bimr,
That is simply wrong. Mass flow rate is NOT choked. Velocity is Choked. Think of depressurizing a pipeline full of methane and pressurized to 1000 psig at 60F through a 2-inch pipe at sea level.
[ul]
[li]Sonic velocity is 1380 ft/sec[/li]
[li]Flow area of pipe is 0.022 ft^2[/li]
[li]Therefore volume flow rate at actual conditions as long as the flow is choked is 2600 MACF/day[/li]
[li]Initial density is 3.732 lbm/ft^3, so mass flow rate is 111.2 lbm/s[/li]
[li]When the line has blown down to 500 psig, density is down to 1.748 lbm/ft^3 which cuts mass flow rate to 52.4 lbm/s.[/li]
[li]At the end of choked flow (about 27 psia), density is now 0.131 lbm/s, velocity is still sonic so mass flow rate is 3.9 lbm/s[/li]
[/ul]
Not a constant "weight rate of flow [kg/s]" at all. It is a constant volume flow rate at actual conditions, but that isn't a terribly useful number.
venividivici,
In gases, BigInch answered your question--to put it in anthropomorphic terms, the flow stream doesn't have the ability to tell the source that downstream pressure dropped further. This "communication" happens at the speed of sound so the flow of the gas is limited to Mach 1.0. In fluid mechanics terms, the flow creates standing shock waves in the flow that have a very large dynamic pressure discontinuity.
You were careful to use the word "fluid" instead of "gas" or "liquid". I'm not sure why. This phenomena is only seen in gases. In liquids the momentum effects disturb the shock waves and a standing wave is not possible (think of pictures of bullets shot into a pool of water to visualize the process). Very high velocity is liquid flow is possible, but the analysis of these flows is way too complex for a sound byte.
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist