Minimum Pressure Ratio Required For Choked Flow To Occur
The minimum pressure ratios required for choked conditions to occur (when some typical industrial gases are flowing) are presented in Table 1. The ratios were obtained using the criteria that choked flow occurs when the ratio of the absolute upstream pressure to the absolute downstream pressure is equal to or greater than k/(k − 1), where k is the specific heat ratio of the gas. The minimum pressure ratio may be understood as the ratio between the upstream pressure and the pressure at the nozzle throat when the gas is traveling at Mach 1; if the upstream pressure is too low compared to the downstream pressure, sonic flow cannot occur at the throat.
| Gas | k = cp/cv | Minimum Pu/Pd required for choked flow |
|---|---|---|
| Dry Air | 1.400 | 1.893 |
| Helium | 1.660 | 2.049 |
| Hydrogen | 1.410 | 1.899 |
| Methane | 1.307 | 1.837 |
| Propane | 1.131 | 1.729 |
| Butane | 1.096 | 1.708 |
| Ammonia | 1.310 | 1.838 |
| Chlorine | 1.355 | 1.866 |
| Sulfur dioxide | 1.290 | 1.826 |
| Carbon monoxide | 1.404 | 1.895 |
Notes:
- Pu = absolute upstream gas pressure
- Pd = absolute downstream gas pressure
- k values obtained from:
- Perry, Robert H. and Green, Don W. (1984). Perry's Chemical Engineers' Handbook, Table 2-166, (6th Edition ed.). McGraw-Hill Company. ISBN 0-07-049479-7.
- Phillips Petroleum Company (1962). Reference Data For Hydrocarbons And Petro-Sulfur Compounds (Second Printing ed.). Phillips Petroleum Company.
Inspection of these values leads to the inference that minimum pressure ratio is the following linear function of specific heat ratio: :Pratio = 0.6057 × k + 1.045.
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