Monday, September 3, 2012

Helical Savonius, axis of twist and axis of rotation

In a simple Savonius wind turbine there are two types of gaps between the buckets that can be specified based on the design requirements: the primary gap and the secondary gap, but it was common to set the secondary gap to zero. In our graduation project, we were asked to create a two-bladed helical Savonius VAWT (90 degrees twist angle) with different values of the primary gap to know the best dimensionless value (gap distance divided by bucket diameter) of the primary gap that gives highest power coefficient. We thought about creating two helical (twisted) blades and then use them with different primary gap values by sliding them in or out on a straight line (as the same method you can think about when you want to change the primary gap for a conventional Savonius). Since we were in the brain storming phases and not taking what we were saying seriously, we didn’t release that this was a mistake. When we tried to draw the cad model of the blades and bottom and top caps to just test how to draw the blades in Autodesk Inventor, we got assembly errors with a large exclamation mark on our faces !!!!. After a wise thinking we found that we should create a couple of blades for each value of the primary gap. To understand why we reached this conclusion, let’s consider the following figure that shows two cases:

§  In the first case: the axis of twist coincides with the axis of rotation. Therefore, the primary and secondary gaps are constant for all sections of the blade. Simply, axis of rotation, axis of twist of the first blade, and axis of twist of the second blade are coincident.
In the second case: the axis of rotation does not coincide with the axis of twist and the axes of twist of both blades also don’t coincide. In other words, we have 3 axes at different locations. Taking the bottom edge profile (buckets) as a reference, we will find that we increased the primary gap and the secondary gap is zero. Moving up along the axis of rotation, we will find that the values of the primary and secondary gaps are changing from section to section. So, for this reason, the turbine will not perform the same at different phase angles.