This paper explores the theoretical and computational challenges associated with modeling of flow, momentum transport, and energy conversion processes in disk rotor drag turbine expanders. This category of expander devices, also known as Tesla turbines, has distinct advantages for Rankine power generation using low temperature heat from renewable source such as solar, waste heat, or geothermal steam or hot water. Specifically, the nozzle and rotor designs and the overall expander can be simple to manufacture, low cost, and durable, making this type of expander an attractive option in green energy technology applications where low maintenance costs and rapid capital investment payback are important qualities. Efficient energy conversion performance in rotor disk drag expanders requires that the nozzle efficiently convert flow exergy to fluid kinetic energy, and the rotor be designed to efficiently convert fluid angular momentum to shaft torque and power. To achieve these goals, modeling and analysis tools must provide the designer with a means to predict the performance of these components that accurately represents the physics, and can be effectively used to illuminate the parametric trends in performance. Two categories of modeling are examined in this paper: (1) computational fluid dynamics (CFD) modeling, and (2) more idealized one- and two-dimensional analysis frameworks. The advantages and disadvantages of these two approaches are examined here for the specific flows of interest in this type of expander design. The implications of model predictions for optimal design of disk rotor expanders for green energy applications are also discussed.

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