382 ME. J. E. PETAVEL ON THE PEESSURE OF EXPLOSIONS. 



The quantity of heat which is transmitted to the walls of the enclosure during the 

 brief period occupied by the cooling of the gas is much greater than would occur in 

 cases met with in ordinary engineering practice. With a gravimetric density of O'l 

 the amount of heat to be absorbed per unit surface of our cylindrical enclosure is some 

 hundred times as large as that which would be absorbed by the cylinder of an ordinary 



gas engine. 



In the case of artillery of large calibre the inner surface of the steel probably 

 attains a temperature close to its melting-point and the correspondingly plastic 

 material yields easily under the combined friction and chemical action of any escaping 

 gas. In the case of small arms, the temperature being limited by the relatively small 

 volume and therefore small thermal capacity of the gaseous mass, practically no 

 erosion takes place. 



To return now to the experimental work. In the following table the time required 

 for the pressure to fall to three quarters, one half, one quarter of its maximum value 

 is given for a number of distinct experiments, whereas the cooling curves for three 

 different diameters of cordite at gravimetric densities of O'l and O'l 5 will be found 

 plotted in fig. 16. It is noticeable that after the first tenth of a second the curves 

 taken under similar conditions, but for various sizes of explosive, lie closely together, 

 showm' r that the diameter has no material effect on the subsequent rate of cooling. 



When we refer, however, to the table, we see that the times required to reach a 

 given fraction of the maximum are different for different diameters. 



This apparent discrepancy is explained by the fact that the total quantity of heat 

 absorbed is primordially a function of time. When the combustion is very rapid, the 

 maximum pressure is reached while the walls of the enclosure are still cold and the 

 percentage fall of pressure per unit time is high. With a slow-burning cordite the 

 surface of the enclosure becomes considerably heated during the combustion of the 

 explosive, and after the maximum the percentage fall of pressure is correspondingly 

 lower. Briefly stated, at any fixed interval of time after ignition the total heat 

 absorbed by the enclosure, and, therefore, the temperature of its inner surface, will be 

 nearly the same for all diameters of the explosive. In consequence, the rate of cooling 

 as measured by the rate of change of pressure at any stated time is unaffected by 

 the speed of combustion. 



The rate of cooling for a given volume of the enclosure does not vary, as is usually 

 assumed, in proportion to the surface, but nearly as the square of the surface. 



It will be noticed that the cooling in the cylinder is about four times as rapid as in 

 the sphere, whereas the ratio of the two surfaces is as 2 '17 to 1. 



In such massive enclosures the heat generated by the explosion is at first entirely 

 absorbed by the inner layers of the steel walls. It does not travel to the outside 

 until some time after the explosion is over, A decrease in the surface has, therefore, 

 a double effect. The heat to be absorbed per unit area and the average thickness of 

 metal through which this heat must be transmitted are both increased. 





