Physics Asked on July 4, 2021
I know that the throttling device reduces pressure of the gas but that’s it. I can’t somehow imagine how the change in cross-sectional area can affect how the particles of gas to lose energy and see a decrease in pressure?! Can someone perhaps visually, using a diagram, even if it’s just doodles, explain how the individual particles are affected? I searched all over and can’t get a satisfying explanation.
A throttling device is basically a flow restrictor which, when flow is asserted through it, exhibits a pressure drop the same way pushing current through an electrical resistor develops a voltage drop across it.
Take the example of a throttling valve on a compressed air tank that is exhausted to ambient pressure.
The compressed gas at high pressure gets forced through a small orifice by that pressure and is exhausted at (lower) ambient pressure. As it leaves the orifice, it is free to expand into that ambient pressure and as it does, its temperature and pressure falls in response to the expansion.
Note that a throttling nozzle is mechanically inefficient in that it "throws away" the pressure difference between the high-pressure source and the low pressure ambient.
Correct answer by niels nielsen on July 4, 2021
The pressure decreases as a result of viscous friction, starting with the no-slip (zero velocity) boundary condition at the wall(s) of the throttling device and propagating into the fluid. This is a dissipative process which results in viscous heating of the fluid, to offset the pressure decrease as a result of expansion cooling. For an ideal gas, it results in no change in temperature for the gas.
Answered by Chet Miller on July 4, 2021
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