Hydrodynamics and heat transfer for large amplitude pulsating laminar flow in channels

The heat transfer at superposition of high-frequency oscillations on a laminar flow of a liquid in flat and rectan-gular channels at a distance from an inlet of a heated site is investigated under boundary conditions on channel walls of the first and second kind. For the flat channel, the obtained analytical expressions for the amplitude and phase profiles of the longitudinal velocity oscillations are used as a function of the dimensionless oscillation frequency in the form of functions of a real variable. It is shown that the mean value taken for the perimeter of the channel and also the period of oscillations, the Nusselt number for large amplitudes of mean velocity oscillations over the cross section can signifi-cantly exceed its stationary value. The limiting value of the ratio of Nusselt numbers for a pulsating and steady flow in the region of high pulsation frequencies is found.     

Formation of vortex wakes at flow separation from plate

The plane nonlinear initial boundary value problem about the separated flow past a plate set in motion at a constant velocity from the state of rest has been considered. Results of a numerical experiment which have allowed us to trace in detail the vortex-wake formation process behind a vertical plate are reported. It is shown that, after the beginning of the plate motion, several stable vortical structures, including a Karman street, form in succession behind the plate. It is found that, on the emergence of the Karman street, there occurs a sharp and substantial growth of vortex-wake intensity and hydrodynamic drag force with a pulsating time behavior. A conclusion about the origination, in this regime, of self-sustained oscillations of the liquid in the vicinity of the plate is drawn.     

Flow visualization of vortex structure in a pulsed rectangular jet

Pulsating jet is visualized using hydrogen bubble method to clarify the vortex nature in the near field of the jet. This study focused on the development in space and time of vortex structures evolution in low aspect-ratio rectangular jet with pulsation. Pulsation means large-amplitude, low-frequency excitation which is expected to increase the mixing and spreading of the jet and to accelerate its transition from a rectangular form to an axisymmetric form. It was deemed appropriate to investigate whether jet characteristics of a pulsating, submerged jet flow can be altered by including pulsations. The difference of the vortex deformation process is discussed in relation to pulsating conditions. Consequently, the pulsation leads to the formation of vortices at regular intervals, which are larger than those occurring in a steady jet. The results show that the streamwise interaction, between leading vortex and trailing vortex rolled up at nozzle lips, strengthens with increasing pulsating frequency. The spanwise drift of the vortex becomes strongly apparent at large amplitude and high frequency conditions. The drifting start position does not change regardless of pulsating condition. The convection velocity of vortex increases at lower frequency and larger amplitude.     

Heat exchange at laminar flow in rectangular channels

Numerical modeling of heat exchange at a laminar stationary and pulsatile flow in rectangular channels with different aspect ratios of side lengths γ has been carried out by a finite difference method for two boundary conditions: a constant wall temperature and a constant heat flux density on the wall. For the boundary condition of the first kind, the similarity of distributions of the heat flux density and shear stress on the walls over the channel perimeter has been established. The reasons for a nonmonotonous dependence of the initial thermal interval length on γ are discussed. For the boundary condition of the second kind, the difference of the Nusselt number averaged over the perimeter at γ → 0 from its value for a flow in a flat channel has been explained. An increase in the Nusselt number averaged over the perimeter and the period of oscillations has been revealed for a pulsatile flow in the quasi-stationary regime at large amplitudes of the oscillations of the velocity averaged over the cross section.     

Hydromagnetic slip flow with heat transfer in an inclined channel

The problem of steady two-dimensional laminar flow in slip flow regime of a viscous incompressible and electrically conducting fluid through an inclined channel of rectangular cross-section in presence of a transverse magnetic field has been considered. The walls of the channel are assumed to have prescribed temperatures and finite conductivities. The expressions for the velocity component, induced magnetic field and the temperature are obtained and their numerical results are shown graphically.     

PIV and LDV evidence of hydrodynamic instability over a liner in a duct with flow

The acoustical behavior and the flow in a rectangular lined channel with grazing flow have been investigated. The liner consists of a ceramic structure of parallel square channels and is locally reacting. In the absence of flow, the liner has a classical behavior: the acoustic transmission coefficient has a minimum at the resonance frequency of the resonators. When the Mach number of the grazing flow increases, the material behavior becomes unclassical in the sense that its acoustic transmission increases strongly around the resonance frequency. To connect this behavior with flow features, the flow itself in the vicinity of a liner has been measured by means of laser velocimetry. Periodic structures have been observed along the liner that are phase-locked with the incident sound wave. The axial and transverse velocity of these structures bear the typical features of an instability. In particular, the wavelength, convection speed, and growth rate are given. This is the first time that an aeroacoustic instability resulting from the interaction of flow and sound over a liner is measured.     

Combustion of n-butanol in a flat minichannel

Combustion of n-butanol was studied experimentally at the two-phase flow in a rectangular channel of 4-mm height. Characteristics of the two-phase flow in channels with close configuration were analyzed, and the conditions of flame spread in the channel were determined. The dependencies of flame propagation velocity on flow velocity of the gas mixture with different oxygen contents were measured.     

Hydrodynamic instability of premixed flame propagating in narrow planar channel in the presence of gas flow

The paper analyses the hydrodynamic instability of a flame propagating in the space between two parallel plates in the presence of gas flow. The linear analysis was performed in the framework of a two-dimensional model that describes the averaged gas flow in the space between the plates and the perturbations development of two-dimensional combustion wave. The model includes the parametric dependences of the flame front propagation velocity on its local curvature and on the combustible gas velocity averaged along the height of the channel. It is assumed that the viscous gas flow changes the surface area of the flame front and thereby affects the propagation velocity of the two-dimensional combustion wave. In the absence of the influence of the channel walls on the gas flow, the model transforms into the Darrieus–Landau model of flame hydrodynamic instability. The dependences of the instability growth rate on the wave vector of disturbances, the velocity of the unperturbed gas flow, the viscous friction coefficients and other parameters of the problem are obtained. It is shown that the viscous gas flow in the channel can lead, in some cases, to a significant increase in instability compared with a flame propagating in free space. In particular, the instability increment depends on the direction of the gas flow with respect direction of the flame propagation. In the case when the gas flow moves in the opposite direction to the direction of the flame propagation, the pulsating instability can appear.     

Effects of inlet conditions on dynamics of a thermal pulse combustor

To increase the pulse combustor load, a higher amount of fuel-air mixture has to be supplied. This increases the flow rate or equivalently, the flow time is reduced. However, an increase in flow rate leads to an early extinction. This implies that obtaining pulsating combustion is difficult at higher loads. The objective of the present work is to explore the possibility of extending the regime of pulsating combustion at higher flow rates by preheating and diluting the reactants. In this work, the effects of preheating and dilution are examined by varying the inlet temperature and inlet fuel mass fraction. Varying these parameters, a map, presenting regime of pulsating combustion from steady combustion to extinction for each value of flow time considered, has been made. Lastly, Hopf bifurcation points of the system have been investigated by determining the eigenvalues of Jacobian matrix of the coupled non-linear system at the fixed point using a specialised package for bifurcation analysis, MATCONT. It has been found that at higher load, pulsating combustion can be achieved at higher inlet temperature and lower inlet fuel mass fraction. Comparing the Hopf points with mapping, it is found that existence of Hopf bifurcation agrees with the birth and death of pulsating combustion. The results indicate that altering the mixture condition at the inlet can be used for controlling chaos and stabilising periodic solutions in thermal pulse combustors and thus increase the range of pulsating combustion to higher power regimes.     

Hydromagnetic Blood Flow of Sisko Fluid in a Non-uniform Channel Induced by Peristaltic Wave

In this paper, a smooth repetitive oscillating wave traveling down the elastic walls of a non-uniform twodimensional channels is considered. It is assumed that the fluid is electrically conducting and a uniform magnetic field is perpendicular to flow. The Sisko fluid is grease thick non-Newtonian fluid can be considered equivalent to blood. Taking long wavelength and low Reynolds number, the equations are reduced. The analytical solution of the emerging non-linear differential equation is obtained by employing Homotopy Perturbation Method(HPM). The outcomes for dimensionless flow rate and dimensionless pressure rise have been computed numerically with respect to sundry concerning parameters amplitude ratio ?, Hartmann number M, and Sisko fluid parameter b1. The behaviors for pressure rise and average friction have been discussed in details and displayed graphically. Numerical and graphical comparison of Newtonian and non-Newtonian has also been evaluated for velocity and pressure rise. It is observed that the magnitude of pressure rise is maximum in the middle of the channel whereas for higher values of fluid parameter it increases. Further, it is also found that the velocity profile shows converse behavior along the walls of the channel against multiple values of fluid parameter.     

微细颗粒壁面沉积的数值研究

针对微细颗粒在壁面上的沉积特性,采用湍流雷诺应力模型结合颗粒拉格朗日轨道模型对无量纲弛豫时间τ~+为0.1～100量级的微细颗粒在壁面的沉积特性进行了研究,考查了流动方式和流速对颗粒沉积速率的影响。研究表明:在垂直向下流动和水平流动情况下,颗粒的沉积速率随着颗粒无量纲弛豫时间τ~+增加而增加,但是垂直向下流动中,当颗粒无量纲弛豫时间τ~+增大到一定值,颗粒的沉积速率基本保持定值。对于相同无量纲弛豫时间τ~+的颗粒,颗粒在壁面上的沉积速率随着摩擦速度u~*的增加而减少。     

方截面鼓泡床气液两相瞬态数值研究

在双流体模型中引入界面浓度输运方程,利用界面浓度和气泡平均Sauter直径模化各相间作用力。引入一附加 湍动能输运方程模化气泡诱导引起的液相湍流。利用该模型对方截面鼓泡床内气液两相流进行三维瞬态数值模拟。计算结 果表明该模型能较好得模拟方截面鼓泡床内气液两相流时均和瞬态流动特征。     

Flow separation behind a rib in a channel with laminar flow

Visualization data and results of combined measurements of flow quantities in flow with separation past a rib at nominally laminar regime of channel flow are reported. In the separation region, the flow is found to be essentially three-dimensional and unsteady, exhibiting a distinct cellular structure and flow zones with transverse motion. It is shown that the rib-induced flow separation gives rise to low-frequency fluctuations of flow velocity and initiates the turbulence transition in the channel flow. The critical Reynolds number at which flow instability starts developing in the channel is estimated. It is shown that at Reynolds numbers higher than the critical Reynolds number the linear integral scale of flow velocity fluctuations in the channel is defined by the duct size.     

不同亚格子模型在亚声速槽道流大涡模拟中的应用对比

湍流边界层流动是一种广泛存在于飞行器内部和外部的流动现象，是基础理论和模型验证的重要研究对象.能够捕捉大部分流动细节且计算量适中的大涡模拟（large-eddy simulation，LES）方法在湍流数值模拟中得到了越来越广泛的应用.文章基于格心有限差分方法，使用4阶紧致中心格式离散N-S方程无黏项，分别应用5种不同的亚格子（subgrid-scale，SGS）模型，即隐式，SM（Smagorinsky model），DSM（dynamic Smagorinsky model），WALE（wall-adapting local eddy-viscosity model）和CSM（coherent structures model），对Re = 3 000，Ma = 0.5的等温壁面槽道流动进行了大涡模拟研究.与实验值和直接数值模拟（direct numerical simulation，DNS）结果对比后发现，流场平均温度、平均密度等热力学量以及平均流向速度对亚格子模型不敏感，不适宜作为判断模型优劣的判据.亚格子模型在壁面附近的耗散越大，壁面摩擦速度以及阻力系数就越小.对于与速度相关的脉动量来说，不同模型得到的结果在壁面和脉动峰值附近误差比较大，中心线附近较小；显式模型结果在流向速度峰值处均高于参考值，而在展向和壁面法向速度脉动峰值处则均偏低.考虑显式的4种模型在壁面附近的涡黏系数分布，DSM和CSM曲线满足涡黏系数与无量纲壁面距离3次方成正比的分布规律，SM曲线斜率偏小而WALE曲线斜率偏大.      

脉动热管的结构改进及其传热特性的实验研究

通过观察脉动热管的运行过程,并分析其存在的缺点,提出对脉动热管的结构进行改进,合理匹配各通道内的 流动阻力,实现工质在热管里的稳定单向流动,以改善加热段的供液情况,提高脉动热管传热性能。并对充灌率、倾角和 通道大小对改进型脉动热管的传热性能的影响进行了实验研究。     

Heat transfer and force balance approaches in bubble dynamic study during subcooled flow boiling of water–ethanol mixture

In this paper, the subcooled flow boiling heat transfer coefficient of pure water, water–ethanol mixture and pure ethanol is determined experimentally in horizontal rectangular channels for various parameters like heat flux, mass flux and channel inlet temperatures. Flow visualization is carried out using high speed camera. The bubble departure diameter, growth period and waiting period of bubbles are determined. Correlations are developed for subcooled flow boiling Nusselt number of water–ethanol mixture based on force balance approach and heat transfer approach. The parameters considered for correlation are grouped as dimensionless numbers by Buckingham π-theorem. The significance of each dimensionless number on heat transfer coefficient is discussed. The correlations developed for subcooled flow boiling heat transfer coefficient are validated with the experimental data. They are found to be in good agreement with the experimental data. It is found that the correlation based on force balance approach predicts the subcooled flow boiling Nusselt number well when compared with that of heat transfer approach correlation.     

Experimental analysis of oscillatory airflow in a bronchiole model with stenosis

As the mechanism of gas transport and exchange in human respiratory ventilation, the complicated processes of mixing and diffusion in airways of human lungs are considered. However the mechanism has not been clarified enough. On the other hand, the analysis of detailed mechanism in the case of artificial ventilation like HFOV (High Frequency Oscillatory Ventilation) is strongly required for the development of clinical treatments on patients with respiration disorder. In HFOV, it is considered that pendelluft becomes one of the important factors of gas transport and exchange because of high frequency ventilation in comparison with natural breathing. As increase of the frequency, the different time constants of lung units generate phase lag of ventilation in airways of lungs. The phase lag of ventilation causes to generate pendelluft. The time constant is determined by compliance and flow resistance of lung unit. In order to investigate the effect of the different time constants induced by the difference of flow resistance in a part of respiratory bronchiole of human lungs, the experimental study has been carried out by using multi-bifurcated micro channels as a model of bronchiole. The flow resistance in the model channels was produced by a stenosis. The velocity distributions of ventilation flows in the channels with and without the stenosis have been measured by using μ-PIV technique. The results obtained show the frequency effects on the flow pattern in the bronchiole model channels and the appearance of pendelluft.     

Electroosmotic Flow, Ionic Currents, and Pressure-Induced Flow in Microsystem Channel Networks: NMR Mapping and Computational Fluid Dynamics Simulations

Using nuclear magnetic resonance (NMR) velocity mapping and NMR current density mapping as well as finite-element computational fluid dynamics methods, transport in microsystem electrolytic cells with increasing complexity has been examined ranging from single straight channels to random-site percolation clusters. This sort of system is considered as paradigm for more or less complex devices in microsystem technology. Corresponding model objects were designed on a computer and milled into ceramic (polar) or polystyrene (nonpolar) matrices. The pore space was filled with electrolyte solutions. Maps of the following quantities were recorded: velocity of hydrodynamic flow driven by external pressure gradients, velocity of electroosmotic flow (EOF), ionic current density in the presence of EOF, ionic current density in the absence of EOF. As far as possible, the experiments were supplemented by computational fluid dynamics simulations. It is shown that EOF, as well as the electric current, leads to recirculation patterns in closed complex structures such as percolation clusters. Remarkably, all transport patterns turned out to be dissimilar, and the occurrence and positions of eddies do not coincide in the different maps. Velocity histograms and the mean velocity as a function of the porosity have been evaluated. An EOF percolation transition is found at the geometrical percolation threshold. The combined application of NMR techniques for the quantitative, noninvasive visualization of the total variety of hydroand electrodynamics in the same channel system promises to become a powerful tool for design purposes in microsystem technology. Authors' address: Rainer Kimmich, Sektion Kernresonanzspektroskopie, Universit?t Ulm, Albert-Einstein-Alee 11, 89069 Ulm, Germany     

Mathematical modelling of heat and mass transfer under conditions of mixed convection in rectangular region with heat source and heat-conducting walls

Numerical modelling of a conjugate convective-conduction heat transfer in a rectangular region with a heat-release source was carried out in the presence of forced flow and mass exchange. The distributions of thermal and hydrodynamic characteristics, which describe the specific peculiarities of flow regimes under study, were obtained. The mutual influence of forced and free-convective flow was analysed. The scales of the effect of determining dimensionless complexes (Gr, Br, Re) on flow regimes were established. The evolution of analysed process was shown.     

Effect of staggered array structure on the flow field of micro gas chromatographic column

A new design of staggered array semi-packed micro gas chromatographic column was presented based on the micro electromechanical system(MEMS)technology.It was a sensor for gas sample analysis.The internal velocity fields of ten types of semi-packed micro gas chromatographic column were studied.The effects of array spacing and dislocation spacing on the flow field distribution were investigated.The results show that on the basis of ensuring the formation of virtual wall,with the increase of array spacing,the maximum velocity difference between the flow channels in the vertical direction decreases gradually,but the velocity difference in the flow channels a and b increases.When the inlet velocity was set to be 0.18 m/s,the maximum velocity difference in the channel of the staggered semi-packed micro gas chromatography column 3(CSAC3)was 0.05610 m/s.The maximum velocity difference in the channel a was 0.09160 m/s.The maximum velocity difference in the channel b was 0.02401 m/s.CSAC3 had a more uniform velocity field distribution,which can effectively suppress the laminar flow effect during chromatographic separation,and had a smaller pressure distribution,which puts forward lower requirements for carrier gas system.The staggered array semi-packed micro gas chromatography column proposed in this paper can effectively improve the velocity field distribution and pressure distribution in the channel,and provide a theoretical basis for the design of the new micro gas chromatography column structure.