Bulb design criteria

Default design for Bulb turbines is related to the specific speed

Nq = n*Q^0.5/H^0.75.
Q is nominal flow and H is nominal head.

Nq range is from 200 - 300.

Default values.
          Runner diameter.
                   Axial velocity for design point flow in ring area, m/s.
                   cm0      = 0.85 * sqrt( 2.0 * g * H ) 

           Runner blade.
                   hubD:   Hub diameter relative to D.
                   hubH:   Hub height relative to hubD.

             H <= 6.0 m.
                   numberOfBlades         = 3
                   cordlength                   = 0.18
                   hubDr                          = 0.5
                   hubHr                          = 0.55

              6.0  m. <  H < = 10.0 m.
                    numberOfBlades         = 4
                    cordlength                   = 0.2
                    hubDr                          = 0.5
                    hubHr                          = 0.6

             10.0  m. <  H < = 15.0 m.
                    numberOfBlades         = 5
                    cordlength                   = 0.18
                    hubDr                          = 0.5
                    hubHr                          = 0.5

              15.0  m. <  H < = 20.0 m.
                    numberOfBlades         = 4
                    cordlength                   = 0.16
                    hubDr                          = 0.55
                    hubHr                          = 0.45

             H  > 20.0 m.
                    numberOfBlades         = 5
                    cordlength                   = 0.16
                    hubDr                          = 0.50
                    hubHr                          = 0.45 

Comments.
Design follows the Alab principles which mean that a design point must be defined.
For Francis type turbines this design point is determined automatically in an iteration cycle aiming at a best point defined in input.
The difference between design point and best point is small for Francis type turbines but big for Kaplan type units. The big difference results in a diverging iteration cycle.
The Bulb turbine design point is therefore set as input and the Alab turbine designer will create the actual geometry used in all following hydraulic calculations.
If the result is unacceptable, add new design point in a manual iteration cycle.

The axial velocity head is high for bulb units and an efficient recovery of this head is important to get good turbine efficiency.
But as draft tube outlet velocity normally is not included in turbine efficiency it may look unrealistic high.

Bulb turbine properties should therefore be presented taking into consideration draft tube outlet loss.

Hint.
Due to the above statement nominal output should be increased by 10 % to meet the actual output.
According to Bulb design principles runner speed vil not influence main dimensions, but blade angles will change.
Increasing speed will increase runner blade friction losses and the efficiency will drop. Low speed will of course give the opposite result.
Low speed will result in a more expensive generator.

Design parameters and dimensions may be changed according to normal Alab functionality.