微通道内流体流动的阻力特性

本文重点研究了微通道内入口效应和粗糙度等因素对流动阻力特性的影响。实验结果表明;对于粗糙度3.5%,水力直径为345μm的微圆管,入口段长度比常规管道短,转捩雷诺数Re_c位于2000～2300,其内入口效应并不影响微管内流体从层流到湍流转捩的拐点位置;内径为112.6μm和169.8μm,高宽比为0.59和0.63以及粗糙分别为1.29%和2.03%的矩形微通道内流动由层流向湍流的转捩发生在Re≈1600～2800之间。实验结果表明,微尺度是微管内流体转捩提前的必要条件,但不是充分条件,拐点提前是由多因素综合作用所造成的结果,其机理有待于进一步研究。     

逆压梯度下层流分离泡转捩的大涡模拟

本文采用有限体积法,发展了适合复杂几何边界条件的可压缩流动大涡模拟程序,通过对槽道湍流的计算,验证了程序的可靠性。对低雷诺数、逆压梯度环境中的层流分离泡转捩现象进行了数值模拟,分析了其统计特性及大尺度相干结构的演化规律．初步的结果表明:大涡模拟可以很好地处理层流分离泡转捩问题,既能给出满意的统计平均结果,又可以通过相干结构的演化细致描述分离泡转捩的动力学过程。     

粗糙微管道内液体流动特性的实验研究

利用实验方法研究了粗糙度对矩形截面微管道内液体流动阻力特性的影响，采用微观粒子图像测速技术测量了粗糙微管道内的流场结构.实验结果表明：在层流状态下，3%—7%的相对粗糙度可以导致微管道内流动阻力的明显增加，在粗糙单元附近形成的压差阻力是导致流动阻力增加的主要原因.粗糙单元还会引起微管道内的流动失稳，导致粗糙微管道内层流向湍流的转捩提前. 关键词： 微管道 粗糙度 微观粒子图像测速 转捩     

压缩拐角激波/湍流边界层干扰特性分析

用大涡模拟方法对Mach数3.0下的压缩拐角激波/湍流边界层干扰问题进行数值研究.对拐角上游平板区域边界层转捩及湍流进行模拟,设定平板区域长度,使得转捩过程于平板区域发生并充分完成,从而在拐角处产生激波/湍流边界层相互干扰,研究激波/湍流边界层的作用机理.研究表明:流场能够在非定常扰动激励下迅速转捩,并于平板区发展为完全湍流;湍流边界层与激波相互作用过程中,拐角附近分离区较层流情况明显减小;展向不同区域分离区大小差异较大,局部区域分离现象消失.     

滑移流区内微环缝槽道中的层流流动与换热

本文针对微环缝槽道采用速度滑移和温度跳跃边界条件求解了不可压缩气体的Ｎ－Ｓ方程和能量方程,理论分析了微环缝槽道在单侧或双侧不同热流密度加热条件下的流动与层流换热特性,讨论了Ｋｎ数、内外径比对流动阻力及换热特性的影响。结果表明：滑移流区微环继通道内的流阻和Ｎｕｓｓｅｌｔ数明显低于连续流区;且随着Ｋｎ数的增加,流阻和Ｎｕｓｓｅｌｔ数均减小;但其随内外径比ｒ＊的变化趋势与连续流区相似。     

布朗运动和湍流扩散作用下槽流中纤维悬浮流动特性的研究

为了探讨槽道中纤维悬浮湍流场特性,本文对修正的雷诺平均纳维-斯托克斯方程、含纤维项的湍动能和耗散率方程、纤维动力学方程以及纤维取向概率密度函数方程进行了数值研究,同时进行了相关实验以验证计算结果.研究结果表明,流场中尤其是近壁区域,纤维取向趋向于与流动方向一致,且该现象随着雷诺数和纤维浓度的减小以及纤维长径比的增大而更为明显.纤维在槽道中分布不均匀,但随着雷诺数的增大和纤维长径比的减小趋于均匀.相对于单相流,纤维悬浮流的流向平均速度剖面更陡峭,且剖面斜率随着纤维浓度、长径比的增大以及雷诺数的减小而变大,纤维的存在使湍流场的湍动能和雷诺应力减小,且减小程度随着纤维浓度和长径比的增大以及雷诺数的减小而增加.流场中的第一法向应力差小于0.05且远小于剪切应力.从壁面到中心,剪切应力增加而第一法向应力差减小.剪切应力和第一法向应力差都随着纤维浓度和长径比的增大而增大.随着雷诺数的增大,剪切应力增大而第一法向应力差减小.纤维浓度对于剪切应力和第一法向应力差的影响比纤维长径比更显著.     

层流叶片在压气机中的应用研究

压气机叶片表面层流流动及转捩现象的捕捉是研究压气机层流叶型的重点和关键。通过基于SST转捩模型的数值方法对某压气机叶型进行数值模拟分析,以精确捕捉普通湍流模型无法捕捉的叶片表面的层流流动及转捩现象,并给定叶片表面转捩的判定依据。在此基础上深入研究低雷诺数环境和高湍流度环境对压气机叶片表面层流流动的影响,验证了在实际压气机环境下,叶片表面的层流流动是可能存在的。为后续压气机层流叶型设计提供数值验证手段和可行性验证基础。     

微通道冷却器内流动和传热特性的数值模拟

为满足固体激光器用微通道冷却器的换热要求, 根据冷却器结构分别建立了二维和三维物理模型, 利用计算流体力学方法首先对比研究两者的流动特性, 然后考察雷诺数和玻片生热量对微通道流动和传热特性的影响。结果表明:对于类似大平板间的矩形微通道层流流动区域, 其流动及传热特性可直接采用二维简化模型进行模拟分析;对于重点关注的转捩区, 采用三维模型模拟分析更好;当雷诺数增大到转捩点, 流体的传热效果得到明显增强;随着雷诺数的增大, 玻片生热量对通道内最低压力需求的影响逐渐减小;不同玻片生热量对微通道流动影响不可忽略, 对努赛尔数和通道总压降基本无影响。     

加热对自然转捩湍斑产生影响的实验研究

本文利用流动显示和高速摄像技术，在湍流度为1％的水洞中，对加热的细长旅成体鱼雷模型表面的流动转捩过程进行了实验研究。实验发现，加热对自然转捩过程中湍流斑的猝发有明显影响，加热减小了湍斑产生的频率，增加了湍斑间的展向间距，使转捩过程中层流转变为完全湍流的过渡区延长．加热对瑞斑产生的纵向位置无明显影响。本文的实验结果正好解释在较高湍流庭环境中的加热减阻机理．     

三维边界层内诱导横流失稳模态的感受性机理

边界层感受性问题是层流向湍流转捩的初始阶段,在转捩过程中起关键性作用,尤其是三维边界层流动.因此,研究三维边界层感受性问题对进一步理解层流向湍流转捩机理以及湍流成因具有重要的理论意义.采用数值方法研究自由来流湍流与三维壁面局部粗糙相互作用下三维边界层的感受性问题,确定是否能在三维边界层内寻找一种新的横流失稳模态;确定在何种条件下三维边界层内能诱导出定常、非定常的横流失稳模态;探索自由来流湍流的强度、展向波数和法向波数以及三维壁面局部粗糙的大小和结构类型等因素在自由来流湍流与三维壁面局部粗糙作用下三维边界层内被激发出的感受性过程中有何影响,并确定何种横流失稳模态在三维边界层感受性过程中占据何种地位.对自由来流湍流与三维壁面局部粗糙作用激发三维边界层内感受性问题的深入研究,将有助于完善流动稳定性与湍流理论,为层流向湍流转捩过程的预测与控制提供合理的理论依据.     

微柱群阻力特性实验研究

以去离子水为工质,流经直径为0.5 aim,高度分别为1.0 mm、0.75 mm、0.5 mm和0.25 mm的圆柱组成的柱群板,其宽度与长宽分别为3.5 mm和40 mm,测量通道进出口压差及流量,研究微柱群内部分别在叉排和顺排时液体流动的阻力特性.研究表明,微柱群内流动阻力系数f,随Re数的增大而逐渐减小,当Re数大于500时,f基本不变;微柱高度和直径之间存在一个有利于流动的最佳比例,该值介于1到1.5之间;顺排微柱群的f明显小于叉排微柱群,其,值为叉排微柱群的0.5倍.     

FRICTIONAL FLOW CHARACTERISTICS OF WATER FLOWING THROUGH RECTANGULAR MICROCHANNELS

Experiments were conducted to investigate the flow characteristics of water flowing through rectangular microchannels having hydraulic diameters of 0.133-0.367 mm and H/W ratios of 0.333-1. Experimental results indicated that the laminar flow transition occurred at Reynolds numbers of 200-700. This critical Re for the laminar transition was strongly affected by the hydraulic diameter, decreasing with corresponding decreases in the microchannel. In addition, the size of the transition range was diminished and fully developed turbulent flow also occurred at much lower Re. The friction behavior of both the laminar and turbulent flow was found to depart from the classical thermqfluid correlations. lite friction factor, f, was found to be proportional to Re^?1.98 rather than Re for the laminar condition, and proportional to Re^?1.72i for turbulent flow. The geometric parameters, hydraulic diameter, and H/W were found to be the most important parameters and had a critical effect on the flow. Generally, increasing the ratio H/W increases the friction factor. The reduction of the microchannel hydraulic radius decreases the friction factor significantly for a given H/W. There exists a special range of ratio H/W (approximately 0.5 mm) at which the experimental data are lower than the predictions obtained from classical correlations. Continued reduction of channel size increases the difference between f_I,expf_1,theo at RE_cri, and the quantity of f_I,exp becomes smaller within the region adjacent to H/W = 0.5, and larger when H /Wis out of this region.     

气泡碰撞过程中形变及破碎现象分析

以水气两相流为研究对象, 采用大涡模拟和流体体积法, 结合破碎准则和第三代涡识别方法对双气泡碰撞过程进行数值模拟。采用单一变量法, 研究气泡直径比、相对偏心距及气泡间相对距离对气泡破碎程度的影响。双气泡碰撞过程中, 两气泡的直径越接近时, 碰撞过程中的气泡破碎程度越弱, 偏心距的变化对气泡破碎程度影响不大。而相对距离小于1时, 随着其增大, 气泡碰撞过程中的破碎程度更加明显。当气泡相对距离大于1时, 气泡破碎程度趋于平缓。研究表明: 第三代涡识别方法能够很好地捕捉两相流湍流流场中漩涡位置, 并能敏锐地反映出湍流的变化。     

EFFECTS OF SURFACE ROUGHNESS ON FORCED CONVECTION AND FRICTION IN TRIANGULAR DUCTS

Experimental investigations had been conducted to study the forced convective heat transfer and pressure drop characteristics of the hydrodynamic fully developed turbulent flow in horizontal equilateral triangular ducts fabricated with Ike same length and hydraulic diameter but different surface roughness of 1.2, 3.0, and 11.5 μm. The experiments were performed with hydraulic diameter-based Reynolds number ranging from 7,000 to 20,000. The entire inner wall of the duct was heated uniformly, while the outer surface was thermally insulated. It was found that the variation of Stanton number (St) with friction factor (f) can be expressed by a relationship of St = C * f, where the constant (C) increases from 0.41 to 0.50 when the surface roughness is increased from 1,2 to 11.5 μm. It was also concluded that the duct with a higher surface roughness will have better heat transfer performance. Nondimensional expressions for the determination of the heat transfer coefficient and friction factor of the equilateral triangular ducts with different surface roughness were also developed.     

Production of long,laminar plasma jets at atmospheric pressure with multiple cathodes

Plasma jets from conventional non‐transferred arc plasma devices are usually operated in turbulent flows at atmospheric pressure. In this paper, a novel non‐transferred arc plasma device with multiple cathodes is introduced to produce long, laminar plasma jets at atmospheric pressure. A pure helium atmosphere is used to produce a laminar plasma jet with a maximum length of >60 cm. The influence of gas components, arc currents, anode nozzle diameter, and gas flow rate on the jet characteristics is experimentally studied. The results reveal that the length of the plasma jet increases with increasing helium content and arc current but decreases with increasing nozzle diameter. As the gas flow rate increases, the length of the plasma jet initially increases and then decreases. Accordingly, the plasma jet is transformed from a laminar state to a transitional state and finally to a turbulent state. Furthermore, the anode arc root behaviours corresponding to different plasma jet flows are studied. In conclusion, the multiple stationary arc roots that exist on the anode just inside the nozzle entrance are favourable for the generation of a laminar plasma jet in this device.     

Thermal performance enhancement in tubes using helically twisted tape with alternate axis inserts

This article presents an investigation on heat transfer enhancement in a round tube inserted with a helically twisted tape. The effects of a helically twisted tape with alternate axis (HTT-A) on heat transfer, friction factor, and thermal performance factor behaviours are reported for the turbulent regime. HTT-A geometries are tape pitch to tube diameter, P/D = 1.0, 1.5, and 2.0; alternate length to pitch length, l/P = 1.0, 1.5, and 2.0; twisted length to tape width, y/W = 3.0; and tape width to tube diameter, w/D = 0.2. The experiment has been performed by varying the volumetric air flow rate in order to adjust Reynolds number ranging from 6 000 to 20 000. The wall of the testing tube is uniformly heated as a constant heat flux while the tests are covered with thermal insulations to reduce heat loss to surroundings. Thermal performance is evaluated by comparing the present experimental results with the results of the modified HTT-A and also those obtained from previous study (conventional helically twisted tape, HTT). The thermal performance of tested tube with HTT-A is evaluated to obtain the degree of heat transfer enhancement and friction factor induced by HTT-A with respect to the plain tube under the same test conditions. Evenly, it is interesting to observe that the tube with HTT-A consistently possesses higher heat transfer and thermal performance factor than those with the HTT around 14.1% and 1.9%, respectively. The HTT-A with the smaller pitch ratio and adjacent twist length provides higher heat transfer rate and friction factor than the one with larger pitch ratio and alternate length as a result of a larger contact surface area, stronger swirl intensity and, thus, better fluid mixing near the tube wall. In the range determined, the tubes with the largest pitch ratio (P/D = 2.0) and smallest alternate length (l/P = 1.0) give the highest thermal performance factor at around 1.35. In addition, the empirical correlations of the Nusselt number, friction factor, and thermal performance factor are also described.     

几何参数对喷射器性能的影响分析

运用计算流体动力学软件Fluent对喷射器进行数值模拟,研究了工作喷嘴喉部直径、等截面混合室直径和长度等几何参数对喷射器性能的影响,并对数值模拟结果进行了分析。研究表明:在一定的工况下,工作喷嘴及等截面混合室对喷射器性能有极大的影响,存在最佳的喉部和混合室直径使得喷射系数最大;随着混合室长度的增加,喷射器的喷射系数将逐渐降低。     

喷射器扩压室结构设计优化研究

扩压室对喷射器内混合流体的降速增压有着重要影响。本文基于真实流体物性,采用气体动力学方法建立改进的一维混合模型并提出扩压室结构设计优化方法。将模型计算结果与文献实验值对比验证了模型的准确性。分析了扩压室结构参数与喷射器膨胀比、喷嘴喉部直径和混合室直径之间的关系。结果表明,扩压室半锥角α随膨胀比pg/pe、扩压室出口直径与混合室出口直径之比dc/dc3以及喷嘴喉部直径与混合室直径之比dg0/dc3的增大而减小;扩压室长度Ld随膨胀比pg/pe、直径比dc/dc3及喷嘴喉部直径dg0的增大而增大,而随着混合室直径dc3的增大而减小或近似保持不变。     

等离子体射流产生与特性的实验研究

本文报道热等离子体射流产生及射流特性的实验研究结果。采用同一个直流等离子体发生器,工作气体流量小时产生出层流等离子体长射流,射流长度随气体流量或弧电流的增加而明显增加;工作气体流量大时则产生出湍流等离子体短射流,此时射流长度几乎与工作气体流量或弧电流无关;在层流与湍流等离子体射流工况之间,存在一个流动状况不稳定的过渡区,此时等离子体射流的平均长度随气流量的加大而减小,但随弧电流的加大而明显加大。层流等离子体长射流有相当好的刚性。     

Flow Mechanisms and Diffusion Combustion of Turbulent Jets

The problem of flow and combustion of turbulent jets of fuel gas in the external medium of an oxidant (air) is solved with regard to the existence of the actual boundary of the turbulent flow region of a jet. Based on the ideas of the friction force of the external flow acting on the boundary of a jet, the entrainment equation for the external medium is derived that closes the system of equations of motion of turbulent jets. The physical meaning of the dissipation rate of the turbulent energy of a jet is interpreted as the work of the friction force. To describe the combustion kinetics, the limit of instantaneous reactions corresponding to the diffusion combustion mode is used. Calculations of the effective reaction rates for reactants and the volumes occupied by them are based on the representation of a turbulent medium as an aggregation of independent turbulent particles—vortices—whose random contacts lead to the mixing and combustion of reacting substances [31]. The concomitant phenomena of flow and combustion are analyzed, including radiation effects. In particular, it is shown that the apparent increase in the combustion temperature with increasing Reynolds number is in fact attributed to the relative decrease of thermal radiation losses. Qualitative agreement is obtained between the results of the theoretical calculations of the length of a combustion torch and experimental data.