Fachgebiet Fluidtechnik

We analyze differently worn surface positions of a screw rotor's contact zone using confocal Raman spectroscopy with 2 μm spatial resolution. The Raman scattering spectra demonstrate signals from magnetite Fe3O4 and hematite α-Fe2O3 whose intensities and energy shifts are associated with the degree of wear of the surface locations. While magnetite is identified at the worn surface areas studied, excluding a possible white etching domain, scattered light signatures from hematite are only detected at surface positions, at which high temperatures and/or high pressure inputs were proposed to be present during the liquid-free wear process of the screw rotors. The findings may be used to gain a more general understanding of tribological layers and to develop microscopic indicators of wear.

Screw-type expanders offer excellent prospects for energy conversion in lower and medium power ranges, for instance as expansion engines in Rankine cycles with regard to either waste or geothermal heat recovery. With the aim of identifying the potential in organic Rankine cycle (ORC) power systems, an oil-flooded twin-screw expander without timing gears was designed and experimentally investigated in an ORC with R245fa as working fluid. Here, the scope for the experimental determination of the expander characteristic map was limited by the test rig specifications. Based on the experimental results, a multi-chamber model of the test twin-screw expander was calibrated and theoretical approaches according to mechanical and hydraulic loss calculation were applied. Consequently, the expander's complete characteristic map could be calculated. Furthermore, relevant mechanisms influencing the operational behaviour of oil-flooded twin-screw expanders were identified and analysed in-depth.

The present paper responds to the operating behavior of an exemplarily selected screw expander for exhaust heat recovery from combustion engines in the lower power range. The application in vehicles for the purpose of heat recovery, which is characterized by the small machine dimensions due to relatively low system mass flows, is a still unexplored area for the screw-type machine. The screw expander is to be used in an organic Rankine cycle (ORC). Compared to other expander designs in general screw expanders are characterized by high efficiency together with relatively low geometrical dimensions. Within the presented investigation mainly the performance characteristic of a screw expander, geometrically designed for a particular operating point, is determined as a function of the system parameters-inlet pressure and temperature-as well as the rotational speed of the expander. Here, based on an iterative coupling of thermodynamic and thermal simulations the influence of the thermal deformation on the machine performance in particular is analyzed. The results of the thermodynamic simulation, mainly based on socalled chamber models, represent the thermodynamic and fluid dynamic performance of the screw machine by means of mass and energy conservation. The information obtained in this way about the temperature distributions and the heat fluxes provides a basis for the subsequent thermal simulation using a finite element method (FEM) calculation. The resulting thermally deformed machine, whose performance-related clearance heights are now changed, is used for the next iteration step within the thermodynamic simulation.

Twin-shaft screw-type expanders offer a high potential for energy conversion in the lower and medium power range, for instance as expansion engines in Rankine cycles for exhaust heat recovery. With regard to minimizing internal leakages and lubricating moving machine parts, an auxiliary liquid or liquid working fluid, for example in organic Rankine cycles (ORC)1, can be fed to the screw expander. In this paper, indicator diagrams of a twin-shaft screw-type expander prototype SE 51.2 designed at the Chair of Fluidics at TU Dortmund University are presented and analyzed in detail. The experimental investigations are carried out on a hot-air test rig with an expander inlet manifold water injection. The time-dependent working chamber pressure is recorded by means of high-resolution absolute pressure transmitters. Hereby, specific aspects of working chamber pressure measurements are mentioned. Based on the indicator diagrams, relevant influence mechanisms on the expander's operationa...

The numerical simulation of rotary positive displacement (PD) machines has proven to be a very challenging task characterized by complex geometries with a time dependent change of the chamber volume along with complex flow physics. In addition, the leakage flow caused by housing clearances, front gaps and the intermesh clearance is of particular interest. The resulting losses have a severe impact on the overall machine efficiency. This paper shows workflow and results for Computational Fluid Dynamics (CFD) simulations of the Screw Expander SE-51.2 of TU Dortmund University. The simulation is performed using ANSYS CFX, where the computational grids for the rotary parts are derived from the grid generator TwinMesh. The SE-51.2 is a gearless, dry running twin screw expander that converts thermal into mechanical energy by decompressing air with pressureratios up to 6:1 by a rotational speed range up to 20,000 rev/min. The volume ratio of the expander is 2.5 at a shaft distance of 51 mm ...

Tribology, as the science and technology of interacting surfaces, typically relies on liquid lubricants which reduce friction and wear. For environmentally friendly tribological purposes and applications requiring a liquid-free performance, solid lubricants, such as MoS2 coatings, play an essential role. It is crucial to understand the interplay between the parameters of the coating synthesis and the characteristics of the coating. The impact of the deposition parameters on the structural, mechanical and frictional properties of MoSx thin films, which are synthesized by high-power impulse magnetron sputtering, are studied. The morphology, topography and stoichiometry (2.02 < x < 2.22) of the films are controlled by, in particular, the bias-voltage and heating power applied during the sputtering process. In combination with a low pulse frequency the hardness and elastic stiffness of the MoSx films are enhanced up to 2 and 90 GPa, respectively. This enhancement is assigned to a ...