Two-phase flow in a horizontal rectangular microchannel

Two-phase flow in a rectangular short horizontal channel 200 μm high was studied experimentally. The use of the fluorescent method made it possible to reveal flow of liquid in the channel and to determine its characteristics quantitatively. The existence of the regime of separate (stratified) flow is established. Based on analysis of previous investigations and newly obtained data, it is shown that a change in the height of the horizontal channel has a substantial effect on the boundaries between the regimes. The region of the churn regime increases with decreasing thickness of the channel.     

Instability of a two-phase flow in a rectangular channel

A two-phase (liquid-gas) flow in a short horizontal slit channel of a rectangular cross section with a height of 440 μm has been experimentally studied using a fluorescent method, which allowed the flow pattern to be monitored and its quantitative characteristics to be measured. It is established that two-phase flow regimes in this channel substantially differ from the classical regimes of flow in extended channels of large cross section. It is demonstrated that the formation of various two-phase flow regimes and the transitions between them in short narrow rectangular slits are determined by instability of the liquid-gas flow at the side-walls of the channel.     

Heat exchange in two-phase flow about a surface located in a rectangular channel

Similitude equations are obtained on the basis of the principle of superposition of separate effects to calculate heat exchange between surfaces with complexshaped cross sections located in a rectangular channel during their cooling by a two-phase flow.Notation T, q temperature and heat flux - T_w mean surface temperature - I, R current and electrical resistance - V volume of the material - , , anda heat-transfer coefficients, thermal conductivity, and linear expansion of the material - relative functions - =_m; ^* = _m ^{* *} _s ^* ; temperature factor - X relative weight content of liquid phase Indices w surface - f incoming flow - v volume - m two-phase flow - angle of attack - s shape of surface - * pertains to surface with swirl vanes Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 40, No. 5, pp. 780–786, May, 1980.     

Oscillatory magnetogasdynamic slip flow in a microchannel

The problem of pressure-driven magnetogasdynamic (MGD) slip flow with small rarefaction through a long microchannel is considered. The flow is driven by a steady or oscillatory pressure gradient. The study of MGD flows in microchannels is of interest since they occur in many electromagnetic microscale devices. In obtaining the microfluidic solutions in the presence of a magnetic field, some additional physical, mathematical, and numerical issues need to be considered. These issues deal with the scaling laws for microscale MGD flows and the relevant parameters such as Mach number, Reynolds number, Hartmann number, magnetic Reynolds number, and Knudsen number. For planar constant area microchannels, it is possible to obtain the analytical solutions for both steady and oscillatory pressure-driven flows at low magnetic Reynolds numbers. The flow field is assumed to be quasi-isothermal, which is a good assumption in the absence of a strong electric field. As physically expected, at higher values of the magnetic field (that is at a higher Hartmann number) the velocity profile in the channel flattens, and the pressure varies nonlinearly along the channel.     

Instabilities of two-phase flows in short rectangular microchannels

Two-phase (liquid-gas) flows in a short horizontal slit channel of rectangular cross section with heights (thicknesses) from 100 to 500 μm have been experimentally studied using the laser-induced fluorescence and schlieren photography methods. It is established that the formation of various two-phase flow regimes and the transitions between different regimes are determined by instabilities of the liquid-gas flow in the side parts of a channel. In a 100-μm-thick channel, a frontal instability has been observed during the liquid-gas interaction in the region of liquid output from the nozzle.     

Boiling flow through diverging microchannel

An experimental study of flow boiling through diverging microchannel has been carried out in this work, with the aim of understanding boiling in non-uniform cross-section microchannel. Diverging microchannel of 4° of divergence angle and 146 μm hydraulic diameter (calculated at mid-length) has been employed for the present study with deionised water as working fluid. Effect of mass flux (118–1182 kg/m²-s) and heat flux (1.6–19.2 W/cm²) on single and two-phase pressure drop and average heat transfer coefficient has been studied. Concurrently, flow visualization is carried out to document the various flow regimes and to correlate the pressure drop and average heat transfer coefficient to the underlying flow regime. Four flow regimes have been identified from the measurements: bubbly, slug, slug–annular and periodic dry-out/ rewetting. Variation of pressure drop with heat flux shows one maxima which corresponds to transition from bubbly to slug flow. It is shown that significantly large heat transfer coefficient (up to 107 kW/m²-K) can be attained for such systems, for small pressure drop penalty and with good flow stability.     

Interaction of a two-phase gas-liquid flow in a Venturi scrubber

Results are given on an investigation of the disintegration of a liquid jet at various gas velocities in the throat of a Venturi scrubber.     

Periodic and aperiodic tumbling of microrods advected in a microchannel flow

We report on an experimental investigation of the tumbling of microrods in the shear flow of a microchannel (dimensions: 40 mm ×  2.5 mm ×  0.4 mm. The rods are 20–30 μm long and their diameters are of the order of 1μm. Images of the centre-of-mass motion and the orientational dynamics of the rods are recorded using a microscope equipped with a CCD camera. A motorised microscope stage is used to track individual rods as they move along the channel. Automated image analysis determines the position and orientation of a tracked rod in each video frame. We find different behaviours, depending on the particle shape, its initial position, and orientation. First, we observe periodic as well as aperiodic tumbling. Second, the data show that different tumbling trajectories exhibit different sensitivities to external perturbations. These observations can be explained by slight asymmetries of the rods. Third, we observe that after some time, initially periodic trajectories lose their phase. We attribute this to drift of the centre of mass of the rod from one to another streamline of the channel flow.     

Calculation of a two-phase flow in an axisymmetric nozzle

A system of differential equations is presented to describe a two-phase flow through an axisymmetric supersonic nozzle. The results for the solution of this system on a Minsk-14 computer are given. The theoretical results are compared qualitatively with the experimental data.     

Calculation of two-phase adiabatic flow in a pebble bed

Experimental and theoretical data for two-phase flows in pebble beds are generalized. The dependence of pressure loss on the main characteristics of flow is treated, as well as the correlation between the steam quality and void fraction and the conditions of similarity of water-vapor and water-gas flows. Analysis of these dependences reveals that the gas phase distribution, which is uniform over the cross section of the pebble bed, may be stable and unstable.     

Nozzle wear in two-phase flow

The results of an investigation of the erosive properties of two-phase nozzle flows are presented. Recommendations are made concerning the design of wear-resistant nozzles.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 21, No. 1, pp. 43–47, July, 1971.     

Fluctuation model of a nonequilibrium two-phase channel flow

An ill-posed Cauchy problem for a model of a nonequilibrium two-phase flow in the barotropic approximation is transformed into a well-posed problem by changing the type of the initial hyperbolic equations. Approximation of fluctuations of the phase velocities by a random delta-correlated process and averaging of the equations over its realizations generate a system of parabolic equations. Results of numerical integration of this system are compared with experiment and calculations by well-known models.Notation a velocity of propagation of acoustic perturbations - frozen velocity of sound - b dimensionless constant - h enthalpy - L tube length, scale of velocity fluctuations - p pressure - R complex - T temperature - t time - velocity - x coordinate - step of the difference scheme - fluctuating component of the velocity, delta-function - dimensionless variable - phase density - ² variance of the velocity - characteristic correlation time - volume phase concentration Institute of Engineering Thermophysics, Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 66, No. 6, pp. 651–656, June, 1994.     

Oscillating two-phase flow in a prestressed thick elastic tube

Summary The pulsating flow of a fluid with dusty particles in a prestressed thick walled elastic tube has been studied. The tube, subjected to a static inner pressure P_i and an axial stretch λ, is taken to be an incompressible, isotropic, elastic material. The fluid with particles is treated as incompressible Newtonian. Employing the theory of small deformation superimposed on large initial deformations, for an axially symmetric perturbed motion the governing equations are obtained in cylindrical polar coordinates. The analytical solutions of the equations of motion for the dust and the fluid have been obtained. Because of the variable character of the coefficients of the resulting equations for the solid body they are solved numerically. The dispersion relation is obtained as a function of the stretch, the thickness ratio and the parameters for dusty particles.     

Simulation of the two-phase flow in a laboratory coal pulveriser

The particle-laden flow in the separator of a laboratory beater wheel mill is computationally and experimentally investigated. The main aim of the present study has been the identification of convenient modelling approaches and the assessment of the modelling predictive capability for this class of problems. For the particulate phase a Lagrangian formulation is adopted. Particle computations are carried out as post-processing, assuming different size class distributions. A modulation of the gas turbulence by the particulate phase is neglected. Different turbulent models are used. The turbulent particle dispersion is modelled by a stochastic approach. Different geometric configurations of the separator are investigated. With the results of the simulation study it became possible to predict the trends correctly.     

Pinched flow fractionation: continuous size separation of particles utilizing a laminar flow profile in a pinched microchannel

A concept of "pinched flow fractionation" for the continuous size separation and analysis of particles in microfabricated devices has been proposed and demonstrated. In this method, particles suspended in liquid were continuously introduced into a microchannel having a pinched segment and were aligned to one sidewall in the pinched segment by another liquid flow without particles. The particles were then separated perpendicularly to the flow direction according to their sizes by the spreading flow profile inside the microchannel. Polymer microbeads were successfully separated, and the effects of the flow rate and channel shapes on the separation performance were examined. Also, separated particles were collected independently by making branches at the end of the pinched segment. Since this method utilizes only the laminar flow profile inside a microchannel, complicated outer field control could be eliminated, which is usually required for other kinds of particle separation methods such as field flow fractionation. Also, this method can be applied both for particle size analysis and for preparation of monodispersed particles, since separation can be rapidly and continuously performed.     

Structural characteristics of a gas–liquid flow in a microchannel with a T-shaped mixer

The results of experimental studies of the structural characteristics of a nitrogen–water mixture flow in a horizontal microchannel provided with a T-shaped mixer are presented. The experiments are performed in a channel with a rectangular cross section of 250 × 315 μm under the conditions of a dominating influence of capillary forces. Structural characteristics of the flow are determined using the two-beam laser scanning and high-speed video capture at a distance of 500 calibers from the inlet in a wide range of reduced gas- and liquid-flow rates. A new method for the identification of flow regimes is proposed based on the statistical treatment of the laser-scanning data, and a map of flow patterns is constructed.     

Reduction in pipe resistance in the flow of a two-phase medium

A discussion is presented of the Farrows-Lindquist effect — a sharp reduction in the resistance of a pipe to the flow of a viscous fluid containing solid matter.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 52, No. 6, pp. 954–956, June, 1987.