Andreas Brümmer

Physics of a dry running unsynchronized twin screw expander

The concerned dry running gearless screw machine (GL 51.2) has already been investigated in detail as a super charger and vacuum blower. Aim of the present paper is the physical explanation of the machine behaviour as twin screw expander. This will be done by extensive experimental results including pressure-angle diagrams and indicator diagrams together with numerical calculations using a multichamber model of the machine. It is shown that the primary loss mechanisms of a screw expander are the inlet throttling during chamber filling as well as the gap mass flows in the expansion start range. As the rotational speed increases, the inlet throttling becomes stronger, while the gap losses become smaller.

Influence of water and oil clearance flow on the operational behavior of screw expanders

Currently, the simulation of multi-phase rotary displacement machines in reasonable accordance with the experimental results is not possible. Clearance sealing, additional frictional losses, heat transfer and lubrication are among the various effects caused by the presence of a liquid, which would all have to be modelled. Moreover, complex processes, such as condensation and evaporation affecting the thermodynamic equilibrium, as well as expansion of the multi-phase gas-liquid-mixture would have to be included. With the purpose of achieving a better understanding of liquid-flooded screw expanders, this paper describes a theoretical evaluation of clearance sealing by means of a liquid, and the resulting frictional losses. The influence of different auxiliary liquids, namely water and oil, is examined. Thus, after introducing the expander geometry and the auxiliary liquids, the results of a thermodynamic analysis are presented. The multi-chamber model-based simulation tool KaSim, that has been developed at the Chair of Fluidics, is applied to analyze the maximum potential of clearance sealing. Subsequently, dry running and liquid-flooded screw expanders are compared, taking clearance sealing as well as frictional losses into account. On the one hand, the study demonstrates that the influence of liquid water on temperature is negligible. On the other hand, the results show that reasonable modelling of oil requires a consideration of temperature-dependent dynamic viscosity for this auxiliary liquid. Finally, an extension of the presented simulation approach is introduced.

Energy potential of dual lead rotors for twin screw compressors

The paper deals with a theoretical investigation into the ways in which fluid properties influence the optimal geometrical parameters of dry running twin screw compressors with uniform and dual lead rotors. Dual lead rotors possess two segments with different rotor leads in order to optimise the progression of chamber volume, clearance size and outlet area. Optimal geometry parameters are determined by means of a multi-chamber model simulation. Fluids are treated as an ideal gas and fluid properties are varied systematically in order to provide guidance on how the machine should be designed for a particular application. The optimal geometry parameters mainly depend on the speed of sound and isentropic exponent of the fluid, because they influence outlet throttling during discharge, and pressure progression due to volumetric compression. In particular, the values for the rotor wrap angle and the internal volume ratio need to be adjusted individually for each compressor application. The results reveal that dual lead compressors possess most potential for fluids with low speed of sound and isentropic exponent.

Indication of worn WC/C surface locations of a dry-running twin-screw rotor by the oxygen incorporation in tungsten-related Raman modes

By comparing the worn and untouched locations of a tungsten-carbide/carbon surface of a dry-running twin-screw rotor, we demonstrate that tungsten-oxide Raman modes become observable only at worn locations and the integral intensity of the Raman line at 680 cm−1, which is related to the incipient oxidation of the tungsten-carbide stretching mode, is enhanced. Its frequency and width moreover change significantly, thus indicating the mechanical distortion of the bonding that has been occurred during the wearing process. The shape of the tungsten-oxide Raman lines, resembling the Voigt function, hints at a surface morphology that is a characteristic for an amorphous solid environment. Our Raman scattering results may be exploited to characterize the degree of wear of coated surfaces and to identify signatures of a tribological layer.

Influence of liquid in clearances on the operational behaviour of twin screw expanders

A lot of effort has been expended on understanding the influences of an injected auxiliary liquid on a twin screw expanders performance. Sealed clearances improve performance on the one hand, but involve considerable frictional losses on the other hand. This paper contributes to an evaluation of these opposing effects with regard to the efficiency of screw expanders. First, thermodynamic analyses using the multi-chamber model-based simulation tool KaSim, developed at the Chair of Fluidics, are presented for a test screw expander in order to show the maximum potential of clearance sealing. This analysis involves thermodynamic simulations for sealed and unsealed clearances and leads to an order of priorities for different clearance types. Second, hydraulic losses within front and housing clearances are calculated, applying an analytical model of incompressible one-phase clearance flow. Subsequently dry and wet screw expanders are evaluated while both clearance sealing and frictional losses are considered for the simulation of a liquid-injected machine.

Calculation of discharge pressure pulsations of a screw compressor using the one-dimensional method of characteristics

This paper presents a method to simulate the discharge process of screw compressors. The calculation is based on the method of characteristics. The main focus is the transient, compressible, dissipative flow through the compressor discharge port. The time-dependent velocity at the outlet is calculated using a circular orifice with time-dependent diameter and is initially validated by an exemplary screw compressor. The one-dimensional model for the working chamber is presented and the transient flow on the discharge side is calculated for different discharge pressures. The calculated results are compared to experimental data.

Geometrical abstraction of screw compressors for thermodynamic optimization

The construction and development of different rotor profiles is an important area in connection with the development of screw compressors for specific applications. Geometrical performance figures (using criteria to describe interdependencies of geometrical parameters for screw compressors) for profile optimization are used in order to achieve specific improvements in performance. During this process, rotor profiles and spatial parameters are the main factors. Compared to data derived from the front section of rotor profiles, these figures which also take spatial parameters into account provide a better evaluation of gap conditions and operating efficiency of the compressors under examination.

Opportunities for variable rotor lead in screw compressors depending on dimensionless numbers

The paper deals with the optimisation of the geometry of screw machines, focussing on the wrap angle of the rotors. The Pi-theorem is used to identify independent thermodynamic and geometric factor...

Measurement and simulation of rarefied Couette Poiseuille flow with variable cross section

Clearance flows are the main loss mechanism in dry running positive displacement vacuum pumps. In order to calculate the operation of those pumps, a detailed knowledge of the clearance mass flow rates is crucial. The dimensions of such pumps and the large pressure range of the operating points require a wide range of gas rarefaction to be taken into account. The clearance flow can be described by a combined Couette Poiseuille flow due to the pressure gradient between two chambers and the rotation of the rotary pistons. These clearance flows are studied experimentally and theoretically in the present work. Therefore, a suitable experimental setup is described together with the requirements of sensors and the necessity of a low leakage. A theoretical approach is presented, and the results are compared to experimental investigations varying the pressure ratio and the circumferential speed of a clearance boundary in a wide range of the gas rarefaction.

Effects of spontaneous condensation in steam-driven screw expanders

Condensation and its effects on turbo machinery operation are well understood and have been widely investigated. However, only little scientific work on condensation in positive displacement machines has been published. Although, depending on machine type, high expansion rates and, as a consequence, significant supersaturation can be achieved for working fluids with a negative saturation vapour curve. In this paper the effects of spontaneous steam condensation in screw expanders are discussed. Classical nucleation theory is used for the thermodynamic simulation of operational behaviour. The study shows at which point during the expansion phase spontaneous condensation can be expected, and typical nucleation rates are determined. Since the calculation of condensation requires a suitable fluid model providing consistent data for the metastable region, the advantages of Pollaks equation of state over the IAPWS property formulation are demonstrated. The impact of released latent heat during expansion on chamber states is depicted. Furthermore, a comparison of purely metastable expansion with equilibrium expansion is provided in order to show the full range discrepancies.