NATIONAL BUREAU OF STANDARDS REPORT

« 4731

SUMMARY REPORT OF INVESTIGATION

OF IGNITION OF LUBRICANTS AND

HYDRAULIC FLUIDS BY SHOCK WAVES

by

K0 E0 McCulloh

Engine Fuels Section

Heat and Power Division

Bureau of Ships «=> Bureau of Yards and Docks

U0 S0 Department of the Navy

U. S. DEPARTMENT OF COMMERCE

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NATIONAL BUREAU OF STANDARDS REPORT

SUMMARY REPORT OF INVESTIGATION

OF IGNITION OF LUBRICANTS AND

HYDRAULIC FLUIDS BY SHOCK WAVES

by

K„ Ew McCulloh

Engine Fuels Section

Heat and Power Division

U0 S 0 Department of the Navy

Bureau of Ships - Order No. 1700S°529

Bureau of Yards and Docks - Order No. I27O3

NBS PROJECT

0305-20-2603 June 25 , 1956

NBS REPORT

4731

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, r

unless permission 25.D.C, Such P'

cal ly prepared If

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

ir in part, is prohibited

standards, Washington

report has been specifireport for its own use,

FOREWORD

This report gives a summation of results obtained in an investigation of the ignition of various lubricants and hydraulic

fluids when subjected to shock waves® The study was part of a larger project supported jointly by the Bureau of Ships (Order

number I7OQS-529 ; and the Bureau of Yards and Docks (order

number 12703)® The investigation was originated to develop

fundamental information on the nature of the explosion and

combustion in hydraulic systems, which have on occasion led

to disastrous fires on board ship.

F, Gs Brickwedde, Chief

Heat and Power Division

F, L, Howard, Chief

Engine Fuels Section

KEY

Materials Under Test

Results of ignition tests on the following fluids are reported;

A. Military Symbol 2190 lubricating oil

B* Petroleum base hydraulic fluid, Spec. MIL-O-56O6

C. Dow Coming Methyl Silicone DC-200

D., General Electric Silicone 81406

E. Celanese Cellulube 220

F. Monsanto Pydraul F-9

G. Carbide and Carbon Ucon Hydrolube 200N

H. Houghton Houghto-Safe 271

I. Fluorolube Standard

The above code letter Resignations of these fluids are

used in this report c

NBS Report No. 4731

June 26, 1956
a
Summary Report of Investigation

of Ignition of Lubricants and

Hydraulic Fluids by Shock Waves

by

K„ E0 McCulloh

Engine Fuels Section

Heat and Power Division

ABSTRACT

Nine lubricants or hydraulic fluids were

tested for susceptibility to ignition by shock

waves , and ignition delay data are presented

for seven of these,, The significance of ignition

delay as a safety criterion is discussed 0

INTRODUCTION

Since heating by shock waves provides one conceivable mechanism

for initiating explosions in pneumatic systems which contain com- bustible lubricants or hydraulic fluids, a method has been developed

for investigating ignition in a shock tube c The purpose of this

study was not only to supply answers to the question of the possibility

of igniting such fluids by shock waves, but also to determine ignition

delays where the answer was affirmative.

In order that an ignition delay be measurable and significant,

it must represent the time interval between well defined events

occurring in an inflammable mixture whose state history during this

period admits of sufficient description to allow correlation with

observed delays. Normal reflection of a shock wave at the closed end

of a shock tube provides a clearly defined initial event which can be characterized as an almost instantaneous irreversible adiabatic

compression. The state of the mixture immediately behind the reflected
shock is determined by the initial temperature, pressure, and com- position of the mixture and the incident shock wave velocity,.

Subsequent states of the mixture during the ignition delay period

are determined by processes which are adiabatic for the mixture as

a whole* Therefore, if inflammation sets in sharply, a properly

instrumented shock tube satisfies the requirements of a system for

the determination of ignition delays*

To this evaluation of the shock tube must be added, however,

the provision that the inflammable mixture should experience only

one-dimensional shock waves and flow patterns, except for small

scale non-one-dimensionality resulting from unavoidable heterogeneity

of the mixture (for example, an inflammable mist). Failure to

observe this precaution has the effect that apparently anomalous

ignition may occur in a portion of the mixture which has been shocked

to an unknown state [1]*

In the present study, ignition delay determinations were restricted to mists produced by atomizing the fluids under test.

Attempts to obtain evidence on ignition of test fluid films on

shock tube walls were inconclusive, but ignition was observed when

steel wool or glass wool moistened with oil was subjected to

sufficiently strong shock waves.

The complicated interplay of the processes which follow passage

of a shock wave through an inflammable mist precludes a detailed

description of the state of the mixture during the ignition delay

period* These processes include acceleration and possibly further

atomization of droplets, heat transfer from the hot air to the cooler

droplets, evaporation of combustible from droplet surfaces, interdiffusion of combustible vapor and air, and initiation of chemical

reaction in regions of the resulting non-uniform gaseous mixture

where temperature and composition are most favorable,

A detailed understanding of these processes is not required

for the evaluation of explosion hazards* The important question

is whether a given fluid can be ignited under conditions which are

attainable in a pneumatic system* In this connection, ignition

delay, considered as a function of the temperature and pressure of

the shocked air, has the significance that it represents the upper

safe limit to the durations of high temperature states produced by

shock waves when combustible material is present*

2
EXPERIMENTAL METHOD

Apparatus

The shock tube used in this work consisted of 18 feet of

polished steel pipe of 2 ll/l6 inch inside diameter, closed at

both ends, and separated into two sealed compartments by an aluminum diaphragm 6 feet from one end Q The 6 foot "chamber”

section contained compressed helium as driver gas, while the

12 foot "channel" contained air at subatmospheric pressure* The

fluid to be tested was introduced at the closed end of the channel.

A sharp, pneumatically driven rod passing axially through the chamber

served to puncture the diaphragm* The dimensions of the tube were

such that the duration of the steady state behind the reflected

shock varied from about 1*8 milliseconds at 1000°K to about lo 0

millisecond at l500°Ke

The end plate of the channel contained an atomizer, a straingage pressure pickup, and a window through which light passed to a photomultiplier* A single-beam oscilloscope received pulses from

the strain-gage pickup and two piezoelectric pickups mounted in the

wall of the channel 2 feet and 4 feet from the end plate* A time

base for this oscilloscope was provided by a 2 ke* horizontal sweep

frequency and a 20 kc* beam-blanking signal* A slow vertical sweep

produced a television-screen image with ten to twenty resolved lines on which the pressure pulses appeared as vertical jumps* These

pulses, corresponding to incident shock wave arrival at the three

stations, enabled determination of the velocity of this wave with

an accuracy of about 1% 0

The outputs of the strain-gage pickup and the photomultiplier

were fed to a dual-beam oscilloscope* Tne two beams were swept by

a common sweep generator which was triggered by the signal from the

piezoelectric pickup located 4 feet from the end plate* A sweep

time of 4 milliseconds was used* Time markers were provided by a

2 kc* sawtooth beam-blanking signal from the sweep output of the

single-beam oscilloscope* Ignition delay was taken as the time

interval between the initial rise of the pressure trace and the

first detectable vertical displacement of the light intensity

trace*

The traces of both oscilloscopes were photographed simultaneously

with a 35-mm* camera*

3
Procedure

For ignition delay determinations, the chamber was evacuated

with a mechanical pump before helium was admitted* At the same time the channel was pumped down to such a pressure that the air

to be discharged later by the atomizer would produce the desired

diaphragm pressure ratio* When the chamber pressure had been

brought up to 23O psig of helium, the test fluid was sprayed

into the channel by passing a predetermined amount of compressed air

through the atomizer. After allowing about ten seconds for calm^

ing and temperature equalization in the channel, the diaphragm was ruptured and the oscilloscope traces were photographed. Pressure

was then released, the shock tube was disassembled, and the contents

were examined for evidence of combustion* The tube was cleaned

thoroughly between ignition trials.

When ignition trials were run on films of the test fluids or

on oily steel wood or glass wool, the steps in the procedure pertaining to the atomizer were omitted. For film ignition trials a thin coating of fluid was spread on the quartz window in the end

plate before assembly. When steel -wool or glass wool was used, an

approximately 1-inch ball of wool, moistened with sufficient oil

to coat the fibers without saturation, was placed near the window

at the end of the channel*

In all cases an initial chamber pressure of 23O psig was

employed, using helium as the driver* The shock strength was controlled by means of the initial channel pressure.

Ignition delays and incident shock velocities were determined

from the photographs. Temperatures and pressures behind reflected

shocks were calculated from incident shock velocities, using tabulations based on the actual thermodynamic properties of air [2],

RESULTS

Fluid Films on Window

Attempts to detect ignition of films of fluids A, B, C, D, F,

and I were inconclusive. Examination of the residual fluid after

shocking showed no clear evidence of combustion. If light emission

corresponding to ignition actually occurred, it was masked by more

intense extraneous light, which was traced to burning aluminum

particles.

4
Aluminum Particles

This interpretation of the extraneous light reported above is

support-ed by the following observations 0 (l) Aluminum particles,

which were difficult to remove with certainty, were always found in

the channel following rupture of an aluminum diaphragm„ (2) Direct

photographs showed streaks of light, resembling luminous particle

tracks, some of which exhibited motion through the object plane of

the camera,, This plane was inside the channel, about two inches

from the closed end„ ( 3) The intensity of this light was enhanced

by intentional introduction of aluminum powder into the channel,,

(4) This light was absent when nitrogen was substituted for air in

the channel9

Although ignition of aluminum particles was not studied in

detail, it was found to occur readily when the temperature following

shock reflection was 1000®K or higher0

Oil on Steel Wool and . Glass Wool

Fluid A and SoA.Eo 30 lubricating oil of undetermined origin

were ignited on steel wool and glass wool when the incident shock

wave velocity was high enough to produce temperatures exceeding

about IO5O® K following normal reflection0 Since a ball of steel or

glass wool creates three-dimensional flow disturbances, the actual

temperatures were uncertain, and ignition delays were not determined „ In some cases burning persisted for several seconds 0 Melting of

glass wool and melting and burning of steel wool were observed when

combustion occurred,.

Mists

Ignition delays for most of the fluids under test are listed in

Table I„ The temperatures and pressures appearing in this table are those for the air behind the reflected shock, calculated from the

incident shock wave velocity without regard to the presence of liquid

droplets

„

In the interest of arriving at safe operating conditions, the

data appearing in this table represent the shortest observed delays.

When duplicate determinations were made, only the result yielding

the shorter delay is listed® In cases where the light intensity did

not show a sharp initial rise, or where it passed through a maximum

and a minimum (possibly as a result of two-stage ignition) before

rising to a value corresponding to sustained combustion, the earliest

appearance of significant intensity was taken as the onset of ignition.

Data are presented only for those trials which resulted in sustained

luminosity,,

5
Fluids H and I are omitted from Table I because well defined

delays were not observed*. Data for G are included, although ignition

was doubtful in this case*.

For fluid H the light intensity rose gradually with apparently

zero delay at 1360°K, At I56O0 luminosity appeared sharply without

observable delay, but at both temperatures light emission decayed to

zero. Only very weak, decaying light emission was observed at 1230°K,

The behavior of Fluid G was somewhat similar to that of H, in

that light emission decayed to a low level following arrival of the

rarefaction wave which terminated the steady high temperature state.

In no case was sustained light emission observed for Fluid I,

Luminosity increased with temperature between 1135 and 1540°K, The

products formed by shocking mists of this fluid caused slight corrosion

of the interior of the shock tube, and fumes smelling like phosgene

were detected.

Confirmatory evidence for the ignition of fluids A, B, C, D, E,

and F is the observation of soot formation. The tendency to form

soot was found to decrease with increasing shock temperature. Fluids

C and D represent special cases 9 since their combustion leads to

formation of silicon dioxide. In the absence of soot formation, this

solid appeared as a white smoke which slowly settled as a white deposit

on the shock tube walls. When conditions favored sooting, a fluffy

gray solid was formed, possibly consisting of aggregates containing

microscopic particles of silicon dioxide and carbon.

Only in the case of fluid B could pressure increases due to

combustion be observed. These fluctuations were small and difficult

to measure because the pressure traces were never smooth. They

probably amounted to no more than 20% of the pressure before ignition

occurred, and their durations were of the order of 0,2 to 0,5 millisecond.

6
Table I Ignition Delays

Fluid

Temperature

°K

Pressure

atm®

Delay

3, milliseconds

A 1070 17.1 (0.6)

b

1150 16o3 (0.4) b

1220 15o4 0*1

B 1060 17.1 ( 0. 3 ) b

1100 16 0 7 1.1

1110 16.6 0«5

1150 16 0 2 0.4

1160 I6 e 0 0.2

1230 15 0 2 0.1

C 1135 I6 e 3 1.2

1230 15®2 0.5

1330 14.2 (0)

D 1060 17® 1 0.8 v 1140 16.2 (0.6)b

1230 15*2 ( 0 . 3 ) 1280 14«»7 (0.2)

1400 13o6 (0)

E 1230 15 o2 (0)

1360 34o 0 0.0

1540 11® 9 0.0

F 1210 15 oh 1.4

I36O 14®0 0.6

I56O lie 7 (0)

G

c 1270 14 ® 9 0.7

1350 14®1 (0)

1530 !2 e 0 0.0

aValues in parentheses uncertain because of difficulty in locating

first appearance of light,,

^Possibly the first stage of two-stage ignition

„

cIgnition uncertain for this fluid®

7
DISCUSSION

Since the ignition delays, as reported in Table I, are somewhat

arbitrarily defined as delays to the first appearance of luminosity,

the conclusions based on these data should be more qualitative than

quantitative© In particular, it is probable that light of detectable

intensity appears before combustion becomes well enough established

to persist in spite of the imposition of quenching conditions, such

as the cooling by adiabatic expansion which follows arrival of a rarefaction wave© For this reason, it is the author’s opinion that

the data in Table I provide a margin of safety, but that the magnitude

of this safety margin cannot be appraised reliably on the basis of

existing knowledge of droplet ignition© Such an appraisal might be

obtained empirically by determining limiting conditions under which

sustained combustion can be initiated in a variable-length shock tube,

in which temperature and its duration can be varied independently®

The following conclusions can be drawn from the results of this

investigation©

1© Three of the fluids (A, B, and D) were definitely ignited in

the shock tube at temperatures as low as 1070°K© Fluid C was ignited

at temperatures above 1135°K, and fluids E and F at 1230°K and higher©

In all of these cases combustion persisted after rarefaction wave

arrival©

2© Combustion, if it occurred, was apparently unsustained for

fluid G and definitely unsustained for fluids H and I, even at

temperatures exceeding l500°Ko

3® Ignition delays (which are characteristic of the fluid) decrease

with increasing temperature, and are more strongly temperature-dependent

than is the duration of the high temperature state (which is determined

by shock tube dimensions)© Therefore, for a given combination of shock

tube and ignitable fluid, there should exist a critical temperature

below which ignition delay exceeds the duration of the high temperature

state© This is the non-explosive region© Above this temperature is

the region of possible explosion, where ignition delay is shorter

than the duration of high temperature conditions®

In the present work the temperatures and pressures of the shocked

air were actually interdependent, since a fixed chamber pressure was

exployed® Therefore, the data do not cover the full range of temperatures and pressures which might be encountered in practical

pneumatic systems® Rather, these delays provide a basis for evaluation

of the fluids under quite special conditions©

8
Schrader [ 3 ] has studied, compression ignition of lubricants

and hydraulic fluids by admitting 4000 psi air into a short test

section through a quick-opening valve, -with the test section

initially at room temperature or preheated to 140, 200, or 250°Fo Since transient conditions under which ignition occurred in this

system were not clearly defined, comparison with the results of

the present investigation is difficult* In Schrader1 s report the

fluids are grouped into four categories of combustion resistance,

ranging from unsatisfactory to non^combustible* The present

results must be coupled with gas dynamic analysis of pneumatic

system conditions before such a classification is possible* Such

an attempt should yield a more detailed understanding of the factors

which make for safety or potential hazard*

9
ACKNQWLE3DG@IEMT

The work on this project was begun by Dr* W* Jc Levedahl.

The assistance of Mr* John He McAuley, who constructed the shock

tube and performed much of the experimental work, is gratefully

acknowledged*

10
REFERENCES

[1] M, Steinberg and W# Ee Kaskan, Fifth °yraposium (International) on Combustion, p. 664, Reinhold Publishing CorpOJ New

York, 1955

o

[2] Glass, Martin, and Patterson, UTIA Report No* 2, pp. 74 and 272,

Institute of Aerophysics, University of Toronto, Canada, 1953

»

[3] A* R. Schrader, EES Report O70243K, NS-075-700, U 0 S. Naval

Engineering Experiment Station, Annapolis, Maryland, 1955o

Washington 25, D„ C„

June 26, 1956

11
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