Novel partial admission radial compressor for CO2 applications

A novel partial admission turbo compressor concept is proposed as an alternative to a conventional radial oil-free CO₂ compressor. The concept aims at the improvement of the overall performance through the reduction of the non-stage windage and cooling losses enabled by compression at significantly reduced shaft speeds. Transient CFD analysis gives fairly optimistic prediction of more than 80% of base stage efficiency at around 1.4 total pressure ratio. The study shows potential for efficiency improvement by optimization of the shape and number of blades. The conceptual compressor may be an interesting alternative for commercial CO₂ applications operating at close to critical pressures, provided that the deceleration of the gas in the diffuser is efficient.     

Oil free turbo-compressors for CO2 refrigeration applications

Feasibility of replacing oil-lubricated compressing equipment in CO₂ based refrigeration systems with oil-free turbo-machinery is assessed. Presented concept enables efficient compression for systems ranging from 0.1 to 5 MW of cooling capacity, provided that the operating pressures are low, i.e. 30/10 bar. Performance of the systems with higher operating pressures, i.e. 77/30 bar is penalized in wide range of capacities due to the excessive windage losses, especially pronounced in the systems with cooling capacities lower than 1 MW. In some cases, possibility of using longer motor should be analyzed. This may require special strategies for rotordynamic issues or driving each impeller with separately mounted motor. It is observed that optimal specific speed of the compressor stage does not always result in optimal overall performance. The trade-off between aerodynamic efficiency and non-stage losses must be found.     

Modeling of a semi-hermetic CO2 reciprocating compressor including lubrication submodels for piston rings and bearings

A comprehensive model for a semi-hermetic CO₂ reciprocating compressor is presented. This comprehensive model is composed of three main sub-models simulating the geometry and kinematics, the compression process, and frictional power loss. Valve and leakage sub-models are included in the compression process model. The frictional power loss model includes the friction at the bearings and between the piston ring and cylinder wall. The predicted results of the comprehensive model are validated using external compressor performance measurements of compressor input power and mass flow rate. The mass flow rate and compressor input power are predicted to within 4.03% and 6.43% mean absolute error, respectively, compared to the experimental datum. Additionally, a parametric study is presented which investigates compressor performance as a function of the stroke-to-bore ratio.     

A new structure and theoretical analysis on leakage and performance of an oil-free R290 rolling piston compressor

Lubricating oil improves the reliability of compressors and systems, whereas increases the system complexity. Compared with other types of compressors that have oil-free models, a rolling piston compressor has more leakage paths and bigger leakage loss. Therefore, the leakage is an important problem to be solved in order to develop an oil-free rolling piston compressor. The paper put forward a new structure of rotary compressor adopting a low pressure shell, connecting the cavities within piston and behind vane to the cavity at suction pressure and using radial compliance mechanisms. Then the leakage models were developed to calculate the mass flow rates within both the present rolling piston compressor without any oil as sealant and the new structure of oil free compressor. Results showed that by the new structure, the influences of leakage on the performance of a R290 oil free rolling piston compressor can be largely decreased.     

A novel linear electromagnetic-drive oil-free refrigeration compressor using R134a

A new type of oil-free moving magnet linear compressor with clearance seals and flexure springs has been designed for incorporation into a vapour compression refrigeration system with compact heat exchangers for applications such as electronics cooling. A linear compressor prototype was built with a maximum stroke of 14 mm and a piston diameter of 19 mm. An experimental apparatus was built to measure the compressor efficiencies and coefficient of performance (COP) of a refrigeration system with the linear compressor, using R134a. The resonant frequency for each operating condition was predicted using the discharge pressure, suction pressure and stroke. Refrigeration measurements were conducted for different strokes under each pressure ratio with a fixed condenser outlet temperature of 50 °C and evaporator temperature ranging from 6 °C to 27 °C. The results show that the COPs are around 3.0 for tests with a pressure ratio of 2.5 (evaporator temperature of 20 °C).     

Aerodynamic optimization of turbomachinery blades using evolutionary methods and ANN-based surrogate models

A fast, flexible, and robust simulation-based optimization scheme using an ANN-surrogate model was developed, implemented, and validated. The optimization method uses Genetic Algorithm (GA), which is coupled with an Artificial Neural Network (ANN) that uses a back propagation algorithm. The developed optimization scheme was successfully applied to single-point aerodynamic optimization of a transonic turbine stator and multi-point optimization of a NACA65 subsonic compressor rotor in two-dimensional flow, both were represented by 2D linear cascades. High fidelity CFD flow simulations, which solve the Reynolds-Averaged Navier-Stokes equations, were used in generating the data base used in building the ANN low fidelity model. The optimization objective is a weighted sum of the performance objectives and is penalized with the constraints; it was constructed so as to achieve a better aerodynamic performance at the design point or over the full operating range by reshaping the blade profile. The latter is represented using NURBS functions, whose coefficients are used as the design variables. Parallelizing the CFD flow simulations reduced the turn-around computation time at close to 100% efficiency. The ANN model was able to approximate the objective function rather accurately and to reduce the optimization computing time by ten folds. The chosen objective function and optimization methodology result in a significant and consistent improvement in blade performance.     

Electrochemical compressor driven metal hydride heat pump

In this study, a new metal hydride heat pump system driven by an electrochemical compressor was proposed. The system uses the electrochemical compressor to generate absorption heating and desorption cooling in two identical LaNi₅ reactors operating at different pressure levels. A thermodynamic model was developed to predict the system performance in terms of various parameters. Modeling shows EC compression efficiency has a great impact on system performance. The system is suitable for cooling application less than 200 W where mechanical compressors are the most inefficient. The high compression efficiency of electrochemical compressor could potentially make the cooling system more competitive than existing metal hydride heat pump systems.     

Loss-efficiency model of single and variable-speed compressors using neural networks

Compressor is the critical component to the performance of a vapor-compression refrigeration system. The loss-efficiency model including the volumetric efficiency and the isentropic efficiency is widely used for representing the compressor performance. A neural network loss-efficiency model is developed to simulate the performance of positive displacement compressors like the reciprocating, screw and scroll compressors. With one more input, frequency, it can be easily extended to the variable speed compressors. The three-layer polynomial perceptron network is developed because the polynomial transfer function is found very effective in training and free of over-learning. The selection of input parameters of neural networks is also found critical to the network prediction accuracy. The proposed neural networks give less than 0.4% standard deviations and ±1.3% maximum deviations against the manufacturer data.     

Thermodynamic analysis on the performance of a variable speed scroll compressor with refrigerant injection

A thermodynamic model for a variable speed scroll compressor with refrigerant injection was developed using continuity, energy conservation and real gas equation. The model included energy balance in the low-pressure shell compressor, suction gas heating, motor efficiency, and volumetric efficiency considering gas leakages as a function of compressor frequency. The developed model was verified by comparing the predicted results for the no injection condition with the experimental data. The deviations of the predicted from the measured values were within 10% for approximately 90% of the experimental data. Based on the model, mass flow rate, suction gas heating, cooling capacity and power consumption of the compressor were estimated and analyzed as a function of frequency. The effects of refrigerant injection on the performance of the compressor were also discussed as a function of frequency, injection conditions, and injection geometry.

Résumé

A thermodynamic model for a variable speed scroll compressor with refrigerant injection was developed using continuity, energy conservation and real gas equation. The model included energy balance in the low-pressure shell compressor, suction gas heating, motor efficiency, and volumetric efficiency considering gas leakages as a function of compressor frequency. The developed model was verified by comparing the predicted results for the no injection condition with the experimental data. The deviations of the predicted from the measured values were within 10% for approximately 90% of the experimental data. Based on the model, mass flow rate, suction gas heating, cooling capacity and power consumption of the compressor were estimated and analyzed as a function of frequency. The effects of refrigerant injection on the performance of the compressor were also discussed as a function of frequency, injection conditions, and injection geometry.     

Thermodynamic model for reciprocating compressors with the focus on fluid dependent efficiencies

Fluids with high global warming potential, which are used in existing refrigeration cycles and heat pumps, will have to be replaced soon by less harmful fluids, but the fluid selection is difficult especially due to the unknown compressor performance. In this work a differential compressor model for reciprocating compressors is introduced which predicts volumetric and isentropic efficiencies quickly and can be easily fitted with measured data at only one operation point of an existing compressor. In order to characterise the influence of different fluids two semi-physical correlations for the valve flows are fitted here, and a procedure of transferring them to different compressors is shown. The model is validated on, in total, 63 measured points based on numerous fluids from one semi-hermetic reciprocating compressor which is part of a heat pump cycle. The calculations lead to mean prediction errors of 3.0% for the isentropic and 2.3% for the volumetric efficiency.     

Robust design using Bayesian Monte Carlo

In this paper, we propose an efficient strategy for robust design based on Bayesian Monte Carlo simulation. Robust design is formulated as a multiobjective problem to allow explicit trade‐off between the mean performance and variability. The proposed method is applied to a compressor blade design in the presence of manufacturing uncertainty. Process capability data are utilized in conjunction with a parametric geometry model for manufacturing uncertainty quantification. High‐fidelity computational fluid dynamics simulations are used to evaluate the aerodynamic performance of the compressor blade. A probabilistic analysis for estimating the effect of manufacturing variations on the aerodynamic performance of the blade is performed and a case for the application of robust design is established. The proposed approach is applied to robust design of compressor blades and a selected design from the final Pareto set is compared with an optimal design obtained by minimizing the nominal performance. The selected robust blade has substantial improvement in robustness against manufacturing variations in comparison with the deterministic optimal blade. Significant savings in computational effort using the proposed method are also illustrated. Copyright © 2007 John Wiley & Sons, Ltd.     

Control optimisation of CO2 cycles for medium temperature retail food refrigeration systems

This paper describes a detailed procedure into the investigation of optimised control strategies for CO₂ cycles in medium temperature retail food refrigeration systems. To achieve this objective, an integrated model was developed composing of a detailed condenser/gas cooler model, a simplified compressor model, an isenthalpic expansion process and constant evaporating temperature and superheating. The CO₂ system can operate subcritically or transcritically depending on the ambient temperature. For a transcritical operation, a prediction can be made for optimised refrigerant discharge pressures from thermodynamic cycle calculations. When the system operates in the subcritical cycle, a floating discharge pressure control strategy is employed and the effect of different transitional ambient temperatures separating subcritical and transcritical cycles on system performance is investigated. The control strategy assumes variable compressor speed and adjustable air flow for the gas cooler/condenser to be modulated to achieve the constant cooling load requirement at different ambient conditions.     

直线压缩机用多孔质气体轴承的仿真与实验

采用气体轴承的主动耗气技术可以实现直线压缩机的无油润滑和非接触运行,以保证运行可靠性。为研究多孔质轴承的结构参数和压缩机设计参数对耗气量的影响,本文以R600a为制冷工质,建立了多孔质气体轴承模型,利用Fluent对耗气量进行了仿真模拟计算,基于该模型模拟分析了多孔质材料厚度、间隙气膜厚度、排气压力、压缩机频率和排量占比对气体轴承耗气量和耗气率的影响,并通过实验测试验证了该模型的准确性。结果表明：气体轴承耗气量的仿真结果和实验测量的误差在±15%以内。根据耗气率给出了最佳的设计参数组合,为直线压缩机用多孔质气体轴承优化设计提供了参考。     

基于转静干涉效应的压气机叶片气动载荷分析

转静叶排的相互作用会使压气机内部流场存在复杂的非定常性。为深入研究压气机叶片的气动载荷特性,以某型航空发动机压气机为研究对象,考虑叶排间的转静干涉效应,利用滑移网格技术对整个叶盘的三维流场展开模拟,求解干涉周期T_b内压气机转子内部的流动规律。同时对叶片气动载荷的非定常特性进行进一步分析,讨论了不同压比、转速对压气机叶片气动载荷的影响。结果表明叶片压力面和吸力面气动载荷波动峰值的主导频率皆为转静干涉频率f₀的倍频,其中一倍频（1×f₀）分量占主导地位。在干涉周期T_b内,叶片表面压力涡发生周期性的迁移与耗散,压力面和吸力面气动载荷的变化呈相反趋势。随着压比的增加,压气机叶片气动载荷逐渐增大,但其脉动幅值和频谱峰值基本不变。转速的升高使得转静干涉的频率增大,增强了压气机叶片气动载荷的非定常特性。研究结果能够应用于叶盘结构的气动优化设计,可为高性能航空发动机压气机的研制提供支持和参考。     

跨临界二氧化碳压缩机热力性能仿真与分析

针对跨临界二氧化碳半封闭式往复式活塞压缩机建立了一个通用数学模型,既包括热力学模块,也包括机械模块。热力学模块主要描述气缸内部的气体压缩过程。机械模块包括运动学模型和曲轴连杆机构模型,考虑了轴承上的功耗损失。采用一台压缩机样机对模型进行了不同运行工况下的实验验证,结果显示压缩机流量和耗功的最大误差分别不超过5%和8%。通过仿真分析了变结构和变工况条件下的压缩机性能,结果表明:在不同的运行工况下,存在最佳缸径行程比;容积效率和等熵效率都随着转速的增加而下降;吸排气阀门内径存在最佳值;对于容积效率的影响,吸气阀间隙比排气阀间隙更大,活塞与汽缸间隙比活塞环与汽缸间隙更大。     

Theoretical study of R32 in an oil flooded compression cycle with a scroll machine

R32 is regarded as a potential alternative for R410A, but it has a low slope of isentropic line, high superheat inside a compressor and thus a high discharge temperature. These disadvantages limit its wider adoption. In order to improve the performance of R32 air conditioner, oil flooded compression with regenerator has been suggested. A single stage oil flooded compressor model is developed to obtain a more accurate system-level improvement. In the compressor model, the heat transfer losses between shell and ambient, suction gas and motor, and high-pressure and low-pressure cylinders are considered. By means of parametric studies, it was found that the novel cycle resulted to be beneficial to increase the compressor internal superheating, to decrease the compressor heat losses and to improve its overall isentropic efficiency while cooling capacity or heating capacity is degraded. COP_h improvement can reach up to 16.4% for an evaporating and condensing temperatures of −25 °C and 45 °C, respectively. The discharge temperature resulted to be lower than 110 °C. In addition, a thorough comparison between R32 and R410A with both novel and baseline systems has been carried out. The results indicate that the novel cycle has potential benefits for applications in R32 air conditioners.     

Investigation on the effects of blade corrosion on compressor performance

Corrosion is a common reason for marine gas turbine blade failure, and salt spray is largely responsible for corrosion of metallic objects near the coastline. It will directly lead to geometrical deviations of the compressor blade, forcing an increase in roughness height of the surface of the blade. By using a three-dimensional numerical method, this paper deals with the corrosion of a 1.5 stage axial compressor. Two different schemes, respectively based on blade geometrical model modification and turbulent near wall functions, were employed in numerical simulations for corrosion rate of the blades. The compressor map, derived from the numerical simulations by adjusting the back pressure, shows that there is a decline of compressor efficiency and pressure ratio when the blade is corroded, which will result in performance degradation of the compressor. The corrosion properties of stainless steel were studied, using the static salt spray corrosion test under laboratory conditions. Additionally, combined with numerical methods, the compressor performance parameters along the extension of operating time were simulated and a binomial relationship between performance degradation and corrosion time was established. This research provides a technical guide for compressor performance prediction under less than ideal conditions.     

Performance analysis of SiO2/PAG nanolubricant in automotive air conditioning system

The performance of an automotive air conditioning system (AAC) needs to be enhanced in order to minimize its environment impact and reduce global fuel consumption. Implementing nanofluid technology using nanolubricants inside compressors can improve its performance. Therefore, this paper presents the development of an AAC system performance test rig. The SiO₂/PAG nanolubricant was prepared in a prior performance experiment and the stability of the colloidal was also attained. The experiment was conducted with initial refrigerant charge ranging from 95 to 125 gram and compressor speeds of 900 to 2100 RPM. The performance of the AAC system was evaluated by determining the heat absorb, compressor work and coefficient of performance (COP). The results found that the maximum increase and the average COP enhancement for SiO₂/PAG nanolubricants are 24% and 10.5%, respectively. It can be concluded that the COP was highest at 0.05% volume concentration for all compressor speeds.     

Potential of the single-screw compressor

The single-screw compressor has the potential to become the dominant form of compressor in the 50–150 kW power range. It is capable of high energy efficiency because losses due to leakage, frictional effects and heat transfer factors are capable of being reduced to a low level. A new gaterotor design for this purpose is described. Low cost is possible because the compressor is compact, the new gaterotor is less expensive to manufacture, the bearings can be simplified, and the casing can become a simple pressure vessel.     

Modelling of the non-reactive deposits impact on centrifugal compressor aerothermo dynamic performance

The centrifugal compressor blockage is considered an important issue in compressor operation and one of the main causes of machine failure. This is normally initiated by the presence of deposits with the process gas yielding to reduce the effective flow area, increase the frictional losses and distort the pressure distribution profile. The influences of flow blockage cover the thermodynamic, aerodynamic and rotordynamic performance parameters of centrifugal compressor as will be investigated further in this study. Accordingly, this paper introduces a novel approach to model the impact of non-reactive deposits flow on the centrifugal compressor aerothermodynamics performance. The developed set of empirical equations in this study provides a new way to derive the equivalent compressor performance map at various degrees of fouling with a consideration of gas properties and stage efficiency variation and without a prior knowledge of the detailed geometrical features. In order to emphasize the validity of the new method, it has been tested with two operating cases and the obtained results were compared with the internal inspection findings from the stage overhauling process. Besides, this approach has been proven to be valid for the modelling of flow blockage effect at the suction side, compressor internals and downstream equipment. Furthermore, a new methodology has been established to assess the possibility of deposits accumulation in the gas path of the compression system based on the operational data of the discharge parameters.