圆锥式分流器的分流均匀性条件分析及结构改进

家用空调器中常用的圆锥式分流器在水平安装、压缩机低频运行的条件下存在分流均匀性严重恶化的问题。提出一种实现环状流的气液两相最低流速的计算方法,并通过优化圆锥式分流器的进出口管径和分配腔结构实现环状流的均匀分配。模拟结果表明,改进后的分流器的分流不均匀度相比于改进前的分流器至少降低51%。对改进后分流器的各出口支路流量进行测试,结果表明改进后分流器的分流均匀性得到明显改善。     

预混喷嘴式分流器性能实验研究

在制冷系统中,性能系数受到多个因素的影响,冷风机的性能是重要因素之一。分流器对节流后的制冷剂等干度、等流量分配的性能直接影响冷风机的综合传热特性。本实验优化设计了一种预混喷嘴式分流器(A)并建立实验台测试分流器性能。实验采用R22制冷剂,设计工况为库温-18℃,分别在0、-4、-8、-12、-16、-18、-20℃,7种库温工况下进行实验测试。测试结果与带喷嘴式分流器(B)、气液分离式分流器(C)、CAL分流器(D)、文丘里式分流器(E)进行对比,得到分流器综合性能从大到小为:ABCDE,分流器A的过热度、不均匀度、传热系数、制冷量均优于其他4种分流器,采用分流器A能够减小冷风机传热温差,降低不可逆损失,提高系统性能表现。     

微通道平行流换热器分配特性及优化研究

微通道平行流换热器内制冷剂分配不均是限制其进一步推广应用的原因之一,当微通道平行流换热器做蒸发器时,制冷剂在换热器入口为气液两相状态,会加剧其分布不均匀性。本文以气液两相R134a制冷剂为工质,提出一种数值仿真模型,并利用前人实验数据验证了模型可靠性。提出通过改变不同扁管在集管内突出深度以改善制冷剂分配特性的4种方案,利用数值仿真模型进行计算,当质量流速为100 kg/(m2?s),制冷剂干度为0.4时,发现通过改变不同扁管在集管内的突出深度可以使液相制冷剂分配特性改善29.4%~52.4%。     

平行流换热器中热流体分布均匀性的研究进展

由于带有一定倾斜角度的百叶窗式翅片、多流程和多管并列的独特结构,平行流换热器会产生两侧流体分布的问题,尤其是有相变的制冷剂流量在多个平行扁管中分配的均匀性变得异常重要,是影响换热器性能的关键,同时现有相关理论预测与实验结果有较大的差距,因此如何改进结构以促进制冷剂、空气更加低耗高效的换热是现在国内外相关研究机构的研究热点。这里回顾了近年来国内外相关理论和实验研究成果,研究表明空气气流分布和单相、两相制冷剂流量在集管中分配的均匀性受工况、结构和流体流动特性影响很大,同时换热器传热是制冷剂和空气相互耦合的结果,任一流体分布不均都会导致部分管段低效甚至失效进而使换热器整体性能衰减。     

垂直U形管内含油制冷剂流动的数值分析

利用EES软件对直接膨胀式地源热泵系统中U形埋管换热器内含油制冷流动状态进剂行了模拟分析。模拟分析结果表明:含油制冷剂压力沿U形埋管先缓慢增加后减少;温度沿U形埋管先增加后减少,进入过热区后急剧增加;在U形埋管上升管段中含油制冷剂不存在泡状流,以弹状流和环状流形式存在,且以环状流为主;最小回油速度随U形埋管管径的增加而增加。计算结果和实验结果的对比表明实验系统能够正常回油。     

分液冷凝器的管程理论设计及热力性能评价

根据分液冷凝器强化换热思想对其管程理论设计方法进行了研究。依据质量流速和干度来判断每一流程中制冷剂的流型,并依此选取Cavallini换热模型公式的方法求其平均换热系数,同时采用Cavallini两相压降模型和Darcy-Weisbach单相压降模型分别确定冷凝区和过冷段的压降。针对一个案例计算了三种管程设计方案下冷凝器管内冷凝换热系数和端压值,并用惩罚因子PF对其综合热力性能进行了评价。计算结果表明:不同的管程设计方案中管内制冷剂的流量分配均匀性存在较大的差异,均匀性越好,其综合热力性能越优。在质量流速为1200~1500 kg/(m2.s)范围内,与同等换热面积的蛇形管冷凝器相比,其中最好的分液冷凝器的PF值减小了48.5%~54.1%,可见设计优良的分液冷凝器的综合热力性能明显优于蛇形管冷凝器。     

平行流冷凝器在12m客车空调中的应用研究

设计了平行流冷凝器,对12m客车空调的管片式冷凝器实现整体替代.在标准空调工况进行的焓差法对比性能实验表明:制冷量Q和能效比EER都略有提高,技术指标满足设计要求.替代后,冷凝器的质量减轻30kg、尺寸减少40%,制冷剂R134a的充灌量减少16%,推进了客车空调系统的轻量小型化,成本效益显著.同时,发现平行流式冷凝器制冷剂侧流动阻力相对于管片式明显增大.     

微通道蒸发器的传热性能实验研究

采用实验的手段,探究了制冷剂侧流速改变(雷诺数)对微通道蒸发器流量分配、压降、传热系数以及制冷剂侧换热特性(努塞尔数)的影响。研究表明:随着制冷剂侧流量的增加,微通道蒸发器第一流程制冷剂的流量分配变得更加均匀,进出口的压降随之增大,传热系数及制冷剂侧努塞尔数也逐渐增大,换热效果不断增强。     

气体导流装置对微通道蒸发器性能的影响

针对微通道蒸发器制冷剂流量分配不均匀造成的换热性能恶化和干蒸现象,本文搭建了双流程微通道蒸发器性能测试实验台,研究导气装置对蒸发器换热性能及扁管中制冷剂分配均匀性的影响,并与常规的双流程微通道蒸发器进行对比。结果表明：由于入口制冷剂流量不变,液相制冷剂蒸发为气体的最大相变潜热不变,导致二者换热量和传热系数差值较小,最大值仅相差0.5%和6.9%。但加导气装置后流动阻力降低,两相段长度较常规结构增幅为87.3%,过热度显著降低,风速为3 m/s时两种结构的过热度降幅为44.4%。各扁管间制冷剂分布趋于一致,均匀性得到提升,干蒸现象得到缓解。     

分流板开孔面积对微通道平行流蒸发器性能的影响

采用数值模拟与实验研究相结合的方法,以R134a为工质,研究了微通道平行流蒸发器内入口处分流板的开孔面积变化时,对蒸发器的流动和换热性能的影响。结果表明:随着分流板上的开孔孔径增大,蒸发器内的流动阻力会降低,制冷剂流量分配的均匀性变差,而制冷量却先增加后减小。分流板的开孔孔径存在一个最佳值,此时蒸发器内的阻力损失低,制冷剂的流量分配均匀性好,两者的匹配性最好,使得蒸发器的制冷量达到最大。此蒸发器中入口分流板的最佳开孔面积约为150.7 mm2。     

Effect of pulsations on two-layer channel flow

The effect of vertical wall vibrations on two-phase channel flow is examined. The basic flow consists of two superposed fluid layers in a channel whose walls oscillate perpendicular to themselves in a prescribed, time-periodic manner. The solution for the basic flow is presented in closed form for Stokes flow, and its stability to small periodic perturbations is assessed by means of a Floquet analysis. It is found that the pulsations have a generally destabilizing influence on the flow. They tend to worsen the Rayleigh–Taylor instability present for unstably stratified fluids; the larger the amplitude of the pulsations, the greater the range of unstable wave numbers. For stably stratified fluids, the pulsations raise the growth rate of small perturbations, but are not sufficient to destabilize the flow. In the latter part of the paper, the basic flow for arbitrary Reynolds number is computed numerically assuming a flat interface, and the motion of the interface in time is predicted. The existence of a time-periodic flow is demonstrated in which the ratio of the layer thicknesses remains constant throughout the motion.     

错流离心分级技术的研究

通过分析分级设备中分级力场方向和介质流体流动方向 ,提出了错流分级和逆流分级的概念 ,并在详细讨论这两种不同分级形式各自特点的基础上 ,设计出一种新型的错流离心分级设备。分级实验结果表明 ,该分级设备不仅适用于微细粉末分级 ,而且能在一次分级过程中获取多个窄级别分级产品     

Numerical Simulation of Self-Excited Oscillation of a Turbulent Jet Flowing into a Rectangular Cavity

The example of a plane jet flow into a rectangular cavity (“dead end”) is used in comparing the capabilities of different approaches to numerical simulation of self-oscillatory turbulent flows characterized by global quasi-periodic oscillation of all flow parameters. The calculations are performed for two flow modes, of which the first one is statistically steady according to the available experimental data, and the second one is self-oscillatory. In both cases, three approaches are used to describe the turbulence, namely, the method of large eddy simulation (LES) in combination with the subgrid model of Smagorinsky, and steady and unsteady Reynolds averaged Navier-Stokes equations (SRANS and URANS) with two well-known differential models of turbulence. In the case of the first flow mode, all three approaches produce qualitatively similar and quantitatively close results. In the case of the second (self-oscillatory) mode, a steady-state solution of Reynolds equations may only be obtained in half the domain using the symmetry boundary conditions; within the framework of the other two approaches, the solutions turn out to be unsteady-state. In so doing, their characteristics calculated using the LES and URANS methods differ significantly from each other; in the case of URANS, they further depend on the model of turbulence employed. The best results as regards the accuracy of prediction of the amplitude-frequency characteristics of self-oscillation are produced by the use of the LES and three-dimensional URANS methods. A similar inference may be made with respect to the mean flow parameters. From this standpoint, the worst results are those obtained from calculations involving the use of the symmetry boundary conditions on the geometric symmetry plane of the flow.__________Translated from Teplofizika Vysokikh Temperatur, Vol. 43, No. 4, 2005, pp. 568–579.Original Russian Text Copyright © 2005 by D. M. Denisikhina, I. A. Bassina, D. A. Nikulin, and M. Kh. Strelets.     

CMC液体垂直向下流动沸腾流型实验

采用透明导电玻璃管，进行了流动沸腾流型实验研究。观察到一种新的流型：闪蒸沸腾。建立了流型判别式。     

Choke-free flow in trapezoidal channels with change in bed elevation

Solutions for choke-free flow in a trapezoidal channel, with rise in bed elevation that may occur with partial filling up of the channel bottom, are discussed. In this analysis, the side slopes of the channel are assumed to be the same before and after the transition. Considering smooth and gradual transition zones, equations of energy and continuity are solved for subcritical and supercritical upstream flow conditions to determine the maximum limiting rise in bed elevation. The ranges of the upstream flow depths are also obtained for the choke-free condition, using the continuity equation of the upstream flow. A list of symbols is given at the end of the paper     

Accelerated and robust finite volume Navier-Stokes solver for all speeds

The paper describes the results obtained from a multigrid accelerated Navier-Stokes solver. The method is based on 2-D explicit cell-centred finite volume Reynolds averaged Navier-Stokes (N-S) flow solver for speeds from near-incompressible Mach numbers to high hypersonic Mach numbers including flows at high angles of attack. The time integration is done using a hybrid 5-stage Runge-Kutta local time stepping scheme. With the help of a simple technique, the capability of the Jameson-Schmidt-Turkel numerical dissipation scheme has been enhanced to include hypersonic flows. The iterative procedure is accelerated significantly by incorporating a multigrid technique which has been used in all computations up to about supersonic speeds. Systematic numerical experiments were conducted to evolve guidelines to generate airfoil grid that could offer reliable flow simulations. The computed results are in very good agreement with experimental data where available, especially from the point of view of predicting large suction peaks and shock locations where considerable departures are often seen in the literature. Further, the highly accelerated computations make this code a useful tool of practical interest in preliminary aerodynamic design.     

Hourglass of constant weight

In contrast to a still common belief, a steadily flowing hourglass changes its weight over the course of time (Shen and Scott in Am J Phys 53(8):787–788, 1985). We will show that, nevertheless, it is possible to construct hourglasses that do not change their weight.     

A continuum model for pumping performance of turbomolecular pumps in all flow regimes

In the present study a continuum model for one-dimensional plane Couette-Poiseuille flow is implemented to turbomolecular pumps in different flow regimes. Pumping performance of various turbomolecular pumps including 6 single rotors, a rotor-stator row, a rotor-stator-rotor row, and a multi-row with 13 alternative rotor-stator rows is considered here. The obtained results show that the model provides good quantitative values for pumping performance of turbomolecular pumps over the whole regimes ranging from molecular flow to transition to slip flow.     

An implicit potential method for incompressible flows

In this paper, we consider a novel numerical scheme for solving incompressible flows on collocated grids. The implicit potential method utilizes an implicit potential velocity obtained from a Helmholtz decomposition for the mass conservation and employs a modified form of Bernoulli's law for the coupling of the velocity–pressure corrections. It requires the solution only of the momentum equations, does not involve the solution of additional partial differential equations for the pressure, and is applied on a collocated grid. The accuracy of the method is tested through comparison with analytical, experimental, and numerical data from the literature, and its efficiency and robustness are evaluated by solving several benchmark problems such as flow around a circular cylinder and in curved square and circular ducts.Copyright © 2013 John Wiley & Sons, Ltd.     

Non-dilatant double-shearing theory applied to granular funnel-flow in hoppers

The storage and efficient withdrawal of material from silos and hoppers is basic to numerous industrial processes. Practising engineers classify two fundamental flows, namely mass-flow and funnel-flow. The former describes the situation when the bulk solid is in motion at every point in the silo or hopper, whenever material is drawn from the outlet. The latter describes the situation when a stable channel forms, called a rat-hole, and the flow is such that only material above the rat-hole is in motion. Funnel-flow occurs whenever the outlet walls are too rough and not sufficiently steeply sloped. Funnel-flow is generally erratic and can give rise either to segregation problems or may lead to complete blockage of the outlet. Here two relevant analytical solutions of the equations for the non-dilatant double-shearing model of granular flow are presented for both plane and axially symmetric funnel-flow. These solutions give rise to flow patterns which are similar to those observed in funnel-flow in the discharge of rectangular and circular cylindrical silos and hoppers.