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zTurbo Details

What makes the design of an ORC turbine so difficult?

A critical component in the ORC technology is the turbo-expander; the difficulties involved in the accurate thermodynamic modeling of organic fluids and, especially, the complex gasdynamic phenomena that are commonly found in ORC turbines may result in relatively low efficiency and in performance reduction at partial loads. In this perspective, a relevant path of development can be outlined in the evaluation of nonconventional turbine architectures, such as the radial-outward or centrifugal turbine.

How zTurbo predicts efficiency for ORC turbines

zTurbo selects the optimal turbine configuration based on reliable loss prediction and accurate  thermo-physical property calculation.  Basically, the designer specifies the turbine type, the working fluid, the operating conditions and a set of geometrical parameters (blade angles, blade chords, tip clearance, etc.) and zTurbo provides a full detailed turbine geometry with relative performance at very low computational cost.

A case study

zTurbo has been applied to various design challenges and proved to be highly accurate in predicting the performance for a given turbine configuration. Just one example may illustrate this. It deals with the design of the following turbine:

n0 =

7500 rpm

flow =

7.8 kg/s

Tt,in =

413 K

pt,in =

2.6 bar

pout =

1.022 bar

βstg =


ris =



The table below illustrates the results of zTurbo against measured parameters (in brackets) for relevant turbine quantities, namely total-to-static efficiency, outlet total pressure, and flow velocity . Given the uncertainty rate affecting the various loss correlation models, zTurbo is found to be in very good accordance with experiments. The turbine geometry calculated by zTurbo, denoted using red lines, practically overlaps with the actual geometry.

Etats =

0.894 (0.913)

Ptout =

1.04 (1.046) bar

vout =

58.55 (59.5) m/s










Input and Outputs

zTurbo uses the following inputs, which can all be varied to find an optimal turbine configuration:

  • loss model
  • speed of revolution option (single speed for all stages or different per stage)
  • expansion ratio (specified by user or computed automatically)
  • inlet conditions option (way of computing blade span at turbine inlet)
  • supersonic nozzle option (specify either nozzle outlet span or outlet blade angle)
  • loading criterion option (zweifel criterion or assigned pitch/chord)
  • tip clearance (either as ratio or in absolute terms)
  • Fluid database used
  • Turbine data (mflow, nstg, rpm, tint, p_int, p_out, dynvisc)
  • Inlet data (inlet angle, blade height, h/D ratio, loading coefficient, admission degree)
  • Data per stage
  • General: rotation speed, expansion ratio, reaction degree, stator-rotor clearance/stator chord, rotor-stator clearance/rotor chord, stator radial chord,
  • Stator: outlet angle, tip clearance / outlet blade, trailing edge / throat, pitch, loss coefficient,
  • Rotor: radial chord, outlet angle, tip clearance / blade height, trailing edge / throat, pitch /chord, loss coefficient

zTurbo produces all the numbers that are needed to evaluate the turbine configuration:

The output of the code consists of two separate files i) a report containing all turbine features, alongside the evaluation of its performance and ii) the shape of the meridional channel, which is crucial information when designing the full-scaled 3D blade geometry. A third file with a preliminary design of the 2D blade to blade profiles will be available in the near future. This can be used for initializing a CFD-based optimization loop.

Who is behind zTurbo?

zTurbo was originally developed by Matteo Pini with the help of Salvatore Vitale (now both part of the Propulsion and Power Group at Delft University of Technology) and Giacomo Persico (Politecnico di Milano).

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