Unsteady simulation of quasi-periodic flows in Organic Rankine Cycle cascades using a Harmonic Balance method

A. Rubino, M. Pini*, P. Colonna

*Corresponding author for this work

Research output: Chapter in Book/Conference proceedings/Edited volumeConference contributionScientificpeer-review

3 Citations (Scopus)
36 Downloads (Pure)


Currently, turbomachinery design optimization methodologies are mainly restricted to steady state approaches, due to the high computational cost associated with time-accurate shape optimization algorithms. However, the possibility to include unsteady effects in turbomachinery optimization can significantly increase the level of accuracy of the design predictions, leading to a more realistic representation of the actual performance and ultimately to a substantial increase in operating efficiency. Unsteady effects are particularly relevant in Organic Rankine Cycle turbines. A trade-off between high-fidelity time-accurate unsteady simulations of the flow solution and computational cost is therefore needed at design level. In this paper, a first application of the harmonic balance method to non-ideal compressible flows is presented. The methodology allows to solve the unsteady flow equations for a set of specified frequencies only, with significant computational time savings. An algorithm is proposed for non uniform time sampling in order to resolve frequencies that do not need to be integral multiple of one fundamental harmonic. This enables the solution of quasi-periodically forced non-linear flow problems, in combination with complex fluid models based on accurate equations of state. The method is applied to the unsteady analysis of a supersonic Organic Rankine Cycle stator with quasi-periodic inlet operating conditions, showing about one order magnitude lower computational cost compared to time-accurate simulations.

Original languageEnglish
Title of host publication4th International Seminar on ORC Power Systems September 13-15th 2017
Subtitle of host publicationMilano, Italy
EditorsV. Dossena, A. Guardone, M. Astolfi
Number of pages8
Publication statusPublished - 2017
Event4th International Seminar on ORC Power Systems - Politecnico Di Milanobovisa Campus, Milano, Italy
Duration: 13 Sept 201715 Sept 2017
Conference number: 4

Publication series

NameEnergy Procedia


Conference4th International Seminar on ORC Power Systems
Abbreviated titleORC 2017
Internet address


  • CFD
  • Harmonic Balance
  • Quasi-periodic
  • Time Sampling
  • Turbomachinery
  • Unsteady


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