Preliminary design method for small scale centrifugal ORC turbines

In the recent years the ORC technology has become widely applied for small to medium power range applications. In this context the development of innovative turbine architectures represents a key theme to make ORC systems more efficient. Aim of the present thesis is the critical evaluation of a multi-stage centrifugal ORC turbine, whose architecture could have substantial advantages over traditional ax- ial and centripetal solutions. The work is focused on the one-dimensional approach, which usually constitutes the first step in the design of a turbomachine, and whose output is the selection of basic design parameters: number of stages, velocity triangles, basic geometry, speed of revolution and finally the estimated turbine performance. Due to the absence of experience and specific literature, traditional design pro- cedures base on similarity rules are difficult to apply. Thus, in order to achieve a preliminary turbine design, a 1-D mean-line code, called zTurbo, has been de- veloped, on the basis of initial assumptions and available models for losses and flow deviations. To perform the design calculation, the program is linked to an external optimization tool and with the FluidProp package for accurate thermo- dynamic properties calculation. Moreover, the mean-line procedure, implemented on the code, is validated with a test-case. On the basis pf this design technique, a innovative preliminary design methodol- ogy, best suited for small-scale turbine, is defined and is finally applied to design three small-scale centrifugal turbines. The results are extensively discussed and a comparison with a reference centripetal turbine for the same application is performed.