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Gaseous Explosions.—The Committee's Report.

10th September 1908
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Page 14, 10th September 1908 — Gaseous Explosions.—The Committee's Report.
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Which of the following most accurately describes the problem?

The first report of the Committee, consisting of Sir IV. H. Preece (Chairman), Mr. Dugald Clerk and Professor Bertram Hopkinson (Joint Secretaries), Professors Bone, Burstall, Callender, Coker, Dalbv, Dixon, Hele-Shaw, Smithells, and Watson, Dr. Harker, Lieut.-Colonel Hoiden, Dr. Petavel, and Captain Sankey, appointed for the investigation of gaseous explosions, with special reference to temperature, was presented before the engineering section of the Association, on Friday last, by Professor Hopkinson. The Committee has considered the subject mainly from the point of view of its great hearing upon the theory of the internal-combustion engine, but its work has by no means entirely been on these lines. Many questions of a kind that might properly engage the attention of the Chemical and Physical Sections have been raised and discussed. The bulk of the report will not, however, be of interest to the majority

of our readers. We have, there fore, extracted only certain parts of the text of the report which deal

e , with the experimental work that /0 OL rone. has been done in the second and third of the three classes into which the experiments may be divided. The three classes are : (1) Constant-pressure expel-In-lents, the gas for which was heated from an external source and was at atmospheric pressure; (2) experiments in which both the pressure and the volume were varied, the gas being heated by compression ; and (3) constant-volume experiments. To the latter category belong the experiments of Mallard and Le Chatelier, Clerk, Langen, Petaval, Hop kinson, and others. In the explosion experiments the gas was heated by internal combustion.

Clerk's Experiments.

" The gas used was the products of an explosion in a gas engine, and therefore consisted of a mixture of CO2, steam, and air. It was first expanded in the ordinary course after the explosion, and was then heated by compression on the next in. strokeof the engine, the valves being kept closed for this purpose. On the next out-stroke the gas was again expanded, then compressed again, and so on, the valves remaining closed and the engine running on its own momentum. An indicator diagram was taken of the whole operation. The change of internal energy in any portion of a comprcssion slroke (13 C in the diagram) is equal to the work clone less the heat lost to the cylinder walls-, in an expansion stroke )C D) it is the work done plus the heat lost. The work can be obtained from the indicator diagram with an accuracy which is only limited by the indicating appliances. The change of temperature can also be calculated from the indi-2ator diagram subject to a knowledge of the temperature at one point. Errors in the latter, however, do not greatly affect the results found for internal energy or volumetric beat because the figure fur the quantity of gas present is affected by these errors in such a way as to cancel out the error in temierature interval. The loss of heat comes in as a correction en the work done, and was estimated by a comparison of the compression line and the immediately following expansion line (II C and C D). The calculation is based on the assumption that the total heat loss from the hot gases during any given portion of a stroloe is the same in expansion and compression if he mean temperature be the same.

In the first compression the temperature of the gas rose to

about 1,100 degrees C (at the point ( During the first threetenths of the following expansion stroke (C I)), the temperature fell to about 700 degrees C. The work done in this part of the expansion was measured and the heat loss determined as above was added. Thus the change of internal energy corresponding to the temperature change 1,140 minus 700 degrees is

obtained. The average volumetric heat over this range is within the errors of experiment equal to the volumetric heat at the mean temperature of 900 degrees C'., which accordingly is by this method determined direct instead of by difference, as is

necessarily the case when the whole internal energy ehauge associated with complete cooling of the gas is measured. A comparison of Clerk's results with those of Messrs lion:nem and Henning is of great interest. Clerk's measurements extended to 1,450 degrees C., but those above 1 900 degrees C. were based on the first expansion line after the explosion when the method for getting heat loss would be of doubtful application, and when, moreover, combustion may have been incomplete. It will be better, therefore, to confine the comparison to temperatures of 1,200 degrees and below. The following table exhibits the internal energies of the mixed gas with which Clerk experimented calculated from Holborn and Iteiming's figures, together with the energy calculated from Clerk's ,alues for the mean volumetric heat. The energies are, as usual, reckoned from 100 degrees C. ; and the energies of an ideal gas with n constant volumetric heat of 4,9 are added for ci.mparistm.

It will be seen that Clerk's results are throughout about LU per cent, higher than the others. The difference between the energy of the real and of the ideal gas, the discovery of which is the true, object of these experiments, is about twice as great in the one case as in the other. It does not seem possible to Lecount for so large a discrepancy by ordinary experimental errors. It must be due either to some systematic error inherent in the method of experiment in one or both cases, or to a difference in the conditions of experiment giving rise to a real difference of internal energy.

Professor Callender has favoured the committee with a note dealing with the constant-pressure experiments. Ile is of opinion that the results obtained by Regnault's method are too low, and that at the higher temperatures reached by Holborn and Henning, the error may possibly amount to as much as 10 per cent. In all these experiments there is a considerable flow of heat from the heater to the calorimeter. This, of course, has to be deducted from the heat registered in the calorimeter in order to find that which has been given up by the hut gas."

Explosion Experiments.

" In so far as the heat-loss and the departure from equilibrium are dependent on surface phenomena, a definite estimate of their amount can be obtained by a comparison of explosiens of the same mixture in vessels of different sizes.

" Many years ago Berthelot tried this experiment, firing hydrogen and oxygen, in explosive proportions, in vessels of 300 c.c. and 4,000 c.c. respectively. It is stated that the pressure reached was very nearly the same, which would show that such part of the cooling and other corrections as depends on the surface of the vessel is small in the ease of this mixture.

"Materials for a more accurate comparison are to Le found in the extensive researches of Mallard and Le Chatelier, and of Langen. The French experimenters worked with a cylindrical vessel 17 cm. by 17 ern., whereas Langen used a sphere 40 cm. diameter. The ratio surface/volume was 2.3 times as ereat iii the first as in the second case.

" The following table shows the results obtained in tem instances, in each of which the composition of the mixt:ire was practically identical in the two sets of experiments P is the pressure reached in the explosion in aterespheres alter correctim4 or cooling in the manner ciescrilsed all:live., when the Ctial temperature is 0 degrees C.'' Loss of Heat.

"That much of the heat-loss goes oa bydirect conduction to the walls, and is, therefore, a surface phenomenon, is ohvicus. But there is reason to believe that the loss 1,y radiatiom which certainly exists in any flame, is practically important. ” Measurements of the temperature reached in an explesien Cy I/lea:1i of a platinum thermometer, under circemstances whieh render kery improbable any loss of heat by conduction teem the as whese temperature is measured, show that that temperature s censiderably lower than is to be expected from the 1w-:it of 'ramble:lion of the gases and the specific heat of the produets. Professor Callenrlar pointed out, in the discussion of these exseriments, that there was probably a good deal of rat-nation,

and stated that he had found that an ordinary Bunsen flame might radiate up to 15 per cent, of its] heat.

"(te Recent experiments, in which the loss of heat during an explosion was directly measured by finding the rise of temperature of the walls, showed that in a certain coal-gas explosion it amounted to about 12 per cent. of the whole heat at the moment of maximum pressure. Estimated by Mallard and Le Chatelier's extrapolation method, the loss was at most 5 per cent.

"The prevailing opinion seems to be that most simple gases cannot he made to radiate by direct heating. If this be so the radiation must take place in the act of combustion. It seems very probable that when, say, hydrogen and oxygen combine a certain part of the energy of combination passes into the form of internal vibrations of the steam molecule, and that a large proportion, if not all, of this part is ultimately radiated away. If' this he the case a definite proportion of the heat produced in combustion is always lost, and a comparison of explosions in vessels of different sizes would not reveal this loss."

Thermal Equilibrium.

• ‘ When an explosive mixture of gases is ignited in a closed vessel the effect of. the change of pressure during the progress of the flame front the point or points of ignition is to raise the temperature round about those points much above the mean temperature, and, on the other hand, the temperature attained at those places which are last reached by the flame, and where the gas is compressed before instead of after ignition, is much below the mean. Even in a.vessel whose walls are impervious to heat the difference of temperature between the points first and last inflamed might amount to 700 degrees fe at the moment of maximum pressure. In a real explosion the cooling effect of the walls causes the temperature to range front perhaps 300 degrees or more above the mean as shown by the pressure) right down to the wall temperature at points close to the metal. '1 he existence of large temperature differences irt the gas close to the walls of an engine cylinder was first experimentally demonstrated by Professor l3urstall with the aid of platinum thermometers.

"If the volumetric heat of the gas were constant the equalisation of these temperature differences by convection and conduction, could it take place without loss of heat, would cause no change of pressure. The volumetric heat is, however, not constant, but may quite possibly be 50 per cent, greater in the hottest than in the coldest part of the mass. The attainment of thermal equilibrium must, in fact, cause a change of pressure. The amount of the change might be the subject of rough calculation, taking an assumed distribution of temperature and assuming values for the volumetric heat. Such a calculation in the present state of knowledge would only be of value as showing the possible order of magnitude of the quantity sought, and the assumptions made could therefore be of a character to make the calculation fairly simple. More accurate knowledge both of temperature distribution and of thermal capacity will enable greater accuracy to be attained in the estimation of this correction, which will be of such a kind that a method of successive approximation can be pursued, the revised values of thermal capacity resulting from its application being applied to a more accurate calculation of the correction if necessary.

The temperature variation set up by the cooling action of the walls is a surface phenomenon, and as such the correction which it necessitates can probably be determined and eliminated by experiments with vessels of different sizes. Thevariation caused by the change of pressure during the period of inflammation is not of this character ; and the necessity for a large correction on this account is quite consistent with the observations of Berthelot, or of Mallard and 1.e Chatelier and of Langen. In these experiments the maximum pressure reached in the exMoslem was measured, and at the time of maximum pressure very large differences of temperature are known to exist at a distance from and quite independent of the walls. "non after maximum pressure, however, the temperatures at points remote from the walls are equalised to a large extent by convection currents. There then remains only the layer of gas near the walls to be considered in this connection. If, therefore, the measurements be postponed until a long enough time has elapsed to admit of this internal equalisation, the correction becomes of the surface kind, and can he dealt with by the method appropriate to corrections of that type. But in that case the heat lost will be too large a quantity to admit of rough estimation ; it must be directly ineesured."

Chemical Equilibrium.

" The view that chemical equilibr:um is not attained until some titre after the moment ca; maximum pressure was first put forward by Clerk in l85, who then expressed the opinion that the greater part of the so-called suppression of heat ' in explosions was to be ascribed to this cause. On the ether hand, Continental writers have almost completely ignored it. For example, I.angen makes pract:cally no reference to this in his paper. It ran hardly be doubted, however, that in many explosions, especially of weak mixtures, a considerable amount 'if the

energy is in the chemical form at the moment of maximum pressure. On the other hand, it seems probable to the committee that the amount of unhurof gas at this moment in such experiments as those of Lang,en was not such as to very greatly affect the results. This helief is based on the supposition that the incomplete combustion is due to the cooling action of the Walls. It seems probable that very shortly after the attainment of maximum pressure, that is, within a time small compared with that required to reach maximum pressure, the transformation of the chemical energy into thermal form is everywhere complete except in a thin surface layer where this transformation is retarded by the cooling action of the walls.

•• hi the discussion of this important matter the Committee have derived great assistance from the experience of l'rofessors Dixon and Done, who have made a special study of the velocity of chemical action in gases. These gentlemen are of opinion that though such action may be of great complexity, involving in many cases several successive molecular operations, yet if it is not retarded by the presence of cold foreign bodies, it will generally be completed within a period which, for the purposes of gas-engine theory, may be regarded as negligibly small. In the simple case of the explosion of hydrogen and oxygen they consider that the complete transformation of the mixed gases .into steam at any given point is complete withiu a time measured by the interval between molecular collisions. When the action is more complicated, as in the explosion of carbon monoxide and oxygen in the presence of water, or in the combustion of hydrocarbons, the period will be larger, but will still be measured by thousandths of a second.

Some direct evidence that incomplete combustion in an explosion is mainly, if not entirely a surface phenomenon is to be found in Hopkinson's measurements of the -temperature at Points within a large explosion vessel by means of a platinum thermometer. A photographic record of the resistance of a fine platinum wire immersed in the gas showed that when the flame reached it the temperature rose in less than 1-40th of a second from 20 degrees C., which was the temperature of the unburnt gas, to about 1,250 degrees C., which was that of the burnt gas, and that it remained at the latter figure quite steadily except in so far as the increase of pressure in the vessel caused it to rise. In other words, these was no increase of thermal energy except that due to work done upon the gas from outside. The mixture was one part of coal gas to nine parts of air—a slow burning mixture—and the time taken to reach maximum pressure was about a quarter of a second, or at least ten times that required for combination of thegases at any one point. It is true that the vessel was of. rather large sire—about six cubic feet capacity —lint, on the other hand, owing to the fact that the platinum wire extended over about 1 cm., so that the flame took an appreciable time to completely envelop it, it is probable that the period of 1-40th of a second, Oven above, is a superior limit which greatly exceeds the actual time taken to effect the combination at any one point.

On the other hand, it cannot be doubted that combustion must be greatly retarded in the neighbourhood of the cold metal walls ; and there is nothing to show that this surface retardation is not sufficient to account for all the phenomena of delayed combustion. A simple calculation based upon the rate of flow of heat per square foot into the metal of a gas-engine cylinder (which is roughly known from measurements of the heat carried away by the jacket water) shows that the mean temmrature of the exposed surface at points separated by sin inch •:trom the cooling water cannot exceed quite a moderate value. Probably about 200 degrees C. is a superior limit for the cylinder liner. similar calculation of a still rougher kind, but still sufficiently accurate to g.ive the order of magnitude of the quantity sought, shows that the fluctuation above and below the rite-all in the course of a cycle is very unlikely to exceed 20 degrees C. The latter conclusion has been confirmed by some experimeniq made by Professor Coker with a preliminary account of which he has favoured the Committee. Measuring the cyclical variation of temperature of the inner surface of a 12h.p. gas engine cylinder by methods similar to those adopted by Professors Callender and Nicholson in their well-known work on the steam engine, he found that the maximum was only 7 degrees F. in excess of the mean, The direct measurements by Professor Hopkinson of the temeerature of the walls of an explosion vessel lined with copper strip also lead to the conclusion that it is quite moderate. This cold metal must obviously profoundly affect the combustion in its neighbourhood. In R /aver of gas of appreciable thickness the combustion will be of a smoulderincf character, depend ing upon the velocity with which the unburnt gas in contact with the walls can diffuse into the hotter regions at a distance from them, and so be brought to the ignition temperature. This layer being cold and highly compressed might account for a considerable fraction of the heat, though its actual thickness may be only a few tenths of a millimetre. it would appear prob, able that the continued burning which undoubtedly goes on after the time of, maximum pressure in many explosions, and probably also occurs during the first portion at least of the expansion stroke of a gas engine, -is mainly of this character."

Results of Observations.

" The temperatures reached in these explosion experiments range from about 1,300 degrees up to 3,000 degrees C. •ferrsperatures of below 1,500 degrees are, however, obtained by the use of weak mixtures, involving slow burning and large cooling corrections, and but little reliance can be placed on the results. Langen made very few observations on mixtures giving a lower temperature than 1,500 degrees, and takes that as the lower limit of the range of temperature to which his observations apply. The extreme upper limit of the constant pressure experiments is 1,400 degrees. The temperature of 3,000 degrees C. is about that reached in the explosion of hydrogen and oxygen in their combining proportions. This is much above the mean temperature ordinarily reached in the gas engine, the upper Inuit of which may be put at about 2,000 degrees C.; though it is probable that 2,500 degrees or more is occasionally reached

Langen, however, places the upper limit of the application of his formula at 1,700 degrees C., on the ground that there is dissociation of the CO5 at higher temperatures than that. There does not seem to bemuch reason for this limitation, for the effects of dissociation (provided that equilibrium is attained) are indistinguishable from those of increasing specific heat, and should be included in the change of energy. Dissociation may give rise to errors in the temperature measurement, but there is reasons to suppose that the dissociation which occurs in the CO-, and steam at a temperature of 2,000 degrees C. is too small to cause any material change of volume, though it may mean considerable absorption of heat. . .

"The Committee are of opinion that values of the energy obtained from explosion records are not subject to any very great errors on account of heat-loss by conduction to the walls of the vessel, nor on account of incomplete combustion, but that they are affected by errors of quite unknown amount due, first, to heat radiated, and, secondly, to the want of thermal equilibrium at the time when the pressure is measured. For the purpose of testing the first of these conclusions, it is very desirable that further experiments should be made on explosions in vessels of greatly different size but of similar form. The opinion entertained by the Committee that incomplete combustion is a surface-phenomenon, on which this conclusion as to the validity of the method is based, also requires further confirmation. As regards the second conclusion, further experiment on the actual amount of heat radiated by burning gas is urgently required, and also experiments to confirm or negative the effect of the nature of the wall surface upon the pressure reached in an explosion. The effect of want of thermal equilibrium can be determined up to a point by calculation ; but before such calculation can be usefully made, it is desirable that further information should be obtained as to the temperature distribution after an explosion, especially in the neighbourhood of the walls. It should not be difficult to get an idea of this sufficiently accurate for the purpose by means of platinum thermometers.

"The most hopeful way, however, of making use of explosions to give definite information as so the properties of gases would appear to he to directly measure the heat lost in the explosion, as if this be done it is possible to defer the pressure measurement until such time as equilibrium conditions, except those that depend on the surface of the vessel, have beets attained."

The report then went on to deal with. the measurement of temperature and the various methods adopted by different experimenters, and wound up with an eight-page appendix by Professor H. L. Callender, M.A., LT-.D., F.R.S., on "The deviation of actual gases front the ideal state, and on experimental errors in the determination of their specific heats.'' Lack of space, however, prevents us from making further mention of these. The Committee was re-appointed, and given a grant for additiitnal research,

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