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This study explores the characteristics of twin-screw pumps when operated in turbine, i.e., reverse mode as an alternative to traditional control valves in piping systems for recovering energy. Experimental and flow simulation methods are employed. By an overset grid technique, unsteady three-dimensional (3D) flow simulations are conducted, ensuring high cell quality and high spatial resolution, particularly in wall proximity and within the gaps. To assess viscosity effects, pump and turbine mode characteristics are analyzed for water and oil. Compared to water, a pronounced reduction of the flow rate dependence on pressure difference is revealed for highly viscous oil. The gap flow in oil is significantly affected by the spindles' rotational speed and direction, whereas water displays minimal influence. Analyzing the variation in gap sizes, a 50% reduction in the minimum flank gap width leads to a slight rise in volumetric efficiency with negligible overall efficiency effects. Our findings underscore that utilizing twin-screw pumps as turbines for high-viscosity fluids can potentially recover up to 50% of the energy instead of dissipating it by using conventional control valves.
Moghaddam et al. (Thu,) studied this question.