The Number of Blades for Crossflow Turbine

By Ahmad Suhendra

Example :

A crossflow turbine is designed with blade angle β1=30 0 (angle between the relative velocity and tangential velocity) and α1=16 0 (angle between the absolute velocity and tangential velocity). The turbine have a rotation speed of 750 rpm on the net head 10 m, flow 100 lt / s, what is the specific speed, absolute and tangential velocity , outside diameter of the disc, blades spacing and number of baldes?

Crossflow turbine requirements are :

• ns = 42 – 170 (classical based on metric HP)
• Ns = 0.86* ns (True metric specific speed)
  
• Nt = 100 -1000 rpm (nominal speed)

1.Specific Speed

ns (rpm) = 3.65 x Nt (rpm) x Q(m^3/s)^ 0.5 / H_netto(m)^ 0.75

= 3.65 * 750 * 0.01^ 0.5 / 10^ 0.75

= 48.7 rpm ----> Ok, Crossflow

2.Absolute Velocity of water

kc = 0.967 Coefficient dependent upon the nozzle

C = kc * (2 * g * Head)^ 0.5

= 0.967 (2 * 9.8 * 10 )^0.5

= 13.54 m/s

3.Tangential velocity

U1 = ( C * Cos α1) / 2

= ( 13.54 * Cos 16 )/ 2

= 6.51 m/s

4. Outside diameter of disc

D1 = 60 * U1 / (phi * Nt)

= 60 * 6.51 / 3.14 * 750

= 0.166 m

Then :

S1 = k * D1----> k = 0.075 – 0.1 (respectively)

Taken k = 0.087

S1 = 0.087 * D1

= 0.087 * 0.166

= 0.0144 m

5. Spacing of blades in disc

t = S1 / Sin β1

= 0.0144 / Sin 30

= 0.0288 m

6.Number of Blades

Z = phi * D1 / t

= 3.14 * 0.166 / 0.0288

= 18 blades (20 were used for design)

Ref : Mockmore C.A and Merryfield Fred " The Banki Water Turbine " , Bulletin Series No:25 Oregon State College Corvallis