CFD simulation of a 300 Hz thermoacoustic standing wave engine

High frequency operation of standing wave thermoacoustic heat engines is attractive for space applications due to compact size and high reliability. To expedite practical use, further improvement and optimization should be based on deep understanding and quantitative analysis. This article focuses on using computational fluid dynamics (CFD) to investigate nonlinear phenomena and processes of a 300 Hz standing wave thermoacoustic engine (SWTE). The calculated model was tested in detail, which indicated that the co-axially stacked tube model was suitable for the simulation of SWTEs. Two methods of imposing temperature gradient across the stack were studied, and the processes of mean pressure increasing, pressure wave amplification and saturation were obtained under the thermal boundary condition of applying heating power. The acoustic fields were given, and the flow vortices and their evolution in both ends of the stack and resonator were observed. Moreover, a comparison between the simulation and experiments was made, which demonstrated the validity and power of the CFD simulation for characterizing complicated nonlinear phenomenon involved in the self-excited SWTEs.     

Experimental study of a thermoacoustically-driven traveling wave thermoacoustic refrigerator

New configurations of traveling wave thermoacoustic refrigerators driven by a traveling wave thermoacoustic engine were introduced and tested in this paper. First, the performance of the refrigerator with different-diameter inertance tubes was investigated experimentally. Then, investigation of substituting a flexible membrane attached to inertial mass for inertance tube was tested. The experimental results show that the substitution could improve the efficiency of the system and lead to a larger cooling power. So far, using helium gas as the working gas, the system could provide 340 W cooling power at the temperature of −20 °C with working frequency of 57 Hz and average pressure of 3.0 MPa. The total COP, i.e., cooling power divided by heating power, is 0.16.     

热声斯特林发动机热动力学特性的CFD研究——第一部分:热声自激振荡演化过程

采用商业计算流体动力学(CFD)软件Fluent6.0对热声斯特林发动机热动力学特性进行了热声自激振荡演化过程的数值模拟研究.主要研究了边界条件、初始条件和数值离散方法对模拟瞬态、可压缩、非线性的热声系统的重要影响,同时给出了两种在回热器内部建立温度梯度的方法,比较了它们对应的不同自激振荡演化过程.模拟结果表明,在高于功产生的临界温度梯度时,波动压力振幅被显著放大.本研究初步验证了该CFD研究的有效性.     

Investigation on the oscillation modes in a thermoacoustic Stirling prime mover: mode stability and mode transition

The purpose of this paper is to investigate the stability of oscillation modes in a thermoacoustic Stirling prime mover, which is a combination of looped tube and resonator. Two modes, with oscillation frequencies of 76 and 528 Hz, have been observed, stabilities of which are widely different. The stability of the high frequency mode (HFM) is affected by low frequency mode (LFM) strongly. Once the LFM is excited when the HFM is present, the HFM will be gradually slaved and suppressed by the LFM. The details of the transition from HFM to LFM have been described. The two stability curves of the two modes have been measured. Mean pressure P_m is an important control parameter influencing the mode stability in the tested system.     

氦气作为增压气体排出贮罐内液氢过程的CFD分析

基于计算流体力学方法,数值模拟了用常温氦气作为增压气体压出贮罐内液氢过程的流动和传热传质特性。构建了基于二维轴对称的VOF多相流以及包含氢气和氦气组分流动的气相多组分数值模型,液氢界面相变传质基于Hertz-Knudsen方程计算。分析了排出过程贮罐内压力、温度、液位及液氢相变率随时间的变化,重点考察气相出现在贮罐出口时间,以及此时气相中氦气含量。发现刚开始增压时,高温氦气和低温氢气传热只发生在氦气进口附近,贮罐内压力增加较慢,液氢界面不存在蒸发现象。随着进入氦气增加,贮罐内气相温度逐渐形成分层,在一定时刻,液面上气体温度开始上升,触发沸腾蒸发,导致压力快速增加。由于贮罐出口液体外流导致的减压效应远小于气相空间的压力增速,贮罐压力急剧增加并超过氦气入口,部分低温气体混合物从入口倒流出贮罐,同时使氦气入口处温度降低。由于贮罐内压力增加,底部液氢出口流量随时间呈线性增加。计算结果揭示了液氢贮罐增压流出过程复杂的流动和传热传质特性,对低温液体的储运有实际工程指导意义。     

气液耦合振动热声发动机的压力特性

采用气液耦合振动是提升热声发动机系统压力振幅以及降低谐振频率的有效方式.根据热声理论,对气液耦合振动热声发动机系统进行了模拟,重点讨论了平均工作压力对压力振幅、压比和谐振频率等性能参数的影响,分析了热声板叠产生声功率以及各部件消耗声功率随平均工作压力的变化情况.进行了相关实验,以验证模拟计算结果的合理性.模拟计算和实验数据均表明,增大平均工作压力可显著提升系统压力振幅,这对于利用其驱动后续负载是有利的.     

Effect of different working gases on the performance of a small thermoacoustic Stirling engine

The performance of a small thermoacoustic Stirling heat engine (TASHE) was investigated with three kinds of working gases experimentally and numerically. The examined performances focused on the operating frequency, onset temperature, pressure amplitude and some temperature characteristics after onset. The working frequency with nitrogen, argon and helium as the working gas was 45 Hz, 42 Hz and 130 Hz, respectively. The engine worked with helium in a much wider range of mean pressure than with nitrogen and argon. There was an optimal mean pressure for the minimum onset temperature for each working media. Using nitrogen and argon as working gas rather than helium, another optimal mean pressure for the highest pressure ratio was obtained in the experiment. The loop dimension was indispensable in determining the frequency and the highest pressure ratio was observed in the resonator cavity.     

液氮温区重力辅助深冷回路热管的实验研究

给出了液氮温区重力辅助深冷回路热管结构设计方案,建立了实验系统,对其启动特性和工作性能进行了实验研究.深冷回路热管以高纯氮作为工作液体,工作温区为90 K～126 K.实验结果表明,深冷回路热管能在重力作用下快速启动,在气体管线高于液体管线20 mm的情况下,最大可传送的功率为11W.     

混合工质热声发动机起振及消振特性试验研究

基于驻波热声发动机,开展了热声发动机在不同安装倾角下起振及消振特性的试验研究。试验结果表明,热声发动机的安装倾角对热声系统的起振温度、消振温度等参数的影响程度与充注工质种类有很大关系。当系统中充注1 MPa氮气时,在试验的5种角度下,最高起振温度484℃,最低起振温度428℃。当系统充注1 MPa氦气时,热声发动机的起振温度和消振温度并无明显变化。该特性为选择合适角度和工质以降低系统起振温度提供了试验参考,并为太阳能驱动的热声发动机的设计和工质的选择提供了试验依据。     

新型组合翅片换热器数值研究

通过对传统多排管翅式换热器的研究分析,提出第1排为平片且后部为开缝翅片的新型组合翅片换热器,并对传统和新型组合翅片2种形式换热器内的空气流动和换热进行数值分析。结果表明,新型组合翅片换热器的综合换热性能比传统翅片的提升5％～8％。     

Experimental and CFD study on a cyclonic classifier with new flow pattern

Physical principle of conventional top-inlet classifier (CTC) with reverse-flow pattern leads to the heavily fine particles entrainment in coarse fraction. Present work concentrates on the flow-field design for less downward airflow at near-wall region of the classifier. A new middle-inlet classifier (NMC) is proposed and analyzed using computational fluid dynamics (CFD) method and powder classification experiments. The results showed that new flow pattern characterized by a pair of vortexes was created in the new classifier. The upper vortex with 80% of the total air volume moves upward and forms the washing effect at near-wall region, which effectively reduces the fine particles entrainment in coarse fraction. The downer vortex with reverse-flow pattern discharges the coarse particles timely. The radial centrifugal sedimentation combined with the axial counter-current washing effect dominates the particle classification in the NMC. Compared to the CTC, classification accuracy index of the NMC with double-vortex averagely increases by 27% with a pressure drop reduction of more than 38%. This work offers a new principle for high-efficiency particle classification and new strategy for improving the classification performance of turbo air classifiers and hydrocyclones.     

不同工作气体的双作用热声发电机的热力性能研究

对一台三缸的双作用热声发电机进行了数值模拟,考察了不同工作介质下系统的热力性能。考察的气体工作介质包括氮气、二氧化碳、氩气、不同组分的氦氩混合气以及氢气。模拟计算表明,系统的热力性能与工质密切相关。采用氢气作为工作介质时,该热声发电机获得的电功最大,三个电机获得总电功最大可达5 145 W,此时系统的热电效率为22.8%。     

太阳能膜蒸馏热工质加热系统CFD计算分析

对太阳能膜蒸馏热工质加热系统进行了简化,建立加装导流板的全玻璃太阳能膜蒸馏热工质加热系统的物理模型,该模型包括全玻璃太阳能集热系统和膜蒸馏热腔两部分,提出二者的耦合集成点,得出耦合边界条件及其非稳态流动的设定方法,建立了太阳能膜蒸馏系统的三维非稳态CFD(计算流体动力学)计算模型,得出求解方法及思路,得到太阳能集热器联接管出口流体温度和流量的变化规律,与膜蒸馏热腔模型耦合计算,得出非稳态情况下膜通量与太阳能辐照强度的变化规律,为下一步太阳能利用与膜蒸馏的耦合优化奠定基础.     

液氢温区冷氦增压系统试验研究

针对某型号火箭冷氦增压系统,建设了液氢温区冷氦增压系统试验平台,通过试验得到了冷氦气源压力和温度、冷氦加热器性能、模拟贮箱压力和温度的变化规律.结果表明:低温制冷机组配合高压低温换热贮罐可以真实模拟箭上的冷氦气源;根据对增压过程中贮箱压力的分析表明排气方式可以真实模拟箭上推进剂消耗过程中贮箱压力的变化情况;另外,试验中压力信号器、电磁阀和贮箱工作正常,验证了火箭冷氦增压系统的可靠性.     

Experimental study on natural circulation precooling of cryogenic pump system with gas phase inlet reflux configuration

A novel gas-phase inlet configuration in the natural circulation system instead of the liquid-phase inlet is introduced to cool down a cryogenic pump system from room temperature to cryogenic temperatures, effectively. The experimental apparatus is illustrated and test process is described. Heat transfer and pressure drop data during the cool-down process are recorded and portrayed. By contrast with liquid-phase inlet configuration, experimental results demonstrate that the natural circulation with the gas-phase inlet configuration is an easier and more controllable way to cool down the pump system and maintain it at cryogenic temperatures.     

Investigation of drill pipe rotation effect on cutting transport with aerated mud using CFD approach

In drilling with aerated mud, appropriate hole cleaning has a great impact on decreasing the drilling time and cost. Thus, investigating the impact of diverse drilling parameters such as pipe rotation on the hole cleaning looks as a key. In this study, the combination of $k-\epsilon$ turbulent model and Eulerian-Eulerian multiphase model was used to investigate the three phase flow in the concentric annulus. Furthermore, the collision of cutting – cutting, cutting-drill pipe and cutting-well was considered in the simulations. In the first step, for validating the computational setup, the simulation outcomes were compared with the previous experimental works. The impact of pipe rotation on hole cleaning at various water and air flow rates, inclination and cutting sizes was scrutinized. The simulation results demonstrated that the impact of pipe rotation on the hole cleaning can be different depending on the different hole conditions. In the low inclination and air rates, rising the pipe rotation from 0 to 75 RPM brings about dropping the cutting concentration, but increasing the pipe rotation from 75 to 125 RPM comes about raising the cutting concentration. On the other hand, in low inclination and high flow rates of air, rising the pipe rotation causes a high accumulation of cutting. Lastly, at high inclination, the cutting concentration in annulus decreases with increasing the pipe rotation and this is independent of air and water flow rates. The simulation outcomes also showed that the impact of pipe rotation on hole cleaning at large cutting size is not as much as small cutting size.     

Numerical and experimental studies on stability of cryogenic turbo-expander with protuberant foil gas bearings

High speed turbo-expander is an essential refrigerating component in cryogenic systems. In addition to thermodynamic performance, its stability in transient high speed operation is also of great concern for its comprehensive performance. Oil free foil bearing has a great application potential in high speed turbomachinery by virtue of its high stability with no extra external supplying gas. In this paper, finite element model of a 150 Nm³/h cryogenic turbo-expander using foil bearing for air separation is configured for a case study. Influence of bearing structural stiffness and damping characteristics on the rotor-foil bearing system are evaluated for stable operation. Static structural stiffness and damping coefficients of the protuberant foil journal bearing and thrust bearings are obtained from the static loading and deflection tests. With the tested coefficients, rotordynamic performances of the rotor-bearing system is analyzed numerically and compared with the transient experimental results.     

Experimental study of hydrogen embrittlement on AISI 304 stainless steels and Rayleigh wave characterization

Many failures due to hydrogen embrittlement or hydrogen damage are widely reported in oil and refinery industry. Despite many ultrasonic testing methods have been developed to assess hydrogen embrittlement, they are applied well to serious hydrogen attack instead of earlier degradation. This paper aims to characterize nascent hydrogen embrittlement of AISI 304 austenitic stainless steels under cathodic hydrogenation using Rayleigh wave. After cathodic hydrogen charging of AISI 304 stainless steel, XRD and metallographic examination show that martensite transformation occurs within the subsurface region of the specimens. Microhardness testing indicates that hydrogen leads to hardening of the material. It is found that Rayleigh wave are better to inspect local degradation than bulk waves. Rayleigh wave velocity of 5 MHz and 10 MHz decreases significantly with cathodic charging time, while longitudinal wave velocity changes not. Acoustic velocity change is due to elastic modulus reduction resulting from hydrogen-induced phase transformation in the subsurface region.     

微通道冷凝器低温回路热管的实验研究

设计了一套以乙烷作为工质,使用微通道冷凝器的低温回路热管原理样机,并对样机的降温过程、传热性能以及再启动特性进行了实验研究。结果表明:在5 W的驱动功率下该低温回路热管可实现快速降温,在降温过程中通过增大蒸发器的加热功率可以加速回路热管的降温;该样机可以在200 K时能够稳定传输50 W的热量,并且随着加热功率的增大,回路热管的热阻不断减小;在回路热管停止工作后,重新施加热量,该样机仍能够正常启动;该样机在本实验中的最大传热功率为54W,此时的热阻为0.46 K/W。