The simulation of turbulent two-phase flow after an abrupt expansion of the pipe in the presence of evaporation of droplets

The results are given of investigation of flow and heat and mass transfer of a gas-droplet flow after an abrupt expansion of the pipe using the Eulerian approach. It is demonstrated that the intensity of heat transfer significantly increases upon addition of evaporating droplets into separated flow (by a factor of more than two compared to single-phase flow at a low value of mass concentration of droplets M _L1 ≤ 0.05). The addition of dispersed phase to a turbulent gas flow leads to an insignificant increase in the length of recirculation zone. Low-inertia droplets (d ₁ ≤ 50 μm) are well entrained into circulation flow and are present in the entire cross section of the pipe. Large particles (d ₁ ≈ 100 μm) pass through the shear layer and do not enter the separated-flow region. Adequate agreement with experimental data is indicative of the adequacy of the developed model for the calculation of gas-droplet separated flow in the case of an abrupt expansion of the pipe.     

Pulsed Third Sound in the Presence of a Superfluid Flow

We report measurements of pulsed third sound in the presence of a uniform superfluid flow at T=0.89 K. The superfluid flow is created using a heater technique. For increasingly large values of power dissipated in the heater used to create the flow, the shapes of the received third sound pulses evolve. The pulses propagating parallel to the flow grow while the pulses propagating against the flow decrease. When the heater power is increased further such that film thinning effects occur, the pulses propagating antiparallel to the flow qualitatively resemble the derivative of the drive pulse. Similar effects were also observed in thin films at T=1.35 K. A possible explanation for the growth and decrease of the pulses will be discussed. The cause of the pulse shapes that resemble the derivative of the drive pulse is not understood.     

Experimental study of the gravity effect on the distribution of refrigerant flow in a multi-pass condenser

Gravity in multi-pass condensers affects the refrigerant flow rate distribution, owing to the gravitational pressure drop that occurs mainly in the U-bend tubes in fin and tube condensers with horizontal tubes. This effect was studied using an experimental approach. A condenser with two ‘nU’ circuits was selected, and the temperature variation of the refrigerant side was measured and compared along each circuit. The critical air velocity, which indicated the initiation of the gravity effect, was found for a given refrigerant flow rate. As the air velocity increased beyond the critical air velocity, the gravity effect (or mal-distribution of the refrigerant flow) developed further. Similarly, the critical refrigerant flow rate was also determined for a given air velocity. As the refrigerant flow rate decreased below the critical refrigerant flow rate, the gravity effect also developed further. The gravity-affected region was shown in the table with rows of air velocities and columns of refrigerant flow rates, and expressed using a single parameter for a given refrigerant flow circuit.     

Heat Transfer in the Neighborhood of the Stagnation Point under Conditions of Hypersonic Heterogeneous Slip Flow past Bodies

This paper integrates extensive experimental and theoretical data on heat transfer on the surface of blunt bodies subjected to hypersonic heterogeneous flow. The value of the Mach number of the carrier gas flow is varied from 2.4 to 6, and those of the Reynolds number calculated by the diameter of the body subjected to flow, from 10⁴to 10⁶. The mass concentration of solid impurities in the flow does not exceed 3%. When the particle diameter varies from 0.15 to 160 m, the rate of particle collisions with the body surface varies from zero to 1350 m/s. An analysis of the experimental data helps develop the phenomenological model of heat transfer on the surface of a blunt body in a hypersonic heterogeneous flow.     

Conditions for the onset of vaporization in superfluid liquid He II in the presence of heat flow

Studies of the vaporization onset associated with heat flow are used as a tool to probe the superfluid thermohydrodynamics during entropy transport. Onset-flux-density data of the heat flow suggest that the normal fluid viscous force dominates during entropy transport from a single source (strip) to bulk He II over a layer of thickness 10 ^–4 cm (upper bound) or less.Work supported by the National Science Foundation.     

Raising the accuracy of operative monitoring by using a variable pressure differential of the flow rate of material and energy carriers in the liquid and gaseous states

Means are considered for reducing the systematic errors in calculating the flow rate of material. For operative monitoring of the day- to- day flow rate, a comparative analysis is made of the numerical solution of the mass flow rate equation and the structure- generalized method for solving the equation.Translated from Izmeritel'naya Tekhnika, No. 5, pp. 37–39, May, 1994.     

Effect of rotation on the flow behaviour in a high-speed cryogenic microturbine used in helium applications

The complex flow characteristics in a high-speed helium microturbine used in cryogenic refrigeration and liquefaction cycles are highly influenced by the effects of rotation. In order to enhance the turbine performance and to improve the preliminary design process of the turboexpander, the flow characteristics within the turbine blade passage need to be investigated at different rotational speeds. Here, three-dimensional unsteady flow analysis of a high speed cryogenic microturbine used in helium applications was carried out using Ansys CFX^®. The loss generated by the various secondary and vortical flows for the different cases was quantified in terms of entropy loss coefficient. The loss generating mechanism was also assessed by analysing the velocity vectors, entropy contours and the behaviour of the vortex cores. With change in speed the influence of scraping flow due to relative casing motion and the blade loading on the flow characteristics was found to vary significantly. At lower speeds, the scraping flow decreases and thus augments the tip leakage flow which in turn interacts with the suction side leg of the leading edge vortex to form a single large vortex. This combined vortex increases the velocity defect and thus leads to increased loss generation. The analysis of the vortex core velocity and the blade loading diagram revealed the need for modifications in blade profile for improved turbine performance. Furthermore, the comparison of the CFD results with the Balje's n_sd_s chart showed remarkable variations, the results of which can be used to modify the chart for the design of efficient cryogenic microturbines for helium applications.     

An anomalous change in the supercooling of vapor in spontaneous condensation

Spontaneous condensation of vapor at transonic velocities in the subsonic part of a flow in contrast to condensation in all the remaining regions of the flow leads not to an increase, but rather to a decrease in temperature and not to a drop, but rather to a rise in the supercooling of vapor. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 71, No. 2, pp. 215–217, March–April, 1998.     

The effect of particles on the gas velocity in a free turbulent flow

The article describes experimental studies carrid out to investigate the interaction between gas and particles in a free turbulent two-phase flow at the outlet from a rather long vertical tube.Notation A cross-sectional area of the flow - A ₀ initial cross-sectional area of the flow - d diameter of the flow - d _p average diameter of particles - I _i initial momentum of the two-phase flow - k mass ratio of particles and gas (k=m _p/m _g) - k ₀ mass ratio of particles and gas in the initial cross-section of the two-phase flow (x=0) - m _g mass flow rate of gas - m _p mass flow rate of particles - r instantaneous radius of the flow - r ₀ radius of the initial cross-section of the flow - r _1/2 normal distance from the flow axis to the point at which the velocity of gas is equal to the half of the axial velocity - R cross-sectional radius of the flow - u velocity - u _a air velocity - u _fa gas velocity on the flow axis - u _g gas velocity - u _av average gas velocity in the initial cross-section for two-phase and single-phase flows - u ₀ gas velocity on the axis of the initial cross-section of the flow - u _p particle velocity - x distance along the axis from the original of coordinates - _g gas density Institute of Nuclear Research Vina, Laboratory of Thermal and Power Engineering, Belgrade, Yugoslavia. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 68, No. 3, pp. 361–365, May–June, 1995.     

Calculating the nonsteady exchange of mass and heat in the laminar flow of dissociating nitrogen tetraoxide in a fuel-cell cluster

We propose a method for the numerical calculation of the nonsteady exchange of mass and heat in the flow of a chemically reactive coolant in channels of complex shape. As an example we present the results from the calculations of heat and mass exchange in the laminar flow of dissociating N₂O₄ for a triangular fuel-cell cluster.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 57, No. 4, pp. 658–667, October, 1989.     

Simulation and numerical investigations of the kinetics of atmospheric aerosol droplets in the wake behind a flat plate

Within the framework of the physicomathematical model of evolution of a polydisperse condensate, numerical investigations of the kinetics of atmospheric aerosol droplets in a supersonic two-phase flow past a flat plate were carried out. The gas flow was described by the Reynolds equations with the use of the two-parameter turbulence model. In view of the smallness of the condensate mass fraction in the incoming flow, the inverse effect of the dispersed phase on the gas was not considered. For various regimes of exposure to a flow, the characteristic features of the spatial distribution of the main parameters of the condensate fractions have been studied: the number densities, radii, temperatures, and averaged velocities of microdrops. The dependence of the dispersed phase dynamics on the Mach number and the incoming flow angle of attack has been investigated and the influence of the allowance for the processes of coagulation/fragmentation on the mass spectrum of droplets is shown. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 82, No. 2, pp. 331–341, March–April, 2009.     

Calculating the hydraulic resistance of foam flows in tubes

Formulas are derived for calculating the hydraulic resistance factor of a foam flow for isothermal flow in a tube with allowance for compressibility, biphasality, and a change in the structure of the foam during its movement.Notation p pressure in a cross section of the tube - p total pressure drop on a section of the tube - p_fr pressure drop due to friction on a section of the tube - u mean flow rate - specific gravity - K expansion ratio - T flow temperature - R universal gas constant - G weight flow rate - weight gas flow rate - ¯d weighted mean diameter of bubble - surface tension coefficient - D tube diameter - L length of section of tube - F cross-sectional area of tube Indices liquid phase - 0 atmospheric conditions for p=9.8·10⁴ Pa Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 4, pp. 624–628, October, 1980.     

Statistical characteristics of the temperature field in the pipe flow of a gas-water mixture

An experimental study of the statistical characteristics of the temperature field for a flow of a water and gas mixture in a pipe discloses a significant increase in the turbulent transport coefficient and a change in the structure of the water flow in the presence of the gaseous phase.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 20, No. 4, pp. 581–587, April, 1971.     

Nonstationary flow in the model channel of a ramjet engine in pulse-periodic energy supply

A study has been made of the influence of the pulse-periodic supply of energy that is equal to the energy released in the combustion of hydrogen in air on the structure of supersonic flow in a channel of variable cross section, modeling the duct of a ramjet engine. The flow has been modeled on the basis of two-dimensional nonstationary gas-dynamic equations. Different flow regimes have been obtained depending on the configuration of the zones of energy supply and the excess-air coefficients. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 81, No. 3, pp. 464–469, May–June, 2008.     

Effect of rotation of the inner cylinder on the equilibrium condition of spherical particles in a turbulent ascending flow in an annular channel

The equilibrium condition of a solid spherical particle in a turbulent flow is considered. A relation between the rotational velocity of the inner cylinder and the critical velocity of the ascending flow in an annular channel is obtained for the range of Reynolds numbers 10³ –5 · · 10⁴.     

Vortex motion in the early stages of unsteady flow around a circular cylinder

In this paper, processes in the early stages of vortex motion and the development of flow structure behind an impulsively-started circular cylinder at high Reynolds number are investigated by combining the discrete vortex model with boundary layer theory, considering the separation of incoming flow boundary layer and rear shear layer in the recirculating flow region. The development of flow structure and vortex motion, particularly the formation and development of secondary vortex and a pair of secondary vortices and their effect on the flow field are calculated. The results clearly show that the flow structure and vortices motion went through a series of complicated processes before the symmetric main vortices change into asymmetric: development of main vortices induces secondary vortices; growth of the secondary vortices causes the main vortex sheets to break off and causes the symmetric main vortices to become “free” vortices, while a pair of secondary vortices is formed; then the vortex sheets, after breaking off, gradually extend downstream and the structure of a pair of secondary vortices becomes relaxed. These features of vortex motion look very much like the observed features in some available flow field visualizations. The action of the secondary vortices causes the main vortex sheets to break off and converts the main vortices into free vortices. This should be the immediate cause leading to the instability of the motion of the symmetric main vortices. The flow field structure such as the separation position of boundary layer and rear shear layer, the unsteady pressure distributions and the drag coefficient are calculated. Comparison with other results or experiments is also made. This work was presented at the First Asian Congress of Fluid Mechanics, Bangalore in December 1980.     

Asymptotic analysis of the flow of shear-thinning foodstuffs in annular scraped heat exchangers

The problem of isothermal flow of a shear-thinning (pseudoplastic) fluid in the gap between two concentric cylinders is considered. A pump provides an axial pressure gradient which causes flow down the device. The outer cylinder is fixed and has scrapers attached to it to cause flow mixing, whilst the inner cylinder rotates about its axis to provide shear and thus thin the fluid. The goal is to determine the optimal distribution of power between rotation and pumping. Although ostensibly the flow is nonlinear and three-dimensional we show that judicious use of fairly straightforward asymptotic methods can yield a great deal of information about the device, including cross-sectional flow predictions and throughput results. Furthermore, these results are derived for a variety of different flow conditions. Some numerical calculations are carried out using a commercial CFD code. These show good agreement with the asymptotic analysis.     

Effect of inlet configuration on the refrigerant distribution in a parallel flow minichannel heat exchanger

The refrigerant R-134a flow distribution was experimentally studied for a round header/ten flat tube test section simulating a brazed aluminum heat exchanger. Three different inlet configurations (parallel, normal, vertical) were investigated. Tests were conducted with downward flow for mass flux from 70 to 130 kg m⁻² s⁻¹ and quality from 0.2 to 0.6. Tubes were flush-mounted in the test section, with no protrusion into the header. It is shown that normal and vertical inlet yielded similar flow distribution. As mass flux or quality increased, however, better results were obtained for normal inlet configuration. The flow distribution was worst for the parallel inlet configuration. Possible explanation is provided based on flow visualization results. Correlations were developed to predict the fraction of liquid or gas taken off by downstream channel as a function of header gas Reynolds number at immediate upstream.     

Numerical Simulation of Thermocapillary Flow Induced by Non-Uniform Evaporation on the Meniscus in Capillary Tubes

Three-dimensional numerical simulation was developed to investigate thermocapillary flow induced by non-uniform evaporation on the meniscus in capillary tubes. Capillary tube radiuses ranging from 0.1 to 1 mm were considered and the working liquid was methanol. The effects of tube size, evaporation heat flux and buoyancy on thermocapillary flow were investigated. The results show that the non-uniform evaporation on the meniscus leads to two opposite temperature gradients along the radial direction, which generate two thermocapillary flow vortexes under the meniscus. For horizontal capillary tubes with r ₀?≥ 0.32 mm, the path-lines in the vertical center plane are asymmetrical, which is attributed to the combined buoyancy and thermocapillary effects. For the vertical capillary tube, with increasing average evaporation heat flux, the steady axisymmetrical flow will gradually transit to a steady asymmetrical flow and eventually becomes a three-dimensional oscillatory flow.     

Surface Phenomena in the Hydrodynamics of an Incompressible Fluid

By taking as an example the outflow of an incompressible fluid through a hole with small geometric parameters under the conditions of supersmall pressures, we attempted to evaluate the effect of surface phenomena on its flow by the similarity method. The Navier–Stokes differential equation was supplemented by parameters allowing for the effect of surface forces. The modified Navier–Stokes equation was subjected to similarity conversion. This yielded a dimensionless group that includes the whole range of variable parameters affecting the fluid flow, namely, the generalized criterion Pv. The graphic dependence of the coefficient of the fluid flow rate on the generalized criterion Pv is presented on the basis of experimental data.