143 research outputs found
Thermodynamic Limit for the Ising Model on the Cayley Tree
While the Ising model on the Cayley tree has no spontaneous magnetization at
nonzero temperatures in the thermodynamic limit, we show that finite systems of
astronomical sizes remain magnetically ordered in a wide temperature range, if
the symmetry is broken by fixing an arbitrary single (bulk or surface) spin. We
compare the behavior of the finite size magnetization of this model with that
of the Ising model on both the Sierpinski Gasket, and the one-dimensional
linear chain. This comparison reveals the analogy of the behavior of the
present model with the Sierpinski Gasket case.Comment: 5 pages, 4 figure
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Improving Screw Compressor Displacement and Efficiency by Increasing the Rotor Profile Depth
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Development and Design of Energy Efficient Oil-Flooded Screw Compressors
It is estimated that about 17% of the world's generated power is used for compression. Thus all, even minor improvement of the efficiency of compressors will substantially reduce CO2 emission. This paper presents development of family of energy efficient oil-flooded screw compressors for Kirloskar Pneumatic Company Ltd. The developmental techniques adopted to improve efficiency such as introduction of superior 'N' rotor profile, rotor clearance management, performance calculation using 3D CCM (Computational Continuum Mechanics), direct parametric interface to CAD (Computer Aided Design), which contains bearing selection for complete 3D solid modelling. Also, contemporary prototyping and experimental investigation is supported by the fully computerised data acquisition and processing. The cumulative improvement of all these elements of the design process resulted in a very efficient machine which guarantees the competitive position of Kirloskar Pneumatic Company Limited in the screw compressor market
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The influence of port shape on gas pulsations in a screw compressor discharge chamber
Gas pulsations in suction and discharge chambers are a significant source of noise in screw compressors. This paper shows how such effects in the discharge chamber are influenced both by the compressor operating conditions and its geometric characteristics. An area function is identified for the discharge port as an important parameter influencing the gas pulsations and it is shown how their amplitude can be reduced by optimization of the port shape
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Geometrical Comparison of Conventional and Gerotor-Type Positive Displacement Screw Machines
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Three Dimensional Numerical Analysis of Screw Compressor Performance
Modern manufacturing methods enable screw compressors to be constructed to very close tolerances where full 3-D numerical calculation of the heat and fluid flow through them is then required to obtain the maximum possible improvements in their design. An independent stand-alone interface program has been developed by the authors in order to generate a numerical grid for this purpose. The interface employs a procedure to produce rotor profiles and an analytical transfinite interpolation method with adaptive meshing to obtain a fully structured 3-D numerical mesh, which is directly transferable to a CFD code. This was required to overcome problems associated with moving, stretching and sliding rotor domains and with robust calculations in domains with significantly different geometry ranges. Some changes had to be made within the solver functions both to enable calculations and to make them faster. These include a means to maintain constant pressures at the inlet and outlet ports and consideration of two-phase flow resulting from oil injection in the working chamber. Modifications implemented to the CFD procedure improved solutions in complex domains with strong pressure gradients. The pre-processor code and calculating method have been tested on a commercial CFD solver to obtain flow simulations and integral parameter calculations. The results of calculations on an oil injected screw compressor are presented in this paper and compared with experimental results
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Effect of air temperature variation on the performance of wet vapour organic rankine cycle systems
A multi-variable optimization program has been developed to investigate the performance of Wet Organic Rankine Cycles (WORC) for generating power from low temperature liquid dominated brines. This cycle model contains a detailed thermodynamic model of a twin-screw expander, and the methods used to match the operation of the expander to the requirements of the cycle are described. Optimum operating conditions are calculated for a particular design point, which specifies the required size of heat exchangers and the port geometry and operating speed of the expander. Performance at off-design conditions can then be optimized within these constraints. This allows a rigorous investigation of the effect of air temperature variation on performance of WORC systems. The capability of the cycle model has been demonstrated for the case of power generation from a brine heat source at 120°C, assuming typical air temperature conditions for Nevada, USA. There are two main findings from the paper. Firstly, optimization of the WORC system using the annual average air temperature of 10.5°C achieves maximum power output with 75% dry working fluid at the inlet to the expander. Secondly, analysis of the off-design performance of the system shows that positive net power output is possible for air temperatures up to around 40°C. The estimated average power output over the course of a year was only 3.4% smaller than the power generated at the average annual temperature of 10.5°C. This confirms that a single calculation of WORC system performance using the average temperature for the region gives a good estimate of the expected average annual power output of the system. For the resource conditions assumed, screw driven WORC systems can be built with net power outputs of the order of 600kW, using standard size machines
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A numerical study of fluid-solid interaction in screw compressors
Efforts are continually being made to produce screw compressors with smaller clearances in order to reduce internal leakage. However, since the compression process induces large pressure differences across the rotors and temperature rise, they deform. A reliable method of estimating the interaction between fluid flow parameters and rotor deflection is thus needed in order to minimise clearances while avoiding contact between the rotors and the casing. A 3-D mathematical procedure is presented here to generate a numerical grid comprising both solid and fluid domains. This can be used to calculate the fluid flow and compressor structural deformation simultaneously by means of a suitable commercial numerical solver. Simulation results demonstrate the effects of change in working clearances, caused by rotor deformation, on compressor performance
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