THE COLLIERY GUARDIAN

AND JOURNAL OF THE COAL AND IRON TRADES-

Vol. CV.

FRIDAY. JANUARY 10, 1913

No. 2715.

THE FLAMELESS OR 11 CONVERGENT ”
COMBUSTION OF CASES.

By Dr. Jean Meunier.

[Specially Contributed.]

An explosive mixture composed of suitable proportions
of any combustible gas and air does not necessarily give
rise to an explosion on ignition, but may burn quietly,
in which case the combustion is flameless and the
gaseous strata burn in contact with a hot, solid body,
which is thus maintained in a state of incandescence.

Various names have been applied to this type of
combustion—“ flameless combustion,” “ combustion by
incandescence,” “ surface combustion,” &c.; but in view
of the molecular mechanism by which it Is produced, a
more suitable name would be “ convergent combustion.”
This molecular mechanism, in fact, enables a simple
conception to be formed of the laws and properties of
the phenomenon, which are of a most remarkable and
unexpected character. Actually, this new mode of
combustion is the exact antithesis of the explosive
method in which ignition, starting from a certain point
in the gaseous medium, is propagated with violence
throughout the whole of the mass by divergence. It is
curious to observe that the dynamics of combustion
exhibit the same antithetical duality found in the
majority of natural phenomena, which occur in both a
positive and negative sense—as, for instance, electricity,
magnetism, &c.

The action of incandescent electrical conductors in
firedamp mixtures, which has long formed the subject
of investigation by the author in collaboration with
M. Couriot, merely forms a particular instance of
convergent combustion.

The laws of the phenomenon can be most easily
studied by the aid of platinum wire. For the investiga-
tion, it is necessary to use a Bunsen burner, adjusted in
such a manner as to burn with a perfectly blue flame.
The platinum wire is inserted into the flame, and when
it has become heated to redness, the rubber tube
supplying the burner with gas is squeezed so as to
extinguish the flame, and is then immediately released,
thus re-admitting gas to the burner. In these circum-
stances, the platinum wire glows with added vigour, and
remains in this state of vivid incandescence without the
gas becoming ignited.

About 50 years ago, St. Claire Deville and Debray
pointed out that a current of gas directed on a still hot
platinum crucible raises it to red heat, and, in certain
circumstances, itself becomes ignited ; but a point which
entirely escaped these workers is that the proportion of
air mingled with the gas plays a leading part in the
phenomenon. At that time the ideas prevailing on the
limits of inflammability, and on the properties of
explosive gaseous mixtures, were of only a very vague
character, and no one had any inkling of the mechanism
of combustion.

Incandescence depends essentially on the composition
of the gaseous mixture concerned. To arrive at a clear
conception of the matter it is necessary to examine the
curve representing the properties of mixtures of gas and
air in relation to their composition.

Take, as an example, coal gas of average composition,
the lower limit of inflammability of which is 10 per
cent., and the upper limit 33 per cent., and mark on the
axis of the abscissas the percentage proportions of gas
from 1 to 100, the corresponding velocities of ignition
being plotted on the axis of the ordinates. These velo-
cities will evidently be represented by zero for propor-
tions from 0 to 10 per cent., and the curve will coincide
with the axis of the abscissas, but will rise suddenly
when 10 per cent, is exceeded, indicating a rapid increase
in the explosive energy. It attains a maximum at 18
to 20 per cent, (proportion of complete combustion) and
thereafter diminishes, intersecting the axis of the abscissas
at an angle of 45 degs. when 33 per cent, is reached, and
preserves this direction in a straight line, of negative
value, up to 100 per cent.

This dip at an angle of 45 degs. is accounted for by
the fact that, in order to bring, say, a 40 per cent,
mixture to ignition (33 per cent.), there must be added
to it an equal volume of a 26 per cent, mixture, the
velocity of ignition of which is thereby neutralised.
Mixtures containing more than 33 per cent, of gas have
an extinctive action, and behave exactly like those in
the flame and in convergent combustion.

50 60

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iO

u.

o

>

<el

o

>

Fig. 1.

Fig. 2.

33

This affords a very simple means of tracing the curves
of inflammability (fig. 1). At the upper limit of
inflammability mark the straight line at an angle of
45 degs. (fig. 2), prolonging it above and below the axis
of the abscissas, and set up the corresponding ordinate
for the proportion of complete combustion—that is to
say, 19 per cent, for coal gas—then joining the point of
intersection to the point corresponding to the lower
limit of inflammability, which is 10 per cent, for coal gas.
Since in the vicinity of a maximum the variation of the
function is slight, the apex of the triangle is rounded
off, and the figurative curve thus obtained is in perfect
harmony with experimental facts.

Mixtures of combustible gas and gaseous medium of
combustion may thus be grouped into three categories:
(1) uninflammable mixtures, in which the proportion of
combustible is too small and the properties of the
medium are retained; (2) explosive mixtures; (3)
extinctive mixtures, in which the properties of the com-
bustible medium are masked.

To plot the curves of inflammability of a com-
bustible, it is sufficient to ascertain the two limits of
inflammability, and to calculate the proportion which
gives complete combustion. The author has devised a
very simple apparatus (fig. 3) which enables the limits
of inflammability of any combustible gas or vapour to
be determined with ease. The field of explosibility lies

between the positive portion of the curve and the axis of
the abscissas. It is very small in the case of firedamp,
the limits of inflammability—6 and 16 per cent. (?)—of
which are very close together; but, on the other hand,
is very wide in the case of hydrogen, oxygen, carbon
monoxide and acetylene, which exhibits an anomaly in
the lower section.

In an ordinary flame, the mixture is of the extinctive
or explosive category, according to its position; but at
present we are not concerned with flames.

With the preceding data, the peculiarities of con-
vergent combustion can be explained without difficulty.

Successive mixtures of the explosive type are pre-
pared in a glass test tube into which, after inversion, is

f®.

90

i8P

300

Ji

11

Fig. 3.

Apparatus for Measuring
the Limits of Inflamma-
bility of Combustible
Gases.

Fig. 4.

Exhausting and Trans-
ferring the Gas.

introduced a hot platinum wire (fig. 5). The incan-
descence of the wire is the more vivid in proportion as
the explosive character of the mixture is increased.
Since this incandescence is persistent and the gas is
stagnant, it follows necessarily that the different strata
of the gas must be powerfully attracted toward the
incandescent wire.

All the phases of incandescence can be obtained simul-
taneously by operating in a different manner. In this
case an ordinary Bunsen burner (fig. 6) is provided with
a glass cylinder, which plays the part of a lamp chimney
and is fitted with a mica rim to protect the glass from
breaking. The lamp is lighted to give a blue flame, and
as soon as the platinum wire is hot it is lowered into the
glass cylinder, and the flame is extinguished by com-
pressing the rubber tube supplying the burner. The
platinum wire continues to glow, and by adjusting the
air supply the brilliance of the incandescence can be
increased or diminished. This incandescence is in
harmony with the above-mentioned curve of inflamma-
bility, thus demonstrating that the intensity of con-
vergent combustion is proportionate to the explosive
intensity. To stop the incandescence, all that is
required is to prevent the further admission of air,
without touching the gas tap at all.

It should be noted as an essential point that, for an
explosive mixture of any given proportions, the incan-
descence is greater in proportion as the wire is thinner.
To demonstrate this, a number of platinum wires of
different gauge (1 mm. to 01 mm.) are attached to a