but were not rigid criteria, especially for the experimental unit. Mowever, the

objectives of lightweight and compactness were still valid and although the actual experimental hardware didn't need to meet these obje ; :ives , the system concept had to be compatible

with them.

3. EVALUATION OF PROPOSED SYSTEMS

Several systems discussed by Carten for gasifying and warming helium were evaluated

in a preliminary study along with a preliminary look at high pressure gas storage. The

systems considered for converting the liquid helium to warm gas used heat exchange with the

following: 1) hot gas from a gas generator, 2) a packed bed heat exchanger and 3) an electric heater. The types of gas generators considered were those using fuel and air,

fuel and an oxidizer and a single component such as a solid rocket motor uses. All of the

gas generators have a similar system requirement; they must transfer heat between the

hot gas and the helium at a rate of 190 kW. This requires a very large, heavy unit.

Instead of using a large heat exchanger the hot gas from the generator could be mixed

directly with the helium. This method has the disadvantage of adding gas to the balloon

that is heavier than air and, therefore, adds to the total weight and required balloon

volume. Also, some products of combustion from the gas generators, such as water, will

freeze out at the desired gas temperature and may endanger the balloon material or interfere with the filling of the balloon. Gas generators that burn fuel and air or oxidizers are in general complex, requiring pumps, controls, etc. and are therefore

expensive.

Electrical heating was considered only briefly but was rejected because there is no means of storing the required electrical energy in a package that is compatible with the

objectives of light weight.

A high pressure gas storage system was considered, as advances in high pressure cylinder

design have been made since Payne's time; however, this system was still four times heavier

than the desired maximum weight.

The storage of heat in a packed bed using direct heat exchange with the helium was then

evaluated considering a number of materials for heat storage. Material selection was based

on a number of criteria. High specific heat was an obvious requirement; high specific weight

was also desired to keep the volume within the design criteria. Since heat stored in any

material is nearly proportional to the temperature, the higher the temperature the more heat

stored per unit mass; therefore, the hot bed material had to be compatible with high temperature - that is, it must not break from thermal shock, it must not melt or deform at high

temperatures and it must not react chemically with a.ir that may be in the system during the

heating of the bed.
Heavy metals in general filled the requirements of specific weight but they have lower

specific heats than some metal oxides; also, some of the heavy metals melt at low temperatures

Some of the metallic sulfates have high specific heats but have other undesirable characteristics such as low melting points or they are corrosive to metal containers.

Two oxides of metals filled all of the desired criteria fairly well. They were aluminum

oxide (A^O^) and beryllium oxide (BeO) . Both have high specific heat at high temperature,

both are compatible with high temperature, neither is highly susceptible to thermal shock as they have low thermal expansion coefficients and neither is degraded by heating in air. Both

oxides are in the medium range of specific weights, but because they filled the other criteria

they were selected for further evaluation. Properties for both are given in table 1.

Table 1. Selected Properties of Aluminum Oxide and

Beryllium Oxide [3]

Specific Heat

at 900 K

at 300 K

Specific Weight

Melting Point

Coefficient of Thermal Expansion

After pricing the aluminum oxide and beryllium oxide, the aluminum oxide was selected

for the experiment even though beryllium oxide has a higher specific heat. The reasons for

selecting aluminum oxide were that it was twelve times less expensive than beryllium oxide

and it was readily available in a number of sizes of balls suitable for hot packed bed use. The packed bed heat exchanger system was selected over the other systems because of the

following advantages: 1) The system is relatively simple as it requires no auxiliary pumps,

no complex speed and fluid flow controls and no ignition sources such as required by gas

generators. 2) The system does not contaminate the inflating gas as the heat storage media

is chemically and physically stable throughout the operating temperature of the hot bed.

3) The preliminary study indicated a weight advantage over systems using gas generators

with heat exchangers.

With the type of heat exchanger selected and the rest of the system constraints defined,

the system functions to be resolved by analysis or experiment were heat transfer in the hot

bed, mixing of the hot gas from the bed with cold gas to get the desired temperature, re- moving the helium from the dewar and control of the helium flow.

4. REDUCED SCALE TESTS AND MATHEMATICAL MODELS

Reduced scale tests were conducted and mathematical models were used early in the investigation to determine the characteristics of certain system functions, such as heat transfer

in the hot bed (see 4.4) and liquid helium transfer from the dewar (see 4.2).

Even though mixing of the hot and cold gas stream was considered one of the areas requiring development, no small scale tests or models were set up; instead, the mixer was developed and tested on the full scale experiment conducted later in the investigation.

This decision was made because a mixer test required a test fixture as complex as the system

for filling a balloon. The mixing experiments are described in section 7.

4.1 Helium Dewar Performance

Since the performance of high quality liquid helium dewar s is quite well known, no experimental verification of dewar performance was required. Estimated dewar performance

was used to analytically determine the best technique to achieve the desired seven days of

liquid storage without service. Two methods of storage were analyzed.

Aluminum Oxide

1.21 J/(g-K)

0.80 J/(g«K)

3.6 to 3.9 g/cm

2310 K

8 X

10~6 K

_1

Beryllium Oxide

1.99 J/(g-K)

1.088 J/(g«K)

3.03 g/cm3

2725 K

8 X 10"6 K

-1
In the first method considered for the helium storage, the liquid helium would be kept

at one atmosphere pressure by venting the boiling liquid. In this system the cold vented gas

would be used to cool a thermal radiation shield in the dewar insulation and about one percent

of the initial liquid would be boiled away each day. Sc, by increasing the initial mass by

seven percent, an adequate supply of helium would be available to fill the balloon after

seven days of storage. In the second method considered for the storage of helium, the dewar would be filled

with liquid in equilibrium with one atmosphere pressure, then it would be sealed for the

duration of the storage period and the pressure would be allowed to rise. The pressure would

reach 15 atmospheres in six days if the ullage at the time the dewar had been filled was ten

percent. Fifteen atmospheres pressure was the aaxxman pressure considered for the analysis

since the weight of the dewar inner vessel became excessive for higher pressures. This

method of storage would be suitable for "blowing down" (removing the helium from) an ALBS

system that has no dewar pressurization scheme (see 4.2) but it requires that the helium be

in the liquid state at high pressure or in the supercritical condition.* However, if the

system were to be used immediately after filling the dewar, pressure would be too low and a device for increasing the pressure would still be needed, in order to remove the helium from

the dewsr.

In the end, selection of the liquid storage method was based on the fluid control system

requirements instead of dewar performance, as both of the helium storage methods described

are feasible and are "state-of-the-art" for liquid helium.

4.2 Removing Helium From the Tewar

The characteristics of removing liquid helium from a dewar were considered as unknown,

even though transferring liquid helium is a daily occurrence. Information on the amount of

externally-supplied helium gas required to expel liquid from a dewar maintained at atmospheric

pressure is meager, as is information on the characteristics of blowing—down dewars of supercritical helium or saturated liquid helium at elevated pressures.

Small scale tests were conducted to determine the amount of helium gas needed to pressure transfer saturated liquid helium in equilibrium at cne atmosphere pressure. The tests were made realizing that the dewar materials and shape would have a significant effect on

the pressurizing gas required; therefore, test results were just used as a guide.

The experimental apparatus for the pressure transfer tests included a 50 liter receiving

dewar, a helium gas supply cylinder fitted with a pressure regulator, a 33 liter dewar of

liquid helium and a transfer line between the two dewars. The test procedure was to pressurize the liquid helium dewar to approximately 34 k?a (5 psig) ; then to measure the time

required to transfer the 33 liters of liquid helium from the supply dewar to the receiving

dewar. The gas consumed to transfer the 33 liters of liquid was calculated using an estimated

gas temperature-room temperature - and the measurements cf the pressure in the helium supply

cylinder before and after the liquid transfer. All the measurements were crude since the

results were just to be used as a guide. From the results of these experiments the Tn-iniTmTm quantity of gas required to pressure transfer the required 45. ^ kg of liquid at the rate of

3 0.16 kg per second was determined to be equivalent to 2,6 m at standard pressure and temperature.The blow-down characteristics of a dewar of saturated liquid helium in equilibrium with

pressures up to critical pressure, and of a dewar of supercritical helium were determined

by analytical models instead of tests. Two separate models were required; one model for the

saturated liquid helium case and one for the supercritical helium case. For saturated liquid

helium the dewar was considered to be 100 percent full of liquid in equilibrium at the

starting pressure, and the mass discharged was determined for exhaust pressures down to 0.5

atmosphere. The results from these analyses are shevn in figure 2 . Also shown in figure 2 for each curve is the ratio Mi/Mo, which is the ratio of the mass of the saturated liquid at the starting pressure to the mass of the liquid required to completely fill the dewar at one

arm: sphere rris;,:;, Tr.is raric vas ii.ar-ir.ed sc that all zi :he systems ;:_ld la compared

to a common base. To get the ratio of the mass discharged to the mass at the common base, we must multiply the abscissa of figure 2 by Mi/Mo. This ratio was calculated for a discharge

pressure of one atmosphere and is shown as curve A on figure 3.

*A fluid that is supercritical is at a temperature and pressure above its critical point. At

the critical point the specific volume of gas and liquid becomes equal, and above this point

the fluid no longer separates into the liquid and gas phases. Helium critical temperature

and pressure are 5.2014 K and 2.245 atmospheres respectively.

5
O 100% Liquid Full @ Saturation

A 100% Vapor Full @ Saturation

Mi = The mass of saturated liquid at

the starting pressure.

Mo = The mass of liquid to fill the

dewar at 1 atm pressure

1 atm

All liquid is

withdrawn (only

vapor remains)

Isentropic vapor

wi thdrawal

0 20 40 60 80 100

MASS DISCHARGED {% of initial fill MASS)

Figure 2. Blow-down characteristics of a dewar of saturated liquid helium.

6
1.0,

3

Both curves show the ratio of mass discharged to the mass required to fill the dewar with saturated liquid at 1 atm

pressure.

For curve A the dewar is filled with liquid in equilibrium

with the starting pressure

For curve B the dewar is filled with supercritical helium

at the same -density as saturated liquid helium at 1 atm

pressure.

1

5 10 DEWAR PRESSURE BEFORE START OF BLOW-DOWN (atm)

15

Figure 3. The mass discharged by the blow-down of a dewar of supercritical and saturated

liquid helium.

In the analysis for the blow-down of a dewar of supercritical helium, the dewar was assumed to be filled completely with liquid in equilibrium at one atmosphere. The mass of

this liquid is the same mass as the base mass used for the saturated liquid helium analysis.

To get supercritical pressure after filling the dewar, heat must be added either by the dewar

heat leak or by an externally powered heater. The mass discharged from the dewar of

supercritical helium was calculated by assuming an exhaust pressure of one atmosphere. The

ratio of discharged mass to the mass used to fill the dewar is shown versus the pressure

before the blow-down by curve B of figure 3. Because the ratio of mass discharged from the

dewars of supercritical helium .^nd saturated liquid helium was calculated using the same base mass at the same thermodynamic conditions, the curves A and B of figure 3 show a direct

comparison based on dewars of equal volume.

It is apparent that little is to be gained by pressures higher than 10 atmospheres

(1.013 X 10^ Pa). It is also apparent that the best one can do with this system is to expel

86 percent of the initial mass. So, for a system designed to supply 45.4 kg of liquid helium,

6.4 kg of helium are going to remain in the dewar and the dewar must be constructed to contain 10 atmospheres pressure. Both the saturated liquid helium storage and the supercritical

helium storage systems require some complexity in valving for flow control to maintain a nearly constant flow rate. A schematic for a blow-down system is shown in figure 4. The

depicted system uses valves opened in a programmed sequence as a means of flow control.

Another method for supplying pressure to discharge the helium from the dewar is to add

heat from the hot bed to the liquid in the dewar at a rate to keep the dewar pressure constant during discharge. This method would require less complex controls for the fluid since

the dewar pressure is constant. Results of the analysis for this system showed that the

7
Flow

Control

Valves

loon

Supercritical Helium Dewar

Figure 4. Schematic of an ALBS using dewar blow-down for liquid removal.

heat flow rate to maintain constant pressure varied four fold in less than four of the five

minutes total flow time. It was apparent that this system would also be complicated requiring a pressure controller to operate a throttle valve which would be used to control flow

to a heat exchanger in the dewar. The heat exchanger would require two more penetrations

through the dewar insulation, thereby reducing storage efficiency.

After considering the described methods of removing the helium, it was decided to use

a gas pressurization system to pressure transfer liquid helium that has been stored at slightly over atmospheric pressure. This system would use a cylinder of high pressure

helium gas, a valve and a pressure regulator. This appeared to be the least complicated,

most reliable and lightest weight method of those studied for removing helium from the dewar.

The system is depicted schematically in figure 5. 4.3 Thermal Shock Test of the Bed Media

A simple test was made to determine if the aluminum oxide spheres selected for the hot

bed media would stand the shock of the thermal cycle when cold helium gas flowed onto a bed

at 1090 K. The spheres were heated to red hot, well above 1090 K, and dropped into a dewar of

liquid nitrogen. This test was conducted on three spheres for three times with no evidence

of fracture. The spheres were subsequently dropped on a concrete floor from a height of one meter and none broke. Thus it was assumed that they would withstand the thermal shock

experienced in the hot bed.

8
4.4 Modeling and Experimental Test of the Small Scale Heat Exchanger

A test program was combined with a mathematical model to determine the heat transfer

characteristics of the packed bed heat exchanger. Results from model and tests were to be

used to design the full scale heat exchanger and to determine the temperature of the gas

discharging from the bed versus the flow rate and versus time. These data would, in turn,

be used to determine the mixing ratio between the hot gas coming from the hot bed and the

cold gas bypassing the bed.

4.4.1 Modeling the Packed Bed Heat Exchanger

When the packed bed was selected as the heat exchanger for the helium, it was decided

that a numerical model of the bed heat exchanger should be developed. The purpose of the

model was two fold. First, it was to be used to determine the heat transfer coefficients

using the experimental data; the coefficients could then be correlated as a function of various parameters in the bed. Second, once there waf a correlation, the model could predict

design criteria for the full scale packed bed.

4.4.1.1 The Mathematical Model

To describe the heat transfer process in a packed bfi requires two equations;

IS

9t ha

Vb V (1)

and

9
(3T 3T \ 3v

*

-r-& + v -r-^ j = ha (T - T ) - P v-^ , (2) dt X dX / b g dX

with the following boundary conditions: at t = 0, T = T, = known function of x and at

g b

x = O5 Tg is a known function of time. Equation (1) is the energy equation for the solid of

the bed and equation (2) is for the gas. These equations are based on the following assumptions :

1. The heat transfer between the solid and the gas can be described us ng an overall heat

transfer coefficient and the difference in local temperature

,

2. The walls of the bed are well insulated and there is no radial ccmponent of the heat

flux.

3. The thermal conductivity between the, spheres in the bed is negligible and the temperature gradient within each sphere is negligible.

If we also assume that the superficial mass velocity G (i.e. Pv

x

) is independent of position

and helium obeys the ideal gas laws, equation (2) can be rewritten as

3T _ a3T _ -JL +

G __S. m h±_ (T T ) +

G |P

dt p dX pC b g 2„ 3x v p CV

Equation (2) can be further simplified if the bed pressure drop is neglected and the

heat content of the gas is small relative to the heat content of the bed (i.e. pC « MC, ): v b

this breaks down at temperatures below 5 F where the heat capacity of the bed is very small.

Then equation (2) becomes

3T —£ = ^5- (T - T ) f h) 3x GC Ub g

; ^'

P

Both equations (3) and (4) were used with equation (I) and gave comparable results,

therefore, for most of the work the simpler equation (4) ''as used. Equations (1) and (4) are nonlinear because h, C, and G were functions of time and/or temperature. To solve these

b equations the central difference algorithm described by Wi^lmott [4] was used. Nonlinear

effects were treated by iteration; temperatures calculated by the previous iteration were

used to evaluate the nonlinear terms for the next iteration. This procedure was repeated

until two iterations gave the same temperature. Convergence was rapid since four or less

iterations were required.

4.4.1.2 Determination of Heat Transfer Coefficients

The experiments were run using the small scale packed bed heat exchanger. The early

runs were made at constant flow and with small temperature differences (50 - 100 K) near

ambient conditions. Under these conditions the physical properties of the gas are nearly

constant; this in turn implies a constant heat transfer coefficient. However, the temperature dependence of the heat capacity of the bed had to be considered.

^Nomenclature is in section 4.4.1.3

10
Data obtained from the experiments included bed temperature profiles along with flow

rates, inlet and exit gas temperatures and pressure drops all as a function of time. To obtain the heat transfer coefficients from these data a trial and error procedure had to be

used which compared experimental and calculated time-temperature profiles; it is basically

the procedure described by Handley and Heggs [5]. The "correct" value of the heat transfer

was the value which gave the best match of the time interval between two arbitrarily chosen

temperatures. These temperatures were chosen in the region where the temperature "front" was leaving the bed, i.e. where the exit temperature was changing rapidly.

Later runs were made with constant flow and temperature differences as large as 900 K. Under these conditions the physical properties of the gas and solid are both changing as the

bed and gas temperature change; thus, the heat transfer coefficient changes. Heat transfer

coefficients from the low temperature difference run were adjusted as a function of temperature

for these runs. Again the comparison between experimental and calculated time-temperature

profiles were made in the region where the temperature "front" was leaving the bed. Some

adjustments to the heat transfer coefficients were made to get the best correlation and the

best fit was considered to be the "correct" heat transfer coefficient.

4.4.1.3 Nomenclature

a =

2 3 heat transfer surface area per unit volume, m /m (= G(l-p)/d for spheres);

Cb = specific heat capacity of solid, J/(kg*K);

c ,c =

p v specific heat capacity of gas, J/(kg*K);

d = rl "i PTTiP*- pt nf ^nhPTP 1! m*

G

, ., 2, superficial mass velocity, kg/(s*m ) ;

h

2 heat transfer coefficient, J/ (m *s*K);

° mass of the bed, kg;

P void fraction;

P

="' pressure, bar;

t time, min;

T , T =

g b temperature of gas and solid respectively, K;

v = Xlinear velocity, m/s;

X - axial distance from the front of the bed, m;

a heat capacity ratio, C /C ; p V

P

3 density of gas, kg/m .

4.4.2 Test Program for the Small Scale Heat Exchanger

The dimensions of the heat exchanger bed were to be determined by the analytical model.

Since the model was to be adjusted by data from tests of the small scale bed, dimensions for

this bed had to be calculated using estimates. To get the small scale bed dimensions, the

bed diameter was arbitrarily selected as 7.6 cm and the heat transfer coefficient was estimated. The model was then used to calculate the bed length, which was 39 cm. The first

small scale bed contained 3.35 kg of 3/8- inch high-density aluminum oxide spheres. The

spheres, which were actually 7.6 mm. diameter, are shown in figure 6. The hot bed was incorporated into a system which is shown schematically in figure 7. With this system the bed

could be heated by gas flowing through the external electric heater and the bed, or it could

be heated by using the internal electric heater. During the tests the inlet gas temperature

could be controlled between 80 K and 350 K by adjusting the control valves controlling the

hot and cold gas flow.

11

Pressure

Figure 7. Schematic of reduced scale hot bed test apparatus.

The first time the small scale test bed was constructed, 7.6 mm diameter spheres were

used. Fourteen tests were run on this configuration. Data from these tests indicated that

a smaller media might suit the application better by increasing the heat transfer between

the bed and the helium. The second bed was built using 1.8 mm diameter aluminum oxide

spheres. Ten tests were conducted with this configuration of the bed. Test results showed

that channeled flow was occurring in the bed and the heat transfer was incomplete and un- predictable. Also, the pressure drop in this bed was much higher than in the bed of 7.6 mm

diameter spheres. The decision was to abandon the small media and use the 7.6 mm diameter

spheres.

The bed was rebuilt using the 7.6 mm diameter spheres and sixteen more tests were made.

Test data and the analytical model appeared to correlate in some of these tests. In other

tests correlation was poor. Figure 8 shows an example of one of the better correlations.

The calculated value for the heat lost from the hot bed and the heat gained by the

helium did not agree in all of the tests. The reason for the discrepancy in the heat balance

was discovered when the bed design was examined and it was concluded that the metal end plates

the electric heater and the metal walls of the bed contained heat equivalent to 20 percent

of the bed capacity and they were participating significantly in the heat transfer. Since

these parts were not instrumented for temperature measurement, their effect had to be

estimated and a reasonable heat balance was calculated. Good heat transfer correlation between the test data and the model was not possible though, as the heat transfer coefficient

at the walls and end plates was not known and it was not the same as in the bed.

Enough data were taken to determine the effects of bed temperature, inlet gas temperature, and flow rate on heat transfer in the bed. Using these data and the analytical model,

the size for the full scale hot bed was predicted. The final bed was then oversized 20 percent to hedge against the unknown effects of the scale up and of the metal parts. The final

bed size selected was 34.8 cm diameter by 40.6 cm long and the bed contained 74 kg of 7.6 mm diameter aluminum oxide spheres.

13
3SC

Q_

z:uj

34C«-

32C «

333"

2e:

X.

V

»=EX? ,=C*LC

HTC= 11653 X

t t

TIME, MINUTES

RUN NUMBER 7, DECEMBER 17, 1374, HE GAS

Figure 8. Bed discharge temperature vs. time, predicted and test data.

5.0 DESIGN OF THE FULL SCALE SYSTEM

The design parameters for pressure transfer of the liquid helium and for the hot bed

heat exchanger were established from the analytical models and the scaled down tests. The

designs to be determined by full scale testing were those for the control system and the

mixer of the hot and cold gas. Also to be determined from the full scale tests were the

effectiveness of the bed insulation and of the electric heater for heating the bed. At this time in the program the decision was made to use a commercial liquid helium

dewar for the balloon filling demonstration. This decision was economic and timely since a special built dewar would cost over $15,000 and could not be delivered until well after the

scheduled balloon filling date. Also, the desired dewar size could be influenced by the

results of the test program and the balloon filling.

14
The system, as first constructed, consisted of a pressurant gas cylinder with a valve

and a pressure regulator, the liquid helium dewar, the liquid transfer system with flow con- trol nozzles, orifices and valves, the hot bed and the gas mixer. A schematic of a similar

system is shown in figure 5. To keep the control system as simple as possible, the selected method for fluid control

used an electric timer set to control electric solenoid operated valves. Solenoid valves

that are either open or closed were selected over modulating valves because they are more

reliable at liquid helium temperatures; also modulating valves require more complex controls.

The valves and the clock were powered by a 24 volt battery.

The pressurant gas cylinder was a fiber glass and epoxy reinforced aluminum vessel. It

3 weighed 12.2 kg with a volume of 0.0108 m . At 31 MPa (4500 psi) it holds equivalent to

3 3.3 m of gas at standard pressure and temperature.

The liquid transfer system originally had two- liquid valves and flow control nozzles.

One nozzle was sized to flow half the volume of the other to meet the start requirement of

reduced flow. Because of the natural flow restriction occurring during the cooldown of the

system, the flow rate in the large flow loop was reduced to the rate required for the first

20 to 30 seconds; the small flow loop was therefore not needed and it was abandoned after

the first two tests. The liquid helium transfer line was vacuum insulated between the bottom of the helium

dewar and the flow control nozzle. The transfer line was not insulated from the valve to the hot bed unit, thereby, the heat transfer with the ambient air was used to convert the

liquid helium to 100 percent vapor before it reached two flow control orifices. With 100

percent vapor at the inlet to the orifices, the flow was more predictable than it. would

have been had it been two phase. These replaceable orifices were used to control the split

in the flow between the hot bed and the cold gas bypass. Therefore, these orifices were

sized to control the temperature of the gas supplied to the balloon.

The hot bed was packed with 74 kg of 7.6 mm diameter aluminum oxide spheres as previously

stated. The hot bed was contained in a stainless steel shell which was wrapped with an insulating blanket. The shell and the shell with the blanket installed are shown in figures

9 and 10. This assembly was enclosed in an outer aluminum container which was vacuum tight.

The space between the inner and outer vessel was filled with a powder developed for high

temperature insulation. The inner vessel was 35 cm in diameter and the inside diameter of

the outer vessel was 45 cm in diameter. The fiber blanket was 6.35 mm thick and the powder

was 43.65 mm thick. The specified thermal conductivity of the blanket was 7.3 x 10 ^ w/(cm«K).

Thermal conductivity for the powder insulation in one atmosphere pressure air was predicted

to be 4.04 x 10 W/(cm*K) with improvement expected in a good vacuum. The insulation space

did not contain a good vacuum as considerable pumping time was required to get a vacuum and

the pumping rate had to be extremely slow to prevent carrying the powder out with the gas. The final pressure in the insulation was believed to be as high as one half atmosphere because the time available for pumping was limited.

The 2.86 cm diameter inlet tube was located at the top of the bed and the 12.7 cm diameter outlet was at the bottom. This orientation was selected to take advantage of

thermal stratification in the residual gas in the bed and thus reduce convective heat transfer in the relatively large discharge tube. Since the discharge tube was large, insulation

was still needed to reduce heat loss by conduction and radiation during the bed heating cycle. An insulating plug 10 cm long that was made from the same material as the insulation blanket

was used for the test program. This plug was manually removed before each test. Systems for

automatic removal of the insulation plug are discussed in section 9. The hot bed was heated using an electric heater that had 640 cm of heated length and was 1.21 cm in diameter. This heater drew 7.5 amperes at 220 volts ac. It was coiled 30.5 cm

in diameter for 6-1/2 turns with the turns spaced closer at the ends of the bed. This closer

spacing at the ends was used to counteract the high heat losses through the inlet and outlet

tubes. An automatic control was used to control the heater temperature. The maximum operating temperature of the heater and the control unit was 1090 K. Four shielded thermocouples were equally spaced axially through the bed for measuring

bed temperature. One thermocouple was strapped to the heating element to be used as the

temperature sensor for the heater control.

15
16
Figure 10. Hot bed inner vessel with the insulation blanket.

17
A small gas deflector and some brass sponges were placed in the inlet end of the hot

bed to help spread the inlet gas flow evenly across the bed. A cross section of the hot bed

is shown in figure 11.

: Heater and Thermocouple Leads

Figure 11. Cross section of the heat exchanger assembly.

6 . 0 INSTRUMENTATION

Temperatures were measured at the nozzle, at both of the orifice inlets, at the gas

discharge tube and at the four points in the hot bed. Five thermocouples were installed

in the gas discharge tube. Four were equally spaced at 3.8 cm radius in the 6.4 cm radius

discharge tube. The fifth was at the center. Carbon resistance-thermometers were used

to measure the temperature at the nozzle inlet and at the orifice inlet. Pressures were measured at the dewar, at the inlet and throat of the nozzle and at the inlet and discharge

of the orifices. The pressure taps at the nozzles and the orifices were placed according

to ASME recommendation for flow measurements in long radius nozzles and in sharp edge

orifices. Flow was calculated from these pressure and temperature measurements.

18
Test data were taken automatically using a minicomputer and an automatic data acquisition

unit. Figure 12 shows an example of the data printout for data taken at 62 and 95 seconds

after the start of a test.

TIME 62. 00 s BED TEMPS. 92U.53 932 .

:--- 7 503 . 55 363.91 K GAS OUTLET TEMPS. 262.20 271. 55 267.^2 26U.<+7 263.10 K

NOZZLE' INLET TEMPS. <t . 0 6 K NO-ZZLE INLET PRESSURES 979. 07 MM HG

ORIFICE IN LET TEMP. 11 K ORIFICE INLET PRESSURE 915.33 MM HG

NOZ DP = 8i*. 76 MM HG DEWAR PRESSURE = 6*+6.11. MM HGG

LARGE NOZZTe ~FLQ iT 13 . 8 3 L bTmI N

3IG ORIFICE F^OW 7.036 L8/MIN

TIME 95. 00 s 5ED TEMPS. 930.97 931 . 53 835,99 89.11 K GAS OUTLET TEM^S. 262.57 272.05 267. 26^.53 262. 71 K

NOZZLE INLET TEMPS. <+ . 0 5 K NOZZLE INLET PRESSURES 950. 10 MM HG

'ORIFICE INLET TEMP. Tf ~ORTfTCE ~TNLET~~P RE SSURE 811. 3<+ MM HG

NOZ DP = 89.83 MM HG DEWAR PRESSURE = "631. 2 Z MM HGG

LARGE NOZZLE FLOW 1<+.17 LB/MlN

BIG

"

OR IF ICEFL OW 6 . 9<*8~~ "LB/MlNFigure 12. Typical data printout frcm the test program.

7. FULL SCALE TEST PROGRAM

The initial phase of the test program was to evaluate the startup transient and to set

the timing of the valves. The remainder of the test program was to develop mixing and to evaluate the effectiveness of the bed for the full duration of the run.

7.1 Startup Transient and Valve liming Tests

Fifty liter? of liquid helium was enough for the startup transient and approximately 20

seconds of relatively steady state flow. Three tests using 50 liters each were required to determine the correct length of the uninsulated line to get temperatures at the orifices

slightly above liquid helium temperature. The orifice inlet temperature of 9 K was the

final temperature obtained with 1.82 m of non-insulated line. four more tests were required

to find the right flow ratio between the hot and cold gas. During the four tests a valve

was added to the hot bed inlet to delay the flow of hot gas. This delay was required because the outlet gas temperature was greater than the upper limit for the first 10 seconds

after start. The gas flow rate was low during these first tests so the dewar pressure was increased

to increase the flow. The last two runs were made with the dewar pressure at the maximum

allowable for the commercial dewars - 103 kPa gauge (15 psig) - but the flow rate was still

20 percent below the design flow of 0.16 kg per second. The flow restriction was in the

transfer line between the dewar and the flow control nozzle. Flow could not be increased

without considerable hardware changes in the system and changes in the dewar since the

19
transfer line was the largest diameter that the dewar fitting would pass. Instead of making

changes to the dewar fittings, the decision was made to proceed with the low flow rate knowing that an increase in the size of the dewar opening and in the liquid helium transfer line

would solve the flow problem.

To prolong heater life, 925K was selected as the design bed temperature instead of

1090 K which was the maximum temperature rating for the heater. The heater required 14 hours

to heat the bed to 925 K. At 925 K bed temperature and 300 K ambient temperature, the outer

wall of the hot bed was 375 K in still air. This was near the estimated temperature for the

insulation system with one atmosphere pressure air in the insulating powder. The outer wall

temperature should be reduced significantly if a vacuum is maintained in the powder insulation. For the test program the outer wall temperature was reduced to 322 K by forcing air across the unit at 2.5 m/s velocity.

7.2 Testing of the Packed Bed and the Gas Mixer

The second phase of testing was to develop the mixer and to control steady state outlet

temperatures between the design limits of 218 to 323 K. The second phase testing started

with a full length test using 400 liters of liquid helium. During this test the outlet

temperature was satisfactory for the first four minutes, then it dropped below the allowable

limit of 218 K for the last 90 seconds. Also, the outlet temperature range was 100 degrees,

so ga; mixing was inadequate.

To improve the mixing of the hot and cold gas a new manifold mixer was designed to inject

equal volumes of the cold gas into three equal areas of the hot gas stream. Also a mixer

was added to the discharge tube. This mixer consisted of three half moon discs oriented at

lv.0° and spaced at one half tube diameter apart. Mixing was satisfactory with these mixers

but the half moons created a high velocity gas stream in one side of the discharge tube. This

high velocity jet was damaging to the plastic tube that was to be used to direct the gas from

the unit to the balloon.

Another gas mixer was designed for the discharge tube. This mixer used three sets of

louvers oriented at 90° and spaced one half tube diameter apart. The first set of the

louvers are shown in place in figure 13. This configuration proved to give good mixing and

uniform discharge velocity.

To increase the gas temperature for the last 90 seconds of the run, a higher flow rate was required to the hot bed. The flow to the hot bed had been previously increased by com- pletely removing the flow control orifice during startup transient tests; therefore, the flow

delay valve was now restricting the flow. This valve was replaced with a larger valve. Then

another 50 liter test was run to evaluate the effects of the larger valve. The larger valve

increased the hot gas flow but it caused a high temperature spike at the beginning, so the

opening of this valve was delayed an additional four seconds. The increased flow to the hot

bed resulted in an increase of 50 K in the average gas cutlet temperature during steady state

conditions.

The increased flow through the hot bed was a potential solution to the 90 second cold

flov problem at the end of the run, but, actually the increase in flow created a worse problem because calculations showed that the hot bed did not contain enough energy for an increase in the gas temperature of 50 K for the full run duration. To eliminate this problem a second cold gas bypass was added to the system to increase the cold gas flow and therefore raduce the average gas outlet temperature. This bypass contained a valve and a flow

control orifice. The bypass was opened 30 seconds after startup and was closed when the

outlet gas temperature dropped to near the lower limit. One test was required to determine

the correct size of orifice so that the discharge gas temperature remained below 260 K during

the steady flow operation. Then, a full length run was made to determine the time at which

the bypass valve was to be closed. During this run the maximum temperature spread at the

gas outlet was 28 K, and except during the initial two second high temperature spike, the

outlet gas temperature remained between 220 and 313 K. The initial high temperature spike

to 355 K was caused by the hot residual gas in the mixing chamber flowing out ahead of the

cold gas entering during the start. Since the duration of the spike was short it was acceptable as the plastic fill tube was not damaged. The addition of the second cold gas bypass

did not increase the total flow rate but just redistributed the flow. As mentioned earlier

the total flow rate was limited by the transfer line in the dewar.

The system was ready to be demonstrated by filling the balloon. Figure 14 is a schematic of this system. Figure 15 is a picture of the heat exchanger and the control valves.

20
-1
22
Figure 15. ALBS heat exchanger and plumb

23
8. FILLING THE BALLOON

The balloon used for the demonstration was 8.2 m high and 8.6 m diameter. The design

3 volume was 287.5 m ; the balloon material was 0.05 mm thick polyethylene. Both the top and

bottom end fittings were aluminum castings. The top fitting contained a large, remotely

controlled valve for exhausting the inflating gas. The bottom end fitting contained a tie

down bolt and a 12.7 cm diameter inlet tube for filling the balloon.

The test procedure for filling the balloon was as follows:

1. The hot bed was heated to 925 K.

2. The test was started by opening the main liquid valve.

3. The gas flow to the hot bed was delayed 14 seconds.

4. The second cold gas bypass flow was delayed 30 seconds and was turned off at 4 minutes, 45 seconds.

The run lasted six minutes, 47 seconds and consumed 355 liters of liquid helium. The overall

average gas temperature at the unit discharge tube was 265 K and the barometric pressure was

626 mm Hg (83.5 kPa) . At this temperature and pressure, 355 liters of liquid weighs 45.8

3 kg and produces 311 m of gas. Figure 16 is an overall view of the system prior to filling.

Figure 17 shows the non- insulated transfer line and the valves during the run. Frost and

drops of liquid air are visible on the non- insulated line from the valve to the heat exchanger. Figure 18 shows the balloon immediately after filling.

The filling of the balloon was successful and no unexpected incidents developed. The

system ran completely automatic after the start switch on the control clock was manually

actuated.

9. DISCUSSION OF THE EXPERIMENTAL SYSTEM

The experimental system was a self-contained package that was automatic once the starting switch was actuated. The two elements of the system that were not developed for airborne use were the helium dewar and the insulation plug in the discharge tube of the hot bed.

The dewar technology is well developed and a dewar can probably be designed and built that

will meet the design criteria of size and weight without a development program.

9.1 Insulation in the Hot Bed Discharge Tube

The insulation plug in the discharge tube of the hot bed must be removed automatically

in the airborne system and the scheme for removal will require some development. Several

schemes that can be explored are presented in figures 19 through 21. The schemes shown in

figures 19 and 20 use powder as the insulator. This may prove to be unreliable depending on

the packing and caking characteristics of the powder over a long period of storage. These

two schemes would require vertical orientation of the heat exchanger at all times.

The scheme shown in figure 21 requires more space but could probably be made to operate

very reliably. The development of the insulation scheme for the discharge tube was excluded

from the program at the time the decision was made to perform only ground demonstrations of

filling the balloon. This decision was made to conserve overall program time.

9.2 ALBS System Weight

The heat transfer unit as built weighed 166 kg, which is about the design objective

weight for the entire cryogenic package. However, the hot bed was built using hardware

available instead of waiting for specially fabricated parts. Hardware such as the heads for

the inner shell of the heat exchanger were much heavier gauge than was required. Valves were selected from industrial stock to expedite delivery and reduce cost. Also, the bed mass was increased twenty percent over the calculated needs. If the bed were resized to the mini- mum weight and the hardware was built using typical aircraft design instead of industrial

design, the estimated weight of the system would be as shown in Table 2. The total weight of 224.7 kg is still 40 percent over the design objective but is not

so large as to be unacceptable. It is still as light as the gas generator systems using

heat exchangers and the system does not contaminate the inflating gas. It is also much less

complicated than most gas generating systems as the only operating parts are the clock and

four solenoid valves.

The estimated size of package for the system is a cylinder 91 cm in diameter by 180 cm

long which is only 8 percent longer than the design objective of 166 cm.

24


Figure 18. The filled balloon.

27
0 0 O 0 0

Powered Butterfly Valve

Mixing Manifold

Mixing Louvers

To Balloon

Figure 19. Discharge tube insulation using powder on a butterfly valve,

28
Tissue Paper Sleeve to Support Powder

Powder insulation

Mixing Manifold

Burst Disc

Mixing Louvers

To Balioon

Figure 20. Discharge tube insulation using contained powder on a burst disc,

29
Insulation Plug Before Run

Figure 21. Discharge tube insulation using a moving fiber plug.

30
Table 2. ALBS Weights

Component

Hot Bed and Plumbing

Bed media

Inner vessel

Outer vessel

Valves

Pipe

Mixer

Pressurization System

Gas cylinder

Valve

Regulators

Liquid and Dewar

Dewar

Liquid

Controls

Clock

Batteries

Estimated Weight

kg

60

129.1

7.9

1.0

1.8

Sub Total

Sub Total

Sub Total

Sub Total

91.8

12.2

0.7

1.6

14.5

68.7

45.8

114.5

1.6

2.3

379

Experiment Weight

kg

7440

22

16

5

9 166

12.2

0.7

1.6

14.5

not

applicable

1.6

2.3

3.9

TOTAL 224.7

10. CONCLUSIONS

The demonstration filling of the balloon is believed to be the first successful fillin]

of a large balloon using liquid helium to supply the inflating gas. The filling demonstration occurred without incident and was totally controlled by the electric clock mechanism

from the time the clock was started until the balloon was full.

The conclusions from the experiments and demonstration balloon filling are: 1) a balloon can be filled using liquid helium and a hot packed bed to convert the liquid helium

to warm gas; 2) the filling of a balloon can be totally automatic and can therefore be

accomplished completely remote, while the balloon is suspended from a parachute; and

3) the system is simple, reliable and lightweight.

Some development work is still required on the high temperature insulation scheme used

in the hot gas discharge tube and to optimize the packaging of the hot packed bed and the

liquid helium.

31
11. REFERENCES

Payne, James C. , Final report on the hurricane positioning device feasibility study,

Project 7776, Task 67642, Air Force Cambridge Research Laboratories, L. G. Hanscom

Field, Bedford, Mass. (1960).

Carten, Andrew S. , An investigation of techniques for launching large balloon systems

from aircraft or rockets in flight, AFCRL-TR-73-0633 , Air Force Cambridge Research

Laboratories, L. G. Hanscom Field, Bedford, Mass. (1973).

Goldsmith, A., Waterman, T. and Hirschborn, H. , Handbook of thermal physical properties

of solid material, The Macmillan Company, New York, N.Y. (1961).

Willmott, A. J., Simulation of a thermal regenerator under conditions of variable mass

flow, International Journal of Heat and Mass Transfer, 11, p. 1105 (1968).

Handley, D. and Heggs, P. J., Momentum and heat transfer mechanisms in regular shaped

packings," Trans. Istn. Chem. Engr., 46, T251 (1968).

32
N8S- MA (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 76-834

2. Gov't Accession

No.

3. Recipient's Accession N<

TITLE AND SUBTITLE

A SYSTEM FOR INFLATING A BALLOON USING HELIUM STORED IN THE

LIQUID PHASE

5. Publication Dan

January 1976

6. Performing Organization C 275.05

7. AUTHOR(S)

C. F. Sindt, W. R. Parrish

8. Performing Organ. Report Ni

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project Task Work Unit N, 2750455

11. Contract/Grant No.Y74-898

Y75-808,Y75-S41,Y76-806

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Air Force Cambridge Research Laboratories

Air Force Systems Command

United States Air Force

Hanscom AFB, Massachusetts 01731

13. Type of Report & Period Covered

Final

14. Sponsoring Agency C

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.)

This report covers the design and development of the prototype of a system to be used to fill a balloon that has been launched from an aircraft. The described system uses a hot packed bed heat exchanger to

gasify 45.4 kg of liquid helium and warm the gas to 260 K for filling the

balloon. The prototype system was successfully demonstrated on the

I

- 3

ground by filling an ambient pressure balloon with 300 m of helium gas

at an average temperature of 260 K in six minutes and 45 seconds.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Balloon; cryogenic balloon inflation; heat transfer;

liquid helium; packed bed heat exchangers.

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