You can get better airflow inside with a crossflow design:

The following data is for construction of a 12" diameter wheel. The dimensions for wheel
diameter and blade curvature can be changed proportionately for other size wheels, though the
angles will remain the same.

The end plates are 1 foot diameter discs, cut from 1/4 inch sheet steel, with keyed hubs welded
in to fit a suitable sized shaft (this is dependant on power requirements). The blades are cut
from standard 4 inch steel water pipe (wall thickness is 0.237 inch). Each blade is cut for a 72
degree arc; this can be measured at 1/5 the circumference of the pipe (for a 4" pipe the distance
along the arc would be 2.83 inches, or 0.236 feet). Each length of pipe suitable for blade pieces
will make only 4 blades (each piece 1/5 the pipe's circumference); since there is some loss of
material with each cut, there would not be enough left over after 4 blades and 5 cuts to allow a
5th blade.

The length of each blade must be 3/4 inch longer than the inside width of the wheel (W2) to
allow enough to stick out beyond each side plate for an 1/8 inch welding tab. The slots on the
side plates can be cut with a welding torch. This should require about a 5/16 inch wide cut. For
a more accurate job, the slots could be milled out with a 0.25 inch mill-bit, assuming a milling
machine is available. Every center-of-radius for the arc of each mil cut will fall on a circle of a
circle of 4.47 inch radius as measured from the center of the wheel (again, this is for a 12 inch
wheel). If the wheel is to have 24 blades, each blade will be placed every 15 degrees apart
around the wheel (i.e every 360/24 degrees); and so each center-of-radius will be 15 degrees
apart around the 4.47 inch radius circle. Once the centers are located for the arc of each blade
slot, the arcs are drawn at 2 inch radii, and the slots can be cut.

For constant speed regulation (something essential for running an A.C. generator), a slide gate
valve would have to be added to the nozzle, plus a centrifugal governor to actuate the valve,
(This little mechanism could cost as much or more as all the other materials combined). For
high heads, the entrance works would be connected directly to a pipe.

Design equations for a cross flow turbine:

Width of Nozzle = W1 = (210)(Q)/(D)(sq rt H)
Inside Width of Turbine = W2 = W1 + 1inch
Length of blades = We + 3/4 inch (for 1/4 inch material)
Optimum RPM = (862)(sq rt H)/(D)

Where:
Q = Flow rate in cubic feet per second
D = Diameter of wheel in inches
H = Head at the wheel in feet
W1 = Inside width of nozzle
W2 = Inside width of turbine runner

Hope that helps!!!

Spaw