There is probably more known about centrifugal pump design than any other
mechanical device made, likewise thermodynamics and heat transfer. That
knowledge is mostly ignored by modern users. the most efficient pump
impeller design is an S curve, with the hooks of the S pointing away from
the direction of rotation. water accelerates as it travels out the
impeller--it doesn't get flung or "scooped". For a given rotation speed,
rotor diameter and case design one impeller works the same as twelve, except
that more impellers provides a balanced thrust against the bearings and
seals, increasing pump life. If your pump is over capacity, the best way to
reduce the flow and decrease the power requirement is to trim the
impellers--a common practice in big pumps. Smooth impellers are good because
they cavitate less. cavitation doesn't just beat up the balde, it also is
hard on bearings and seals.
slower flow does not increase heat transfer. Heat transfer is a function of
the mass flow rate and the differential temperature. Orifices in a car
cooling system improve cooling because they increase the pressure of the
system upstream of them which decreases the bubble size of any boiling in
the system. When steam has an opportunity to create a film on a heat
transfer surface (like a cylinder sleeve) the heat transfer goes all
buggywhompers (a technical term). Small bubbles are good--they increase
surface turbulence and disturb laminar films (stagnant coolant) on the heat
transfer surface. This is called nucleate boiling, and it's one reason why
our car cooling systems don't run away as the car gets hotter (the other is
higher heat transfer rate as the delta-T gets higher)--if the cooling
efficiency didn't increase they would do so as the temperature rises. Bulk
boiling is bad--that's why when a car overheats it all goes bad fast. The
heat transfer goes away.
Nuclear power plants are basically heat transfer devices and pumps. As a
former Nuke I have more useless information about this stuff than I could
ever use.
|