﻿Orifices, and Entrainment of Air by the Issuing Jet. 985 



orifices must be so disposed that the air-entraining power 

 of each individual jet is not reduced by mutual interference 

 of the jets. 



It will be clear from the figure that there is a limit of size 

 below which the orifice cannot be reduced without the intro- 

 duction of disturbing factors adversely affecting the greater 

 degree of air-entrainment normally accompanying reduction 

 of the orifice. This is first seen in the region of F in curve 9, 

 where the degree of air-entrainment is somewhat less than 

 its anticipated value, as shown by the broken line. From 

 curves 9 and 10 it is seen that over the range of pressures 

 up to about 16 cm. of water, the degree of air-entrainment 

 effected by the jet issuing from the small orifice (no. 10) is 

 very much less than that effected by the jet issuing from the 

 larger orifice (no. 9). 



A similar phenomenon to that described has been observed 

 in the case of coal-gas issuing from a fine orifice. In such 

 a case a flame burning at an orifice under conditions corre- 

 sponding to those shown at F in fig. 7 would be very sensitive 

 to slight changes of pressure, the degree of air-entrainment 

 varying considerably with a slight increase of pressure. The 

 phenomenon probably explains, at least in part, the existence 

 of low-pressure sensitive flames, to which attention has been 

 recently directed, and which were studied in considerable 

 detail by Chichester Bell *. 



With a view to ascertaining the cause of the apparently 

 anomalous bel aviour of jets issuing from orifices 9 and 10, 

 similar experiments on air-entrainment were performed with 

 air jets issuing from the comparatively much longer channels 

 nos. 1 A-5 A, 5 B, and 5 C, particulars of which have been 

 given in fig. 5 and Table III. The curves corresponding to 

 those in fig. 6, in which the total volumes of air-entrainment 

 were plotted as ordinate against the pressures as abscissa 

 showed well-marked points of inflexion in the cases of 

 channels 3 A, 4 A, 5 A, 5 B, and 5 C, these points corresponding 

 to pressures respectively equal to 2'5, 5*0, 7*5, ll'O, and 

 13 cm. of water. 



• The curves for channels nos. 1 A-5 A showing the degree 

 of air-entrainment per unit volume of air in the jet are 

 shown in fig. 8. The curves generally resemble those given 

 in fig. 7. One distinction is of importance. A comparison 

 of curves 3 A and 4 A with curves 9 and 10 in fig. 7 indicates 



* See, e. g., ' Nature/ vol. cviii. p. 532 (1921). Phil. Trans. Part 2, 

 pp. 383-422 (1886). 



Phil. Mag. S. 6. Vol. 44. No. 263. Nov. 1922. 3 S 



