Question

Why is it that when a steadily flowing gas flows from a larger diameter pipe to a smaller diameter pipe, what causes the pressure in the smaller pipe to drop?

Answer 1

@TheSmartOne maybe?

Answer 2

@sleepyjess @Michele_Laino @e.mccormick

Answer 3

they key to getting this, i always think, is to be able to see the distinction between static and other types of pressure in a gas or fluid. the link down below does a great job of bringing that out.

first of all the static pressure has to drop in the narrower pipe. there can be no alternative. because the gas moves quicker through the smaller area (conservation of mass), even allowing for compressibility, a force has to have acted, and that force is caused by the pressure drop. same when it slows down on leaving a narrower pipe.

that i believe is the original historic observation , and the basis of the most common derivations of related equations.

looking at it in terms of energy, the gas's energy will be a mix between internal (general molecular vibration ==> static pressure) energy and broader kinetic energy of its flow (which creates dynamic pressure). this adjusts as a flow enters a narrower pipe. thanks to energy conservation, more kinetic energy with faster flow = less internal vibration (still kinetic in a gas but applied on a molecule by molecule basis) energy. the static pressure drops.

see here for some info on how the pressures are measured:
http://www.tsi.com/uploadedFiles/_Site_Root/Products/Literature/Manuals/flow_and_pressure.pdf