1 86 



NATURE 



[December 8, 1910 



try," on " Fruit Culture in Japan," and on "The Art of 

 l-'lower Arrangement," as well as others of special interest 

 to all who study things Japanese. Altogether, the 

 magazine makes very good reading, and if it maintains 

 the standard of the issue which we have been considering 

 it will take a high place among publications on the Far 

 East. H. D. 



RADIATION FROM HEATED GASES.' 

 On the Radiation from Gases. 



T N the first and second reports of the committee reference 

 was made to the part played by radiation in the cool- 

 ing of the products of an explosion, and to its bearing on 

 the measurements of volumetric and specific heat with 

 which those reports were principally concerned. The 

 general question of radiation from heated gases has, how- 

 ever, from the point of view of the committee, an interest 

 and importance of its own which are sufficient to justify 

 a detailed study of it in its wider aspects. Radiation 

 plays a part comparable with that of conduction in deter- 

 mining the heat-flow from the gas to the cylinder walls in 

 the gas engine, and it is this flow of heat which is the 

 most important peculiarity of the gas engine, and to which 

 are chiefly due the leading characteristics of its design. 

 Even to the uninstructed eye the most obvious features 

 about large internal-combustion engines are the arrange- 

 ments for cooling, and the great size and weight for a 

 given power which is necessitated mainly by those arrange- 

 ments. The difficulties which the designer has to meet are 

 due in the main to the stresses set up by the temperature 

 gradients which are necessary to sustain the flow of heat. 

 In the present state of the art it is probable that the most 

 Important service which science could render to the gas- 

 engine constructor would be to establish definitely the 

 principles upon which depends the heat-flow from hot gases 

 into cold metal with which they are in contact, and thus 

 to enable him to predict the effect upon heat-flow of 

 changes in the temperature, densitv, or composition of the 

 charge, and in the state of the cylinder walls. 



The committee does not propose in this report to deal 

 with the whole of this large question, but will confine its 

 attention to one important factor in heat-flow, namely, 

 radiation. The subject is a wide one. which has excited 

 much attention among physicists and chemists, and on 

 several important points agreement has not yet been 

 reached. No attempt will therefore be made to do more 

 than state shortly the experimental facts, and to define the 

 issues which have been raised in regard to the explanation 

 of these facts. 



Practical Effects of Radiation. 



It is believed that the first instance in which radiation 

 from a flame was used in an industrial process, with 

 knowledge of its importance, was the regenerative glass 

 furnace of Frederick Siemens, which he described at the 

 Iron and Steel Institute in 1884. Here the combustible 

 gas was burnt in a separate chamber, and the hot pro- 

 ducts of combustion were led into the furnace. The 

 objects to be heated were placed on the floor of the furnace 

 out of contact with the stream of flame which flowed 

 above them. They would therefore receive heat only by 

 radiation, and it was supposed that this r.-adiation came 

 in a large measure from the flame. Siemens, however, 

 was of opinion (in 1884) that the radiation was due to 

 incandescent particles of carbon, and that there was little 

 radiation from a non-luminous flame. ^ 



In 1890 Robert von Helmholtz measured the radiation 

 from a non-luminous coal-gas flame 6 mm. diameter, and 

 found it to be about 5 per cent, of the heat of combustion.' 

 The radiation from a luminous flame was greater, but not 



1 From the Third Remrt of the British Assf^ciation Committee, cons-sting 

 of Sir W. H. Preece (Cha-Vman), Mr. Dugald Cleric and Prof. Bertram 

 "onkmson (Joint Secretaries), Profs. Bone, B'lrstall. Callendar, Coker. 

 Dalby, and Dixon, D . Gla^ehrook. Profs. Petavel, Smithells, and Watson, 

 T)T. HarkT, Lient.-Col. Holden, Capt. Sankev and Mr. D. L. Chapman, 

 appointed for the Investigation of Gaseous Kxplosions, with special reference 

 to Temperature. Presented at the Sheffield meeting of the Association, 

 19T0 



2 Capt. Sankey has prepared an abstract of papers relating to the Siemens 

 furnace. 



3 "XJie Licht- und Warmestrahlung verbrennender Case," Robert von 

 Helmholt7. (Berlin, 1890.) 



NO. 2145, VOL. 85] 



very much greater — rising to a maximum of iij per cent. 

 for an ethylene flame. Discussing the Siemens furnace in 

 the light of these results, R. von Helmholtz calculated thai 

 radiation from the flame in the furnace could only account 

 for a small fraction of the actual heat transmission. He 

 pointed out, however, that a large flame would probablv 

 radiate energy at a greater rate than a small one. Ikit 

 while admitting that for this reason gaseous radiation 

 might play a part in the heat transmission, he suggested 

 that a more important agent was radiation from the rcKif 

 of the furnace, which received heat by direct contact with 

 the hot gas, and so reached a very high temperature. Hi- 

 showed by calculation that a comparatively small excess 

 of temperature in the roof over that of the floor would 

 cause a sufficient flow of heat. 



But though the discussions on the Siemens furnace and 

 the work of Helmholtz show that the idea that a flame, 

 even if non-luminous, might radiate large amounts of 

 heat, was a familiar one to many people twenty years ago, 

 its possible importance in causing loss of heat during and 

 after a gaseous explosion, and in determining the heat- 

 flow in a gas engine, does not appear to have been 

 appreciated until quite recently. Prof. Callendar was 

 probably the first to direct attention to its significance in 

 this connection. In the discussion on a paper about 

 explosions, read before the Royal Society in 1906, he said 

 that he had found a non-luminous Bunsen flame to radiate- 

 15 to 20 per cent, of its heat of combustion, and expressed 

 the opinion that the loss from this cause in a closed-vessel 

 explosion would be of the same order.' 



There are, in fact, several points about the behaviour 

 of gas engines which suggest the importance of radiation^ 

 as a cooling agent. The particular matter which attracted 

 Callendar 's attention was the effect of speed on thermal 

 efficiency. His experiments showed that a part of the loss" 

 of efficiency in an internal-combustion motor, as compared' 

 with the corresp>onding air-cycle, was independent of the 

 speed at which the engine was run. The loss of heat per 

 cycle could, to a first approximation, be represented by 

 an expression of the type A-|-B/n, where n is the number 

 of revolutions per minute and A and B are constants. 

 The term A represents a constant loss of heat per ex- 

 plosion, and among the many causes contributing to this' 

 constant loss of heat, radiation from the flame is probably 

 important.^ 



Another phenomenon which is difficult to explain, except; 

 as the result of radiation, is the effect of strength of^ 

 mixture on heat-loss. The following table shows some' 

 results which were obtained by Hopkinson upon a 40I 

 horse-power gas engine ' : — ' 



Percentage of gas in cylinder contents 8"^ ifo percent. 



Total h^at-loss per minute 1510 230oB.Th.U. 



Total heat-loss as percentage of total 



heat-eupply 29 34 ner cent. 



Temperature of piston 300° C. 430° C. 



It will be observed that the proportion of heat-loss 

 the walls increases very materially as the strength 

 mixture is increased. If the transfer of heat were wholt 

 due to conduction it might be expected, apart from til 

 disturbing influence of speed of ignition, which in thi 

 case was not very important, that the percentage of heat 

 loss would rather diminish with increase of charge, becaus 

 the temperature with the stronger mixture should 

 relatively less on account of the increase of volumetrir 

 heat. The increased temperature of piston and valve- 

 would work in the same direction. The existence ol 

 radiation, however, which increases more rapidly in pro- 

 portion to the temperature, would account for the increasnl 

 heat-flow. The practical importance of questions of thi- 

 kind is illustrated by these figures, from which it appear- 

 that the piston is 50 per cent, hotter, though the charge of 

 gas is onlv increased 30 per cent. 



More direct evidence of the importance of radiation is 

 furnished by experiments on the effect of the surface of 

 the walls. In the second report of the committee refer- 

 ence was made to the belief, which is widely spread amon-i 

 those who are concerned with the practical design and 

 operation of gas engines that pKjlishing the interior of th'^ 



1 Hopkinson, Proc Rov. Soc, A. vol. Ixxvii., p. 400. 



2 I'roc. Inst. Automobile Eng. , Tune, 1907. 



3 Proc. Inst. C.E., v.,1. clxxvii. (1909). 



