液体阵列射流冲击冷板工质与传热结构研究进展

液体阵列射流冲击冷却是解决高热流密度散热问题的最有效技术之一,能够有效地对目标表面进行散热,具有散热能力高、能效比高和噪声低的优点,在散热方面具有巨大优势。本文简述了国内外对阵列射流冲击的研究进展,从换热工质和射流冲击冷板的换热结构两个方面,指出了其对液体阵列射流冲击换热特性的影响,并介绍了倾斜射流和旋流射流两种新型阵列射流方式。综合分析了常用的液体换热工质和纳米流体换热工质在射流冲击过程中强化换热的原理,介绍了喷嘴孔型、喷嘴的排列方式和冲击表面结构三种阵列射流结构。分析表明,不同孔型的喷嘴会影响流体的射流速度和湍流特性,不同的喷嘴排列方式会对射流流体的相互作用和有效冲击面积产生影响,不同的冲击表面会影响射流工质的循环混合,这些都将对射流冷板的换热特性产生很大影响。指出了解影响液体阵列射流冲击效果的主要因素,是改善和提高射流换热性能的根本方法。     

不同工作因数下方波冲击射流的换热特性

应用数值模拟方法研究了不同工作因数下方波冲击射流的换热及流动特性,并分析了冲击靶面换热特性的变化规律。研究了冲击靶面换热特性随Reynolds数、脉冲频率、喷嘴距冲击靶面距离与喷嘴直径之比等参数的变化规律,重点分析了不同工作因数对冲击射流滞止区域换热与流动特性的影响,并将数值计算结果与连续冲击射流、脉动冲击射流实验结果进行对比验证。计算结果表明:当工作因数为0.5与0.7时,冲击靶面滞止区域Nusselt数非常接近;当工作因数为0.5时,壁面射流区域Nusselt数比工作因数为0.7时提高了10%;工作因数为0.9时,冲击靶面Nusselt数比连续冲击射流提高3%;工作因数为0.7时,相对于工作因数为0.5、0.9及连续冲击射流时,冲击靶面滞止区域存在强烈的涡结构变化。     

圆形自由表面水射流冲击换热特性

对圆形自由表面水射流冲击换热特性进行了系统的实验研究,总结了喷距、射流出口速度以及相变等因素对换热的影响,并对其形成机理进行了分析,得到了驻点传热系数以及局部传热系数径向分布的关联式.     

柱状微结构表面强化沸腾换热研究综述

沸腾换热是一种非常高效的热传递方式,不论在地面常重力环境还是空间微重力环境下都有十分重要和广泛的应用。强化表面结构是一种有效的无源强化换热技术,而强制对流和射流冲击作为高效的直接冷却方式,在有源强化换热技术中被认为是最有发展前景的冷却方式。结合无源强化换热技术和有源强化换热技术同时进行强化沸腾换热是进一步提高换热能力的有效途径。以电子器件高效冷却技术为背景,对自主开发的微米级柱状微结构表面强化沸腾换热研究现状进行了综述,包括常重力条件下池沸腾、流动沸腾、射流冲击、流动-喷射复合式沸腾换热以及微重力条件下的池沸腾换热。同时,与其他强化沸腾换热表面结构进行了对比,总结并分析了各种强化表面结构及换热方式的优缺点,为下一步的学术研究和工业应用提供相应的参考。     

固体推进剂对射流刺激的易损性响应

以空心装药为标准射流源,研究了3种典型固体推进剂在空心装药射流冲击下的易损性响应特性,分析了配方、射流源的冲击方向(轴向或径向)、装药尺寸等对固体推进剂易损性响应的影响。结果表明,推进剂对轴向射流刺激的响应程度大于径向响应;在千克级药量和长径比为4∶1和1∶1条件下,3种典型推进剂对射流刺激的响应程度差别不大,且均有较强反应发生;HTPB复合推进剂、改性双基推进剂、NEPE推进剂的反应等级依次为爆炸、部分爆轰和部分爆轰。     

多孔介质中受限层流冲击射流的流动与换热特性

基于两区(two-domain)模型采用基于预处理的时间推进法对铺设有多孔介质层的恒温平板在受限层流冲击射流作用下的流动与换热特性进行了研究,其中多孔区域动量方程采用Brinkman-Forchheimer拓展Darcy模型,能量方程则采用局部热平衡(LTE)模型,并对porous/fluid交界面切应力跳跃条件对多孔介质冲击射流的影响进行了分析。流体的控制方程采用基于密度的有限体积法来求解,并针对于多孔区域低速流动特点采用相对应的预处理矩阵来消除控制方程的刚性。还对Reynolds数、孔隙率、Darcy数、热导率比、多孔介质层厚度等参数的变化对流动结构及换热特性的影响进行了分析。研究结果表明,在目前的计算条件下,在其他参数一定时,Reynolds数、孔隙率对通道内流动结构的影响有限;Darcy数、多孔介质层厚度则对流动结构的影响很大;上述参数对受冲击平板的总体换热性能均有明显的影响。在受冲击平板上铺设适当厚度的高渗透率、高热导率的多孔材料能有效地增强换热性能。     

Ag纳米流体浸没射流冲击换热特性

采用超声膜扩散法一步制备出水基Ag纳米流体作为实验工质,并对不同质量分数的Ag纳米流体在受限浸没阵列射流冲击针肋热沉中的流动和换热特性进行了实验研究.结果表明:采用超声膜扩散法制备的Ag纳米颗粒粒径分布均匀,平均粒径只有4.8 nm;表面活性剂对纳米流体的黏度影响较大;相同射流速度下,与基液(水+表面活性剂)相比,Ag...     

表面微结构对阵列微射流沸腾换热的影响

与单相射流相比,阵列式微射流沸腾换热耦合了分布式射流与气液相变两种高效传热模式,在高热通量电子器件冷却领域具有重要的应用前景。本文创新性提出一种具有顶部浸入式阵列射流柱与底部微针肋阵列结构耦合的微射流沸腾换热系统,采用无水乙醇为工质,研究了入口过冷度、入口Re、热通量对射流沸腾换热影响特性;采用电刷镀制备了镍/石墨烯微纳复合结构,研究了该复合结构对微针肋阵列表面射流沸腾换热的影响规律,揭示了镍过渡层引入的附加热阻以及蘑菇状微纳复合结构对气泡脱离的抑制是换热削弱的主要原因。为克服上述弊端,采用激光对镍/石墨烯微纳复合结构表面进行了刻蚀,发现激光刻蚀消除了镍/石墨烯微纳复合结构导致的附加热阻及其气泡脱离抑制效应,其最大传热系数达到30787.0W/(m2·K),较镍/石墨烯微纳复合结构表面和针肋阵列光滑表面传热系数分别提高了140.7%和119.8%。本文的研究结果表明,微纳复合结构对沸腾换热的影响取决于制备工艺及其结构形貌,激光刻蚀较电刷镀形成的微纳复合结构在微射流沸腾换热强化方面更具优势,为表面微纳结构强化沸腾换热系统设计、制备和运行提供科学参考。     

射流式涡发生器强化矩形螺旋通道内流体换热机理

提出利用射流式涡流发生器(JVG)强化螺旋通道内流体的换热。采用三维激光多普勒测速仪(LDV)测量了曲率为0.134并安装了JVG的矩形截面螺旋通道内流体的流动特性,实验结果与数值模拟结果吻合较好。获得了安装JVG的螺旋通道内复合二次涡旋的演变规律以及射流在螺旋通道内的衰减过程。结果表明,射流的冲击和卷吸作用改变了单一螺旋通道内背离壁面(common-flow-up, CFU)结构的离心二次涡旋,在射流的起始段形成了一对冲向壁面(common-flow-down, CFD)结构的二次涡旋。随着流动的发展,CFD涡旋经历了快速形成、缓慢分解并逐渐耗散的过程。射流速比ε_j在1.48～4.02范围内时,射流在螺旋通道内沿主流方向的作用距离可达40d_h～74d_h(d_h为螺旋通道当量直径)。射流提高了通道换热壁面附近处速度场与温度场的协同性,实现了换热强化。研究范围内,换热壁面平均Nusselt数的最大值相对于单一螺旋通道提高了28%～248%。     

文丘里管在给水排水工程中的应用

文章通过对文丘里管工作原理的分析,阐述了文丘里管的原理、构造组成、特点及应用,探讨了特别是在给水排水工程中,用于投加设备的水射器,用于气浮的射流气浮设备,用于曝气的射流曝气设备,用于换热的混合式换热器等方面的应用优势。     

Study of horizontal sonic gas jets in gas–solid fluidized beds

Existing jet penetration correlations have all been developed for sub‐sonic gas jets and have been found to perform poorly when employed in sonic and supersonic regimes. In the present study, triboelectric probes were used to measure both the penetration depth and expansion angle of sonic gas jets. Experiments were conducted using nozzles of different size and geometry, different particle types, different injection gases, and different fluidization velocities. All these data were used to develop a new, general correlation to predict the penetration depth of sonic gas jets. The effects of these variables on jet expansion angle were also studied.     

Experimental study of mixing performances using steady and unsteady jets

This paper deals with the experimental study of mixing characteristics by means of steady and unsteady jets. Investigations were carried out with one and two steady jets, and with two alternate intermittent jets, supplying 0.1 m³ and 70 m³ tanks. From the data obtained from several sensors put into the tank, the signification of the mixing time as applied to the tank is discussed. A Reech-Froude similarity appears to be a good way in scaling the results obtained on either 0.1 m³ or 70 m³ models. Combining the specific momentum of the jets and the tank characteristics, a mixing time factor M is deduced which permits the evaluation of the mixing time. The efficiency of different configurations of jets are then compared through a degree of transient inhomogeneity and required power.     

PARTICLE CONCENTRATION AND SIZE MEASUREMENTS IN TWO-PHASE TURBULENT COAXIAL JETS

Particle concentration and particle size distributions have been measured for two-phase (solid/air) turbulent coaxial jets using the Laser Diffraction Method (LDM) and a tomography data transform technique. Effects of velocity ratio, particle loading ratio, and particle size on the dispersions of gas and particles were determined. Experimental results show that the gas disperses much more rapidly than the particles and particle dispersion decreases with increasing in particle size. Increasing velocity ratio significantly increases gas dispersion, while effects of other variables are less significant. The mean particle size at the jet edge is about 15-20% smaller than that at the jet centerline. The turbulent Schmidt number Sc_p for two-phase turbulent coaxial jets ranges from 1.4 to 1.5.     

The effects of surface active agents on jet breakup in liquid-liquid systems

An experimental study has been made of the effects of anionic, cationic and nonionic surface active agents on jet length, jet contraction and drop size resulting from the formation and breakup of liquid jets in another immiscible liquid, while undergoing mass transfer. Correlations previously established for uncontaminated systems with various rates of mass transfer successfully fitted the present data, when used with the diminished value of interfacial tension caused by the surfactant. Variables included nozzle size, flow rate, and type and concentration of surfactant. Data were obtained from measurements on over 3,000 drops, plus jet lengths and jet diameters from 501 photographs.     

流化床中射流机制和双射流相互作用

在文献的基础上, 通过实验和模拟的方法研究了射流机制和双射流的相互作用. 采用一个300 mm′51 mm的两维气固流化床,内置两个垂直射流, 使用多路毕托管系统测量射流穿透深度. 使用描述气固流态化的双流体模型进行模拟,用改进的IPSA求解模型方程,通过数值模拟, 讨论了射流产生的机理, 再现了双射流, 并发现双射流的相互作用可分为三类：孤立射流、过渡射流和互作用射流,提出了相应的射流间距判据. 发现影响双射流穿透深度最主要的因素是射流动量、两相间曳力、射流间距和床层表观气速, 建议使用Froude数、Reynolds数、床层表观气速、射流间距和喷口直径来关联不同区域的射流穿透深度. 得到了一个关联式并与文献中的关联式或实验数据做了比较.     

Vortex Structures in Combustion of Hydrogen in a Supersonic Cocurrent Air Stream

The existence of vortex zones at the outer boundary of the hydrogen plume in a cocurrent air flow is confirmed. An analysis of experimental results and a comparison with available calculations show that the formation of such zones can be associated with the wave structure arising in the case of nonisobaric outflow of the air stream. The size of vortex zones of combustion increases along the plume, and these zones can improve mixing of the fuel and air jets.     

The formation and breakup of molten oxide jets under periodic excitation

The experiments on the capillary breakup of slag jets at high temperatures are presented in this article. The impact of external excitations on the disintegration process was investigated in a furnace with optical access filmed at frame rates up to 10,000 fps. A synthetic calcia‐alumina slag was used to form jets at different temperatures (1570–1660°C) and jet velocities (0.6–1.4 ms^?1). The impact of external vibration on the breakup was evident: for low jet velocities, the jet length decreased, the droplet size increased, satellite droplet formation was hindered, and a distinct “pumping mechanism” was observed. For jets with higher velocity, the jet length decreased by 30%, the droplet generation frequency increased from 20 to 250 droplets per second, the drop sizes were uniform, and satellite formation was also suppressed. In this case, the ideal case in which the volume of one wave instability forms one droplet was achieved. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3350–3361, 2014     

Breakup of non-newtonian emulsion jets

The breakup of non-Newtonian emulsion jets into drops was experimentally studied by ejecting both O/W and W/O emulsions vertically downward into stagnant air through nozzles. Breakup lengths of non-Newtonian emulsion jets were found to be almost equivalent to those of Newtonian jets. Experimental breakup data establish that the static surface tension of the oil phase can be used as the surface tension of W/O emulsion jets, whereas the dynamic surface tension of aqueous surfactant solutions is used as that of O/W emulsion jets. Diameters of drops formed from non-Newtonian emulsion jets are in good agreement with the prediction from the stability theory previously developed by the authors. When the rheological index in a power law model is appreciably smaller than unity and the Ohnesorge number is significantly large, however, drop sizes are larger than the prediction because of the profile relaxation in jets. The critical velocity of emulsion jets, either O/W or W/O emulsion, is significantly lower than that of homogeneous Newtonian jets.     

T型撞击流混合器内流动特性的PIV研究

采用粒子图像测速技术对入射管直径为3 mm、混合腔直径为16 mm的T型撞击流混合器内的流动特性进行了研究,考察了不同流速比和撞击轴线上方空间条件下混合腔内的速度和湍流动能分布. 结果表明,在相同入射管直径和流速下,撞击驻点位于混合腔中心处,无因次化的速度和湍流动能分布趋势基本一致. 高湍流动能区主要集中在撞击点附近区域,其无因次化数值是传统Rushton涡轮搅拌槽叶端处的3倍. 流速比对撞击驻点位置影响显著;减小撞击轴线上方空间可增加高湍流动能分布区域,利于物料混合.     

Flow and Heat Transfer Characteristics of Confined Noncircular Turbulent Impinging Jets

A three dimensional computational fluid dynamic investigation is carried out to predict the turbulent flow and surface heat transfer under an impinging air jet issuing normally from a single noncircular orifice in a plate held parallel to the target surface. Static pressure distributions, velocity fields and local as well as average Nusselt number on the impinged surface are presented for square, elliptic, and rectangular orifices and compared with those for a circular orifice. Effects of jet Reynolds number as well as spacing between the nozzle plate and the impinged surface are examined using a two-layer κ-η turbulence model. Results show flow structure similarities between the characteristics of rectangular and elliptic jets of equal aspect ratio. Further, it is observed that noncircular impinging jets can provide higher average heat transfer rates than corresponding circular jets for certain geometric parameters viz. nozzle-to-plate spacing and the size of the averaging area used to compute the average Nusselt number.