Calculation of clearances in twin screw compressors

Clearances between rotating and stationary parts in a screw compressor are set to ensure the efficient operation and allow for thermal deformation without unwanted contacts. The change in clearances is caused by both pressure and temperature changes within the machine. If clearances are too large, the increased leakage flows will reduce efficiency. However, if the nominal clearances are too small, contacts between the rotating and stationary parts can occur as a consequence of rotor and casing deformations. In order to determine the operational clearances, a numerical analysis of deformation of screw compressor rotors and casing has to be performed. This paper discusses how the temperature of rotor and casing surfaces calculated from the one-dimensional chamber model in the SCORG could be used as a boundary conditions for a steady state thermal and structural analysis of a screw compressor solid parts. Deformations of rotors and casing under temperature load were calculated using a commercial Finite Element Analysis code ANSYS. Operational clearance are estimated from these deformations and some recommendations for further work are proposed

Spectral Properties of AGN with Very Weak [O III] Lines

The spectral properties of a sample of 58 Active GalacticNuclei (AGN) spectra, in which emission [O~III] $lambdalambda$4959, 5007 AA lines are weak or totally absent, are analyzed. In order to investigate thephysical reason for the [O~III] emission suppression, the spectral propertiesof the weak [O~III] spectra sample are compared with the same properties of asample of 269 spectra with the strong [O~III] lines. The spectra are obtainedfrom Sloan Digital Sky Survey (SDSS) Database. It is found that the objectswith the weak or absent [O~III] $lambdalambda$4959, 5007 AA linesgenerally have the high continuum luminosities (log($lambda$L$_{5100}$) $>$45), that they are very rare at smaller redshifts ($z <$ 0.3) and that theyusually have strong starburst influence. From the sample with weak or absent[O~III] lines, two boundary subgroups may be distinguished: the subgroup witha strong H$beta$ narrow component and subgroup with a very weak or negligibleH$beta$ narrow component. The physical causes for the [O~III] linessuppressing are probably different in these two subgroups: the [O~III] linesare absent in objects with strong narrow H$beta$ probably because of strongstarburst (SB) activity, which produces high density of the gas, while in theobjects with the negligible narrow H$beta$, the reason for [O~III] and narrowH$beta$ suppression may be a low covering factor

Spectral monitoring of AGNs: Preliminary results for Ark 564 and Arp 102B

We present preliminary results of the long term spectral monitoring of two active galactic nuclei with different broad line shapes: Ark 564 and Arp 102B. Ark 564 is a bright nearby narrow line Syfert 1 (NLS1) galaxy with relatively narrow permitted optical emission lines and a high FeII/H${\beta}$ ratio, while Arp 102B is a nearby broad-line radio galaxy with broad double-peaked Balmer emission lines. The spectra of Ark 564 were observed during 11-year period (1999-2009) and the spectra of Arp 102B in the 12-year period (1998-2009), with SAO 6-m and 1-m telescopes (Russia) and the GHAO 2.1-m telescope (Cananea, Mexico).Comment: Presented on "8th Serbian Conference on Spectral Line Shapes in Astrophysics". In revised version minor changes in the tex

Normal Rack Grid Generation Method for Screw Machines with Large Helix Angles

Improving the efficiency of the screw machine is highly significant for industry. Numerical simulation is an important tool in developing these machines. The 3D computational fluid dynamic simulation can give a valuable insight into the flow parameters of screw machines. However, it is currently difficult to generate high quality computational grids required for screw rotors with large helix angle. This is mainly due to the excessively high cell skewness of the rotors with large helix angel, which would introduce errors in numerical simulation. This paper presents a novel grid generation algorithm used for the screw rotors with large helix angel. This method is based on the principles developed for the grid generation in transverse cross-section. Such mesh is generated by SCORGTM using normal rack grid generation method which means numerical meshes are generated in a plane normal to the pitch helix line. The mesh lines are then parallel to the helix line and thus an orthogonal mesh will be produced. The main flow and leakage flow directions are orthogonal to the mesh, potentially reducing numerical diffusion. This developed algorithm could also be employed for single screw machines

Influences of precipitation and temperature trend on maize yields

Maize yield for three decade period of the last century (1961-1990) were in Hungary for 15% higher than in Croatia (means 3.81 and 4.39 t/ha, respectively) and this trend was continued in the 1996-2007 period. However, amplitude of maize yields (differences among year (for the 1996-2007) in Hungary were higher (from 3.60 to 7.56 t/ha) than in Croatia (from 3.86 to 6.92 t/ha). Aim of this study was testing maize yield, precipitation and air-temperature variations in four Counties (Croatia: Vukovarsko-Srijemska =VSC and Virovitiţko-Podravska =VPC; Hungary: Békés = BC and Fejér =FC). Mean yield in VSC for 1996-2007 period was for 16% higher than in VPC. Yields in three less favorable years (LFY: 2000, 2003 and 2007) were considerably lower (means 5.22 and 4.41 t/ha, for VSC and VPC, respectively) than in three more favorable years (MFY: 1997, 2002 and 2005) years (means 7.50 and 7.00, respectively). Precipitation (means of two sites: Osijek and Virovitica) in 3-months period June-August was in LFY for 58% lower than in MFY (129 mm and 305 mm, respectively). At the same time, air-temperatures were for 2.0°C higher (22.7 and 20.7°C, respectively). Mean yield in BC for 1996-2007 period was for 21% higher than in FC. Yields in three LSY were considerably lower (means 3.78 and 3.79 t/ha, for BC and FC, respectively) than in three MFY (means 6.13 and 7.30 t/ha, respectively). Precipitation (means of two sites: Békéscsaba and Székesfehérvár) in 3-months period June-August was in LFY for 51% lower (115 mm and 235 mm, respectively) and air-temperatures were for 1.9°C higher (22.1 and 20.2°C, respectively) than in MFY. Precipitation and temperature trends for LFY and MFY in two sites of both countries were similar with emphasis that in Hungary they were negligible lower

The flux ratio of the [OIII] 5007,4959 lines in AGN: Comparison with theoretical calculations

By taking into account relativistic corrections to the magnetic dipole operator, the theoretical [OIII] 5006.843/4958.511 line intensity ratio of 2.98 is obtained. In order to check this new value using AGN spectra we present the measurements of the flux ratio of the [OIII] 4959,5007 emission lines for a sample of 62 AGN, obtained from the Sloan Digital Sky Survey (SDSS) Database and from published observations. We select only high signal-to-noise ratio spectra for which the line shapes of the [OIII] 4959,5007 lines are the same. We obtained an averaged flux ratio of 2.993 +/- 0.014, which is in a good agreement with the theoretical one.Comment: Accepted for publication in the MNRA

Non-equilibrium hysteresis and spin relaxation in the mixed-anisotropy dipolar coupled spin-glass LiHo0.5_{0.5}Er0.5_{0.5}F4_{4}

We present a study of the model spin-glass LiHo$_{0.5}$Er$_{0.5}$F$_4$ using simultaneous AC susceptibility, magnetization and magnetocaloric effect measurements along with small angle neutron scattering (SANS) at sub-Kelvin temperatures. All measured bulk quantities reveal hysteretic behavior when the field is applied along the crystallographic c axis. Furthermore avalanche-like relaxation is observed in a static field after ramping from the zero-field-cooled state up to $200 - 300$ Oe. SANS measurements are employed to track the microscopic spin reconfiguration throughout both the hysteresis loop and the related relaxation. Comparing the SANS data to inhomogeneous mean-field calculations performed on a box of one million unit cells provides a real-space picture of the spin configuration. We discover that the avalanche is being driven by released Zeeman energy, which heats the sample and creates positive feedback, continuing the avalanche. The combination of SANS and mean-field simulations reveal that the conventional distribution of cluster sizes is replaced by one with a depletion of intermediate cluster sizes for much of the hysteresis loop.Comment: 6 pages, 4 figure

Grid Generation for Screw Compressors with Variable Geometry Rotors

An algebraic grid generation algorithm is presented in this paper which enables the per- formance of twin screw compressors with variable rotor geometry to be predicted, by means of Computational Fluid Dynamics (CFD). It is based on a method, previously developed by the authors, for compressors with standard uniform pitch rotors and con- stant cross-section profile, which has now been extended to include rotors with variable pitch and/or variable profile. By its use with the commercial CFD solver ANSYS CFX, it has been possible to obtain performance predictions for three variants of an oil-free 3/5 screw compressor, namely uniform helical rotors, variable pitch rotors and variable profile rotors. The variable pitch and variable profile rotors achieve steeper internal pres- sure rise and a larger discharge area for the same pressure ratio. Variable pitch rotors also showed lower leakage rates due to reduced sealing line length in the high pressure domain. This grid generating procedure advances the ability to evaluate both existing and novel compressor configurations

Phase-field modeling of pitting and mechanically-assisted corrosion of Mg alloys for biomedical applications.

A phase-field model is developed to simulate the corrosion of Mg alloys in body fluids. The model incorporates both Mg dissolution and the transport of Mg ions in solution, naturally predicting the transition from activation-controlled to diffusion-controlled bio-corrosion. In addition to uniform corrosion, the presented framework captures pitting corrosion and accounts for the synergistic effect of aggressive environments and mechanical loading in accelerating corrosion kinetics. The model applies to arbitrary 2D and 3D geometries with no special treatment for the evolution of the corrosion front, which is described using a diffuse interface approach. Experiments are conducted to validate the model and a good agreement is attained against in vitro measurements on Mg wires. The potential of the model to capture mechano-chemical effects during corrosion is demonstrated in case studies considering Mg wires in tension and bioabsorbable coronary Mg stents subjected to mechanical loading. The proposed methodology can be used to assess the in vitro and in vivo service life of Mg-based biomedical devices and optimize the design taking into account the effect of mechanical deformation on the corrosion rate. The model has the potential to advocate further development of Mg alloys as a biodegradable implant material for biomedical applications. STATEMENT OF SIGNIFICANCE: A physically-based model is developed to simulate the corrosion of bioabsorbable metals in environments that resemble biological fluids. The model captures pitting corrosion and incorporates the role of mechanical fields in enhancing the corrosion of bioabsorbable metals. Model predictions are validated against dedicated in vitro corrosion experiments on Mg wires. The potential of the model to capture mechano-chemical effects is demonstrated in representative examples. The simulations show that the presence of mechanical fields leads to the formation of cracks accelerating the failure of Mg wires, whereas pitting severely compromises the structural integrity of coronary Mg stents. This work extends phase-field modeling to bioengineering and provides a mechanistic tool for assessing the service life of bioabsorbable metallic biomedical devices