Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

NATIONAL BUREAU OF STANDARDS REPORT

7358

AN INVESTIGATION OF THE FIRE-INDUCED FLOW REGIME AT

THE SINGLE VENTILATION OPENING IN A MODEL ROOM ENCLOSURE

by

George D. Van Arsdale

<5JSp>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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The functions of the National Bureau of Standards are set forth in the Act of Congress, March 3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and maintenance of the national standards of measurement and the provision of means and methods for making measurements consistent

with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials, devices, and structures; advisory services to government agencies on scientific and technical problems; invention and development of devices to serve special needs of the Government; and the development of standard practices, codes, and specifications. The work includes basic

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July 1957 to June 1960 (Includes Titles of Papers Published in Outside Journals 1950 to 1959) (S2. 25); avail- able from the Superintendent of Documents, Government Printing Office, Washington 25, D. C.
NATIONAL BUREAU OF STANDARDS REPORT

MBS PROJECT MBS REPORT

1002-11-10122 October 16, 1961 7358

AN INVESTIGATION OF THE FIRE INDUCED FLOW REGIME AT

THE SINGLE VENTILATION OPENING IN A MODEL ROOM ENCLOSURE

by

George D. Van Arsdale

Participant in

Junior Scientist and Engineer Programs

1961

IMPORTANT NOTICE

NATIONAL BUREAU OF STANDARDS REPORTS are usually preliminary or progress accounting documents

intended for use within the Government, Before material in the reports is formally published it is subjected

to additional evaluation and review. For this reason, the publication, reprinting, reproduction, or open-literature

listing of this Report, either in whole or in part, is not authorized unless permission is obtained in writing from

the Office of the Director, National Bureau of Standards, Washington 25, D. C. Such permission is not needed,

however, by the Government agency for which the Report has been specifically prepared if that agency wishes

to reproduce additional copies for its own use. <NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

An Investigation of the Fire-Induced Flow Regime at

the Single Ventilation Opening in a Model Room Enclosure

by

George D. Van Arsdale

ABSTRACT

Three methods for determining the boundary

between entering air and exiting smoke at the

single ventilation opening in a model room enclosure are presented: temperature profile,

optical distortion due to thermal gradients, and

differential COp concentration. Preliminary tests

with the first two methods gave reasonable and

consistent results.

A technique for measuring smoke and flame

velocities near ventilation openings has been

explored. By means of interrupted streak photographs of small, inert particles, velocities up to

about 10 fps in one plane may be measured.

BACKGROUND

As part of a project exploring the use of models for investigating fire behavior in buildings, it was desirable to

study the flow of air and smoke at the opening in a model room

enclosure. It had been found that the weight rate of combustion

of the fiberboard fuel cribs within the enclosure was related

JL to the "ventilation parameter," A H

2

, where A is the area and

H is the height of the opening. This parameter is of the same

form as the expression for non-compressible gas flow through an

orifice Q = k A(2gH) 2

, where Q is volume rate of flow, k is a constant, A is the cross sectional area, g is the gravitational

constant and H is the static head change.

It had been observed in these burning experiments that in

the case of a single opening in an enclosure, air would enter

the enclosure through the lower portion of the opening, and

smoke would leave through the upper portion. At any given time,

a fairly well defined boundary existed between the two, but the

boundary was observed to vary with the particular burning stage

i.e., ignition, steady-state burning, and decay. It would be

expected that the boundary would be elevated as the oxygen in

the enclosure was consumed and steady-state was approached.
2

The velocity distribution, when integrated to obtain total

air and gas flow, should give an indication of the combustion

procresses involved. Some of the data might indicate a change

ih mechanism, depending upon the degree of ventilation. In

addition, knowledge of smoke (and flame) velocities at the

opening in the model room enclosure should be useful in

estimating the effects at windows and doors during fires in

full-size rooms.

This report summarizes preliminary studies dealing with:

I. Boundary Location, and II. Velocity Distribution.

PART I. DETERMINATION OF THE BOUNDARY BETWEEN ENTERING AIR

AND EXITING SMOKE.

1. Introduction

The boundary between the air and the smoke may be defined

by either the point of zero velocity or the point of maximum

temperature gradient. While it is not known whether or not

these points (or actually lines) are coincident, they are in

close proximity. The boundary could be defined as the point

where clear air stops and smoke begins. However, this would be

extremely difficult to detect, even photographically, because

of the ’'neutral" color and lack of contrast of the smoke. With

all of these definitions, the random fluctuations of the turbulent flames must be considered.

t a. Temperature Profile

Since the smoke and gases leaving the enclosure will be

at a higher temperature than the ambient air which is entering

the enclosure, it is relatively simple to measure the temperature of the gas at the opening, and determine at what height

the first (and only) sudden change occurs. A convenient way

for measuring this temperature profile is by means of a series

of stationary thermocouples. However, since the temperature

measuring devices should not interfere unduly with the gas flow,

a better arrangement seemed to be a single thermocouple mounted

on a traverse mechanism. This would not be limited in precision

by thermocouple junction spacing, and the recording of data would

be simpler.
- 3

-

The apparatus used in this experiment consisted of a No. 24 gage (0.020 in.) chrome 1 -a lumel thermocouple probe

mounted on a traversing mechanism. The probe was made of 1/4"

o.d. porcelain two-hole-tubing and presented a very small

obstruction to the gas flow. The traversing mechanism was

built around a hand-operated hydraulic jack, the piston movement of which was read on an attached scale. The thermocouple leads were connected directly to the input of a millivoltrange, dual channel recording potentiometer without cold junction

compensation.

The thermocouple was made to traverse the opening in

about four minutes. A second thermocouple probe with a 4.5 cm

separation was installed and its output recorded simultaneously.

A typical temperature profile during the initial stages of

burning is shown in Figure 1, no attempt having been made to

measure absolute temperature. The height of the boundary, h,

is clearly defined.

b. Optical Distortion

It is well known that thermal gradients produce a bending

of light rays which results in distortion in the image of

objects. This is due to the changes of refractive index of the

air and the resulting prism effect. This principle is employed

in Schlieren photography, and explains the "dancing" of hot

pavement. A very simple way of determining the location of the

boundary is to observe the distortion of a grid, by the smoke

and flames. Through the use of photography, fairly accurate

measurements may be made conveniently. However, since the grid

and the hot gases are not in the same plane, parallax errors

must be considered. By placing the camera at the same level

as the boundary, the error will be small and the use of a telephoto lens (half of a pair of 7 x; 50 binoculars for example),

should reduce the error to negligible proportions.

Several preliminary photographs were made with an Exakta

35

TM single-lens-reflex camera, coupled with a pair of

7 x 50 binoculars. With several different combinations of

grid and illumination, good results were obtained. The camerabinocular combination had an effective opening of f 7*0.

Using the setup shown in Figure 2, the result of a typical

optical distortion test is shown in Figure 3*
c. Differential CO^ Concentration

The products of combustion which leave the opening of

the enclosure contain, say 10% CO^, whereas the ambient air

contains less than 1%. C0

? is an excellent absorber of infrared radiation, especially that with a wavelength of 4.2

microns. By placing an infrared source on one side of the

box, and a detector (i.e., a thermocouple) on the other side,

the height of the boundary could be measured. A series of

fixed detectors or a single movable one could be used. The

same sort of scheme could be worked out for visible radiation,

but there is insufficient natural contrast to be detected

photoelectrically . Any material which was introduced into

the enclosure for the purpose of producing dense smoke would

seriously affect the combustion processes.

While this method would probably give good results,

other methods appear more expedient for this particular series

of experiments.

2. Discussion

Reasonable and consistent results have been obtained by

both temperature profile and optical distortion methods. A

summary of the results from a limited number of boundary determination tests is shown in Figure 4. The optical method

has the advantages of measuring the boundary over the whole

front of the enclosure, and of not disturbing the gas flow at

all. It gives a nearly instantaneous value of the boundary

location, but the time period can be varied by changing camera

exposure time. Some distortion of the grid may be traced to

the lens system, but this may be accounted for by comparing

the photograph with one taken without any fire. Distortion

may be reduced by using a smaller lens opening on the camera

and adjusting the exposure time accordingly.

While the optical method is definitely superior for

instantaneous measurements, a variation of the thermocouple

method offers possibilities for continuous measurements. Two

thermocouples, displaced vertically from each other about 5 nim,

could be differentially connected so as to respond to temperature difference. The height of the probes could be regulated

by a servo system to maintain a maximum temperature difference

and thus measure boundary height as a continuous function of time.

Either Schlieren photography or interf erommetry could also

be employed to measure this boundary, but the expense involved

does not appear justified.
- 5 -

PART II. TECHNIQUE FOR MEASURING SMOKE AND FLAME VELOCITIES

NEAR VENTILATION OPENINGS.

L. Introduction

Gas velocity measurements often present many problems.

The most common method is to measure the pressure drop across

an orifice through which the fluid is flowing and compute the

velocity from Bernoulli's equation. Another method involves

measurement of dynamic pressure with a Pitot-static tube [l].

While the first method is suitable for a closed duct, with

gas of uniform temperature and composition, it would be

difficult to apply at the entrance to a model room enclosure.

The second method, while possible, was not considered practical

due to the slow response time of available manometers sufficiently sensitive to measure the small pressure changes

involved. A method which is often used when low velocities

are to be measured is the hot-wire anemometer. This device,

however, is temperature-dependent and compensating arrangements appear to be inadequate. A cup-type anemometer or a calibrated "fan blade" offer too much resistance to be practical.

Faced with this array of unsuitable methods, it was

decided to turntoaflow visualization technique. If small

particles are released in a flowing stream, they will tend to

follow the streamlines. The more closely the densities of the

particles and the gas match, the more the particles will

follow the gas flow. Ideally, "marked" ( i . e ., ionized or radioactive) molecules of gas could be used as they would follow

the streamlines exactly, and cause a minimum of flow interference.

In practice, however, these would be difficult to work with. It

then becomes necessary to accept some loss in accuracy and use

solid particles.

2. Test Method

In this test method, the distance which small particles

travel in a given time was measured, and their velocity

computed. The particles were introduced into the gas flow by

a dispenser of the type shown in Figure 5 * All of their initial

velocity was in a direction in which the flowing gases have no

velocity component. The distance which the particles travelled

was measured from a photograph, taken at a distance of about

2 feet. Light was provided by a source located beneath the

enclosure opening, which projected a narrow (i.e., 1-inch) band

of light upward. A mirror (or second source) located above and

in front of the enclosure served to provide more even illumination. [ 2]

.
- 6 -

The timing was provided by a mechanical auxilliary shutter

which was placed in front of the camera. This device is

essentially a pitchless "fan,” driven by a constant speed nrotror

,

and was provided with a protective guard housing.

The whole apparatus was set up as in Figure 6, and provided with thermal radiation shields to protect the camera,

dispenser, etc. It was necessary that the shield opposite

the camera be a dull black, in order to provide a contrasting

background for the particles. A scale for distance measurements and a card bearing the test number was inserted into

the camera's field when each photograph was taken.

3. Materials and Procedures

The powder which was used in most of the preliminary tests

was commercial perlite although magnesium oxide powder or

commercial talcum powder may also be used [3]- The perlite was

passed through sieves in order to obtain a fairly uniform size

sample. It was found that the best sizes were between #200

sieve (7** microns) and #70 sieve (210 microns). While finer

particles follow the streamlines better, they tend to cake in

the dispenser. In a humid climate such as is common in

Washington, D. C., in the summer, it is helpful to store the

powder in a desiccator prior to use. It would be possible to

use a less inert particle, such as sodium bicarbonate, but

this decomposes very rapidly at the high temperatures (** 725°)

encountered. For some measurements, the luminescence of such

particles would help, and thereby justify their use.

The powder dispenser essentially shakes powder into a stream of air which carries the powder out of the delivery

tube. There are several adjustments which may be used to

vary the powder flow. The particle velocity is controlled by

the air supply pressure, while the amount is controlled by

the screen size, the vibrapor impulse, and the differential

pressure between the magazine and the delivery tube (fixed in

this series). The delivery tube should be aimed more or less

toward the camera, and can be adjusted in height as needed.

The chopper which was used in the preliminary tests consisted of two 30-degree blades driven by an 1800 rpm synchro--*

nous motor. A "light" period of 1/72 sec is followed by a I/36O sec "dark" period; thus a particle which moved 5/6 in.

in each dash, was traveling at 5 fps. The camera shutter was

left open for 1/5 sec., in order that each particle leave

"dashes" on the photograph. The camera was placed so that the

chopper blade traveled at right angles to the camera's focal

plane shutter, reducing the foreshortening effect.
- 7

-

Any good camera capable of focusing at the required

distance is satisfactory. In the preliminary tests, an

Exakta single-lens reflex, 3 5 mm camera was used. A good

black: and while film such as Plus-X or Tri-X should be used,

with exposure depending upon film and illumination. It was

found that f.22 at 1/5 sec gave good results with Tri-X and

two photoflood lamps. A telephoto lens is helpful, as

larger particle images are easier to measure. The measurements may be made directly from a projected negative, or

from the negative itself with a microscope.

The illumination should be concentrated in a narrow band,

so as to keep all illuminated particles within the camera's

depth of field. A slit opening, fitted with a cylindrical

condensing lens would ensure such a band. However, heat

produced by incandescent lamps must be considered in the design

of their housing.

Under some conditions, it is desirable to provide the

camera with a filter, e.g., light blue, to reduce the image

of orange flames. This also protects the camera lens from

heat, flying ash particles, etc.

4. Discussion

The method of velocity measurement described herein

offers a potentially flexible and accurate means for studying

the flow in the vicinity of flames. It has been used for

this purpose by several workers in the field, c.f. Lewis and

von Elbe f3l- The principal merits for this method lie in

its simplicity (apparatus which can be operated by one man)

,

and its minimal disturbance of the flow patterns. A sample

record of fire-induced particle tracks at the opening of an

enclosure is shown in Figure 7.

The powder dispenser gradually evolved from a gravity

feed type. In this case, it was noted that many particles

failed to make the nearly 180° direction change necessary to

follow the rising gas flow. In spite of the seemingly overwhelming effect of the "drag" or aerodynamic lifting force

(see appendix), the mass and initial velocity of^ the particles

seems to have some effect. This may be aggravated by the

tendency for flames to travel up into the vertical delivery

tube and to fuse particles together within the tube.
8

One difficulty inherent in the forced-feed dispenser is

the effect of its air stream on the flow patterns which are

being studied. As the air is directed into the upper part of

the opening, in the horizontal direction, the particles are

carried away from the enclosure without becoming part of the

hot gas flow. The stream of supply air, however, helps keep

the delivery tube (1/4-in. copper) cool. The tube itself

causes some interference with the gas flow, but this is small,

and not in the region being studied. The important feature

of the dispenser is the restriction of initial particle velocity

to a direction in which the velocity of the hot gases has no

significant component.

The chopper is quite straightforward, the blade geometry

having been selected to give a reasonable interval. A speed

check with a stroboscopic tachometer indicated that the chopper

was running exactly at synchronous speed for constant line

frequency.

There is some question as to the best film to use. At

the short distances suggested, a medium speed film such as

Plus-X is probably best, although at a greater distance the

more sensitive Tri-X is better. The film should be processed

according to the manufacturer's directions, and kept free of

dust particles.

The band lighting was used to obscure particles which

are out of the camera's depth of field. It also served to

keep the background darker, thus providing maximum contrast

between the particles and the background. The heat produced

by photoflood lamps was a major problem in their use. Although

flash lighting eliminated this problem, it was bothersome. A

stroboscope could be used, but its flash duration is far too

short for the chopper. The stroboscope itself could be used

to provide the timing interval, but this would produce a series

of dots, rather than dashes on the photograph. In some cases,

dots could be used satisfactorily, but it is extremely difficult to distinguish individual particles among an array of dots.

The question of filter use was not completely resolved.

In some cases a blue filter seemed to help. In other tests,

the photographs taken without a filter were far superior.

Apparently, the effect of a filter depends upon the emission

of the particle (if any) and the emission of the flames.

These would vary with temperature.
- 9

-

5. Conclusions

The method of velocity measurement described is ready

to be tried in full-scale tests. Some refinements will be

necessary before good reliability can be maintained, but

this should not require a great deal of effort.

The velocities of air, smoke, and flame in the same

region may be measured with a fair degree of accuracy. Before

many tests are made, a calibration of the system in a duct

would be in order. A certain amount of inaccuracy is inherent

in the system because of the difference in densities of the

gases and the particles. However, this is not considered of

great magnitude in the application for which the method was

developed

.

6. References

[

1]

. R. T. Gautreau, NBS Technical Report No. 7008, 3 Nov.

I960.

[2]

. M. Allen and A. J. Yerman, "Neutral Density Beads for

Flow Visualization" ASME Symposium on Flow Visualization,

New York, 30 Nov. I960.

C 3 J - B. Lewis and G. von Elbe, Journal of Chemical Physics,

Vol U, (199-3) 75-84.

[4]. V. Streeter, Fluid Mechanics, McGraw-Hill, New York,

(1958).
10

7. Appendix

Fluid Force on Particles

Assumptions: (only approximate)

U= 5 fps, T = 1500°F p and \i of smoke

p and p of dry air, d = wetted perimeter/4

- 125

4 cm

R =

pTJd _ 4 1.47 x 10"3 4 x 2.54

_1 121 = 4180

this is in the so-called transition range

r 2

Fdrag C d A LZ

2g

with a particle having p 7.85 lb/ft^, D = .0058

(will just pass through #100 sieve)

= 0.4 from Tables - Streeter [4]. p. 170

F

d

= (0.4) (1.83 x 10' 7

) (7.85) (25)

2 (32.2)

-7- = 2.23 x 10 lbs

Using the same assumptions, the gravitational force is

F

g = pv = #

r

3

P = ^(i728) /^8

}

3

; (7.85)

= 4.63 x 10 12 lb

Since 2.23 x 10 ^ » 4.63 x lO*"^

2

, gravity effects

are small compared with the force of the fluid.

USCOMM-NBS-DC
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FIG. 1

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FIG-6. VELOCITY MEASUREMENT

SET- UP

SHIELD

LI GHT
Ftfi 7

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PHOTO&RAPH

OF

F(fiE-lMbUCEb

PARTICLE

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OF

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camera

Distance

z

FE5T

U. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, beaetury

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

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