Organic Rankine cycle turbogenerator cooling – Optimization of the generator water jacket heat exchange surface

AUTORZY: Dawid Zaniewski, Piotr Klimaszewski, Piotr Klonowicz, Łukasz Witanowski, Piotr Lampart, Łukasz Jędrzejewski, Tomasz Suchocki

Discharge of excess heat from the micro-turbine electric generator is an important issue for the micro-turbine operation and should be considered during the design process. The manuscript is intended to present a new tool for rapid shape optimization of the heat exchanging surface of the ORC (Organic Rankine Cycle) micro-turbine electric generator. The optimization of the water jacket geometry of a 10 kWe ORC micro-turbine generator is performed by making use of a modified, with respect to the models presented in the literature, 0D heat transfer model of the finned heat exchange surface of the cooling water jacket, and a genetic algorithm. As a result of optimization, an apparent reduction in fin size and increased fin density at the finned wall surface is achieved, yielding a significant increase of the heat transfer coefficient and a temperature decrease of the electric generator stator. The temperature at the shell surface tangential to the electric generator stator was found to decrease by over 6°C. A considerable reduction of temperature gradients around the water jacket wall is also reported.

The results of optimization from the 0D model are verified by numerical simulation using a full 3D CFD model of fluid flow and conjugate heat transfer (CHT) in Ansys CFX for the original and optimized geometries. The CFD CHT model was first validated using experiment-based correlations for heat transfer in liquid flow over a circular cylinder pipe for a wide range of Reynolds numbers.

The obtained temperature decrease in the optimized ORC electric generator stator for the nominal operating point of the ORC generator is found similar for both 0D and 3D models. The temperatures calculated in the 0D model at the shell surface tangential to the electric generator stator deviate from respective CFD values by 0.5–1.5°C. Another operational case beyond the nominal operating point has been analysed making use of the 3D CFD method as well, showing that the optimized geometry is characterized by an improved heat exchange performance not only for the nominal operating point.

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