水平微细管道内R717沸腾换热特性研究

搭建了氨(R717)沸腾换热测试台,对内径3 mm水平光管内R717的沸腾换热特性进行了测试,分析热流密度、干度、饱和温度及质量流率对沸腾换热及换热方式的影响。实验热流密度15～40 kW/m~2,质量流率40～160 kg/(m~2·s),饱和温度-5、0和5℃,干度0.1～0.9。结果表明:在氨制冷剂管内沸腾换热的过程中,质量流率过低和热流密度过高会导致干涸传热恶化,换热形式由核态沸腾换热向气态氨制冷剂强制对流换热转变,同时也影响干涸的起始干度;在干涸发生前,沸腾换热系数随着干度的增加而增大,逐渐达到峰值;在干涸发生后,传热恶化导致换热系数急剧降低;饱和温度升高会加快核态沸腾气泡生成速率,强化沸腾换热,但干涸的起始干度随着饱和温度升高而降低。     

R32与R290在小管径内的沸腾压降与换热特性研究

搭建5 mm内螺纹小铜管内水平单管沸腾实验台,实验工况为饱和蒸发温度20℃,热流密度25 kW/m~2,质量流速100～300 kg/(m~2·s),干度0.1～0.9。对比研究替代制冷剂R32与R290在不同干度和质量流速下的沸腾压降与换热特性。结果表明:两种制冷剂的摩擦压降均随质量流速的增加而显著增大,R290的摩擦压降比R32平均大59.4%,且随着干度的增大摩擦压降的差值呈上升趋势。R32与R290的沸腾换热系数与质量流速呈正相关,在0.1～0.6中低干度区域内,R32的沸腾换热系数明显大于R290,但在干度大于0.6的高干度区域内,R32的沸腾换热系数仅比R290大9.8%,两者数值较为接近。通过对SunMishima压降关联式与Li换热关联式进行修正,提高了关联式的准确性,修正后实验值与预测值的压降平均相对偏差为15.93%,沸腾换热系数平均相对偏差为16.71%。     

一个水基SiO2纳米流体核态池沸腾数学模型

基于水基SiO_2纳米流体沸腾实验研究结果,在双流体多相流模型和热流分区模型(RPI模型)的基础上建立了一个水基SiO_2纳米流体核态池沸腾数学模型。结果表明:汽化核心密度、气泡脱离直径和纳米流体润湿角是纳米流体换热性能提升的主要原因,所建立的新的汽化核心密度、气泡脱离直径模型预测结果与实验数据吻合非常好,证明该模型的可靠性,给精确预测水基SiO_2纳米流体核态沸腾换热特性提供了重要参考。     

“球囊夹紧法”取出锁骨下动脉支架推送杆断裂残端一例

针对水平光滑管和微肋管,基于FLUENT平台对制冷剂管内沸腾传热特性进行了数值模拟,研究质量流量、热流密度及干度等因素对制冷剂R245fa沸腾换热系数的影响。模拟结果表明:沸腾换热系数随着制冷剂质量流速与热流密度的增加而提高;随着干度的增加,换热系数先增加再降低,并在x=0.7时达到极大值;相比光滑管,微肋管内制冷剂的沸腾传热系数能提高10%~25%。     

水平管内制冷剂沸腾传热特性的数值模拟

针对水平光滑管和微肋管,基于FLUENT平台对制冷剂管内沸腾传热特性进行了数值模拟,研究质量流量、热流密度及干度等因素对制冷剂R245fa沸腾换热系数的影响。模拟结果表明:沸腾换热系数随着制冷剂质量流速与热流密度的增加而提高;随着干度的增加,换热系数先增加再降低,并在x=0.7时达到极大值;相比光滑管,微肋管内制冷剂的沸腾传热系数能提高10%~25%。     

不凝结气体对多壁碳纳米管换热表面的池沸腾性能的影响

为揭示不凝结气体对多壁碳纳米管（Multi-walled Carbon Nanotube, MWCNT）纳米结构表面核态池沸腾过程的影响，使用气体沉积法（Chemical Vapor Deposition, CVD）在硅表面制作MWCNT纳米结构表面，并使用光滑硅表面进行对比实验研究。实验操作中，将驱气前后的工作液体应用于两种表面的池沸腾实验，换热表面过热度控制在0℃-35℃，工作液体过冷度分为40℃和50℃。实验结果表明，液体中含气量的变化对MWCNT纳米结构表面影响较小，而对光滑硅表面的影响较大；对比硅表面，MWCNT纳米结构表面能够有效提升沸腾传热效果，对于驱气后的工作液体提升效果更为明显。     

微细管内R290流动沸腾换热与干涸特性研究

对内径为1、2、3 mm的水平不锈钢圆管内R290两相流动沸腾换热特性进行了理论与实验研究。分析了热流密度为15~35 kW/m~2、质量流率为76~200 kg/(m~2·s)、饱和温度为16~36℃、干度为0~1时的管内传热特性。研究结果表明:热流密度的增加促进管内核态沸腾,换热得到强化,从而导致换热系数随之增加;质量流率的增加促进管内由核态沸腾换热向对流换热转化,换热系数也随之增加;饱和温度的增加促进管内气泡核心的形成速率加快,强化管内沸腾换热;管径的减小导致微尺度效应增加,从而导致换热系数随之增加;在整个换热过程中干涸前平均换热系数、干涸过程中的平均换热系数分别占总换热系数的40%、37%。     

预热微通道内制冷剂对沸腾换热系数影响的实验研究

在内径为2 mm的水平不锈钢微通道内对制冷剂R290的沸腾换热特性进行了实验研究。实验工况为:制冷剂质量流率分别为150和330 kg/(m~2·s),测试段热流密度分别为43和76 k W/m~2,制冷剂干度的范围为0.1-0.7,测试段制冷剂的饱和温度为15和26℃,测试段制冷剂的入口干度范围为0-0.65。在相同干度情况下,将制冷剂进入测试段前未进行预热而获得的换热系数与制冷剂进入测试段前进行预热后获得的换热系数进行了对比。研究结果表明:制冷剂进入测试段前进行预热会使换热系数产生偏差,偏差的平均值达到了14.2%;在实验范围内,随着制冷剂在测试段入口以及制冷剂在测试段内干度的上升,预热所引起的换热系数偏差将逐步下降。     

强润湿性液体池沸腾传热的实验研究和机理分析

对强润湿性液体的池沸腾传热实验而言，本文提出了行之有效的实验程序，并严格按照实验程序进行了R113池沸腾传热的实验研究，具体研究了表现老化和液体过冷度对池沸腾传热曲线及起沸点的影响，实验中观察到了三个反常现象，最后，从强润湿性液体的沸腾传热机理的角度对其给出了相应的解释。     

CO_2单管池沸腾换热实验研究

在CO2跨临界水-水热泵试验台添加池沸腾实验段,进行CO2单管池沸腾换热实验,用威尔逊分析法对实验数据进行处理。研究表明:CO2池沸腾换热效果优于管内沸腾换热效果;同时,结合实验数据,拟合在饱和压力3.2和3.4MPa下的CO2池沸腾换热系数与热流密度和对比压力之间的关联式。     

Performance assessment of some ice TES systems

In this paper, a performance assessment of four main types of ice storage techniques for space cooling purposes, namely ice slurry systems, ice-on-coil systems (both internal and external melt), and encapsulated ice systems is conducted. A detailed analysis, coupled with a case study based on the literature data, follows. The ice making techniques are compared on the basis of energy and exergy performance criteria including charging, discharging and storage efficiencies, which make up the ice storage and retrieval process. Losses due to heat leakage and irreversibilities from entropy generation are included. A vapor-compression refrigeration cycle with R134a as the working fluid provides the cooling load, while the analysis is performed in both a full storage and partial storage process, with comparisons between these two. In the case of full storage, the energy efficiencies associated with the charging and discharging processes are well over 98% in all cases, while the exergy efficiencies ranged from 46% to 76% for the charging cycle and 18% to 24% for the discharging cycle. For the partial storage systems, all energy and exergy efficiencies were slightly less than that for full storage, due to the increasing effect wall heat leakage has on the decreased storage volume and load. The results show that energy analyses alone do not provide much useful insight into system behavior, since the vast majority of losses in all processes are a result of entropy generation which results from system irreversibilities.     

Natural-gas fueled spark-ignition (SI) and compression-ignition (CI) engine performance and emissions

Natural gas is a fossil fuel that has been used and investigated extensively for use in spark-ignition (SI) and compression-ignition (CI) engines. Compared with conventional gasoline engines, SI engines using natural gas can run at higher compression ratios, thus producing higher thermal efficiencies but also increased nitrogen oxide (NO_x) emissions, while producing lower emissions of carbon dioxide (CO₂), unburned hydrocarbons (HC) and carbon monoxide (CO). These engines also produce relatively less power than gasoline-fueled engines because of the convergence of one or more of three factors: a reduction in volumetric efficiency due to natural-gas injection in the intake manifold; the lower stoichiometric fuel/air ratio of natural gas compared to gasoline; and the lower equivalence ratio at which these engines may be run in order to reduce NO_x emissions. High NO_x emissions, especially at high loads, reduce with exhaust gas recirculation (EGR). However, EGR rates above a maximum value result in misfire and erratic engine operation. Hydrogen gas addition increases this EGR threshold significantly. In addition, hydrogen increases the flame speed of the natural gas-hydrogen mixture. Power levels can be increased with supercharging or turbocharging and intercooling. Natural gas is used to power CI engines via the dual-fuel mode, where a high-cetane fuel is injected along with the natural gas in order to provide a source of ignition for the charge. Thermal efficiency levels compared with normal diesel-fueled CI-engine operation are generally maintained with dual-fuel operation, and smoke levels are reduced significantly. At the same time, lower NO_x and CO₂ emissions, as well as higher HC and CO emissions compared with normal CI-engine operation at low and intermediate loads are recorded. These trends are caused by the low charge temperature and increased ignition delay, resulting in low combustion temperatures. Another factor is insufficient penetration and distribution of the pilot fuel in the charge, resulting in a lack of ignition centers. EGR admission at low and intermediate loads increases combustion temperatures, lowering unburned HC and CO emissions. Larger pilot fuel quantities at these load levels and hydrogen gas addition can also help increase combustion efficiency. Power output is lower at certain conditions than diesel-fueled engines, for reasons similar to those affecting power output of SI engines. In both cases the power output can be maintained with direct injection. Overall, natural gas can be used in both engine types; however further refinement and optimization of engines and fuel-injection systems is needed.     

Effect of co-cultivation of Chlamydomonas reinhardtii with Azotobacter chroococcum on hydrogen production

Chlamydomonas reinhardtii cc124 and Azotobacter chroococcum bacteria were co-cultured with a series of volume ratios and under a variety of light densities to determine the optimal culture conditions and to investigate the mechanism by which co-cultivation improves H₂ yield. The results demonstrated that the optimal culture conditions for the highest H₂ production of the combined system were a 1:40 vol ratio of bacterial cultures to algal cultures under 200 μE m^?2 s^?1. Under these conditions, the maximal H₂ yield was 255 μmol mg^?1 Chl, which was approximately 15.9-fold of the control. The reasons for the improvement in H₂ yield included decreased O₂ content, enhanced algal growth, and increased H₂ase activity and starch content of the combined system.     

Decomposition of limestone in a solar reactor

The thermal decomposition of limestone has been selected as a model reaction for developing and testing an atmospheric open solar reactor. The reactor consists of a cyclone gas/particle separator which has been modified to let the concentrated solar energy enter through a windowless aperture. The reacting particles are directly exposed to the solar irradiation. Experimentation with a 60 kW reactor prototype was conducted at PSI's 90m² parabolic solar concentrator, in a continuous mode of operation. A counter-current flow heat exchanger was employed to preheat the reactants. Eighty five percent degree of calcination was obtained for cement raw material and 15% of the solar input was converted into chemical energy (enthalpy).The technical feasibility of the solar thermal decomposition of limestone was experimentally demonstrated. The use of solar energy as a source for high-temperature process heat offers the potential of reducing significantly the CO₂ emissions from lime producing plants. Such a solar thermochemical process can find application in sunny rural areas for avoiding deforestation.     

Exergetic evaluation of 5 biowastes-to-biofuels routes via gasification

This paper presents the exergy analysis results for the production of several biofuels, i.e., SNG (synthetic natural gas), methanol, Fischer–Tropsch fuels, hydrogen, as well as heat and electricity, from several biowastes generated in the Dutch province of Friesland, selected as one of the typical European regions. Biowastes have been classified in 5 virtual streams according to their ultimate and proximate analysis. All production chains have been modeled in Aspen Plus in order to analyze their technical performance. The common steps for all the production chains are: pre-treatment, gasification, gas cleaning, water–gas-shift reactions, catalytic reactors, final gas separation and upgrading. Optionally a gas turbine and steam turbines are used to produce heat and electricity from unconverted gas and heat removal, respectively. The results show that, in terms of mass conversion, methanol production seems to be the most efficient process for all the biowastes. SNG synthesis is preferred when exergetic efficiency is the objective parameter, but hydrogen process is more efficient when the performance is analyzed by means of the 1st Law of Thermodynamics. The main exergy losses account for the gasification section, except in the electricity and heat production chain, where the combined cycle is less efficient.     

液压系统常见的故障诊断及处理

任何工程机械式液压设备使用时出现故障是不可避免的。但是怎样确定故障的原因及找到好的解决方法，这是使用者最关心的问题。讲述了液压系统常见的故障及其排除方法。     

Biohydrogen production by Anabaena sp. PCC 7120 wild-type and mutants under different conditions: Light, nickel, propane, carbon dioxide and nitrogen

Increasing awareness of environmental problems caused by the current use of fossil fuel-based energy, has led to the search for alternatives. Hydrogen is a good alternative and the cyanobacterium Anabaena sp. PCC 7120 is naturally able to produce molecular hydrogen, photosynthetically from water and light. However, this H₂ is rapidly consumed by the uptake hydrogenase.This study evaluated the hydrogen production of Anabaena sp. PCC 7120 wild-type and mutants: hupL⁻ (deficient in the uptake hydrogenase), hoxH⁻ (deficient in the bidirectional hydrogenase) and hupL⁻/hoxH⁻ (deficient in both hydrogenases) on several experimental conditions, such as gas atmosphere (argon and propane with or without N₂ and/or CO₂ addition), light intensity (54 and 152 ??Em⁻²s⁻¹), light regime (continuous and light/dark cycles 16 h/8 h) and nickel concentrations in the culture medium.In every assay, the hupL⁻ and hupL⁻/hoxH⁻ mutants stood out over wild-type cells and the hoxH⁻ mutant. Nevertheless, the hupL⁻ mutant showed the best hydrogen production except in an argon atmosphere under 16 h light/8 h dark cycles at 54 ??Em⁻²s⁻¹ in the light period, with 1 ??M of NiCl₂ supplementation in the culture medium, and under a propane atmosphere.In all strains, higher light intensity leads to higher hydrogen production and if there is a daily 1% of CO₂ addition in the gas atmosphere, hydrogen production could increase 5.8 times, related to the great increase in heterocysts differentiation (5 times more, approximately), whereas nickel supplementation in the culture medium was not shown to increase hydrogen production. The daily incorporation of 1% of CO₂ plus 1% of N₂ did not affect positively hydrogen production rate.     

Economie d'énergie en trigénération

Trigeneration is defined as the production of three useful forms of energy—heat, cold and power—from a primary source of energy such as natural gas or oil. For instance, trigeneration systems typically produce electrical power via a reciprocating engine or gas turbine and recover a large percentage of the heat energy retained in the lubricating oil, exhaust gas and coolant water systems to maximize the utilization of the primary fuel. The heat produced can be totally or partially used to fuel absorption refrigerators. Therefore, trigeneration systems enjoy an inherently high efficiency and have the potential to significantly reduce the energy-related operation costs of facilities. In this paper, we describe a model of characterization of trigeneration systems trough the condition of primary energy saving and the quality index, compared to the separate production of heat, cold and power. The study highlights the importance of the choice of the separate production reference system on the level of primary energy saving and emissions reduction.     

Investigation of the thermodynamic and kinetic properties of La–Fe–B system hydrogen-storage alloys

La–Fe–B hydrogen-storage alloys were prepared using a vacuum induction-quenching furnace with a rotating copper wheel. The thermodynamic and kinetic properties of the La–Fe–B hydrogen-storage alloys were investigated in this work. The P–C–I curves of the La–Fe–B alloys were measured over a H₂ pressure range of 10⁻³ MPa to 2.0 MPa at temperatures of 313, 328, 343 and 353 K. The P–C–I curves revealed that the maximum hydrogen-storage capacity of the alloys exceeded 1.23 wt% at a pressure of approximately 1.0 MPa and temperature of 313 K. The standard enthalpy of formation ΔH and standard entropy of formation ΔS for the alloys' hydrides, obtained according to the van't Hoff equation, were consistent with their application as anode materials in alkaline media. The alloys also exhibited good absorption/desorption kinetics at room temperature.     

Mineralogical characterization of the intraseam layers of Lofoi lignite deposits in Florina basin (western Makedonia,northwest Greece)

The mineralogical composition of intraseam layers from Lofoi lignite deposits (northwest Greece) is the subject of the present study. The samples were examined by means of X-ray diffraction (XRD), thermo-gravimetric (TG/DTG) and differential thermal analysis (DTA), and Fourier transform infrared (FT-IR) spectrometry. The clay minerals prevail in most samples, with illite-muscovite being the dominant phase, and kaolinite and chlorite being the other major clay components. No smectite was found. Quartz and feldspars, dominate in two cases. The studied materials are characterized as clays to clayey sands, showing significant similarities with the intraseam layers of the adjacent Achlada lignite deposits.