Numerical investigations on unsteady flow of a scroll expander for compressed air energy storage

本工作以适应用于微型压缩空气储能（micro-CAES）系统的涡旋膨胀机为研究对象,采用计算流体力学（CFD）的方法对涡旋膨胀机工作过程进行非定常数值模拟,得到膨胀机内部温度场、压力场和速度场的分布,研究了吸气温度对涡旋膨胀机性能的影响规律及工作腔流场分布特点,结果显示：膨胀机吸气温度的升高,能够增加单位质量流量的输出功;随着吸气温度的下降,动涡旋盘所受轴向气体力增大,径、切向气体力减小;膨胀机工作过程中工作腔内的温度分布并不是沿涡旋盘半径方向逐渐下降,两侧背压腔存在较大的机械能损耗,背压腔温度会高于上游排气腔。该研究结果为涡旋膨胀机排气结构的设计提供了理论依据。     

三角转子膨胀机在有机朗肯循环中应用分析

基于中低温太阳能驱动的有机朗肯循环应用背景,分析三角转子机械作为膨胀机的运行特征,通过建立含泄漏影响的运行过程热力学模型对膨胀机性能进行计算,分析以R245fa为工质在单个膨胀周期内膨胀机内部的热力学性能,包括进气角度、形状因子、偏心距和轴距系数等型线设计参数对膨胀机性能的影响。结果显示,三角转子膨胀机可实现较大的膨胀压比,所讨论的因素会对质量流量、输出功率、膨胀压比和容积效率产生影响,适当选择结构参数可优化膨胀机的性能。     

Simulation and experimental research on energy conversion efficiency of scroll expander for micro‐Compressed Air Energy Storage system

A scroll expander was applied to the Micro‐Compressed Air Energy Storage system, and its energy conversion efficiency was investigated. In order to study the variation mechanism of the volume, mass, pressure and temperature of the air in different chambers, the mathematical model of the expansion process was developed on the base of the geometric model, mass conservation equation, ideal gas equation and energy conservation equation. Then, the mathematical model was implemented in Matlab, and the simulated energy conversion efficiency defined as the ratio between the output shaft power of the scroll expander and the input compressed air power was obtained. Furthermore, a test system was built in order to validate the mathematical model and study the improvement of the energy conversion efficiency. The prototypes of the scroll expander with different cross‐sectional areas of the intake port or the discharge port were fabricated and tested in the experiments. Results show that the simulated torque and energy conversion efficiency agree well with the experimental results. Also, there is a small deviation between the expansion process and the ideal isentropic process due to the gas leakage, intake and discharge loss. In addition, the air supply pressure and the cross‐sectional area ratio of the discharge port to the intake port are two important parameters for the improvement of the energy conversion efficiency. The experiments show that the energy conversion efficiency varies from 23% to 36% at the air supply pressure of 0.35 to 0.65 MPa, indicating that it is proportional to the air supply pressure. It can also be concluded from the experiments that when the air pressure is higher than 0.45 MPa, the ideal ratio range can be determined as 0.6‐0.8. Copyright © 2013 John Wiley & Sons, Ltd.     

Applicability of scroll expander and compressor to an external power engine: Conceptual design and performance analysis

A scroll expander and scroll compressor for a 10‐kW class Ericsson engine utilizing solar energy as heat source has been suggested. Orbiting scroll members of the expander and compressor were designed to have a base plate with double‐sided wrap structure for compact size and axial gas force cancellation. For axial compliance of the scroll machines, a back pressure chamber was created on each fixed scroll. In order to compensate for the thermal expansion, flexible coupling between the orbiting scroll base plate and the scroll plate holders and sliding keys between the fixed scroll and the supporting frame were designed. Common shafts were shared by the expander and compressor for direct power transmission. For high‐ and low‐side pressures of 6 and 2.5 MPa, respectively, and expander inlet temperature of 700°C, the proposed engine efficiency was estimated to be 7.3%. The engine efficiency strongly depends on the expander and compressor efficiencies as well as on the regenerator efficiency. The shaft output of the designed scroll expander was calculated to be 38.84 kW, while the input power for the scroll compressor was 27.97 kW, yielding 10.87 kW for the engine output. Copyright © 2011 John Wiley & Sons, Ltd.     

Testing and modeling a scroll expander integrated into an Organic Rankine Cycle

Organic Rankine Cycles (ORC’s) are particularly suitable for recovering energy from low-grade heat sources. This paper first presents the results of an experimental study carried out on a prototype of an open-drive oil-free scroll expander integrated into an ORC working with refrigerant HCFC-123. By exploiting the overall expander performance measurements, the eight parameters of a scroll expander semi-empirical model are then identified. The model is able to compute variables of first importance such as the mass flow rate, the delivered shaft power and the discharge temperature, and secondary variables such as the supply heating-up, the exhaust cooling-down, the ambient losses, the internal leakage and the mechanical losses. The maximum deviation between the predictions by the model and the measurements is 2% for the mass flow rate, 5% for the shaft power and 3 K for the discharge temperature. The validated model of the expander is finally used to quantify the different losses and to indicate how the design of the expander might be altered to achieve better performances. This analysis pointed out that the internal leakages and, to a lesser extent, the supply pressure drop and the mechanical losses are the main losses affecting the performance of the expander.     

涡旋式膨胀机的非线性数学模型及动态仿真

为了设计涡旋式膨胀机控制系统,根据涡旋式膨胀机机理分析,以膨胀机的进气管道、进气腔、膨胀腔和排气腔等组建的静态数学关系为基础,建立了阀门开度与膨胀机进气管道压力以及膨胀机内部功率与转子转速两组非线性微分方程,通过膨胀机进气管道压力与膨胀机内部功率之间的关系,得到了阀门开度与转子转速的动态模型．仿真结果表明：所建立的动态模型符合涡旋式膨胀机的特性,在一定程度上证明了模型的正确性和有效性．     

Experimental investigation of a scroll expander for an organic Rankine cycle

This paper presents experimental investigation of the performance of an organic Rankine cycle (ORC) with scroll expander which utilizes renewable, process and waste heats. An ORC test bench is built with a scroll expander‐generator unit modified from a refrigeration compressor‐electrical drive unit. A detailed experimental investigation within the test bench is performed with the organic working fluid R134a. The results show that scroll expander can effectively be used in low‐power ORC to generate mechanical work or electricity from low‐temperature thermal sources (e.g. 80–200 °C, respectively). The experiments are performed under fixed intake conditions into the expander. The pressure ratio and the load connected to the expander‐generator unit were varied. It is found that an optimum pressure ratio and an optimum angular speed co‐exist. When operating optimally, the expander's isentropic efficiency is the highest. The optimum angular speed is around 171 rad/s which corresponds to a generated voltage of 18.6 V. The optimum pressure ratio is about 4. The isentropic efficiency at optimum operation is found in the range of 0.5 to 0.64, depending on the intake conditions. The volumetric efficiency overpasses 0.9 at optimum operation and degrades significantly if the load is increased over the optimum load. A regenerative ORC equipped with the studied expender‐generator unit that operates under 120 °C heat source and has an air cooled condenser generates 920 W net power with efficiencies of 8.5% energetically and 35% exergetically. Copyright © 2014 John Wiley & Sons, Ltd.     

基于CFD数值模拟下的蒸汽喷射器性能预测

建立了蒸汽喷射器热力学模型,并验证了该模型性能及结构计算的可靠性。并针对实际情况中,对该模型进行了动态模拟。结果显示：当工作流体的温度升高时,工质流体的质量流量都会增加,引射系数存在峰值,峰值所对应温度为热力学模型设定温度值;当引射流体的温度升高时,其质量流量也会随之增大,而工作流体的质量流量则较稳定,因此引射流体温度与压力的升高可以改善喷射器的性能;当背压升高时,在一定压力范围内,工质流体的质量流量都趋于稳定,而当背压超过热力学模型设定背压值时,引射流体的质量流量便随背压的升高而急剧下降,喷射器性能严重恶化,故认为该压力值为喷射器的临界背压。本文研究结果对喷射器的设计计算具有一定的指导作用。 关键词：蒸汽喷射器;热力学模型;数值模拟;引射系数     

气膜冷却对高压涡轮叶栅损失特性的影响研究

为获得全气膜气冷涡轮叶栅的损失特性,采用试验及数值仿真方法,研究了不同冷气流量、不同叶栅出口马赫数条件下冷气射流对叶栅损失的影响。通过叶栅槽道静压云图及叶片表面压力分布等试验及数值仿真结果对比,验证了通冷气叶栅性能仿真分析方法的准确性。结果表明：同一冷气流量比下,通冷气叶栅能量损失系数随着马赫数的增大先减小后增大,在设计马赫数附近损失最低;通冷气叶栅能量损失系数随着冷气流量的增大而增大,且前后腔均通冷气时能量损失系数最大,前腔单独通冷气时能量损失系数最小;通冷气叶栅能量损失系数随着冷气与主流温比增大而增大。     

低品位热能驱动有机朗肯循环的变工况特性

构建有机朗肯循环变工况分析模型,研究热源条件对系统变工况性能的影响规律。结果表明:随着热源温度升高,系统的最佳蒸发压力线性增大,而涡旋膨胀机的等熵效率逐渐减小。相比额定工况,热源温度变化-30.0K与30.0K时,净输出功率变化了-32.4%与18.4%,热效率降低了4.0%与11.4%,热回收效率变化幅度分别为-9.8%及8.9%;当热源温度从423增大至483K时,系统不可逆损失的变化率为-37.1%与45.5%,火用效率的变化率为6.7%与-17.5%。相比热源流量,热源温度对系统变工况性能的影响更大。     

耦合有机朗肯循环的液化空气储能系统性能研究

为解决液化空气储能系统(LAES)压缩热利用不完全的问题,构建了耦合有机朗肯循环的液化空气储能系统(ORC-LAES)。对ORC-LAES系统建立热力学性能计算模型,在设计参数下分析压缩机出口压力、膨胀机入口压力、加压水初温、加压水流量比及膨胀机级数对ORC-LAES系统性能的影响。结果表明,当压缩机出口压力由6 MPa上升到16 MPa、加压水初温从293 K上升到323 K时,系统的循环效率、火用效率和液化率均下降;当膨胀机入口压力由8 MPa上升到18 MPa时,系统循环效率和火用效率均增加;当加压水流量比由0.51上升到0.96时,系统循环效率和火用效率先增加再减少,流量比为0.71时,系统的循环效率和火用效率达到最大;在压缩热利用上耦合有机朗肯循环要优于增加膨胀机级数;ORC-LAES系统与LAES系统相比,循环效率提高4.8%,火用效率提升5.1%。     

Transcritical CO2 refrigerator and sub‐critical R134a refrigerator: A comparison of the experimental results

This paper describes experiments comparing a commercial available R134a refrigeration plant subjected to a cold store and a prototype R744 (carbon dioxide) system working as a classical ‘split‐systems’ to cool air in residential applications in a transcritical cycle. Both plants are able to develope a refrigeration power equal to 3000 W. The R744 system utilizes aluminium heat exchangers, a semi‐hermetic compressor, a back‐pressure valve and a thermostatic expansion valve. The R134a refrigeration plant operates using a semi‐hermetic reciprocating compressor, an air condenser followed by a liquid receiver, a manifold with two expansion valves, a thermostatic one and a manual one mounted in parallel, and an air cooling evaporator inside the cold store. System performances are compared for two evaporation temperatures varying the temperature of the external air running over the gas‐cooler and over the condenser. The refrigeration load in the cold store is simulated by means of some electrical resistances, whereas the air evaporator of the R744 plant is placed in a very large ambient. The results of the comparison are discussed in terms of temperature of the refrigerants at the compressor discharge line, of refrigerants mass flow rate and of coefficient of performance (COP). The performances measured in terms of COPs show a decrease with respect to the R134a plant working at the same external and internal conditions. Further improvements regarding the components of the cycle are necessary to use in a large‐scale ‘split‐systems’ working with the carbon dioxide. Copyright © 2009 John Wiley & Sons, Ltd.     

夏季工况条件下空调/冷柜一体机性能实验研究

空调冷柜一体机系统是通过中间冷却器将空调与冷柜耦合,可以将空调系统中的部分制冷剂节流至中间冷却器对冷柜系统中的制冷剂进行过冷以提升其系统性能。实验研究了夏季工况条件下冷柜温度、室外环境温度及质量流量比对一体机系统制冷量及COP的影响。实验结果表明:在夏季工况条件下,冷柜系统的制冷量和COP随质量流量比的增大而增大,但质量流量比大于12%后其增速放缓;空调系统制冷量随质量流量比的增大而减小,而其COP随质量流量比的增大而略有增大。综合分析认为夏季工况条件下,质量流量比控制在8%～12%时可以提高空调及冷柜系统COP,同时空调器制冷量衰减也较小。     

Numerical model for predicting thermodynamic cycle and thermal efficiency of a beta-type Stirling engine with rhombic-drive mechanism

This study is aimed at development of a numerical model for a beta-type Stirling engine with rhombic-drive mechanism. By taking into account the non-isothermal effects, the effectiveness of the regenerative channel, and the thermal resistance of the heating head, the energy equations for the control volumes in the expansion chamber, the compression chamber, and the regenerative channel can be derived and solved. Meanwhile, a fully developed flow velocity profile in the regenerative channel, in terms of the reciprocating velocity of the displacer and the instantaneous pressure difference between the expansion and the compression chambers, is derived for calculation of the mass flow rate through the regenerative channel. In this manner, the internal irreversibility caused by pressure difference in the two chambers and the viscous shear effects due to the motion of the reciprocating displacer on the fluid flow in the regenerative channel gap are included. Periodic variation of pressures, volumes, temperatures, masses, and heat transfers in the expansion and the compression chambers are predicted. A parametric study of the dependence of the power output and thermal efficiency on the geometrical and physical parameters, involving regenerative gap, distance between two gears, offset distance from the crank to the center of gear, and the heat source temperature, has been performed.     

Power and efficiency optimization for combined Brayton and inverse Brayton cycles

A thermodynamic model for open combined Brayton and inverse Brayton cycles is established considering the pressure drops of the working fluid along the flow processes and the size constraints of the real power plant using finite time thermodynamics in this paper. There are 11 flow resistances encountered by the gas stream for the combined Brayton and inverse Brayton cycles. Four of these, the friction through the blades and vanes of the compressors and the turbines, are related to the isentropic efficiencies. The remaining flow resistances are always present because of the changes in flow cross-section at the compressor inlet of the top cycle, combustion inlet and outlet, turbine outlet of the top cycle, turbine outlet of the bottom cycle, heat exchanger inlet, and compressor inlet of the bottom cycle. These resistances control the air flow rate and the net power output. The relative pressure drops associated with the flow through various cross-sectional areas are derived as functions of the compressor inlet relative pressure drop of the top cycle. The analytical formulae about the relations between power output, thermal conversion efficiency, and the compressor pressure ratio of the top cycle are derived with the 11 pressure drop losses in the intake, compression, combustion, expansion, and flow process in the piping, the heat transfer loss to the ambient, the irreversible compression and expansion losses in the compressors and the turbines, and the irreversible combustion loss in the combustion chamber. The performance of the model cycle is optimized by adjusting the compressor inlet pressure of the bottom cycle, the air mass flow rate and the distribution of pressure losses along the flow path. It is shown that the power output has a maximum with respect to the compressor inlet pressure of the bottom cycle, the air mass flow rate or any of the overall pressure drops, and the maximized power output has an additional maximum with respect to the compressor pressure ratio of the top cycle. When the optimization is performed with the constraints of a fixed fuel flow rate and the power plant size, the power output and efficiency can be maximized again by properly allocating the fixed overall flow area among the compressor inlet of the top cycle and the turbine outlet of the bottom cycle.     

Experimental investigation on the internal working process of a CO2 rotary vane expander

This paper presents the experimental investigation on the internal working process especially the internal leakage of a rotary vane expander prototype, which was developed to replace the throttling valve to improve the Coefficient of Performance (COP) of the transcritical CO₂ refrigeration cycle. The pressure diagram as a function of the rotation angle (p–θ diagram) was recorded by two pressure sensors arranged within the expansion chamber, based on which the features of the working process were analyzed and effects of the some improvement measures on the internal leakage were discussed. Compared with the ideal p–θ diagram, the recorded diagrams presented more rapid decrease in the pressure during expansion process, which was attributed to serious leakage within the expander. Further analysis of the recorded p–θ diagrams showed that three adjacent vanes instead of two formed an integrated working chamber, which implied that the in-between vane did not contact the cylinder wall at all, and this guess was proved by the high speed video recording of the running rotor together with the vanes during operation of the expander in CO₂ system. By arranging springs in the vane slots, tight contact between the vanes and the cylinder wall was ensured and hereby the working process was improved. The gap between the suction and discharge ports was another important leakage path and installation of a seal there could increase the pressure difference through the expander by 1.5–2.5 MPa by decreasing the leakage directly from the high-pressure suction chamber to the low-pressure discharge chamber. Performance test of the modified expander prototype with springs in the slots and seal at the seal arc showed that the volumetric efficiency increased from 17% to 30% and the isentropic efficiency from 9% to 23% at the speed of 800 rpm.     

Performance analysis in near-critical conditions of organic Rankine cycle

According to fluid critical temperature and heat source temperature, organic Rankine cycle (ORC) is recognized in two categories: subcritical ORC and supercritical ORC. For a given heat source, some organic fluids not only can be used in subcritical ORC, but also can be used in supercritical ORC. For heat source with temperature of 90 °C, HFC125, HFC143a and HF218 can be used in both ORCs. Performance of the three substances in both cycles, especially in near-critical conditions is studied with expander inlet temperature of 85 °C and hot water mass flow rate of 1 kg/s. The results show that when fluids go in supercritical ORC from subcritical ORC, cycle thermal efficiency varies continuously, while mass flow rate and net power generation vary discontinuously. Maximum net power generation in near-critical conditions of subcritical ORC is higher than that of supercritical ORC. For HFC125 and HFC143a, outlet temperature of hot water decreases with the increase of heating pressure ratio. For HF218, outlet temperature of hot water increases firstly and decreases secondly with the increase of heating pressure ratio, which leads to an increase of net power generation with the increase of heating pressure ratio in high heating pressure ratio conditions.     

Development of a double acting free piston expander for power recovery in transcritical CO2 cycle

To replace the throttling valve with an expander is considered as an efficient method to improve the performance of the transcritical CO₂ refrigeration cycle. This paper presents the design and experimental validation of a double acting free piston expander, in which a slider-based inlet/outlet control scheme is used to realize a full expansion process for the expander. The power extracted from the expansion process is utilized by an auxiliary compressor, which is arranged in parallel with the main compressor. A design model is developed to determine the geometric parameters of the expander together with the auxiliary compressor. An expander prototype is manufactured and validated experimentally in the air test system, mainly by means of analyzing the dynamic pressures in the expander chamber. The experimental results show that the expander can work stably in a wide range of pressure differences/ratios at the frequency approximately linear with the pressure difference through the expander. The p–t diagrams in the expander indicate that the slider-based inlet/outlet control scheme enables the expander to have the proper suction, expansion and discharge processes. However, the prototype at high frequency doesn’t present isobaric suction process, which results in insufficient gas suction and therefore decrease in the expander efficiency. With the p–t diagrams at various frequencies compared, the optimal working frequency is found to range from 10 to 17 Hz in the air system. The isentropic efficiency of 62% is obtained from the p–V diagram analysis. Further validation of the expander in the CO₂ system will be conducted in the near future.     

影响Zeuch法喷油速率测量精度的关键因素研究

为提高汽油直喷喷油器喷油速率的测量精度,基于Zeuch法原理开发了喷油速率测量系统。研究发现容器测量腔内容器刚度和含气量都会对Zeuch喷油速率测量精度产生重要影响,同时通过试验与仿真计算对这两个关键因素进行定量的系统的研究。结果表明:低刚度容器在相同喷油条件下体积膨胀相对高刚度容器更大,由此测得的喷油速率曲线相对偏低,测量精度降低;当含气量较大时,腔内压力变化曲线中的压力增量呈线性减小,波动振幅增大,同时波动频率减小,这会导致喷油速率与实际值相比偏低且波动较大。基于研究结果,提出相关修正方法来提高喷油速率测量精度。经优化后的喷油速率测量系统的测量精度大大提高,具有较高的重复性。     

旋流式气液同轴式喷油器在加压空间中流量特性的试验研究

在增压条件下对一种旋流式气液同轴喷油器的流量特性进行试验研究.在不同的环境背压条件下,分别研究了气液比、环境背压对燃油流量系数、雾化空气流量系数的影响.结果表明:本喷油器的燃油流量系数稳定在0.2971,不受气液比、环境背压等因素的影响;雾化空气流量系数随环境背压与喷油器雾化空气通道进口压力的压力比的增大而减小,与气液比无关.根据试验数据整理出雾化空气流量系数拟合公式,能够对试验数据作出准确的预测.