Axial Impulse Turbines
Barber-Nichols Inc. ( BNI ) is a premier designer and manufacturer of high specific energy axial flow impulse turbines. First, BNI has extensive experience with both partial and full admission impulse turbine designs that cover a wide Specific Speed (NS ) range. 
Next, Barber-Nichols designs and manufactures axial flow impulse turbines that cover temperatures ranging from cryogenic to 1,370° C (2,500° F). Finally, BNI employs unique Electrochemical Machining techniques to cost effectively manufacture highly efficient turbine wheels with extremely tight blade spacing.
The term axial flow indicates that the flow path through the turbine is basically parallel to the shaft with little change in radius. The term impulse is used by turbine designers to indicate that the head drop across the turbine occurs mainly in the nozzle and not the rotor. When the head drop occurs in the nozzle, essentially all of the energy available to the turbine is converted to velocity. This velocity is then directed toward the rotor where it is deflected or turned, imparting a force on the rotor blades. Impulse turbines have an advantage compared to reaction turbines (where some of the energy is expanded in the rotor) in that they reach their maximum efficiency at a lower rotor tangential velocity. This can be important for very high energy across a turbine stage as the tangential (tip speed) velocity may be limited for structural considerations.
Partial Admission turbines are used in the low specific speed regime in order to maintain reasonable blade heights. Specific Speed, a dimensionless parameter, can be used to determine weather the turbine should be partial or full admission. If the NS is between 3 and 30 a partial admission turbine will perform the best. The full admission turbine can be used from NSvalues of 30 to 300. The most commonly used definition of N S(using US customary units) is:
N S = RPM (Q) 1/2 / H 3/4
( Where Q = ft 3 /sec & H = Feet of Head Across the Turbine )
A partial admission turbine means that the turbine nozzles only fill part of the circumference or arc around the turbine rotor. This arc of admission can be as low as one nozzle covering approximately 2% of the arc to as high as 100% which of course is full admission. The reason to use partial admission is that the rotor blade height becomes very small when the flow rates are low. Very short blades have losses due to leakage and a poor aerodynamic performance. There are some specific losses associated with partial admission but improved blade geometry outweighs these losses.
Axial impulse turbines are ideally suited for applications where you need to keep the speed of the machine and the tangential velocity of the rotor low. In other words, they are used in a lower specific speed range. For power generation systems it is advantageous to operate at 3,600 rpm in order to drive a synchronous generator. In aerospace or high density power systems where the driving fluid is very energetic resulting in very high enthalpy drop across the turbine, the impulse turbine can operate efficiently at a lower tangential velocity than reaction turbines. Also, the impulse turbine can operate partial admission without significant losses as compared to reaction turbines.
Axial flow impulse turbines can be designed to operate efficiently at both sub and supersonic velocities; additionally, they can accommodate extremely high stage pressure ratios. BNI has designed and produced high energy power systems using high pressure steam at pressure ratios as high as ninety to one. The impulse design lends itself to these high expansion ratios because essentially all expansion occurs in the nozzles. The rotor blades do not have to accept large changes in volume as the flow transits the rotor.
The above displayed Specific Speed / Specific Diameter Turbine Chart was developed by O.E. Balje. His book entitled "Turbomachines: A Guide to Design, Selection, and Theory" is an excellent reference for anybody working in this field. Additionally, please Contact Barber-Nichols, Inc. to speak with an engineer about your project.