Adaption of the single-channel approach to use the CFD code MULTALL as an effective tool in the preliminary design of radial inflow turbines with vaneless spiral casing

Authors

Piero Danieli  - University of Padova, Italy
Massimo Masi - University of Padova, Italy

Abstract

This work deals with the CFD tools available to engineers as support for the preliminary design of radial inflow turbines. In particular, the problem of using rotor single-channel computations to the non-periodic domain of turbines with vaneless spiral casing is crucial to accelerate the CFD computations performed in the preliminary phase of a new design. Modern CFD packages suggest different approaches to this problem. On the other hand, in a previous publication, the present authors suggested a modelling approach which permits to reduce the single blade passage non-periodic domain to a fluid-dynamically similar periodic domain, which does not need for any preliminary CFD to define the required boundary conditions. Also, it was verified that this approach fits the capabilities of the CFD code MULTALL. This open-source code is conceived for single-channel steady state computations of succeeding blade rows (axial machines) or concentric blade stars (radial machines) and makes it possible fast computations even on inexpensive today’s laptops. Although the implemented physics is rather basic if compared to the one available in the modern commercial CFD packages, it is supposed that such a drawback is overcompensated by the speed of computations as far as the preliminary design phase is concerned. In fact, in the preliminary design it is deemed as most important the possibility to rapidly check a large set of designs, in order to limit the space that must be explored in the succeeding optimisation. Accordingly, the aim of the paper is to validate the periodic-domain calculation approach applied in MULTALL as a reliable tool to support the preliminary design of radial inflow turbines. To this end, the aerodynamic performance of a nano-turbine as predicted by MULTALL, is compared with the prediction obtained from the state-of-the-art CFD code Star CCM+. To make a fair comparison, two sets of computations were performed with the latter: preliminary simulations on a coarse grid of the complete turbine geometry allowed the extraction from the flow field of the boundary conditions required to perform very detailed computations on a non-periodic slice of the entire turbine including only one rotor blade passage. The results demonstrate that the periodic-domain approach permits to predict turbine’s global performance in acceptable agreement with those obtained from the computations on the domain accounting for the complete turbine. Most important, the non-periodic domain approach leads to predictions in very good agreement with those obtained with the state-of-the-art CFD code applied to the single-channel of the truly non-periodic domain. These findings support the validity of the proposed modelling approach in conjunction with MULTALL for the radial inflow turbines preliminary design.