"

Aerona~tical and Astronautical Engineering Department

University of Illinois Urbana, Illinois •

Technical Report AAE 85-2

UILU ENG 85-0502

Behavior of the Lean Methane-Air

Flame at Zero-Gravity

by

Kurt A. Noe and Roger A. Strehlow

Aeronaut~cal and Astronautical Engineering Department

University of Illinois at Urbana-Champaign

Fi.na1 Report

NASA NCC 3-35

for the period

November 1983 through December 1984

February 1985

50272-101

REPORT DOCUMENTATION 1.1.....[PORT NO.

PAGE

•• TItle ancl Subtitle

Behavior of Lean Methane-Air Flame at Zero-Gravity

7. Autt!orll)

t, Performl"C O,.anization Name and Addren

Department of Aeronautical and Astronautical

Engineering, University of Illinois

105 Transportation Bldg., 104 S. Mathews

Urbana, IL 61801

12. Sponsorine O..anlutian Name and Add'-tSl>

NASA Lewis Research Center

210000 Brookpark Road

Cleveland, Ohio 44135

15. Supplementary Notu

J. "Kip/ent', 4cce..ion No.

i. Report Date

February 1985

a. Perform;... O..anlzetl"" Rept. No.

AAE 85-2

10. ProjectITask/Worlt Unit Na.

N11. Contrect(C) or Grant(G) Na.

(C) NASA Nee 3-35

(G)

13. Type of Report & Period Covered

Final

Nov. 1983 - Dec. 198

14.

16. Abstract(Limit:200warcts) A specl.ai r:rgnasoeen desl.gnect anaconstruc.tl:d LoU U~ Ub~U

with the NASA Lewis Research Center Airborne Research Laboratory to study

the effect of gravity on the behavior of lean fuel-air flames in a standard

(one end c.losed) 50.4 mm. (2") internal diameter tube when the mixtures are

ignited at the open end. Design c.onstraints are discussed and the procedure for using the rig is described. The propagation of lean methane-air

flames over the range of 5.05 to 6.1 volume percent methane was studied.

The lean limit at zero gravity was found to be 5.10% methane and the flame

was found to extinguish as previously observed for downward propagating

flames at one g. It was observed that g-jitter could be maintained at

less than + 0.04 g for most zero g trajectories. The flames were found to

be sporadically cellularly unstable at zero g. The effects of g-jitter

were studied. Upward propagation at two g yielded the expected stable

flame cap propagation. When the g level was increased from zero g at

ignition to one g downward during propagation of the flame, the flame

extinguished at rather low g levels when the methane concentration was less

than 5.86% CH4 (the downward lean limit at one g) and did not extinguish at

higher methane concentrations.

17. Document Analysis a. Oe5CriptOrs

Combustion flammability limits, Zero gravity

b. Identifien;/Open·Ended Terms

c. COSATI Field/Group

lL A"ailability Statement

Release unlimited

T19, Security Class (This Report) I21. No. of PAile.

jUnclassified ii + 40 I-=-:-::------=------------tl~=--~~-=----- I20. Security Class (This Page) 22. Price

!Unclassified

(See ANSI-Z39.1S) SH InstructIons on Re~~r$e OPTIONAL FORM 272 (4-77)

(Formerly NTI>'35)

Department of Commerce

Table of Contents

Abstract ii

I Introduction 1

II Experimental Apparatus and Procedure 3

2.1 NASA Lewis' Learjet Facility 3

2.2 The Experimental Rig 6

2.3 Electronics and Automatic Sequencing 11

2.4 Tube Filling Methods 15

2.5 In Flight Procedure 16

III Results and Analysis 18

3.1 Lean Limit at Zero-Gravity for this Apparatus 18

3.2 Behavior of the Lean Methane-Air Flame at Zero-Gravity 21

3.3 Cellular Instability 28

3.4 Miscellaneous Gravity Loadings 31

IV Conclusions 38

References 40

i
Abstract

A special rig has been designed and constructed to be compatible

with the NASA Lewis Research Center Airborne Research Laboratory to

allow the study of the effect of gravity on the behavior of lean

fuel-air flames near their lean limit in a standard 50.4 mm (2")

internal diameter tube when the mixtures are ignited at the open

end and propagate towards the closed end of the tube. Design constraints including safety constraints are discussed and the final

design and the procedure for using the rig are described.

The rig was used to study the propagation of lean methane-air

flames over the range of 5.05 to 6.1 volume percent methane. The

lean limit at zero gravity was found to be 5.10% methane and the

flame was found to extinguish in a manner previously observed for

downward propagating flames at one g. It was observed that g-jitter

could be maintained at less than +0.04 g on most zero g trajectories.

All of the propagating lean limit flames were found to be sporadically

cellularly unstable at zero g. There was no observable correlation

between the occurence of g-jitter and the lean limit. average propagation speed of the flame through the tube or the occurrence of

cellular instability. Some experiments on upward propagation at two

g yielded the expected stable flame cap propagations up the tube with

a longer skirt. In a few experiments in which the g level was increased

from zero g at ignition to one g downward during propagation of the

flame, the flame extinguished at rather low g levels when the methane

concentration was less than 5.86% CH4 (the downward lean limit at

one g) and did not extinguish at higher methane concentrations.

ii
I

INTRODUCTION

This study examined the behavior of the lean limit methane-air

flame in a standard flammability limit tube (SFLT) at zero-g. Previously, a similar study had been performed in the 2.2 second drop

tower at NASA Lewis Research Center (Strehlow and Reuss, 1980). This

previous work was restricted, however, by a lack of sufficient time

at zero-g. For flames of this type, propagation speeds are on the

order of 10 em/sec. Since the standard tube is approximately 1 meter

to 1.5 meters in length, 2.2 seconds is not sufficient time in which

to observe the entire flame history. Note that the propagation speed

is the rate of propagation of the flame tip and not the burning

velocity; these two quantities are equal only for flat flames. A

new facility which is being used in this study, namely the NASA

Lewis Research Center Learjet Airborne Research Laboratory, provides

sustained zero-g for upwards of 20 seconds. This allows flames to be

examined throughout their entire lifetime. This improvement is of

utmost importance since the definition of a flammability limit requires

that the near limit flame propagate the entire length of a standard

flammability limit tube. Thus, this present study has enabled us to

determine for the first time the lean limit, as defined by the U.S.

Bureau of Mines (Coward and Jones, 1952), for methane at zero-g.

In addition to determining the zero-g lean limit for this apparatus,

other characteristics of the zero-g flame's behavior are also examined.

These characteristics include flame speed and structure as a function

of composition, and the method of extinction.

1
Besides zero-g flames, this study examines the behavior of

flames in transitional gravity and the effect of small gravity fluctuations, or g-jitter, on flames. In the case of transitional gravity,

flames have been observed during the transition from zero-g to

sufficiently large positive gravity. Of some interest is the behavior

of flames which are ignited in zero-g and propagate downward as

gravity loading is applied. In the case of transitional gravity,

flames have been observed during the transition from zero-g to

sufficiently large positive gravity. Of some interest is the behavior

of flames which are ignited in zero-g and propagate downward as

gravity loading is applied. In the case of small gravity fluctuations,

their effect on flame speed and shape is investigated. In both of

these cases, mainly qualitative observations are reported because

the data for these flames are not as extensive as in the zero-g

case.

2
II

EXPERIMENTAL APPARATUS AND PROCEDURE

2.1 NASA Lewis' Learjet Facility

Research for this study was carried out at NASA Lewis Research

Center's Airborne Research Laboratory in Cleveland, Ohio. This

facility includes a Learjet Model 25 airplane which has been equipped

specifically for research purposes. The original seating configuration

has been changed to include seating for only three persons; this

allows cabin space for the research apparatus (see Fig. 1). Electrical

buses are available to the researcher and include AC and DC supplies

at various voltages and power ratings. For more information on

this facility the NASA Learjet Model 25 Airborne Research Laboratory

Experimenter's Handbook (NASA, no date available) may be consulted.

The zero-g environment is obtained in the Lear Jet facility by

flying along Keplerian trajectories. Transitional gravities are

obtained by adjusting a part of this trajectory, and small gravity

fluctuations are the inevitable result of imperfections in the

trajectories caused by the minor disturbances of normal flight.

Due to the nature of this facility stringent safety requireme~ts

have to be met. In addition to conventional air safety requirements,

zero-g gravity flight, as well as the use of combustibles, leads to

additional requirements. Due to the absence of gravity all items on

board have to be securely fastened to prevent floating objects which

might impair the testing procedure, damage research equipment, or cause

injury to the researchers. For this reason all e1ectircal leads are

secured through the use of cannon-plugs and all mechanical equipment is

3
cockpit curtain

cabin

do or

-

experimental accelerometer

rig package

I

I I

r ""-

electrical

j

seat for

package researcher Itl I

seat for

researcher 112

/

\1--- ,

Fig. 1. Aircraft cabin layout.

4
either bolted or pinned in place. The researchers also have to be

securely belted to their seats in order to avoid injury. Combustion

safety requirements and how they were satisfied are discussed in

Section 2.4.

In addition to requirements imposed by zero-g conditions, standards

of structural integrity have to be complied with. These standards,

which meet or exceed FAA standards for this type of aircraft, consist of

load factors which must not produce yield stresses in the materials

used, and construction specifications. The load factors which must

not cause the materials to yield are as follows (NASA, no date

available) :

Load Direction

Forward

Down

Up

Side

Aft

Load Factor

9.0g

7.0g

2.0g

1.5g

1.5g

These requirements were established to insure the structural integrity

of the installation. Note that these load requirements must also be

met by all welds and bolted fastenings.

The construction specifications deal mostly with fitting the

equipment into the experimental racks. These standardized racks were

designed and built by NASA to allow for easy installation into the

aircraft and the experimental apparatus must be designed to be fully

contained in these racks. Another construction specification is that

all fasteners have to be secured by self-locking nuts or safety

wire.

5
2.2 The Experimental Rig

Along with the safety requirements, constraints due to available

space and construction specifications and the difficulty of mo~ement

by the researchers in flight led to the design of a system which

would be as self-sufficient as possible. It was also desirable to have

a system which could be operated from a stationary, seated position,

with few actual tasks to be performed by the researcher. With these

factors in mind an automated system became essential. Additionally,

the system had to be simple in order to allow adjustments to be made

easily, and to allow fast and easy repair. To achieve this simplicity,

This automated system also required a suitable

a mechanically automated system was chosen instead of an electronically

automated system.

test apparatus.

In order to have a self-contained system it is necessary that a

number of standard flammability limit tubes be filled before flight

and ignited and filmed one by one during the required trajectories. To

allow for the filming of these flames by a single set of stationary

cameras, a carousel configuration for the tubes was designed. Eight

tubes are placed along the perimeter of a circular metal plate which

can be rotated with respect to the vertical axis, inside the experimental

rack. The rotating plate can then be locked in place, in one of

eight positions, insuring proper positioning for firing.

It is imperative for safety purposes that the mixtures in the

filled tubes cannot be ignited aCCidentally, and to prevent the

possibility of a tube shattering due to excess pressure, it is also

required that ignition of a filled tube occurs only when one end is

open. These factors necessitate ignition of a single tube, only

when that tube is in its proper firing position with an open end.

6
and only during a test run. Since the mixtures in the tubes are

ignited electrically the system was designed to allow electrical

contact only with the circuit of the tube in the firing (and filming)

position.

To insure against igniting the mixture in a closed tube a second

electrical safeguard is employed. While the tube in firing position

is the tube which is supplied with current, the line carrying the

current is connected to an open circuit which can only be closed when

a microswitch is triggered. This microswitch is triggered when the

microswitch arm makes contact with a sliding valve plate only when

the valve plate is in the open position.

The sliding valve plate is pushed to its open position by a

piston/pressure cylinder arrangement and pulled closed by a spring

mechanism. This system allows the valve plate to be activated by an

automated process rather than by the researcher. The piston/cylinder

arrangement consists of a high-pressure air cylinder which releases

pressurized air through an electrically controlled solenoid valve into

the piston housing, actuating the piston. The piston in turn pushes the

sliding valve to its open position. Through another port of the

solenoid valve the pressurized air is exhausted, allowing the piston.

and thus the sliding valve. to slide back into the closed position.

The tubes used in this study are modified versions of standard

flammability limit tubes (SFLT's). The SFLT was defined by Coward

and Jones (1952) as a two-inch inner diameter tube approximately four

to six feet in length. This tube is open at one end (also the end

at which ignition takes place) and closed at the other end.

There are eight SFLT's in the carousel arrangement (see Fig. 2).

These tubes are approximately 28 inches in length - their length was

7
i

c

•

•

• ;

l

Fig. 2. Experimental rig (sho~~ here with patch hoses).

I

•

•

•

•

•

•

•

• ..;; • .,

-.i

...

..

...

...

l~ •

f •

j: •

.

ill • !;C"'-

•

8

ORfGINAL PAQE

BLACK AND WHITE PHOTOGR~H
shortened to fit in the available space. They are made of standard

clear plexiglass tubing which has sufficient optical quality to allow

visual light photography of the flames. These tubes have the ignition

source at the end which opens and an O-ring sealed, removable plexiglass cap at the other end. This removable cap facilitates cleaning

of the tubes.

The ignition end of each of the tubes is modified to hold the

removable igniters (see Fig. 3).

through the tube wall at this end.

A circular hole has been bored

At this hole, the tube has attached

to it a plexiglass collar, with a hole of the same diameter, which

has been fused to the tube's outer wall. This collar is fitted with

bolts which serve not only to hold the igniters in place but also

to carry the current to the igniter coils. Another feature at the

ignition end of the tubes is the inclusion of a pressure release valve

which, when electrically triggered, opens the tube to the atmosphere.

The reason for including this valve is to allow the pressure of the

mixture in the tube to come to equilibrium with the cabin pressure.

By opening this valve approximately one second prior to opening the

tube for ignition, the pressures are equalized. This prevents sudden

expulsion of mixture when the main sliding valve is opened. Such a

late release of mixture causes flow inside the tube during the ignitioL

process and disturbs flame propagation.

In order to insure ignition the nichrome coils are chemically

treated. The coils are dipped in a solution of nitrocellulose and

acetone and allowed to dry. When current is then passed through the

coil, heating the coil, the nitrocellulose residue ignites in a small

burst of flame providing ample energy to ignite_ the flammable mixtures.

Thi~ insures that an ignition limit will not be observed in these

experiments.

9
copper 1eau---__~--__~~

conducting washer

igni ter co il

copper lead

.. ·1 19n1ter COl

- - - - -1----_

-- - - +------~

- - - - - +--_-L_~

con.du~ting ./ ",-as"er~

Fig. 3. A removable igniter (not t~ scale).

10
In addition to the tubes and their igniter systems, digital

voltmeters, which receive as a signal the real-time output of three

orthogonally positioned accelerometers, are an integral component of

the experimental rig. In order to determine the effect of gravity

on flames, continuous gravity readings are essential. Therefore,

by placing these voltmeters, or g-meters, on the experimental rig

in a position where they can be filmed, the gravity loads can be

determined at any time during flame propagation. Along with the gmeters, a cabin pressure meter was placed on the experimental rig,

allowing continuous pressure readings to be recorded.

2.3 Electronics and Automatic Sequencing

The electronics package used in this experiments serves two main

purposes. It regulates the research power supplied by the aircraft

for use in the experiment, and it controls the automatic sequencing.

These two functions are quite interrelated since all but one of the

experimental system's electrical components playa part in the automatic sequencing process. The electrical package also contains

two 28V high speed cameras and a pressure condenser.

The cameras, focused on the tube in firing position in the

experimental rig, are positioned one above the other so that one

camera films the bottom two-thirds of the tube while the other camera

films the top two-thirds of the tube. The area of overlap is the

area of the rig in which the g-meters have been positioned, allowing

both cameras to record real time gravity readings. The cameras run

on a current of 2 A each (4 A total), with a starting current of

5 A each (10 A total). The pressure condenser measur~s the cabin

pressure, and is wired to the digital pressure meter located on the

1 1
experimental rig in the image plane.

The electrical power is supplied by the aircraft in both

alternating and direct currents. This particular experimental con~

figuration uses only the 10 A-28 V DC and the 10 A-lIS V-60 Hz AC

supplies. The current is carried from the aircraft's research power

supply to the elctrical package through shielded wire with all

connections made by cannon-plugs. Both the 28 V and 115 V lines are

connected to a single pull-double throw master on/off switch. In

this arrangement all system power

single switch. From this switch,

may be shut down by throwing a

the 28 V line is passed through

a 10 A fuse and the 115 V line is passed through a 1 A fuse. This

prevents any possible power overloads to tne system circuitry.

After passing through the fuses the two lines are connected to their

respective indicator lights. These lights indicate whether or not

current is being supplied to the system by the 28V and 115 V lines.

From its indicator light the 28 V line is connected to the

ignition circuit. This is the sole electrical component which is

not directly controlled by the automatic sequencing system. This

circuit includes a 38,000 microfarad capacitor in series with a

100 ohm, high power resistor which controls the flow of current to

the capacitor. This combination has a charging time constant of

about 15 seconds which is sufficient to insure that capacitor is

fully charged prior to the firing of the next tube. The 38.000

microfarad capacitor was specifically chosen because its discharge

was strong enough to always provide ignition, while not being so

strong as to burn out the nichrome wire igniters. A DC voltmeter

is patched across the leads of the capacitor to indicate whether or

not the capacitor is fully charged.

12

The leads of the capacitor
are attached to shielded wire which connects by cannon-plug to the

experimental rig. On the experimental rig the ignition circuit

is completed as the lines from the two capacitor leads are connected

to the two knife-switches. These knife-switches are wired to the

leads of the igniter of the tube which is in firing position.

Consequently, during ignition the capacitor is discharged through the

circuit and thus through the igniter.

The automatic sequencing system consists of the timer box,

sequencing on/off switch, and sequencing indicator light. The most

important component of the automatic sequencing system is the timer

box. The timer box contains a low-speed motor, its axle, and four

aluminum disk/microswitch combinations attached to the axle. The

low-speed (2 rpm) timer drive motor is fastened to the timer box,

and the motor axle extends through the timer box. The motor axle

passes through the centers of the aluminum disks which are then

fastened to the axle by set screws. Therefore, the disks rotate

at 2 rpm when the timer drive motor is running. These circular disks

have been machined along their perimeters to contain raised and

depressed regions. Microswitches have been fastened inside the timer

box in such a way that the microswitch arms extend to the edges of

the disks. In this way the raised regions will engage the microswitch arms and trigger the microswitches while the depressed regions

will not. By machining the depressions to a fixed arc length along

the perimeter of the disks, the microswitches can be engaged for a

fixed time interval. Furthermore, by adjusting the angular position

of these disks with respect to the motor axle the exact timing of

the triggering of the microswitches can be controlled. The sequence

in which the camera motors and other electrical components are

activated is represented graphically in Figure 4.

13
Sl-Rl S2-R2 53 54

,.I ,I

1\\\ \\

o - O-r-lI~

60 - 5_

240 20~

180 - 15-

120 - 10-

- ~H <"C l""C 3:0 <'"0 H HZ ;p;::::; H:t ~~ ;PH 3: 00 t""'t'IJ 0.0 r CfJ

:3:tIJ ::r:H <CfJ ::r:>-3 Otlj

~X "-30

;::.

t:JCfJ "-30 Ai Ai ~3 C ;::. Z

00 t-3 X :;o~ 0 tlj

H

< POh'ER

tlj

0

Fig. 4. Sequencing diagram. ON OFF

14 OUG••'''. ot POOR QUAUf'Y
2.4 Tube Filling Methods

Flame behavior as a function of mixed composition is perhaps the

most important aspect of this study. Consequently, an accurate flow

regulation and tube filling system is necessary. In addition, the use

of flammable methane necessitates that care be taken in all steps

of the filling process to insure safety.

The method employed to insure that the mixtures are lean is

to test mixtures in the composition test tube. The composition

test tube is a single, portable SFLT, identical to those used in the

rig, which has its own ignition circuit and can thus be ignited on

the ground. All mxtures used to fill the tubes in the experimental

rig also fill the composition test tube which is then ignited on the

ground. In this way a visual inspection of the behavior and tip

velocity of the flame in the composition test tube is used to insure

that the mixture that has been placed in the tubes in the rig has

the desired composition. Carefully calibrated rotometers gauged

the flows of the methane and air. These rotometers were calibrated

under constant back pressure by measuring fluid displacement.

safety consideration is that the filling is always done outside

Another

of the airplane and at a safe distance from any aircraft. Additionally, all exhaust gases are vented to the outside of the hangar in

which the filling takes place.

All the tubes have two quick-disconnect valves fused into the

tube walls, one near the top of the tube and one near the bottom of

the tube. These quick-disconnect plug and socket connectors allow

flow to pass through them only when properly engaged and sealed.

The tubes can be filled by connecting the fill hose to the quickdisconnect socket at one end of the tube.

15

The gas mixture flows from
the fill hose into the tube at that end, and the gas which was in

the tube is exhausted at the tube's other end through a similar

quick-disconnect socket. In order that more than one tube may be

filled with the same mixture, patch hoses with quick-disconnect plugs

were constructed. These patch hoses can be used to patch two or

more tubes together so that the exact same mixture fills all of them.

The procedure is to choose the composition and the number of tubes

to be filled with that mixture. Each of the tubes to be filled is

then patched to the mixing chamber with the composition test tube

connected last. The desired mixture is allowed to flow into the

tubes for a period of time which causes the displacement of ten times

the volume of the system to be filled. All hoses are standard 1/8

inch tygon tubing.

2.5 In Flight Procedure

The aircraft cabin is arranged so that system components are

visible to, and within easy reach of the researcher. The researcher

is seated alongside the test rig, close enough so that any manipulation

or adjusting of the rig may be performed. The electrical rack, ineluding cameras, is installed and fastened at a fixed distance in

front of the test rig, allowing the cameras to be focused at the

center of the tube in firing position. All indicator lights are

visible to the researcher and allan/off switches are within easy

reach. When a test run is to take place the cabin windows are closed

and a curtain is drawn behind the cockpit, darkening the cabin for

filming purposes.

The actual procedure during a test run consists of checking

indicator lights to make sure the system is ready for testing, darkening

16
the cabin, and then, when the desired g conditions have been attained,

initiating the automatic sequencing by depressing the push-button

sequencing switch. Following a test run the cabin may be illuminated

and the tube carousel must be rotated to place and lock the next

tube into the firing position. At this time adjustments may be

made to prepare the system for another test cycle.

In the case of an emergency, an emergency operational checklist

has been devised. The researcher's responsibility in this case is

to shut down all system power by shutting off the master on/off switch -

terminating the sequencing, and to inform the crew of the emergency

situation.

17
III

RESULTS AND ANALYSIS

3.1 Lean Limit at Zero-Gravity for this Apparatus

Initially, the most important goal of this investigation was

to determine a lean limit for methane-air mixtures in this apparatus

at zero-g. Since this apparatus uses SFLT's, a limit found in it is

a limit as defined by the U.S. Bureau of Mines (Coward and Jones.

1952).

In order to better understand the results obtained at zero-g,

flame behavior at earth's gravity (one-g) in this apparatus was also

investigated. In this way a comparison can be made between the zero-g

and one-g behavior, and the effect of gravity on the lean limit can

be examined. The one-g results are helpful in determining the

influence that the apparatus has on the limits obtained. since data

for one-g limit behavior can be found for several other experimental

configurations.

Extensive laboratory tests were performed with this apparatus

at one-g. The lean limit for methane-air mixtures in this apparatus

for upward propagation at one-g was found to be 5.25% methane in air.

This limit agrees well with the limit of 5.3% established by Levy

(1965), and the limit obtained by Strehlow and Reuss (1980) of 5.27%.

The agreement with the data of Strehlow and Reuss is significant

since zero-g data from this experiment will be compared to the zero-g

results obtained by Strehlow and Reuss.

The zero-g experimental data for this apparatus was obtained

for mixtures ranging in composition from 5.7% methane to 5.05% methane.

18
This range was sufficient to allow the determination of the lean limit

at zero-g. All flames in mixtures whose compositions were above the

one-g limit (5.25%) propagated the length of the tube. Below the

one-g upward limit, flames in mixtures as low as 5.10% propagated

the length of the tube (see Table 1). From these results it is

concluded that the lean limit for this apparatus at zero-g is 5.10%

methane in air.

Accuracy in obtaining the zero-g lean limit in this experiment

is insured by the experimental methods used. By observing entire

flame histories, the extent of flame propagation can be determined

exactly. In addition, the use of g-meters to measure acceleration

in three orthogonal directions allows consideration of gravity

distrubances in the determination of a flame's behavior. Finally,

the use of SFLT's in this experiment allows the lean limit to be

determined through a standardized method. More extensive testing,

yielding more data for flames in mixtures of composition near 5.10%,

would have been useful; however, the fact that the behavior of the

flames is consistent in mixtures whose composition was less than 5.10%

or greater than 5.11% indicate that the behavior was reproducible.

19
Table 1

Extent of Propagation Versus Composition

at Zero-Gravity

LEAN LIMIT AT ZERO GRAVITY

Percent Methane Date Propagation/Extinction

5.25* 7/31 Full Propagation

5.25* 8/1 Full Propagation

5.25* 8/1 Full Propagation

5.24 8/1 Full Propagation

5.22 8/2 Full Propagation

5.20 8/2 Full Propagation

5.20 9/13 Full Propagation

5.15 9/13 Full Propagation

5.11 9/13 Extinguished

5.10 9/14 Full Propagation

5.10 9/14 Extinguished

5.08 9/14 Extinguished

5.05 9/13 Extinguished

*5.25% Methane in Air is the one-gravity upward lean limit for

this apparatus.

20
Comparison to the zero-g data of Strehlow and Reuss (1980)

is somewhat difficult since their value for the lean limit depends

on the assumption that unsteady motion would necessarily lead to

extinction of the flame, and since their data includes mixtures whose

compositions differ greatly from one to the next. Strehlow and

Reuss observed a mixture of 5.22% supporting a steady flame which

is assumed to propagate the full length of the tube, and a mixture

of 5.10% supporting a flame of unsteady motion which is assumed to

extinguish. With no data for mixtures with composition between

5.22% and 5.10%, the limit could be at any composition in the range

from 5.10% to 5.22%.

Since apparatus dependence is of little importance in the

comparison of Strehlow and Reuss' data and this experiment's data

(the one-g limits differed by only .02%), the results can be compared

easily. The lean limit of 5.10% obtained in this experiment agrees

with the range of compositions which represent the lean limit in

Strehlow and Reuss' experiment. Unfortunately, with only an estimate

as to the exact lean limit for Strehlow and Reuss' configurat~on

any more detailed comparison is impossible.

3.2 Behavior of the Lean Methane-Air Flame at Zero-Gravity

The lean methane-air flame propagating through an SFLT at zero-g

exhibits behavior which could be described as a combination of the

characteristics of upward propagating one-g flames and downward

propagating one-g flames. For example, the zero-g flame may be

described as a slightly flattened hemispherical flame cap with a

21
relatively short flame skirt. This shape is similar to that of the

one-g upward propagating flame which has a hemispherical flame cap

and long flame skirt, while also having some of the characteristic

flatness of the one-g downward propagating flame. In addition, the

propagation speed of the zero-g flame is less than that of the one-g

upward propagating flame and greater than that of the one-g downward

propagating flame.

Propagation speeds for flames in this apparatus at zero-g have

been determined from measurements taken from the filmed data for flames

with compositions ranging from 5.7% methane to the lean limit of 5.1%.

They are tabulated in Table 2, and plotted in Fig. 5 along with

one-g upward data from this experiment, and zero-g and one-g upward

data from the Strehlow and Reuss (1980) study. The plotted results

emphasize the effect that the apparatus can have on flame behavior.

The apparatus used in this experiment yielded slightly higher propagation speeds than the apparatus used by Strehlow and Reuss for both

the case of zero-g and one-g upward propagation.

The zero-g propagation speeds tabulated and plotted here represent

average propagation speeds taken over a flame's entire history.

Propagation speeds were also calculated at every 1/24 second, by

measuring the distance the flame propagated between frames. These

"interval" speeds have been tabulated in Table 3 by listing maximum and

minimum interval speeds, along with the average propagation speeds, for

each flame. This fairly broad range of interval speeds for a given

flame represents the effect of flame front oscillations on that flame's

propagation, and while the range of interval speeds for a given flame

22
• Ig, rig

A Ig, Strehlow

and Reuss

•

•

•

•

Fig. 5. Propagation speed vs. composition.

23
Table 2

Tip Propagation Speed Versus Mixture Composition

VELOCITY CHARACTERISTICS

Composition (% Methane)

5.70

5.70

5.70

5.70

5.50

5.50

5.40

5.40

5.35

5.35

5.35

S.3S

5.33

5.30

5.30

5.30

5.30

5.27

5.25

5.25

5.25

5.24

5.22

5.20

5.20

5.15

5.15

5.10

24

Tip Speed (em/sec)

14.77

14.00

14.34

11. 81

15.14

8.43

10.79

15.62

10.06

10.22

10.51

16.73

13.80

9.16

11.49

15.62

9.24

11.89

7.81

7 .98

8.79

7 .72

8.63

7 .39

8.69

7 .87

7 .72

8.40
Table 3

Minimum and Maximum Interval Propagation

Speeds, as well as Average Propagation

Speed, Versus Composition at Zero-Gravity

MAXIMUM AND MINIMUM INTERVAL SPEEDS

% Methane v. v. v

1- 1-min ave

max

5.70 18.03 8.62 11.81

5.70 20.77 10.97 14.77

5.70 19.20 10.19 14.00

5.70 19.59 9.80 14.34

5.50 13.32 4.70 8.43

5.50 20.77 10.97 15.14

5.40 16.07 7 .45 10.79

5.40 21.55 10.19 15.62

5.35 13.32 7 .45 10.06

5.35 18.03 5.88 10.22

5.35 14.50 7.84 10.51

5.35 26.26 11.76 16.73

5.33 23.91 10.19 13.80

5.30 12.15 7 .05 9.16

5.30 16.85 7.05 11.49

5.30 23.91 9.80 15.62

5.30 15.28 5.09 9.24

5.27 19.99 9.01 11. 89

5.25 12.15 4 .70 7.98

5.25 10.97 5.09 8.79

5.24 11.36 5.49 7 .72

5.22 11.76 5.88 8.63

5.20 11.76 4.31 7 .39

5.20 11.76 5.49 8.69

5.15 10.58 5.09 7.87

5.15 13.32 5.09 7 .72

5.10 12.15 5.49 8.40

25
may be large, the average propagation speed for any single flame is

usually consistent with the rest of the propagation speed data.

The burning velocity, or normal burning velocity, of a flame has a

greater physical significance than the propagation speed, since the

burning velocity is the velocity at which an adiabatic, strictly

one-dimensional flat flame would propagate. Unfortunately, the burning

velocities of the zero-g flames observed in this experiment were

extremely difficult to analyze. This is due to their complex flame

shapes and the fact that the 2-D images recorded on film may not

properly represent the actual 3-D flame shape. Nevertheless, the

burning velocities were calculated as accurately as possible. For a

single flame, the burning velocities measured at different times in

the flame's history were generally consistent. Unfortunately, the

burning velocities were not as consistent from one flame to the next

(see Table 4).

As mentioned earlier, the structure of the flames at zero-g

possessed characteristics of both the one-g upward propagating and

the one-g downward propagating flames whose shapes have been described

by Strehlow (1984). While the zero-g flame had a cap-and-skirt shape

similar to the one-g upward propagating flame, the cap was flatter and

the skirt was shorter, similar to the nearly flat one-g downward

propagating flame. In addition, the zero-g flame front often contained

cells, a phenomenon exhibited by one-g downward propagating flames.

This phenomenon was unexpected for the zero-g case and will be discussed

in greater detail in a later section.

26
Table 4

Normal Burning Velocities for Flames at

Zero-Gravity

NORMAL BURNING VELOCITIES

Composition (% methane) Burning velocity (mm/sec)

5.70 47

5.70 46

5.70 51

5.70 49

5.50 35

5.50 29

5.35 39

5.35 24

5.35 43

5.35 45

5.25 26

5.25 33

5.25 35

5.15 36

27
A final characteristic of flame propagation at zero-g is the

method of extinction. In this respect, the zero-g flame behaved

somewhat like the one-g downward propagating flame. At zero-g,

extinction begins with the flame skirt gradually shortening until

only a flame cap remains. The flame cap then becomes flatter and

slightly distorted. At this point the extinction process becomes

rapid. This, along with the relative dimness of the flame makes the

remainder of the extinction process difficult to observe. There

seems to be a reduction in the diameter of this flat flame front just

prior to the immediate extinction of the entire flame front. The

behavior is similar to the extinction behavior of the one-g downward

propagating flame as described by Jarosinski et al. (1982), but

does not include the flame "rise" due to buoyancy that they observed

just prior to extinction.

3.3 Cellular Instability

Cellular instability was observed in this experiment, in lean

methane-air flames propagating at zero-g. While cellular structure

is a common phenomenon in the one-g downward propagating flame,

especially for very lean mixtures, its presence in the zero-g flame

had not previously been observed or expected.

In these experiments, the cellular flame front has two or more

distinct flame "cells" which varied in size and shape. These cells

actually resemble individual flame fronts propagating side by side.

Markstein (1964) postulated that these cells are caused by "selectivediffusion" of the deficient species in the mixture as influenced by

flame curvature and flow conditions ahead of the flame. Recently,

however, Sivashinsky (1983) has noted that cellular instability

28
is really a thermodiffusive phenomenon associated with the presence

of a deficient light species in the approach mixture. As Strehlow

(1984) states, "when any sufficiently light species of the fueloxidizer pair is sufficiently deficient, any concave perturbation

of a flat flame shape will grow to become a deep trough, because

preferential diffusion of the light and deficient species toward

the reaction zone will deplete that species in the neighborhood

of the perturbation and cause the local burning velocity to be reduced."

The flames which will be unstable to this process are those of a

light fuel in a lean mixture or a heavy fuel in a rich mixture.

Consequently, a lean mixture of methane, a light fuel, would be unstable to this type of perturbation.

Typically, the behavior of the cells observed in the zero-g flames

of this study was similar to the cellular behavior reported by Markstein (1964) for flames in wide vertical tubes. In the zero-g flames

which exhibited cellular instability, a trough would develop, separating the flame front into cells. The largest of these would propagate

faster than the smaller cells, and would grow larger, eventually

enveloping the smaller cells. This new flame front would then either

propagate as a stable flame. 0 .... again brea.k into cells repeating

the process (see H'ig. 6). In this manner, a cellular instability

would often develop, die out, and appear again in the same flame.

None of the propagating flames exhibited cellular instability throughout their entire history.

Cellular instabilities were observed at zero-g in flames over

the entire range of mixture compositions tested, 5.70% to 5.10%.

The probability of occurrence had no apparent dependence on the

leanness of these lean mixtures (all flames in 5.70% mixtures exhibited cellular instability). The instabilities occurred throughout

29
stable flame

Photographs of

instability.

right to left)

comparison.

Fig. 6 . the development of a cellular

(time increases top to bottom,

Note the stable flame for

30 ORIGINAL PAGE

kACK AND WHITE PHOTOGRAPW
the tube, and consequently cannot be attributed solely to the ignition

process or to wall cooling effects. Additionally, small gravity

fluctuations, or g-jitter had no noticeable effect on the occurrence

of the instabilities. The flow field ahead of the flame could not

be correlated with these effects since the cells appeared near both

the tube center and the tube wall, and it was found that a flame could

be well behaved before and after the instability ap~eared, with the

overall burning speed unchanged by the occurrence of the instability.

As we have seen, cellular instabilities occur in flames propagating downward at one-g in lean limit mixtures, in flames in lean

mixtures at zero-g and in flames propagating upward at one-g in wide

vertical tubes. The first two flames have relatively slow burning

velocities, while the third has a high burning velocity but a large

surface area. Therefore the cellular instability may be related to

the lifetime of an element of the flame. This is the time it takes

for an element of the flame to propagate as a wave from the tip, the

point at which the flame is held, to the end of the skirt. The relation between cellular instability and wave element lifetime would

explain the occurrence of the instability in flames with low burning

velocities in standard tubes, and in flames with higher burning

velocities in tubes of large cross-section. In both cases, the

longer lifetime of the flame element may be allowing the flames

to become cellularly unstable.

3.4 Miscellaneous Gravity Loadings

In addition to the zero-g flame, two other methane-air flames

were examined in this study. The first of these is the flame prop agating downward while the gravity load is increased continuously

31
from zero-g to one-g; this loading will be referred to as transitional

gravity. Besides examining these two additional flames, the effect of

g-jitter in the zero-g case will be discussed.

A two-g environment was obtained by flying the aircraft at a

continuously increasing angle-of-attack. Unfortunately, the quality

of these two-g trajectories was not as good as that of the zero-g

trajectories. Consequently, the amount of useable data at two-g was

very small. Nonetheless, the useable data was consistent and it

yielded some general observations relative to the behavior of two-g

upward propagating flames.

A methane-air flame propagating upward at two-g is a stable

flame which exhibits a significantly higher propagation speed than the

one-g upward propagating flame. These propagation speeds ranged from

34.4 em/sec to 37.4 em/sec for the two mixture compositions tested,

5.35% and 5.55%. These speeds are approximately 10 em/sec higher

than those of the stable one-g upward propagating flame of similar

composition and approximately 22 ern/sec higher than the average

propagation speed of all zero-g flames of similar composition. The

two-g upward propagating flame also had a longer flame skirt than the

one-g upward propagating flame; this behavior is predictable given

the relation between propagation speed and flame skirt length as

proposed by Strehlow (1984).

Another flame which was studied is the flame which is ignited

at zero-g and which propagates while a continuous gravity loading is

applied. This transitional gravity environment was obtained when

part of a Keplerian trajectory was modified. The gravity loading

for each trajectory was recorded on film, allowing the individual

trajectories to be different while still yielding valuable results.

32
A particularly interesting example of a flame in this environment

is the downward propagating flame whose mixture composition is between

the zero-g lean limit of 5.10% and the one-g downward lean limit of

5.85%. This flame, ignited at zero-g, will begin propagation in the

zero-g environment, and as it propagates, the gravity load is increased.

At some point, the flame will be too lean to propagate down~ard at

the given gravity, and the flame will extinguish.

Flames in mixtures with compositions greater than the one-g

downward limit of 5.85%, did not extinguish with the gravity loads

applied. For mixtures of less than 5.85% methane, extinction occurred

at moderately low gravities for all flames (see Table 5). Considering

the fact that the mechanisms causing extinguishment may have been

initiated at a lower gravity load than that which was recorded at

extinction, the gravity loads which cause extinction in these flames

may actually be lower than those recorded and tabulated. Nevertheless)

the loads recorded at extinction are less than one-half-g for all

flames, including a flame in a 5.80% mixture. For mixtures nearer

the zero-g lean limit, extinguishment occurred at gravity loads as

low as one-tenth-g.

The method of extinction for a flame propagating downward in

transitional gravity is very similar to that of the one-g downward

propagating flame as described by Jarosinski et a1. (1982). In our

case, as gravity loading was increased the flame flattened to a

disk-shaped flame front which would propagate with low frequency

oscillations at a gradually decreasing speed. Just prior to extinction, the disk-shaped flame would slow markedly, its diameter

decreasing, until the small disk of flame would extinguish completely

(see Fig. 7).

33
Table 5

Gravity Loads at Extinction for Flames in

Transitional Gravity

Composition (% Methane)

6.10

5.90

5.80

5.40

5.40

5.30

34

Gravity at Extinction

No Extinction

No Extinction

0.43 - g

0.43 - g

0.30 - g

0.10 - g
a d

b e

c f

Fig. 7. Photographs of a flame extinguishing under a

transitional gravity load. (time increasing

top to bottom, right to left)

, " [') '" ("E- '.) . ·\l·.... L rt'\-..

\J' .... '. .... H01OGRAP~

BL.ACK AND WHiTE. t'

35
The availability of gravity measurements at all times during

a flame's propagation allows an examination of the effect of g-jitter,

small gravity fluctuations, on the flame behavior. These gravity

fluctuations are the inevitable result of obtaining zero-g through

flight.

The effect of g-jitter on propagation speed is minimal. For a

typical flame, a slight change in gravity may be followed by a large

change in speed, while a large change in gravity may be followed by

a small change in speed, and occasionally a change in gravity would

not produce any change in speed. Moreover, the time lapse between a

change in gravity and the change in speed was inconsistent in those

cases where correlations were attempted. This leads one to the conelusion that g-jitter had little effect on a flame's propagation speed.

Another consideration of the effect of g-jitter on flame behavior

is the relation between g-jitter and cellular instability. As was

mentioned earlier, g-jitter had little noticeable effect on the

occurrence of cells, since cellular instabilities appeared in even

fifteen-thousandths of a g. Thus it appears that g-jitter has little

This is true for both flame shape and

appreciable and reproducible effect on the behavior of the lean

methane-air flame at zero-g.

flame speed.

In the case of g-jitter, the gravity loads range approximately

from plus four-hundredths of a g to minus four-hundredths of a g.

These loads have no appreciable effect on the behavior of lean methaneair flames. Conversely, in the case of transitional gravity, a gravity

load of one-tenth of a g may cause extinction of a flame in a near

limit methane-air flame. These results could indicate the presence of a

36
critical gravity, below which the effects may be neglected and above

which the effects are similar to that of the one-g environment. More

extensive testing is needed, however, to allow prediction of a critical

gravity for extinction.

37
IV

CONCLUSIONS

For this study of lean limit methane-air flames at zero-gravity, a

new facility, the NASA Lewis Research Center Airborne Research Laboratory, was used, and an experimental apparatus was designed and

constructed specifically for use in this facility. The quality of the

results indicates that the facility provided an excellent test environment for a number of test configurations, and that the apparatus

was well designed for use in the facility.

In this study the behavior of lean limit methane-air flames at

zero-g was investigated. Several goals were accomplished. A lean

limit for methane in air at zero-g was obtained. Flame structure,

flame speed, and the extinction process were observed for the zero-g

flame. Cellular instabilities in the zero-g flame were observed and

investigated. Finally, the effects of three distinct gravity loadings

on the lean methane-air flame were examined.

The initial goal of this study was to determine a lean limit for

methane in air at zero-g. A standardized method, utiliZing standard

flammability limit tubes, was used to obtain the limit, which was found

to be 5.10%. This value for the lean limit agrees with the value

obtained in a previous, less comprehensive experiment.

The behavior of the lean methane-air flame at zero-g was markedly

similar to the behavior of a lean methane-air flame propagating downward at one-g. The zero-g flame exhibited some of the characteristic

flatness of the one-g downward propagating flame, and had a similar

propagation speed to the one-g downward propagating flame.

38

More
significantly, however, the zero-g flame extinguished by a mechanism

similar to that which causes extinction in the one-g downward propagating flame; specifically, the limit flame at zero-g is drastically

affected by heat loss to the tube walls. AdditionallYt cellular

instabilities occurred in the zero-g flame. While frequently observed

in the one-g downward propagating flame, cellular instability had not

previously been reported in zero-g flames. Occurrence of these

instabilities was shown to be independent of the ignition process,

g-jitter, and, within the range tested, mixture composition. Since

the lean methane-air system is known to be cellularly unstable except

in special cases, some evidence was presented to support the possibility

that the cellular instability may be related to the lifetime of a flame

element.

The behavior of the lean methane-air flame was also examined for

three additional gravity loadings, including a two-g loading, the random

loading of g-jitter, and a transitional gravity loading from zero-g to

one-g. The two-g flame is a stable flame exhibiting a high propagation

speed. The effect of small gravity fluctuations, or g-jitter, on all

the flames tested was found to be negligible. These gravity loads were

in the range of plus/minus four-hundredths of a g. The flames propagating downward in the transitional loading from zero-g to one-g were

particularly interesting. In mixtures with compositions below the one-g

downward limit, all flames extinguished at relatively low gravity loads

- below one half g for all extinguishing mixtures, and as low as

one-tenth of a g for one mixture whose composition was near the

lean limit. The extinction behavior of flames in transitional gravity

was very similar to the extinction behavior of a one-g downward propagating flame.

39
REFERENCES

Coward, H. F., and Jones, G. W., "Limits of Flammability of Gases

and Vapors," Bureau of Mines Bulletin 503, 1952.

Jarosinski, J., Strehlow, R. A., and Azarbarzin, A., "The Mechanisms

of Lean Limit Flame Extinguishment of an Upward Propagating Flame

in a Standard Flammability Limit Tube", Nineteenth Symposium

(International) on Combustion, 1982.

Levy, A., Proc. Roy. Soc., London A-283, 134, 1965.

Markstein, G. H. Non-steady Flame Propagation, New York, Macmillan,

1964.

NASA, NASA Learjet Model 25 Airborne Research Laboratory Experimenter's

Handbook, Cleveland.

Sivashinsky, G. I., "Instabilities Pattern Formation and Turbulence

in Flames," Ann. Rev. Fluid Mech., 15:179-199, 1983.

Strehlow, R. A., and Reuss, D. L., "Effect of a Zero-g Environment on

Flammability Limits Determined Using a Standard Flammability

Limit Tube Apparatus," NASA Contract Report 3259, 1980.

Strehlow, R. A., Combustion Fundamentals, New York, McGraw-Hill, 1984

40
RECENT AERONAUTICAL AND ASTRONAUTICAL ENGINEERING DEPARTMENT TECHNICAL REPORTS

Technical

Report Number

ME 83-1

UILU ENG 83 0501

ME 83-2

UILU ENG 83 0502

ME 83-3

UILU ENG 83 0503

ME 83-4

UILU ENG 83 0504

ME 83-5

UILU ENG 83 0505

ME 83-6

UILU ENG 83 0506

ME '83-7

UILU ENG 83 0507

ME 83-8

UILU ENG 83 0508

Title

MNOR03: An Efficient Rotational Nonequilibrium

cw HF Chemical Laser Model

Concepts of a General Substructuring System

for Structural Dynamics Analyses

Approximate Hodal Control of DistributedParameter Structures

Users Guide for Program MNOR03

Nonlinear Interactions Between the Pumping

Kinetics, Fluid Dynamics and Optical Resonator

of cw Fluid Flow Lasers

Effects of the HF Rate Package and the Optical

Resonator on cw HF Chemical Laser Performance

The Effects of Cavity Losses on the Performanc~

of a Subsonic cw HF Chemical Laser

The Dynamic Synthesis of General Nonconservative

Structures From Separately Identified Substructure

Models

Author

--

L. H. Sentman

P. Schmidt

A. L. Hale

L. V. Warren

A. L. Hale

G. A. Rahn

L. H. Sentman

P. Schmidt

L. H. Sentman

M. H. Nayfeh

L. H. Sentman

P. F. Schmidt

G. M. Marinos

L. H. Sentman

P. Renzoni

S. Townsend

M. H. Nayfeh

K. K. King

A. L. Hale

L. A. Bergman

ME 83-9

UILU ENG 83 0509

ME 83-10

UILU ENG 83 0510

Finite Element Model for Nonaxisymmetric Structure A. R. Zak

With Rate Dependent Yield Conditions

Nonlinear Interactions Between the Pumping Kinetics, L. H. Sentman

Fluid Dynamics and Optical Resonator of cw Fluid M. H. Nayfch

How Lasers
RECENT AERONAUTICAL AND ASTRONAUTICAL ENGINEERING DEPARUIENT TECHNICAL REPORTS (c

Technical

Report Number

ME 84-1

UILU ENG 84 0501

ME 84-2

UILU ENG 84 0502

ME 84-3

UILU ENG 84 0503

ME 84-4

UILU ENG 84 0504

ME 84-5

UILU ENG 84 0505

ME 84-6

UILU ENG 84 0506

ME 84-7

UILU ENG 84 0507

Title

Trailing Edge Flow Conditions as a Factor in

Airfoil Design

Time-Dependent Oscillations in a cw HF Chemical

Laser Unstable Resonator

Stochastic Rotor Blade Dynamics

Sensing Angle of Attack

An Introduction to the Concept of Ang1e-ofAttack Feedback in Manual Control

Random Response of Turbine Structures

Under Seismic Excitation

Finite Element Model for Gap Contact

Problems

\

Author

A. I. Ormsbee

M. D. Maughmer

L. H. Sentman

S. Townsend

G. Tsioulos

J. Bichanich

M. H. Nayfeh

K. K. King

Y. K. Lin

J. E. Prussing

N. M. Karayanak

N. M. Karayanak

Y. K. Lin

R. C. Y. Hong

A. R. Zak
RECENT AERONAUTICAL AND ASTRONAUTICAL ENGINEERING DEPARTMENT TECHNICAL REPORTS (c

Technical

Report Number

ME 85-1

UI LU ENG 85 0501

ME 85-2

UILU ENG 85 0502

ME 85-3

UILU ENG 85 0503

Title

Low Thrust, Optimal, Time-free Transfer Between

Coaxial, Aligned, Noncoplanar Elliptical Orbits

Behavior of the Lean Methane-Air Flame at Zero

Gravity

An Experimental Study of Fabry-Perot and Stable

Resonator cw HF Chemical Laser Performance

Author

G. M. Marinos

L. H. Sentman

K. A. Noe

R. A. Strehlow

L. H. Sentman

G. Tsioulos

J. Bichanich

D. Carroll