3.0T MRI快速电影相位对比序列测量非搏动稳态流体模型流速

目的通过建立稳态流体模型,验证3.0 T MRI快速电影相位对比(fast cine PC)序列进行血流动力学测量的准确性。方法将直径为3.0mm的医用塑料管固定在水模中,塑料管的一端连接高压注射器,高压注射器以不同流速(0.5、1.0、1.5、2.0、2.5、3.0、3.5、4.0 mL/s)向管腔内注射0.9%氯化钠溶液,管的另一端连入量筒。将流体模型中心置于头部8通道线圈中心,平行放置的医用塑料管与磁体长轴一致。应用3.0TMRI扫描仪进行fastcine PC序列扫描,扫描层面垂直于所测管腔长轴。在不同流速、不同观测者(2名)、不同流动方向(相向和相反)、不同倾斜角度(0°、20°、30°、45°)对流速进行测量。对图像进行后处理。测定感兴趣区流速,进行统计学分析。结果实际流动方向与流速编码方向相同时测得的流速值分别为6.75、13.90、21.14、28.69、36.52、42.48、50.49、57.10 cm/s;实际流动方向与流速编码方向相反时测得的流速值分别为8.09、14.76、22.53、29.79、36.61、44.75、51.08、54.94 cm/s。管腔内实际流速为42.46 cm/s时,不同倾斜角度测得的流速值分别为42.48、42.77、42.38、42.94、42.47、42.62、42.59 cm/s。测量速度与实际速度间差异无统计学意义,且呈正相关(r=0.99,P<0.000 1)。2名观察者的测量值之间差异无统计学意义(t=-0.87,P=0.41>0.05)。液体流动方向与流速编码方向相同、相反测得的流速与实际流速两两之间差异无统计学意义(F=3.51,P=0.06>0.05)。不管管腔在各个方向有无角度,测得的流速与实际流速之间差异均无统计学意义(t=2.01,P=0.09>0.05)。结论 3.0 T fast cine PC序列可以准确测量稳态流体模型流速,为人体血流动力学检测的临床应用提供了可靠的实验依据。

基于稳态流体的粉煤灰水力分级及矿物学特征研究

粉煤灰颗粒细、密度小且粒径分布范围较广,对流体介质有良好的跟随性,常规分级设备内部流场的不均匀性对颗粒分级扰动大,造成粉煤灰精细化分级困难。通过稳态水力分级装置提供的均匀流场对-400+600目粒级的粉煤灰进行精细化分级,结果表明:可得到平均粒径(d_(50))分别为57.85、56.15、52.49、46.54、39.53μm的粉煤灰样品,且分级产物的密度依次为2.94、2.84、2.24、1.95、1.81 g/cm^(3),可实现粉煤灰在粒径和密度上的精细化分级。同时通过分析分级产物的元素分布可知,产物中Al_(2) O_(3)质量分数随粉煤灰粒径减小而增大,SiO_(2)质量分数随粉煤灰粒径减小先增大后减小,CaO、TiO_(2)、Fe_(2) O_(3)质量分数均随粉煤灰粒径减小而减小。

边界层理论中非稳态流体方程的数值解

非稳态流体方程是边界层流问题中的一个典型方程,是一个定义在半无限区间上的一维三阶非线性方程,采用有限差分法求解它的数值解.方法是通过将半无限区间上的三阶非线性微分方程转化成有限区间上的二阶微分形式,并构造出相应的有限差分方程来求得数值解.结果表明该方法是有效的.

基于稳态流体模型的齿轮箱强制润滑系统研究

强制润滑系统对于提高齿轮箱的寿命预期起较大的作用,传统的粗放估算、简单设计已经满足不了使用条件对产品的实际需求。所阐述的基于稳态流体模型的强制润滑系统设计充分利用流体仿真技术发展,对本系统采用全新的设计方法,结合试验验证,并经产品使用反馈,明显改善了齿轮箱的润滑冷却,预期对齿轮箱长期稳定运行具有较大的促进作用。

非稳态流体力作用下两相流管束流弹失稳研究

流弹失稳会引起传热管振幅过大而发生磨损破坏,是两相流作用下蒸汽发生器管束流致振动的重要机理。为了较为准确地预测两相流作用下圆柱管的失稳临界流速,对试验测量的两相流非稳态流体力进行参数拟合,建立了气-水两相流作用下单管的动力学模型。通过无量纲化,运用Galerkin方法对方程变量进行离散后,联立求解方程得到了不同空泡份额的临界流速。数值结果表明,数值解与试验测得失稳临界流速较为吻合,验证了该模型可用于两相流传热管临界流速的预测。

DTM-BF方法和可渗透收缩壁面上带滑移速度的非稳态磁流体力学流动

研究了运动的粘性导电流体中可渗透收缩壁面上非稳态磁流体边界层流动,利用解析和数值方法对问题进行了研究,并考虑了壁面速度滑移的影响.提出了一个新的解析方法(DTM-BF),并将其应用于求解带有无穷远边界条件的非线性控制方程的近似解析解.对所有的解析结果和数值结果进行了对比,结果显示两者非常吻合,从而证明了DTM-BF方法的有效性.另外,对不同的参数,得到了控制方程双解和单解的存在范围.最后,分别讨论了滑移参数、非稳态参数、磁场参数、抽吸/喷注参数和速度比例参数对壁面摩擦、唯一解速度分布和双解速度分布的影响.

基于COMSOL对涂布垫片流体仿真分析研究

为探索锂离子电池涂布垫片流体仿真分析方法,进行结构优化,应用COMSOL软件的流体分析功能对涂布垫片流道进行了流体分析。按照一定的初始条件进行蠕动流分析,通过计算模拟涂布垫片的流道,从流道的流速和压力分布云图以及剪切速率曲线看出涂布垫片流道结果。针对不理想的结果,通过结构优化设计来改善流道流速、压力和剪切速率曲线,然后对优化后的结构再次进行仿真分析。结果表明,优化后流速更顺畅,压力更均匀,剪切速率波动减少,从而证明了优化设计的合理性。

水泥混凝土路面板底脱空区水运动规律的分析模型

在不考虑路面板横向曲率变化、地基的阻尼效应、水的粘性和假设脱空区内始终充满水的前提下,将车辆荷载作用下水泥混凝土路面板底脱空区内滞留水的流体运动问题,换变为求解非稳态的第一类边界条件的Laplace方程,其数值解采用有限元和中心差分相结合的方法求解。为预防和控制板底脱空的出现和扩展,提高水泥混凝土路面使用性能和寿命提供了理论基础和分析手段。

Finite element modeling of convective pore-fluid flow in fluid-saturated porous rocks within upper crust:An overview

Convective pore-fluid flow (CPFF) plays a critical role in generating mineral deposits and oil reservoirs within the deep Earth. Therefore, theoretical understanding and numerical modeling of the thermodynamic process that triggers and controls the CPFF are extremely important for the exploration of new mineral deposits and underground oil resources. From the viewpoint of science, the CPFF within the upper crust can be treated as a kind of thermodynamic instability problem of pore-fluid in fluid-saturated porous media. The key issue of dealing with this kind of problem is to assess whether a nonlinear thermodynamic system under consideration is supercritical. To overcome limitations of using theoretical analysis and experimental methods in dealing with the CPFF problems within the upper crust, finite element modeling has been broadly employed for solving this kind of problem over the past two decades. The main purpose of this paper is to overview recent developments and applications of finite element modeling associated with solving the CPFF problems in large length-scale geological systems of complicated geometries and complex material distributions. In particular, two kinds of commonly-used finite element modeling approaches, namely the steady-state and transient-state approaches, and their advantages/disadvantages are thoroughly presented and discussed.

Dynamic simulation research on injection and withdrawal performance of underground salt cavern natural gas storage

Owing to perfect impermeability,dynamics stability,flexible and efficient operation mode and strong adjustment,underground salt cavern natural gas storage is especially adapted to be used for short-term dispatch.Based on characteristics of gas flow and heat transfer,dynamic mathematic models were built to simulate the injection and withdrawal performance of underground salt cavern gas storage.Temperature and pressure variations of natural gas in gas storage were simulated on the basis of building models during withdrawal operation,and factors affecting on the operation of gas storage were also analyzed.Therefore,these models can provide theore-tic foundation and technology support for the design,building and operation of salt cavern gas storage.

Numerical Analysis of the Thermal Behavior of a Hermetic Reciprocating Compressor

A numerical model to predict the temperature field in a hermetic reciprocating compressor for household refrigeration appliances is presented in this work. The model combines a high resolution three-dimensional heat conduction formulation of the compressor＇s solid parts, a three-dimensional CFD （computational fluid dynamics） approach for the gas line domain and lumped formulations of the shell gas and the lubrication oil. Heat transfer coefficients are determined by applying CFD to the gas line side and correlations from the literature on the shell gas and oil side, respectively. The valve in the gas line simulation is modelled as a parallel moving fiat plate. By means of an iterative loop the temperature field of the solid parts acts as boundary condition for the CFD calculation of the gas line which returns a cycle averaged quantity of heat to the solid parts. Using an iteration method which is based on the temperature deviation between two iteration steps, the total number of iterations and consequently the computational time can be reduced. The loop is continued until a steady-state temperature field is obtained. Calculated temperatures of the solid parts are verified by temperature measurements of a calorimeter test bench.

Simulation of Steady-State and Dynamic Behaviour of a Plate Heat Exchanger

The present paper deals with both the steady-state and dynamic simulation of a plate heat exchanger, in counter-flow arrangement. A CFD （computational fluid dynamics） program FLUENT has been used to predict the temperature distribution in steady-state conditions in plate heat exchanger as well as fluid temperatures at exit of flow channels in transient condition. The results are presented for the heat exchanger, which is simulated according to the configuration of the plate heat exchanger used in the experiment. The simulated results obtained by the CFD model have been compared with the experimental data from the literature, which shows that the CFD model developed in this study is capable of predicting the steady-state and transient performance of the plate heat exchangers satisfactorily.

A new alternative approach for investigating acidization dissolution front propagation in fluid-saturated carbonate rocks

The natural phenomenon associated with the chemical dissolution of dissolvable minerals of rocks can be employed to develop innovative technology in mining and oil extracting engineering. This paper presents a new alternative approach for theoretically dealing with chemical dissolution front （CDF） propagation in fluid-saturated carbonate rocks. Note that the CDF is represented by the porosity front in this study. In this new approach, the porosity, pore-fluid velocity and acid concentration are directly used as independent variables. To illustrate how to use the present new approach, an aeidization dissolution system （ADS） consisting of carbonate rocks, which belongs to one of the many general chemical dissolution systems （CDSs）, is taken as an application example. When the acid dissolution capacity （ADC） number （that is defined as the ratio of the volume of the carbonate rock dissolved by an acid to that of the acid） approaches zero, the present new approach can be used to obtain analytical solutions for the stable ADS. However, if the ADC number is a nonzero finite number, then numerical solutions can be only obtained for the ADS, especially when the ADS is in an unstable state. The related theoretical results have demonstrated that： （1） When the ADS is in a stable state and in the case of the ADC number approaching zero, the present new approach is mathematically equivalent to the previous approach, in which the porosity, pore-fluid pressure and acid concentration are used as independent variables. However, when the ADS is in an unstable state, the use of the present new approach leads to a free parameter that needs to be determined by some other ways. （2） The existence of a non-step-type dissolution front within a transient region should at least satisfy that none of the ADC number, injected acid velocity and reciprocal of the dissolution reaction rate is equal to zero in the stable ADS.

Low Speed Axial Compressor Stall Margin Improvement by Unsteady Plasma Actuation

This research investigates the use of single dielectric barrier discharge(SDBD) actuators for energizing the tip leakage flow to suppress rotating stall inception and extend the stable operating range of a low speed axial compressor with a single rotor.The jet induced by the plasma actuator adds momentum to the flow in the tip region and has a significant impact on the tip-gap flow.Experiments are carried out on a low speed axial compressor with a single rotor.The static pressure is measured at both the rotor inlet and outlet.The flow coefficient and pressure rise coefficient are calculated.Then the characteristic line is acquired to show the overall performance of the compressor.With unsteady plasma actuation of 18kV and 60W the compressor stability range improvement is realized at rotor speed of 1500 r/min – 2400 r/min.

Simulation of Unsteady State Performance of a Secondary Air System by the 1D-3D-Structure Coupled Method

This paper describes the calculation method for unsteady state conditions in the secondary air systems in gas turbines. The 1D-3D-Structure coupled method was applied. A 1D code was used to model the standard components that have typical geometric characteristics. Their flow and heat transfer were described by empirical correlations based on experimental data or CFD calculations. A 3D code was used to model the non-standard components that cannot be described by typical geometric languages, while a finite element analysis was carried out to compute the structural deformation and heat conduction at certain important positions. These codes were coupled through their interfaces. Thus, the changes in heat transfer and structure and their interactions caused by exterior disturbances can be reflected. The results of the coupling method in an unsteady state showed an apparent deviation from the existing data, while the results in the steady state were highly consistent with the existing data. The difference in the results in the unsteady state was caused primarily by structural deformation that cannot be predicted by the 1D method. Thus, in order to obtain the unsteady state performance of a secondary air system more accurately and efficiently, the 1D-3D-Structure coupled method should be used.

Dynamics of Front Driven by Convection Field in Bistable Media

The front dynamics driven by a convection field in a model of FitzHugh-Nagumo type is studied both analytieMly and numerically. Saddle-node bifurcation induced by the convection field is found by using a singular perturbation analysis of front solutions. Convection field accelerates the B1och front propagating opposite the direction of convection field, but inhibits the Bloch front propagating along the direction of convection field. In addition convection field drives Ising front to travel opposite the direction of convection field.

Experimental study on the motion of a spherical particle in a plane traveling sound wave

We present an experimental study on the motion of a spherical droplet in a plane traveling sound wave.The experiments were performed in the test section of a tunnel with two loudspeakers at the two ends of the tunnel.By adjusting the amplitude ratio and the phase difference between the two speakers,a plane traveling sound wave field can be achieved in the test section of the tunnel,which we checked by measuring the amplitudes and phases of the sound pressure along the tunnel and by simultaneously measuring the velocity field of the air flow at three different locations in the tunnel.When a liquid droplet was introduced in the test section,the motion of the droplet and the velocity of the air flow around the droplet were recorded by high speed cameras,from which we analyze and obtain the ratio of the velocity amplitudes and the phase difference between the particle motion and the fluid motion.The experimental data confirm the theoretical result from the wave equations in the long-wavelength regime,i.e.,when the particle size is much smaller than the wavelength.Moreover,we showed that in this regime,the theory on particle motion in an unsteady uniform fluid,when the history term is included,also yields the same results that are in agreement with the experimental data and the wave equation.Our result extends the parameter range over which the theory on particle motion in unsteady fluid is checked against experiments,especially to the range of particle-fluid density ratio that is of important practical applications.