Imagine a pipe whose diameter changes (pipe reduction or pipe expansion)
Let's define two different places in that pipe:
point 1: with a pressure P1, velocity V1 and diameter D1;
and point 2: with a pressure of P2, velocity V2 and and D2.
Knowing that the fluid flows from point 1 to point 2, the pipe is horizontal (no level change) and D2 > D1: applying a material balance we know that V2 < V1 then applying an energy balance (i.e. Benouilli's equation) we know that P2 > P1 (pressure drop in the pipe has been considered very low and irrelevant). Therefore the fluid is flowing from a low pressure place (point 1) to a high pressure place (point 2) !!! How can that be ?
Let's define two different places in that pipe:
point 1: with a pressure P1, velocity V1 and diameter D1;
and point 2: with a pressure of P2, velocity V2 and and D2.
Knowing that the fluid flows from point 1 to point 2, the pipe is horizontal (no level change) and D2 > D1: applying a material balance we know that V2 < V1 then applying an energy balance (i.e. Benouilli's equation) we know that P2 > P1 (pressure drop in the pipe has been considered very low and irrelevant). Therefore the fluid is flowing from a low pressure place (point 1) to a high pressure place (point 2) !!! How can that be ?