乙醇对换热器结霜特性影响的试验研究

分析凝缩性物质的抑霜机制,搭建乙醇喷射换热性能可视化试验系统,观测乙醇喷射比(乙醇与空气中水蒸气质量比)和换热器管壁温度对换热器结霜特性的影响,分析传热系数、管外侧压损、霜层厚度等随乙醇喷射比的变化,得到不同条件下乙醇对结霜过程的影响规律。结果表明,当载冷剂进口温度为77 K,乙醇喷射比为0.6时,换热器传热系数是未喷射乙醇时的2倍,在30 s时霜层厚度仅为未喷射乙醇时的1/10;当载冷剂进口温度为240 K,乙醇喷射比分别为0.1和0.2时,换热器传热系数与未喷射乙醇相比分别提升30%和60%;当载冷剂进口温度为77 K,乙醇喷射比分别为0.4和0.6时,换热器传热系数与未喷射乙醇相比分别提升10%和20%。

Effects of Y on hot tearing susceptibility of Mg-Zn-Y-Zr alloys

The hot tearing susceptibility of MgZn2.5YxZr0.5 （x=0.5, 1, 2, 4, 6） alloys was evaluated by thermodynamic calculations based on Clyne-Davies model. The microstructure and morphology of hot tearing regions of the alloys were observed by X-ray diffraction and scanning electron microscopy. The solidification temperature and shrinkage stress during the solidification of MgZn2.5YxZr0.5 alloys in the“T”type hot tearing permanent-mold were acquired with the attached computer. The effect factors of hot tearing susceptibility of MgZn2.5YxZr0.5 alloys, such as the solidification temperature interval, the variation of solid fraction in vulnerable region, the residual liquid fraction in the final stage, the type of the second phase of the alloys were discussed based on the above calculation and observation. The results demonstrated that the hot tearing susceptibility in the investigated alloys was found as follows：MgZn2.5Y2Zr0.5＆gt;MgZn2.5Y0.5Zr0.5＆gt;MgZn2.5Y4Zr0.5＆gt;MgZn2.5Y6Zr0.5＆gt;MgZn2.5Y1Zr0.5. The highest hot tearing susceptibility of MgZn2.5Y2Zr0.5 alloy related to the following reasons： the largest freezing range, the biggest changing of the variation of solid fraction in vulnerable region, the least liquid film in the final stage of solidification, the formation of the second phase which worsens the liquid flow and interdendritic feeding after dendrite coherency.

Consolidation Properties of Highly Plastic Clay During Osmotic Pressure Consolidation Test

For the very soft clay with high water content, its void ratio, compressibility coefficient and permeability varied with stress during consolidation. It is necessary to use large strain consolidation based on the permeability-void ratio relationship and effective stress-void ratio relationship to analyze these properties. To overcome the disadvantages of conventional oedometer test, and determine the effective stress-void relations of this kind of soil, osmotic pressure consolidation test for highly plastic clay study and the expression of permeability-void ratio are performed. Therefore, the decided properties will be reasonably used for solving the large strain consolidation equation.

Hot tearing susceptibility of Mg-xZn-2Y alloys

Effects of Zn content (0, 0.5%, 1.5% and 4.5%) on the hot tearing characteristics of Mg?2%Y alloy were studied in aconstrained rod casting (CRC) apparatus attached with a load cell and data acquisition system. The experimental results indicate thatthe hot tearing susceptibility (HTS) is affected by the content of Zn. The Zn-free base alloy shows the lowest HTS. The HTS ofMg?xZn?2Y alloys increases with increasing Zn content, reaches the maximum at 1.5% Zn, and then decreases with further Znaddition. The high HTS observed in the alloy with 1.5% Zn is attributed to its high force release rate and large force drop duringsolidification. The hot cracks of casting are initiated and propagate along the dendritic or grain boundaries. The predictions of HTS ofMg?xZn?2Y alloys using ProCAST software are in good agreement with the results obtained by experimental measurements.

Eliminating shrinkage defects and improving mechanical performance of large thin-walled ZL205A alloy castings by coupling travelling magnetic fields with sequential solidification

ZL205 A alloys with large thin-walled shape were continuously processed by coupling travelling magnetic fields(TMF)with sequential solidification,to eliminate the shrinkage defects and optimize the mechanical performance.Through experiments and simulations,the parameter optimization of TMF and the influence on feeding behavior,microstructure and properties were systematically studied.The results indicate that the magnetic force maximizes at the excitation current of 20 A and frequency of 200 Hz under the experimental conditions of this study,and increases from center to side-walls,which is more convenient to process thin-walled castings.TMF can break secondary dendritic arm and dendrites overlaps,widen feeding channels,prolong the feeding time,optimize the feeding paths,eliminate shrinkage defects and improve properties.Specifically,for as-cast state,TMF with excitation current of 20 A increases ultimate tensile strength,elongation and micro-hardness from 186 MPa,7.3%and 82.1 kg/mm^(2) to 221 MPa,11.7%and 100.5 kg/mm^(2),decreases porosity from 1.71%to 0.22%,and alters brittle fracture to ductile fracture.

Stress-strain behavior of plastic concrete using monotonic triaxial compression tests

The mechanical behavior of plastic concrete used in the cut-off walls of earth dams has been studied. Triaxial compression tests on the specimens in various ages and mix designs under different confining pressures have been done and the stress-strain behavior of such materials and their strength parameter changes have been experimentally investigated. It has been observed that increasing the confining pressures applied on the specimens causes the material behavior to be alike the more ductile materials and the compressive strength increases considerably as well. Moreover, a parametric study has been carded out to investigate the influence of essential parameters on the shear strength parameters of these materials. According to the research, increasing the coarse to fine aggregates ratio leads to the increase of compressive strength of the specimens as well as the increase of the cohesion and internal friction angle of the materials. Furthermore, the bentonite content decrease and the cement factor increase result in an increase of the cohesion parameter of plastic concretes and decrease of the internal friction angle of such materials.

Self-desiccation mechanism of high-performance concrete

Investigations on the effects of W/C ratio and silica fume on the autogenous shrinkage and internal relative humidity of high performance concrete (HPC), and analysis of the self-desiccation mechanisms of HPC showed that the autogenous shrinkage and internal relative humidity of HPC increases and decreases with the reduction of W/C respectively; and that these phenomena were amplified by the addition of silica fume. Theoretical analyses indicated that the reduction of RH in HPC was not due to shortage of water, but due to the fact that the evaporable water in HPC was not evaporated freely. The reduction of internal relative humidity or the so-called self-desiccation of HPC was chiefly caused by the increase in mole concentration of soluble ions in HPC and the reduction of pore size or the increase in the fraction of micro-pore water in the total evaporable water (Tr/Tte ratio).

Effects of coagulation bath temperature on structure and performance of poly(vinyl butyral) hollow fiber membranes via thermally induced phase separation

Poly （vinyl butyral） （PVB） hollow fiber membranes were fabricated via thermally induced phase separation （TIPS）. The effects of coagulation bath temperature （CBT） on the structure and performance of membranes were investigated in detail. The morphologies of the membranes were studied by scanning electron microscopy （SEM）, the performances of water permeability, rejection, breaking strength and elongation were measured, respectively. The results indicate that all the membranes have the asymmetric morphology and the thickness of the skin layer decreases and the pore size of the outer layer increases with the increase of CBT. The permeability of membranes prepared at air gap 1.0 cm and take-up speed 0.253 m/s increases from 1.047×10-7 to 5.909×10-7 m3/（m2·s-kPa） with the CBT increasing from 20 ℃ to 40℃, and sharply increases to 35.226×10 7 m3/（m2.s.kPa）once the CBT arrives at 50 ℃. While the carbonic ink rejections have no significant decrease, totally exceed 98%, but that of acid-maleic acid copolymer greatly decreases with the increase of CBT. Both the breaking strength and elongation decrease with the increase of CBT.

Experimental study on creep of high-performance concretes

In order to investigate the compression creep of two kinds of high-performance concrete mixtures used for prestressed members in a bridge,an experimental test under laboratory conditions was carried out.Based on the experimental results,a power exponent function was used to model the creep degree of these high-performance concretes(HPCs) for structural numerical analysis,and two series parameters of this function for the HPCs were given with the optimum method of evolution program.The experimental data were compared with CEB-FIP 90 and ACI 92 models.Results show that the two code models both overestimate the creep degree of two HPCs,so it is recommended that the power exponent function should be used for the creep analysis of bridge structure.

Effects of fused hirudin on activity of thrombin and function of platelets

Objective: To investigate whether fused hirudin peptide has both antithrombin and antiplatelet functions. Methods: The core region of fused hirudin was the C-terminal tail of hirudin(hirudin_ 53-64),which could bind to the anion binding exosite (ABE) of thrombin.Arg-Pro-Pro-Gly-Phe(RPPGF) amino acid sequence,a metabolite of bradykinin,was added to the N-terminus of hirudin_ 53-64.It bound to the active site of thrombin.Additionally,Arg-Gly-Asp(RGD)amino acid sequence,an inibitor of glycoprotein Ⅱb/Ⅲa( GP Ⅱb/Ⅲa) receptor,was linked to C-terminus of hirudin_ 53-64.This 26-animo acid-fused hirudin peptide was artificially synthesized,purified and analysed. Results: Fused hirudin peptide significantly lengthened the activated partial thromboplastin time(APTT),thrombin time(TT)and prothrombin time(PT) and inhibited the amidolytic activity of thrombin.The ADP-induced platelet aggregation was markedly inhibited by fused hirudin peptide. Conclusion: Fused hirudin peptide has activity of antithrombin as well as antiplatelet.Therefore bifunctional anticoagulation peptide has capacity to target various components of haemostatic process and may become more powerful antithrombosis agent.

Design of Monolithic Concrete Ground Floors

It is known that the slabs on soil constitute one of the most difficult types of structures despite their apparent simplicity. The objective of this paper is to give a general survey of the design of ground supported slabs with the interposition of a suitable subbase. A solution is proposed with the following characteristics： （1） complete suppression of joints; （2） conventional reinforcement with meshes in the upper and lower fiber of the slab in order to confront and distribute cracking that is caused by hindrance of free contractions and expansions; （3） effective confrontation of problems of bulging. The proposal is in effect on one hand for industrial floorings and on the other hand for concrete pavings with large durability requirement.

Axial compressive behavior and design method of tubed circular steel reinforced concrete short columns

To study the behavior and design of tubed circular steel reinforced concrete (TCSRC) short column under axial compressive loads, a nonlinear finite element model (FEM) has been developed to simulate this kind of structure. Depending on the FEM results, an elastic-plastic analysis was carried out to clarify the status of steel tube, then a simplified procedure was proposed to predict the compressive axial load strength. The results obtained from this procedure were compared with the test results. It is found that they agree well each other.

《向苍天呼吁》的“黑洞”隐喻说解读

詹姆斯·鲍德温是美国黑人小说家、散文家、戏剧家和社会批评家。鲍德温的《向苍天呼吁》聚焦于20世纪30年代居住在纽约黑人区哈莱姆的一个黑人牧师家庭。黑人文学中的隐喻性一直是永恒的主题。非裔美国文学批评家休斯顿·A.贝克,把黑人文本置于黑人文化的传统中,提出了＂黑洞＂隐喻说。文本的浓缩性,＂黑洞＂仪式和神话渗透是贝克的＂黑洞＂隐喻说的关键词。《向苍天呼吁》的语言的极度浓缩体现了语言的狂欢,＂黑洞＂仪式契合了主人公约翰的心路成长历程,文本中对于神话的借鉴表现了神话渗透。《向苍天呼吁》的隐喻性和神秘性,继承了黑人作品语言的隐喻性,探寻了主人公完整自我的心路历程,体现了浓厚的神话渗透。

Effect of high temperature on compression property and deformation recovery of ceramic fiber reinforced silica aerogel composites

Ceramic fiber reinforced silica aerogel composites are novel insulation materials in the thermal protection field for hypersonic vehicles. Before the aerogel composites are applied in load-bearing structures, it is necessary to investigate their mechanical properties including load-bearing and deformation recovery capabilities. High temperature from service conditions will have important effects on the mechanical properties of thermal protection materials. In this paper, compression tests including loading and unloading stages were conducted for ceramic fiber reinforced silica aerogel composites at room temperature and elevated temperatures(300℃, 600℃ and 900℃). Influences of thermal exposure to high temperature and high temperature service environment on the compression property and deformation recovery were both investigated. Scanning electron microscopy(SEM), Fourier transform infrared spectroscopy(FT-IR) and X-ray diffraction(XRD) were applied to help understand the mechanisms of mechanical property variations. The experimental results show that the compression modulus and strength both increase with the increasing thermal exposure temperature and testing temperature,but the deformation recovery capability decreases. The micro structure changes caused by thermal sintering are considered as the main reason for the property variations.Viscous flow and matter transport due to high temperature resulted in the fusion of aerogel particles. This made the particle skeleton thicker and stronger, which led to higher stiffness and strength of the composites. However, matrix cracks induced by the formation and fracture of larger pores made unrecoverable deformation more serious. In the tests at elevated temperatures,the aggregation of aerogel particles in a fused state got more severe because of the addition of mechanical load. As a result, the degradation of deformation recovery capability became more significant.