Analytical study of slightly eccentric core–annular flow

An analytical study was conducted on the flow of a rigid core surrounded by an annular liquid layer through a horizontal tube. Special emphasis was placed on the question how the buoyancy force on the core, caused by a possible density difference between the core and the annular layer, is counterbalanced. The analysis was carried out under the following restrictions: Stokes flow (no inertia), gap between the core and tube wall small compared with the tube radius, and eccentricity small compared with the gap between the core and the tube wall. A perturbation calculation was carried out with the amplitude of the wave at the core–annular interface as small parameter. The restoring (levitation) force is due to the viscous force exerted by the azimuthal velocity components on the core.     

Numerical modeling of precessing vortex core in the presence of local heat sources

Based on the results of numerical simulation of a nonstationary, nonaxisymmetric turbulent swirling gas flow in a tube with local sources of heat release, it is shown that a precessing vortex core (PVC) appears at supercritical values of the swirl parameter as a result of the development of instability of a left-handed bending mode. The dependence of the PVC frequency on the mass flow rate of the gas and the heat-source power has been studied. As the heat-source power increases, the frequency of precession grows while the amplitude of vortex core oscillations drops.     

Dispersion of a solute in pulsatile non-Newtonian fluid flow through a tube

The unsteady dispersion of a solute by an imposed pulsatile pressure gradient in a tube is studied by modeling the flowing fluid as a Casson fluid. The generalized dispersion model is applied to study the dispersion process, and according to this process, the entire dispersion process is expressed in terms of two coefficients, the convection and the dispersion coefficients. This model mainly brings out the effects of yield stress and flow pulsatility on the dispersion process. It is observed that the dispersion phenomenon in the pulsatile flow inherently differs from the steady flow, which is due to a change in the plug flow radius during a cycle of oscillation. Also, it was found that the dispersion coefficient fluctuates due to the oscillatory nature of the velocity. It is seen that the dispersion coefficient changes cyclically, and the amplitude and magnitude of the dispersion coefficient increases initially with time and reaches a non-transient state after a certain critical time. It is also seen that this critical time varies with Womersley frequency parameter and Schmidt number and is independent of yield stress and fluctuating pressure component. It is observed that the yield stress and Womersley frequency parameter inhibit the dispersion of a solute. It is also observed that the dispersion coefficient decreased approximately 4 times as the Womersley frequency parameter increases from 0.5 to 1. The study can be used in the understanding of the dispersion process in the cardiovascular system and blood oxygenators.     

Stokes flow through a coiled tube

Summary Stokes flow through a coiled tube with arbitrary cross-section is investigated by perturbation expansions in the limit where the helical pitch is either small or large compared to the radius of the helical centerline. The problem is formulated using two alternative non-orthogonal helical coordinate systems applicable to these two opposing limits. The solutions of the respective zeroth-, first-, and second-order perturbation problems are computed by finite element methods for unidirectional, axisymmetric, and two-dimensional Stokes flow. Computations are carried out for tubes with circular and square cross-sections oriented in different ways. The results illustrate the effect of the tube geometry on the hydraulic conductivity and kinematic structure of the flow.     

Transient flow of rarefied gas through a short tube

A transient rarefied gas flow through a short tube is studied on the basis of the direct simulation Monte Carlo method. The mass flow rates through both inlet and outlet cross sections are calculated as a function of the time in the free-molecular, transitional and hydrodynamic regimes with respect to the gas rarefaction. Two values of the pressure ratio, i.e. 0.1 and 0.5, and two values of the aspect ratio, i.e. 1 and 5, are considered. A characteristic time equal to that needed to cross the tube radius with the most probable molecular speed is defined. A typical interval of time to establish the steady flow rates was calculated. It was found that this time is about ten and forty characteristic times for the shorter and longer tubes, respectively. The flow field for the longer tube is analyzed in details.     

Experimental investigation on a thermoacoustic engine having a looped tube and resonator

The purpose of this paper is to study the impact of regenerator hydraulic radius, resonator length, and mean pressure on the characteristics of the tested thermoacoustic engine, which has a looped tube and resonator. Two different acoustic oscillations are observed in the tested engine [Yu ZB, Li Q, Chen X, Guo FZ, Xie XJ, Wu JH. Investigation on the oscillation modes in a thermoacoustic stirling prime mover: mode stability and mode transition. Cryogenics 2003;43(12):687-91]. In this paper, they are called two acoustic modes, high frequency mode (with a frequency independent of the resonator length) and low frequency mode (with a frequency depending on the resonator length). Experimental results indicate that the relative penetration depth (the ratio of penetration depth over hydraulic radius) plays an important role in the excitation and pressure amplitude of the two acoustic modes. For each tested regenerator hydraulic radius, there is a measured optimal relative penetration depth, which leads to the lowest onset temperature difference. Note that, in the tested engine, the measured optimal relative viscous penetration depths are in the range 3-5 (for low frequency mode). Furthermore, experimental results also show that the resonator length affects the presence of high frequency mode in this engine.     

An analysis of peristaltic transport for flow of a Jeffrey fluid

Summary The peristaltic mechanism of a Jeffrey fluid in a circular tube is investigated. The rheological effects and compressibility of the fluid are taken into account. The modeled equations are solved using perturbation technique when the ratio of the wave amplitude to the radius of the pore is small. In the second order approximation, a net flow due to a travelling wave is obtained and effects of Reynolds number, relaxation and retardation times, compressibility of the fluid and tube radius are studied. It is noticed that for the Jeffrey fluid the back flow only occurs for large values of the relaxation time and small values of the retardation time (less than 10 in the present analysis). Another interesting observation is that oscillatory behavior of the net flow rate in the Jeffrey fluid is less than that of a Maxwell fluid. Several results of other fluid models can be deduced as the limiting cases of our situation.     

Oscillatory non-Newtonian flow in tubes of slowly varying radius

Summary The paper studies low Reynolds number flow of a non-Newtonian fluid in an axisymmetric tube of slowly varying radius which is subjected to an axial oscillatory pressure gradient. It is observed that the leading approximation is affected by the visco-elastic coefficient. In the higher approximation particular attention is centered around the steady streaming components for both small and large values of the frequency of oscillation. On the overall the combined effect of visco-elast c and cross viscosity parameter is to induce a radial pressure gradient. For the velocity components the Newtonian and non-Newtonian effects are of the same magnitude when the frequency is small; but when this frequency is large the non-Newtonian effects swamp the flow velociites. When the results are applied to a locally constricted tube, flow reversal is possible downstream of this constriction. The most striking feature however is the condition of zero velocity at a locally constricted tube for the steady streaming velocities—upstream of this constriction the velocity is positive while downstream it is negative. In pathophysiology thrombus formation in constrictions is believed to be caused by aggregation of platelets due to endothelium damage. The condition of zero steady streaming velocity at the constriction is another possible explaination of platelet accumulation and possible blood cloting.With 2 Figures     

Flow induced vibrations of a triangular tube array in various arrangements of orientation and natural frequency

Abstract

Crossflow induced vibrations of a triangular tube array with a pitch ratio 1.33 were investigated experimentally. The streamwise and cross‐stream displacements of a monitored tube in the array were simultaneously measured by two accelerometers to examine the tube response to the cross flow in a water tunnel. The experiment was aimed to study the effects of the array orientation, and the tube's natural frequency on the flow induced vibration of the tube array. It is shown by amplitude diagrams that fluid elastic vibrations exist when the reduced velocity is above a critical value. The critical reduced velocity is found to be sensitive to the orientation of the test array. Based on the measured data of critical reduced velocity, the tube array in a triangular pattern (at a 30‐deg orientation with respect to the flow direction in the experiment) is found to be more stable than when in a rotated triangular pattern (0‐deg orientation). Furthermore, it is illustrated that the discrepancy in natural frequency of the tubes delays the occurrence of the fluid elastic vibrations of the tube array. With all the tubes in the test array having the same natural frequency, the orbits of the tube that exhibits fluid elastic vibrations are an organized, elliptic shape. The corresponding spectra are line‐dominated with peaks at the natural frequency and its harmonics, suggesting that the tube vibration is an organized oscillator. Without the same natural frequency as the surrounding tubes, the monitored tube exhibits fluid elastic vibration at larger reduced velocity, vibrating in a relatively random orbit.     

Nonlinear waves in a prestressed elastic tube filled with a layered fluid

In this work, we studied the propagation of weakly nonlinear waves in a prestressed thin elastic tube filled with an incompressible layered fluid, where the outer layer is assumed to be inviscid whereas the cylindrical core is considered to be viscous. Using the reductive perturbation technique, the propagation of weakly nonlinear waves in the longwave approximation is studied. The governing equation is shown to be the Korteweg-de Vries-Burgers' (KdV-B) equation. A travelling wave type of solution to this evolution equation is sought and it is observed that the formation of shock wave becomes evident with increasing core radius parameter.     

Wiener–Hopf approach for predicting the transmission loss of a circular silencer with a locally reacting lining

The propagation of sound in an infinite rigid circular cylindrical duct with an inserted expansion chamber whose walls are treated with an acoustically absorbent, locally-reacting material is investigated rigorously through the Wiener–Hopf technique. The expansion chamber is separated from the central airway which contain a uniform main gas flow by a perforated cylindrical screen which also increases the silencing performance. The influence of the expansion chamber radius, lining impedances, the mean flow and the acoustical impedance of the central perforated tube on the transmission loss are displayed graphically.     

Elastic-plastic deformation of a centrally heated cylinder

Summary Subject of the investigation is the deformation of a perfectly plastic cylinder with uniform temperature inside its cylindrical core and zero surface temperature. The calculation is based on Tresca's yield condition and the flow rule associated to it. For small radii of the hot core, a plastic region appears at the center expands towards the surface of the cylinder with increasing core temperature. The other possibility is that, depending on the core radius, two plastic regions form one after the other or simultaneously, in the core and at the surface and join in the fully plastic state. The image points of the cylinder lie on different edges and sides of the Tresca-prism in stress space.     

Modeling the Effect of Bubbles on a Pattern and Heat Transfer in a Turbulent Polydisperse Upward Two-Phase Flow after Sudden Enlargement in a Tube

In this work, the numerical modeling of the flow pattern and heat transfer in a polydisperse bubbly turbulent flow after sudden enlargement in a tube is performed. The pattern of average and fluctuation twophase flows at small volumetric gas flow rate ratios (β ≤ 10%) is qualitatively similar to the one-phase liquid flow pattern. It is shown that small bubbles are present almost throughout the entire cross section of a tube, while great bubbles generally pass through the flow core and the shear mixing layer. The addition of air bubbles to a one-phase liquid flow appreciably intensifies heat transfer (up to two times), and these effects become stronger with an increase in the diameter of bubbles and the volumetric gas flow rate ratios gasratios.     

声流对单管道周围换热特性的影响

通过仿真与实验的方式,研究声波影响管道外部的流动和换热特性。在仿真中,对输入声压级相同时,不同频率的声流速度进行研究,结果表明频率对管壁处平均声压级的影响较小,但对管壁周围切向速度的影响较大。基于声波对流场分布的影响,在实验中研究不同声压级和频率对单根钢管换热的影响,结果表明声压级不变,频率越低,钢管换热效果越好;频率不变,声压级越大,钢管换热效果越好。仿真与实验结果表明：管道周围的换热效果与声流速度的大小成正比,而声流大小与频率和激励速度直接相关,激励速度越大,频率越低,钢管附近声流运动越剧烈,采用低频高声压级的声波能够加速钢管的换热过程,该研究为声波影响管道换热特性提供了理论依据。     

Mechanical effects in a vortex device with a rotating core

The process of the appearance of forced rotation of an axial core mounted in a modified vortex tube in the direction opposite to the rotation of the air vortex and the precession of its axis have been studied. It has been established that dynamical bending of a metal axial core arises in the process of rotation which causes mechanical wear of its end part and fracture in the fastening area of the bearing without residual curvature of the core axis. The excitation of rotation and observed force effects are not related to the mechanical action of rotating air flow on the axial core.     

Experimental study on the performance of an inverter heat pump with a bypass orifice

An economical, fixed geometry, bypass orifice was invented to control more precisely the refrigerant flow in an inverter heat pump system. Flow characteristics of the bypass orifice were investigated as a function of orifice geometry and operating conditions. Experimental results of bypass orifices were compared with those of capillary tubes. The bypass orifice showed the best flow trend as a function of frequency: the flow slope of the bypass orifice with respect to frequency was the highest among the orifice, capillary, and bypass orifice. The performance of an inverter heat pump with a bypass orifice and then a capillary tube was measured with a variation of frequency in the psychrometric calorimeter. It was observed that the performance of the inverter heat pump was enhanced with the application of the bypass orifice in the system instead of a capillary tube. The improvement of COP in an inverter heat pump with a bypass orifice versus the capillary tube was prevalent at the low frequency region.     

Surface-tension-driven flow on a moving curved surface

The leading-order equations governing the flow of a thin viscous film over a moving curved substrate are derived using lubrication theory. Three possible distinguished limits are identified. In the first, the substrate is nearly flat and its curvature enters the lubrication equation for the film thickness as a body force. In the second, the substrate curvature is constant but an order of magnitude larger; this introduces an extra destabilising term to the equation. In the final regime, the radius of curvature of the substrate is comparable to the lengthscale of the film. The leading-order evolution equation for the thin film is then hyperbolic, and hence can be solved using the method of characteristics. The solution can develop finite-time singularities, which are regularised by surface tension over a short lengthscale. General inner solutions are found for the neighbourhoods of such singularities and matched with the solution of the outer hyperbolic problem. The theory is applied to two special cases: flow over a torus, which is the prototype for flow over a general curved tube, and flow on the inside of a flexible axisymmetric tube, a regime of interest in modelling pulmonary airways.     

Wave characteristics of falling liquid film on a vertical circular tube

A falling film with waves on a vertical circular tube has been analyzed using the integral approach. A theoretical model of free surface deflection has been developed. The nonlinear equation obtained in the present work is similar to the deflection equation of a wave on a flat plate. It becomes exactly the same as the wave equation on a flat plate in the case of an infinite radius. This study shows that the wave characteristics depend on the parameters such as wave number, dimensionless wave velocity, tube radius and Reynolds number. As the tube radius decreases, the intensity of the wavy processes increases. The velocity distribution of the falling film was investigated in the present work. The cylindrical model appears to be more appropriate over the Cartesian model when the film thickness to tube diameter ratio is large. Calculated values are in good agreement with other published data.     

Fully developed steady flow in a slightly curved annular pipe

Summary A theoretical study of steady flow in a slightly curved annular pipe is presented. The equations of motion are expanded in powers of a non-dimensional radius ration . It is shown that the presence of the axial core affects essentially the axial flow and the wall shear stress. In addition the secondary flow is modified depending on the size of the annular gap.     

双向进气型脉管制冷机的线性理论分析

采用线性模型对双向进气型脉管制冷机的性能进行了分析.得到了通过回热器的焓流、质量流量及脉管冷端压力和流量相位差的代数计算公式.详细分析了工作频率对脉管制冷机制冷量与COP的影响,并对额定输入压力振幅和额定输入功率两种工况分别进行了优化分析.所得结果与其它文献完全一致.