Abstract
In the Institute of Thermal Turbomachinery (ITSM) at Stuttgart University, a three-stage model steam turbine was used to study the complex steam flow through the last stages of low pressure steam turbines. The conditions in this test rig were such that condensation occurred after the first stage, so that wet steam was prevalent in the last stage. In order to determine the inlet conditions for the last stage and for the validation of results of a numerical simulation of the condensing steam flow through the model turbine, both steam quality and the droplet size spectrum were measured in front of the stator of the last stage using a miniature combined optical/pneumatic probe. However, while the measured steam quality matched the numerical results quite well, there were considerable differences between the computed and the measured droplet size spectrum. The latter showed considerably larger droplets than predicted by computational fluid dynamics.
In this paper, the measurement principle (light extinction) is presented and the inherent limitations for the determination of very small droplets with diameters of less than 0.2 µm are discussed. Issues related to this challenge are the availability of UV-wavelengths and the optical length used for the measurements. The reliability of the inversion algorithm used to analyse the light extinction data is shown by comparison with data from experiments using polystyrene suspensions with known particle size and concentration.
Thereafter, the validity of the measurement results is reassessed based on numerical results and data from repeated measurements. It can be shown that the experimental results are reliable, implying that the numerical results suffer from inaccuracies. Apart from modelling issues, one reason for this could be that the inherent unsteadiness in turbine flows is neglected in steady computational fluid dynamics computations.
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