Static vs Dynamic pressure

Question

According to Bernoulli's law, the narrower the tube where the fluid is flowing the lower its static pressure and the higher its dynamic pressure. Does this make any difference? If the dynamic pressure increases when the static decreases, then nothing changes at all. So what happens then?

If the pressure decreases in narrower tubes, why is the refrigerator's evaporator thicker than the condenser if we want it to be at lower pressure to absorb heat? If it increases in narrower tubes, how does the air in carburetors suck fuel if it was lower in pressure than the air?

Answer 1

This is really three separate questions.

1. Static and dynamic pressure: It is the static pressure that really matters in practical situations. The dynamic pressure is related to the kinetic energy of the fluid which, when it changes, causes a corresponding change in the static pressure.

2. Condenser/evaporator application: The basic Bernoulli equation applies to situations in which viscous pressure decreases are negligible. In the compressor/evaporator application, there is a valve between the compressor and evaporator that features a very large viscous pressure drop. This pressure change is much larger than the static pressure change associated with the decrease in kinetic energy accompanying the increased diameter. So the viscous pressure decrease dominates. The larger diameter is necessary to accommodate the much higher coolant vapor volume after the valve.

3. Carburator application: The narrower tube is accompanied by a decrease in air pressure in the throat of the carburetor. This lower pressure of the air provides the driving force for sucking in fuel.

Answer 2

1. Even if you call the term dynamic pressure, and has units of pressure, it is not a pressure at all. It is a necessary contribution to the total static pressure, though, And it is the static pressure that matters when you move a piston in a gas. The static pressure is also related thermodynamically to the temperature, in the first case as the average speed of the molecules, and the second as the average force, or average transfer of momentum, per unit area. That is, a molecule colliding with an expanding cylinder makes positive work and then decrease its kinetic energy. The same if you think about it in terms of static pressure and change in volume.

2. & 3. Additionally, in the two examples, what you consider an actual pressure, with all the properties you intuit about pressure, is the static pressure.