Mechanical turning momentum (torque) is generated by flows which press against one side of the turbine blades. Commonly, turbines have blades where a groove is effectively created between two successive blades. In effect, the driving pressure of a turbine is applied to one face of this virtual groove. With this arrangement, the leading face represents the "pressure" side and the trailing face represents the "suction" side. The generation of torque is based on the difference of pressure between these two wall faces. This pressure difference is maximised if there is no suction side at all, that is, when there is no pressure at all on the "suction" side. This is possible along the surfaces of a cone-shaped turbine which has saw tooth-like grooves as already described.
These turbine "blades" have a pressure-side which faces in a radial direction relative to the direction of rotation. Each groove has a 'bottom' or inner side which faces in a tangential direction. Water flow which moves diagonally outwards effectively flows parallel to that inner face. The pressure-side plus the inner-side, form the contours of an asymmetric saw tooth shaped groove. Each inner-side extends from the inner edge of the pressure-side to the outer edge of the following pressure-side. These triangular shaped grooves effectively have no backside wall.
In Figure 07.05.08, the cross-sectional view shows several axial levels marked with the dotted lines A to H. The plan-view diagram shown at the top of the Figure indicates where these levels extend horizontally. At inlet level A, the radius is 12 cm and a ring-shaped cross-sectional surface is available for water to enter between the round turbine face and the round cone-shaped wall of the housing (drawn here across a sector of 30 degrees).
Further up, these tooth-shaped blades extend further out of the surface of the turbine cone. At point B, the inner edge still has a radius of nearly 12 cm, while the outer edge extends further out into the ring-shaped groove. Here for example, twelve turbine "blades" are shown, and in the 60 degree sector B, there are two of these "saw-teeth".
Level C marks the junction between the turbine-inlet area (TE) to the main body of the turbine (T). The turbine "teeth" at this level have a radius of 16 cm and this level has the deepest grooves. This sector of 60 degrees has two of these teeth TS.
Further up, the outer circumference becomes greater and the notches become longer. If the cross-sectional area for water flow were to remain constant, then the notches would need to be correspondingly shallower. In sectors D, E and F, which again span a 60 degree sector, two turbine-blades are shown in each sector.
As sector H covers only 30 degrees, it contains just one tooth. At this top level, which has a radius of 24 cm, is located the turbine outlet, where water should exit, forming a homogenous flat jet. Consequently, the contours of the turbine rotor grooves should be ring-shaped. Also, the water which previous ran along the inner side of a cone-shaped wall, now is contained in a space between that wall and the inner turbine cone. These surfaces can effectively be a nozzle and this long groove can have additional divider walls (shown as thick red lines), to enlarge the pressure-surfaces in this area.
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