It's to do with the direction of flow through the impeller.
If the fluid goes through the impeller in the same direction as flow (i.e. like a propeller), it's called axial flow. If it goes through the impeller at right angles to the flow direction it's called radial flow, if it's somewhere between these two it's called mixed flow. ESP impellers are either radial or mixed flow.
Radial flow impellers are less good at dealing with gas in the fluid, as they tend to gas lock, but you get a high increase in pressure per impeller. Axial impellers are excellent at dealing with gas (think of aircraft propellers) but they have a very low delta P per stage.
To get the required pump pressure increase with axial flow impellers we can do two things: either use thousands of axial impellers running at typical ESP speeds, resulting in a axial flow ESP hundreds of feet long, (which I certainly wouldn't like to try running in hole!)
OR
A similar number of impellers to an normal ESP, but running at much higher speeds than a normal ESP. ESPs are usually limited to about 3600rpm as the electrical power supply is ususally 60Hz. So use a power fluid to spin a turbine at higher speeds and drive the pump with the turbine. (I guess you could try to get electrical power at a higher frequency than 60Hz, but that would be hard and expensive to do). i.e. the Wier HSPs running on Captain run at 5000-7500rpm and go up to 10,000rpm for gassy slugs.
Or, for gassy crudes, use downhole gas separators and normal ESPs, which is what Schlumberger or Centrilift will try to sell you.....
