From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Page discussing the principles of gas flow through an orifice and limiting pressure ratios.
| Identifier | ExFiles\Box 36\1\ scan 020 | |
| Date | 1st February 1919 guessed | |
| -8- Contd. Starting with P2 = P1 and gradually reducing P2, the fraction D is gradually reduced, and the flow is gradually increased up to a certain limit. Beyond this point the flow would appear from the first equation to diminish because the expression D^(2/n) - D^((n+1)/n) diminishes again on further reducing D.{John DeLooze - Company Secretary} Actually however the flow is not diminished, but remains constant on further reducing P2 below the limit and is now given by the second equation. The value of this limiting pressure ratio D or P2/P1 is ( 2 / (n+1) )^(n / (n-1)). In the case of air expanding adiabatically through an orifice n = 1.405 and the value of the limiting pressure ratio is .527. This means that so long as P2 is greater than .527 P1, the flow is increased on diminishing P2, and is according to the first equation, but so soon as P2 is further diminished beyond .527 P1, the flow remains constant, according to the second equation. The reason of this anomaly lies in the expansive property of a gas and the velocity produced in the stream due to the work of expansion. At any stage in the expansion corresponding to a pressure P, both the velocity and the specific volume of the gas are determinate, the former by the amount of work done by the expanding gas and the latter by the law of expansion PV^n = const. Hence the area of cross section of the ideal stream at every stage is determinate. This area, calculated thus, undergoes first a diminution and | ||